Mutant frequency and molecular analysis of in vivo lacI mutations in the bone marrow of Big Blue (R) rats treated with 7,12-dimethylbenz[a]anthracene

Recently, we evaluated lacl mutations in lymphocytes and mammary tissue of Big Blue (BB) rats exposed to 7,12-dimethylbenz[a]anthracene (DMBA). The re sults on the time course of mutant induction suggested that the lacl gene m ay manifest a tissuespecific increase in mutant frequency (MF). To test whe ther a tissue-specific increase in lacl MF is dependent on the cell prolife ration rate of a tissue, we examined rapidly proliferating bone marrow cell s for DMBA-induced loci mutations. Seven-week-old female BE rats were given single doses of 0, 20, and 130 mg/kg DMBA by gavage and the lad MFs in the bone marrow were measured over a period of 14 weeks following treatment. B one marrow cells had a remarkably low average background MF (3.1 +/- 1.6 x 10(-6) plaque-forming units) and the DMBA-induced loci MFs were significant ly higher than control MFs for both doses and at all time points (P < 0.01) . The lacl MF in the bone marrow increased for 2 weeks and then remained re latively constant; 20 and 130 mg/kg DMBA produced 34- and 106-fold increase s in MF over control MF. DNA sequencing revealed that the majority of DMBA- induced lacl mutations were bose-pair substitutions and that A:T -> T:A (48 %) and G:C -> T:A (24%) transversions were the predominant types. Thus, the different loci mutation fixation times observed for bone marrow (2 weeks), mammary(10 weeks), and lymphocytes (6 weeks) suggest that the loci gene ma nifests a tissuespecific mutation fixation time, which may depend on the ce ll proliferation rate of a tissue. In addition, the relatively low spontane ous MF in bone marrow compared with that in other tissues may be useful for increasing the sensitivity of the assay for detecting induced MFs in BE ra ts. Published 2000 Wiley-Liss, Inc.(dagger)