The cellular mismatch repair system is able to repair mismatches within MLV retroviral double-stranded DNA at a low frequency

Eukaryotic cells possess several distinct mismatch repair pathways. A mismatch can be introduced in retroviral double-stranded DNA by a pre-existing mutation within the primer binding site (PBS) of the viral RNA genome. In order to evaluate mismatch repair of retroviral double-stranded DNA, Moloney leukemia virus (MLV)-based vectors with a mutation in their PBS were used to infect mismatch repair-competent as well as mismatch repair-deficient cell lines. If the target cells were capable of repairing the mismatch before an infected cell divided, the mismatch within the PBS could be repaired to the wild-type or mutant PBS. If the target cells were unable to repair the mismatch, half the cells in the colony should contain the mutant PBS while the other half should contain the wild-type PBS. To evaluate these predictions, individual colonies were isolated and analyzed by PCR. Almost all mismatch-deficient cell colonies analyzed (cell lines HCT 116 and PMS2–/–) contained both the wild-type and mutated PBS, therefore, mismatches within retroviral double-strand DNA could not be repaired by the mismatch-deficient cells. In contrast, mismatches in ~25% of the mismatch repair-competent cell clones analyzed (cell lines HeLa and PMS2+/+) were repaired, while 75% were not. Therefore, the cellular mismatch repair system is able to repair mismatches within viral double-stranded DNA, but at a low frequency.

真核细胞具备多条独特的错配修复通路。病毒RNA基因组的引物结合位点（PBS）中预先存在的突变，可在逆转录病毒双链DNA中引入碱基错配。为评估逆转录病毒双链DNA的错配修复情况，研究人员采用引物结合位点（PBS）携带突变的莫洛尼白血病病毒（MLV）载体，分别感染错配修复功能正常与错配修复缺陷的细胞系。若靶细胞能在受感染细胞分裂前完成错配修复，则PBS区域的碱基错配可被修复为野生型或突变型PBS序列。若靶细胞无法修复该错配，则菌落中一半的细胞将携带突变型PBS，另一半则携带野生型PBS。为验证上述预测，研究人员分离得到单个菌落并通过聚合酶链式反应（PCR）进行分析。对所有被分析的错配修复缺陷细胞菌落（HCT 116细胞系与PMS2–/–细胞系）的检测结果显示，几乎所有菌落均同时存在野生型与突变型PBS，由此证实错配修复缺陷细胞无法修复逆转录病毒双链DNA中的碱基错配。与之相反，在被分析的错配修复功能正常的细胞克隆（HeLa细胞系与PMS2+/+细胞系）中，约25%的碱基错配得到了修复，剩余75%则未被修复。综上，细胞错配修复系统可修复病毒双链DNA中的碱基错配，但修复频率较低。