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National Institute of Environmental Health (NIEHS) Sciences Study of Somatic Mutation Load in Clones of Single Human Cells

Accumulation of genetic changes with time and proliferation of cells is inevitable. We report here the genome-wide magnitude and spectra of mutations accrued in skin fibroblasts over the lifetime of healthy humans. We found that every cell contains at least one somatic chromosomal rearrangement and 600 - 13,000 base substitutions, similar to the median mutation load in human cancers. The mutation spectra and correlation of changes with epigenomic features also resemble many human cancers. Interestingly, the magnitude of ultraviolet light-characteristic mutations, representative of environmental mutagenesis, in sun-exposed skin was comparable to the number of mutations attributed to endogenous DNA damage estimated in unexposed cells. Our methodology allows delineating the precise contributions of environmental and endogenous factors to the accrual of genetic changes in the human body. This is fundamental to understanding the etiology, and improving the prognosis and prevention of cancers and other genetic diseases.