Cyclophosphamide-induced in vivo sister chromatid exchange in Mus Musculus. II: Effect of age and genotype on sister chromatid exchange, micronuclei and metaphase index.

In vivo cyclophosphamide-induced sister chromatid exchanges (SCEs) micronuclei, and metaphase indices were assessed in two age groups (10.8 +/- 0.9 weeks' an 33.1 +/- 1.3 weeks' old) of female mice from three genetic strains (C3H/S, C57BL/6J, and Balb/c). In general, older animals showed diminished SCE induction over their younger counterparts. The relative difference between individuals of the two ages is strain-dependent. Unlike C57BL/6J and Balb/c, strain C3H/S showed significantly lower SCE values in the older animals at every cyclophosphamide treatment. It may reflect on the possible involvement of genetic determinant(s) for the component(s) of SCE formation during aging. Frequencies of micronuclei, however, were consistently higher in older animals than in their younger counterparts. Furthermore, cytotoxicity of cyclophosphamide, as reflected in metaphase indices, was also higher in older animals. Lower metaphase indices associated with higher micronuclei levels in older individuals may suggest a decline in the rate of cellular replication in these animals. Furthermore, the lower metaphase indices associated with lower SCE values, and increasing micronuclei levels accompanied by decreasing SCE frequencies in older animals, may reflect reduced DNA repair ability during aging. These results support the hypothesis of genotype-dependent decline in the rate of DNA repair and replication during aging, particularly under stressed conditions.