High-frequency structural gene deletion as the basis for functional hemizygosity of the adenine phosphoribosyltransferase locus in Chinese hamster ovary cells.

The CHO-AT3-2 Chinese hamster ovary cell line is functionally hemizygous for the adenine phosphoribosyltransferase (APRT; EC 2.4.2.7) locus. Class 1 APRT +/- heterozygotes, such as CHO-AT3-2, can be isolated at high spontaneous frequencies from wild-type CHO cell populations. Simon et al. [Simon, A. E., Taylor, M. W., Bradley, W. E. C. & Thompson, L. (1982) Mol. Cell. Biol. 2, 1126-1133] have proposed that a high-frequency event that inactivates one APRT allele might be responsible for both the spontaneous generation of class 1 APRT +/- heterozygotes and the high-frequency occurrence of APRT- mutants in class 2 APRT +/- heterozygote populations. This event appears to occur at only one of the two APRT alleles. To investigate the nature of this high-frequency event, and to determine the genetic basis for functional hemizygosity of the APRT locus in CHO-AT3-2 cells, we have mapped the APRT locus by using CHO-AT3-2-mouse somatic cell hybrids. Our data confirm that CHO-AT3-2 cells have a single functional APRT allele, which is located on the Z7 chromosome. Karyotypic analysis of CHO-AT3-2 revealed an interstitial deletion on the long arm of the Z4 chromosome, in the very region where the other APRT allele should be located. To determine whether the Z4q interstitial deletion had resulted in physical loss of the APRT gene, DNA from CHO-AT3-2-mouse cell hybrids that had either lost or retained the Z4q- chromosome was analyzed for the presence of CHO APRT coding sequences. Our data suggest that allele-specific high-frequency structural gene deletion events involving the long arm of chromosome Z4 are responsible for the spontaneous generation of functional hemizygosity at the APRT locus in CHO cells.