Modulation of O6-alkylating agent induced clastogenicity by enhanced DNA repair capacity of bone marrow cells

The murine bone marrow micronucleus assay has been used to examine (1) the potentiation of fotemustine and streptozotocin induced-clastogenicity by the O-6-alkylguanine-DNA alkyltransferase (ATase) inactivator O-6-benzylguanine (O-6-beG) and (2) the level of protection afforded against this potentiation by retrovirus-mediated expression of an O-6-beG-resistant mutant of human ATase (hATPA/GA) in mouse bone marrow. Both fotemustine and streptozotocin induced significantly higher levels of micronucleated polychromatic erythrocytes (p < 0.001 for the highest doses studied) compared to those seen in vehicle-treated animals. The number of micronuclei produced by either agent was dramatically elevated by pretreatment with O-6-beG (p < 0.001), Furthermore, in myeloablated mice reconstituted with bone marrow expressing the O-6-beG-resistant hATPA/GA as a result of retroviral gene transfer, the frequency of micronucleus formation following exposure of mice to otherwise clastogenic doses of fotemustine or streptozotocin, in the presence of O-6-beG, was highly significantly reduced (p < 0.001 for both agents) relative to that in mock transduced controls. These data clearly implicate O-6-chloroethyl- and O-6-methylguanine as clastogenic lesions in vivo and establish ATase as a major protective mechanism operating to reduce the frequency of such damage. The potentiation of drug induced clastogenicity by O-6-beG suggests that the clinical use of this inactivator in combination with O-6-alkylating agents, could substantially increase the risk of therapy related malignancy. Nevertheless the use of hATPA/GA as a protective mechanism via gene therapy may overcome this risk. (C) 1998 Elsevier Science B.V. All rights reserved.