Cloned mouse lymphocytes permit analysis of somatic mutations that occur in vivo

As part of our mouse model of somatic mutation, we have begun to characterize spontaneously occurring hypoxanthine phosphoribosyltransferase (HPRT) -deficient mouse lymphocytes. Lymphocytes were cloned by in vitro exposure of spleen cells from male C57B1/6 mice to the mitogen concanavalin A, conditioned medium containing lymphocyte growth factors, and thioguanine (TG), in a limiting dilution assay. The 17 TG-resistant clones recovered were all highly deficient in HPRT activity and were found by analysis of surface antigens to be representative of the major subclasses of T lymphocytes. Southern analysis of lymphocyte genomic DNA detected alterations of the hprt gene in 12/17 of the HPRT-deficient lymphocyte clones. Of these 12, 2/17 were lacking the entire hprt locus, 7/17 lacked part of the locus, and 3/17 had other, unidentified alterations.