title: Mechanism of mutation at the aprt locus in Chinese hamster ovary cells: analysis of heterozygotes and hemizygotes.
abstract: A two-step model to explain the high frequency of mutation at the diploid adenine phosphoribosyltransferase (aprt) locus in CHO cells has been proposed previously (Simon et al., Mol. Cell. Biol. 2:1126-1133, 1982). This model indicates that two distinct classes of aprt heterozygotes can be isolated. Class 1 heterozygotes, the most abundant class, were defined as those which arose spontaneously and were capable of undergoing mutation to the APRT- phenotype only at a low frequency (putative point mutation). Class 2 heterozygotes arose from a mutation and gave rise at a high frequency to APRT- cells. This high-frequency event has been identified as a deletion of the wild-type allele (A. E. Simon and M. W. Taylor, Proc. Natl. Acad. Sci. U.S.A. 80:810-814, 1983). In this paper we report further analysis of class 1 heterozygotes with respect to genetic structure, gene products, and karyotype. Our study indicated that class 1 heterozygotes contain two different types of mutants. About half have only one copy of the aprt gene and an unaltered karyotype, indicating that a deletion (similar to the high-frequency second-step event observed for class 2 heterozygotes) rather than a loss of the chromosome was responsible for the generation of the aprt+/- genotype. The remainder of the previously designated class 1 heterozygotes still contained two copies of the aprt gene (within the limits of the quantitation technique used) and arose presumably by a point mutation. One of this group, D423, was characterized with respect to aprt gene products and found to produce an electrophoretic variant in addition to the wild-type protein. APRT- mutants derived from D423 retained the same number of aprt gene copies as D423 and still synthesized a protein that comigrated with wild type, unlike APRT- mutants derived from class 2 heterozygotes. D423 and the other heterozygotes with two aprt genes therefore did not fit into either class 1 or 2 and are now designated class 3. The model we present suggests that only one of the two aprt alleles present in wild-type cells can undergo the deletion.
mesh_terms: D015075:2-Aminopurine / Q000031:analogs & derivatives; D000225:Adenine / Q000031:analogs & derivatives / Q000494:pharmacology; D000228:Adenine Phosphoribosyltransferase / Q000235:genetics*; D000818:Animals; D002460:Cell Line; D002872:Chromosome Deletion; D006224:Cricetinae; D003412:Cricetulus; D004351:Drug Resistance; D005260:Female; D005347:Fibroblasts / Q000187:drug effects; D006579:Heterozygote; D008957:Models, Genetic; D009154:Mutation; D010053:Ovary; D010430:Pentosyltransferases / Q000235:genetics*
chemical_list: D015075:2-Aminopurine; C007300:2,6-diaminopurine; D010430:Pentosyltransferases; D000228:Adenine Phosphoribosyltransferase; D000225:Adenine; C006654:8-azaadenine