Evidence base of the molecular theory
Modern pathological anatomy views any tumor as a genetic disease of somatic cells. Even if initial genetic alterations occurred in germ cells, tumor development is always rooted in DNA damage and epigenetic changes. Various carcinogenic agents lead to a uniform outcome precisely by acting on the genome.
Four undeniable facts support this concept:
- There is a direct correlation between specific chromosomal mutations and the development of particular types of malignant neoplasms.
- When genetic material from oncogenic viruses is transfected into healthy cells, the latter develop a typical tumor phenotype.
- The vast majority of carcinogens known to science exhibit pronounced mutagenic properties.
- Specific epigenetic alterations are consistently detected in the cells of an established tumor.
Seven mechanisms of malignant progression
For normal tissue to transform into malignant tissue, cells must acquire a series of pathological properties. There are seven key mechanisms driving tumor progression:
- Autonomy of growth. Cells acquire the ability to proliferate without external stimulatory signals. The primary driver of this phenomenon is the activation of cellular oncogenes.
- Insensitivity to anti-growth signals. The tumor ignores molecules that normally block cell growth (e.g., transforming growth factor beta — TGF-β, as well as cyclin-dependent kinase inhibitors).
- Evasion of apoptosis. Resistance to programmed cell death develops. This most commonly occurs due to inactivation of the critical p53 gene or hyperactivation of anti-apoptotic genes.
- Limitless replicative potential. Cancer cells overcome natural cellular senescence mechanisms and successfully avoid mitotic catastrophe, gaining the capacity for unlimited division.
- Sustained angiogenesis. Like healthy tissue, tumors require nutrients and oxygen, as well as pathways to clear metabolic wastes. To achieve this, they stimulate the continuous growth of new blood vessels.
- Capacity for invasion and metastasis. The process of tumor dissemination is initiated by both the malignant cell itself and its microenvironment. Metastasis is the primary cause of death in cancer patients.
- Genomic instability. Under the influence of carcinogens or due to deregulated proliferation, DNA repair defects arise. This leads 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 repair. The main targets include proto-oncogenes, tumor suppressor genes (anti-oncogenes), genes regulating cell death, and DNA repair genes. Mutator genes and the enzyme telomerase also play an important role.
The molecular mechanisms of carcinogenesis, tumor progression, and the difference between sporadic and hereditary forms of cancer are explained by the Gatekeeper theory. It divides the genes involved in tumor growth into three functional groups:
- Gatekeepers. Key genes specific to each individual organ (e.g., Rb1, APC, NF1, MTNI, VCT, PCT). The loss or mutation of such a gene leads to neoplasia in a strictly defined organ, which explains the phenomenon of organ specificity in tumors.
- Caretakers. Antiblastoma defense genes associated with DNA synthesis and repair. They are universal and important for multiple tissue types simultaneously. Their alterations are characteristic of hereditary cancer syndromes.
- Guardsmen. Genes that ensure autonomous growth. They are not specific to any particular type of cancer predisposition.
Briefly: answers to topical questions
Through which molecular mechanisms does a tumor cell evade apoptosis?
A tumor cell evades apoptosis through:
- Inactivation of the p53 gene — preventing apoptosis from triggering, allowing cells with damaged DNA to survive.
- Activation of anti-apoptotic genes.
- Mutations in pro-apoptotic signaling pathways that disrupt the transmission of the programmed cell death signal.
- Alterations in genes regulating apoptosis, including BCL2.
Which molecules normally block cell growth but are ignored during carcinogenesis?
Normally, cell growth is blocked by specific signaling molecules and cell cycle regulators, to which the tumor cell loses sensitivity. These molecules include:
- Transforming growth factor beta (TGF-β) — suppresses normal cell proliferation.
- Cyclin-dependent kinase inhibitors — for example, the p27 protein, which regulates the cell cycle and prevents uncontrolled division.
Which genes belong to the group of caretakers?
The caretaker (Caretaker) group includes antiblastoma defense genes that are typically associated with DNA synthesis and repair processes. These genes share the following characteristics:
- They are key for multiple tissue types simultaneously rather than being specific to a single organ.
- Their alterations and mutations form the basis for the development of hereditary tumors and various cancer syndromes.
Which growth factors provide sustained angiogenesis in the tumor?
Sustained angiogenesis in the tumor is driven by a complex system of growth factors that stimulate the proliferation of vascular wall elements. These include:
- Vascular endothelial growth factor (VEGF).
- Fibroblast growth factors — components of the Volkmann factor produced by malignant cells.
- Insulin-like growth factors (IGF).
- Platelet-derived growth factor.
- Epidermal growth factor.
- Angiogenin and angiopoietins.
Hypoxia-inducible factor (HIF) is also involved in the regulation of angiogenesis.
What stages does the invasion and metastasis process of a tumor cell include?
The process of invasion and metastasis is described by the metastatic cascade theory and includes four sequential stages:
- Formation of a metastatic tumor subclone — acquisition of the necessary properties for migration by cells.
- Intravasation — detachment of the malignant cell from the primary node, invasion into the wall of a blood or lymphatic vessel, and entry into its lumen.
- Circulation of the tumor embolus — movement of the cell within the blood or lymph stream (embolism) followed by adhesion to the vascular endothelium.
- Extravasation and metastasis formation — invasion of the cell through the vessel wall from the inside, exit into the surrounding tissue, proliferation, and growth of the secondary tumor node.
What is the molecular difference between proto-oncogenes and tumor suppressor genes?
The molecular difference lies in their normal function and mechanism of involvement in carcinogenesis:
| Characteristic | Proto-oncogenes | Tumor suppressor genes |
|---|---|---|
| Normal function | Regulators of cell proliferation and differentiation | Inhibit cell proliferation |
| Pathological mechanism | Upregulation or modification of function turns a proto-oncogene into an oncogene | Loss or mutation leads to the loss of suppressor function and the development of neoplasia |
| Genetic manifestation | Conversion into an oncogene is a dominant trait | For suppressor genes, sources indicate loss or mutation; loss of heterozygosity of the corresponding chromosomal region has also been described |
To remember the Gatekeeper theory, imagine a castle: Caretakers repair the walls (DNA repair) throughout the castle, Gatekeepers guard specific doors (organ specificity), and Guardsmen ensure the independence and autonomy of the invaders (tumor).
Class and exam questions
Why is cancer called a genetic disease of somatic cells?
Because the development of any tumor is based on DNA mutations and epigenetic alterations specifically in somatic cells, even if an external carcinogen served as the initial trigger.
Which genes are the primary targets of carcinogens?
Carcinogens damage genes regulating the cell cycle and homeostasis: proto-oncogenes, tumor suppressor genes, apoptosis genes, and DNA repair genes.
What is the essence of the Gatekeeper theory?
The theory explains the molecular mechanisms of carcinogenesis through the interaction of three groups of genes: "gatekeepers" (responsible for organ specificity), "caretakers" (DNA repair, associated with hereditary cancer), and "guardsmen" (ensure autonomous growth).
What else to study on this topic
- The role of the p53 gene and anti-apoptotic genes in tumor survival.
- Mechanisms of cellular oncogene activation.
- The influence of the microenvironment on invasion and metastasis.
- Functions of the telomerase enzyme in carcinogenesis.
- Epigenetic alterations in tumor cells.
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