What are oncogenes and how do they work
In a healthy organism, the genes we call oncogenes perform a crucial physiological function. They encode specific proteins that ensure normal proliferation (division) and differentiation of cell populations. The products of such genes include various growth factors, nuclear proteins, GTPases, and protein kinases.
It is important to understand that tumor development cannot be linked to a mutation in just a single oncogene. Malignant transformation requires the combined interaction of several oncogenes, as well as genes responsible for regulating the cell cycle.
Main examples of oncogenes and their functions:
- c-erbB — responsible for the synthesis of the epidermal growth factor receptor.
- erbA — encodes a receptor sensitive to steroid hormones.
- MYC — a multi-activity gene acting as a potent stimulator of cell division.
- BCR-ABL — a pathological chimeric gene. It produces a hybrid protein with unregulated tyrosine kinase activity.
- Aberrant JAK2 kinase — triggers transcription factors of the STAT family. As a result, the cell begins to autonomously maintain proliferation, even without external growth factors.
- Cyclin and cyclin-dependent kinase genes — ensure the entry of a quiescent cell into the mitotic cycle.
Tumor suppressor genes (anti-oncogenes)
While oncogenes stimulate cell division, tumor suppressor genes (or anti-oncogenes) perform the exact opposite function. Under normal conditions, they inhibit excessive cell proliferation, acting as a kind of biological safeguard.
The pathogenesis of tumor growth is closely linked to the damage of these structures. Genetic mutations lead to the loss of suppressor function. As a result, the cell loses its inhibitory mechanisms, creating ideal conditions for multiple divisions and unlimited growth of the transformed cell pool.
Guardian of the genome: p53 protein
The p53 protein is one of the most important regulators of the cell cycle. Its activation and structural modification occur when the cell is exposed to adverse factors: viral invasion, hypoxia, or direct DNA damage.
The mechanism of action of p53 is that it specifically binds to certain DNA regions and suppresses cell growth in the G1 phase. This blocks the cell cycle until the abnormalities are fully repaired. In damaged cells, the concentration of p53 increases sharply, giving the cell the necessary time for DNA repair.
If the damage is critical and unrepairable, p53 normally initiates apoptosis — programmed "cell suicide". In tumor cells, this defense mechanism is broken: mutations in the p53 gene are detected in nearly 50% of all neoplasms.
The role of p27 in the cell cycle
Another significant tumor suppressor is the p27 protein. Its mechanism of action is based on direct interaction with cyclins and specific Cdpk proteins (Cyclin dependent protein kinase). By forming a complex with them, p27 reliably blocks the cell's entry into the S phase of the cell cycle, preventing premature duplication of genetic material.
The clinical significance of this protein is especially high in practical oncology. In particular, the assessment of its activity is used for diagnosing breast cancer. It has been proven that a decrease in p27 levels is a prognostically unfavorable sign indicating an aggressive course of the disease.
Briefly: answers to topical questions
What are the main anti-oncogenes known, besides p53 and p27?
In addition to p53 and p27, sources list the following as anti-oncogenes/tumor suppressor genes: Rb — a suppressor gene; loss of function of suppressor genes leads to cancer, with retinoblastoma given as an example of Rb gene loss; BRCA1/BRCA2 — tumor suppressor genes in breast cancer, whose products are involved in DNA repair and cell cycle regulation; loss of their function leads to impaired DNA repair, cell cycle control, and apoptosis; APC — an anti-oncogene associated with the control of cell number and intercellular contacts.
What mechanisms exist for the inactivation of tumor suppressors other than genetic mutations?
Besides genetic mutations, sources indicate an epigenetic mechanism for the inactivation of suppressor genes: selective hypermethylation of promoters leads to the "silencing" of suppressor genes and serves as an alternative to gene mutation. Epigenetic changes are reversible hereditary alterations in gene expression without mutations in the DNA sequence. Mechanisms of epigenetic regulation include DNA methylation and post-translational histone modifications; chromatin changes via histone acetylation and methylation can lead to transcriptional activation or repression.
It is easy to remember the balance through a car analogy: oncogenes are the "gas pedal" (stimulate division), and tumor suppressors are the "brake pedal" (stop the cycle). If the gas is stuck (hyperactivation of an oncogene) or the brakes fail (suppressor mutation), the cell rushes at high speed into tumor transformation.
Class and exam questions
Can a mutation in just a single oncogene lead to cancer?
No, the development of a full-fledged tumor always requires the interaction of multiple mutated oncogenes and genes regulating proliferation.
In which phase does the p53 protein arrest the cell cycle?
The p53 protein blocks cell division in the G1 phase, giving it time to repair damaged DNA until the abnormalities are completely resolved.
What happens if the p53 protein realizes that the DNA cannot be repaired?
In such a case, p53 initiates apoptosis — the process of programmed cell "suicide" to prevent the proliferation of the mutant clone.
Why is the p27 protein level determined clinically?
It is used in breast cancer diagnostics. A decrease in p27 levels is a prognostically unfavorable sign.
What else to study on this topic
- Mechanisms of activation of STAT family transcription factors
- The role of the BCR-ABL chimeric gene and its unregulated tyrosine kinase activity
- Interaction of cyclins and cyclin-dependent kinases (Cdks) in the mitotic cycle
- DNA repair processes during cell cycle arrest in the G1 phase
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