Gr. Douglas et al., SEQUENCE SPECTRA OF SPONTANEOUS LACZ GENE-MUTATIONS IN TRANSGENIC MOUSE SOMATIC AND GERMLINE TISSUES, Mutagenesis, 9(5), 1994, pp. 451-458
As a critical step in determining whether transgenic mouse gene mutati
on systems are suitable models for the detection and quantification of
induced gene mutations in vivo, spontaneous mutant frequencies and mu
tation spectra have been characterized for liver, bone marrow, and mal
e germ cells of the lacZ transgenic mouse strain 40.6. The lacZ transg
ene is carried on a recombinant bacteriophage lambda shuttle vector th
at is recovered from mouse genomic DNA, and analysed in vitro for muta
tions that occurred in the mouse tissues. Mutations are detected visua
lly as clear or pale blue plaques when X-gal is the substrate for beta
-galactosidase; whereas, the wild-type plaques are dark blue. There wa
s no statistical difference in the mutant frequency among the three ti
ssues studied, the pooled mutant frequency being 2.23 +/- 0.41 per 10(
5) pfu. The predominant type of mutation was GC --> AT transitions, wi
th most occurring in 5'-CpG dinucleotides, suggesting that the deamina
tion of 5-methylcytosine is the main mechanism of mutagenesis. There w
as, however, a statistically significant difference in the base pair s
ubstitution mutation spectrum for the liver and bone marrow when mutat
ions were grouped according to GC or AT base-pairs. The proportion of
transition versus transversion mutations was also statistically differ
ent among the three tissues, resulting mainly from the fact that germ
cells were different from both bone marrow and liver. A lower number o
f spontaneous transitions in male germ cells was accompanied by an inc
rease in transversions, with the proportion of GC --> AT transitions i
n 5'-CpG sites also declining. Only nine of the 66 independent mutatio
ns sequenced were deletions, ranging from 1 to 553 bp. The relatively
low proportion of deletions appears to reflect the deletion size limit
ations of bacteriophage-based systems. Most deletions had short repeat
or duplicate base sequences at their termini or were in monotonic bas
e repeats, which suggests strand slippage as the mechanism for their o
ccurrence.