Three groups of American scientists, independently of each other, managed for the first time to establish a link between mutations in certain genes and the likelihood of developing autism spectrum disorders in a child, reports The New York Times. In addition, the researchers found scientific confirmation of a previously identified direct relationship between the age of parents, especially fathers, and the risk of developing autism in offspring.

All three groups focused on a rare group of genetic mutations called "de novo". These mutations are not inherited, but occur during conception. As genetic material, blood samples were taken from family members in which the parents were not autistic, and the children developed various autism spectrum disorders.

The first group of scientists, led by Matthew W. State, professor of genetics and child psychiatry at Yale University, whose work was published April 4 in the journal Nature, analyzed the presence of de novo mutations in 200 people diagnosed with autism, whose parents , brothers and sisters were not autistic. As a result, two children were found with the same mutation in the same gene, while they were not linked by anything other than the diagnosis.

"It's like when playing darts to hit the same point on a target twice with a dart. The probability that the detected mutation is associated with autism is 99.9999 percent," Professor State quotes the publication.

A team led by Evan E. Eichler, a professor of genetics at the University of Washington, examined blood samples from 209 families with autistic children and found the same mutation in the same gene in one child. In addition, two autistic children from different families were identified who had identical de novo mutations, but in different genes. There were no such coincidences in non-autistic subjects.

A third group of researchers, led by Professor Mark J. Daly of Harvard University, found several cases of de novo mutations in the same three genes in autistic children. At least one mutation of this type is present in the genotype of any person, but, according to Daly, autists, on average, have much more of them.

All three groups of researchers also confirmed the previously observed association between parental age and child autism. The older the parents, especially the father, the higher the risk of de novo mutations. After analyzing 51 mutations, the team led by Professor Eichler found that this kind of damage occurs in male DNA four times more often than in female DNA. And even more often if a man is over 35 years old. Thus, scientists suggest that it is the damaged paternal genetic material received by the offspring during conception that is the source of those mutations that entail the development of autistic disorders.

Scientists agree that the search for ways to prevent this development of events will be long, and the study of the genetic nature of autism is just beginning. In particular, Eichler and Daly's teams found evidence that genes in which de novo mutations are found are involved in the same biological processes. "But this is just the tip of the tip of the iceberg," says Professor Eichler. "The main thing is that we all agreed on where to start."

CHAPTER 5 VARIATION OF THE ORGANISM

CHAPTER 5 VARIATION OF THE ORGANISM

Total information

The variability of an organism is the variability of its genome, which determines the genotypic and phenotypic differences in humans and causes the evolutionary diversity of its genotypes and phenotypes (see Chapters 2 and 3).

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 by the fusion of the maternal and paternal genomes (see Chapters 2 and 12).

In the course of ontogenesis, the genome of an individual's organism and the information encoded in it undergo continuous transformations under the influence of environmental factors. Changes in the genome can be transmitted from generation to generation, causing the variability of the traits and phenotype of the organism in offspring.

At the beginning of the XX century. German zoologist W. Hacker identified the direction of genetics, devoted to the study of connections 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 passed 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, chromosomal and genomic mutations (see below).

Let us sequentially consider various classes and types of organism variability at different stages of its 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, provide the emergence of molecular life, the emergence of a new qualitative state - one of the properties of biological matter.

In fig. 23 shows the results of interaction of two parental genomes during gamete fertilization.

According to the fertilization formula: zygote = egg + sperm, the beginning of the development of the zygote is the moment of formation of a double (diploid) when two haploid sets of parental gametes meet. It is then that molecular life arises and a chain of sequential reactions is launched based on first the expression of genes of the zygote genotype, and then the genotypes of the daughter somatic cells that emerged from it. Individual genes and groups of genes in the genotypes of all cells of the body begin to "turn on" and "turn off" during the implementation of the genetic program of ontogenesis.

