Mutations causing deficiency of APRT in fibroblasts cultured from humans heterozygous for mutantAPRT alleles

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Abstract
Frequent mutation to adenine analog resistance in diploid human cells reflected heterozygosity for recessive alleles affecting expression of the adenine phosphoribosyltransferase (APRT) locus. Cells from both parents of APRT-deficient sibs were heterozygous and had rates of spontaneous mutation to 2,6-diaminopurine (DAP) resistance of 6.0 × 10−5 and 16 × 10−5 per cell generation. Spontaneous DAP-resistant mutants were not observed in cultures of homozygous cells. Almost all mutants of proven heterozygous cultures were APRT deficient and could not use adenine for growth. Frequent DAP-resistant mutations identified heterozygous strain 438, which carried an allele encoding a partially defective form of APRT. All DAP-resistant mutants of strain 438 were partially APRT deficient and could use adenine for growth. The frequency of MNNG-induced DAP-resistant mutants in homozygous strains was approximately the square of the induced frequency in heterozygous strains.