Activity of X-linked genes in stem and differentiated Mus musculus X Mus caroli hybrid cells.

Hypoxanthine phosphoribosyltransferase-deficient (HPRT-) F9-derived teratocarcinoma stem cells carrying an SV40 genome (12-16TG cells) were fused with Mus caroli (M. car.) spleen cells, and a stem cell hybrid containing reduced numbers of M. car. chromosomes was isolated (BC6 stem cell). The BC6 cells containing an active X chromosome from each parental cell were induced to differentiate in retinoic acid, and differentiated clones were isolated. Most differentiated clones retained both parental X chromosomes in active form. One differentiated clone, BC6-13, grew equally well in hypoxanthine/aminopterin/thymidine (HAT) selective medium (which requires an active M. car. HPRT (E.C.2.4.2.8) locus) or in 6-thioguanine (6TG, which would require either loss or inactivation of the M. car. HPRT locus). Using cDNA probes for HPRT and phosphoglycerate kinase (PGK) (E.C.2.7.2.3) loci and biochemical assays for HPRT and PGK enzymes, it was shown that BC6-13 cells, whether grown in nonselective medium, HAT medium, or 6TG-containing medium, retain the HPRT and PGK genes of both parental cells, but the M. car. forms of HPRT and PGK were inactivated in cells treated with 6TG. 6-Thioguanine seems to act as an inducer, one effect of which is X chromosome inactivation, which seems to be complete and irreversible as early as 24 h after addition of 6TG to BC6-13 cells.