Detection of a de novo mutation in the dystrophin gene and its significance for medical genetic counseling in Duchenne muscular dystrophy (clinical observation). medical genetics
CHAPTER 5 VARIABILITY OF THE ORGANISM
CHAPTER 5 VARIABILITY OF THE ORGANISM
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The variability of an organism is the variability of its genome, which determines the genotypic and phenotypic differences of a person and causes the evolutionary diversity of its genotypes and phenotypes (see Chapters 2 and 3).
The intrauterine development of the embryo, embryo, fetus, further postnatal development of the human body (infancy, childhood, adolescence, adolescence, adulthood, aging and death) are carried out in accordance with the genetic program of ontogenesis, formed during the merger of the maternal and paternal genomes (see Chapters 2 and 12).
During ontogenesis, the genome of an individual's organism and the information encoded in it undergo continuous transformations under the influence of factors environment. The changes that have arisen in the genome can be transmitted from generation to generation, causing the variability of the characteristics and phenotype of the organism in descendants.
At the beginning of the XX century. the German zoologist W. Hacker singled out the direction of genetics devoted to the study of the relationships and relationships between genotypes and phenotypes and the analysis of their variability, and called it phenogenetics.
Currently, phenogenetics distinguish two classes of variability: non-hereditary (or modification), which is not transmitted from generation to generation, and hereditary, which is transmitted from generation to generation.
In turn, hereditary variability can also be of two classes: combinative (recombination) and mutational. Variability of the first class is determined by three mechanisms: random encounters of gametes during fertilization; crossing over, or meiotic recombination (exchange of equal sections between homologous chromosomes in the prophase of the first division of meiosis); independent divergence of homologous chromosomes to the poles of division during the formation of daughter cells during mitosis and meiosis. The variability of the second
class is due to point, chromosome and genomic mutations (see below).
Let us sequentially consider various classes and types of variability of an organism on different stages his individual development.
Variability during fertilization of gametes and the beginning of the functioning of the genome of the nascent organism
The maternal and paternal genomes cannot function separately from each other.
Only two parental genomes, united in a zygote, ensure the emergence of molecular life, the emergence of a new quality state- one of the properties of biological matter.
On fig. 23 shows the results of the interaction of two parental genomes during the fertilization of gametes.
According to the fertilization formula: zygote \u003d egg + sperm, the beginning of the development of the zygote is the moment of the formation of a double (diploid) when two haploid sets of parental gametes meet. It is then that molecular life is born and a chain of successive reactions is launched based first on the expression of the genes of the genotype of the zygote, and then on the genotypes of the daughter somatic cells that emerged from it. Individual genes and groups of genes in the composition of the genotypes of all body cells begin to “turn on” and “turn off” during the implementation of the genetic program of ontogenesis.
The leading role in the ongoing events belongs to the egg, which has in the nucleus and cytoplasm everything necessary for the nucleation.
Rice. 23. The results of the interaction of two parental genomes during the fertilization of gametes (pictures from www.bio.1september.ru; www.bio.fizteh.ru; www.vetfac.nsau.edu.ru, respectively)
of life and continuation of life structural and functional components nuclei and cytoplasm (essence biological matriarchy). The spermatozoon, on the other hand, contains DNA and does not contain components of the cytoplasm. Having penetrated into the egg, the DNA of the sperm cell comes into contact with its DNA, and thus the main molecular mechanism that functions throughout the life of the organism is “turned on” in the zygote: the DNA-DNA interaction of two parental genomes. Strictly speaking, the genotype is activated, represented by approximately equal parts of the nucleotide sequences of DNA of maternal and paternal origin (without taking into account the mtDNA of the cytoplasm). Let's simplify what has been said: the beginning of molecular life in the zygote is a violation of the constancy of the internal environment of the egg (its homeostasis), and all subsequent molecular life multicellular organism- the desire to restore the homeostasis exposed to environmental factors or the balance between two opposite states: stability On the one side and variability with another. These are the cause-and-effect relationships that determine the emergence and continuity of the molecular life of an organism in the course of ontogenesis.
Let us now turn our attention to the results and significance of the variability of the organism's genome as a product of evolution. First, consider the question of the uniqueness of the genotype of the zygote or parent cell of all cells, tissues, organs and systems of the body.
Fertilization itself occurs by chance: one female gamete is fertilized by only one male gamete out of 200-300 million spermatozoa contained in the male ejaculate. It is obvious that each egg and each sperm are distinguished from each other by many genotypic and phenotypic traits: the presence of altered or unchanged in composition and combinations of genes (results of combinative variability), different sequences of DNA nucleotide sequences, different sizes, shape, functional activity (mobility), gamete maturity, etc. It is these differences that make it possible to speak about the uniqueness of the genome of any gamete and, consequently, the genotype of the zygote and the whole organism: the random fertilization of gametes ensures the birth of genetically unique organism individual.
In other words, the molecular life of a person (as well as the life of a biological being in general) is a "gift of fate" or, if you like, a "divine gift", because instead of a given individual with the same
probability could be born genetically different - his siblings.
Now let's continue our reasoning about the balance between stability and variability of hereditary material. IN broad sense, maintaining such a balance is the simultaneous preservation and change (transformation) of the stability of the hereditary material under the influence of internal (homeostasis) and external environmental factors (reaction rate). Homeostasis depends on the genotype due to the fusion of two genomes (see Fig. 23). The reaction rate is determined by the interaction of the genotype with environmental factors.
Norm and range of reaction
The specific way in which an organism reacts to environmental factors is called reaction norm. It is the genes and genotype that are responsible for the development and range of modifications of individual traits and the phenotype of the whole organism. At the same time, far from all the possibilities of the genotype are realized in the phenotype; phenotype - a particular (for an individual) case of the implementation of the genotype in specific environmental conditions. Therefore, for example, between monozygotic twins with completely identical genotypes (100% of common genes), noticeable phenotypic differences are revealed if the twins grow up in different conditions environment.
The reaction rate can be narrow or wide. In the first case, stability a separate feature(phenotype) is preserved almost regardless of the influence of the environment. Examples of genes with a narrow reaction norm or non-plastic genes serve as genes encoding the synthesis of antigens of blood groups, eye color, curly hair, etc. Their action is the same under any (compatible with life) external conditions. In the second case, the stability of an individual trait (phenotype) changes depending on the influence of the environment. An example of genes with a wide response rate or plastic genes- genes that control the number of erythrocytes in the blood (different for people going uphill and people going down the mountain). Another example of a broad reaction rate is color change skin(sunburn), associated with the intensity and time of exposure to ultraviolet radiation on the body.
Speaking of reaction range, should be borne in mind the phenotypic differences that appear in an individual (his genotype) depending on
"depleted" or "enriched" environmental conditions in which the organism is located. According to the definition of I.I. Schmalhausen (1946), "it is not traits that are inherited, as such, but the norm of their reaction to changes in the conditions of existence of organisms."
Thus, the norm and range of the reaction are the limits of the genotypic and phenotypic variability of the organism when environmental conditions change.
It should also be noted that from internal factors, influencing the phenotypic expression of genes and genotype, the sex and age of the individual have a certain significance.
External and internal factors that determine the development of traits and phenotypes are included in the three groups of main factors indicated in the chapter, including genes and genotype, mechanisms of intermolecular (DNA-DNA) and intergenic interactions between parental genomes, and environmental factors.
Undoubtedly, the basis of an organism's adaptation to environmental conditions (the basis of ontogeny) is its genotype. In particular, individuals with genotypes that do not provide suppression of the negative effects of pathological genes and environmental factors leave fewer offspring than those individuals who unwanted effects are suppressed.
It is likely that the genotypes of more viable organisms include special genes (modifier genes) that suppress the action of "harmful" genes in such a way that alleles of the normal type become dominant instead of them.
NON-HEREDITARY VARIABILITY
Speaking about the non-hereditary variability of the genetic material, we will again consider an example of a wide reaction norm - a change in the color of the skin under the influence of ultraviolet radiation. “Sunburn” is not passed down from generation to generation, i.e. is not inherited, although plastic genes are involved in its occurrence.
In the same way, the results of injuries are not inherited, cicatricial changes tissues and mucous membranes in case of burn disease, frostbite, poisoning and many other signs caused by the action of environmental factors only. At the same time, it should be emphasized that non-hereditary changes or modifications are associated with hereditary
natural properties given organism, because they are formed against the background of a specific genotype in specific environmental conditions.
Hereditary combinative variability
As stated at the beginning of the chapter, in addition to the mechanism of random meetings of gametes during fertilization, combinative variability includes the mechanisms of crossing over in the first division of meiosis and independent divergence of chromosomes to the poles of division during the formation of daughter cells during mitosis and meiosis (see Chapter 9).
Crossing over in the first division of meiosis
Through the mechanism crossing over the linkage of genes with a chromosome is regularly disturbed in the prophase of the first division of meiosis as a result of mixing (exchange) of genes of paternal and maternal origin (Fig. 24).
At the beginning of the XX century. at the opening of the crossing over T.Kh. Morgan and his students suggested that crossing over between two genes can occur not only in one, but also in two, three (respectively, double and triple crossing over) and more points. Crossing-over suppression was noted in the areas immediately adjacent to the exchange points; this suppression is called interference.
In the end, they calculated: for one male meiosis, there are from 39 to 64 chiasms or recombinations, and for one female meiosis - up to 100 chiasms.
