Melanoma mutations are caused by deamination of cytosine in pyrimidine dimers

The majority of sunlight-associated melanomas carry a unique UV-related C to T mutation signature at dipyrimidine sites. UVB radiation is known to induce cyclobutane pyrimidine dimers (CPDs) as the major form of DNA damage, but the mechanism of how these DNA lesions lead to mutations is unknown. To map the genome-wide distribution of CPDs at single base resolution, we developed a new method, circle damage sequencing (circle-damage-seq). We found that in human cells CPDs form in a specific tetranucleotide sequence context (5’PyPy<>PyT/A), but this alone does not explain the mutation patterns. To test whether mutations arise at CPDs by cytosine deamination, we next applied a modification of circle-damage-seq and mapped UVB-induced cytosine-deaminated CPDs. We observed that the transcription start sites (TSS) are the sequences most protected from CPDs and deaminated CPDs. The strongest spike of CPDs and deaminated CPD lesions was found immediately upstream of the TSS, suggesting a mutation-promoting role of bound transcription factors. Most significantly, the genome-wide dinucleotide and trinucleotide sequence specificity of deaminated CPDs matched the prominent mutation signature found in melanomas. Our data identifies the cytosine-deaminated CPD as the leading premutagenic lesion responsible for the vast majority of mutations in sun-exposure-linked melanomas. DNA adducts and deaminations were mapped genome-wide in MEF and HDF cells treated with a dose of 1000 J/m2 and 4000 J/m2 UVB.