Chapter 24: Cancer Genetics

Cancer is fundamentally an acquired genetic disease predominantly affecting somatic cells, arising from a process of multistep clonal evolution where a single ancestral cell accumulates a critical number of driver mutations (typically between two and eight). This malignancy is defined by two properties: uncontrolled cell growth and division (proliferation) and the ability to spread to distant tissues (metastasis). These dysfunctions stem from genetic defects in fundamental cellular aspects, including DNA repair, cell-cycle regulation, signal transduction, and programmed cell death (apoptosis). A hallmark of malignant cells is extreme genomic instability, known as the mutator phenotype, which drives the accumulation of numerous non-contributing passenger mutations and gross chromosomal abnormalities, such as aneuploidy, deletions, and specific translocations like the Philadelphia chromosome associated with Chronic Myelogenous Leukemia (CML). The essential defects involve two major gene classes: proto-oncogenes, which normally promote cell division but, upon gain-of-function alteration (e.g., point mutations that lock the ras family proteins in an "on" state), become dominant cancer-causing oncogenes; and tumor-suppressor genes (TSGs), which police the cell cycle at checkpoints (G1/S, G2/M, M) and initiate apoptosis. TSGs, particularly the globally important TP53 gene, often called the “guardian of the genome,” require a loss-of-function in both alleles, frequently through loss of heterozygosity (LOH), to contribute to tumorigenesis. Furthermore, cancer progression is significantly influenced by epigenetic alterations, including a general decrease in DNA methylation alongside the hypermethylation of specific TSG promoters, which changes gene expression patterns. While only a small fraction of cancers (5 to 10 percent) are hereditary, often involving inherited mutations in TSGs like BRCA1 or RB1, the majority are accelerated by environmental exposure to carcinogens (including radiation, chronic inflammation, and especially tobacco smoke) and infection by tumor viruses (such as HPV or HTLV-1), which introduce mutations or interfere with TSG function.