Rice. 24. The scheme of crossing over in the first division of meiosis (according to Shevchenko V.A. et al., 2004):
a - sister chromatids of homologous chromosomes before meiosis; b - they are during pachytene (their spiralization is visible); c - they are during diplotene and diakinesis (arrows indicate the places of crossing-over-chiasma, or exchange sites)
As a result, they concluded that the linkage of genes with chromosomes is constantly broken during crossing over.
Factors Affecting Crossing Over
Crossing over is one of the regular genetic processes in the body, controlled by many genes, both directly and through the physiological state of cells during meiosis and even mitosis.
Factors that affect crossover include:
Homo- and heterogametic sex ( we are talking O mitotic crossing over in males and females of such eukaryotes as Drosophila and silkworm); thus, in Drosophila, crossing-over proceeds normally; in the silkworm - either also normal or absent; in humans, attention should be paid to the mixed (“third”) sex and specifically to the role of crossing over in anomalies in the development of sex in male and female hermaphroditism (see Chapter 16);
Structure of chromatin; to the crossover frequency different areas chromosomes affects the distribution of heterochromatic (centromeric and telomeric regions) and euchromatic regions; in particular, in the pericentromeric and telomeric regions, the frequency of crossing over is reduced, and the distance between genes, determined by the frequency of crossing over, may not correspond to the actual one;
The functional state of the body; as age increases, the degree of chromosome spiralization and the rate of cell division change;
Genotype; it contains genes that increase or decrease the frequency of crossing over; The “lockers” of the latter are chromosomal rearrangements (inversions and translocations), which impede the normal conjugation of chromosomes in the zygotene;
Exogenous factors: exposure to temperature, ionizing radiation and concentrated salt solutions, chemical mutagens, drugs and hormones, as a rule, increase the frequency of crossing over.
The frequency of meiotic and mitotic crossing over and CHO is sometimes used to judge the mutagenic effect of drugs, carcinogens, antibiotics, and other chemical compounds.
Unequal crossing over
In rare cases, during crossing over, breaks are observed at asymmetric points of sister chromatids, and they exchange
are separated by unequal sections - this is unequal crossover.
At the same time, cases are described when during mitosis mitotic conjugation (mismatch) of homologous chromosomes is observed and recombination occurs between nonsister chromatids. This phenomenon is called gene conversion.
The importance of this mechanism cannot be overestimated. For example, as a result of incorrect pairing of homologous chromosomes along flanking repeats, doubling (duplication) or loss (deletion) of the chromosome region containing the PMP22 gene can occur, which will lead to the development of hereditary autosomal dominant motor sensory neuropathy Charcot-Marie-Tous.
Unequal crossing over is one of the mechanisms for the occurrence of mutations. For example, the peripheral protein myelin is encoded by the PMP22 gene, located on chromosome 17 and having a length of about 1.5 million bp. This gene is flanked by two homologous repeats about 30 kb long. (repeats are located on the flanks of the gene).
Especially many mutations as a result of unequal crossing over occur in pseudogenes. Then either a fragment of one allele is transferred to another allele, or a fragment of a pseudogene is transferred to a gene. For example, such a mutation is noted when the pseudogene sequence is transferred to the 21-hydroxylase gene (CYP21B) in adrenogenital syndrome or congenital hyperplasia adrenal cortex (see chapters 14 and 22).
In addition, due to recombinations during unequal crossing over, multiple allelic forms of genes encoding antigens can be formed. HLA class I.
Independent divergence of homologous chromosomes to division poles during the formation of daughter cells during mitosis and meiosis
Due to the process of replication preceding mitosis of the somatic cell, the total number of nucleotide sequences of DNA is doubled. The formation of one pair of homologous chromosomes comes from two paternal and two maternal chromosomes. When these four chromosomes are distributed into two daughter cells, each of the cells will receive one paternal and one maternal chromosome (for each pair of chromosomes), but which of the two, the first or second, is unknown. Occurs
random distribution of homologous chromosomes. It is easy to calculate: due to various combinations of 23 pairs of chromosomes, the total number of daughter cells will be 2 23, or more than 8 million (8 χ 10 6) variants of chromosome combinations and genes located on them. Consequently, with a random distribution of chromosomes into daughter cells, each of them will have its own unique karyotype and genotype (its own version of the combination of chromosomes and genes linked to them, respectively). The possibility of a pathological distribution of chromosomes into daughter cells should also be noted. For example, hitting one of two daughter cells with only one (paternal or maternal origin) X chromosome will lead to monosomy (Shereshevsky-Turner syndrome, karyotype 45, XO), hitting three identical autosomes will lead to trisomy (Down syndromes, 47,XY ,+21; Patau, 47,XX,+13 and Edwads, 47,XX,+18; see also chapter 2).
As noted in Chapter 5, two paternal or two maternal chromosomes can simultaneously enter one daughter cell - this is uniparental isodisomy for a specific pair of chromosomes: Silver-Russell syndromes (two maternal chromosomes 7), Beckwith-Wiedemann (two paternal chromosomes 11) , Angelman (two paternal chromosomes 15), Prader-Willi (two maternal chromosomes 15). In general, the volume of chromosome distribution disorders reaches 1% of all chromosomal disorders in a person. These disorders are of great evolutionary importance, because they create a population diversity of human karyotypes, genotypes, and phenotypes. Moreover, each pathological variant is a unique product of evolution.
As a result of the second meiotic division, 4 daughter cells are formed. Each of them will receive either one maternal or paternal chromosome from all 23 chromosomes.
To avoid possible errors in our further calculations, we will take it as a rule: as a result of the second meiotic division, 8 million variants are also formed male gametes and 8 million options female gametes. Then the answer to the question, what is the total volume of variants of combinations of chromosomes and genes located on them when two gametes meet, is the following: 2 46 or 64 χ 10 12 , i.e. 64 trillion.
The formation of such a (theoretically possible) number of genotypes at the meeting of two gametes clearly explains the meaning of the heterogeneity of genotypes.
The value of combinative variability
Combinative variability is important not only for the heterogeneity and uniqueness of the hereditary material, but also for the restoration (repair) of the stability of the DNA molecule when both of its strands are damaged. An example is the formation of a single-stranded DNA gap opposite an unrepaired lesion. The resulting gap cannot be unmistakably corrected without involving a normal DNA strand in the repair.
Mutational variability
Along with the uniqueness and heterogeneity of genotypes and phenotypes as a result of combinative variability, a huge contribution to the variability of the human genome and phenome is made by hereditary mutational variability and the resulting genetic heterogeneity.
Variations in the nucleotide sequences of DNA can be purely conditionally divided into mutations and genetic polymorphism(see chapter 2). At the same time, if genotype heterogeneity is a constant (normal) characteristic of genome variability, then mutational variability- this is usually his pathology.
In favor of the pathological variability of the genome, for example, unequal crossing over, incorrect divergence of chromosomes to the poles of division during the formation of daughter cells, the presence of genetic compounds and allelic series testify. In other words, hereditary combinative and mutational variability is manifested in humans by significant genotypic and phenotypic diversity.
Let's clarify the terminology and consider general issues mutation theory.
GENERAL QUESTIONS OF THE THEORY OF MUTATIONS
Mutation there is a change structural organization, quantity and / or functioning of the hereditary material and the proteins synthesized by it. This concept was first proposed by Hugh de Vries
in 1901-1903 in his work "Mutational Theory", where he described the main properties of mutations. They:
Arise suddenly;
Passed down from generation to generation;
Inherited by dominant type (manifested in heterozygotes and homozygotes) and recessive type (manifested in homozygotes);
Not directed (“mutates” any locus, causing minor changes or affecting vital signs);
By phenotypic expression are harmful (most mutations), beneficial (extremely rare), or indifferent;
Occur in somatic and germ cells.
In addition, the same mutations can occur repeatedly.
mutation process or mutagenesis, there is a continuously ongoing process of the formation of mutations under the influence of mutagens - environmental factors that damage hereditary material.
First continuous mutagenesis theory proposed in 1889 by a Russian scientist from St. Petersburg University S.I. Korzhinsky in his book "Heterogenesis and Evolution".
As is commonly believed at the present time, mutations can manifest themselves spontaneously, without visible changes. external causes, but under the influence of internal conditions in the cell and the body - this spontaneous mutations or spontaneous mutagenesis.
Mutations caused artificially by exposure external factors physical, chemical or biological nature, are induced mutations, or induced mutagenesis.
The most common mutations are called major mutations(for example, mutations in the genes of Duchenne-Becker myodystrophy, cystic fibrosis, sickle cell anemia, phenylketonuria, etc.). Now commercial kits have been created that allow you to automatically identify the most important of them.
Newly occurring mutations are called new mutations or mutations. de novo. For example, these include mutations that underlie a number of autosomal dominant diseases, such as achondroplasia (10% of cases are familial forms), Recklinghausen type I neurofibromatosis (50-70% are familial forms), Alzheimer's disease, Huntington's chorea.
Mutations from the normal state of a gene (trait) to a pathological state are called straight.
Mutations from pathological condition gene (trait) to normal state called reverse or reversions.
The ability to reverse was first established in 1935 by N.V. Timofeev-Ressovsky.
