THE USE OF METHYLTHIOADENOSINE PHOSPHORYLASE-ACTIVITY TO SELECT FOR HUMAN-CHROMOSOME-9 IN INTERSPECIES AND INTRASPECIES HYBRID-CELLS

Methylthioadenosine phosphorylase (MTAP) is an enzyme that functions i n a salvage pathway for adenine synthesis. The locus that encodes MTAP activity has been mapped to human chromosome 9 (9q12-9pter) by analys is of mouse x human somatic cell hybrids. Cells that have MTAP activit y will stop proliferating, and eventually die in the presence of azase rine, an inhibitor of de novo purine synthesis, but can be rescued by the addition of methylthioadenosine (MTA) to the culture medium. Some mouse and human tumor cells lack MTAP activity and can not grow in the presence of azaserine and MTA. We fused MTAP competent human Fibrobla st cells to MTAP deficient mouse L-cells and selected for somatic cell hybrids, containing MTAP activity in medium containing azaserine and MTA. In a separate experiment, a CHO cell x human fibroblast somatic c ell hybrid, containing a normal copy of human chromosome 9, was used t o prepare microcells, which were fused to an MTAP-deficient human leuk emic cell line, CCRF-CEM. Somatic cell and microcell hybrids were show n to retain human chromosome 9 by fluorescence in situ hybridization u sing probes that hybridize to the interferon-alpha and -beta 1 genes o n human chromosome 9 (9p21), and the centromere of human chromosome 9. This is the first report of complementation for MTAP activity being u sed to select for somatic cell hybrids and microcell hybrids that reta in a human chromosome 9.