Spontaneous and induced chromosomal damage and mutations in Bloom Syndrome mice

Bloom Syndrome (BS) is characterized by both cancer and genomic instability, including chromosomal aberrations, sister chromosome exchanges, and mutations. Since BS heterozygotes are much more frequent than homozygotes, the issue of the sensitivity of heterozygotes to cancer is an important one. This and many other questions concerning the effects of BLM (the gene responsible for the BS) are more easily studied in mice than in humans. To gain insight into genomic instability associated with loss of function of BLM, which codes for a DNA helicase, we compared frequencies of micronuclei, somatic mutations, and loss of heterozygosity (LOH) in Blmtm3Brd homozygous, heterozygous, and wild-type mice carrying a cII transgenic reporter gene. It should be noted that the Blmtm3Brd is inserted into the endogenous locus with a partial duplication of the gene, so some function of the locus may be retained. The cII reporter gene was introduced from the Big Blue mouse by crossing them with Blmtm3Brd mice. All measurements were made on F2 mice from this cross. The reticulocytes of Blmtm3Brd homozygous mice had more micronuclei than heterozygous or wild-type mice (4.5, 2.7, and 2.5 per thousand, respectively; P < 0.01) but heterozygotes did not differ significantly from wild-type. Unlike spontaneous chromosome damage, spontaneous mutant frequencies did not differ significantly among homozygous, heterozygous, and wild-type mice (3.2 x 10(-5), 3.1 x 10(-5), and 3.1 x 10(-5), respectively; P > 0.05). Mutation measurements were also made on mice that had been treated with ethyl-nitrosourea (ENU) because Bloom Syndrome cells are sensitive to ethylating agents. The ENU-induced mutation frequency in Blmtm3Brd homozygous, heterozygous, and wild mice were 54 x 10(-5), 35 x 10(-5), and 25 x 10(-5) mutants/plaques, respectively. ENU induced more mutations in Blmtm3Brd homozygous mice than in wild-type mice (P < 0.01), but not significantly more in heterozygous mice (P = 0.06). Spontaneous LOH did not differ significantly among the genotypes, but ENU treatment induced much more LOH in Blmtm3Brd homozygous mice, as measured by means of the Dlb-1 test of Vomiero-Highton and Heddle. Hence, these Blmtm3Brd mice resemble Bloom Syndrome except that they have normal frequencies of spontaneous mutation. The fact that these mice have elevated rates of both cancer and chromosomal aberrations (as shown by more micronuclei and LOH) but normal rates of spontaneous mutation, shows the greater importance of chromosomal events than mutations in the origin of their cancers.