Tumor suppressor genes. Mechanisms for preventing tumorigenesis

Introduction.

Carcinogenesis is a multi-stage process of accumulation of mutations and other genetic changes leading to disruptions of key cellular functions, such as regulation of proliferation and differentiation, natural cell death (apoptosis), morphogenetic reactions of the cell, and also, probably, to the ineffective functioning of specific and nonspecific antitumor immunity factors . Only a combination of such changes, acquired, as a rule, as a result of a rather long evolution of neoplastic clones, during which the selection of cells with the necessary characteristics occurs, can ensure the development of a malignant neoplasm. The likelihood of several genetic changes occurring in one cell increases sharply when the systems that control the integrity of the genome are disrupted. Therefore, mutations leading to genetic instability are also an integral stage of tumor progression. Moreover, some congenital anomalies of genetic control systems are a factor predetermining the inevitable occurrence of a neoplasm: they so increase the probability of the appearance of various oncogenic mutations in each cell of the body that an individual, sooner or later, in some of the cells of the proliferating clone, under selection pressure, will necessarily accumulate the necessary a combination of changes and a tumor is formed.

Significant progress in understanding the mechanisms of carcinogenesis is associated with the discovery first of oncogenes and protoncogenes, and then - tumor suppressors And mutator genes. Oncogenes are cellular or viral (introduced by a virus into a cell) genes, the expression of which can lead to the development of a tumor. Proto-oncogenes are normal cellular genes, the enhancement or modification of whose function turns them into oncogenes. Tumor suppressors (antioncogenes, recessive tumor genes) are cellular genes, the inactivation of which sharply increases the likelihood of tumors, and restoration of function, on the contrary, can suppress the growth of tumor cells. It should be noted that the so-called “mutator” genes classified as tumor suppressors, i.e. genes whose dysfunction in one way or another increases the rate of occurrence of mutations and/or other genetic changes may not affect the growth of neoplastic cells. However, their inactivation so greatly increases the likelihood of various oncogenic mutations that the formation of a tumor becomes only a matter of time.

Belonging to oncogenes or tumor suppressors is determined by several criteria: a) the natural nature of changes in the structure and/or expression of a given gene in the cells of certain or various neoplasms; b) the occurrence at a young or young age of certain forms of tumors in individuals with inherited germinal (i.e., occurring in the germ cell) mutations of a given gene; c) a sharp increase in the incidence of tumors in transgenic animals, either expressing an activated form of a given gene - in the case of oncogenes, or carrying inactivating mutations ("knockouts") of a given gene - in the case of tumor suppressors; d) the ability to cause morphological transformation and/or unlimited growth (oncogenes) in cells cultured in vitro, or suppression of cell growth and/or the severity of signs of transformation (tumor suppressors).

The last two decades have been characterized by the rapid discovery of more and more new oncogenes and tumor suppressors. To date, about a hundred potential oncogenes (cellular and viral) and about two dozen tumor suppressors are known. Genetic events leading to the activation of proto-oncogenes or the inactivation of tumor suppressors have been described. It has been discovered that the mechanism of action of viral oncogenes is associated with the activation of cellular proto-oncogenes (retroviruses) or inactivation of tumor suppressors ( DNA viruses) . Changes in oncogenes and tumor suppressors, characteristic of certain forms of human tumors, were identified, including highly specific anomalies used for diagnosis (Tables 1, 2).

Table 1.
Some changes in proto-oncogenes characteristic of human tumors

Proto-oncogene	Protein function	Changes	Neoplasms*
ERBB1 (EGF-R)	receptor tyrosine kinase	gene amplification and overexpression	glioblastomas and other neurogenic tumors
ERBB2 (HER2)	receptor tyrosine kinase		mammary cancer
PDGF-Rb	receptor tyrosine kinase	chromosomal translocations forming chimeric genes TEL/PDGF-Rb, CVE6/PDGF-Rb, encoding permanently activated receptors	chronic myelomonocytic leukemia, acute myeloblastic leukemia
SRC	non-receptor tyrosine kinase	mutations in codon 531 that abolish negative regulation of kinase activity	part of colon tumors in late stages
K-RAS,N-RAS,H-RAS	participates in the transmission of mitogenic signals and regulation of morphogenetic reactions	mutations at codons 12,13,61 causing the formation of a permanently activated GTP-bound form of Ras	60-80% of pancreatic cancer cases; 25-30% of various solid tumors and leukemias
PRAD1/cyclinD1	regulates the cell cycle	gene amplification and/or overexpression	breast and salivary gland cancer
C-MYC	transcription factor, regulates cell cycle and telomerase activity	a) chromosomal translocations that move the gene under the control of regulatory elements of immunoglobulin genes; b) amplification and/or overexpression of a gene; protein stabilizing mutations	a) Burkitt's lymphoma b) many forms of neoplasms
CTNNB1 (beta-catenin)	a) transcription factor that regulates c-MYC and cyclin D1; b) binding to cadherin, it participates in the formation of adhesive contacts	mutations that increase the amount of beta-catenin unrelated to E-cadherin, which functions as a transcription factor	hereditary adenomatous polyposis of the colon;
BCL2	suppresses apoptosis by regulating the permeability of mitochondrial and nuclear membranes	chromosomal translocations that move the gene under the control of regulatory elements of immunoglobulin genes	follicular lymphoma
ABL	regulates cell cycle and apoptosis	chromosomal translocations leading to the formation of chimeric BCR/ABL genes, the products of which stimulate cell proliferation and suppress apoptosis	all chronic myeloid leukemias, some acute lymphoblastic leukemias
MDM2	inactivates p53 and pRb	gene amplification and/or overexpression	some osteosarcomas and soft tissue sarcomas

