A Novel BaEVRless-Pseudotyped γ-Globin Lentiviral Vector Drives High and Stable Fetal Hemoglobin Expression and Improves Thalassemic Erythropoiesis In Vitro

The Optimized γ-Globin Lentiviral Vector GGHI-mB-3D Leads to Nearly Therapeutic HbF Levels In Vitro in CD34+ Cells from Sickle Cell Disease Patients

We have previously demonstrated that both the original γ-globin lentiviral vector (LV) GGHI and the optimized GGHI-mB-3D LV, carrying the novel regulatory elements of the 3D HPFH-1 enhancer and the 3' β-globin UTR, can significantly increase HbF production in thalassemic CD34⁺ cells and ameliorate the disease phenotype in vitro. In the present study, we investigated whether the GGHI-mB-3D vector can also exhibit an equally therapeutic effect, following the transduction of sickle cell disease (SCD) CD34⁺ cells at MOI 100, leading to HbF increase coupled with HbS decrease, and thus, to phenotype improvement in vitro. We show that GGHI-mB-3D LV can lead to high and potentially therapeutic HbF levels, reaching a mean 2-fold increase to a mean value of VCN/cell of 1.0 and a mean transduction efficiency of 55%. Furthermore, this increase was accompanied by a significant 1.6-fold HbS decrease, a beneficial therapeutic feature for SCD. In summary, our data demonstrate the efficacy of the optimized γ-globin lentiviral vector to improve the SCD phenotype in vitro, and highlights its potential use in future clinical SCD trials.

Success Stories and Challenges Ahead in Hematopoietic Stem Cell Gene Therapy: Hemoglobinopathies as Disease Models

Gene therapy is a relatively novel field that amounts to around four decades of continuous growth with its good and bad moments. Currently, the field has entered the clinical arena with the ambition to fulfil its promises for a permanent fix of incurable genetic disorders. Hemoglobinopathies as target diseases and hematopoietic stem cells (HSCs) as target cells of genetic interventions had a major share in the research effort toward efficiently implementing gene therapy. Dissection of HSC biology and improvements in gene transfer and gene expression technologies evolved in an almost synchronous manner to a point where the two fields seem to be functionally intercalated. In this review, we focus specifically on the development of gene therapy for hemoglobin disorders and look at both gene addition and gene correction strategies that may dominate the field of HSC-directed gene therapy in the near future and transform the therapeutic landscape for genetic diseases.

Gene Therapy for Sickle Cell Disease - Moving from the Bench to the Bedside

Gene therapy as a potential cure for sickle cell disease (SCD) has long been pursued given that this hemoglobin disorder results from a single point mutation. Advances in genomic sequencing, increased understanding of hemoglobin regulation and discoveries of molecular tools for genome modification of hematopoietic stem cells have made gene therapy for SCD possible. Gene addition strategies using gene transfer vectors have been optimized over the last few decades to enable expression of normal or anti-sickling globins as strategies to ameliorate SCD. Many hurdles had to be addressed prior to clinical translation including collection of sufficient stem cells for gene-modification, increasing expression of transferred genes to a therapeutic level and conditioning patients in a safe manner that enabled adequate engraftment of gene-modified cells. The discovery of genome editors that make precise modifications has further advanced the safety and efficacy of gene therapy and a rapid movement to clinical trial has undoubtedly been supported by lessons learned from optimizing gene addition strategies. Current gene therapies being tested in clinical trial require significant infrastructure and expertise given the needs to harvest cells from and administer chemotherapy to patients who often have significant organ dysfunction and that gene-modification takes place ex vivo in specialized facilities. For these therapies to realize their full potential they would need to be portable, safe and efficient making an in-vivo based approach attractive. Additionally, adequate resources for SCD screening and access to standardized care are critically important for gene therapy to be a viable treatment option for SCD.

High level of fetal-globin reactivation by designed transcriptional activator-like effector

The fetal-to-adult hemoglobin switch has been a focus of a long-standing effort to potentially treat sickle cell disease and β thalassemia by induction of fetal hemoglobin. In a continuation of this effort, we designed specific transcriptional activator-like effectors (TALEs) to target both the Gγ and Aγ-globin promoters. We fused the TALEs to a LIM domain binding protein (Ldb1) dimerization domain, followed by a T2A green fluorescent protein (GFP) cassette, which were assembled into a lentiviral vector. To prevent deletions caused by the repeats of TALEs during the lentivirus packing process, we changed the TALE encoding DNA by codon optimization. Intriguingly, 5 of 14 TALEs showed forced reactivation of fetal-globin expression in human umbilical cord blood-derived erythroid progenitor (HUDEP-2) cells, with a significant increase in the γ-globin mRNA level by more than 70-fold. We also observed a more than 50% reduction of β-globin mRNA. High-performance liquid chromatography analysis revealed more than 30% fetal globin in TALE-induced cells compared with the control of 2%. Among several promoters studied, the β-globin gene promoter with the locus control region (LCR) enhancer showed the highest TALE expression during CD34 erythroid differentiation. At day 19 of differentiation, 2 TALEs increased fetal-globin expression more than 40-fold in the mRNA level and up to 70% of the total globin protein. These TALEs have potential for clinical translation.

