INCREASING DNA-REPAIR METHYLTRANSFERASE LEVELS VIA BONE-MARROW STEM-CELL TRANSDUCTION RESCUES MICE FROM THE TOXIC EFFECTS OF 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA, A CHEMOTHERAPEUTIC ALKYLATING AGENT

The chloroethylnitrosourea (CNU) alkylating agents are commonly used f or cancer chemotherapy, but their usefulness is limited by severe bone marrow toxicity that causes the cumulative depletion of all hematopoi etic lineages (pancytopenia), Bone marrow CNU sensitivity is probably due to the inefficient repair of CNU-induced DNA damage; relative to o ther tissues, bone marrow cells express extremely Low levels of the O- 6-methylguanine DNA methyltransferase (MGMT) protein that repairs cyto toxic O-6-chloroethylguanine DNA lesions, Using a simplified recombina nt retroviral vector expressing the human MGMT gene under control of t he phosphoglycerate kinase promoter (PGK-MGMT) we increased the capaci ty of murine hone marrow-derived cells to repair CNU-induced DNA damag e. Stable reconstitution of mouse bone marrow with genetically modifie d, MGMT-expressing hematopoietic stem cells conferred considerable res istance to the cytotoxic effects of 1,3-bis (2-chloroethyl)-1-nitrosou rea (BCNU), a CNU commonly used for chemotherapy. Bone marrow harveste d from mice transplanted with PGK-MGMT-transduced cells showed extensi ve in vitro BCNU resistance. Moreover, MGMT expression in mouse bone m arrow conferred in vivo resistance to BCNU-induced pancytopenia and si gnificantly reduced BCNU-induced mortality due to bone marrow hypoplas ia. These data demonstrate that increased DNA alkylation repair in pri mitive hematopoietic stem cells confers multilineage protection from t he myelosuppressive effects of BCNU and suggest a possible approach to protecting cancer patients from CNU chemotherapy-related toxicity.