Complementation of the Radiosensitive Phenotype in Severe Combined Inimunodeliden t Mice by Human Chromosome 81

Severe combined immunodeficient (seid) C.B-17 mice are deficient in variable (diversity) joining region recombination, the process of assem bling the immunoglobulin and T-cell receptor genes from gene segments, thereby creating much of the enormous diversity of antigen-binding ca pacity, seid mice are also sensitive to ionizing radiation, as a result of their deficiency in double-strand break repair. Here we report the complemen tation of the radiation-sensitive seid phenotype by transferring human chromosome 8 into seid cells. Somatic cell hybrids were generated by fusing seid cells with human HT-1080 cells, resulting in radioresistant hybrids with several human chromosomes. One of the identified human chromosomes in the radioresistant seid cell line 4.61, which retains only two human chromosomes, is a rearranged 8/21 translocation. Proof that chromosome 8 confers the complementation was achieved by transferring only human chromosome 8 into seid cells by microcell-mediated chromo some transfer (scid/hu8 cell line). The presence of chromosome 8 in our scid/hu8 cell line was monitored by fluorescence in situ hybridization and polymerase chain reaction. We demonstrated the radioresistance of this hybrid not only to high dose rate but also to low dose rate radiation. We also showed that transference of human chromosome 8 to seid cells fully complements the DNA double-strand break repair deficiency and the high sensitivity of seid cells to radiation-induced chromosome aberrations. Mapping the seid gene to human chromosome 8 is an important first step in cloning the seid gene, which will enhance our understanding of doublestrand break repair pathways in humans.