* Italics indicate hereditary forms of diseases that arise from mutations in germ cells. In other cases, mutations occur in somatic cells that form tumors

Table 2.
Forms of human tumors arising from inactivation of certain tumor suppressors and mutator genes

Gene	Protein function	Neoplasms*
p53	transcription factor; regulates the cell cycle and apoptosis, controls genome integrity	Li-Fraumeni syndrome and most forms of sporadic tumors
INK4a-ARF	inhibition of Cdk4**, activation of p53**	hereditary melanomas And
Rb	controls entry into S phase by regulating the activity of the transcription factor E2F	hereditaryretinoblastoma
TbR-II	type 2 receptor for the cytokine TGF-b	hereditary and sporadic colon cancers
SMAD2, SMAD 3	transmit a signal from activated TGF-b receptors to Smad4	cancer of the colon, lung, pancreas
SMAD4/DPC4	transcription factor; mediates the action of the cytokine TGF-b, leading to the activation of Cdk inhibitors - p21WAF1, p27KIP1, p15INK4b	juvenile hamartomatous polyposis of the stomach and intestines; various forms of sporadic tumors
E-cadherin	participates in intercellular interactions; initiates signaling that activates p53, p27KIP1	hereditary stomach cancers and many forms of sporadic tumors
APC	binds and destroys cytoplasmic beta-catenin, prevents the formation of beta-catenin/Tcf transcription complexes	hereditary adenomatous polyposis and sporadic colon tumors
VHL	suppresses the expression of the VEGF gene (vascular endothelial growth factor) and other genes activated during hypoxia	von Hippel-Lindau syndrome (multiple hemangiomas); clear cell carcinomas of the kidney
WT1	transcription factor; binding to p53, modulates the expression of p53-responsive genes	hereditary nephroblastomas (Wilms tumor)
PTEN/MMAC1	phosphatase; stimulates apoptosis by suppressing the activity of the PI3K-PKB/Akt signaling pathway	Cowden's disease (multiple hamartomas); many sporadic tumors
NF1 (neurofibromin)	protein of the GAP family; converts the ras oncogene from active to inactive form	neurofibromatosis type 1
NF2 (merlin)	participates in interactions between the membrane and the cytoskeleton	neurofibromatosis type 2; sporadic meningiomas, mesotheliomas and other tumors
BRCA1	increases the activity of p53 and other transcription factors by binding to RAD51 is involved in the recognition and/or repair of DNA damage	various forms of sporadic tumors
BRCA2	transcription factor with histone acetyl transferase activities; binding to RAD51 participates in DNA repair	hereditary tumors of the breast and ovaries; various forms of sporadic tumors
MSH2, MLH1, PMS1, PMS2	repair of unpaired DNA sections (mismatch repair)	nonpolyposis cancer of the colon and ovaries; many sporadic tumors

* Italics indicate hereditary forms of diseases that arise from mutations in germ cells.
** The INK4a/ARF locus encodes two proteins: p16 INK4a - an inhibitor of cyclin-dependent kinases Cdk4/6 and p19 ARF (Alternative Reading Frame) - a product of an alternative reading frame that, by binding p53 and Mdm2, blocks their interaction and prevents p53 degradation. Deletions and many point mutations in the INK4a/ARF locus simultaneously inactivate the suppressor activities of both of these proteins.

However, for a long time, knowledge about each of the oncogenes or tumor suppressors seemed discrete, largely unrelated. It is only in very recent years that a general picture has begun to emerge, showing that the vast majority of known proto-oncogenes and tumor suppressors are components of several common signaling pathways that control the cell cycle, apoptosis, genome integrity, morphogenetic reactions and cell differentiation. Obviously, changes in these signaling pathways ultimately lead to the development of malignant tumors. provides information about the main targets of oncogenes and tumor suppressors.

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Although the regulation of cell proliferation is complex and not yet sufficiently studied, it is already obvious: normally, in addition to the system that stimulates proliferation, there is a system that stops it.

Suppressor genes

Soon after the discovery of the first oncogenes, reports appeared about the existence of another class of oncology-associated genes, the loss or suppression of whose activity also leads to the development of tumors.

These genes are called suppressor genes (other names are antioncogenes, recessive tumor genes, tumor suppressors).

In unchanged cells, suppressor genes suppress cell division and stimulate their differentiation. In other words, if proto-oncogenes encode proteins that stimulate cell proliferation, then proteins of suppressor genes normally, on the contrary, inhibit proliferation and/or promote apoptosis.

Mutations in such genes lead to suppression of their activity, loss of control over proliferation processes and, as a consequence, to the development of cancer. However, it should be kept in mind that the physiological function of antioncogenes is to regulate cell proliferation and not to prevent tumor development.

Unlike oncogenes, which act dominantly, changes in antioncogenes are recessive in nature, and inactivation of both gene alleles (copies) is necessary for tumor transformation.

Therefore, the genes of this half-mile group are also called “recessive cancer genes.”

The identification of antioncogenes began with the discovery of the Rb gene (retinoblastoma gene), congenital mutations of which cause the development of retinoblastoma. In the early 70s of the XX century, E. A. Knudson (1981) established that about 40% of retinobpastomas occur in infancy (on average 14 months), and these tumors are usually bilateral (in the retina of both eyes).