Subsequent mutations in a gene that suppress the primary mutant phenotype are called suppressor. The suppression may be intragenic(restores functional activity squirrel; the amino acid does not match the original, i.e. there is no true reversibility) and extragenic(the structure of tRNA changes, as a result of which the mutant tRNA includes another amino acid in the polypeptide instead of the one encoded by the defective triplet).
Mutations in somatic cells are called somatic mutations. They form pathological cell clones (a set of pathological cells) and, in the case of the simultaneous presence of normal and pathological cells in the body, lead to cellular mosaicism (for example, in Albright's hereditary osteodystrophy, the expressivity of the disease depends on the number of abnormal cells).
Somatic mutations can be either familial or sporadic (non-familial). They underpin the development malignant neoplasms and premature aging processes.
Previously, it was considered an axiom that somatic mutations are not inherited. In recent years, the transmission from generation to generation of hereditary predisposition of 90% of multifactorial forms and 10% of monogenic forms of cancer, manifested by mutations in somatic cells, has been proven.
Mutations in germ cells are called germline mutations. It is believed that they are less common than somatic mutations, underlie all hereditary and some congenital diseases, are transmitted from generation to generation, and can also be familial and sporadic. The most studied area of general mutagenesis is physical and, in particular, radiation mutagenesis. Any sources of ionizing radiation are detrimental to human health; as a rule, they have a powerful mutagenic, teratogenic and carcinogenic effect. The mutagenic effect of a single dose of irradiation is much higher than with chronic irradiation; a radiation dose of 10 rad doubles the mutation rate in humans. It has been proven that ionizing radiation can cause mutations that lead to
to hereditary (congenital) and oncological diseases and ultraviolet to induce DNA replication errors.
The greatest danger is chemical mutagenesis. There are about 7 million chemical compounds in the world. In the national economy, in production and in everyday life, approximately 50-60 thousand chemical substances. About one thousand new compounds are introduced into practice every year. Of these, 10% are able to induce mutations. These are herbicides and pesticides (the proportion of mutagens among them reaches 50%), as well as a number of medicines(some antibiotics, synthetic hormones, cytostatics, etc.).
There is still biological mutagenesis. Biological mutagens include: foreign proteins of vaccines and sera, viruses ( chicken pox, measles rubella, poliomyelitis, herpes simplex, AIDS, encephalitis) and DNA, exogenous factors (deficiency in protein nutrition), histamine compounds and its derivatives, steroid hormones (endogenous factors). Enhance the effect of external mutagens commutagens(toxins).
In the history of genetics, there are many examples of the importance of relationships between genes and traits. One of them is the classification of mutations depending on their phenotypic effect.
Classification of mutations depending on their phenotypic effect
This classification of mutations was first proposed in 1932 by G. Möller. According to the classification were allocated:
amorphous mutations. This is a condition in which the trait controlled by the abnormal allele does not occur because the abnormal allele is not active compared to the normal allele. These mutations include the albinism gene (11q14.1) and about 3000 autosomal recessive diseases;
antimorphic mutations. In this case, the value of the trait controlled by the pathological allele is opposite to the value of the trait controlled by the normal allele. These mutations include the genes of about 5-6 thousand autosomal dominant diseases;
hypermorphic mutations. In the case of such a mutation, the trait controlled by the pathological allele is more pronounced than the trait controlled by the normal allele. Example - goethe-
rozygous carriers of genome instability disease genes (see Chapter 10). Their number is about 3% of the world's population (almost 195 million people), and the number of diseases themselves reaches 100 nosologies. Among these diseases: Fanconi anemia, ataxia telangiectasia, pigment xeroderma, Bloom's syndrome, progeroid syndromes, many forms of cancer, etc. At the same time, the frequency of cancer in heterozygous carriers of the genes for these diseases is 3-5 times higher than in the norm, and in the patients themselves ( homozygotes for these genes) the incidence of cancer is ten times higher than normal.
hypomorphic mutations. This is a condition in which the expression of a trait controlled by a pathological allele is weakened compared to a trait controlled by a normal allele. These mutations include mutations in pigment synthesis genes (1q31; 6p21.2; 7p15-q13; 8q12.1; 17p13.3; 17q25; 19q13; Xp21.2; Xp21.3; Xp22), as well as more than 3000 forms of autosomal recessive diseases.
neomorphic mutations. Such a mutation is said to be when the trait controlled by the pathological allele is of a different (new) quality compared to the trait controlled by the normal allele. Example: the synthesis of new immunoglobulins in response to the penetration of foreign antigens into the body.
Speaking about the enduring significance of H. Möller's classification, it should be noted that 60 years after its publication, the phenotypic effects of point mutations were divided into different classes depending on their effect on the structure protein product gene and/or the level of its expression.
In particular, Nobel laureate Victor McKusick (1992) identified mutations that change the sequence of amino acids in a protein. It turned out that they are responsible for the manifestation of 50-60% of cases of monogenic diseases, and the remaining mutations (40-50% of cases) are mutations affecting gene expression.
A change in the amino acid composition of the protein manifests itself in a pathological phenotype, for example, in cases of methemoglobinemia or sickle cell anemia due to mutations in the betaglobin gene. In turn, mutations affecting the normal expression of the gene were isolated. They lead to a change in the amount of the gene product and are manifested by phenotypes associated with a deficiency of one or another protein, for example,
in cases hemolytic anemia, caused by mutations of genes localized on autosomes: 9q34.3 (adenylate kinase deficiency); 12p13.1 (triose phosphate isomerase deficiency); 21q22.2 (phosphofructokinase deficiency).
The classification of mutations by W. McKusick (1992) is, of course, a new generation of classifications. At the same time, on the eve of its publication, the classification of mutations depending on the level of organization of the hereditary material was widely recognized.
Classification of mutations depending on the level of organization of hereditary material
The classification includes the following.
Point mutations(violation of the structure of the gene at its different points).
Strictly speaking, point mutations include changes in the nucleotides (bases) of one gene, leading to a change in the quantity and quality of the protein products synthesized by them. Changes in bases are their substitutions, insertions, displacements or deletions, which can be explained by mutations in the regulatory regions of genes (promoter, polyadenylation site), as well as in coding and non-coding regions of genes (exons and introns, splicing sites). Base substitutions lead to three types of mutant codons: missense mutations, neutral mutations, and nonsense mutations.
Point mutations are inherited as simple Mendelian traits. They are common: 1 case per 200-2000 births is primary hemochromatosis, non-polyposis colon cancer, Martin-Bell syndrome and cystic fibrosis.
An extremely rare (1:1,500,000) point mutation is severe combined immunodeficiency (SCID) resulting from an adenosine deaminase deficiency. Sometimes point mutations are formed not under the influence of mutagens, but as errors in DNA replication. At the same time, their frequency does not exceed 1:10 5 -1:10 10, since they are corrected with the help of cell repair systems by almost
Structural mutations or chromosome aberrations (violate the structure of chromosomes and lead to the formation of new gene linkage groups). These are deletions (losses), duplications (doublings), translocations (movements), inversions (180° rotation) or insertions (inserts) of hereditary material. Such mutations are characteristic of somatic
cells (including stem cells). Their frequency is 1 per 1700 cell divisions.
A number of syndromes caused by structural mutations are known. The most famous examples are: "Cat's cry" syndrome (karyotype: 46, XX, 5p-), Wolff-Hirschhorn syndrome (46, XX, 4p-), translocation form of Down syndrome (karyotype: 47, XY, t (14; 21) ).
Another example is leukemia. When they occur, a violation of gene expression occurs as a result of the so-called separation (translocation between the structural part of the gene and its promoter region), and, therefore, protein synthesis is disturbed.
Genomic(numerical) mutations- violation of the number of chromosomes or their parts (lead to the emergence of new genomes or their parts by adding or losing whole chromosomes or their parts). The origin of these mutations is due to the nondisjunction of chromosomes in mitosis or meiosis.
In the first case, these are aneuploids, tetraploids with undivided cytoplasm, polyploids with 6, 8, 10 pairs of chromosomes or more.
In the second case, this is the non-separation of paired chromosomes involved in the formation of gametes (monosomy, trisomy) or the fertilization of one egg by two spermatozoa (dispermy or triploid embryo).
Their typical examples have already been cited more than once - these are Shereshevsky-Turner syndrome (45, XO), Klinefelter's syndrome (47, XXY), regular trisomy in Down syndrome (47, XX, +21).
E.V. Tozliyan, pediatric endocrinologist, geneticist, candidate of medical sciences, Separate structural subdivision "Scientific Research Clinical Institute of Pediatrics" SBEI HPE Russian National Research Medical University named after. N.I. Pirogov of the Ministry of Health of the Russian Federation, Moscow Keywords
Key words: children, Noonan syndrome, diagnostics.
key words: children, Noonan syndrome, diagnostics.
The article describes the Noonan syndrome (Ulrich-Noonan syndrome, terneroid syndrome with a normal karyotype) - a rare congenital pathology, is inherited in an autosomal dominant manner, is familial, but there are also sporadic cases. The syndrome suggests the presence of a phenotype characteristic of the Shereshevsky-Turner syndrome in female and male individuals with a normal karyotype. Presented clinical observation. The complexity of differential diagnostic search, lack of awareness of clinicians about this syndrome and the importance of an interdisciplinary approach.
