Abstract A125: Profound impact of the nucleotide excision repair proteins XPC and CSB, but not ERCC1, on the resistance to an antitumor alkylator destabilizing local DNA duplex structure

Abstract

Abstract Backgound: Nucleotide excision repair (NER) is one of our most versatile DNA repair pathways. NER plays an important role in the repair of platinum adducts, and expression levels of the ERCC1 endonuclease, the last step in the NER process, is widely considered as a useful clinical marker for NER activity. S23906 is an antitumor alkaloid forming bulky monofunctional adducts with the capacity to destabilize local base pairing, which is a unique feature among anticancer agents. The purpose of the present study was to determine the influence of NER on the sensitivity to this unusual agent. Experimental: A panel of primary and transformed cell lines derived from patients with xeroderma pigmentosum or Cokayne syndrome was used to determine the role of each step in the NER pathway on the response to S23906. KB carcinoma cells with acquired resistance to S23906 were used for further characterization. Results: We here show that cells deficient for either global genome NER or transcription-coupled NER are profoundly sensitive to S23906, which for cells deficient in XPC, CSB and XPA far exceed the sensitivity observed for cisplatin and UV, two classical NER substrates. In contrast, cells deficient in ERCC1 and XPF, the last step in the NER pathway, showed modest sensitivity to S23906, compared to wt cells. Acquired resistance to S23906 was accompanied by cross-resistance to cisplatin and UV irradiation as well as by an important increase in NER activity as determined by unscheduled DNA synthesis and transcriptional recovery. Accordingly, resistant cells show up to 5-fold upregulation of XPC, CSB and XPA proteins. In contrast, no detectable alterations were observed for ERCC1. Conclusions: These results show that upregulation of NER activity in cells with acquired resistance to S23906 is associated with upregulation of XPC and CSB, the two proteins needed for the initial steps of the NER pathway, but without detectable changes in ERCC1 expression, which is needed for the last step of the pathway. Our results suggest that ERCC1 is neither rate-limiting nor representative of NER activity, and is unlikely to be of value as a predictive marker for bulky monofunctional compounds. We further suggest that the value of ERCC1 as a predictive marker for platinum compounds and other cross-linking agents may be based on its functions in recombination repair, cross link repair and telomere maintenance, rather than its role in NER. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):A125.