Tumor suppressor gene

Tumor suppressor models.

A tumor suppressor gene is a gene that reduces the likelihood that a cell in a multicellular organism will become a cancer cell. Tumor suppressor genes are found in normal cells and are usually inhibit excessive cell proliferation. A mutation or deletion of a tumor suppressor gene will increase the probability that a tumor will develop by losing its function. In this way, an altered tumor suppressor gene is similar to an oncogene.

In normal cells, proteins encoded by tumor suppressor genes arrest cell cycle progression in response to DNA damage or growth suppression signals from the extracellular milieu (contact inhibition). When tumor suppressor genes are mutated or inactive, cells cannot respond normally to cell cycle checkpoints, or are unable to perform programmed cell death if DNA damage is too extensive. This leads to an increase in mutations and the inability of the cell to exit the cell cycle when it should become quiescent. When both alleles of a tumor suppressor gene are inactive, and there are other changes in the cell that keep it growing and dividing, cells can become tumorigenic. In many tumors, these genes are absent or inactivated, so negative regulators of cell proliferation are not involved, contributing to the abnormal proliferation of tumor cells.

Unlike oncogenes (which produce tumors by activating normal proto-oncogenes present in the cell), tumor suppressor genes are involved in the tumor process if they undergo mutations that inactivate them, that is, if there is a loss of function; this type of mutation has a recessive effect, since to eliminate the activity, both alleles have to be mutated (see also Knudson's hypothesis). For this reason, tumor suppressor genes are sometimes called recessive tumor genes.

Identification of tumor suppression genes

The first data on these genes were obtained from somatic cell hybridization experiments. Fusion of normal cells with tumor cells resulted in hybrid cells, which contained the chromosomes of both parents. In many cases, the hybrid cells were not capable of forming tumors in animals. Thus, it appeared that there were genes from the normal cell progenitor that suppressed tumor development, but the biochemical characterization of genes at the molecular level was made from analysis of rare heritable forms of cancer in humans.

The model of tumor suppressor genes was first proposed by Alfred Knudson in the 1970s to explain the hereditary mechanism of retinoblastoma, an autosomal dominant disease. Knudson proposed that, in families affected by the hereditary form of retinoblastoma, a first hit event occurs in the germ line that inactivates one of the two RB1 alleles. Since the other allele would remain active, this would only produce a 50% decrease in the amount of active protein, which has a negligible effect. Knudson then proposed that the loss of the second allele of RB1 (second hit) must occur in a somatic tissue for a tumor to develop in it. This leads to a paradox: although the transmission of the predisposition to develop a tumor is dominant (because one mutated allele is enough to transmit the predisposition), the development of the tumor itself is recessive (because two mutated alleles are needed to produce the tumor). The RB1 gene (located at 13q14.1-q14.2) was cloned in 1986, one of the first successes of the positional cloning method. As predicted by Knudson's hypothesis, when retinoblastoma tumors were analyzed using cDNA as probe in Northern blots, it was observed that there were tumors in which the messenger RNA was completely absent, while in others mRNA of abnormal size was observed. In other cases, the size of the mRNA appeared normal, but sequencing of the mRNA revealed the presence of point mutations that affected the function of the protein. In no case were normal RB1 mRNAs observed in retinoblastoma tumors.

Functions of Tumor Suppressor Gene Products

The proteins encoded by most tumor suppressor genes inhibit cell proliferation or survival. Therefore, inactivation of tumor suppressor genes leads to tumor development by knocking out negative regulation proteins. In several cases, tumor suppressor proteins inhibit the same regulatory pathways that are activated by oncogene products. Several tumor suppressor genes encode transcriptional regulatory proteins. Other products of these genes regulate the progression of the cell cycle and are capable of acting as oncogenes. Tumor suppressor gene products inhibit cell proliferation, so their functional loss makes the cell proliferate more easily.

The products of these genes act through very diverse mechanisms:

• By inhibiting cell progression through the cell cycle.
• Making cells enter in apoptosis.
• Maintaining the stability of the genome (replication, repair and segregation).

Unlike proto-oncogenes, tumor suppressor genes need both alleles to be inactivated for cell behavior to be altered. Tumor suppressor genes belong to different types of proteins, such as growth factors, cell adhesion factors, cell cycle control, transcription factors, DNA repair...