Historical facts
For the first time about unusual syndrome mentioned by O. Kobylinski in 1883 (photo 1).
oldest known clinical case Noonan syndrome, described in 1883 by O. Kobylinski
The disease was described in 1963 by American cardiologist Jacqueline Noonan, who reported on nine patients with valve stenosis. pulmonary artery, short stature, hypertelorism, moderate intellectual decline, ptosis, cryptorchidism, and skeletal disorders. Dr. Noonan, who practiced as pediatric cardiologist at the University of Iowa, noticed that children with a rare type of heart disease - pulmonic valve stenosis - often had typical physical anomalies in the form of short stature, pterygoid neck, wide-set eyes and low-lying ears. Boys and girls were equally amazed. Dr. John Opitz, former student Noonan, was the first to introduce the term "Noonan syndrome" to describe the condition of children who had signs similar to those described by Noonan. Later, Noonan wrote the article "Hypertelorism with the Turner phenotype", and in 1971 the name "Noonan syndrome" was officially recognized at the symposium of cardiovascular diseases.
Etiology and pathogenesis
Noonan syndrome is an autosomal dominant disorder with variable expressivity (Fig. 1). The Noonan syndrome gene is localized to long shoulder chromosome 12. The genetic heterogeneity of the syndrome cannot be ruled out. Sporadic and familial forms of the syndrome with an autosomal dominant form of inheritance have been described. In family cases, the mutant gene is inherited, as a rule, from the mother, since due to severe malformations genitourinary system men with this condition are often infertile. Most of the reported cases are sporadic, caused by de novo mutations.
. Autosomal dominant inheritance pattern
The described combinations of Noonan syndrome with type I neurofibromatosis in several families led to the suggestion possible connection two independent loci 17q11.2 of chromosome 17. Some patients have microdeletions in the 22q11 locus of chromosome 22; in these cases clinical manifestations Noonan syndromes are combined with hypothyroidism of the thymus and DiGeorge's syndrome. A number of authors discuss the involvement of putative genes of lymphogenesis in the pathogenesis of the syndrome due to the presence of facial and somatic anomalies similar to Turner's syndrome and a high incidence of pathology. lymphatic system.
Most common cause Noonan syndrome is a mutation of the PTPN11 gene, which is found in approximately 50% of patients. The protein encoded by the PTPN11 gene belongs to a family of molecules that regulate the response of eukaryotic cells to external signals. Largest number mutations in Noonan syndrome are localized in exons 3,7 and 13 of the PTPN11 gene, encoding protein domains responsible for the transition of the protein to the active state.
Possible ideas about pathogenesis are represented by the following mechanisms:
RAS-MAPK-path is very important way signal transduction, through which extracellular ligands—certain growth factors, cytokines, and hormones—stimulate cell proliferation, differentiation, survival, and metabolism (Fig. 2). After ligand binding, receptors on the cell surface are phosphorylated at the sites of their endoplasmic region. This binding involves adapter proteins (eg, GRB2) that form a constitutive complex with guanine nucleotide exchange factors (eg, SOS) that convert the inactive GDP-bound RAS to its active GTP-bound form. The activated RAS proteins then activate the RAF-MEKERK cascade through a series of phosphorylation reactions. As a result, activated ERK enters the nucleus to change the transcription of target genes and corrects the activity of endoplasmic targets to induce adequate short-term and long-term cellular responses to the stimulus. All genes involved in Noonan syndrome encode proteins integral to this pathway, and mutations disease-causing, usually amplify the signal passing through this path.
. RAS-MAPK signaling pathway. Growth signals are transmitted with activated growth factor receptors to the nucleus. Mutations in PTPN11, KRAS, SOS1, NRAS, and RAF1 are associated with Noonan syndrome, and mutations in SHOC2 and CBL are associated with a Noonan syndrome-like phenotype.
Clinical characteristics of Noonan syndrome
The phenotype of patients with Noonan syndrome resembles Turner's syndrome: a short neck with a pterygoid fold or low hair growth, short stature, hypertelorism of the palpebral fissures (photo 2). Facial micro-anomalies include antimongoloid incision of the palpebral fissures, downward lateral canthus, ptosis, epicanthus, low-lying auricles, folded curl auricles, malocclusion, cleft uvula soft palate, gothic sky, micrognathia and microgenia. The thorax of the thyroid form with hypoplastic widely spaced nipples, the sternum protrudes in the upper part and sinks in the lower. About 20% of patients have moderately severe pathology of the skeleton. Most common funnel deformity chest, kyphosis, scoliosis; less often - a decrease in the number of cervical vertebrae and their fusion, resembling anomalies in Klippel-Feil syndrome.
. Phenotypes of Noonan syndrome
Patients with Noonan syndrome usually have blond, thick, curly hair with unusual crown growth, often dark spots on the skin, hypertrichosis, degeneration of the nail plates, anomalies in the eruption and arrangement of teeth, a tendency to form keloid scars, increased skin extensibility. A third of patients have peripheral lymphedema, more often lymphedema of the hands and feet is manifested in children early age. A frequent sign is a pathology of vision (myopia, strabismus, moderate exophthalmos, etc.). Growth retardation occurs in approximately 75% of patients, is more pronounced in boys and is usually insignificant. Growth retardation manifests itself in the first years of life, less often there is a slight deficit in growth and weight at birth. From the first months of life there is a decrease in appetite. Bone age usually lags behind the passport.
A characteristic feature of the syndrome is unilateral or bilateral cryptorchidism, which occurs in 70–75% of male patients; in adult patients, azoospermia, oligospermia, degenerative changes testicles. Nevertheless, puberty occurs spontaneously, sometimes with some delay. In girls, there is often a delay in the formation of menstruation, sometimes - violations menstrual cycle. Fertility may be normal in both sexes.
Mental retardation is detected in more than half of patients, usually minor. Behavioral features, disinhibition, attention deficit disorder are often noted. Speech is usually better developed than other intellectual spheres. The degree of intellectual decline does not correlate with the severity somatic disorders[Marincheva G.S., 1988]. IN isolated cases describes the malformations of the central nervous system(hydrocephalus, spinal hernias), thromboembolic infarcts of the brain, possibly associated with vascular hypoplasia.
vices internal organs with Noonan syndrome are quite characteristic. The most typical are cardiovascular anomalies: valvular stenosis pulmonary artery (about 60% of patients), hypertrophic cardiomyopathy(20–30%), structural anomalies mitral valve, atrial septal defects, tetralogy of Fallot; coarctation of the aorta has been described only in male patients.
In a third of patients, malformations of the urinary system are recorded (hypoplasia of the kidneys, doubling of the pelvis, hydronephrosis, megaureter, etc.).
Quite often, with Noonan syndrome, increased bleeding is noted, especially with surgical interventions V oral cavity and nasopharynx. Various coagulation defects are found: insufficiency of the platelet system, a decrease in the level of coagulation factors, especially XI and XII, an increase in thromboplastin time. There are reports of a combination of Noonan syndrome with leukemia and rhabdomyosarcoma, which may indicate a slight increase in the risk of malignancy in these patients.
Table 1 presents the features of the phenotype in Noonan syndrome, which change with the age of the patient. Table 2 shows the correlation between phenotype and genotype in Noonan syndrome.
Table 1. Typical facial features in patients with Noonan syndrome by age
| Forehead, face, hair | Eyes | Ears | Nose | Mouth | Neck | |
| Newborn* | High forehead, low hairline in the occipital region | Hypertelorism, downwardly inclined palpebral fissures, epicanthal fold | – | Short and wide recessed root, upturned tip | Deeply recessed philtrum, high broad peaks of the vermilion border of the lips, micrognathia | Excess skin on the back of the head |
| Breast (2–12 months) | Large head, high and protruding forehead | Hypertelorism, ptosis, or thick drooping eyelids | – | Short and wide recessed root | – | – |
| Child (1-12 years old) | Rough features, long face | – | – | – | – | – |
| Teenager (12-18 years old) | Myopathic face | – | – | The bridge is tall and thin | – | Obvious neck fold formation |
| Adult (>18 years old) | Distinctive facial features are refined, the skin appears thin and translucent | – | – | Protruding nasolabial fold | – | – |
| All ages | – | Blue and green irises, diamond-shaped eyebrows | Low, rear-rotated ears with thick folds | – | – | – |
table 2. Correlations between genotype and phenotype in Noonan syndrome*
| The cardiovascular system | Height | Development | Skin and hair | Other | |
| PTPN11 (approx. 50%) | More pronounced stenosis pulmonary trunk; less - hypertrophic cardiomyopathy and atrial septal defect | Lower growth; lower concentration of IGF1 | Patients with N308D and N308S have mild decline or normal intelligence | – | More pronounced hemorrhagic diathesis and juvenile myelomonocytic leukemia |
| SOS1 (approx. 10%) | Less atrial septal defect | Higher growth | Less decline in intelligence, delayed speech development | Similar to cardiocutaneous facial syndrome | – |
| RAF1 (approx. 10%) | More severe hypertrophic cardiomyopathy | – | – | More birthmarks, lentigo, spots of coffee with milk | – |
| KRAS (<2%) | – | – | More severe cognitive delay | Similar to cardio-cutaneous-facial syndrome | – |
| NRAS (<1%) | – | – | – | – | – |
Data from laboratory and functional studies
There are no specific biochemical markers for the diagnosis of Noonan syndrome. In some patients, a decrease in spontaneous nocturnal secretion of growth hormone with a normal response to pharmacological stimulation tests (clopheline and arginine), a decrease in the level of somatomedin-C and a decrease in the response of somatomedins to the introduction of growth hormone are detected.
