Abstract
Nijmegen breakage syndrome(NBS), characterized by high sensitivity to ionizing radiation (IR) and predisposition to lymphoid cancer, is phenotypically similar to Ataxia telangiectasia (AT). It is well known that NBS1, the protein responsible for NBS, is cooperative with ATM, mutated in AT, for IR-induced DNA damage response such as double-strand break repair and cell cycle checkpoints. Recently, it was also reported that NBS shares common clinical signs with ATR-defective Seckel syndrome, such as microcephaly, and NBS cells is similarly defective in response to hydoxyurea (HU) treatment. Since ATR, a family gene of ATM kinase, functions in stalled replication fork after HU treatment or UV exposure, NBS1 might associate with ATR during stalling replication forks. Our result showed that NBS1 formed the discrete foci in UV-irradiated cells. To investigate the mechanism of its foci formation, we have generated a series of NBS1 mutant and determined a crucial domain for the foci formation. As a result, NBS1 mutant lacking FHA domain was able to form UV-induced foci, while FHA domain is essential for interaction with histon H2AX at damaged sites to form IR-induced foci. This was confirmed by observation that H2AX knockout cells can form NBS1 foci at the UV-exposed sites. The H2AX-independent foci formation was confined to S-phase cells after UV exposure. These results indicated that NBS1 is recruited to UV-induced damage sites by different mechanism from IR-induced foci formation, probably for functions of ATR in response to UV damages.