If such patients were cured of retinobpastomas, then many of them developed osteosarcoma in adolescence, and skin melanoma in adulthood. In most cases, the nature of the disease was hereditary.

In an attempt to explain why phenotypically identical tumors are either sporadic or hereditary in nature, A. Knudson formulated the “two-hit” (mutation) hypothesis. The author suggested that in the case of a hereditary form of the tumor, a mutation (the first blow) in retinoblasts is passed on to the child from one of the parents.

If a second mutation (second hit) occurs in one of these cells, the retina (i.e., already having a mutation), very often (in 95% of patients) a tumor develops. In the case of a sporadic tumor, children do not inherit the mutant allele of the gene, but they have two independent mutations in both alleles (copies) of one of the retinoblasts, which also leads to the development of a tumor.

Therefore, according to A. Knudson’s hypothesis, patients of the first group have one congenital and one acquired mutation, while in patients of the second group both mutations are acquired.

Due to the fact that in hereditary retinoblastomas, changes in the region of chromosome 13 (13ql4) were detected. it has been suggested that the retinoblastoma susceptibility gene (Rb) is localized at this location in the genome. This gene was subsequently isolated.

Both of its alleles turned out to be inactivated in the cells of both hereditary and sporadic retinobpastomas, but in hereditary forms, all cells of the body had congenital mutations of this gene.

Thus, it became clear that the two mutations postulated by A. Knudson, necessary for the development of retinobpastomas, occur in different alleles of the same Rb gene. In cases of inheritance, children are born with one normal and one defective Rb allele.

A child, a carrier of an inherited allele of the mutant Rb gene, has it in all somatic cells, and is completely normal. However, when an acquired mutation occurs, the second (normal) copy (alele) of the gene in retinoblasts is lost and both copies of the gene become defective.

In cases of sporadic tumor occurrence, mutations occur in one of the retinoblasts and both normal alleles in Rb are lost. The end result is the same: that retinal cell that has lost both normal copies of the Rb gene. and those that have lost the remaining normal give rise to retinoblastoma.

Patterns identified during the study of the Rb gene. in particular, the connection with hereditary forms of tumors and the need to affect both alleles (the recessive nature of the manifestation of mutations), began to be used as criteria in the search and identification of other tumor suppressors.

The group of well-studied classical tumor suppressors that are inactivated by a two-hit mechanism includes the WT1 gene (Wilms Tumor 1), the inactivation of which predisposes 10-15% to the development of nephroblastoma (Wilms tumor), the neurofibromatosis genes (NF1 and NF2) and the anti-oncogene DCC (deleted in colon carcinoma) is a gene that is inactivated in colon cancer.

However, the main representative of antioncogenes is the p53 suppressor gene, which normally provides constant control of DNA in each individual cell, preventing the appearance of harmful mutations, including tumor-causing ones. In humans it is located on chromosome 17.

The physiological functions of p53 are to recognize and correct errors that invariably occur during DNA replication under a wide variety of stresses and intracellular disorders: ionizing radiation, overexpression of oncogenes, viral infection, hypoxia, hypo- and hyperthermia, various disorders of cellular architecture (increased number of nuclei, changes cytoskeleton), etc.

The above factors activate p53; its product - the p53 protein - tightly controls the activity of proto-oncogenes in the regulation of the cell cycle and causes either a stop in the reproduction of abnormal cells (temporary, to eliminate damage, or irreversible), or their death, launching a program of cell death - apoptosis, which eliminates the possibility of accumulation of genetically modified cells in the body (Fig. 3.4). Thus, the normal form of the p53 gene plays an important protective role, being a “molecular policeman” or “guardian of the genome” (D. Lane).

Mutations can lead to inactivation of the suppressor gene53 and the appearance of an altered form of the protein, the targets of which are more than 100 genes. The main ones include genes whose products cause arrest of the cell cycle in its various phases; apoptosis-inducing genes; genes that regulate cell morphology and/or migration and genes that control angiogenesis and telomere length, etc.

Therefore, the consequences of complete inactivation of such a multifunctional gene cause the simultaneous appearance of a whole set of characteristic properties of a neoplastic cell. These include decreased sensitivity to growth inhibitory signals, immortalization, increased ability to survive in unfavorable conditions, genetic instability, stimulation of neoangiogenesis, blocking cell differentiation, etc. (Fig. 3.4).

Rice. 3.4. Security functions of the p53 suppressor gene [Zaridze D.G. 2004].

This, obviously, explains the high frequency of p53 mutations in neoplasms - they allow one to overcome several stages of tumor progression in one step.

Mutation of the p53 gene is the most common genetic disorder inherent in malignant growth, and is detected in 60% of tumors of more than 50 different types. Terminal (occurring in the germ cell and inherited) mutations in one of the alleles of the p53 gene can initiate the initial stages of carcinogenesis of various, often primary multiple, tumors (Li-Fraumeni syndrome), or can arise and be selected during tumor growth, providing its heterogeneity.

The presence of a mutated p53 gene in a tumor determines a worse prognosis in patients compared to those in whom the mutant protein is not detected, since tumor cells in which p53 is inactivated are more resistant to radiation and chemotherapy.

Mutator genes

Inhibition of the activity of suppressor genes that control apoptosis and/or the cell cycle lifts the ban on the proliferation of cells with various genetic changes, which increases the likelihood of the appearance of oncogenic cell clones. This group of genes is usually called “watchmen”.