Diagnosis Criteria
The diagnosis of "Noonan syndrome" is made on the basis of clinical signs, in some cases the diagnosis is confirmed by the results of a molecular genetic study. Criteria for diagnosing the syndrome include the presence of a characteristic face (with a normal karyotype) in combination with one of the following features: heart disease, short stature or cryptorchidism (in boys), delayed puberty (in girls). To detect cardiovascular pathology, it is necessary to conduct an ultrasound examination of the heart with a dynamic determination of the size of the cavities and the walls of the ventricles. Prenatal diagnosis of the disease is possible with the help of ultrasound monitoring, which makes it possible to detect heart defects and anomalies in the structure of the neck.
Differential Diagnosis
In girls, the differential diagnosis is made primarily with Turner syndrome; The diagnosis can be clarified by cytogenetic examination. Phenotypic signs of Noonan syndrome are found in a number of other diseases: Williams syndrome, LEOPARD syndrome, Dubovitz, cardiofacio-cutaneous syndrome, Cornelia de Lange, Cohen, Rubinstein-Taybi, etc. Accurate identification of these diseases will be possible only when conducting molecular genetic studies of each syndrome with significant clinical material that is currently being actively developed.
Treatment
Treatment of patients with Noonan syndrome is aimed at eliminating defects of the cardiovascular system, normalizing mental functions, stimulating growth and sexual development. For the treatment of patients with dysplasia of the valves of the pulmonary artery, among other methods, balloon valvuloplasty is successfully used. In order to stimulate mental development, nootropic and vascular agents are used. Drugs aimed at stimulating sexual development are indicated mainly for patients with cryptorchidism. Chorionic gonadotropin preparations are used in age dosages. At an older age - in the presence of hypogonadism - testosterone preparations. In recent years, recombinant forms of human growth hormone have been used in the treatment of patients with Noonan syndrome. Clinical data are confirmed by an increase in the level of somatomedin-C and specific binding protein during therapy. The final height of patients receiving long-term growth hormone therapy, in some cases, exceeds the average height of family members.
Forecast for life is determined by the severity of cardiovascular pathology.
Prevention disease is based on the data of medical genetic counseling.
Medical genetic counseling
In medical genetic counseling, one should proceed from the autosomal dominant type of inheritance and a high (50%) risk of recurrence of the disease in the family with inherited forms. In order to identify the nature of the type of inheritance, it is necessary to conduct a thorough examination of the parents, since the syndrome can manifest itself with minimal clinical symptoms. Currently, molecular genetic diagnosis of the disease has been developed and is being improved by typing mutations in the genes: PTPN11, SOS1, RAF1, KRAS, NRAS, etc. Methods for prenatal diagnosis of the disease are being developed.
Clinical observation
Boy G., 9 years old (photo 3), was observed at the place of residence by a geneticist with a diagnosis of chromosomal pathology?, Williams syndrome (a peculiar phenotype, thickening of the mitral valve cusps, hypercalcemia once every 3 years)?.
. Peculiarities of the phenotype of a child with Noonan syndrome (an elongated facial skeleton with “chubby cheeks”, a short neck, pterygoid folds on the neck, a shortened nose with nostrils open forward, puffy lips, a sloping chin, an anti-Mongoloid incision of the palpebral fissures, malocclusion, macrostomia)
Complaints on reduced memory, fatigue, reduced growth rates.
Family history : parents are Russian by nationality, not related by blood and not having occupational hazards, healthy. The height of the father is 192 cm, the height of the mother is 172 cm. In the pedigree of cases of mental illness, epilepsy, developmental delays were not noted.
History of life and disease : a boy from the 2nd pregnancy (1st pregnancy - m / a), which proceeded with the threat of interruption throughout, accompanied by polyhydramnios. The birth was the first, on time, rapid, birth weight - 3400 g, length - 50 cm. He screamed immediately, Apgar score - 7/9 points. At birth, the neonatologist drew attention to the unusual phenotype of the child, recommended the study of the karyotype, the result is 46, XY (normal male karyotype). Congenital hypothyroidism was suspected, a thyroid profile study was performed, the result was a normal thyroid status. Further, the child was observed by a geneticist with a presumed diagnosis of "Williams syndrome". Early postnatal period - without features. Motor development by age, the first words - by the year, phrasal speech - at 2 years 3 months.
At the age of 8, he was consulted by an endocrinologist about reduced growth rates, fatigue, and reduced memory. An x-ray examination of the hands revealed a moderate lag in bone age (BC) from the passport one (BC corresponded to 6 years). The study of the thyroid profile revealed a moderate increase in thyroid-stimulating hormone with a normal level of free T4 and other indicators; Ultrasound of the thyroid gland - without pathology. Hormone therapy was prescribed, followed by dynamic observation.
Taking into account the uncertainty of the diagnosis at the place of residence, the geneticist referred the child to the Moscow Regional Consultative and Diagnostic Center for Children in order to clarify the diagnosis.
Objective research data:
Height - 126 cm, weight - 21 kg.
Physical development is below average, harmonious. Growth Sds corresponds to -1 (normal -2 + 2). Phenotype features (photo 3): elongated facial skeleton with “chubby cheeks”, short neck, pterygoid folds on the neck, low hair growth on the neck, short nose with nostrils open forward, puffy lips, sloping chin, anti-Mongoloid incision of the palpebral fissures, malocclusion , macrostomia, nipple hypertelorism, asymmetry of the chest, incomplete skin syndactyly of the 2nd or 3rd fingers on the feet, pronounced hypermobility of the interphalangeal joints, brittle, dry nails. On the internal organs - without features. Sexual development - Tanner I (which corresponds to the prepubertal period).
Data from laboratory and functional studies:
Clinical analysis of blood and urine is the norm.
Biochemical analysis of blood - indicators within the normal range.
Thyroid profile (TSH) - 7.5 μIU / ml (normal - 0.4-4.0), other indicators are normal.
Somatotropic hormone (STH) - 7 ng / ml (norm - 7-10), somatomedin-C - 250 ng / ml (norm - 88-360).
Ultrasound of the thyroid gland - without pathology.
Ultrasound of the internal organs - without features.
ECG - sinus tachycardia, the normal position of the electrical axis of the heart.
Echocardiography - MVP of the 1st degree with minimal regurgitation, myxomatous thickening of the mitral valve cusps, an additional chord in the cavity of the left ventricle.
R-graphy of the spine - right-sided scoliosis of the thoracic spine, I degree.
R-graphy of the hands with the capture of the forearms - bone age 7–8 years.
EEG patterns of epileptic activity were not registered.
MRI of the brain - without pathological changes.
Audiogram - without pathology.
DNA diagnostics: molecular genetic study - no deletions of the studied loci of the critical region of chromosome 7 were detected; Gly434Ary (1230G>A) mutation was found in the 11th exon of the SOS1 gene (PTPN11 gene analysis - no mutations were found), which is typical for Noonan syndrome.
Expert advice:
Endocrinologist- subclinical hypothyroidism, incomplete drug compensation.
Optometrist- astigmatism.
Neurologist- vegetative dystonia. neurotic reactions.
Cardiologist- functional cardiopathy.
Orthopedic surgeon- violation of posture. Chest deformity.
Geneticist Noonan syndrome.
Taking into account the phenotype of the child, the history data, the results of additional studies, the diagnosis of Noonan syndrome was made, which was confirmed by the result of a molecular genetic study.
Thus, the presented clinical observation demonstrates the complexity of differential diagnostic search, the need to integrate individual signs into the general phenotype of a particular pathological condition for targeted timely diagnosis of certain forms of hereditary diseases, and the importance of molecular genetic methods to clarify the diagnosis. Timely diagnosis, clarification of the genesis of each syndrome are especially important, as they allow you to find the best approach to the treatment of these conditions, the prevention of possible complications (up to the child's disability); prevention of the recurrence of hereditary diseases in affected families (medical genetic counseling). This dictates the need for doctors of various specialties to clearly navigate the flow of hereditary pathology.
Bibliography:
- Baird P., De Jong B. Noonan's syndrome (XX and XY Turner phenotype) in three generations of a family // J. Pediatr., 1972, vol. 80, p. 110–114.
- Hasegawa T., Ogata T. et al. Coarctation of the aorta and renal hupoplasia in a boy with Turner/Noonan surface anomalies and a 46, XY karyotype: a clinical model for the possible impairment of a putative lymphogenic gene(s) for Turner somatic stigmata // Hum. Genet., 1996, vol. 97, p. 564–567.
- Fedotova T.V., Kadnikova V.A. et al. Clinical-molecular-genetic analysis of the Noonan syndrome. Materials of the VI Congress of the Russian Society of Medical Genetics. Medical Genetics, Supplement to No. 5, 2010, p.184.
- Ward K.A., Moss C., McKeown C. The cardio-facio-cutaneous syndrome: a manifestation of the Noonan syndrome? // Br. J. Dermatol., 1994, vol. 131, p. 270–274.
- Municchi G., Pasquino A.M. et al. Growth hormone treatment in Noonan syndrome: report of four cases who reached fi nal height // Horm. Res., 1995, vol. 44, p. 164–167.
