Chapter 25: Genetic Basis of Cancer

Cancer arises from a combination of genetic mutations and epigenetic alterations that accumulate over time, transforming normal cells into malignant tumors capable of uncontrolled division and metastasis. While inherited mutations account for approximately ten percent of cancer cases, the majority develop from somatic mutations induced by environmental carcinogens such as ultraviolet radiation and tobacco smoke. The disease progresses through a multistep model requiring disruption of both oncogenes and tumor-suppressor genes, two fundamental classes of cancer-related genes that function through opposing mechanisms. Proto-oncogenes normally regulate cell growth and division, but when mutated through missense changes, gene amplification, chromosomal translocations, or viral integration, they become oncogenes that hyperactivate growth pathways like the Ras signaling cascade, promoting excessive proliferation. Tumor-suppressor genes such as rb and p53 enforce critical cell cycle checkpoints and trigger protective responses to DNA damage including repair mechanisms, growth arrest, or programmed cell death. The rb protein controls transition through the restriction point by regulating the e2f transcription factor, while p53 functions as a cellular guardian mutated in approximately half of all cancers. Cancer development typically requires inactivation of both copies of a tumor-suppressor gene through loss-of-function mutations, chromosomal loss, or epigenetic silencing, a principle illustrated by the two-hit hypothesis observed in inherited cancers like retinoblastoma and familial breast cancer associated with brca1 and brca2 mutations. Beyond genetic changes, epigenetic mechanisms including aberrant dna methylation, histone modifications, and chromatin remodeling can silence tumor-suppressor genes or activate oncogenes without changing the underlying dna sequence. These epimutagenic processes frequently involve dysfunction of chromatin-modifying proteins and exposure to environmental agents such as benzene and polycyclic aromatic hydrocarbons. Emerging epigenetic therapies, particularly dna methyltransferase inhibitors, offer potential to reverse pathological gene silencing and restore normal cellular regulation in malignancies such as leukemia. The complete transformation to cancer requires cumulative disruption of multiple regulatory pathways, reflecting the complex multigenic nature of malignant disease.