Along with this, a number of genes specialized in recognizing and restoring (repairing) DNA damage, which can cause genetic instability and the development of cancer, have been identified. Such genes are called “caretakers” or mutator genes.

They do not directly induce malignant transformation of the cell, but contribute to the development of a tumor, since inactivation of the function of thiutator genes so increases the rate and probability of the occurrence of various oncogenic mutations and/or other genetic changes that the formation of a tumor becomes only a matter of time.

The physiological function of mutator genes is to detect DNA damage and maintain genome integrity by activating repair systems to restore the original normal DNA structure.

Therefore, they are also called DNA repair genes. It has been established that inactivation of such genes leads to disruption of DNA repair, a large number of mutations accumulate in the cell and the probability of reproduction of cellular variants with various genetic disorders sharply increases.

In this regard, in cells with defective mutator genes, a high level of genetic instability occurs and, accordingly, the frequency of spontaneous or induced genetic changes (gene mutations, chromosomal translocations, etc.) increases, against which cancer arises.

Hereditary forms of neoplasms associated with congenital gene mutations, the products of which do not ensure the functioning of repair systems, have been described. Of this group, the most studied genes are BRCA1 and BRCA2, MSH2, MSH6, MLH1, PMS2 and XPA, HRB, etc.

The BRCA1 and BRCA2 genes (Breast Cancer 1 and 2) were first identified as genes whose inherited mutations are associated with hereditary forms of breast cancer.

In women with terminal mutations of one of the alleles of the BRCA1 gene, the risk of developing breast cancer during life is about 85%, ovarian cancer - about 50%, and the risk of developing colon and prostate tumors is also higher.

With terminal mutations of the BRCA2 gene, the risk of developing breast tumors is slightly lower, but its occurrence is more frequent in men. The BRCA1 and BRCA2 genes behave like classical tumor suppressors: to initiate tumor growth, in addition to a congenital mutation in one of the alleles, inactivation of the second allele is also necessary, which occurs already in the somatic cell.

With congenital heterozygous mutations of the MSH2, MLH1, MSH6 and PMS2 genes, Lynch syndrome develops. Its main feature is the occurrence of colon cancer at a young age (the so-called hereditary nonpolyposis coporectal cancer) and/or ovarian tumors.

The predominant localization of tumors in the intestine is associated with the highest proliferative potential of cells at the bottom of the intestinal crypts and the possibility of more frequent occurrence of mutations, which are normally corrected by repair systems.

Therefore, when these genes are inactivated, rapidly reproducing intestinal epithelial cells do not recover, but accumulate a set of mutations in proto-oncogenes and antioncogenes, critical for cancer development, faster than slowly reproducing cells.

Terminal heterozygous mutations in the genes of the XPA family lead to the appearance of xeroderma pigmentosum, a hereditary disease with increased sensitivity to ultraviolet radiation and the development of multiple skin tumors in areas of solar insolation.

The human genome contains at least several dozen tumor suppressor and mutator genes, the inactivation of which leads to the development of tumors. More than 30 of them have already been identified, for many the functions performed in the cell are known (Table 3.2).

Table 3.2. Basic characteristics of some tumor suppressor and mutator genes.

Most of them, by regulating the cell cycle, apoptosis or DNA repair, prevent the accumulation of cells with genetic abnormalities in the body. Tumor suppressors have also been identified with other functions, in particular, controlling morphogenetic reactions of the cell and angiogenesis.

The discovered genes do not exhaust the list of existing tumor suppressors. It is assumed that the number of antioncogenes corresponds to the number of oncogenes.

However, studying their structure and function in primary human tumors is associated with great technical difficulties. Such research turns out to be beyond the capabilities of even the world's leading laboratories. At the same time, the classification of some genes into the category of oncogenes or antioncogenes is rather conditional.

In conclusion, it should be noted that the concept of oncogene and antioncogene for the first time in the history of oncology made it possible to combine the main directions of research into carcinogenesis.

It is believed that almost all known carcinogenic factors lead to damage to proto-oncogenes, suppressor genes and their functions, which ultimately leads to the development of a malignant neoplasm. This process is shown schematically in Figure 3.5.

Rice. 3.5. Scheme of the main stages of carcinogenesis [Moiseenko V.I. et al., 2004].

It is also necessary to emphasize that a normal differentiated cell of any tissue cannot be subject to tumor transformation, since it no longer participates in cell division, but performs a specialized function and ultimately dies apoptotically.

Disturbances in gene structure can occur without visible effects. Every second in the human body, which consists of 100 trillion cells, about 25 million cells divide.

This process is carried out under the strict control of a complex of molecular systems, the mechanisms of functioning of which, unfortunately, have not yet been fully established. It is estimated that each of the approximately 50 thousand genes in a human cell undergoes spontaneous disturbances about 1 million times during the life of the body.

Oncogenes and anti-oncogenes account for less than 1% of identified mutations, while the remaining genetic disorders are “noise”. In this case, almost all violations are recorded and eliminated by genome repair systems.

In the rarest cases, the normal structure of the altered gene is not restored, the protein product it encodes and its properties change, and if this anomaly is of a fundamental nature and affects key potential oncogenes and/or antioncogenes, cell transformation becomes possible.

In this case, some of the mutated cells may survive, but a single effect of a carcinogen on the DNA structure is not enough for tumor transformation to occur in them. It must be assumed that, with rare exceptions (for example, in virus-induced carcinogenesis), for cancer to occur, the coincidence of 4-5 mutations in one cell, independent of one another, is necessary.

