Nonhomologous end joining (NHEJ) and homologous recombination (HR) are known as DNA double-strand break (DSB) repair pathways. It has been reported that DNA-PK, a member of PI3 kinase family, promotes NHEJ and aberrant DNA-PM causes NHEJ deficiency. However, in this study, we demonstrate that a wild-type cell line treated with DNA-PK inhibitor and a mutant cell line with dysfunctional DNA-PM showed decreased HR efficiency in fish cells (Medaka, Oryzias latipes). Previously, we reported that the radiation-sensitive mutant RIC1 strain has a defect in the Histone H2AX phosphorylation after gamma-irradiation. Here, we showed that a DNA-PM inhibitor, NU7026, treatment resulted in significant reduction in the number of gamma H2AX foci after gamma-irradiation in wild-type cells, but had no significant effect in RIC1 cells. In addition, RIC1 cells showed significantly lower levels of DNA-PM kinase activity compared with wild-type cells. We investigated NHEJ and HR efficiency after induction of DSBs. Wild-type cells treated with NU7026 and RIC1 cells showed decreased HR efficiency. These results indicated that aberrant DNA-PM causes the reduction in the number of gamma H2AX foci and HR efficiency in RIC1 cells. We performed phosphorylated DNA-PKcs (Thr2609) and 53BP1 focus assay after gamma-irradiation. RIC1 cells showed significant reduction in the number of phosphorylated DNA-PKcs foci and no deference in the number of 53BP1 foci compared with wild-type cells. These results suggest that low level of DNA-PM activity causes aberrant DNA-PKcs autophosphorylation in RIC1 cells. It is known that 538P1 is involved in both DNA-PM dependent and independent NHEJ. Therefore we suggest that DNA-PM independent NHEJ repair DSBs under the condition of decreased DNA-PK activity, which causes reduction of HR efficiency. (C) 2012 Elsevier Inc. All rights reserved.