Published online 1 April 2008 Nucleic Acids Research, 2008, Vol. 36, No. 9 2939–2947 doi:10.1093/nar/gkn146 A polycomb group protein, PHF1, is involved in the response to DNA double-strand breaks in human cell 1 1 1 1 1 Zehui Hong , Jie Jiang , Li Lan , Satoshi Nakajima , Shin-ichiro Kanno , Haruhiko Koseki2 and Akira Yasui1,* 1Department of Molecular Genetics, Institute of Development, Aging and Cancer, Tohoku University, Seiryomachi 4-1, Aobaku, Sendai 980-8575 and 2Laboratory of Developmental Genetics, Riken Research Center for Allergy and Immunology, Suehiro-cho 1-7-22, Tsurumi-ku, Yokohama 230-0045, Japan Received December 18, 2007; Revised March 4, 2008; Accepted March 17, 2008 ABSTRACT DNA double-strand breaks (DSBs) represent the most toxic DNA damage arisen from endogenous and exogenous genotoxic stresses and are known to be repaired by either homologous recombination or nonhomologous end-joining processes. Although many proteins have been identified to participate in either of the processes, the whole processes still remain elusive. Polycomb group (PcG) proteins are epigenetic chromatin modifiers involved in gene silencing, cancer development and the maintenance of embryonic and adult stem cells. By screening proteins responding to DNA damage using laser micro-irradiation, we found that PHF1, a human homolog of Drosophila polycomb-like, Pcl, protein, was recruited to DSBs immediately after irradiation and dissociated within 10 min. The accumulation at DSBs is Ku70/Ku80-dependent, and knockdown of PHF1 leads to X-ray sensitivity and increases the frequency of homologous recombination in HeLa cell. We found that PHF1 interacts physically with Ku70/Ku80, suggesting that PHF1 promotes non- homologous end-joining processes. Furthermore, we found that PHF1 interacts with a number of proteins involved in DNA damage responses, RAD50, SMC1, DHX9 and p53, further suggesting that PHF1, besides the function in PcG, is involved in genome maintenance processes. INTRODUCTION DNA double-strand breaks (DSBs) can be caused by both cell-intrinsic sources, such as replication errors or reactive oxygen species, and a variety of extrinsic factors, includ- ing ionizing radiation (IR) and radiomimetic chemicals. DSBs representing the most toxic DNA lesions, if left unrepaired, may cause cell death and genomic instability. Ineﬃcient or inaccurate repair may lead to mutation and/or chromosome rearrangement, and predisposition to cancer (1–5). DSBs also represent obligatory inter- mediates of physiological DNA rearrangement processes taking place during the development and maturation of the adaptive immune system, V(D)J recombination and immunoglobulin (Ig) heavy-chain class switch recombina- tion (CSR) (6). Therefore, defects in the repair of these DNA breaks can cause profound immuno-deﬁciencies (7). Eukaryotes cells have evolved two major pathways for repairing DSBs, homologous recombination (HR) and nonhomologous end joining (NHEJ). Both pathways are conserved from yeast to mammals and function in complementary ways to repair DSBs (1,5,8). During HR, DSBs are repaired through a precise pathway that uses homologous sequence usually provided by the sister chromatid during replication as for template. In contrast, NHEJ is an error-prone repair pathway that joins ends together without the requirement for signiﬁcant sequence homology (1,5,8). Once DSBs are produced, cells trigger a series of signaling pathway including cycle regulation, transcription, histone modiﬁcation and apoptosis that have direct or indirect eﬀect on DSB repair. Following DNA damage, the DNA damage sensors ATM/ATR and DNA-PK phosphorylate CHK1 and CHK2 to regulate cell cycle checkpoint, phosphorylate P53 to activate apoptosis signal pathway, phosphorylate H2AX and a number of proteins involved in DSB repair such as NBS1 and SMC1 (5,9). Besides phosphorylation of H2AX, recently, histone ubiquitinations, acetylations and methyl- ations have been implicated in the DNA damage checkpoint and DSBs repair pathways (10). Although the last few years a wealth of new information has been produced about DSBs damage response and DNA repair, and many novel proteins involved in the process have been identiﬁed, the process still remains elusive. *To whom correspondence should be addressed. Tel: +81 22 717 8465; Fax: +81 22 717 8470; Email: ayasui@idac.tohoku.ac.jp 2008 The Author(s) This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.