The most dangerous combination is considered to be the activation of oncogenes and the inactivation of anti-oncogenes, when the autonomization of the proliferative signal is combined with breakdowns of the cell cycle control mechanisms.

That is why most malignant tumors are characterized by their development as age increases; abnormalities in the genome accumulate and can lead to the induction of the tumor process. This can also be confirmed by the gradual development of some malignant tumors: precancer, dysplasia, cancer in situ and cancer, as well as experimental studies.

We presented the main genes whose protein products help a normal cell turn into a cancerous one, and the genes whose protein products prevent this.

Of course, in addition to those listed, many other oncogenes and suppressor genes have been discovered, which in one way or another are associated with the control of cell growth and reproduction or affect other cellular characteristics.

Obviously, in the coming years we will expect other important discoveries of the mechanisms of malignant growth and the role of tumor suppressors and

If proteins encoded by oncogenes contribute to the development, then mutations in tumor suppressor genes promote malignancy through a different mechanism and with loss of function of both alleles of the gene.

Tumor suppressor genes very heterogeneous. Some of them actually suppress tumors by regulating the cell cycle or causing growth inhibition through cell-cell contact; Tumor suppressor genes of this type are CCCs, since they directly regulate cell growth.

Other tumor suppressor genes, “janitor” genes, are involved in the repair of DNA breaks and maintain the integrity of the genome. The loss of both alleles of genes involved in DNA repair or chromosomal breakage leads to cancer indirectly, allowing the accumulation of subsequent secondary mutations, both in proto-oncogenes and in other tumor suppressor genes.

Most products tumor suppressor genes highlighted and described. Because tumor suppressor genes and their products protect against cancer, it is hoped that understanding them will ultimately lead to improved anticancer therapies.

Tumor suppressor genes:
1. Tumor suppressor gene RB1: gene functions: p110 synthesis, cell cycle regulation. Tumors with gene pathology: retinoblastoma, small cell lung carcinoma, breast cancer.

2.: gene functions: p53 synthesis, cell cycle regulation. Diseases due to gene pathology: Li-Fraumeni syndrome, lung cancer, breast cancer, many others.

3. Tumor suppressor gene DCC: gene functions: Dcc receptor, decreased cell survival in the absence of a survival signal from its neutrino ligand. Diseases caused by gene pathology: colorectal cancer.

4. Tumor suppressor gene VHL: gene functions: synthesis of Vhl, part of the forms of the cytoplasmic destruction complex with APC, which normally, in the presence of oxygen, inhibits the induction of blood vessel growth. Diseases due to gene pathology: Hippel-Lindau syndrome, clear cell renal carcinoma.

5. Tumor suppressor genes BRCA1, BRCA2: gene functions: synthesis of Brcal, Brca2, chromosome repair in response to double DNA breaks. Diseases due to gene pathology: breast cancer, ovarian cancer.

6. Tumor suppressor genes MLH1, MSH2: gene functions: synthesis of Mlhl, Msh2, repair of nucleotide mismatches between DNA strands. Diseases caused by gene pathology: colorectal cancer.

Antioncogenes (or tumor suppressor genes) are genes encoding key regulatory proteins, the loss of which leads to disruption of the control of cell proliferation. Most of the identified antioncogenes in normal cells are regulators (factors) of the process of cellular gene transcription, presumably acting to enhance cell differentiation programs as opposed to proliferation programs.

Proteins encoded by a group of suppressor genes (p53, KB, C-LR! (p21), p15, p16, etc.) are directly involved in the process of cell division, controlling their entry into one or another phase of the cell cycle. Loss The activity of such genes ultimately provokes unregulated cell proliferation.

Thus, along with the activation of oncogenes, disruptions in the functioning of tumor suppressor genes are decisive in the initiation of tumorigenic processes, affecting the progression of the cell cycle, regulating differentiation and programmed cell death, i.e. the natural process of their death, the so-called apoptosis. If the majority of altered proto-oncogenes act as dominant factors from a genetic point of view, tumor suppressor genes usually act recessively.

Structural and functional changes in tumor suppressors, as in oncogenes, can be the result of point mutations in the coding and regulatory regions of the gene, insertions or deletions that cause disturbances in the protein reading process, changes in their configuration or modulation of protein expression (product formation during cellular synthesis). Loss of functions of anti-^ncogenes in tumor cells occurs as

usually as a result of inactivation of both alleles. It is assumed that the loss of one allele as a result of deletion creates the possibility of fatal recessive mutations in the remaining one (Knudsen theory). But there are exceptions to this rule: for example, for p53, the existence of mutations with dominant properties has been shown. Germinal (inherited) recessive mutations of one of the two antioncogene alleles may be the basis of a hereditary predisposition to cancer.

Experimental studies have established that inactivation of an antioncogene as a result of simultaneous disturbances in the corresponding loci of paired chromosomes (mutations in one and deletions in the other) can be eliminated by introducing a wild-type allele (i.e., structurally unchanged, intact), which is the basis for scientific developments in the field of gene _terall_n tumors_.

In addition to loss of gene function as a result of mutation or deletion, inactivation of the α-suppressor gene can occur due to hypermethylation of the DNA sequence encoding the gene. This is a characteristic method of inactivation of certain genes belonging to the group of kinase inhibitors that regulate the sequence and rate of cell cycle phases, for example p/6 and p15.

