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

All three groups focused on a rare group of genetic mutations called "de novo." These mutations are not inherited, but arise 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, a 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 , the siblings were not autistic. As a result, two children were discovered with the same mutation in the same gene, and nothing more connected them except the diagnosis.

“It’s like hitting the same point on the target twice when playing darts. The probability that the discovered mutation is associated with autism is 99.9999 percent,” the publication quotes Professor State.

A team led by Evan E. Eichler, a genetics professor 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. No such coincidences were observed among 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, Daly believes, autistic people, on average, have significantly more of them.

All three groups of researchers also confirmed the previously observed connection between the age of parents and autism in the child. How older parents, first of all, the father, the higher the risk of “de novo” mutations. After analyzing 51 mutations, the team led by Professor Eichler found that this type of damage was four times more common in male DNA than in female DNA. And even more often if the man’s age exceeds 35 years. Thus, scientists suggest that it is the damaged paternal genetic material received by the offspring at conception that is the source of those mutations that lead to the development of autistic disorders.

Scientists agree that the search for ways to prevent such developments will take a long time; research into the genetic nature of autism is just beginning. In particular, Eichler and Daly's teams found evidence that the genes in which de novo mutations were found are involved in the same biological processes. “But this is just the tip of the iceberg,” says Professor Eichler. “The main thing is that we all agree on where to start.”

CHAPTER 5 VARIABILITY OF THE ORGANISM

CHAPTER 5 VARIABILITY OF THE ORGANISM

General information

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

During ontogenesis, the genome of an individual’s body and the information encoded in it undergo continuous transformations under the influence of factors environment. Changes that occur in the genome can be transmitted from generation to generation, causing variability in the characteristics and phenotype of the organism in descendants.

At the beginning of the 20th century. German zoologist W. Hacker identified a branch 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, phenogeneticists 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 There are also 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 division poles during the formation of daughter cells during mitosis and meiosis. Variability of the second

class is caused by point, chromosomal and genomic mutations (see below).

Let us sequentially consider the various classes and types of variability of the organism at different stages of its individual development.

Variability during gamete fertilization and the beginning of 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 origin of molecular life, the emergence of a new qualitative state - one of the properties of biological matter.

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

According to the fertilization formula: zygote = egg + sperm, the beginning of zygote development 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 first on the expression of the genes of the zygote genotype, and then on the genotypes of the daughter somatic cells that emerged from it. Individual genes and groups of genes within 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 that take place belongs to the egg, which has in the nucleus and cytoplasm everything necessary for germination.

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

development and continuation of life, the structural and functional components of the nucleus and cytoplasm (the essence biological matriarchy). The sperm contains DNA and does not contain cytoplasmic components. Having penetrated the egg, the sperm DNA 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: DNA-DNA interaction of two parental genomes. Strictly speaking, the genotype is activated, represented by approximately equal parts of DNA nucleotide sequences of maternal and paternal origin (without taking into account 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 exposed to environmental factors or the balance between two opposite states: stability On the one side and variability on the other. These are the cause-and-effect relationships that determine the emergence and continuity of the molecular life of an organism during ontogenesis.

Now let us pay attention to the results and significance of the variability of the genome of an organism as a product of evolution. First, let's 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 sperm contained in a man’s ejaculate. It is obvious that each egg and each sperm are distinguished from each other by many genotypic and phenotypic characteristics: the presence of altered or unchanged genes in composition and combinations (results of combinative variability), different sequences of DNA nucleotide sequences, different sizes, shapes, functional activity (motility), 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 entire organism: the accident of fertilization of gametes ensures the birth of a genetically unique individual organism.

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

there was a possibility that genetically different brothers and sisters could have been born.

Now let's continue our discussion about the balance between stability and variability of hereditary material. In a broad sense, maintaining such a balance is the simultaneous preservation and change (transformation) of the stability of hereditary material under the influence of internal (homeostasis) and external environmental factors (reaction norm). Homeostasis depends on the genotype caused by 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 the body reacts in response 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 characteristics and the phenotype of the entire organism. At the same time, not all the capabilities of the genotype are realized in the phenotype, i.e. phenotype - a particular (for an individual) case of the implementation of a genotype in specific conditions environment. Therefore, for example, between monozygotic twins who have completely identical genotypes (100% common genes), noticeable phenotypic differences are revealed if the twins grow up in different conditions environment.

The norm of reaction can be narrow or broad. In the first case, the stability of an individual trait (phenotype) is maintained almost regardless of environmental influences. Examples of genes with a narrow reaction norm or nonplastic genes are genes encoding the synthesis of blood group antigens, eye color, hair curl, 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 broad reaction rate or plastic genes- genes that control the number of red blood cells (different for people going up a mountain and people going down a mountain). Another example of a broad reaction norm is color change skin(tanning), associated with the intensity and time of exposure to ultraviolet radiation on the body.

