Chapter 19: The Genetics of Cancer

The chapter presents cancer as a multistep process requiring sequential accumulation of mutations in individual cells, with the clonal evolution model demonstrating that all cells within a tumor originate from a single transformed ancestor. The distinction between driver mutations that confer selective growth advantages and passenger mutations that accumulate without directly contributing to malignancy provides a framework for understanding tumor heterogeneity and evolution. Central to cancer development are two major gene categories: proto-oncogenes that regulate normal cell growth and division, which become dangerous when activated through gain-of-function mutations, and tumor-suppressor genes that normally restrict proliferation, requiring inactivation of both alleles to promote cancer progression. The chapter details how cancer cells circumvent cell cycle checkpoints, particularly the G1/S and G2/M transitions, often through dysregulation of cyclin and cyclin-dependent kinase complexes or loss of checkpoint proteins such as p53, a critical guardian that normally triggers apoptosis in damaged cells. Genomic instability emerges as a hallmark of cancer cells, manifesting through chromosomal rearrangements, aneuploidy, and defective DNA repair mechanisms affecting genes like BRCA1, MSH2, and MLH1. Epigenetic alterations, including abnormal DNA methylation patterns and histone modifications, further disrupt gene regulation independent of DNA sequence changes. The chapter explores metastasis as the ability of cancer cells to invade and establish tumors in distant tissues through altered cell adhesion and increased protease activity. Hereditary cancer syndromes illustrate how inherited mutations in specific genes predispose individuals to malignancy, though additional somatic mutations remain necessary for tumor formation. The chapter concludes by examining environmental carcinogens including tobacco smoke, ultraviolet and ionizing radiation, and oncogenic viruses that initiate or promote cancer development through mutagenic and epigenetic mechanisms.