542 Xeroderma Pigmentosum C mutations increase oxidative DNA damages by inhibiting Base Excision Repair System’s expression and activity

Abstract

Xeroderma Pigmentosum C (XPC) is a rare genodermatosis which manifests clinically as pronounced photosensitivity, abnormal pigmentation and numerous skin cancers. It is caused by mutations in XPC gene that initiates global-genome nucleotide excision repair (GG-NER), a repair system responsible for bulky DNA adducts removal. Interestingly, development of internal and skin cancers in non-photo-exposed areas in parallel to an increase in oxidative DNA damages are observed upon these mutations. Therefore, we propose in our work to study XPC role in the modulation of BER, a major route for repairing oxidative DNA damage, at transcriptomic and proteomic levels along with repair activity using normal and XPC-mutated primary fibroblasts. To do so we used real time q-PCR and western blot to check effect of different XPC mutations on BER enzymes (OGG1, MYH in particular) at basal level and upon ultraviolet radiation type B (UVB) stress. Meanwhile, for studying the effect on BER’s activity, we measured the kinetic of repair of 7, 8-dihydro-8-oxoguanine (8-oxoguanine) using comet assay associated with formamidopyrimidine glycosylase (Fpg). A decrease in basal level and stress-induced level of BER was observed at gene and protein expression levels. Interestingly OGG1 and MYH, both of which are involved in 8-oxoguanine repair, were significantly downregulated. Finally, upon comet-fpg, a higher prolonged 8-oxoguanine oxidative lesions, persisting for more than 24 hours, were seen in XPC-mutated fibroblasts compared to the control especially after UVB-stress. Upon this, XPC deficient cell lines exhibit a reduction in 8-oxoguanine excision capacity, in agreement with the reduced genomic and proteomic levels BER enzymes. This may explain the higher incidence of skin cancer, especially in non-photo-exposed areas where impaired BER leads to an accumulation of lesions in the genome upon an increased induction and poorer repair of oxidative lesions.