Lentiviral Vector Pseudotypes: Precious Tools to Improve Gene Modification of Hematopoietic Cells for Research and Gene Therapy

Viruses have been repurposed into tools for gene delivery by transforming them into viral vectors. The most frequently used vectors are lentiviral vectors (LVs), derived from the human immune deficiency virus allowing efficient gene transfer in mammalian cells. They represent one of the safest and most efficient treatments for monogenic diseases affecting the hematopoietic system. LVs are modified with different viral envelopes (pseudotyping) to alter and improve their tropism for different primary cell types. The vesicular stomatitis virus glycoprotein (VSV-G) is commonly used for pseudotyping as it enhances gene transfer into multiple hematopoietic cell types. However, VSV-G pseudotyped LVs are not able to confer efficient transduction in quiescent blood cells, such as hematopoietic stem cells (HSC), B and T cells. To solve this problem, VSV-G can be exchanged for other heterologous viral envelopes glycoproteins, such as those from the Measles virus, Baboon endogenous retrovirus, Cocal virus, Nipah virus or Sendai virus. Here, we provide an overview of how these LV pseudotypes improved transduction efficiency of HSC, B, T and natural killer (NK) cells, underlined by multiple in vitro and in vivo studies demonstrating how pseudotyped LVs deliver therapeutic genes or gene editing tools to treat different genetic diseases and efficiently generate CAR T cells for cancer treatment.

Comparative analysis of lentiviral gene transfer approaches designed to promote fetal hemoglobin production for the treatment of β-hemoglobinopathies

β-Hemoglobinopathies are among the most common single-gene disorders and are caused by different mutations in the β-globin gene. Recent curative therapeutic approaches for these disorders utilize lentiviral vectors (LVs) to introduce a functional copy of the β-globin gene into the patient's hematopoietic stem cells. Alternatively, fetal hemoglobin (HbF) can reduce or even prevent the symptoms of disease when expressed in adults. Thus, induction of HbF by means of LVs and other molecular approaches has become an alternative treatment of β-hemoglobinopathies. Here, we performed a head-to-head comparative analysis of HbF-inducing LVs encoding for: 1) IGF2BP1, 2) miRNA-embedded shRNA (shmiR) sequences specific for the γ-globin repressor protein BCL11A, and 3) γ-globin gene. Furthermore, two novel baboon envelope proteins (BaEV)-LVs were compared to the commonly used vesicular-stomatitis-virus glycoprotein (VSV-G)-LVs. Therapeutic levels of HbF were achieved for all VSV-G-LV approaches, from a therapeutic level of 20% using γ-globin LVs to 50% for both IGF2BP1 and BCL11A-shmiR LVs. Contrarily, BaEV-LVs conferred lower HbF expression with a peak level of 13%, however, this could still ameliorate symptoms of disease. From this thorough comparative analysis of independent HbF-inducing LV strategies, we conclude that HbF-inducing VSV-G-LVs represent a promising alternative to β-globin gene addition for patients with β-hemoglobinopathies.

Hematopoietic stem cell gene therapy: The optimal use of lentivirus and gene editing approaches

Due to pioneering in vitro investigations on gene modification, gene engineering platforms have incredibly improved to a safer and more powerful tool for the treatment of multiple blood and immune disorders. Likewise, several clinical trials have been initiated combining autologous hematopoietic stem cell transplantation (auto-HSCT) with gene therapy (GT) tools. As several GT modalities such as lentivirus and gene editing tools have a long developmental path ahead to diminish its negative side effects, it is hard to decide which modality is optimal for treating a specific disease. Gene transfer by lentiviruses is the platform of choice for loss-of-mutation diseases, whereas gene correction/addition or gene disruption by gene editing tools, mainly CRISPR/Cas9, is likely to be more efficient in diseases where tight regulation is needed. Therefore, in this review, we compiled pertinent information about lentiviral gene transfer and CRISPR/Cas9 gene editing, their evolution to a safer platform for HSCT, and their applications on other types of gene disorders based on the etiology of the disease and cell fitness.