UVA-induced DNA damage and mutations in melanocytes and melanoma mutations
Melanomas carry characteristic mutational signatures associated with solar UVB radiation-induced cyclobutane pyrimidine dimers (CPDs) that contain deaminated cytosines. However, there are several other mutation signatures, including those found in melanomas from non-sun-exposed body sites, that have unknown origins. To test if these signatures are linked to UVA radiation from the sun, we exposed human melanocytes to UVA and to UVB for comparison. We mapped DNA damage in the form of CPDs or 8-oxoguanine (8-oxoG) genome-wide at base resolution. We then determined mutational patterns in single melanocyte cell clones by whole genome sequencing. UVA-induced CPDs occurred overwhelmingly at TT sequences resembling melanoma signature SBS7d. We did not observe rising CPD levels after cessation of radiation (dark CPDs). However, the UVA-induced TT-CPDs did not score as mutagenic in the mutation analysis. 8-oxoG was present in melanocytes but was not substantially increased after UVA. G/C to T/A mutations were prominent in melanocyte single cell clones with no major shift after UVA radiation. These mutations matched SBS18, a signature present in melanomas. Our data suggest that melanocytes carry an endogenous but UVA-independent load of oxidative base lesions and their associated mutations that may be associated with a subset of melanoma mutations. Overall design: UVA-induced DNA damage was mapped in genome-wide using CD-seq in human melanocyte cells.