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Tumor Suppressor Genes

Q: Which tumor suppressor genes mutate in colorectal cancer?

In colorectal cancer, the APC and TP53 (p53) genes frequently mutate. APC (APC) — mutation participates in the molecular genetic mechanism of carcinogenesis along the "adenoma-carcinoma" sequence.TP53 (TP53) — mutation is also characteristic of the "adenoma-carcinoma" sequence; alterations in this gene are found in almost all human tumors.

Q: What functions does the p53 protein perform in a healthy cell?

In a healthy cell, the normal ("wild-type") p53 protein functions as a transcription factor and DNA-binding protein. Cell cycle control — regulates the G1 to S phase transition; upon DNA damage, it activates the transcription of the WAFI protein (p21), which inactivates cyclins.DNA repair and synthesis — ensures restoration of the genome structure during cell cycle arrest.Induction of apoptosis — initiates programmed cell death if DNA damage is irreparable.Cell differentiation — participates in processes of cellular specialization.

Antioncogenes

For medical students2 min readUpdated 2026-10-10

Tumor suppressor genes (antioncogenes) are genes that inhibit cellular proliferation and trigger apoptosis in response to DNA damage. The loss of their function deprives the cell of division control, which is a key step in malignant transformation.

The p53 geneNamed the "Molecule of the Year" in 1993 (often referenced as 1995). Blocks the cell cycle in the G1 phase for DNA repair.

The Rb geneLocalized to chromosome 13. Its mutation causes retinoblastoma in 1 out of 20,000 children.

Two allelesA tumor develops only when both alleles of a tumor suppressor gene lose their function.

EpigeneticsPromoter hypermethylation can "turn off" antioncogenes without mutations in the DNA sequence.

Main Guardians of the Genome: p53 and Rb

Tumor suppressor genes act as cellular brakes. Their main task is to halt proliferation and trigger programmed cell death (apoptosis) in response to critical damage.

A key representative is the p53 gene. Normally (wild-type), it responds to nuclear DNA damage by blocking the cell cycle in the G1 phase, giving the cell time for repair. If restoring the DNA structure is impossible, p53 initiates apoptosis. However, mutant p53 behaves in a dominant-negative fashion. Not only does it fail to stop division, but it also stimulates the proliferation of cells with a defective genome, leading to genomic instability.

Another important antioncogen is Rb, localized on chromosome 13. Its inactivation causes retinoblastoma. Tumor development requires damage to both alleles. In the hereditary form (40% of cases), the first mutation is inherited from the parents (germline), while the second occurs spontaneously. In the sporadic form (60%), both mutations happen randomly in a single somatic cell.

Apoptosis Regulation and Tumor Survival

Malignant transformation largely depends on the balance between tumor suppressor genes and oncogenes. The general principle of carcinogenesis is the accumulation of mutations in oncogenes alongside the loss of tumor suppressor function.

The regulation of apoptosis plays a massive role. For example, the bcl2 gene is a cellular oncogene with anti-apoptotic activity. Its overexpression protects tumor cells from death. A classic example is B-cell follicular lymphoma, where chromosomal translocation t(14;18) places the bcl2 gene in the region of actively transcribed genes regulating immunoglobulin heavy chain synthesis. A similar apoptosis-resistance mechanism is characteristic of small cell lung cancer.

Interestingly, the death of a tumor cell during chemotherapy or radiation therapy is achieved through the activation of p53 against the background of active cellular oncogenes (such as c-fos and c-myc). If p53 is mutated while bcl2 and oncogene expression are elevated, the cell divides uncontrollably and the tumor progresses.

Epigenetic Mechanisms and MicroRNAs

In addition to direct mutations, the activity of tumor suppressor genes can be suppressed by epigenetic changes—reversible modifications that do not alter the DNA sequence.

DNA methylation: An imbalance is observed in cancer cells. Global hypomethylation causes chromosomal instability, whereas selective promoter hypermethylation leads to the "silencing" of tumor suppressor genes, acting as a full alternative to mutation.
Histone code: Acetylation and methylation of histone tails alter chromatin compaction density, thereby activating or repressing transcription.

Another level of control is provided by microRNAs (miRNAs)—short non-coding molecules (~22 nucleotides) integrated into the RISC complex. They suppress gene expression at the post-transcriptional level. In carcinogenesis, miRNAs play a dual role:

As suppressors: their deficiency leads to the overproduction of oncoproteins.
As oncogenes: their overexpression (5- to 100-fold in brain or breast tumors) blocks normal tumor suppressor function.

The miRNA expression profile is tissue-specific, which aids in tumor classification and targeted therapy development.

Mnemonic

To remember the mechanism of retinoblastoma development, imagine a car with two brake pedals (the two alleles of the Rb gene). A tumor will only develop if both pedals break. If one pedal remains intact, it compensates for the braking function.

Frequently asked questions

Which tumor suppressor genes mutate in colorectal cancer?

In colorectal cancer, the APC and TP53 (p53) genes frequently mutate.

APC (APC) — mutation participates in the molecular genetic mechanism of carcinogenesis along the "adenoma-carcinoma" sequence.
TP53 (TP53) — mutation is also characteristic of the "adenoma-carcinoma" sequence; alterations in this gene are found in almost all human tumors.

Which tumor suppressor genes are associated with hereditary breast cancer?

Hereditary breast cancer is associated with germline mutations in breast cancer susceptibility genes:

BRCA1 — the gene product participates in DNA repair and cell cycle regulation; locus 17q21.
BRCA2 — the gene product participates in DNA repair and cell cycle regulation; locus 13q12.3.
CHEK2 (referred to as CHEK).
NBS1.
TP53.

What functions does the p53 protein perform in a healthy cell?

In a healthy cell, the normal ("wild-type") p53 protein functions as a transcription factor and DNA-binding protein.

Cell cycle control — regulates the G1 to S phase transition; upon DNA damage, it activates the transcription of the WAFI protein (p21), which inactivates cyclins.
DNA repair and synthesis — ensures restoration of the genome structure during cell cycle arrest.
Induction of apoptosis — initiates programmed cell death if DNA damage is irreparable.
Cell differentiation — participates in processes of cellular specialization.

Why is mutant p53 so dangerous for the cell?

It acts in a dominant-negative manner. Instead of halting the division of a defective cell in the G1 phase, it stimulates its proliferation, leading to genomic instability.

How does radiation therapy work at the molecular level?

It causes DNA damage, which activates the normal p53 gene in the presence of active oncogenes (c-fos, c-myc). This combination triggers apoptosis in the tumor cell.

How does hereditary retinoblastoma differ from sporadic retinoblastoma?

In the hereditary form (40% of cases), the first Rb mutation is present in all cells from birth, requiring only a single spontaneous hit. In the sporadic form (60%), both mutations occur randomly in a single somatic cell.

Go deeper

DNA repair mechanisms in the G1 phase of the cell cycle
Chromosomal translocation t(14;18) in B-cell follicular lymphoma
The role of cellular oncogenes c-fos and c-myc in carcinogenesis
RNA-induced silencing complex (RISC) and post-transcriptional regulation
Histone code: the influence of acetylation and methylation on transcription