Sun exposure shapes distinct mutational profiles in human skin cells

Surprisingly little is known about how ultraviolet (UV) radiation exposure affects individual skin cell types. Prior studies relied on bulk sequencing of whole biopsies, masking cell type-specific mutational patterns, and often used narrow UV spectra, non-physiological doses, or focused only on immediate DNA damage rather than lasting mutations. To delineate the enduring genetic effects of UV-radiation, we measured somatic mutations and cellular viability in melanocytes, keratinocytes, and fibroblasts exposed to physiologically relevant simulated solar radiation. We exposed primary neonatal melanocytes, keratinocytes, and fibroblasts to simulated solar radiation under three conditions: no irradiation (control), 5 minutes (~70.2 J/m²), and 10 minutes (~129.9 J/m²), corresponding to ~0, 0.42, and 0.83 Minimal Erythema Dose (MED). Cell counts were measured on days 1 (pre-irradiation), 3, and 5 (post-irradiation) to assess viability and proliferation. For mutational profiling, single cells from each condition were sorted, clonally expanded, and subjected to exome and transcriptome sequencing using G&T-seq. In total, we analyzed 9 fibroblast clones (n=4, 2, 3), 13 keratinocyte clones (n=5, 3, 5), and 12 melanocyte clones (n=4 per condition) to quantify UV-induced mutational burden. All three cell types exhibited a dose-dependent reduction in cell counts following irradiation, though with differing magnitude. Keratinocytes showed the most pronounced decline, whereas melanocytes displayed a dose-dependent decrease but to a lesser extent. Fibroblasts were the most resilient, with the smallest reduction in cell count and the fastest post-irradiation recovery. Mutational profiling revealed a corresponding dose-dependent increase in mutation burden (mutations per megabase) and in the fraction of canonical UV-signature substitutions (C>T and CC>TT) across all cell types. After 10 minutes of simulated solar radiation, median mutation burdens reached 1.30 mut/Mb in keratinocytes, 0.52 mut/Mb in melanocytes, and 0.36 mut/Mb in fibroblasts. The cells also showed distinct mutation signature and gene expression profiles. Together, these findings highlight both the physiological and genomic consequences of solar radiation in distinct skin cell types. The differences in mutational landscapes point to cell type-specific mutational processes and DNA repair mechanisms, providing a framework for understanding how UV exposure shapes the genomic architecture of human skin.