A NOVEL LACI TRANSGENIC MUTATION-DETECTION SYSTEM AND ITS APPLICATIONTO ESTABLISH BASE-LINE MUTATION FREQUENCIES IN THE SCID MOUSE

To assess DNA mutations in vivo, we have established a new transgenic mouse line, BC-1, carrying a lacI target gene for mutation detection w ithin a bacteriophage shuttle-vector. The lacI gene was positioned wit hin sequences derived from a rearranged murine immunoglobulin gene loc us, a feature that distinguishes the BC-1 transgene from other shuttle vector systems. As mutations in lacI transgenes likely reflect mutati ons occurring throughout the genome, these systems have been successfu lly used to investigate spontaneous and induced mutations in a variety of tissues, An important additional application of the transgenic sys tems is the characterization of lad mutations occurring in murine stra ins having specific DNA repair defects, For this study, scid (severe c ombined immunodeficiency) mice were selected as animals with this muta tion have a defect in double-strand DNA break repair. To determine wha t impact the scid mutation might have on spontaneous mutation frequenc ies within DNA recovered from various tissues, these mice were crossed with the BC-1 line. Interestingly, mutation frequencies within BC-1/s cid mouse DNA were not significantly different from those of BC-1 cont rol mice. Furthermore, spontaneous lad mutations obtained from BC-I an d from BC-1/scid liver DNA were similar in spectrum. As spontaneous BC -1 liver mutations were similar to those reported previously for other lacI systems, such as the Big Blue(R) transgenic line, this suggested that the nature of the DNA sequences flanking the reporter gene did n ot modify lad mutation rate or character.