The leading role in the events taking place belongs to the egg cell, which has in the nucleus and cytoplasm everything necessary for embryo-

Rice. 23. Results of interaction of two parental genomes during gamete fertilization (figures are from www.bio.1september.ru; www.bio.fizteh.ru; www.vetfac.nsau.edu.ru, respectively)

structural and functional components of the nucleus and cytoplasm (essence biological matriarchy). The sperm cell contains DNA and does not contain cytoplasmic components. Penetrating into the egg, the sperm's DNA comes into contact with its DNA, and thus the main molecular mechanism functioning throughout the life of the organism “turns on” in the zygote: the DNA-DNA interaction of two parental genomes. Strictly speaking, the genotype is activated, which is represented by approximately equal parts of the nucleotide sequences of DNA of maternal and paternal origin (excluding the mtDNA of the cytoplasm). Let us 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 the entire subsequent molecular life of a multicellular organism is the desire to restore homeostasis or balance between two opposite states, which is exposed to environmental factors, or the balance between two opposite states: stability 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 during ontogenesis.

Now let's pay attention to the results and significance of the variability of the organism's genome as a product of evolution. First, let us consider the question of the uniqueness of the genotype of the zygote or the progenitor 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 a man's ejaculate. It is obvious that each egg and each sperm is distinguished from each other by many genotypic and phenotypic traits: the presence of altered or unaltered genes in composition and combinations (results of combinative variability), different sequences of DNA nucleotide sequences, different sizes, shapes, functional activity (mobility), maturity of gametes, etc. It is these differences that allow us to speak about the uniqueness of the genome of any gamete and, consequently, the genotype of the zygote and the whole organism: the randomness of fertilization of gametes ensures the birth of a genetically unique organism of an individual.

In other words, the molecular life of a person (like 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

it is likely that genetically different ones could be born - his siblings.

Now we will continue our reasoning about the balance between stability and variability of the hereditary material. In a 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 rate of reaction is determined by the interaction of the genotype with environmental factors.

Rate and range of response

The specific way the body reacts in response to environmental factors is called normal reaction. It is genes and genotype that are responsible for the development and range of modifications of individual traits and phenotypes of the whole organism. At the same time, far from all the possibilities of the genotype are realized in the phenotype, i.e. phenotype is a particular (for an individual) case of the realization of a genotype in specific environmental conditions. Therefore, for example, between monozygous twins with completely identical genotypes (100% of common genes), noticeable phenotypic differences are revealed if the twins grow up in different environmental conditions.

The reaction rate can be narrow or wide. In the first case, the stability of an individual trait (phenotype) is maintained practically regardless of the influence of the environment. Examples of genes with a narrow reaction rate or non-plastic genes there are 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 red blood cells (different for people going uphill and people going downhill). Another example of a wide reaction rate is a change in the color of the skin (sunburn) associated with the intensity and time of exposure to ultraviolet radiation on the body.

Talking about reaction range, one should bear in mind the phenotypic differences that appear in the individual (his genotype) depending on

"Depleted" or "enriched" environmental conditions in which the body is located. According to the definition of I.I. Schmalhausen (1946), "it is not traits as such that are inherited, but the norm of their reaction to changes in the conditions of existence of organisms."

Thus, the norm and the range of the reaction are the limits of the genotypic and phenotypic variability of the organism when the environmental conditions change.

It should also be noted that of the internal factors influencing the phenotypic manifestation of genes and genotype, the sex and age of the individual are of a certain importance.

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.

Of course, the basis for the adaptation of an organism 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 whose undesirable 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 instead of them alleles of the normal type become dominant.

INHERENT VARIABILITY

Speaking about the non-hereditary variability of genetic material, let us again consider an example of a wide reaction rate - a change in the color of the skin under the influence of ultraviolet radiation. "Sunburn" is not passed on from generation to generation, that is, not inherited, although plastic genes are involved in its occurrence.

In the same way, the results of trauma, cicatricial changes in tissues and mucous membranes during burns, frostbite, poisoning and many other signs caused by the action of exclusively environmental factors are not inherited. At the same time, it should be emphasized: non-hereditary changes or modifications are associated with hereditary

natural properties of this organism, because they are formed against the background of a specific genotype in specific environmental conditions.

Hereditary combinative variability

As mentioned at the beginning of the chapter, in addition to the mechanism of random encounters 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 division poles during the formation of daughter cells during mitosis and meiosis (see Chapter 9).

Crossing over in the first division of meiosis

Due to the mechanism crossing over The linkage of genes to the chromosome is regularly disrupted 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 (double and triple crossing over, respectively) and more points. Suppression of crossing over was noted in the areas immediately adjacent to exchange points; this suppression was called interference.

