Pre-clinical Development of a Lentiviral Vector Expressing the Anti-sickling βAS3 Globin for Gene Therapy for Sickle Cell Disease

Preclinical efficacy of a modified gamma-globin lentivirus gene therapy in Berkeley sickle cell anemia mice and human xenograft models

We previously showed correction of sickle cell anemia (SCA) in mice utilizing a lentiviral vector (LV) expressing human γ-globin. Herein, we made a G16D mutation in the γ-globin gene to generate the G16D mutation (GbGM) LV to increase fetal hemoglobin formation. We also generated an insulated version of this LV, GbGMI, inserting a 36-bp insulator from the Foamy virus in the long terminal repeats of the LV. Preclinical batches of GbGM and GbGMI LV showed both were highly efficacious in correcting SCA in mice, with sustained gene transfer in primary transplanted SCA mice and high hematopoietic stem cell (HSC) transduction in colony-forming unit-spleen in secondary transplanted mice. CRISPR-mediated targeting of the proviruses into the LMO2 proto-oncogene showed remarkably reduced LMO2 activation by both insulated and uninsulated LV, compared to the SFFV γ-RV vector targeted to the same locus. We therefore used the GbGM LV to perform preclinical human CD34+ gene transfer. We assessed gene transfer and engraftment of human HSCs in two immunocompromised mouse models: persistent stable GbGM-transduced cell engraftment was comparable to that of untransduced cells with no detrimental effects on hematopoiesis up to 20 weeks post transplant. These robust preclinical studies in mouse and human HSCs allowed its translation into a clinical trial.

Pharmacological and pre-clinical safety profile of rSIV.F/HN, a hybrid lentiviral vector for cystic fibrosis gene therapy

RATIONALE AND OBJECTIVE

Cystic fibrosis (CF) is caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. CFTR modulators offer significant improvements, but ∼10% of patients remain nonresponsive or are intolerant. This study provides an analysis of rSIV.F/HN, a lentiviral vector optimised for lung delivery, including CFTR protein expression, functional correction of CFTR defects and genomic integration site analysis in preparation for a first-in-human clinical trial.

METHODS

Air-liquid interface cultures of primary human bronchial epithelial cells (HBECs) from CF patients (F508del/F508del), as well as a CFTR-deficient immortalised human lung epithelial cell line mimicking class I (CFTR-null) homozygous mutations, were used to assess transduction efficiency. Quantification methods included a novel proximity ligation assay for CFTR protein expression. For assessment of CFTR channel activity, Ussing chamber studies were conducted. The safety profile was assessed using integration site analysis and in vitro insertional mutagenesis studies.

RESULTS

rSIV.F/HN expressed CFTR and restored CFTR-mediated chloride currents to physiological levels in primary F508del/F508del HBECs as well as in a class I cells. In contrast, the latter could not be achieved by small-molecule CFTR modulators, underscoring the potential of gene therapy for this mutation class. Combination of rSIV.F/HN-CFTR with the potentiator ivacaftor showed a greater than additive effect. The genomic integration pattern showed no site predominance (frequency of occurrence ≤10%), and a low risk of insertional mutagenesis was observed in an in vitro immortalisation assay.

CONCLUSIONS

The results underscore rSIV.F/HN as a promising gene therapy vector for CF, providing a mutation-agnostic treatment option.

Cyclosporin H Improves the Transduction of CD34+ Cells with an Anti-Sickling Globin Vector, a Possible Therapeutic Approach for Sickle Cell Disease

Sickle cell disease (SCD) is a debilitating monogenic disease originating from mutations in the hemoglobin beta chain gene producing an abnormal hemoglobin HbS. The polymerization of HbS is responsible for the sickling of erythrocytes leading to anemia and vaso-occlusive events. Gene therapy is a promising treatment of SCD, and two different gene therapy drugs, using gene editing or gene transfer, have already reached the marketing stage. There is still a need to improve the efficacy of gene therapy in SCD, particularly when using anti-sickling beta-globin gene transfer strategies, which must outcompete the pathological HbS. One possibility is to increase transduction by inhibiting lentiviral restriction factors such as interferon-induced transmembrane proteins (IFITMs). This can be achieved by the addition of cyclosporin H (CsH) during the transduction process. This strategy was applied here in CD34+ hematopoietic progenitor and stem cells obtained from cord blood (CB). A first series of experiments with lentiviral vector coding for a green fluorescent protein (GFP) gene confirmed that the addition of CsH enhanced transgene expression levels and vector copy number per cell (VCN), while CD34+ cells remained viable and functional. Notably, the production of colony-forming cells (CFC) remained unaffected unless very high VCN values were reached. In a second step, CD34+ cells obtained from the CB of newborns with homozygous (n = 2) or heterozygous (n = 1) SCD mutations were transduced with the GLOBE-AS3 lentiviral vector coding for the HbAS3 anti-sickling beta globin. As with GFP, GLOBE-AS3 lentiviral transduction was clearly enhanced by CsH, leading to VCN > 2 and therapeutic levels of expression of the HbAS3. Moreover, the process did not affect the viability or functions of CFC. The combination of CB progenitors, the GLOBE-AS3 vector, and CsH is thus shown here to be a promising approach for the treatment of SCD.