Schizophrenia is one of the most mysterious and complex diseases, and in many ways. It is difficult to diagnose - there is still no consensus on whether this disease is one or many similar to each other. It is difficult to treat it - now there are only drugs that suppress the so-called. positive symptoms (like delirium), but they do not help return the person to a full life. Schizophrenia is difficult to study - no other animal except humans suffers from it, so there are almost no models for studying it. Schizophrenia is very difficult to understand from a genetic and evolutionary point of view - it is full of contradictions that biologists cannot yet resolve. However, the good news is that in recent years things have finally seemed to get off the ground. We have already talked about the history of the discovery of schizophrenia and the first results of its study by neurophysiological methods. This time we will talk about how scientists are looking for the genetic causes of the disease.
The importance of this work is not even that almost every hundredth person on the planet suffers from schizophrenia, and progress in this area should at least radically simplify the diagnosis, even if it is not possible to create a good medicine right away. The importance of genetic research lies in the fact that they are already changing our understanding of the fundamental mechanisms of inheritance of complex traits. If scientists do manage to understand how such a complex disease as schizophrenia can “hide” in our DNA, this will mean a radical breakthrough in understanding the organization of the genome. And the significance of such work will go far beyond clinical psychiatry.
First, some raw facts. Schizophrenia is a severe, chronic, disabling mental illness that usually affects people at a young age. It affects about 50 million people worldwide (slightly less than 1% of the population). The disease is accompanied by apathy, lack of will, often hallucinations, delirium, disorganization of thinking and speech, and motor disorders. Symptoms usually cause social isolation and reduced performance. The increased risk of suicide in patients with schizophrenia, as well as concomitant somatic diseases, lead to the fact that their overall life expectancy is reduced by 10-15 years. In addition, patients with schizophrenia have fewer children: men have an average of 75 percent, women - 50 percent.
The last half century has been a time of rapid progress in many areas of medicine, but this progress has hardly affected the prevention and treatment of schizophrenia. Last but not least, this is due to the fact that we still do not have a clear idea about the violation of which biological processes is the cause of the development of the disease. This lack of understanding has meant that since the introduction of the first antipsychotic drug chlorpromazine (trade name: Aminazine) to the market more than 60 years ago, there has not been a qualitative change in the treatment of the disease. All currently approved antipsychotics for the treatment of schizophrenia (both typical, including chlorpromazine and atypical ones) have the same main mechanism of action: they reduce the activity of dopamine receptors, which eliminates hallucinations and delusions, but, unfortunately, has little effect on negative symptoms. like apathy, lack of will, thought disorders, etc. We don’t even mention side effects. A common disappointment in schizophrenia research is that drug companies have long been cutting funding for antipsychotics, even as the total number of clinical trials continues to rise. However, the hope for clarification of the causes of schizophrenia came from a rather unexpected direction - it is associated with unprecedented progress in molecular genetics.
Collective responsibility
Even the first researchers of schizophrenia noticed that the risk of getting sick is closely related to the presence of sick relatives. Attempts to establish the mechanism of inheritance of schizophrenia were made almost immediately after the rediscovery of Mendel's laws, at the very beginning of the 20th century. However, unlike many other diseases, schizophrenia did not want to fit into the framework of simple Mendelian models. Despite the high heritability, it was not possible to associate it with one or more genes, therefore, by the middle of the century, the so-called "syntheses" began to become more and more popular. psychogenic theories of disease development. In agreement with psychoanalysis, which was extremely popular by the middle of the century, these theories explained the apparent heritability of schizophrenia not by genetics, but by the characteristics of upbringing and an unhealthy atmosphere within the family. There was even such a thing as "schizophrenogenic parents."
However, this theory, despite its popularity, did not last long. The final point on the question of whether schizophrenia is a hereditary disease was put by psychogenetic studies conducted already in the 60-70s. These were primarily twin studies, as well as studies of adopted children. The essence of twin studies is to compare the probabilities of the manifestation of some sign - in this case, the development of the disease - in identical and fraternal twins. Since the difference in the effect of the environment on twins does not depend on whether they are identical or fraternal, the differences in these probabilities should mainly come from the fact that identical twins are genetically identical, while fraternal twins have, on average, only half the common variants of genes.
In the case of schizophrenia, it turned out that the concordance of identical twins is more than 3 times higher than the concordance of fraternal twins: for the first it is approximately 50 percent, and for the second - less than 15 percent. These words should be understood as follows: if you have an identical twin brother suffering from schizophrenia, then you yourself will get sick with a probability of 50 percent. If you and your brother are fraternal twins, then the risk of getting sick is no more than 15 percent. Theoretical calculations, which additionally take into account the prevalence of schizophrenia in the population, estimate the contribution of heritability to the development of the disease at the level of 70-80 percent. For comparison, height and body mass index are inherited in much the same way - traits that have always been considered closely related to genetics. By the way, as it turned out later, the same high heritability is characteristic of three of the four other major mental illnesses: attention deficit hyperactivity disorder, bipolar disorder and autism.
The results of twin studies have been fully confirmed in the study of children who were born to patients with schizophrenia and were adopted in early infancy by healthy adoptive parents. It turned out that their risk of developing schizophrenia is not reduced compared to children raised by their schizophrenic parents, which clearly indicates the key role of genes in etiology.
And here we come to one of the most mysterious features of schizophrenia. The fact is that if it is so strongly inherited and at the same time has a very negative effect on the fitness of the carrier (recall that patients with schizophrenia leave at least half as many offspring as healthy people), then how does it manage to remain in the population for at least ? This contradiction, around which in many respects the main struggle between different theories takes place, has been called the "evolutionary paradox of schizophrenia"
Until recently, it was completely unclear to scientists what specific features of the genome of patients with schizophrenia predetermine the development of the disease. For decades, there has been a heated debate not even about which genes are changed in patients with schizophrenia, but about what is the general genetic "architecture" of the disease.
It means the following. The genomes of individual people are very similar to each other, with differences averaging less than 0.1 percent of nucleotides. Some of these distinguishing features of the genome are quite widespread in the population. It is conventionally considered that if they occur in more than one percent of people, they can be called common variants or polymorphisms. These common variants are believed to have appeared in the human genome over 100,000 years ago, before the first migration from Africa of the ancestors of modern humans, so they are commonly found in most human subpopulations. Naturally, in order to exist in a significant part of the population for thousands of generations, most of the polymorphisms should not be too harmful to their carriers.
However, in the genome of each of the people there are other genetic features - younger and rarer. Most of them do not provide carriers with any advantage, so their frequency in the population, even if they are fixed, remains insignificant. Many of these traits (or mutations) have a more or less pronounced negative effect on fitness, so they are gradually removed by negative selection. Instead, as a result of a continuous mutation process, other new harmful variants appear. In sum, the frequency of any of the new mutations almost never exceeds 0.1 percent, and such variants are called rare.
So, the architecture of a disease means exactly which genetic variants - common or rare, having a strong phenotypic effect, or only slightly increasing the risk of developing a disease - predetermine its occurrence. It is around this issue that, until recently, the main debate about the genetics of schizophrenia took place.
The only fact indisputably established by molecular genetic methods regarding the genetics of schizophrenia over the last third of the 20th century is its incredible complexity. Today it is obvious that the predisposition to the disease is determined by changes in dozens of genes. At the same time, all the "genetic architectures" of schizophrenia proposed during this time can be combined into two groups: the "common disease - common variants" (CV) model and the "common disease - rare variants" model (common disease - rare variants", RV). Each of the models gave its own explanation of the "evolutionary paradox of schizophrenia."
RV vs. CV
According to the CV model, the genetic substrate of schizophrenia is a set of genetic traits, a polygene, akin to what determines the inheritance of quantitative traits such as height or body weight. Such a polygene is a set of polymorphisms, each of which only slightly affects the physiology (they are called "causal", because, although not alone, they lead to the development of the disease). To maintain a fairly high incidence rate characteristic of schizophrenia, it is necessary that this polygene consists of common variants - after all, it is very difficult to collect many rare variants in one genome. Accordingly, each person has dozens of such risky variants in his genome. In sum, all causal variants determine the genetic predisposition (liability) of each individual to the disease. It is assumed that for qualitative complex features, such as schizophrenia, there is a certain threshold value of predisposition, and only those people whose predisposition exceeds this threshold value develop the disease.
Threshold model of disease susceptibility. A normal distribution of predisposition plotted on the horizontal axis is shown. People whose predisposition exceeds the threshold value develop the disease.
For the first time, such a polygenic model of schizophrenia was proposed in 1967 by one of the founders of modern psychiatric genetics, Irving Gottesman, who also made a significant contribution to proving the hereditary nature of the disease. From the point of view of adherents of the CV model, the persistence of a high frequency of causal variants of schizophrenia in the population over many generations can have several explanations. First, each individual such variant has a rather minor effect on the phenotype, such "quasi-neutral" variants may be invisible to selection and remain common in populations. This is especially true for populations with low effective size, where the influence of chance is no less important than selection pressure - this includes the population of our species.
On the other hand, assumptions have been made about the presence in the case of schizophrenia of the so-called. balancing selection, i.e., the positive effect of "schizophrenic polymorphisms" on healthy carriers. It's not that hard to imagine. It is known, for example, that schizoid individuals with a high genetic predisposition to schizophrenia (of which there are many among close relatives of patients) are characterized by an increased level of creative abilities, which may slightly increase their adaptation (this has already been shown in several works). Population genetics allows for a situation where the positive effect of causal variants in healthy carriers may outweigh the negative consequences for those people who have too many of these "good mutations", which led to the development of the disease.
