DNA mismatch repair (MMR) is a highly-conserved DNA repair mechanism, whose primary role is to remove DNA replication errors preventing them from manifesting as mutations, thereby increasing the overall genome stability. Defects in MMR are associated with increased cancer risk in humans and other organisms. Here, we characterize the interaction between MMR and a proofreading-deficient allele of the human replicative DNA polymerase delta, PolδD316A;E318A, which has a higher capacity for strand displacement DNA synthesis than wild type Polδ. Human cell lines overexpressing PolδD316A;E318A display a mild mutator phenotype, while nuclear extracts of these cells exhibit reduced MMR activity in vitro, and these defects are complemented by overexpression or addition of exogenous human Exonuclease 1 (EXO1). By contrast, another proofreading-deficient mutant, PolδD515V, which has a weaker strand displacement activity, does not decrease the MMR activity as significantly as PolδD316A;E318A. In addition, PolδD515V does not increase the mutation frequency in MMR-proficient cells. Based on our findings, we propose that the proofreading activity restricts the strand displacement activity of Polδ in MMR. This contributes to maintain the nicks required for EXO1 entry, and in this manner ensures the dominance of the EXO1-dependent MMR pathway.

中文翻译：

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人类DNA聚合酶δ双突变D316A; E318A在体外干扰DNA错配修复

DNA错配修复（MMR）是一种高度保守的DNA修复机制，其主要作用是消除DNA复制错误，从而防止它们表现为突变，从而提高总体基因组稳定性。MMR的缺陷与人类和其他生物体的癌症风险增加有关。在这里，我们表征了MMR和人类复制性DNA聚合酶δ的校正不足等位基因PolδD316A; E318A之间的相互作用，与野生型Polδ相比，其具有更高的链置换DNA合成能力。过度表达PolδD316A; E318A的人细胞系表现出轻度的突变表型，而这些细胞的核提取物在体外表现出降低的MMR活性，这些缺陷可以通过过表达或添加外源人核酸外切酶1（EXO1）来弥补。相比之下，另一个校对缺陷的突变体PolδD515V具有较弱的链置换活性，却没有像PolδD316A; E318A那样显着降低MMR活性。此外，PolδD515V不会增加MMR精通细胞的突变频率。根据我们的发现，我们提出校对活性限制了MMR中Polδ的链置换活性。这有助于维持进入EXO1所需的切口，并以这种方式确保了依赖EXO1的MMR途径的优势。