There are currently three known ways of inactivating these genes:

• Due to punctual mutations; the most frequent are those that lead to changes in the gene reading framework, mutations that lead to stop condoms (STOP condoms) and amino acid change mutations.
• Deletion; deletion often also includes neighboring genes of the tumor suppressant gene.
• By methylation. DNA cytosine can be methylated if it is in a position before a guanine. This CpG sequence (p=phosphorus) is repeated in regions called CpG islands, which are found in the promoter regions of the so-called genes housekeepers or genes that are expressed in all cells. If these regions are methylated, they do not allow the expression of the gene so in practice the result is the same as if the gene was not or was mutated.

The contribution of these genes to tumor suppression is basically done in three ways:

1. Repression of genes that are essential in the continuation of the cell cycle. If these genes are not expressed, the cell cycle will stop, and the division of the cell will effectively be inhibited.
2. Relationship between cell cycle and DNA damage. The more lesions exist in the DNA the less the cell will be divided, since the alterations in the DNA are detected by the control points of the cell cycle, which produce a stop in the progression of the cycle. If the damage can be repaired, the cell can continue its progression through the cell cycle and divide.
3. If the damage cannot be repaired, the cell must initiate apoptosis, programmed cell death, to eliminate the threat it poses to most of the organism: if it acquires tumorigenic mutations and divides, it will pass the new characteristics to the daughter cells, increasing the population of dangerous cells.

Like oncogenes, tumor suppressor genes have diverse roles in regulating growth, cell differentiation, and programmed cell death (apoptosis).

Role of oncogenes and suppressor genes in tumor development

The development of cancer is a multi-step process in which healthy cells gradually progress to become cancer cells. At these stages, both the activation of oncogenes and the inactivation of tumor suppressor genes are critical steps in tumor initiation and progression. Over time, cumulative damage to various genes is responsible for increased ability to proliferate, invade other tissues and generate metastasis, characteristic of cancer cells. The role of numerous genetic alterations is best understood in colon carcinomas. These tumors involve mutations of oncogenes or tumor suppressor genes with four distinct activities:

• Oncogenes ras o raf that affect the ERK pathway.
• Tumor suppressant proteins involved in signaling TGF-β.
• Wnt Tumor Surprising Components.
• p53.

With the various lesions that have been seen in surgical samples in the development of colon cancer, genetic alterations can be related to the different stages of tumor progression.

Types of suppressor genes according to their function

Gatekeeper tumor suppressor genes

They are found in autosomal dominant hereditary cancer syndromes. Its function is to directly regulate cell growth. They block tumor development by regulating the transition of cells through existing NO checkpoints in the cell cycle or by stimulating programmed cell death, with control of cell division and survival. Loss-of-function mutations in gatekeeper genes result in uncontrolled cellular mutation.

Guardian tumor suppressor genes encode:

• Regulators of various control points of the cell cycle.
• The programmed cell death mediators.

Retinoblastoma: RB1

Retinoblastoma is the prototype of diseases caused by a mutation in a tumor suppression gene; It is a rare tumor that originates in the retina of infants and has an incidence of approximately 1/20,000 newborns.

It can be hereditary in 40% of cases; these children inherit a mutant allele (first event or hit) at the retinoblastoma locus (RB1) via germ cells. A somatic mutation or other alteration in a single retinal cell results in loss of function of the remaining normal allele, initiating tumor development. This disorder is dominantly inherited due to the presence of a high number of primordial retinoblasts and their rapid rate of proliferation, making it highly likely that a somatic mutation (second hit) occurs in one or more of the existing retinoblasts. Because the chances of the second event in the hereditary form are so high, this event often occurs in more than one cell, so heterozygotes for this disease often have multiple tumors affecting both eyes. On the other hand, the appearance of the second event is a casual phenomenon and does not occur in all cases; therefore, the penetrance of the retinoblastoma gel is high but incomplete.

The remaining 60% of cases are sporadic (non-hereditary), in this case both RB1 alleles of a cell have been inactivated independently. Retinoblastoma is usually only located in one eye. One difference between hereditary and sporadic tumors is the fact that the average age of patients when the sporadic form begins is in early childhood, that is, later than that of infants with the hereditary form.

On the other hand, although Rb has been identified in a rare childhood cancer, it is also involved in some more common adult tumors. The Rb gene is inactive in bladder, breast, and lung carcinomas. Thus, Rb gene mutations contribute to the development of an important part of the most common human cancers. In addition, the Rb protein is an important target of the oncogenic proteins of several DNA tumor viruses, such as human papillomavirus.

