Therapeutic Effect of HOXB4-Expanded Stem Cells in Mice with Beta Thalassemia Given a Non-Myeloablative Conditioning Regimen.
Authors: Bakovic Silvia , Rosten Patricia M. , Eaves Connie J. , Humphries R. Keith 
Published: October 2019

Abstract
The ultimate promise of gene therapy for patients with hemoglobinopathies depends on the development of safe strategies for achieving 2 goals. One is to obtain efficient and permanent correction of the gene defect in autologous hematopoietic stem cells (HSCs). The second is to develop methods for the pre-transplant amplification of transduced HSCs to high levels to ensure that they will outcompete the large residual endogenous HSC population remaining in non-myeloablated hosts (e.g. previous experiments have shown that a minimum of ~5 × 106 normal adult mouse bone marrow (BM) cells (~500 HSC) is required to achieve a level of chimerism of 20% in mice given 200 cGy). The ability of HOXB4 to promote HSC self-renewal divisions in short term culture prior to their use as transplants offers an attractive approach to achieve this latter goal. As a first test we transduced day-4 5FU BM cells from normal mice with a MSCV-HOXB4-IRES-GFP or control MSCV-IRES-GFP virus and then transplanted the cells either before or after 7 days maintenance in vitro into normal recipients given 250 cGy. Mice transplanted with an estimated 50 HSCs immediately after transduction with either virus reached equivalent low levels of chimerism (~10%) showing that HOXB4 does not impart an in vivo selective growth advantage under sublethal conditions. After ex vivo culture, the GFP transduced cells yielded an even lower level of chimerism (~5%), in contrast recipients of cultured HOXB4-transduced cells attained much higher stable levels of lympho-myeloid chimerism (~50%), indicative of a marked expansion of the HSCs pre-transplant and their retention of robust competitive repopulating potential. We then applied this approach to a gene therapy model of severe β-thalassemia in mice bearing a homozygous deletion of the β-major globin gene (β-MDD). To model a transplant of genetically corrected cells, BM cells were harvested from day-4 5FU pre-treated congenic wild-type donors and transduced with the HOXB4 virus. Cells were then cultured for 10 days and the progeny of 200K starting cells transplanted into 3 β-MDD and 4 normal recipients given 200 cGy. Transplantation of 500K freshly harvested day-4 5FU BM cells into 4 similarly conditioned control mice failed to produce significant chimerism (1–3% at 5 months). In contrast, all 4 control recipients of ex vivo expanded HOXB4-transduced cells exhibited significant stable chimerism (21±6% at 5 months). Similar levels of chimerism were also achieved in all 3 β-MDD recipients (18–76%), one of which was sustained at 34% at 5 months (52% in the RBCs). This was associated with substantial improvement in the Hct (36% vs 23% in untreated β-MDD), Hb (10.5 vs 5 g/dl) and RBC morphology. Southern blot analyses performed on 53 individual in vitro-expanded myeloid colonies generated from FACS-selected GFP+ marrow cells from this mouse 2 months post-transplant showed 19 distinct integration patterns indicating reconstitution from polyclonal expanded HSCs. This conclusion was further confirmed by proviral integration site analyses, which identified 13 separate integration sites from 9 colonies that had unique proviral patterns. These data demonstrate the curative potential of ex vivo expanded HSCs in a preclinical model of β-thalassemia treated with non-myeloablative conditioning. They also underscore the potential of HOXB4 as a potent tool to achieve the HSC expansions required.