Currently, the search for tumor suppressor genes is extremely widespread.

Specific deletions of certain chromosomal regions have been identified in various types of tumors. The relationship of such deletions to tumor development is often referred to as “functional loss of a tumor suppressor gene.”

To identify chromosomal regions that claim to be potential anti-oncogenes, screening for heterozygous deletions is widely used. The deletion of one of the heterozygous alleles can be detected during a comparative analysis of RSC products (po!utegave

cNat geasTtp) or KET.P (gea^psIop Gga^tep! 1en§Y ro1utogPet) of normal and tumor DNA during electrophoretic separation. Loss of heterozygosity (1oz8 o!" be1er21205Yu - bOH) is regarded as the loss of one of the two alleles in tumor DNA when compared with the DNA of a normal somatic cell.

At present, slightly more than ten antioncosenes are known. Disturbances in antioncogenes occur in approximately 90% of human tumors. For each specific tumor, the spectrum of genetic changes is individual in nature, but nevertheless, certain patterns are observed in violations of individual genes or their clusters, which give reason to associate them with the development or nature of the progression of a particular pathology. One of the prerequisites for tumor growth is disruption of the regulation of cell division. It should be emphasized that changes in the complex chain of cell cycle control, mediated by the participation of one or another tumor suppressor, can occur at different stages of the cycle and be associated with the development of different histological types of tumors.

This chapter discusses the currently most well-known tumor suppressor genes, possible mechanisms of their action and participation in proliferative processes.

The p53 gene is one of the most studied representatives of the group of suppressor genes, which currently play an important role in the induction and progression of tumor growth. The multipotent p53 gene is involved in a number of important processes in cell life. It is localized on chromosome 17 (17p13) and encodes a transcription factor that ensures the production and functioning of proteins that control cell division. Three regions can be distinguished in the p53 protein: the I-terminal region containing the transcriptional activation domain, the central region containing the specific DNA-binding domain, and the C-terminal region containing the multifunctional domain [19].

During the growth and division of normal cells, there is a constant accumulation of violations of the primary structure of DNA as a result of natural mutagenesis or errors in the process of its doubling (DNA replication). A special system for eliminating them, including a chain of repair proteins, works in certain phases of the cell cycle. Induction of p53 causes cell cycle arrest followed by damage repair or natural cell death, thus preventing the disruption of genome integrity and the acquisition of a tumor phenotype.

The p53 protein controls the correct progression of the cell cycle at a number of control points (Fig. 3.1). The path leading to a delay in the cell cycle in phase 01, where one of the central roles belongs to the IUAP1 (p21) gene, has been more studied. The p53 gene activates the transcription of the p21 protein, which is one of the inhibitors of cyclinase kinase (CKA) complexes - regulators of the cell cycle. In this case, p53 is not only involved in the regulation of phase 01, but also takes part in the regulation of phase 02 and mitosis itself. In response to disturbances in the DNA duplication process at the checkpoint of entry into phase 02 or in response to disturbances in the formation of the mitotic spindle at the mitotic control point, p53 induction occurs.

In addition, p53 itself regulates DNA repair and replication by directly binding to a number of proteins involved in DNA processes. The exact pathway linking DNA damage and p53 activation is unknown. It is assumed that it includes the products of the suppressor gene BKCA1 (Lgeas! kanser azoaaGec! §epe I), as well as the ATM protein (a(ax1a 1e1an§]ec:a5]a &epe), which “recognizes” damage in DNA and activates p53 ( Fig, 3.2).

Another consequence of p53 activation is natural, programmed cell death, or aptosis. The p53 gene can cause apoptosis, associated or not associated with activation of transcription of target genes. In the first case, p53 activates the transcription of the BAX gene and similar genes that inhibit proteins that have an anti-apoptotic effect (for example, the BCL-2 oncogene). In addition, p53 activates the transcription of the MBM2 gene, the product of which, by binding to the p53 protein, inhibits its ability to activate the transcription of other target genes, thus providing negative self-regulation. It has been shown that p53 induction causes cell cycle arrest at 01 or apoptosis depending on a number of factors, the most important of which are the cell type, the concentration of growth factors, the level of expression of the suppressor genes KB, AIR and (or) the transcription factor E2P, the expression of a number of viral proteins, etc. .

Inactivation of p53 gives cells a greater selective advantage in proliferation. Impaired p53 function as a result of point mutations, deletions, formation of a complex with another cellular regulator, or changes in intracellular localization leads to the loss of suppressive properties and stimulates the tumor process. When studying tumors of various histogenesis, it was found that in a large percentage of cases both p53 alleles are inactivated - one as a result of point mutations, the other as a result of deletions.

p53 mutations are the most common genetic disorder recorded in various tumors

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For the occurrence of tumors in humans, activation of oncogenes alone is not enough, since uncontrolled cell division is prevented by suppressor genes (Rb, p-53, APC genes), which ensure that cell mitosis stops at checkpoints. At the first checkpoint, DNA damage repair occurs as the G1/S control mechanism blocks DNA replication. When repair processes are disrupted, apoptosis is induced. At the second checkpoint, the G2/M control mechanism inhibits mitosis until replication is complete.

This ensures genome stability. In the case of mutations, suppressor genes acquire a recessive trait on both alleles, the activity of their proteins decreases sharply, a cell with genetic damage realizes the property of uncontrolled reproduction and creates a clone of similar descendants. An explanation for the formation of recessive suppressor genes was given by Knudson, who proposed a hypothesis of carcinogenesis known as the “two-hit” theory. Its essence is this: one allele of a recessive suppressor gene is inherited from parents (“first blow”), and the second is the result of a mutation (“second blow”). Knudson's hypothesis is confirmed by cytogenetic or molecular studies of some tumors.