In the end, they calculated: one male meiosis accounts for 39 to 64 chiasmas or recombinations, and one female meiosis accounts for up to 100 chiasmas.

Rice. 24. Crossover scheme in the first division of meiosis (according to Shevchenko V.A. et al., 2004):

a - sister chromatids of homologous chromosomes before the onset of meiosis; b - they are the same during pachytene (their spiralization is visible); c - they are the same 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 disrupted 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 affecting crossing over include:

Homo- and heterogametic sex (we are talking about mitotic crossing over in males and females of such eukaryotes as fruit fly and silkworm); so, in Drosophila, crossing over proceeds normally; in the silkworm, it is either normal or absent; in humans, attention should be paid to the mixed ("third") sex and specifically to the role of crossing over in case of sex developmental anomalies in male and female hermaphroditism (see Chapter 16);

Chromatin structure; the frequency of crossing over in different parts of chromosomes is influenced by the distribution of heterochromatin (pericentromeric and telomeric parts) and euchromatin 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; with increasing age, the degree of spiralization of chromosomes and the rate of cell division change;

Genotype; it contains genes that increase or decrease the frequency of crossing over; The "inhibitors" 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 SCO is sometimes judged on 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 unequal between each other - this is unequal crossing over.

At the same time, cases have been described when mitotic conjugation (incorrect pairing) of homologous chromosomes is observed during mitosis and recombination occurs between non-sister chromatids. This phenomenon is called gene conversion.

The importance of this mechanism can hardly be overestimated. For example, as a result of incorrect pairing of homologous chromosomes in flanking repeats, duplication (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-Toes.

Unequal crossing over is one of the mechanisms of mutation. 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, a similar mutation is observed when the pseudogene sequence is transferred to the 21-hydroxylase (CYP21B) gene in adrenogenital syndrome or congenital adrenal hyperplasia (see chapters 14 and 22).

In addition, due to recombinations during unequal crossing over, multiple allelic forms of genes encoding HLA class I antigens can be formed.

Independent divergence of homologous chromosomes to the poles of division during the formation of daughter cells during mitosis and meiosis

Due to the replication process preceding mitosis of the somatic cell, the total number of DNA nucleotide sequences doubles. The formation of one pair of homologous chromosomes occurs 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 the chromosome set), but which of the two, the first or the second, is unknown. Occurs

the random nature of the 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 combinations of chromosomes 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 variant of the distribution of chromosomes into daughter cells should also be noted. For example, getting into one of the two daughter cells of only one (paternal or maternal in origin) X chromosome will lead to monosomy (Shereshevsky-Turner syndrome, karyotype 45, XO), hitting three identical autosomes will lead to trisomy (Down syndrome, 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 a uniparental isodisomy for a specific pair of chromosomes: Silver-Russell syndromes (two maternal chromosomes 7), Beckwitt-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 humans. These violations 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 one of either the maternal or the paternal chromosome from all 23 chromosomes.

To avoid possible errors in our further calculations, let us take it as a rule: as a result of the second meiotic division, 8 million variants of male gametes and 8 million variants of female gametes are also formed. 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 as follows: 2 46 or 64 χ 10 12, i.e. 64 trillion.

The formation of such (theoretically possible) number of genotypes when two gametes meet clearly explains the meaning of the heterogeneity of genotypes.

The meaning 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 in the event of damage to its both strands. An example is the formation of a single-stranded DNA gap opposite an unrepaired lesion. The gap that appears cannot be corrected without error without the involvement of 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, hereditary mutational variability and the resulting genetic heterogeneity make a huge contribution to the variability of the human genome and phenome.

Variations in DNA nucleotide sequences can be conventionally divided into mutations and genetic polymorphism (see Chapter 2). At the same time, if heterogeneity of genotypes is constant (normal) characteristics of genome variability, then mutational variability- this is, as a rule, his pathology.

Pathological variability of the genome is evidenced, for example, by unequal crossing over, incorrect divergence of chromosomes to the division poles during the formation of daughter cells, and the presence of genetic compounds and allelic series. In other words, hereditary combinative and mutational variability is manifested in humans by significant genotypic and phenotypic diversity.

Let us clarify the terminology and consider general questions of the theory of mutations.

