Evidence obtained by induced mutation frequency analysis for functional hemizygosity at theemt locus in CHO cells

Abstract

The ease of selection of autosomal recessive mutants from the established, quasi-diploid cell line, CHO, has led to the hypothesis that this line may have lost or inactivated one copy of many of its genes, resulting in functional hemizygosity. If this hypothesis is correct for CHO, it may be related to the long growth history of this particular line, and other Chinese hamster lines of more recent derivation ought to show much lower mutation frequencies. We have, therefore, examined five other established lines and a primary diploid fibroblast culture and compared their mutation frequencies to that of CHO. The recessive marker chosen for this study was resistance to the protein synthesis inhibitor emetine (Emtr). Our results show that while the frequencies of an X-linked (thioguanine resistance) or a dominant (ouabain resistance) mutation vary by no more than two- to fivefold, the frequency of Emtr is at least 30- to 130-fold higher in CHO than in the other Chinese hamster cells tested. These results are consistent with the hypothesis that CHO is functionally hemizygous at the emt locus, although a primary culture and the other cell lines tested are dizygous.