Abstract
Hypoxanthine-guanine phosphoribosyltransferase (HPRT) enzyme activities may be elevated in genetically unstable chromosome-mediated gene transferents selected for transfer of the HPRT gene. Increased levels of HPRT polypeptides in unstable mouse L cell gene transferents were demonstrated by two-dimensional gel electrophoresis and immunoprecipitation. No additional polypeptides were found to be overexpressed. HPRT mRNA levels were elevated 10- to 15-fold in the unstable gene transferent GT427C. Southern blot hybridization experiments showed that overexpression of HPRT correlated with a 5- to 15-fold amplification of HPRT gene sequences in two unstable cell lines. Stabilized gene transferents displayed reduced HPRT copy numbers. The amplification of HPRT gene sequences in the unstable transferent GT427C was associated with the presence of multiple minute chromosome fragments. An average of 9.6 fragments was found per metaphase, but the variation was considerable, ranging from 0 to 53. We conclude that genomic DNA sequences may be amplified in unstable chromosome-mediated gene transferents and that such amplification may be associated with the occurrence of multiple chromosomal fragments.