MISMATCH REPAIR DEFECTS AND HUMAN TUMOR RADIOSENSITIZATI

DNA错配修复（MMR）基因（尤其是MLH1、MSH2和PMS2）的突变或表达缺失在许多类型的散发性人类结肠癌中发现的频率越来越高，以及先前观察到的与遗传性非息肉病性结直肠癌（HNPCC）的因果关系。mmr缺陷的人类肿瘤细胞已经显示出对许多不同类型的临床活性化疗药物的耐药性，这指出了对mmr缺陷肿瘤的新治疗方法的潜在需求。在mmr缺陷细胞中观察到的耐药性可能归因于“损伤耐受性”，即细胞无法检测或对化疗诱导的DNA损伤作出反应。嘌呤类似物6-硫鸟嘌呤（6-TG）在实验中被用来定义mmr缺陷细胞的“耐损伤”表型。我们实验室最近的数据表明，MLH1-、mmr缺陷细胞在电离辐射（IR）暴露后克隆性存活减少，G2/M阻滞减少。此外，我们发现MLH1-（以及初步发现的MSH2-）、mmr缺陷细胞在暴露于卤代胸苷（dThd）类似物、溴脱氧尿嘧啶（BrdUrd）或碘脱氧尿嘧啶（IdUrd）后表现出“损伤耐受性”；与遗传匹配的野生型（MMRT）肿瘤细胞相比，将药物掺入DNA的水平高出2-3倍以上。因此，我们发现在IR之前使用BrdUrd或IdUrd治疗可显著增强MLH1-， mmr缺陷细胞的IR诱导的细胞毒性（放射致敏），但在匹配的MLH1+， mmr有效的肿瘤细胞中则没有。我们假设有可能利用卤化胸苷类似物介导的放射增敏来开发针对核磁共振缺陷的人类癌症的治疗策略。这些临床前研究将是开发针对mmr缺陷肿瘤的I/Il期肿瘤特异性治疗策略的第一步。

Mutations or loss of expression of DNA mismatch repair (MMR) genes especially MLH1, MSH2 and PMS2) have been found with an increasing frequency in many types of sporadic human colon cancers, along with the causal relationship previously observed with hereditary nonpolyposis colorectal cancers (HNPCC). MMR-deficient human tumor cells have demonstrated resistance to many different types of clinically active chemotherapy drugs, pointing out the potential need for new treatment approaches for MMR-deficient tumors. The observed drug resistance in MMR-deficient cells may be attributed to "damage tolerance", an inability of the cell to detect or respond to chemotherapy-induced DNA damage. The purine analog, 6-thioguanine (6-TG) is used experimentally to define the "damage tolerant" phenotype of MMR-deficient cells. Recent data from our laboratory suggest that MLH1-, MMR-deficient cells have reduced clonogenic survival and reduced G2/M arrest following ionizing radiation (IR) exposures. Additionally, we found that MLH1- (and, preliminarily, MSH2-), MMR-deficient cells show "damage tolerance" following exposures to the halogenated thymidine (dThd) analogs, bromodeoxyuridine (BrdUrd) or iododeoxyuridine (IdUrd); resulting in greater than 2-3 fold higher levels of incorporated drug into DNA, compared to genetically-matched wild type (MMRT) tumor cells. Consequently, we found that treatment with BrdUrd or IdUrd prior to IR resulted in a significant enhancement in IR-induced cytotoxicity (radiosensitization) in MLH1-, MMR-deficient cells, but not in the matched MLH1+, MMR-proficient tumor cells. We hypothesize that it may be possible to develop treatment strategies to target MMR-deficient human cancers using halogenated thymidine analog-mediated radiosensitization. These preclinical studies will be the first step in the development of a Phase I/Il tumor-specific treatment strategy to target MMR-deficient tumors.