Consideration of the liver of embryonic lacZ transgenic mice as an analogue of the mouse coat colour spot test: preliminary data and technical problems.
Pregnant lacZ+ transgenic mice (Muta Mouse) were treated with ENU (25 mg/kg) by oral gavage on day 10.5 of pregnancy. This dose of ENU is the optimal dose observed by other investigators for activity in the mouse coat colour spot test. Day 10.5 of pregnancy represents the stage when the embryonic liver first becomes visually discernable. By day 15.5, when the maternal and embryonic livers were analysed for lacZ- mutation frequency, the embryonic liver is the largest tissue in the embryo (approximately 25 mg). These experiments therefore represent treatment of a small pool of progenitor hepatocytes just as they are entering into an intense wave of cell division--optimum conditions for the fixation of mutations. Exposure to ENU led to an average fourfold, and a maximum tenfold increase in mutation frequency in the embryonic livers. This relatively weak response is consistent with lacZ- mutants not having a growth advantage; unlike in the mouse spot test, clonal amplification of lacZ- mutants cannot be separately scored. The level of mutation in the control embryonic livers was lower than that of the maternal control livers, but the group sizes were too small to conclude this definitively. Embryonic livers from each individual mother showed a range of mutation frequencies that were not obviously related to that of the maternal liver. On a treatment group basis, ENU was non-mutagenic to maternal livers. Half of the embryonic livers yielded DNA that failed to package, despite repeated attempts and re-isolation of the DNA from the liver. The cause of this unexpected finding is not clear. What is clear is that it was not due to the ENU treatment because eight of 22 control embryonic livers behaved similarly. These preliminary results suggest that further research is required in order to establish a practical transgenic analogue of the mouse coat colour spot test.
Morrison
V
V
Zeneca Central Toxicology Laboratory, Macclesfield, Cheshire, UK.
Tinwell
H
H
Ashby
J
J
D000818:Animals; D004252:DNA Mutational Analysis; D005038:Ethylnitrosourea / Q000494:pharmacology; D005260:Female; D007763:Lac Operon; D008099:Liver / Q000187:drug effects / Q000196:embryology* / Q000201:enzymology; D051379:Mice; D008822:Mice, Transgenic; D009152:Mutagenicity Tests; D009154:Mutation; D011247:Pregnancy
D005038:Ethylnitrosourea