From Normal to Pathological
Proto-oncogenes are natural structural elements of the genome. In a healthy organism, they strictly control the cell cycle, governing proliferation and differentiation. In mature tissues, these genes are predominantly inactive. Their physiological activation occurs only under specific conditions: during embryogenesis or in foci of reparative regeneration when tissues need to recover.
However, under the influence of exogenous (chemical, physical, biological) or endogenous carcinogens, primary DNA damage occurs in a somatic cell. If this process is accompanied by a failure in DNA repair systems, the mutations become fixed in the genome. As a result, the normal proto-oncogene transforms into a cellular oncogene, the primary promoter of tumor growth.
Mechanisms of Proto-Oncogene Activation
For a cell to begin dividing uncontrollably, a proto-oncogene must be activated. This process occurs via several pathways:
- Point mutations: localized structural alterations in the DNA sequence.
- Amplification: a multiple increase in the copy number of a specific gene. Additional small marker chromosomes may even appear in cells. A prime example is c-neu amplification in breast cancer or N-myc in neuroblastoma.
- Translocation: the relocation of a chromosomal segment containing a proto-oncogene into an active zone, close to strong enhancers (transcriptional boosters).
- Insertional activation: a viral mechanism. Retroviruses (RNA viruses) integrate into the host cell genome and modulate the activity of neighboring cellular proto-oncogenes via their own genes.
- Epigenetic alterations: changes in gene expression without altering the DNA sequence itself (e.g., DNA demethylation or histone acetylation).
Classification and Functions of Oncoproteins
Activated cellular oncogenes encode the synthesis of specific molecules—oncoproteins. Their primary task in a tumor cell is the continuous transmission of mitogenic signals from the cell membrane to the nucleus. Structurally and functionally, they are homologous to normal elements of signaling pathways:
- Growth factor analogs: e.g., c-sis.
- Growth factor receptor analogs: c-erb-B, c-fms.
- Proteins with tyrosine kinase activity: c-abl, c-src.
- Small molecular weight G-protein analogs: c-ras. They interact with GTP and affect the adenylate cyclase system.
- Proteins with serine-threonine kinase activity: c-raf, associated with protein kinase C.
- Nuclear transcription factors: c-myc, c-fos, c-jun. They localize directly in the nucleus and directly affect DNA.
- Mitochondrial proteins: the c-bcl-2 family, which regulate (block) cellular apoptosis.
The Role of Chromosomal Translocations
Translocations play a pivotal role in the development of certain hematologic malignancies by placing proto-oncogenes under the control of powerful cellular promoters.
- Burkitt lymphoma: a reciprocal translocation occurs between chromosomes 8 and 14. The c-myc proto-oncogene is transferred to the immunoglobulin heavy chain gene locus. Active immunoglobulin genes begin to act as enhancers, causing c-myc overexpression.
- Chronic myeloid leukemia (CML): the Philadelphia chromosome is formed. A segment of chromosome 9 containing the c-abl gene is translocated to chromosome 22. A chimeric c-abl-bcr gene is formed, whose protein product possesses pathological tyrosine kinase activity.
Tumor Morphogenesis
The accumulation of genetic errors leads to fundamental disruptions: proliferation suffers, apoptosis is blocked, and cell differentiation is impaired. Morphologically, this manifests in stages:
- Pre-tumor dysplasia: atypical cells appear, tissue architecture is disrupted, but there is no invasion yet.
- Non-invasive tumor (Carcinoma in situ): the tumor grows but does not breach the basement membrane.
- Invasive tumor: cells acquire immortality (immortalization) and invade surrounding tissues.
- Metastatic tumor: tumor cells spread to distant organs.