Evidence Base of the Molecular Theory
Modern pathology views any tumor as a genetic disease of somatic cells. Even if primary genetic alterations occurred in germ cells, tumor development is always driven by DNA damage and epigenetic changes. Various carcinogenic agents lead to a common endpoint precisely by altering the genome.
Four undeniable facts support this concept:
- There is a direct correlation between specific chromosomal mutations and the development of distinct types of malignant neoplasms.
- Transfection of genetic material from oncogenic viruses into healthy cells induces a typical tumor phenotype.
- The vast majority of known carcinogens exhibit prominent mutagenic properties.
- Specific epigenetic alterations are consistently detected in established tumor cells.
Seven Mechanisms of Malignant Progression
For normal tissue to transform into malignancy, cells must acquire a series of pathological traits. There are seven key mechanisms driving tumor progression:
- Self-sufficiency in growth signals. Cells acquire the ability to proliferate without external stimulatory signals, primarily driven by the activation of cellular proto-oncogenes.
- Insensitivity to antigrowth signals. Tumors ignore molecules that normally block cell growth (e.g., transforming growth factor-beta [TGF-β] and cyclin-dependent kinase inhibitors).
- Evading apoptosis. Resistance to programmed cell death develops, most commonly due to inactivation of the critical TP53 gene or upregulation of anti-apoptotic genes.
- Limitless replicative potential. Cancer cells bypass natural cellular senescence and avoid mitotic catastrophe, gaining the capacity for unlimited division.
- Sustained angiogenesis. Like healthy tissue, tumors require nutrients, oxygen, and metabolic waste removal, driving the continuous sprouting of new blood vessels.
- Tissue invasion and metastasis. The spread of tumor cells is initiated by both the malignant cells and their microenvironment. Metastasis is the primary cause of death in cancer patients.
- Genomic instability. DNA repair defects arise due to carcinogens or unregulated proliferation, leading to an avalanche-like accumulation of mutations in proto-oncogenes and tumor suppressor genes.
Target Genes and the Gatekeeper Theory
Mutations in carcinogenesis are not random—they specifically target genes regulating the cell cycle, differentiation, apoptosis, and DNA replication. Key targets include proto-oncogenes, tumor suppressor genes (antioncogenes), cell death regulators, and DNA repair genes. Mutator genes and the enzyme telomerase also play critical roles.
The molecular mechanisms of carcinogenesis, tumor progression, and the distinction between sporadic and hereditary forms of cancer are explained by the Gatekeeper theory. It categorizes genes involved in tumor growth into three functional groups:
- Gatekeepers. Key genes specific to each organ (e.g., RB1, APC, NF1, MTN1, VHL). Loss or mutation of such a gene leads to neoplasia in a strictly defined organ, explaining the phenomenon of organ specificity in tumors.
- Caretakers. Antineoplastic protection genes involved in DNA synthesis and repair. They are universal and vital across multiple tissue types. Mutations in these genes are characteristic of hereditary cancer syndromes.
- Guardsmen. Genes that maintain autonomous growth. They are not specific to any particular type of cancer predisposition.