Talking about response range, one should keep 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 the characteristics as such that are inherited, 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 among the internal factors that influence the phenotypic manifestation of genes and genotype, the gender and age of the individual are of 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 an organism’s adaptation to environmental conditions (the basis of ontogenesis) is its genotype. In particular, individuals with genotypes that do not suppress the negative effects of pathological genes and environmental factors leave fewer offspring than those individuals in whom 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 alleles of the normal type become dominant instead.

NON-HERITABLE VARIABILITY

Speaking about non-hereditary variability of genetic material, let us again consider an example of a broad reaction norm - a change in skin color under the influence of ultraviolet radiation. “Tan” is not passed on 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, scar changes in tissues and mucous membranes due to burn disease, frostbite, poisoning and many other signs caused solely by 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 a given organism, because they are formed against the background of a specific genotype under specific environmental conditions.

Hereditary combinative variability

As stated 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 meiotic division

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 20th century. when opening the crossing over T.H. Morgan and his students suggested that crossing over between two genes can occur not only at one, but also at two, three (double and triple crossing over, respectively) and more points. Suppression of crossing over was noted in areas immediately adjacent to the exchange points; this suppression was called interference.

Ultimately, it was calculated: for one male meiosis there are from 39 to 64 chiasmata or recombinations, and for one female meiosis there are up to 100 chiasmata.

Rice. 24. 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 the onset of meiosis; b - they are during pachytene (their spiralization is visible); c - they are also during diplotene and diakinesis (arrows indicate places of crossing-over-chiasma, or areas of exchange)

As a result, they concluded: the linkage of genes to chromosomes is constantly disrupted during crossing over.

Factors influencing 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 influencing crossing over include:

Homo- and heterogametic sex ( we're talking about O mitotic crossing over in males and females of such eukaryotes as Drosophila and silkworm); Thus, in Drosophila crossing over proceeds normally; at 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 of sex development in male and female hermaphroditism (see Chapter 16);

Chromatin structure; to the crossing over frequency in different areas chromosomes are affected by the distribution of heterochromatic (pericentromeric and telomeric regions) and euchromatic regions; in particular, in 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;

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; “lockers” of the latter are chromosomal rearrangements (inversions and translocations), which complicate the normal conjugation of chromosomes in zygotene;

Exogenous factors: exposure to temperature, ionizing radiation and concentrated salt solutions, chemical mutagens, drugs and hormones, which usually increase the frequency of crossing over.

The frequency of meiotic and mitotic crossing over and SCO is sometimes used to judge the mutagenic effect of drugs, carcinogens, antibiotics and others. chemical compounds.

Unequal crossing over

In rare cases, during crossing over, breaks are observed at asymmetrical points of sister chromatids, and they exchange

are divided into unequal areas among themselves - this is unequal crossing over.

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

The importance of this mechanism is difficult to overestimate. For example, as a result of incorrect pairing of homologous chromosomes along the flanking repeats, doubling (duplication) or loss (deletion) of the chromosome region containing the PMP22 gene may occur, which will lead to the development of hereditary autosomal dominant motor-sensory neuropathy Charcot-Marie-Tooth.

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 approximately 30 kb in length. (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 a pseudogene sequence is transferred to the 21-hydroxylase gene (CYP21B) 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 division poles during the formation of daughter cells during mitosis and meiosis

Due to the replication process that precedes mitosis of a 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 cell will receive one paternal and one maternal chromosome (for each pair of chromosome set), but which of the two, the first or the second, is unknown. Takes place

random distribution of homologous chromosomes. Easy to count: due to different combinations of 23 pairs of chromosomes total quantity 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 the 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). It should be noted that there is a pathological variant of the distribution of chromosomes into daughter cells. For example, the entry into one of two daughter cells of only one (paternal or maternal in origin) X chromosome will lead to monosomy (Shereshevsky-Turner syndrome, karyotype 45, XO), the entry of three identical autosomes will lead to trisomy (Down syndrome, 47,XY ,+21; Patau, 47,ХХ,+13 and Edvadsa, 47,ХХ,+18; see also chapter 2).

As noted in Chapter 5, two paternal or two maternal chromosomes of origin can simultaneously enter one daughter cell - this is uniparental isodisomy for a specific pair of chromosomes: Silver-Russell syndrome (two maternal chromosomes 7), Beckwitt-Wiedemann syndrome (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 disorders are of great evolutionary significance, because they create 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 either 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 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 variant combinations of chromosomes and genes located on them when two gametes meet, the following: 2 46 or 64 χ 10 12, i.e. 64 trillion.

