failed repair and disease (BRCA, p53)

Every cell suffers thousands of DNA injuries a day, and the only reason this does not dissolve our genes into chaos is that repair systems fix the damage continuously. The dark side of this dependence is what happens when repair itself breaks down: mutations pile up unchecked, and the cell drifts toward cancer. Many of the most important hereditary cancer syndromes are, at heart, inherited failures of DNA repair.

Several diseases make the link vivid. Xeroderma pigmentosum, a defect in nucleotide excision repair, leaves patients unable to clear UV damage, causing extreme sun sensitivity and a vastly raised skin-cancer risk. Lynch syndrome, a defect in mismatch repair, causes hereditary colorectal cancer. And the BRCA1 and BRCA2 genes encode proteins essential for the accurate, homologous-recombination route of double-strand-break repair; people who inherit a broken copy carry a sharply elevated lifetime risk of breast and ovarian cancer, because their cells must rely on the error-prone backup and so accumulate damaging rearrangements.

Standing behind all of this is p53, often called the guardian of the genome. p53 is a protein that senses DNA damage and responds by halting the cell cycle to buy time for repair, or — if the damage is too severe — ordering the cell to commit suicide so a dangerously mutated cell never divides. It is the single most commonly mutated gene in human cancers. The deeper lesson is double-edged: defective repair makes cells vulnerable to cancer, yet that same vulnerability has become a treatment opportunity, since drugs that block a tumour's last remaining repair backup can selectively kill BRCA-deficient cancer cells while sparing healthy ones.