The second basic model of the genetic architecture of schizophrenia is the RV model. She suggests that schizophrenia is a collective concept and that each individual case or family history of the disease is a separate quasi-Mendelian disease associated in each individual case with unique changes in the genome. In this model, causal genetic variants are under very strong selection pressure and are quickly removed from the population. But since a small number of new mutations occur in each generation, a certain balance is established between selection and the emergence of causal variants.
On the one hand, the RV model can explain why schizophrenia is very well inherited, but its universal genes have not yet been found: after all, each family inherits its own causal mutations, and there are simply no universal ones. On the other hand, if we are guided by this model, then we have to admit that mutations in hundreds of different genes can lead to the same phenotype. After all, schizophrenia is a common disease, and the occurrence of new mutations is rare. For example, data on sequencing of father-mother-child triplets show that in each generation, only 70 new single-nucleotide substitutions occur per 6 billion nucleotides of the diploid genome, of which, on average, only a few can theoretically have any effect on the phenotype, and mutations of other types - an even rarer occurrence.
However, some empirical evidence indirectly supports this model of the genetic architecture of schizophrenia. For example, in the early 1990s, it was discovered that about one percent of all patients with schizophrenia had a microdeletion in one of the regions of the 22nd chromosome. In the vast majority of cases, this mutation is not inherited from parents, but occurs de novo during gametogenesis. One in 2,000 people is born with this microdeletion, which leads to a variety of abnormalities in the body, called "DiGeorge syndrome." Those suffering from this syndrome are characterized by severe impairment of cognitive functions and immunity, often accompanied by hypocalcemia, as well as problems with the heart and kidneys. A quarter of people with DiGeorge syndrome develop schizophrenia. It would be tempting to suggest that other cases of schizophrenia are due to similar genetic disorders with catastrophic consequences.
Another empirical observation indirectly supporting the role de novo mutations in the etiology of schizophrenia is the relationship of the risk of getting sick with the age of the father. So, according to some data, among those whose fathers were over 50 years old at the time of birth, there are 3 times more patients with schizophrenia than among those whose fathers were under 30. de novo mutations. Such a connection, for example, has long been established for sporadic cases of another (monogenic) hereditary disease - achondroplasia. This correlation has most recently been confirmed by the aforementioned triplet sequencing data: de novo mutations are associated with the age of the father, but not with the age of the mother. According to the calculations of scientists, on average, a child receives 15 mutations from the mother, regardless of her age, and from the father - 25 if he is 20 years old, 55 if he is 35 years old and more than 85 if he is over 50. That is, the number de novo mutations in the child's genome increases by two with each year of the father's life.
Together, these data seemed to indicate quite clearly the key role de novo mutations in the etiology of schizophrenia. However, the situation actually turned out to be much more complicated. Even after the separation of the two main theories, for decades the genetics of schizophrenia stagnated. Almost no reliable reproducible evidence has been obtained in favor of one of them. Neither about the general genetic architecture of the disease, nor about specific variants that affect the risk of developing the disease. A sharp jump has occurred over the past 7 years and it is associated primarily with technological breakthroughs.
Looking for genes
The sequencing of the first human genome, the subsequent improvement in sequencing technologies, and then the advent and widespread introduction of high-throughput sequencing finally made it possible to gain a more or less complete understanding of the structure of genetic variability in the human population. This new information immediately began to be used for a full-scale search for genetic determinants of predisposition to certain diseases, including schizophrenia.
Similar studies are structured like this. First, a sample of unrelated sick people (cases) and a sample of unrelated healthy individuals (controls) of approximately the same size are collected. All these people are determined by the presence of certain genetic variants - just in the last 10 years, researchers have the opportunity to determine them at the level of entire genomes. Then, the frequency of occurrence of each of the identified variants is compared between groups of sick people and a control group. If at the same time it is possible to find a statistically significant enrichment of one or another variant in carriers, it is called an association. Thus, among the vast number of existing genetic variants are those that are associated with the development of the disease.
An important measure that characterizes the effect of a disease-associated variant is OD (odds ratio), which is defined as the ratio of the chances of getting sick in carriers of this variant compared to those people who do not have it. If the OD value of a variant is 10, this means the following. If we take a random group of carriers of the variant and an equal group of people who do not have this variant, it turns out that in the first group there will be 10 times more patients than in the second. At the same time, the closer the OD is to one for a given variant, the larger the sample is needed in order to reliably confirm that the association really exists - that this genetic variant really affects the development of the disease.
Such work has now made it possible to detect more than a dozen submicroscopic deletions and duplications associated with schizophrenia throughout the genome (they are called CNV - copy number variations, one of the CNVs just causes the DiGeorge syndrome already known to us). For the CNVs that have been found to cause schizophrenia, the OD ranges from 4 to 60. These are high values, but due to their extreme rarity, even in total, they all explain only a very small part of the heritability of schizophrenia in the population. What is responsible for the development of the disease in everyone else?
After relatively unsuccessful attempts to find CNVs that would cause the development of the disease not in a few rare cases, but in a significant part of the population, supporters of the "mutation" model had high hopes for another type of experiment. They compare in patients with schizophrenia and healthy controls not the presence of massive genetic rearrangements, but the complete sequences of genomes or exomes (the totality of all protein-coding sequences). Such data, obtained using high-throughput sequencing, makes it possible to find rare and unique genetic features that cannot be detected by other methods.
The cheapening of sequencing has made it possible in recent years to conduct experiments of this type on rather large samples, including several thousand patients and the same number of healthy controls in recent studies. What is the result? Alas, so far only one gene has been found, in which rare mutations are reliably associated with schizophrenia - this is the gene SETD1A, encoding one of the important proteins involved in the regulation of transcription. As in the case of CNV, the problem here is the same: mutations in the gene SETD1A cannot explain any significant part of the heritability of schizophrenia due to the fact that they are simply very rare.
Relationship between the prevalence of associated genetic variants (horizontal axis) and their impact on the risk of developing schizophrenia (OR). In the main plot, the red triangles show some of the disease-associated CNVs identified so far, the blue circles show the SNPs from GWAS. The incision shows areas of rare and frequent genetic variants in the same coordinates.
There are indications that there are other rare and unique variants that influence susceptibility to schizophrenia. And further increase in samples in experiments using sequencing should help to find some of them. However, while the study of rare variants may still provide some valuable information (especially this information will be important for creating cellular and animal models of schizophrenia), most scientists now agree that rare variants play only a minor role in heritability. schizophrenia, and the CV model is much better at describing the genetic architecture of the disease. The confidence in the correctness of the CV model came first of all with the development of GWAS-type studies, which we will discuss in detail in the second part. In short, studies of this type have uncovered the very common genetic variability that describes a large proportion of the heritability of schizophrenia, the existence of which was predicted by the CV model.
Additional support for the CV model for schizophrenia is the relationship between the level of genetic predisposition to schizophrenia and the so-called schizophrenia spectrum disorders. Even early researchers of schizophrenia noticed that among relatives of patients with schizophrenia, there are often not only other patients with schizophrenia, but also "eccentric" personalities with oddities of character and symptoms similar to schizophrenic, but less pronounced. Subsequently, such observations led to the concept that there is a whole set of diseases that are characterized by more or less pronounced disturbances in the perception of reality. This group of diseases is called the schizophrenia spectrum disorder. In addition to various forms of schizophrenia, these include delusional disorders, schizotypal, paranoid and schizoid personality disorders, schizoaffective disorder and some other pathologies. Gottesman, proposing his polygenic model of schizophrenia, suggested that people with subthreshold values of predisposition to the disease may develop other pathologies of the schizophrenic spectrum, and the severity of the disease correlates with the level of predisposition.
If this hypothesis is correct, it would be logical to assume that the genetic variants found to be associated with schizophrenia would also be enriched among people with schizophrenia spectrum disorders. To assess the genetic predisposition of each individual, a special value is used, called the level of polygenic risk (polygenic risk score). The level of polygenic risk takes into account the total contribution of all common risk variants identified in the GWAS, present in the genome of a given person, to the predisposition to the disease. It turned out that, as predicted by the CV model, the values of the polygenic risk level correlate not only with schizophrenia itself (which is trivial), but also with other diseases of the schizophrenia spectrum, and higher levels of polygenic risk correspond to severe types of disorders.
And yet one problem remains - the phenomenon of "old fathers". If much of the empirical evidence supports the polygenic model of schizophrenia, how does one reconcile with it the long-established association between age at fatherhood and children's risk of developing schizophrenia?
An elegant explanation of this phenomenon was once put forward in terms of the CV model. It has been suggested that late fatherhood and schizophrenia are not cause and effect, respectively, but are two consequences of a common cause, namely the genetic predisposition of late fathers to schizophrenia. On the one hand, a high level of susceptibility to schizophrenia may correlate in healthy men with later fatherhood. On the other hand, it is clear that a father's high predisposition predetermines an increased likelihood that his children will develop schizophrenia. It turns out that we can deal with two independent correlations, which means that the accumulation of mutations in male spermatozoa precursors may have almost no effect on the development of schizophrenia in their offspring. Recent modeling results, taking into account epidemiological data, as well as fresh molecular data on frequency de novo mutations are in good agreement with this explanation of the phenomenon of "old fathers".
