Nucleotide Excision Repair in Human Cell Lines Lacking Both XPC and CSB Proteins

Numerous studies have shown that in human and other mammalian cells lines the XPC protein is required for nucleotide excision repair of UV induced cyclobutene pyrimidine dimers and (6-4) photoproducts from nontranscribed genomic DNA via global repair and the CSB protein is required for repair of lesions from transcribed DNA via transcription-coupled repair (TCR). Therefore, it is generally assumed that a XPC-/-CSB-/- mutant would be totally defective in nucleotide excision repair. Here we describe the construction of XPC-/-/CSB-/-human cell lines that, contrary to expectations, perform robust TCR of both CPD and (6-4)PP. The respective XPC/CSB genes were mutated in cell lines derived from Xeroderma Pigmentosum patients as well as from normal human fibroblasts and repair was analyzed at the whole genome level using the XR-seq method. As predicted, XPC-/- cells exhibited TCR only and CSB-/- cells exhibited only global repair. However, the XPC-/-/CSB-/- cell lines, although having greatly reduced repair, exhibited TCR. Mutating the CSA gene to generate a triple mutant XPC-/-/CSB-/-/CSA-/- cell line eliminated all residual repair activity. Together, these findings provide new insights into the mechanistic features of mammalian nucleotide excision repair.