Truncated Erythropoietin Receptors Confer an In Vivo Selective Advantage in Gene-Modified Erythroid Cells Expressing Fetal Hemoglobin Due to BCL11A Interference

Hematopoietic stem cell gene therapy for hemoglobin disorders, such as sickle cell disease, requires high-level gene marking and robust therapeutic globin expression in erythroid cells (>20% of γ- or β-globin production) for widespread successful clinical application. We previously demonstrated that lentiviral transduction of a truncated human erythropoietin receptor (thEpoR) gene allows for erythropoietin-dependent selective proliferation of gene-modified human erythroid cells during in vitro differentiation (ASH 2017). In this study, we sought to evaluate whether thEpoR can enhance the phenotypic effect of a therapeutic vector in erythroid cells in xenograft mouse and autologous non-human primate transplantation models.

To investigate this hypothesis, we designed lentiviral vectors encoding both thEpoR and BCL11A-targeting micro RNA-adapted short hairpin RNA (shmiBCL11A), driven off an erythroid specific ankyrin 1 (ANK1) promoter. Both selective proliferation and high-level fetal hemoglobin (HbF) induction were observed in in vitro erythroid differentiation cultures using transduced human CD34+ cells. Healthy donor CD34+ cells were transduced with shmiBCL11A vector, thEpoR-shmiBCL11A vector, and GFP vector (control). Transduced cells were transplanted into immunodeficient NBSGW mice. Five months post-transplant, xenograft bone marrow cells were evaluated for human cell engraftment (human CD45+) and vector copy number (VCN) in both human CD34+ progenitor cells and glycophorin A+ (GPA+) erythroid cells. HbF production was also measured in GPA+ erythroid cells by reverse phase HPLC. We observed efficient transduction in transduced CD34+ cells in vitro (VCN 2.1-5.1) and similar human cell engraftment among all groups (84-89%). The VCN with thEpoR-shmiBCL11A transduction was 3-fold higher in human erythroid cells when compared to CD34+ cells (p<0.01), but not with shmiBCL11A or GFP vectors. HbF levels were significantly elevated in thEpoR-shmiBCL11A vector (43±6%, p<0.01) when compared to no transduction control (1±0%), but not for either shmiBCL11A vector (3±1%) or GFP vector (1±0%). These data demonstrate selective proliferation of gene-modified erythroid cells, as well as enhanced HbF induction with thEpoR-shmiBCL11A transduction.

We then performed autologous rhesus CD34+ cell transplantation using either shmiBCL11A vector (142562 and RA0706, n=2, compared to a GPA promoter-derived shmiBCL11A vector) or thEpoR-shmiBCL11A vector (ZL50 and ZM24, n=2, compared to a Venus-encoding vector). Transduced CD34+ cells were transplanted into autologous rhesus macaques following 2x5Gy total body irradiation. Efficient transduction was observed in CD34+ cells in vitro among all 4 macaques (VCN 3.8-8.7) using a high-density culture protocol (Uchida N, Mol Ther Methods Clin Dev. 2019). In shmiBCL11A transduction animals, engraftment of gene-modified cells (VCN 0.2-1.0) and robust HbF induction (14-16%) were observed 1 month post-transplant. However, VCN and HbF levels were reduced down to VCN ~0.1 and HbF ~0.4% in both animals 6 months post-transplant. In contrast, a thEpoR-shmiBCL11A transduction animal (ZL50) resulted in engraftment of gene-modified cells (VCN 0.8-1.0) and robust HbF induction (~18%) 1 month post-transplant, with both gene marking and HbF levels remaining high at VCN 0.6-0.7 and HbF ~15% 4 months post-transplant. These data suggest that shmiBCL11A transduction results in transient HbF induction in gene-modified erythroid cells, while thEpoR-based selective advantage allows for sustained HbF induction with shmiBCL11A.

In summary, we developed erythroid-specific thEpoR-shmiBCL11A expressing vectors, enhancing HbF induction in gene-modified erythroid cells in xenograft mice and rhesus macaques. While further in vivo studies are desirable, the use of thEpoR appears to provide a selective advantage for gene-modified erythroid cells in gene therapy strategies for hemoglobin disorders.