Defective base excision repair in the response to DNA damaging agents in triple negative breast cancer

DNA repair defects have been increasingly focused on as therapeutic targets. In hormone-positive breast cancer, XRCC1-deficient tumors have been identified and proposed as targets for combination therapies that damage DNA and inhibit DNA repair pathways. XRCC1 is a scaffold protein that functions in base excision repair (BER) by mediating essential interactions between DNA glycosylases, AP endonuclease, poly(ADP-ribose) polymerase 1, DNA polymerase β (POL β), and DNA ligases. Loss of XRCC1 confers BER defects and hypersensitivity to DNA damaging agents. BER defects have not been evaluated in triple negative breast cancers (TNBC), for which new therapeutic targets and therapies are needed. To evaluate the potential of XRCC1 as an indicator of BER defects in TNBC, we examined XRCC1 expression in the TCGA database and its expression and localization in TNBC cell lines. The TCGA database revealed high XRCC1 expression in TNBC tumors and TNBC cell lines show variable, but mostly high expression of XRCC1. XRCC1 localized outside of the nucleus in some TNBC cell lines, altering their ability to repair base lesions and single-strand breaks. Subcellular localization of POL β also varied and did not correlate with XRCC1 localization. Basal levels of DNA damage correlated with observed changes in XRCC1 expression, localization, and measure repair capacity. The results confirmed that XRCC1 expression changes indicate DNA repair capacity changes but emphasize that basal DNA damage levels along with protein localization are better indicators of DNA repair defects. Given the observed over-expression of XRCC1 in TNBC preclinical models and tumors, XRCC1 expression levels should be assessed when evaluating treatment responses of TNBC preclinical model cells.

DNA修复缺陷作为治疗靶点的研究日益受到关注。在激素受体阳性乳腺癌中，XRCC1缺陷型肿瘤已被鉴定，并被提议作为联合疗法的靶点——这类联合疗法可损伤DNA并抑制DNA修复通路。XRCC1是一种支架蛋白，通过介导DNA糖基化酶、AP内切酶、聚ADP核糖聚合酶1、DNA聚合酶β（POL β）以及DNA连接酶之间的必要相互作用，在碱基切除修复（base excision repair, BER）中发挥功能。XRCC1的缺失会导致BER缺陷，并使细胞对DNA损伤剂产生超敏反应。目前尚未在三阴性乳腺癌（triple negative breast cancer, TNBC）中评估BER缺陷，而这类癌症亟需新的治疗靶点与疗法。为了探究XRCC1作为TNBC中BER缺陷标志物的潜力，我们分析了TCGA数据库中的XRCC1表达情况，并检测了TNBC细胞系中XRCC1的表达与定位。TCGA数据库显示，TNBC肿瘤中XRCC1呈高表达；TNBC细胞系的XRCC1表达存在差异，但大多呈现高表达。部分TNBC细胞系中，XRCC1定位于细胞核外，这会改变细胞修复碱基损伤与单链断裂的能力。POL β的亚细胞定位同样存在差异，且与XRCC1的定位无相关性。基础水平的DNA损伤与观测到的XRCC1表达、定位变化以及修复能力测定结果相关。本研究结果证实，XRCC1的表达变化可反映DNA修复能力的改变，但同时强调，基础DNA损伤水平结合蛋白质定位情况，是DNA修复缺陷更可靠的标志物。鉴于在TNBC临床前模型与肿瘤中观测到XRCC1过表达，在评估TNBC临床前模型细胞的治疗响应时，应检测XRCC1的表达水平。