Need Help?
Copied to clipboard!

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. We also took a whole-genome sequencing approach to evaluate the load, sources, and patterns of genome changes in 18 primary human chondrocyte clones from donors with and without osteoarthritis (OA). Findings in chondrocyte clones largely agreed with a recent study of 100 single-cell sequenced chondrocytes. We compared genome changes in chondrocytes with clonally-expanded human skin fibroblasts sequenced in our previous studies. We demonstrated that skin fibroblasts show a higher burden of somatic mutations, with an increased rate of mutation accumulation per cell division. Motif-centered analyses of mutation catalogues identified only endogenous sources of mutations in chondrocytes, as opposed to skin fibroblasts which also showed a heavy burden of UV-induced mutations. Spontaneous deamination of meCpG and mutagenesis by exposure to small epoxides and SN2 electrophiles showed higher mutagenic activities in chondrocytes compared to skin fibroblasts. Chondrocytes showed ubiquitous prevalence of indels in homonucleotide runs of ≥5 bases, while skin fibroblasts showed high contributions of UV-associated deletions of ≥5bp not in repeats. Structural variants in rearrangement hotspots colocalized with human common fragile sites in skin fibroblasts, but not in chondrocytes. Together, our study comprehensively recorded genome instability features in chondrocytes and highlighted the unique mutagenesis landscapes of two mesenchymal cell types.