UVA-induced mutagenesis was investigated in human pol eta-deficient (XP-V) cells through whole-exome sequencing. In UVA-irradiated cells, the increase in the mutation frequency in deficient cells included a remarkable contribution of C>T transitions, mainly at potential pyrimidine dimer sites. A strong contribution of C>A transversions, potentially due to oxidized bases, was also observed in non-irradiated XP-V cells, indicating that basal mutagenesis caused by oxidative stress may be related to internal tumours in XP-V patients. The low levels of mutations involving T induced by UVA indicate that pol eta is not responsible for correctly replicating T-containing pyrimidine dimers, a phenomenon known as the ‘A-rule’. Moreover, the mutation signature profile of UVA-irradiated XP-V cells is highly similar to the human skin cancer profile, revealing how studies involving cells deficient in DNA damage processing may be useful to understand the mechanisms of environmentally induced carcinogenesis.

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Xeroderma Pigmentosum variant (XP-V) is an autosomal recessive disease with an increased risk to develop cutaneous neoplasms in sunlight exposed regions. These cells are deficient in the translesion synthesis DNA polymerase eta. Eleven skin tumors from a genetic cluster of XP-V patients had their exome sequenced. Mutational signatures identified for most tumors were related to ultraviolet exposure, such as C>T transitions targeted to pyrimidine dimers. However, four samples carry different mutational signatures, with C>A mutations associated with tobacco usage. Basal cell carcinomas showed a distinct C>A mutation spectra reflecting a novel mutational signature. Higher levels for retroposon insertions were detected in the XP-V tumors, compared to non-XP skin tumors. The results reveal other possible causes for XP-V tumors and the involvement of polymerase eta in suppressing retrotransposition. The expected high mutation burden, found in most of these tumors, renders these XP patients good candidates for immunotherapy with checkpoint blockers.

Xeroderma pigmentosum variant (XP-V) is an autosomal recessive disease with an increased risk of developing cutaneous neoplasms in sunlight-exposed regions. These cells are deficient in the translesion synthesis DNA polymerase eta, responsible for bypassing different types of DNA lesions. From the exome sequencing of eleven skin tumors of a genetic XP-V patients’ cluster, classical mutational signatures related to sunlight exposure, such as C>T transitions targeted to pyrimidine dimers, were identified. However, basal cell carcinomas also showed distinct C>A mutation spectra reflecting a mutational signature possibly related to sunlight-induced oxidative stress. Moreover, four samples carry different mutational signatures, with C>A mutations associated with tobacco chewing or smoking usage. Thus, XP-V patients should be warned of the risk of these habits. Surprisingly, higher levels of retrotransposon somatic insertions were also detected when the tumors were compared to non-XP skin tumors, revealing other possible causes for XP-V tumors and novel functions for the TLS polymerase eta in suppressing retrotransposition. Finally, the expected high mutation burden found in most of these tumors renders these XP patients good candidates for checkpoint blockade immunotherapy.

Nucleotide excision repair (NER) is one of the main pathways for genome protection against structural DNA damage caused by sunlight, which in turn is extensively related to skin cancer development. The mutation spectra induced by UVB were investigated by whole‐exome sequencing of randomly selected clones of NER‐proficient and XP‐C‐deficient human skin fibroblasts. As a model, a cell line unable to recognize and remove lesions (XP‐C) was used and compared to the complemented isogenic control (COMP). As expected, a significant increase of mutagenesis was observed in irradiated XP‐C cells, mainly C>T transitions, but also CC>TT and C>A base substitutions. Remarkably, the C>T mutations occur mainly at the second base of dipyrimidine sites in pyrimidine‐rich sequence contexts, with 5′TC sequence the most mutated. Although T>N mutations were also significantly increased, they were not directly related to pyrimidine dimers. Moreover, the large‐scale study of a single UVB irradiation on XP‐C cells allowed recovering the typical mutation spectrum found in human skin cancer tumors. Eventually, the data may be used for comparison with the mutational profiles of skin tumors obtained from XP‐C patients and may help to understand the mutational process in nonaffected individuals.