7. Role in carcinogenesis of genes regulating DNA repair and apoptosis.

During carcinogenesis, mutations in genes responsible for DNA repair and apoptosis, leading to a decrease in the activity of the corresponding enzymes, contribute to an increase in the instability of the tumor cell genome. In addition, along with uncontrolled proliferation, due to low activity or disappearance of genes,

regulating apoptosis (bcl-2, bac), there is a sharp increase in the number of tumor cells.

8. Relationship between the tumor and the body. Paraneoplastic syndrome. Mechanisms of antitumor resistance of the body.

The relationship between the tumor and the body is very diverse and contradictory. On the one hand, the organism, which is the external environment for the tumor, creates the necessary conditions for its existence and growth (providing, for example, its blood supply), and on the other hand, it counteracts its development with greater or lesser success.

Tumor development is an interactive process (acts of “aggression” of the tumor alternate with reactive “countermeasures” of the body). The outcome of this struggle is predetermined by the enormous potential for “aggressiveness” of the tumor, on the one hand, and the limited protective resources of the body, on the other.

Immune protection. Not every clone of tumor cells that arises in the body turns into a malignant tumor. The body has certain, albeit limited, means of counteraction. At the first stages, a system of so-called natural nonspecific resistance operates, capable of eliminating a small number (from 1 to 1000) of tumor cells. This includes natural killer cells - large granular lymphocytes, making up 1 to 2.5% of the entire population of peripheral lymphocytes, and macrophages. Specific antitumor immunity usually develops too late and is not very active. Spontaneous tumors in animals and humans are weakly antigenic and easily overcome this barrier. However, in some cases it appears to be able to play a significant role.

Paraneoplastic syndrome is a manifestation of the generalized effect of a tumor on the body. Its forms are varied - a state of immunosuppression (increased susceptibility to infectious diseases), a tendency to increase blood clotting, cardiovascular failure, muscular dystrophy, some rare dermatoses, reduced glucose tolerance, acute hypoglycemia in large tumors and others. One of the manifestations of paraneoplastic syndrome is the so-called

cancer cachexia (general exhaustion of the body), which occurs in a period close to the terminal stage and is often observed in cancer of the stomach, pancreas and liver

It is characterized by loss of body weight, mainly due to increased breakdown of skeletal muscle proteins (partially myocardium, as well as depletion of fat depots, accompanied by aversion to food (anorexia) and changes in taste. One of the causes of cachexia is increased (sometimes by 20-50 %) energy consumption, apparently due to hormonal imbalance.

The mechanisms of antitumor resistance can be conditionally divided according to the stage and factor of carcinogenesis into three main generalized types:

1. Anti-carcinogenic, addressed to the stage of interaction of the carcinogenic (causal) factor with cells, organelles, macromolecules.

2. Anti-transformation, aimed at the stage of transformation of a normal cell into a tumor cell and inhibiting it.

3. Anticellular, addressed to the stage of transformation of the formation of individual tumor cells into a cell colony - a tumor.

Anticarcinogenic mechanisms are represented by three groups. Group 1 includes anti-carcinogenic mechanisms acting against chemical carcinogenic factors:

1. Reactions of inactivation of carcinogens: a) oxidation using nonspecific oxidases of microsomes, for example polycyclic hydrocarbons; b) reduction using microsomal reductases, for example aminoazo dyes - dimethylaminoazobenzene, o-aminoazotoluene; c) dimethylation - enzymatic or non-enzymatic; d) conjugation with glucuronic or sulfuric acid using enzymes (glucuronidase sulfatase);

2. Elimination of eso- and endogenous carcinogenic agents from the body in the composition of bile, feces, urine;

3. Pinocytosis and phagocytosis of carcinogenic agents, accompanied by their neutralization;

4. Formation of antibodies against carcinogens as haptens;

5. Inhibition of free radicals by antioxidants.

The 2nd group includes anti-carcinogenic mechanisms acting against biological etiological factors - oncogenic viruses:

1. Inhibition of oncogenic viruses by interferons;

2. Neutralization of oncogenic viruses with specific antibodies. The third group of anti-carcinogenic mechanisms is represented by mechanisms acting against physical carcinogenic factors - ionizing radiation. The main ones among them are the reactions of inhibition of the formation and inactivation of free radicals (antiradical reactions) and lipid and hydrogen peroxides (antiperoxide reactions), which are, apparently, “mediators” through which ionizing radiation, at least in part, realizes its tumor-causing effect. influence. Antiradical and antiperoxide reactions are provided by vitamin E, selenium, glutathione-disulfide system (consisting of reduced and oxidized glutathione), glutathione peroxidase (breaking down lipid and hydrogen peroxide), superoxide dismutase, which inactivates the superoxide anion radical, catalase, which breaks down hydrogen peroxide.

Anti-transformation mechanisms

Due to these mechanisms, the transformation of a normal cell into a tumor cell is inhibited.

