Involvement of DNA polymerase μ in the repair of a specific subset of DNA double-strand breaks in mammalian cells

The repair of DNA double-strand breaks (DSB) requires processing of the broken ends to complete the ligation process. Recently, it has been shown that DNA polymerase mu (pol mu) and DNA polymerase lambda (pol mu) are both involved in such processing during non-homologous end joining in vitro. However, no phenotype was observed in animal models defective for either pol mu and/or pol mu. Such observations could result from a functional redundancy shared by the X family of DNA polymerases. To avoid such redundancy and to clarify the role of polk in the end joining process, we generated cells over-expressing the wild type as well as an inactive form of polk (pol mu D). We observed that cell sensitivity to ionizing radiation (IR) was increased when either polk or pol mu D was over-expressed. However, the genetic instability in response to IR increased only in cells expressing pol mu D. Moreover, analysis of intrachromosomal repair of the I-Scel-induced DNA DSB, did not reveal any effect of either polk or pol mu D expression on the efficiency of ligation of both cohesive and partially complementary ends. Finally, the sequences of the repaired ends were specifically affected when polk or pol mu D was over-expressed, supporting the hypothesis that polk could be involved in the repair of a DSB subset when resolution of junctions requires some gap filling.