High-frequency mutation at the adenine phosphoribosyltransferase locus in Chinese hamster ovary cells due to deletion of the gene.

Evidence for a two-step model to explain the high-frequency expression of the recessive phenotype at the autosomal adenine phosphoribosyltransferase (APRT; EC 2.4.2.7) locus in Chinese hamster ovary (CHO) cells was given by Simon et al. [Simon, A. E., Taylor, M. W., Bradley, W. E. C. & Thompson, L. (1982) Mol. Cell. Biol. 2, 1126-1133]. This model proposed a high-frequency event, leading to allelic inactivation or a loss of gene function, and a low-frequency event, causing a structural alteration of the APRT protein. Either event could occur first, resulting in two classes of heterozygotes. We have analyzed the low-frequency event that gave rise to the class 2 aprt heterozygote D416 and the high-frequency event that led to APRT- cells derived from D416. Genomic Southern blots of Msp I- or Hpa II-digested DNA from wild-type CHO, aprt heterozygote D416, and two APRT- cell lines derived from D416 indicate a loss of a specific Msp I/Hpa II recognition sequence at one aprt locus in the heterozygote that correlates with the production of the electrophoretically altered APRT protein found in D416. The APRT- mutants are homozygous for the loss of this Msp I/Hpa II site. By using an additional CHO gene as an internal control, it was determined that the APRT- mutants contain only a single copy of the altered aprt gene. Thus, the high-frequency event that produces APRT- mutants derived from D416 is not an inactivation event but rather a deletion of the wild-type aprt gene.