GENETIC-MAPPING USING MICROCELL-MEDIATED CHROMOSOME TRANSFER SUGGESTSA LOCUS FOR NIJMEGEN BREAKAGE SYNDROME AT CHROMOSOME 8Q21-24

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder ch aracterized by microcephaly, short stature, immunodeficiency, and a hi gh incidence of cancer. Cultured cells from NBS show chromosome instab ility, an increased sensitivity to radiation-induced cell killing, and an abnormal cell-cycle regulation after irradiation. Hitherto, patien ts with NBS have been divided into the two complementation groups V1 a nd V2, on the basis of restoration of radioresistant DNA synthesis, su ggesting that each group arises from a different gene, However, the pr esence of genetic heterogeneity in NBS has been considered to be contr oversial. To localize the NBS gene, we have performed functional compl ementation assays using somatic cell fusion between NBS-V1 and NBS-V2 cells, on the basis of hyper-radiosensitivity, and then have performed a genomewide search for the NBS locus, using microcell-mediated chrom osome transfer followed by complementation assays based on radiosensit ivity, We found that radiation resistance was not restored in the fuse d NBS-V1 and NBS-V2 cells and that only human chromosome 8 complements the sensitivity to ionizing radiation, in NBS cell lines. In compleme ntation assays performed after the transfer of a reduced chromosome, m erely the long arm of chromosome 8 was sufficient for restoring the de fect. Our results strongly suggest that NBS is a homogeneous disorder and that the gene for NBS is located at 8q21-24.