RELATIVE SENSITIVITY OF THE ENDOGENOUS HPRT GENE AND LACL TRANSGENE IN ENU-TREATED BIG BLUE(TM) B6C3F1 MICE

Three-week-old Big Blue(TM) (BE) B6C3F1 mice were given a single i.p. injection of ENU. Three weeks later, splenic T cells were isolated fro m each animal by ficoll gradient centrifugation and divided into two s amples. One sample was cultured to measure hprt(-) mutation and the ot her was used to extract DNA for lacl(-) analysis. T cells From BE mice exposed to 0, 4.5, 13.5, and 40 mg ENU/kg (9 or 10 animals per group) displayed dose-related increases in the frequency of both hprt(-) and lacl(-) mutations. Within each treatment group, the ENU-induced mutat ion frequency (average observed mutation frequency minus average contr ol frequency) was remarkably similar at the two loci. Th is suggests t hat treatments that increase mutation frequency at the endogenous hprt gene also produce similar incremental increases at the BB lad transge ne. However, because of the ten-fold higher spontaneous mutation rate at lad, the Fold-increase over background produced by ENU at this locu s was significantly less than the fold-increase produced at hprt. For example, the 4.5 mg ENU/kg treatment produced a 5.2-fold increase abov e background at hprt (P = 0.001), whereas only a 1.5-fold increase was produced at lacl (P = 0.140). Consequently, mutagenic insults that pr oduce up to a fivefold increase in mutation frequency at an endogenous locus may be difficult to detect at the lad transgene. Finally, the E NU-induced response at hprt in BE mice was identical to that in generi c B6C3F1 mice, suggesting that there are no inherent differences betwe en transgenic and normal mice in their response to this mutagenic agen t. (C) 1995 Wiley-Liss, Inc.