The Chinese hamster ovary (CHO-K1) cell mutant XRS-6 is defective in rejoining of DNA double-strand breaks and is hypersensitive to X-rays, γ-rays, and bleomycin. Radiation resistance or sensitivity of somatic cell hybrids constructed from the fusion of XRS-6 cells with primary human fibroblasts strongly correlated with the retention of human chromosome 2 isozyme and molecular markers. Discordancies between some chromosome 2 markers and the radiation resistance phenotype in some of the hybrid cells suggested the location of the X-ray repair cross complementing 5 (XRCC5) gene on the p arm of chromosome 2. Introduction of human chromosome 2 by microcell-mediated chromosome transfer into the radiation-sensitive XRS-6 cells resulted in hybrid cells in which the radiation sensitivity was complemented. The chromosome 2p origin of the complementing human DNA in the microcell hybrids was supported by fluorescent in situ hybridization analysis of human metaphases using human DNA amplified from the hybrids by inter-Alu-PCR as chromosome-painting probes. XRCC5 is therefore provisionally assigned to human chromosome 2p.
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