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

Abstract

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 μ (pol μ ) and DNA polymerase λ (pol λ ) 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 μ and/or pol λ . 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 pol μ in the end joining process, we generated cells over-expressing the wild type as well as an inactive form of pol μ (pol μ D). We observed that cell sensitivity to ionizing radiation (IR) was increased when either pol μ or pol μ D was over-expressed. However, the genetic instability in response to IR increased only in cells expressing pol μ D. Moreover, analysis of intrachromosomal repair of the I-SceI-induced DNA DSB, did not reveal any effect of either pol μ or pol μ 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 pol μ or pol μ D was over-expressed, supporting the hypothesis that pol μ could be involved in the repair of a DSB subset when resolution of junctions requires some gap filling.