Uncovering the Role of DNA Repair Impairment in UVA‐Induced Mutagenesis in Human Xeroderma Pigmentosum Variant Cells

Ultraviolet A (UVA) radiation induces DNA damage both directly, by forming cyclobutane pyrimidine dimers (CPDs), and indirectly, by generating oxidative stress. Cells rely on nucleotide excision repair (NER) and translesion synthesis (TLS) to tolerate these lesions. Xeroderma pigmentosum variant (XP-V) cells, deficient in DNA polymerase eta (pol eta), exhibit a heightened risk of skin cancer due to impaired TLS. While XP-V patients are considered NER-proficient, our findings challenge this assumption by demonstrating that UVA-induced oxidative stress impaired NER activity, leading to increased C > T transitions at CPD sites. Whole-exome sequencing of UVA-irradiated XP-V cells revealed a substantial rise in mutations, with a distinct C > T signature characteristic of defective CPD repair. Notably, pretreatment with the antioxidant N-acetylcysteine (NAC) mitigated this effect, reducing C > T transitions through enhanced NER function and decreasing C > A transversions via its antioxidant properties. These results redefine the mutagenic landscape of XP-V cells, revealing that oxidatively generated damage to NER proteins-rather than TLS deficiency alone-contributes to their elevated mutation burden. Our findings suggest that antioxidant strategies may partially protect XP-V patients from UVA-driven mutagenesis enhancing the cells' DNA repair capacity, ultimately reducing skin cancer and contributing to better overall health outcomes.