The Big Blue (R) transgenic mouse mutation detection assay: the mutation pattern of sectored mutant plaques

There are mutational artifacts in the Big Blue(R) assay and it is important to characterize the source and nature of these mutations. Differences were reported in the mutation patterns of a small sample of 23 sectored and 91 circular mutant plaques derived from skin using the Big Blue(R) transgenic mouse mutation detection system [G.R. Stuart, N.J. Gorelick, J.L. Andrews, J.G. de Beer, B.W. Glickman, The genetic analysis of lad mutations in secto red plaques from Big Blue(TM) transgenic mice, Environ. Mel. Mutagen 28 (19 96) 385-392.]. We have extended these observations by analyzing 46 sectored and 224 circular mutant plaques derived from seven tissues. The frequency of sectored mutant plaques is estimated to be 16% with no significant varia tion with tissue type. However, the patterns of mutation for sectored mutan ts and mouse-derived mutations differed significantly (p = 0.04). Base subs titutions in sectored mutant plaques do not show the asymmetries found in c ircular mutants consistent with integration of a GC rich transgene into the AT rich mammalian genome. Sectored mutants have mutation patterns consiste nt with a mixture of mouse, in vitro and Escherichia coli-derived mutations . Data on the relative frequencies of different mutant plaque morphologies suggests that overlapped plaques are substantially contaminated by sectored plaques at recommended plating densities. (C) 1999 Elsevier Science B.V. A ll rights reserved.