It is a tumor suppressor protein that controls the G1/S checkpoint of the cell cycle. This gene is located on chromosome 13. The RB1 gene product is a phosphoprotein called p110 Rb1 that exhibits hypophosphorylation and then hyperphosphorylation at different stages of the cell cycle. In its hypophosphate state, it blocks cell cycle progression at the border between G1 and S phases, inhibiting the initiation of S phase by binding to transcription factors that stimulate DNA synthesis, with their inactivation. As the protein becomes phosphorylated, it releases its protein-binding elements, facilitating the initiation of S phase in the cell; it then undergoes progressive dephosphorylation over the course of the cell cycle, causing it to again block the initiation of S phase in the subsequent cell cycle. Loss of this gene deprives cells of an important mitotic checkpoint and allows uncontrolled proliferation. Thus, the RB1 gene is a prototypical gatekeeper tumor suppressor gene.

Investigating DNA polymorphisms in the region near the RB1 locus, it was found that children with retinoblastoma who are heterozygous for polymorphic loci near the RB1 gene in normal tissues had tumors that contained alleles corresponding to only one of the RB1 loci. its two homologous chromosome 13s, revealing a loss of heterozygosity. This is the phenomenon of tumor mutation of the second allele, the wild type. It is an essential step for the expression of hereditary cancers. Loss of heterozygosity is the most common mutational mechanism through which the function of the remaining normal RB1 allele is altered in heterozygotes. Loss of heterozygosity may be due to complete deletion of chromosome 13 or interstitial deletion, although there are also other mechanisms such as mitotic recombination or nondisjunction.

Li-Fraumeni syndrome: TP53

There are rare “familial cancers” in which there is a history of many different forms of cancer, affecting several relatives at early ages, and with hereditary transmission in an autosomal dominant pattern. This highly variable phenotype is known as Li-Fraumeni syndrome (LFS, Li-Fraumeni syndrome). Affected members of LFS families have been found to carry a mutant form of the TP53 gene in the form of a germ cell mutation. Because the TP53 gene is inactivated in the sporadic forms of many of the cancers that occur in LFS, it is considered to be a candidate for the defective gene that causes LFS.

This gene encodes the p53 protein, which is a DNA-binding protein and is an important component of the cellular response to DNA damage. It is also a transcription factor that activates the transcription of genes that interrupt cell division and that facilitate the repair of DNA alterations. p53 is also involved in the induction of apoptosis in cells that have suffered irreparable DNA damage. Therefore, loss of function of this protein allows cells with altered DNA to survive, leading to the potential spread of oncogenic mutations. Therefore, the TP53 gene is known as the "guardian of the genome".

Neurofibromatosis type 1: NF1

NF1 is a fairly common autosomal disease that affects the peripheral nervous system and is characterized by the appearance of a high number of benign neurofibromas. There are also some rare malignancies that have a higher incidence in a minority of NF1 patients. Among the malignant tumors, the following can be highlighted: neurofibrosarcoma, astrocytoma, malignant tumors originating in Schwann cells and childhood chronic myeloid leukemia; these tumors are rare in patients without NF1. The abnormal cell growth observed in NF1 suggests that the normal gene may be involved in the regulation of cell division in neural tissue.

The NF1 gene is located on chromosome 17. The normal product produced by this gene interacts with an unknown member of the RAS gene family to regulate proliferative activity in normal cells. The mutant NF1 gene may be unable to regulate growth in the normal cells from which neurofibromas are derived, resulting in overgrowth with tumor formation. Therefore, the NF1 gene is a suppressor gene of

“caretakers” or “maintenance” genes (caretakers)

They are found in autosomal dominant cancer syndromes. They are involved in the repair of DNA alterations and in the maintenance of the integrity of the genome. Loss of function of caretaker genes allows the accumulation of mutations in both oncogenes and caretaker genes, which together lead to the initiation and promotion of cancer.

Caregiver tumor suppressor genes encode:

• Proteins responsible for the detection and repair of mutations.
• The proteins involved in normal chromosomal disjunctions during mytosis.
• Components of the programmed cell death device.

Family breast cancer: BRCA1 and BRCA2

Breast cancer has a strong genetic component; the risk of breast cancer in a woman increases up to 3 times if she has a relative affected in the first degree and up to 10 times if she has more than one relative in the first degree affected by this disease.

