がんは単一細胞の体細胞変異から発生する

A single mutation is insufficient to transform a healthy cell into a cancerous cell. Cancer development requires at least five to six independent and rare genetic alterations occurring in succession over time. This multi-step process, explained by multi-hit theory, allows cells to progressively gain oncogenicity through accumulation of mutations in different cancer-critical genes.

Somatic mutations develop in body cells during an organism's lifetime and are not inherited, accounting for the majority of the over 570 frequently mutated cancer genes. Inherited mutations, by contrast, are passed from parents to offspring. Most cancers arise from somatic mutations acquired in individual cells rather than from germline mutations present at birth.

Multi-hit theory describes cancer as a sequential process where a single mutant cell acquires additional mutations in different genes over time. Each mutation provides a selective advantage—such as faster division, increased mutation rates, or escape from apoptosis—allowing the cell line to accumulate more mutations and eventually become malignant.

Cancer-causing mutations include point mutations, indels, large-scale chromosomal deletions, and translocations. For example, aristolochic acid exposure causes A to T base substitutions, while breast cancer BRCA1 and BRCA2 mutations involve indels and deletions. CML features the Philadelphia chromosome, a translocation between chromosomes 9 and 22.

Cancer cell evolution mirrors Darwinian evolution, where each cancer-critical mutation is naturally selected and fixed in the population. Cells with advantageous mutations outcompete neighboring cells, establishing clonal populations. This evolutionary process explains why cancer development typically requires substantial time and multiple independent mutations.

Cancer typically requires numerous independent, rare genetic mutations and epigenetic changes accumulating in a single cell's lineage over time. This extended timeline explains the increased cancer incidence with age. For example, median diagnosis ages are 61 years for breast cancer, 68 years for colorectal cancer, and 70 years for lung cancer.

Frequently mutated cancer-critical genes in colon carcinoma include APC, c-Myc, K-Ras, and p53. These genes regulate cell cycle control, metabolism, and apoptosis. Mutations in these genes allow cells to escape normal growth regulation and accumulate additional mutations, driving tumor formation and progression.