Determining the response of tumour cells to UV light and cisplatin

Nucleotide excision repair (NER) recognizes and repairs sunlight-induced DNA damage as well as cisplatin induced intrastrand DNA cross-links. When treated with cisplatin, NER capacity of a tumour has been shown to be predictive of outcome in some human cancers. Similarly, in vitro, the response of tumours cells to cisplatin is dependent on NER, where polymorphisms in some NER components alter the outcome. NER consists of two sub-pathways, transcription-coupled (TC-) NER and global-genomic (GG-) NER, differing only in their recognition of DNA lesions. While GG-NER deficiency, resulting in xeroderma pigmentosum (XP), increases rates of cancer in man, TC-NER deficiency, resulting in Cockayne syndrome (CS) does not. Both these syndromes have multiple genetic causes whose study has allowed a more complete understanding of NER. In primary human fibroblasts, keratinocytes and mouse embryonic fibroblasts TC-NER and not GG-NER is important in the avoidance of apoptosis after UV and cisplatin. However, mouse embryonic stem cells do not require TC-NER to recover from this type of DNA damage and so it may be difficult to predict the role of this repair pathway in all cellular contexts. We assessed the importance of TC-NER in the cisplatin response of tumour cells by manipulating TC-NER and measuring DNA repair and sensitivity to DNA damaging agents UV-C and cisplatin. Targeting TC-NER by transient transfection of siRNA against Cockayne syndrome B (CSB), xeroderma pigmentosum A (XPA) and XPA-binding protein 2 (XAB2) sensitized tumour cells to cisplatin. This sensitization was largely independent of p53 tumour suppressor and mismatch repair status of these cells. As a preliminary in vivo test, we generated a colon cancer cell line in which CSB expression was stably reduced and used this cell line in a tumour xenograft model. Treating the animais with a singleweek regimen of cisplatin resulted in a decrease in tumour volume initially and delayed tumour re-growth compared to control tumours. Taken together, these results indicate that TC-NER is a potential target in combined cisplatin tumour therapy. This work also suggests that TC-NER capacity may be clinically predictive.