MGMT Expression in Murine Bone Marrow Is a Major Determinant of Animal Survival After Alkylating Agent Exposure

Abstract

Myelosuppression is commonly observed after alkylating agent chemotherapy due to low levels of O ⁶-alkylguanine DNA alkyltransferase protein (AGT) in hematopoietic progenitors. Mice that lack AGT in all organs, O ⁶-methylguanine-DNA methyltransferase gene knockout (MGMT^-/-) mice are extremely hypersensitive to the methylating agent N-methyl-N-nitrosourea(MNU) and exhibit a 10-fold reduction in the LD₉₀. To determine whether bone marrow damage was the cause of the increased lethality, we transplanted 1 × 10⁶ wild-type marrow into MGMT^-/- mice and MGMT^-/- marrow into wild-type mice and observed survival after MNU. Lethally irradiated MGMT^-/- mice given ≥25 mg/kg MNU 3 weeks after transplant of wild-type cells survived >30 days (n = 11), whereas this dose was lethal to control MGMT^-/- mice 9-12 days post treatment (n = 5). Conversely, lethally irradiated wild-type mice transplanted with MGMT^-/- cells died after only 20-60 mg/kg MNU within 8-12 days (n = 6). No significant toxicities were found in other organs. Additionally, in an in vivo post transplant competition model, wild-type long-term repopulating cells had a >200-fold competitive survival advantage over MGMT^-/- cells, and after MNU treatment completely repopulated the mouse when transplanted at only one-tenth the cell number. We also observed a strong selection for transplanted marrow-derived wild-type stromal elements in the MGMT^-/- background after drug treatment. These data indicate that alkylating agent hypersensitivity of MGMT^-/- mice results from hematopoietic damage at the stem level. Thus, DNA repair involving AGT in hematopoietic cells is required for normal host survival following exposure to methylating and chloroethylating agents.