Abstract 3108: Exploring base excision repair variants as potential defining events in cancer patient populations and characterization of models for pre-clinical studies

Abstract Background Deficiencies in base excision repair (BER) may increase the cell's dependence on alternative repair pathways and so define a patient population for novel inhibitors of DNA damage repair. Notably, cancer associated molecular events in DNA polymerase beta (POLα) have been described in various disease backgrounds including colorectal cancer (CRC), gastric, breast and prostate (1-5). Although functional consequence has been linked to certain POLα mutations and splice variants, their prevalence in defined disease segments remains largely undefined. Aim We set out to ascertain the prevalence of POLα deficiencies in CRC, and subsequently investigated protein and gene expression profiling as alternative methods for defining BER defects in cancer. Methods/Results A comprehensive study of clinical (CRC; N=64), tumour explant (CRC; N=36) and cell line (mixed origin; N=280) samples revealed that cancer-associated mutations in the polymerase encoding domain of POLα (1-3) are not common events in CRC. Additionally, a previously described splice variant of POLα lacking part of the polymerase domain (3-4), was assayed and found to be expressed at similar levels in CRC and normal tissue. This would suggest that expression of the POLα splice variant is not a defining element of CRC in western populations. Expression profiling using Fluidigm Taqman revealed a subset of BER genes more highly expressed in CRC clinical samples and mixed cell lines compared with normal tissue. While protein expression profiles did not reflect relative gene expression in selected cell line models, we observed an additional low molecular weight band in a subset of cells profiled for POLB expression. Furthermore, we report variations in the expression of the scaffold protein XRCC1 in colorectal and breast cell lines and propose that this could also provide a mechanism for variation in DNA repair capacity. Summary Although from our studies sequencing and transcript analysis of POLα do not provide suitable markers of BER deficiency in CRC, we report aberrant banding patterns in POLα by Western blotting and variation in XRCC1 protein expression in a subset of cell lines analyzed. We hypothesize that these variants may confer a BER deficient phenotype. 1. Lang, T. et al. (2004Proc. Natl Acad. Sci. USA, 101, 6074-6079. 2. Tan, Xiao-Hui, et al Cancer Letters: 2005: 220: 101-114 3. Wang, L et al Cancer Research, 1992; 52:4824-4827 4. Bhattacharyya, N. et al. (1997) Proc. Natl Acad. Sci. USA, 94, 10324-10329 Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3108. doi:1538-7445.AM2012-3108