Gene-environmental influence of space and microgravity on red blood cells with sickle cell disease

A fundamental question in human biology and for hematological disease is how do complex gene-environment interactions lead to individual disease outcome? This is no less the case for sickle cell disease (SCD), a monogenic disorder of Mendelian inheritance, both clinical course, severity, and treatment response, is variable amongst affected individuals. New insight and discovery often lie between the intersection of seemingly disparate disciplines. Recently, opportunities for space medicine have flourished and have offered a new paradigm for study. Two recent Nature papers have shown that hemolysis and oxidative stress play key mechanistic roles in erythrocyte pathogenesis during spaceflight. This paper reviews existing genetic and environmental modifiers of the sickle cell disease phenotype. It reviews evidence for erythrocyte pathology in microgravity environments and demonstrates why this may be relevant for the unique gene-environment interaction of the SCD phenotype. It also introduces the hematology and scientific community to methodological tools for evaluation in space and microgravity research. The increasing understanding of space biology may yield insight into gene-environment influences and new treatment paradigms in SCD and other hematological disease phenotypes.

Gene therapies on the horizon for sickle cell disease: a clinician's perspective

INTRODUCTION

Sickle cell disease (SCD) is a monogenic disorder that exerts several detrimental health effects on those affected, ultimately resulting in significant morbidity and early mortality. There are millions of individuals globally impacted by this disease. Research in gene therapy has been growing significantly over the past decade, now with two FDA approved products, aiming to find another cure for this complex disease.

AREAS COVERED

This perspective article aims to provide a clinician's insight into the current landscape of gene therapies, exploring the novel approaches, clinical advances, and potential impact on the management and prognosis of SCD. A comprehensive literature search encompassing databases such as PubMed, Web of Science and Google Scholar was employed. The search covered literature published from 1980 to 2024, focusing on SCD and curative therapy.

EXPERT OPINION

After careful evaluation of the risks and benefits associated with the use of gene therapy for affected patients, the need for a cure outweighs the risks associated with treatment in most cases of SCD. With advances in current technologies, gene therapies can increase access to cures for patients with SCD.

A novel high-titer, bifunctional lentiviral vector for autologous hematopoietic stem cell gene therapy of sickle cell disease

A major limitation of gene therapy for sickle cell disease (SCD) is the availability and access to a potentially curative one-time treatment, due to high treatment costs. We have developed a high-titer bifunctional lentiviral vector (LVV) in a vector backbone that has reduced size, high vector yields, and efficient gene transfer to human CD34+ hematopoietic stem and progenitor cells (HSPCs). This LVV contains locus control region cores expressing an anti-sickling βAS3-globin gene and two microRNA-adapted short hairpin RNA simultaneously targeting BCL11A and ZNF410 transcripts to maximally induce fetal hemoglobin (HbF) expression. This LVV induces high levels of anti-sickling hemoglobins (HbAAS3 + HbF), while concurrently decreasing sickle hemoglobin (HbS). The decrease in HbS and increased anti-sickling hemoglobin impedes deoxygenated HbS polymerization and red blood cell sickling at low vector copy per cell in transduced SCD patient CD34+ cells differentiated into erythrocytes. The dual alterations in red cell hemoglobins ameliorated the SCD phenotype in the SCD Berkeley mouse model in vivo. With high titer and enhanced transduction of HSPC at a low multiplicity of infection, this LVV will increase the number of patient doses of vector from production lots to decrease costs and help improve accessibility to gene therapy for SCD.

An empowered, clinically viable hematopoietic stem cell gene therapy for the treatment of multisystemic mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II), or Hunter syndrome, is a rare X-linked recessive lysosomal storage disorder due to a mutation in the lysosomal enzyme iduronate-2-sulfatase (IDS) gene. IDS deficiency leads to a progressive, multisystem accumulation of glycosaminoglycans (GAGs) and results in central nervous system (CNS) manifestations in the severe form. We developed up to clinical readiness a new hematopoietic stem cell (HSC) gene therapy approach for MPS II that benefits from a novel highly effective transduction protocol. We first provided proof of concept of efficacy of our approach aimed at enhanced IDS enzyme delivery to the CNS in a murine study of immediate translational value, employing a lentiviral vector (LV) encoding a codon-optimized human IDS cDNA. Then the therapeutic LV was tested for its ability to efficiently and safely transduce bona fide human HSCs in clinically relevant conditions according to a standard vs. a novel protocol that demonstrated superior ability to transduce bona fide long-term repopulating HSCs. Overall, these results provide strong proof of concept for the clinical translation of this approach for the treatment of Hunter syndrome.

