Micronucleated erythrocyte frequency in control and azidothymidine-treated Tk+/+, Tk+/− and Tk−/− mice

The first step in the activation of the anti-retroviral nucleoside analogue azidothymidine (AZT) involves its conversion to a 5′-monophosphate. In this study, we have evaluated the role of cytosolic thymidine kinase (Tk), the major enzyme involved in phosphorylating thymidine and its analogues, in the nuclear DNA damage produced by AZT in neonatal mice. Tk+/+, Tk+/− and Tk−/− mice were treated intraperitoneally with 200 mg/kg/day of AZT on postnatal days 1 through 8, and micronuclei were measured in peripheral blood 24 h after the last dose. AZT treatment increased the micronucleus (MN) frequencies to similar extents in both the reticulocytes (RETs) and normochromatic erythrocytes (NCEs) of Tk+/+ and Tk+/− mice; AZT did not increase the frequency of micronucleated RETs (MN-RETs) or micronucleated NCEs (MN-NCEs) in Tk−/− mice. Unexpectedly, neonatal Tk−/− mice treated with the vehicle had significantly elevated MN frequencies for both RETs and NCEs relative to Tk+/+ and Tk+/− mice (e.g., ∼3.4% MN-RETs and ∼4.8% MN-NCEs in Tk−/− mice versus ∼0.7 and ∼ 0.6% MN-RETs and MN-NCEs in neonatal Tk+/+ mice). Additional assays performed on untreated Tk−/− mice showed that elevated spontaneous MN frequencies persisted until at least 20 weeks of age, which approaches the average lifespan of Tk−/− mice. These results indicate that metabolism by Tk is necessary for the genotoxicity of AZT in neonatal mice; however, the genotoxicity of AZT is not altered by reducing the Tk gene dose by half. The elevated spontaneous MN frequencies in Tk−/− mice suggest the presence of an endogenous genotoxic activity in these mice.