Transgenic mice with integrated shuttle vectors containing the LacZ mutational target gene were used to study spontaneous mutational events in vivo. The transgenic mouse strain used carries the LacZ transgene on the X chromosome and was previously found to be characterized by ∼ 25-fold higher spontaneous mutation frequency in liver and brain compared with at least three other transgenic mouse strains. To determine the nature of in vivo spontaneous mutational events, 35 mutant LacZ genes isolated from liver and brain of mice from strain 35.5 were analyzed at the DNA sequence level. The results obtained indicate that single base-pair changes were predominant in both liver and brain. However, in liver the majority of mutations were transitions whereas in brain transversions were predominantly observed. Six mutants appeared to contain multiple dispersed mutations, separated by as much as 44 bp. Mutations were generally located within a 500 bp region encoding the active site of the β-galactosidase protein. Our results indicate that spontaneous mutations at the LacZ transgene are tissue specific and dependent on the chromosomal position of the LacZ transgene.
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