Thus, at the moment we can assume that there are almost no convincing arguments in favor of the "mutational" RV model of schizophrenia. So the key to the etiology of the disease lies in which particular set of common polymorphisms causes schizophrenia in accordance with the CV model. How geneticists are looking for this set and what they have already discovered will be the subject of the second part of our story.
Arkady GolovAll body proteins are written in cellular DNA. Only 4 types of nucleic bases - and countless combinations of amino acids. Nature made sure that each failure was not critical and made redundant. But sometimes the distortion still creeps in. It's called mutation. This is a violation in the recording of the DNA code.
Useful - rare
Most of these distortions (more than 99%) are negative for the organism, which makes the theory of evolution untenable. The remaining one percent is not able to provide an advantage, since not every mutated organism gives offspring. Indeed, in nature, not everyone has the right to reproduce. Cell mutation occurs more often in males - and males, as you know, die more often in nature without giving offspring.
The women are to blame
However, man is an exception. In our species, it is most often triggered by the irresponsible behavior of females. Smoking, alcohol, drugs, STDs - and a limited supply of eggs, exposed to negative effects from early childhood. If it exists for men, then for women, even a small glass can provoke violations of the proper formation of eggs. While European women enjoy freedom, Arab women abstain - and give birth to healthy children.
Not spelled correctly
Mutation is a permanent change in DNA. It can affect a small area or a whole block in the chromosome. But even a minimal violation shifts the DNA code, forcing the synthesis of completely different amino acids - therefore, the entire protein encoded by this site will be inactive.
Three types
A mutation is a violation of one of the types - either inherited, or a de novo mutation, or a local mutation. In the first case, it is. In the second, it is a violation at the level of the sperm or egg, as well as the consequence of exposure to hazardous factors after fertilization. Hazardous factors are not only bad habits, but also unfavorable environmental conditions (including radiation). A de novo mutation is a disturbance in all the cells of the body, as it arises from an abnormal source. In the third case, local, or does not occur in the early stages and does not affect all cells of the body, with a high degree of probability it is not transmitted to offspring, in contrast to the first and second types of disorders.
If problems arose in the early stages of pregnancy, then a mosaic disorder occurs. In this case, some of the cells are affected by the disease, some are not. With this species, there is a high probability that the child will be born alive. Most of the genetic disorders cannot be seen, because in this case miscarriages often occur. The mother often does not even notice the pregnancy, it looks like a delayed period. If the mutation is harmless and occurs frequently, it is called a polymorphism. This is how blood types and colors of the iris originated. However, polymorphism can increase the likelihood of certain diseases.
Neurological and mental disorders account for 13% of the global burden of disease, directly affecting over 450 million people worldwide. The prevalence of these disorders is likely to continue to rise as a result of the increasing life expectancy of the population. Unfortunately, nearly half of patients with schizophrenia currently do not receive appropriate medical care, in part because the early symptoms of schizophrenia are often confused with those seen in other psychiatric disorders (such as psychotic depression or bipolar disorder). Other disorders such as Rett syndrome (RTT) and neurofibromatosis type II (NF2) require a multidisciplinary approach and treatment in specialized medical centers. In addition, most of these disorders are complex, resulting from the interaction of genetic and environmental factors.
Based on data from dual studies, the heritability of some psychiatric disorders is high. This applies to autism and schizophrenia, with inherited factors on the order of 90% and 80% respectively. However, these diseases also often occur as isolated cases, with only one affected child born to unaffected parents with no family history of the disease. One possible explanation for this phenomenon is the appearance of mutations de de novo where the mutations occur during spermatogenesis or oogenesis (germline mutations) and are therefore present in the patient but not detectable in the unaffected parent. This genetic mechanism has recently been the focus of attention in explaining part of the genetic basis for neurodevelopmental disorders.
Given the fact that the human genome is estimated to contain approximately 22,333 genes, over 17,800 genes are expressed in the human brain. Mutations affecting almost any of these genes, when combined with environmental factors, can contribute to neurological and psychiatric brain disorders. Recent studies have identified a number of causal mutations in genes and have shown the significant role that genetics play in neurological and psychiatric disorders. These studies have shown the involvement of rare (<1% частоты) точечных мутаций и вариаций числа копий (CNVs, то есть геномных делеций или дублирования от>1 kb to several Mb) that may arise in gene-free regions, or that may affect a single gene, or include a contiguous set of genes in the genetic etiology of autism, schizophrenia, intellectual disability, attention deficit disorder, and other neuropsychiatric disorders.
It has long been known that neurological and psychiatric disorders appear in the same families, suggesting heritability with a major genetic component of the disease. For some neurological disorders, such as NF2 or RTT, a genetic cause has been identified. However, for the vast majority of neurological and psychiatric disorders, such as schizophrenia, autism, bipolar disorder, and restless leg syndrome, the genetic causes remain largely unknown. Recent developments in DNA sequencing technologies have opened up new possibilities for our understanding of the genetic mechanisms underlying these disorders. Using massive parallel DNA sequencing platforms (also called “next generation”), one sample (experiment) can look for mutations in all genes of the human genome.
Known value De Novo mutations (i.e. acquired mutations in offspring) in mental disorders such as mental retardation (ID), autism and schizophrenia. Indeed, in many recent genome studies, analysis of the genomes of affected individuals and comparison with those of their parents has shown that rare coding and non-coding variations de novo significantly associated with the risk of autism and schizophrenia. It has been suggested that a large number of new cases of these disorders are due in part to mutations de novo, which can compensate for allelic losses due to severely reduced reproductive capacity, thereby maintaining high rates of these diseases. Surprisingly, mutations de novo quite common (on the order of 100 new mutations per child), with only a few (on the order of one per child) in coding regions.
Mutations de novo outside of coding regions, such as in promoter, intron or intergenic regions, may also be associated with disease. However, the challenge is to determine which of these mutations is pathogenic.
Several main lines of evidence must be taken into account when evaluating the pathogenicity of an observed De Novo mutations: De Novo mutation rate, gene function, mutation impact, and clinical correlations. The main questions now can be formulated as follows: how many genes will be involved in neurological and mental disorders? What specific gene pathways are involved? What are the consequences of mutations de novo for genetic counseling? These questions need to be answered in order to improve diagnosis and develop treatments.
The role of mutations de novo in human diseases is well known, especially in the field of oncological genetics and dominant Mendelian disorders such as Kabuki and Schinzel-Giedon syndromes. Both of these syndromes are characterized by severe intellectual disability and congenital facial anomalies, and have recently been found to be caused by mutations de novo V MLL2 genes And SETBP1, respectively. Recently Sanders research et al., Neale et al., O "Roak et al. confirmed the contribution De Novo mutations in the etiology of autism. Each study identified a list of mutations de novo, present in probands, but only a few genes have been identified with several de novo (CHD8, SCN2A, KATNAL2 And NTNG1). Protein-interaction and pathway-based analyzes from these studies showed a significant relationship and common biological pathway between genes carrying mutations. de novo in cases of autism. Protein networks involved in chromatin remodeling, ubiquitination, and neuronal development have been identified as potential targets for autism susceptibility genes. Finally, these studies show that 1,000 or more genes can be interpreted as those in which they can occur as infiltrating mutations that contribute to autism.
Technological advances in DNA sequencing have essentially revolutionized the study of genetic variation in the human genome and have made it possible to identify many types of mutations, including single base pair substitutions, insertions/deletions, CNVs, inversions and re-expansions, as well as those considered somatic and germline mutations. All of these types of mutations have been shown to play a role in human disease. Single nucleotide mutations appear to be mainly of "paternal origin", while deletions may be mainly of "maternal origin". This can be explained by differences between male and female gametogenesis. For example, in a study of neurofibromatosis, 16 of 21 mutations consisted of deletions of maternal origin, and 9 of 11 point mutations were of paternal origin.
Various types of mutations can be passed on from parent to child or acquired spontaneously. The mechanism driving the latter has received attention in recent years due to the importance of this type of mutation in diseases such as schizophrenia and autism. Mutation rate de novo, seems to dominate with the age of the father. The rate here increases with paternal age, possibly due to the effects of reduced DNA replication efficiency or repair mechanisms expected to deteriorate with age. Therefore, the risk of the disease should increase with increasing age of the father. This has been found to occur in many cases, including Crouzon's syndrome, multiple endocrine neoplasia type II, and neurofibromatosis type I. More recently, O'Roak et al. observed a marked paternal component of 51 mutations de novo, identified in a sequencing study of 188 parents-children with cases of sporadic autism. These results are similar to those observed in recent reports on CNN n novo with intellectual disability. This correlation can be explained by the significantly higher number of mitotic cell divisions in germ cells or spermatocytes prior to meiosis during the lifetime of males compared to what occurs during oogenesis in females.
Based on the established number of cell divisions that occur in oogenesis (birth to menopause) compared to spermatogenesis (puberty to the end of life), James F. Crow calculated that at age 30, the average number of chromosome repetitions from zygote to sperm production is 16.5 times higher than from zygote to egg production.
Genetic mosaicism is due to the occurrence de novo mitotic mutations, manifests itself very early in the development of the embryo and is defined as the presence of multiple cell clones with a certain genotype in the same person. Somatic and germline mosaicism exists, but germline mosaicism can facilitate the transmission of what can be passed on by mutation de novo offspring.
Spontaneous mutations that occur in somatic cells (during mitosis, after fertilization) can also play a role in the genesis of diseases associated with developmental disorders.
