MISMATCH REPAIR DEFECTS AND HUMAN TUMOR RADIOSENSITIZATI

DNA错配修复(MMR)基因的突变或表达缺失(尤其是MLH1、MSH2和PMS2)在许多类型的散发性人类结肠癌中被发现的频率越来越高，以及先前观察到的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对许多不同类型的临床活性化疗药物表现出耐药性，这表明有必要为MMR缺陷的肿瘤寻找新的治疗方法。在MMR缺陷细胞中观察到的耐药性可能归因于“损伤耐受”，即细胞无法检测或对化疗引起的DNA损伤做出反应。嘌呤类似物6-硫代鸟嘌呤(6-TG)被实验用来定义MMR缺陷细胞的“损伤耐受”表型。我们实验室的最新数据表明，MLH1，MMR缺陷细胞在电离辐射(IR)暴露后，克隆存活率降低，G2/M期停滞减少。此外，我们发现MLH1-(以及初步的MSH2-)，MMR缺陷细胞在暴露于卤代胸苷(DThd)类似物溴脱氧尿苷(BrdUrd)或碘脱氧尿苷(IdUrd)后显示出“损伤耐受性”；导致DNA中掺入药物的水平比基因匹配的野生型(MMRT)肿瘤细胞高2-3倍以上。因此，我们发现，在IR之前用BrdUrd或IdUrd治疗可以显著增强MLH1-MMR缺陷细胞的IR诱导的细胞毒性(放射增敏)，但对匹配的MLH1+MMR熟练的肿瘤细胞却没有。我们推测，利用卤代胸苷类似物介导的放射增敏，有可能开发针对MMR缺陷型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.