These include:

1. Antimutation mechanisms, which are a function of cellular enzyme systems for DNA repair, eliminating damage, “errors” of DNA (genes) and thereby maintaining gene homeostasis; 2. Anti-oncogenic mechanisms, which are a function of special cellular genes - antagonists of oncogenes and are therefore called antioncogenes. Their action is reduced to suppressing cell reproduction and stimulating their differentiation. The presence of antioncogenes in normal cells is evidenced by the experiments of the group of E. Stanbridge and co-workers. They introduced a normal chromosome (the 11th pair from a human cell) into a Williams tumor cell. As a result, the tumor cells underwent transformation into normal cells. An indirect argument in favor of antioncogenes is the absence of such a gene (the so-called Rb gene) in the 13th pair of chromosomes in retinoblastoma cells and in their normal precursors - retinal cells.

Anticellular mechanisms

These mechanisms are activated from the moment the first blastoma cells are formed. They are aimed at inhibiting and destroying individual tumor cells and tumors as a whole. Factors that include anticellular antitumor mechanisms are antigenic and “cellular” foreignness of tumors. There are two groups of anticellular mechanisms: immunogenic and non-immunogenic

1. Immunogenic anticellular mechanisms are functions of the immune system, which carries out the so-called immune surveillance of the constancy of the antigenic composition of tissues and organs of the body. They are divided into specific and nonspecific.

Specific immunogenic mechanisms include cytotoxic action, growth inhibition and destruction of tumor cells: a) immune T-killer lymphocytes; b) immune macrophages with the help of factors secreted by them: macrophage-lysine, lysosomal enzymes, complement factors, growth inhibitory component of interferon, tumor necrosis factor; c) K-lymphocytes, which have Fc receptors for immunoglobulins and, due to this, exhibit affinity and cytotoxicity for tumor cells that are coated with IgG. Nonspecific immunogenic mechanisms. These include nonspecific cytotoxic effects, inhibition and lysis of tumor cells: a) natural killer cells (NK cells), which, like K-lymphocytes, are a type of lymphocytes lacking the characteristic markers of T- and B-lymphocytes; b) nonspecifically activated (for example, under the influence of mitogens, PHA, etc.); c) nonspecifically activated macrophages (for example, under the influence of BCG or bacteria, endotoxins, especially lipopolysaccharides from gamo-negative microorganisms) with the help of tumor necrosis factor (TNF), interleukin-1, interferon, etc. secreted by them; d) “cross” antibodies.

2. Non-immunogenic anticellular factors and mechanisms.

These include: 1) tumor necrosis factor, 2) allogeneic inhibition, 3) interleukin-1, 4) keylon inhibition, 5) lipoprotein-induced carcinolysis, 6) contact inhibition, 7) labrocytosis, 8) regulating influence of hormones.

Tumor necrosis factor. Produced by monocytes, tissue macrophages, T- and B-lymphocytes, granulocytes, mast cells. Causes destruction and death of tumor cells. Interlekin-1 (IL-1). The mechanism of the anti-blastoma action of IL-1 is associated with the stimulation of K-lymphocytes, killer T-lymphocytes, the synthesis of IL-2, which in turn stimulates the reproduction and growth of T-lymphocytes (including killer T-lymphocytes), activation of macrophages, the formation of γ-interferon and , perhaps partly through pyrogenic action. Allogeneic inhibition. In relation to tumor cells, this is the suppression of vital activity and their destruction by surrounding normal cells. It is assumed that allogeneic inhibition is due to the cytotoxic effect of antigens of histoincompatible metabolites and differences in membrane surfaces. Keylon inhibition. Keylons are tissue-specific inhibitors of cell proliferation, including tumor cells. Carcinolysis induced by lipoproteins. Carcinolysis is the dissolution of tumor cells. The fraction of n-lipoproteins has a specific oncolytic effect. This fraction has no lytic effect on auto-, homo- and heterologous normal cells.

Contact braking. It is believed that cyclic nucleotides - cyclic adenosine-3, 5-monophosphate (cAMP) and cyclic guanosine-3,5-monophosphate (cGMP) - are involved in the implementation of the phenomenon of contact inhibition.

An increase in cAMP concentration activates contact inhibition. Against,

cGMP inhibits contact inhibition and stimulates cell division. Labrocytosis. Carcinogenesis is accompanied by an increase in the number of mast cells (mast cells) producing heparin, which inhibits the formation of fibrin on the surface of tumor cells (fixed and circulating in the blood). This prevents the development of metastases by inhibiting the transformation of a cancer cell embolus into a cellular thrombo-embolus. Regulatory influence of hormones. Hormones have a regulatory effect on the body's anti-blastoma resistance. A characteristic feature of this effect is its diversity, depending on the dose of the hormone and the type of tumor. The question arises: why, despite such powerful anticellular mechanisms directed against the tumor cell, the latter often persists and turns into a blastoma? This happens because the causes of tumors simultaneously (long before the development of the tumor) cause immunosuppression. The resulting tumor, in turn, itself potentiates immunosuppression. It should be noted that immunosuppression that has arisen outside of connection with the action of carcinogens, for example, hereditary T-immune deficiency (with Wiskott-Aldrich syndrome, etc.), as well as acquired (used during organ transplantation or developing during organ transplantation or developing during treatment with cytostatics) sharply increases the risk of tumor development. Thus, immunosuppression during organ transplantation increases the risk of tumor development by 50-100 times. A number of other phenomena also prevent the destruction and, on the contrary, promote the preservation of tumor cells: antigenic simplification; antigen reversion - the appearance of embryonic antigen proteins to which the body has innate tolerance; the appearance of special antibodies that protect tumor cells from T-lymphocytes and are called “blocking” antibodies.