The seid Factor on Human Chromosome 8 Restores V(D)J Recombination in Addition to Double-Strand Break Repair1

The murine severe combined immune deficiency mutation (seid) is characterized by a lack of Band T-lymphoid cells due to a defect in lymphoid V(D)J recombination. Moreover, defective rejoining of DNA double-strand breaks (dsb) in seid cells also results in a marked increase in sensitivity to ionizing radiation. Recently, the putative human homo logue of the murine seid gene locus, ÃŒIYKCI, was assigned to human chromosome Sqll, based on the radiation sensitivity of seid cells as compared to scid:human cell hybrids carrying portions of human chro mosome 8. Given the precedent (e.g., ataxia-telangiectasia) for genes other than the affected one being able to complement radiation defects, we were interested in determining if the ViDi.I recombination defect was also corrected by the IIYKCI locus. The V(D)J recombination analysis using extrachromosomal DNA substrates in control seid cells (SC3VA2) versus complemented cells (RD13B2) indicates that the radiation sensitivitycomplemented cells (RD13B2) are also fully complemented for the V(D)J recombination reaction, whereas the control (uncomplemented) cells (SC3VA2) fail to carry out V(D)J recombination normally. Slightly over 60% of the radiation-induced dsb are rejoined even in seid cells, and this alternative pathway is temperature sensitive. Only the remaining 30-35% of dsb require the introduction of the HYRC1 locus, and this pathway is not temperature sensitive. This merely partial con tribution of the seid factor to the repair process suggests the presence of another pathway of dsb repair. Our results indicate that the IIYKCI locus, assigned to human chromosome 8qll, encodes the seid factor, which is involved in all V(D)J recombination coding joint formation and in 30-35% of dsb repair by the temperature-resistant pathway.