The formation of such a (theoretically possible) number of genotypes when two gametes meet 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 strands are damaged. An example is the formation of a single-stranded DNA gap opposite an unrepaired lesion. The resulting gap cannot be accurately corrected without involving the 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 DNA nucleotide sequences can be purely conventionally divided into mutations and genetic polymorphism (see Chapter 2). At the same time, if the heterogeneity of genotypes is constant (normal) characteristics of genome variability, then mutational variability- this is, as a rule, its pathology.

Pathological variability of the genome is supported, for example, by unequal crossing over, incorrect divergence of chromosomes to division poles during the formation of daughter cells, 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's clarify the terminology and consider general questions mutation theories.

GENERAL ISSUES IN MUTATION THEORY

Mutation there is a change structural organization, the amount and/or functioning of the hereditary material and the 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 basic properties of mutations. They:

Appear suddenly;

Passed on from generation to generation;

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

They have no direction (“mutates” any locus, causing minor changes or affecting vital signs);

According to their phenotypic manifestation, they can be 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, is a continuously ongoing process of the formation of mutations under the influence of mutagens - environmental factors that damage hereditary material.

For the first time theory of continuous mutagenesis proposed in 1889 by Russian scientist from St. Petersburg University S.I. Korzhinsky in his book “Heterogenesis and Evolution”.

As is currently believed, mutations can appear spontaneously, without visible external causes, but under the influence of internal conditions in the cell and body - these are 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 muscular dystrophy, cystic fibrosis, sickle cell anemia, phenylketonuria, etc.). Commercial kits have now been created that make it possible 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 of the disease are familial forms), Recklinghausen neurofibromatosis type I (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 a pathological state of a gene (trait) to a normal state are called reverse or reversions.

The ability to revert was first established in 1935 by N.V. Timofeev-Ressovsky.

Subsequent mutations in the gene that suppress the primary mutant phenotype are called suppressor. Suppression may be intragenic(restores the functional activity of the protein; the amino acid does not correspond to the original one, 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 expressiveness of the disease depends on the number of abnormal cells).

Somatic mutations can be either familial or sporadic (non-familial). They underlie the development of 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 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 or sporadic. The most studied area of ​​general mutagenesis is physical and, in particular, radiation mutagenesis. Any sources of ionizing radiation are harmful to human health; they, as a rule, have a powerful mutagenic, teratogenic and carcinogenic effect. 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, approximately 50-60 thousand are constantly used. chemicals. About one thousand new compounds are introduced into practice every year. Of these, 10% are able to induce mutations. These include herbicides and pesticides (the share of mutagens among them reaches 50%), as well as a number medicines(some antibiotics, synthetic hormones, cytostatics, etc.).

There is also biological mutagenesis. Biological mutagens include: foreign proteins of vaccines and serums, viruses (varicella, measles rubella, polio, herpes simplex, AIDS, encephalitis) and DNA, exogenous factors (poor protein nutrition), histamine compounds and its derivatives, steroid hormones (endogenous factors ). Strengthen the effect of external mutagens comutagens(toxins).

The history of genetics has many examples of the importance of connections 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, the following were identified:

Amorphous mutations. This is a condition in which the trait controlled by the pathological allele is not expressed because the pathological allele is inactive compared to the normal allele. Such 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. Such 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 - gete-

rosygotic carriers of genes for diseases of genome instability (see Chapter 10). Their number is about 3% of the Earth's population (almost 195 million people), and the number of diseases themselves reaches 100 nosologies. Among these diseases: Fanconi anemia, ataxia telangiectasia, xeroderma pigmentosum, Bloom's syndrome, progeroid syndromes, many forms of cancer, etc. Moreover, the frequency of cancer in heterozygous carriers of the genes for these diseases is 3-5 times higher than normal, and in patients themselves ( homozygotes for these genes), the incidence of cancer is tens of 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 the trait controlled by a normal allele. Such 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 occur when the trait controlled by the pathological allele is of a different (new) quality compared to the trait controlled by the normal allele. Example: 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 the effect they have on the structure of the protein product of the gene and/or its level of expression.

In particular, Nobel laureate Victor McKusick (1992) identified mutations that change the amino acid sequence of 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) account for 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 of the betaglobin gene. In turn, mutations affecting normal gene expression were identified. They lead to a change in the amount of the gene product and are manifested by phenotypes associated with the deficiency of a particular protein, for example,

in cases hemolytic anemia, caused by mutations of genes localized on autosomes: 9q34.3 (adenylate kinase deficiency); 12p13.1 (triosephosphate isomerase deficiency); 21q22.2 (phosphofructokinase deficiency).

The classification of mutations by V. 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 became widely accepted.

Classification of mutations depending on the level of organization of hereditary material

The classification includes the following.