Genetic linkage studies in families with familial and early-onset breast cancer led to the discovery of mutations in two genes that increase susceptibility to breast cancer, BRCA1 on chromosome 17 and BRCA2 on chromosome 17. 13. Taken together, both loci account for approximately one-half to one-third of dominantly transmitted familial breast cancer cases. Mutations in these genes are also associated with an increased risk of ovarian cancer in heterozygous females.

The products of these genes are nuclear proteins contained in the same multiprotein complex. This complex is involved in the cellular response to double-stranded DNA fragmentation. Tumor tissue from those heterozygotes for the BRCA1 and BRCA2 mutations show loss of heterozygosity with loss of the normal allele.

Familial Colon Cancer: APC, MLH, MSH2, and MSH6

1. Family colon polyposis: Colorectal cancer is a malignant tumor of the colon and rectum epithelium, and is one of the most common forms of cancer. A small proportion of colon cancer cases is due to the dominant autosomal disorder called family adenomatous polyposis (FAP, familial adenomatous polyposis) and a variant of it, Gardner syndrome. The responsible gene is the APC, which is located in chromosome 5. This gene encodes a cytoplasmic protein that regulates the bifunctional protein called β-catenine. This protein acts as a link between the cytoplasmic portion of transmembrane cell adhesion molecules and the [actine] skeleton, and is also a transcription activator. In normal conditions, when the colonic epithelium remains intact and cell proliferation is not necessary, most of the β-catenine is forming a large protein complex. The APC gene induces phosphorylation and degradation of β-catenine not linked to the complex, making the concentrations of this free protein in the cell low. The loss of the APC gene causes the accumulation of free cytoplasmic β-catenine that suffers translocation to the nucleus, where the transcription of cell proliferation genes activates. Therefore APC is a suppressant gene of guardian tumors.
2. Cancer of hereditary colon not associated with polyposis (HNPCC): is characterized by the dominant autosomal transmission of colon cancer, which begins during the adult stage although at a young age and without association with the adenomatous polyps that are observed in the FAP. HNPCC is a group of five family carcinogenic syndromes caused by mutations in one of five specific DNA repair genes responsible for reconstitution of DNA segments in which there has been an alteration in base pairing. MLH, MSH2 and MSH6 genes are jointly responsible for most HNPCCs. HNPCC genes are suppressants of prototypical tumors. The dominant autosomal hereditary pattern of HNPCC takes place through the inheritance of a mutant allele followed by the mutation or inactivation of the remaining normal allele in a somatic cell. On a cellular level, the most striking phenotype of cells that lack both alleles in one of these genes causes an increase in the number of specific mutations and the instability of DNA segments that contain unique repetitive sequences, such as microsatellite polymorphisms. Microsatellite DNA is considered to be very vulnerable to incorruption in the matching of the bases because when short DNA repetition sequences are synthesized in tandem it is easier for a slide of the mold chain to occur.

Caregiver genes in autosomal recessive chromosome instability syndromes

These autosomal recessive disorders, such as xeroderma pigmentosa, Fanconi anemia… are due to the loss of function of proteins necessary for normal DNA repair or replication. Therefore, the genes altered in the syndromes of chromosomal instability can be considered as tumor suppressor genes caregivers.

Although chromosome instability syndromes are rare autosomal recessive disorders, those heterozygotes for these genetic defects are more common and appear to have an increased risk of malignancy.

Tumor suppressor genes involved in chromatin regulation

In addition to tumor suppressor genes that protect against mutation or DNA instability, there are other types that function by regulating the state of chromatin or transcription. When these types of genes are mutated, other genes that contributed to the development of cancer are expressed or repressed. For this reason, there is a broad class of tumor suppressor genes that typically function in the realm of epigenetics.

For example, DNA methylation is a very important regulator of gene transcription. Specifically, DNA hypermethylation is one of the major epigenetic modifications responsible for repressing transcription via the promoter region of tumor suppressor genes. In glioblastoma, the IDH1 gene mutation causes a large amplification of DNA methylation, which affects the expression of many other genes that ultimately cause cancer. There are also examples of aberrations affecting histone methyltransferases (e.g., MLL), the SWI/SNF chromatin remodeling complex, and several other proteins that repress other genes (e.g., Polycomb). The development of cancer treatments based on these mutations represents a very important goal in the field of precise medicine.

Examples of Tumor Suppressor Genes