MODULATION OF O-6-ALKYLATING AGENT INDUCED CLASTOGENICITY BY ENHANCEDDNA-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-clastogeni city by the O-6-alkylguanine-DNA alkyltransferase (ATase) inactivator O-6-benzylguanine (O-6-beG) and (2) the level of protection afforded a gainst this potentiation by retrovirus-mediated expression of an O-6-b eG-resistant mutant of human ATase (hATPA/GA) in mouse bone marrow. Bo th fotemustine and streptozotocin induced significantly higher levels of micronucleated polychromatic erythrocytes (p < 0.001 for the highes t doses studied) compared to those seen in vehicle-treated animals. Th e number of micronuclei produced by either agent was dramatically elev ated by pretreatment with O-6-beG (p < 0.001), Furthermore, in myeloab lated mice reconstituted with bone marrow expressing the O-6-beG-resis tant hATPA/GA as a result of retroviral gene transfer, the frequency o f micronucleus formation following exposure of mice to otherwise clast ogenic doses of fotemustine or streptozotocin, in the presence of O-6- beG, was highly significantly reduced (p < 0.001 for both agents) rela tive 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 redu ce the frequency of such damage. The potentiation of drug induced clas togenicity by O-6-beG suggests that the clinical use of this inactivat or in combination with O-6-alkylating agents, could substantially incr ease the risk of therapy related malignancy. Nevertheless the use of h ATPA/GA as a protective mechanism via gene therapy may overcome this r isk. (C) 1998 Elsevier Science B.V. All rights reserved.