GENERAL QUESTIONS OF MUTATION THEORY

Mutation there is a change in the structural organization, quantity and / or functioning of the hereditary material and proteins synthesized by it. This concept was first proposed by Hugo de Vries

in 1901-1903 in his work "Mutation theory", where he described the main properties of mutations. They:

Come on suddenly;

Passed down from generation to generation;

Inherited according to the dominant type (manifested in heterozygotes and homozygotes) and recessive type (manifested in homozygotes);

Have no direction ("mutates" any locus, causing minor changes or affecting vital signs);

By phenotypic manifestation, they are harmful (most mutations), useful (extremely rare) or indifferent;

They arise in somatic and germ cells.

In addition, the same mutations can occur repeatedly.

Mutation process or mutagenesis, is a continuous process of formation of mutations under the influence of mutagens - environmental factors that damage hereditary material.

First continuous mutagenesis theory proposed in 1889 by the Russian scientist from St. Petersburg University S.I. Korzhinsky in his book Heterogenesis and Evolution.

As is generally believed at the present time, mutations can manifest themselves spontaneously, without apparent external reasons, but under the influence of internal conditions in the cell and the body, these are spontaneous mutations or spontaneous mutagenesis.

Mutations caused artificially by the action of external factors of a physical, chemical or biological nature are induced mutations, or induced mutagenesis.

The most common mutations are called major mutations(for example, mutations in genes of Duchenne-Becker muscular dystrophy, cystic fibrosis, sickle cell anemia, phenylketonuria, etc.). Commercial kits have now been created to automatically identify the most important of them.

Newly arisen mutations are called new mutations or mutations de novo. For example, these include mutations underlying a number of autosomal dominant diseases, such as achondroplasia (10% of cases are familial), Recklinghausen type I neurofibromatosis (50-70% are familial), Alzheimer's disease, and Huntington's chorea.

Mutations from the normal state of a gene (trait) to a pathological state are called straight.

Mutations from a pathological state of a gene (trait) to a normal state are 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 suppressive. Suppression can be intragenic(restores the functional activity of the protein; the amino acid does not correspond to the initial one, i.e. there is no true reversibility) and extragenous(the structure of tRNA changes, as a result of which the mutant tRNA includes a different 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 expressiveness of the disease depends on the number of abnormal cells).

Somatic mutations can be familial or sporadic (non-familial). They underlie the development of malignant neoplasms and premature aging processes.

It was previously 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 the germ cells are called germinal 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 powerful mutagenic, teratogenic and carcinogenic effects. The mutagenic effect of a single dose of radiation is much higher than that of chronic radiation; 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, about 50-60 thousand chemicals are constantly used. 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 drugs (some antibiotics, synthetic hormones, cytostatics, etc.).

There is still biological mutagenesis. Biological mutagens include: foreign proteins of vaccines and sera, viruses (chickenpox, measles rubella, poliomyelitis, herpes simplex, AIDS, encephalitis) and DNA, exogenous factors (inadequate protein nutrition), histamine compounds and its derivatives, steroid hormones (endogenous factors ). Enhance the action of external mutagens comutagens(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 according to their phenotypic effect

This classification of mutations was first proposed in 1932 by G. Möller. According to the classification, the following were allocated:

Amorphous mutations. This is a condition in which a trait controlled by a pathological allele does not appear, since the pathological allele is inactive 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 meaning of the trait controlled by the pathological allele is opposite to the value of the trait controlled by the normal allele. These mutations include 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 genes for genome instability diseases (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, ataxiateleangiectasia, xeroderma pigmentosa, Bloom's syndrome, progeroid syndromes, many forms of cancer, etc. Moreover, the frequency of cancer in heterozygous carriers of the genes of these diseases is 3-5 times higher than in the norm, and in the patients themselves ( homozygotes for these genes) the frequency 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 the genes for the synthesis of pigments (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 a trait controlled by a pathological allele will have a different (new) quality compared to a trait controlled by a 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 G. 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 of the protein product of the gene and / or the level of its expression.

