Evidence for dominant mutations reducing HGPRT activity

Abstract

HERITABLE variants (which we shall refer to as mutants) occur in mammalian cells with high frequency, despite the apparently recessive phenotype of the variant and despite the diploid or heteroploid nature of the cell line. For example, the frequency of 8-azaguanine (8AG)-resistant mutants¹ or of hypoxanthine–guanine phosphoribosyl transferase (HGPRT) negative mutants² does not seem to decrease as an exponential function of the increase in ploidy. Explanations for this high frequency have ranged from the presence of regulatory mutations¹, to mechanisms such as non-disjunction or somatic recombination^2,3. If the phenotype was due to a dominant mutation, the high frequency of mutants would be expected. We explore this hypothesis here and present two experimental criteria for establishing the presence of dominant mutations which reduce enzyme activity: we restore activity by frameshift mutations or we transfer the mutant phenotype by fusing irradiated mutants with wild-type cells.