Ž . Mutation Research 459 2000 89–97 www.elsevier.comrlocaterdnarepair Community address: www.elsevier.comrlocatermutres Accelerated publication XRCC3 is required for efficient repair of chromosome breaks by homologous recombination Mark A. Brenneman a,1 , Anne E. Weiss a , Jac A. Nickoloff b , David J. Chen c, ) a Life Sciences DiÕision, Los Alamos National Laboratory, Los Alamos, NM, USA b Department of Molecular Genetics and Microbiology, UniÕersity of New Mexico School of Medicine, Albuquerque, NM, USA c Life Sciences DiÕision, Lawrence Berkeley National Laboratory, MS 74-157, 1 Cyclotron Road, Berkeley, CA 94720, USA Received 4 October 1999; received in revised form 17 December 1999; accepted 20 December 1999 Abstract XRCC3 was originally identified as a human gene able to complement the DNA damage sensitivity, chromosomal instability and impaired growth of the mutant hamster cell line irs1SF. More recently, it has been cloned, sequenced and found to bear sequence homology to the highly conserved eukaryotic repair and recombination gene RAD51. The phenotype of irs1SF and the identification of XRCC3 as a member of the RAD51 gene family have suggested a role for XRCC3 in Ž . repair of DNA damage by homologous recombination. Homologous recombinational repair HRR of a specifically induced Ž . chromosomal double-strand break DSB was assayed in irs1SF cells with and without transient complementation by human XRCC3. Complementation with XRCC3 increased the frequencies of repair by 34- to 260-fold. The results confirm a role for XRCC3 in HRR of DNA DSB, and the importance of this repair pathway for the maintenance of chromosomal integrity in mammalian cells. q 2000 Elsevier Science B.V. All rights reserved. Keywords: XRCC3; Homologous recombination; DNA double-strand breaks; DNA repair; Chromosome breaks; Chromosomal stability 1. Introduction XRCC3 is one of a group of genes originally identified by their ability to complement certain mu- tated rodent cell lines for hypersensitivity to ionizing Ž . radiation and other DNA damaging agents , and hence designated ‘‘X-ray repair cross-complement- ing’’. Four of these genes are now known to function ) Corresponding author. Tel.: q 1-510-495-2861; Fax: q 1-510- 486-6816; e-mail: djchen@lbl.gov 1 Present address: Department of Molecular Genetics and Mi- crobiology, University of New Mexico School of Medicine, 915 Camino de Salud, Albuquerque, NM 87131, USA. Ž . in the repair of DNA double-strand breaks DSB through the process of nonhomologous end-joining Ž . NHEJ . The product of the XRCC4 gene functions w x as an accessory to DNA ligase IV 1,2 . XRCC5, XRCC6 and XRCC7 encode, respectively, the Ku80 subunit, the Ku70 subunit and the catalytic subunit Ž .Ž of DNA-dependent protein kinase DNA-PK re- wx. viewed in Ref. 3 . XRCC2 and XRCC3 constitute a subset apart from the XRCC mutants linked to defects in NHEJ, as distinguished by the phenotypes of the mutant hamster cell lines in which they were first defined w x Ž w x. 4–9 reviewed in Ref. 10 . XRCC2 and XRCC3 mutant cells show only moderate hypersensitivity to 0921-8777r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0921-8777 00 00002-1