| The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) recognizes and repairs toxic and mutagenic damage induced by the clinically used alkylating agents temozolomide (TMZ) and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) by removing O6-alkylguanine lesions. AGT may also protect against toxicities and mutations incurred following cyclophosphamide (CP) by repairing lesions introduced by its metabolites acrolein and chloroacetaldehyde (CAA). The role of AGT in protecting from mutations introduced by TMZ, BCNU and CP in vivo is unknown. Of concern is the fact that these agents have been associated with therapy-induced leukemia, underscoring the importance of determining the role of AGT in removing mutagenic damage introduced by these agents. Additionally, in attempts to sensitize tumors over-expressing AGT to BCNU, clinical trials are currently underway administering the potent AGT inactivator O 6-benzylguanine (BG) prior to BCNU; however, the potential increase in mutations and long-term toxicities associated with this treatment are unknown.; To examine the role of AGT in protecting from in vivo toxicities and mutations induced by TMZ, BCNU, CP, acrolein and CAA, mutant frequencies were determined in the Hprt gene of splenic T-lymphocytes in C57BL/6 mice (Agt+/+ and AGT-/-) following an intraperitoneal injection of these agents. Agt-/- mice had significantly more mutations following TMZ than Agt+/+ mice, stressing Agt's importance in protecting against TMZ-induced mutations in vivo. Conversely, Agt status did not significantly alter mutation frequencies following BCNU, BG plus BCNU, CP, acrolein or CAA when administered at their respective MTD in Agt-/- mice.; Nf1+/- mice are more susceptible to alkylating agent-induced leukemia. In long-term toxicity and leukemia studies conducted in Nf1+/- mice, BG plus BCNU did not result in leukemia; however, mice treated with BG plus BCNU died significantly earlier than mice receiving BCNU alone, concomitant with severe pulmonary toxicities. These results support a role for Agt in preventing BCNU-induced pulmonary toxicities. Lastly, Agt deficiency did not alter long-term survival following CP. Unexpectedly, no leukemia and fewer tumors were found in Agt-/- mice following CP compared to Agt+/+ mice. These results suggest inactivation of AGT may increase mutations associated with TMZ, lung damage following BCNU and, conversely, enhance the efficacy of CP. |
