Development of a New Generation, Forward-Oriented Therapeutic Vector for Hemoglobin Disorders

Ameliorating hemoglobin disorders such as sickle cell disease (SCD) using hematopoietic stem cell (HSC) gene therapy is under development. Unlike in other diseases, therapeutic globin vectors have demanding requirements including high-level β-globin expression, tissue specificity among erythroid cells, long-term persistence, and high-level modification at the HSC level. These demanding requirements necessitate the inclusion of complex genetic elements including the locus control region (LCR), β-globin promoter, β-globin gene, and the 3' untranslated region (3'UTR), all now feasible using lentiviral vectors. The additional requirement of intron 2 for high-level β-globin expression dictates a reverse-oriented globin-expression cassette to prevent loss by RNA splicing during viral preparation. This reverse-orientation is in contrast to all other therapeutic vectors under clinical development. Current reverse-oriented globin vectors can drive phenotypic correction in mouse models for both b-thalassemia and SCD, while they are limited by lower viral titers and lower transduction efficiency in primary human HSCs, limiting their prospects, especially in SCD. We hypothesized that the reverse-orientation impedes both viral preparation and vector transduction, as despite deletion of cryptic polyadenylation (polyA) signals to optimize a conventional reverse-oriented globin vector, titers were still 10-fold lower than a standard GFP-vector.

We thus designed a forward-oriented globin-expressing vector, which was further optimized by minimizing the size of the LCR, inclusion of a large segment of the β-globin promoter and an enhancer region of the 3'UTR lacking the polyA signal. Viral titers of the forward-oriented vectors (1.0±0.2x10e9 IU/mL) were 6-fold higher than the optimized vector in the reverse orientation (1.6±0.2x10e8 IU/mL, p<0.01), and comparable to a standard GFP-marking vector (1.9±0.2x10e9 IU/mL, p<0.01). The forward-oriented vector demonstrated 3-4 fold higher transduction efficiency among human erythroid cells derived from transduced CD34⁺ cells in in vitro culture (34±0% vs 13±1%, p<0.01) and in xenografted mice (31±9% vs 7±5%, p<0.05).

To evaluate transduction efficiency for long-term HSCs, we transduced rhesus CD34⁺ cells with our optimized reverse-oriented vector and our forward-oriented vector including GFP or YFP genes (instead of β-globin gene) in a competitive repopulation assay following 10 Gy total body irradiation. In two animals, GFP and YFP signals from both vectors were detected exclusively in red blood cells, documenting tissue specificity. Gene marking levels were 10-fold higher out to 4 years with the forward-oriented vector, compared to the reverse-oriented vector, and were comparable to standard GFP or YFP-marking vectors in 2 other animals.

We then replaced the GFP gene with the β-globin gene containing intron 2 in the forward-oriented vector construct. To positively select intron-2-containing β-globin vectors, essential viral components (packaging signal, rev response element (RRE), or central polypurine tract (cPPT)) were deleted in the backbone of the forward-oriented vector, and the deleted viral components were inserted into intron 2 of the β-globin gene. We observed that half of the forward-oriented vectors lost intron 2 during vector preparation when no elements were included into intron 2. Insertion of the RRE resulted in positive selection of intron-2-containing β-globin vectors. We confirmed β-globin expression from the forward-oriented vector by hemoglobin A production in human erythroid cells derived from transduced peripheral blood mononuclear cells from SCD patients. Finally, human β-globin expression was detected in rhesus erythroid cells following transplantation of transduced CD34⁺ cells in 2 animals.

In summary, we have developed a clinically relevant forward-oriented globin-expressing vector, which has 6 fold higher viral titers and 4-10 fold higher transduction efficiency for hematopoietic repopulating cells, as compared to the optimized reverse-oriented vector. RRE insertion allowed positive selection of intron-2-containing β-globin vectors, and human β-globin production was observed in transplanted rhesus macaques with the forward-oriented β-globin vector transduction. These findings bring us closer to a curative gene therapy for hemoglobin disorders.