CONSIDERATION OF THE LIVER OF EMBRYONIC LACZ TRANSGENIC MICE AS AN ANALOG OF THE MOUSE COAT COLOR SPOT-TEST - PRELIMINARY DATA AND TECHNICAL PROBLEMS

Pregnant lacZ(+) transgenic mice (Muta(TM)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 th e mouse coat colour spot test. Day 10.5 of pregnancy represents the st age when the embryonic liver first becomes visually discernable. By da y 15.5, when the maternal and embryonic livers were analysed for lacZ( -) mutation frequency, the embryonic liver is the largest tissue in th e embryo (similar to 25 mg). These experiments therefore represent tre atment of a small pool of progenitor hepatocytes just as they are ente ring into an intense wave of cell division - optimum conditions for th e fixation of mutations. Exposure to ENU led to an average fourfold, a nd a maximum tenfold increase in mutation frequency in the embryonic l ivers. This relatively weak response is consistent with lacZ(-) mutant s not having a growth advantage; unlike in the mouse spot test, clonal amplification of lacZ(-) mutants cannot be separately scored. The lev el of mutation in the control embryonic livers was lower than that of the maternal control livers, but the group sizes were too small to con clude 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 no n-mutagenic to maternal livers. Half of the embryonic livers yielded D NA 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 beca use eight of 22 control embryonic livers behaved similarly. These prel iminary results suggest that further research is required in order to establish a practical transgenic analogue of the mouse coat colour spo t test.