Nucleotide excision repair of UVA-induced DNA damage: regulation in sunlight-induced melanoma

The causative link between UV exposure and melanoma development is well known, however the mechanistic relationship remains incompletely characterised. UVA and UVB components of sunlight are implicated in melanomagenesis; however the majority of studies have focused on the effects of UVB and UVC light. Melanoma tumour sequencing has revealed an overrepresentation of mutations signature of unrepaired UVA-induced DNA damage, suggestive of a lack of repair. Repair of UVA-induced DNA damage is thought to occur primarily through the Nucleotide Excision Repair (NER) pathway, which recognises and repairs damage either coupled to transcription (Transcription Coupled Repair; TCR), or through global genome scanning (Global Genome Repair; GGR). Current literature suggests NER is deficient in melanoma, however the cause of this remains unknown; and whether reduced NER activity in response to UVA may be involved in melanoma development remains uncharacterised. One melanocyte and four melanoma cell lines were used in this study. Cells were UVA-irradiated, and DNA damage levels assessed by immunodetection of Cyclobutane Pyrimidine Dimer (CPD) and (6-4) Photoproduct ((6-4)PP) lesions. Expression of NER pathway components following UVA treatment was quantified by qPCR and western blot. Methylation status of GGR transcripts was determined by bisulphite sequencing. UVA did not induce detectable induction of (6-4)PP lesions, consistent with previous studies. Repair of CPDs induced by UVA was complete within 48 hours in normal melanocytes, whereas >40% of lesions remained in melanoma cell lines. This was coupled with a delayed and attenuated induction of GGR transcripts in melanoma cells. Preliminary data suggests that the GGR transcripts are methylated; however the role of this in dynamic regulation of NER requires further study. These findings support that NER activity is reduced in melanoma cells due to deficient GGR. These results warrant further investigation into the role of NER in UVA-induced melanomagenesis, and have implications for melanoma treatment.