Gene expression in euploid human hybrid cells: ouabain resistance is codominant

Single-step mutations to ouabain resistance have been induced in Lesch-Nyhan diploid fibroblasts carrying a variant G6PD isozyme (OuaR/HGPRT-/G6PDA). Two of these mutant strains, differing in the degree of ouabain resistance, were fused with wild-type human diploid fibroblasts (OuaS/HGPRT+/G6PDB) and hybrids isolated in doubly selective medium, ouabain + HAT (HOT). Four hybrids, identified by G6PD heteropolymers and stable tetraploid karyotypes, expressed ouabain resistance intermediate to that of the parents. The degree of ouabain resistance was influenced by the specific mutant allele of the OuaR parent, indicating both mutant and wild-type alleles are simultaneously expressed in hybrid cells. Intraspecific hybrids of this kind are especially suited to studies of gene regulation because all relevant genes present and the number of alleles at each locus is known.