6-Methylmercaptopurine riboside resistance in human lymphocytes in the in vivo somatic cell mutation test.

Additional drug-resistance markers are being investigated to broaden the in vivo somatic cell mutation test in human lymphocytes (PBL). The adenosine kinase (AK) locus was chosen for study because Gupta and Singh [Gupta RS, Singh B: Mutat Res 113:441-454, 1983] have demonstrated that in Chinese hamster ovary cells, mutants affected at this locus are obtained at a very high spontaneous frequency and that the response of this locus to different types of mutagens was comparable to that of the hypoxanthine-guanine phosphoribosyl transferase locus. The adenosine analog 6-methylmercaptopurine riboside (MeMPR) was used as the selective agent for obtaining AK-deficient mutants. Cultures of mitogen-stimulated PBL were set up in the presence (test) and absence (control) of the selective agent. Resistant cells capable of synthesizing DNA in the presence of MeMPR were labeled with tritiated thymidine and enumerated autoradiographically. The variant frequency (Vf) was calculated as the ratio of the number of labeled nuclei in the test relative to that in the control. Human PBL were found to be sensitive to MeMPR inhibition of DNA synthesis and exhibited a dose-dependent decrease in Vf with increasing concentrations of MeMPR. However, no leveling off of the dose-response curve was observed. Thus the background level of Vf was probably lower than the practical detection limit of the test (4.0 X 10(-7) with a 50-ml blood sample). It was concluded that, because of the autosomal recessive nature of the AK gene, the background Vf in human PBL is too low to allow a useful baseline to be established for the in vivo somatic mutation test.