Reduced Intensity BCNU/O6-BG Selection of MGMT-P140K Transduced Hematopoietic Stem Cells Is Efficient at Reduced Toxicity and Leads to an Unrestricted Oligoclonal Differentiation of Modified Murine Repopulating Stem Cells.

Successful selection of gene modified hematopoietic stem cells against the majority of non-modified cells may increase the efficiency and safety of clinical gene therapy. Especially if a reduction of the intensity of the myelotoxic pre-transplant conditioning treatment is sought, selection of the initially low percentage of retrovirally gene modified stem cells is required. In addition to the decreased drug related toxicity, such a procedure reduces the risk of genotoxicity caused by insertional mutagenesis simply by diminishing the likelihood of transplanting stem cells that carry unwanted insertion sides compared to strategies that attempt to increase efficiency by increasing vector dose and/or the number of engrafted gene-modified cells. To date, the mutant O⁶-methylguanine-DNA methyltransferase (MGMT) enzyme that confers resistance to nitrosoureas such as BCNU is the drug-resistance gene that allows most efficient selection at the stem cell level. In the murine model, MGMT selection by BCNU and O⁶-BG has mostly been performed at LD 50 dose levels of the selecting agents, that is associated with very considerable toxicity. We now established minimal dosage requirements of transplanted cells as well as of BCNU and O⁶-BG that allow an efficient selection of murine long-term hematopoiesis. Bone marrow cells from 5-FU treated C57 BL/6J mice were transduced with an MGMT/IRES/eGFP encoding retroviral vector and transplanted at two dilutions (1x10⁵ and 4x10⁵) into 84 lethally irradiated syngeneic recipient mice. Starting 4 weeks post-transplant, the mice were treated monthly with two reduced dosages of O⁶-BG and BCNU (either 10 mg/kg O⁶-BG and 2,5 mg/kg BCNU or 20 mg/kg O⁶-BG and 5 mg/kg BCNU) as compared to the commonly used regimens. Kinetics, differentiation and clonality of the transduced hematopoiesis were monitored by FACS and LAM-PCR analysis of serial peripheral blood samples over a total of 5 rounds of selection. 3 of 48 mice died during the selection procedure. Although in 97% of all mice GFP⁺ blood cells were detectable 4 weeks after transplantation, an efficient selection (>twofold increase in transduced cells) was only seen in mice with >3% GFP⁺ initial blood cells. The percentage of transduced hematopoiesis in 9 of 24 mice treated with the higher BCNU and O⁶-BG dosage increased at least twofold as compared to 5 of 24 treated with the lower dosage and 1 of 24 mice without selection. Using Poisson statistics, the frequency of selectable hematopoietic units was calculated to be as low as 1 per 5x10⁵ cells transplanted. Nevertheless, using LAM-PCR we detected stable oligoclonal hematopoiesis after transplantation of 4x10⁵ cells for more than 6 months, suggesting that a subfraction of repopulating cell clones present were not selectable, or that the contribution of individual clones was small. No differences in lineage differentiation of the GFP⁺ hematopoiesis as determined in lineage marker expression by FACS could be observed after selection as compared to unselected control mice. In summary, our results demonstrate that a reduced dosage of BCNU and O⁶-BG with lower toxicity allows selection of MGMT expressing murine hematopoietic stem cells with unperturbed multilineage differentiation potential. Moreover, under these conditions, a subfraction of transduced repopulating cell clones was selectable.