DNA-SEQUENCE ANALYSIS OF SPONTANEOUS MUTATIONS AT A LACZ TRANSGENE INTEGRATED ON THE MOUSE X-CHROMOSOME

Transgenic mice with integrated shuttle vectors containing the LacZ mu tational target gene were used to study spontaneous mutational events in vivo. The transgenic mouse strain used carries the LacZ transgene o n the X chromosome and was previously found to be characterized by app roximately 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 muta nt LacZ genes isolated from liver and brain of mice from strain 35.5 w ere 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 appea red to contain multiple dispersed mutations, separated by as much as 4 4 bp. Mutations were generally located within a 500 bp region encoding the active site of the beta-galactosidase protein. Our results indica te that spontaneous mutations at the LacZ transgene are tissue specifi c and dependent on the chromosomal position of the LacZ transgene.