Abstract
Azathioprine (Aza), a prodrug of 6‐mercaptopurine, is used in human medicine to prevent transplant rejection and for the treatment of autoimmune diseases. Extremely high HPRT lymphocyte mutant frequencies (MFs) are found in humans and mice chronically treated with Aza, and these elevated MFs appear to be caused by selection and amplification of pre‐existing HPRT mutant lymphocytes. In the present study, we investigated if in vivo selection by Aza also promotes the germ‐line transmission of Hprt mutants. Fifty‐five male C57BL/6 mice were treated with 10 mg/kg Aza three times/week for 24 weeks; 10 control mice were treated with the vehicle. Each of these males then was bred to unexposed females for a total of 8 weeks. Analysis of the Aza‐treated males after the breeding period indicated that 12 had highly elevated Hprt lymphocyte MFs (1 × 10^−4^–2.5 × 10^−1^ vs. normal MFs of <1 × 10^−5^), indicating that the Aza treatment successfully selected somatic cell mutants. The female offspring from the breeding were sacrificed at 28 days of age and Hprt MFs were measured in spleen lymphocytes. Most of the 364 female offspring (332 from Aza‐treated fathers) had Hprt MFs of 0–6 × 10^−6^, but seven of the offspring had moderately elevated MFs of 16 × 10^−6^–55 × 10^−6^. Since one of these mice was fathered by a control male, these relatively high MFs appear to be part of the normal variation in lymphocyte Hprt MF. The present results provide no evidence that long‐term Aza treatment promotes high levels of germ‐line Hprt mutation transmission in mice. Environ. Mol. Mutagen., 2007. Published 2007 Wiley‐Liss, Inc.