SELECTION FOR G156A O-6-METHYLGUANINE DNA METHYLTRANSFERASE GENE-TRANSDUCED HEMATOPOIETIC PROGENITORS AND PROTECTION FROM LETHALITY IN MICETREATED WITH O-6-BENZYLGUANINE AND 1,3-BIS(2-CHLOROETHYL)-1-NITROSOUREA

A retroviral gene therapy approach was developed to protect early hema topoietic progenitors from 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) , a stem cell toxin, and O-6-benzylguanine (BG), an inhibitor of a key BCNU resistance protein, O-6-alkylguanine DNA alkyltransferase (AGT). The retroviral vector MFG was used to transfer the G156A MGMT (Delta MGMT) cDNA, encoding a mutant AGT that is resistant to inhibition by B G, into murine bone marrow-derived hematopoietic progenitors, Followin g transplantation into lethally irradiated mice, the transduced cells were subjected to in vivo BG and BCNU treatment to examine the ability to enrich for transduced cells expressing Delta AGT. Transplantation of Delta MGMT-transduced cells resulted in Delta AGT expression in 30% of bone marrow nucleated cells 13 weeks after transplantation, After one cycle of BG and BCNU, Delta AGT expression was observed in 60% of bone marrow cells, and the percentage of colony-forming units (culture ; CPU-C) containing proviral sequence increased from 67 to 100%. CFU-C obtained from BG and BCNU-treated Delta MGMT animals up to 23 weeks a fter transplantation were more resistant to combination BG and BCNU th an CFU-C from mice transplanted with lacZ-transduced cells and treated with BG and BCNU or from mice transplanted with Delta MGMT-transduced cells and left untreated, The degree of drug resistance in Delta MGMT -transduced hematopoietic progenitors to BG and BCNU was much greater than we observed previously with wild-type MGMT gene transfer and trea tment with BCNU alone, Furthermore, whereas 21 of 22 mice transplanted with Delta MGMT-transduced cells survived in vivo BG and BCNU adminis tration, only 3 of 13 mice transplanted with lacZ-transduced progenito rs survived similar drug treatment. Thus, Delta MGMT-transduced murine bone marrow cells selectively survive in vivo BG and BCNU exposure, r esulting in prolonged enrichment for the transduced cells and protecti on from mortality induced by this drug combination.