In particular, the Nobel laureate Victor McCusick (1992) isolated 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 rest of the mutations (40-50% of cases) are the share of 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 caused by mutations in the beta gene. In turn, mutations have been isolated that affect the normal expression of the gene. They lead to a change in the amount of the gene product and are manifested by phenotypes associated with a deficiency of a particular protein, for example,

in cases hemolytic anemia, caused by mutations in 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. McCusick (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 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 protein products synthesized by them. Base changes are their substitutions, insertions, movements, or deletions, which can be explained by mutations in the regulatory regions of genes (promoter, polyadenylation site), as well as in coding and noncoding regions of genes (exons and introns, splicing sites). Base substitutions give rise 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 in 200-2000 births is primary hemochromatosis, non-polyposis colon cancer, Martin-Bell syndrome and cystic fibrosis.

Point mutations, which are extremely rare (1: 1,500,000), are severe combined immunodeficiency (SCID) resulting from adenosine deaminase deficiency. Sometimes point mutations are formed not when exposed to mutagens, but as errors in DNA replication. Moreover, 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 (disrupt the structure of chromosomes and lead to the formation of new linkage groups of genes). These are deletions (losses), duplications (doublings), translocations (displacements), inversions (180 ° rotation) or insertions (insertions) of hereditary material. Such mutations are characteristic of somati-

cells (including stem cells). Their frequency is 1 in 1700 cell divisions.

A number of syndromes associated with structural mutations are known. The most famous examples: “cat cry” syndrome (karyotype: 46, XX, 5p-), Wolf-Hirschhorn syndrome (46, XX, 4p-), translocation form of Down syndrome (karyotype: 47, XY, t (14; 21) ).

Another example is leukemia. With them, 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 disrupted.

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 during mitosis or meiosis.

In the first case, these are aneuploids, tetraploids with undivided cytoplasm, polyploids with 6, 8, 10 pairs of chromosomes and more.

In the second case, this is the non-division of paired chromosomes involved in the formation of gametes (monosomy, trisomy) or the fertilization of one egg by two spermatozoa (dyspermia or triploid embryo).

Their typical examples have already been cited more than once - this is Shereshevsky-Turner syndrome (45, XO), Klinefelter's syndrome (47, XXY), regular trisomy in Down syndrome (47, XX, +21).

Reprogenetics, the largest genetic laboratory in the United States, in collaboration with leading scientists from China, a number of New York institutes and medical centers specializing in the field of PGD, have published the results of new studies, which claim that mutations can be found in embryos after in vitro fertilization (IVF) ...

To conduct the study, a small (sparing) biopsy is sufficient, only about 10 embryonic cells, while most of the new (De Novo) mutations that cause a disproportionately high percentage of genetic diseases can be detected using PGD. The uniqueness of the method lies in the development of a new original screening process for the extended whole genome.

New (De Novo) mutations occur only in germ cells and in embryos after fertilization. As a rule, these mutations are not present in the blood of the parents and even comprehensive screening of the carrier parents will not be able to detect them. Standard PGD cannot detect these mutations because the tests are not sensitive enough or focus only on very narrow specific regions of the genome.

"These results represent an important step in the development of whole genome screening to find the healthiest embryos in PGD," says Santiago Munné, Ph.D., founder and director of Reprogenetics and founder of Recombine. "This new approach can detect almost all genomic changes, and thus eliminate the need for further genetic testing during pregnancy or after birth, while ensuring that the healthiest embryo is selected for transfer to the mother-to-be."

It is also scientifically proven that the new method reduces the error rate by 100 times (compared to previous methods).

“It is remarkable that new (De Novo) mutations can be detected with such high sensitivity and extremely low error rates using a small number of embryonic cells,” says Brock Peters, Ph.D. and lead scientist in the study. "The developed method is effective not only from a medical point of view, but also from an economic point of view, and we look forward to continuing our research in this area."

New mutations can lead to serious congenital brain disorders such as autism, epileptic encephalopathy, schizophrenia, and others. Since these mutations are unique to a particular sperm and egg that are involved in the creation of the embryo, genetic analysis of the parents cannot detect them.

"Up to five percent of newborns suffer from diseases caused by a genetic defect," says Alan Berkeley, MD, professor, director of the Department of Obstetrics and Gynecology at the New York University Fertility Center. "Our approach is comprehensive and aims to identify perfectly healthy embryos. This can significantly alleviate some of the emotional and physical stressors of IVF, especially for couples at risk of passing on genetic disorders."