Point mutations(violation of the gene structure 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 the protein products they synthesize. 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 the coding and non-coding regions of genes (exons and introns, splicing sites). Base substitutions result in 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 - 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 due to exposure 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 (disturb 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 (insertions) of hereditary material. Such mutations are characteristic of somatic

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

There are a number of syndromes caused by structural mutations. The most famous examples: “cry of the cat” syndrome (karyotype: 46,ХХ,5р-), Wolf-Hirschhorn syndrome (46,ХХ,4р-), translocation form of Down syndrome (karyotype: 47, ХУ, t (14;21) ).

Another example is leukemia. When they occur, gene expression is disrupted as a result of the so-called separation (translocation between the structural part of the gene and its promoter region), and, consequently, protein synthesis is disrupted.

Genomic(numerical) mutations- violation of the number of chromosomes or their parts (lead to the appearance of new genomes or their parts by adding or losing whole chromosomes or their parts). The origin of these mutations is due to chromosome nondisjunction 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 sperm (dispermia or triploid embryo).

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

The largest genetic laboratory in the United States, Reprogenetics, 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 that claim that mutations can be detected in embryos after in vitro fertilization (IVF). .

To conduct the study, a small (sparing) biopsy, only about 10 embryonic cells, is sufficient, while most 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 process extended whole genome screening.

New (De Novo) mutations occur only in germ cells and in embryos after fertilization. Typically, these mutations are not present in the parents' blood, and even comprehensive screening of carrier parents will not 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 are important step in the development of whole-genome screening aimed at finding 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, thereby eliminating the need for further genetic testing during pregnancy or after birth, while ensuring that the healthiest embryo is selected for transfer to the expectant mother."

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

"It is remarkable that novel (de novo) mutations can be detected with such high sensitivity and exceptionally low error rates using a small number of embryonic cells," says Brock Peters, Ph.D., lead scientist on the study. "The developed method is effective not only with medical point point of view, but also economically and we look forward to continuing our research work in this area."

New mutations can lead to serious congenital brain disorders such as autism, epileptic encephalopathies, schizophrenia and others. Because these mutations are unique to the particular sperm and egg that create the embryo, genetic testing of the parents cannot detect them.

"Up to five percent of newborns suffer from a disease caused by a genetic defect," says Alan Berkley, MD, professor and 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 greatly alleviate some of the emotional and physical stress of IVF, especially for couples at risk of passing on genetic disorders."

The article was translated specifically 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 may be a multiplication of entire sets of chromosomes (polyploidy) or an increase (trisomy) or decrease (monosomy) in the number of individual chromosomes;

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

V) gene mutations, not leading to changes in chromosomes that can be detected using 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 of hemoglobin genes) has made a great contribution to the understanding of their molecular nature. The results of these studies and the results of analysis of chromosome structure using high-resolution differential staining methods have led to a blurring of the line between chromosomal and gene mutations. We now know that deletions and insertions are possible at the molecular level and that unequal crossing over can change the microstructure. Differential staining methods have made it possible to detect previously indistinguishable chromosomal rearrangements under the microscope. It should be remembered that chromosomal changes detected by differential staining differ by several orders of magnitude

5 Mutations 143

from changes such as deletions 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 germ and somatic cells. Those of them that arise in germ cells are transmitted to individuals of the next generation and, as a rule, are found in all cells of the descendants who become their carriers. Somatic mutations can only be detected in the progeny of the corresponding mutant cell, which leads to a “mosaic” individual. Phenotypic consequences will appear only if these mutations interfere with the implementation of specific functions inherent in these mutant cells.

Mutation frequencies. One of the parameters most often used when studying the mutation process is frequency emergence mutations(or mutation rate). In relation to humans, it is defined as the probability of a mutation event occurring during the life of one generation. As a rule, this refers to the frequency of mutations in fertilized eggs. The issue of mutation rates in somatic cells is discussed in Section. 5 1.6.

To compare the quantity de novo mutations for each case, parallel sequencing of the exomes of the subject and his parents (trio) was performed. Especially many (relative to the control group) in patients with congenital heart disease, nonsynonymous substitutions were found in genes involved in methylation, demethylation and recognition of methylation of lysine 4 of histone 3, as well as those responsible for ubiquitinylation of H2BK120, which is necessary before methylation of H3K4. The peculiarity of these genes is that each of the mutations in them leads to disruption of the expression of several genes that play an important role in the development of the organism.

It is 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 that are common to both diseases (autism and congenital heart disease), and have never been previously detected in normal conditions. The authors suggest that other hereditary diseases may develop through a similar mechanism.

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.

Caption for the picture
De novo mutations in the H3K4 and H3K27 metabolic pathways. The figure lists genes in which mutations affect methylation, demethylation, and recognition of histone modifications. Genes carrying mutations such as frameshifts and splice sites are marked in red; genes carrying nonsynonymous substitutions are shown in blue. The designation SMAD (2) means that this mutation was detected in two patients at once. Genes whose products work together are circled in a rectangle.