Somatic mutations and mosaicism

Somatic mutations are DNA changes that arise in your body cells after conception, not in the germline. They affect only the descendants of the cell where they happen. Every cell division carries a small chance of a copying error. And outside mutagens, like UV light, tobacco carcinogens, and reactive oxygen species, damage your DNA throughout life. So your mutation burden rises roughly in step with age, about 40 single-letter changes per year in gut and liver stem cells (Blokzijl et al., 2016, Nature). When the resulting patchwork of genetically different cell lineages becomes detectable, it is called somatic mosaicism. If a mutation gives a cell even a small growth edge, its clone can expand beyond random drift. That is clonal expansion. The best-studied example is clonal hematopoiesis (CH): mutations in DNMT3A, TET2, and ASXL1 let one blood-cell clone take over. Jaiswal et al. (2014, NEJM) found CH in under 1% of people below 40, rising to about 9 to 18% after age 70. CH carried an 11-fold higher blood-cancer risk, a 2-fold higher coronary heart disease risk, and 40% higher all-cause death. In humans this link is associational. But mouse models support a causal route, where mutant white blood cells worsen atherosclerosis through inflammation. Somatic mutations feed genomic instability, one of the primary hallmarks of aging (López-Otín et al., 2023, Cell). Whether they directly drive aging, or just come along with it, is still under study.

Last reviewed: June 22, 2026

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