The article was specially translated for the IVF School program, based on materials

The following types of mutations are distinguished:

a) genomic mutations, leading to a change in the number of chromosomes. Genomic mutations often occur in plants. In this case, there can be a multiplication of whole sets of chromosomes (polyploidy) or an increase (trisomy) or a decrease (monosomy) in the number of individual chromosomes;

b) chromosomal mutations(see Section 2.2), in which the structure of chromosomes is disrupted, and their number in the cell remains unchanged. Chromosomal mutations can be detected by microscopic examination.

v) gene mutations, not leading to changes in chromosomes detected with a microscope; these mutations can only be detected by genetic analysis of phenotypic changes (see section 3.6).

The study of mutations in humans at the level of proteins and DNA (especially mutations in hemoglobin genes) has made a great contribution to understanding their molecular nature. The results of these studies and the results of analysis of the structure of chromosomes using high-resolution methods of differential staining led to a blurring of the line between chromosomal and gene mutations. We now know that deletions and insertions are also possible at the molecular level and that unequal crossing over can alter microstructure. Differential staining methods made it possible to detect previously indistinguishable chromosomal rearrangements under a microscope. It should be remembered that chromosome changes detected during differential staining differ by several orders of magnitude.

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from changes such as deletion of structural genes. Therefore, the distinction between structural chromosomal aberrations and gene mutations is useful for practical purposes.

Cells in which mutations occur. except type genetic damage, it is extremely important localization. Mutations can occur in both sex and somatic cells. Those of them that arise in the germ cells are passed on to the individuals of the next generation and, as a rule, are found in all the cells of the descendants who have become their carriers. Somatic mutations can be found only in the offspring of the corresponding mutant cell, which leads to the "mosaic" of the individual. Phenotypic consequences will manifest themselves only if these mutations interfere with the implementation of specific functions inherent in these mutant cells.

Frequencies of mutations. One of the parameters most commonly used in the study of the mutation process is frequency emergence mutations(or the rate of mutation). As applied to humans, it is defined as the probability of a mutational event occurring during the lifetime of one generation. As a rule, this refers to the frequency of mutations in fertilized eggs. The question of the frequencies of mutations in somatic cells is discussed in Sec. 5 1.6.

To compare the quantity de novo mutations for each of the cases, parallel sequencing of the exomes of the subject and his parents (trio) was carried out. Especially many (in relation to the control group) nonsynonymous substitutions in CHD patients were found in the genes involved in methylation, demethylation and recognition of lysine 4 methylation of histone 3, as well as those responsible for H2BK120 ubiquitinylation, which is necessary for H3K4 methylation. The peculiarity of these genes is that each of the mutations in them leads to a violation of the expression of several genes at once, which play an important role in the development of the organism.

It seems interesting that according to the results of a similar study conducted on patients with autism, genes involved in the recognition of H3K4 methylation (СHD7, CHD8 and others) were also included in the list of candidates. The work lists mutations common to both diseases (autism and congenital heart disease), and never previously found in the norm. The authors suggest that other hereditary diseases can develop by a similar mechanism.

A source
Nature. 2013 May 12. De novo mutations in histone-modifying genes in congenital heart disease. Zaidi S, Choi M, Wakimoto H, Ma L, Jiang J, Overton JD, Romano-Adesman A, Bjornson RD, Breitbart RE, Brown KK, Carriero NJ, Cheung YH, Deanfield J, Depalma S, Fakhro KA, Glessner J, Hakonarson H, Italia MJ, Kaltman JR, Kaski J, Kim R, Kline JK, Lee T, Leipzig J, Lopez A, Mane SM, Mitchell LE, Newburger JW, Parfenov M, Pe "er I, Porter G, Roberts AE, Sachidanandam R, Sanders SJ, Seiden HS, State MW, Subramanian S, Tikhonova IR, Wang W, Warburton D, White PS, Williams IA, Zhao H, Seidman JG, Brueckner M, Chung WK, Gelb BD, Goldmuntz E, Seidman CE , Lifton RP.

Figure caption
De novo mutations in the H3K4 and H3K27 metabolic pathways. The figure lists genes whose mutations affect methylation, demethylation, and recognition of histone modifications. Genes carrying mutations such as reading frameshift and at splice sites are marked in red; genes carrying nonsynonymous substitutions are shown in blue. The designation SMAD (2) means that this mutation was found in two patients at once. Genes whose products work in a complex are highlighted in a rectangle.