Evaluation of Mono- and Bi-Functional GLOBE-Based Vectors for Therapy of β-Thalassemia by HBBAS3 Gene Addition and Mutation-Specific RNA Interference

Therapy via the gene addition of the anti-sickling βAS3-globin transgene is potentially curative for all β-hemoglobinopathies and therefore of particular clinical and commercial interest. This study investigates GLOBE-based lentiviral vectors (LVs) for βAS3-globin addition and evaluates strategies for an increased β-like globin expression without vector dose escalation. First, we report the development of a GLOBE-derived LV, GLV2-βAS3, which, compared to its parental vector, adds anti-sickling action and a transcription-enhancing 848-bp transcription terminator element, retains high vector titers and allows for superior β-like globin expression in primary patient-derived hematopoietic stem and progenitor cells (HSPCs). Second, prompted by our previous correction of HBBIVSI-110(G>A) thalassemia based on RNApol(III)-driven shRNAs in mono- and combination therapy, we analyzed a series of novel LVs for the RNApol(II)-driven constitutive or late-erythroid expression of HBBIVSI-110(G>A)-specific miRNA30-embedded shRNAs (shRNAmiR). This included bifunctional LVs, allowing for concurrent βAS3-globin expression. LVs were initially compared for their ability to achieve high β-like globin expression in HBBIVSI-110(G>A)-transgenic cells, before the evaluation of shortlisted candidate LVs in HBBIVSI-110(G>A)-homozygous HSPCs. The latter revealed that β-globin promoter-driven designs for monotherapy with HBBIVSI-110(G>A)-specific shRNAmiRs only marginally increased β-globin levels compared to untransduced cells, whereas bifunctional LVs combining miR30-shRNA with βAS3-globin expression showed disease correction similar to that achieved by the parental GLV2-βAS3 vector. Our results establish the feasibility of high titers for LVs containing the full HBB transcription terminator, emphasize the importance of the HBB terminator for the high-level expression of HBB-like transgenes, qualify the therapeutic utility of late-erythroid HBBIVSI-110(G>A)-specific miR30-shRNA expression and highlight the exceptional potential of GLV2-βAS3 for the treatment of severe β-hemoglobinopathies.

Prostaglandin E2 as transduction enhancer affects competitive engraftment of human hematopoietic stem and progenitor cells

Ex vivo gene therapy (GT) is a promising treatment for inherited genetic diseases. An ideal transduction protocol should determine high gene marking in long-term self-renewing hematopoietic stem cells (HSCs), preserving their repopulation potential during in vitro manipulation. In the context of the improvement of a clinically applicable transduction protocol, we tested prostaglandin E2 (PGE2) as a transduction enhancer (TE). The addition of PGE2 shortly before transduction of human CD34+ cells determined a significant transduction increase in the in vitro cell progeny paralleled by a significant reduction of their clonogenic potential. This effect increased with the duration of PGE2 exposure and correlated with an increase of CXCR4 expression. Blockage of CXCR4 with AMD3100 (plerixafor, Mozobil) did not affect transduction efficiency but partially rescued CD34+ clonogenic impairment in vitro. Once transplanted in vivo in a competitive repopulation assay, human CD34+ cells transduced with PGE2 contributed significantly less than cells transduced with a standard protocol to the repopulation of recipient mice, indicating a relative repopulation disadvantage of the PGE2-treated CD34+ cells and a counter-selection for the PGE2-treated cell progeny in vivo. In conclusion, our data indicate the need for risk/benefit evaluations in the use of PGE2 as a TE for clinical protocols of GT.

Gene Therapy for Hemoglobinopathies

β-Thalassemia and sickle cell disease are autosomal recessive disorders of red blood cells due to mutations in the adult β-globin gene, with a worldwide diffusion. The severe forms of hemoglobinopathies are fatal if untreated, and allogeneic bone marrow transplantation can be offered to a limited proportion of patients. The unmet clinical need and the disease incidence have promoted the development of new genetic therapies based on the engineering of autologous hematopoietic stem cells. Here, the steps of ex vivo gene therapy development are reviewed along with results from clinical trials and recent new approaches employing cutting edge gene editing tools.

Erythroid lineage-specific lentiviral RNAi vectors suitable for molecular functional studies and therapeutic applications

Numerous genes exert multifaceted roles in hematopoiesis. Therefore, we generated novel lineage-specific RNA interference (RNAi) lentiviral vectors, H23B-Ery-Lin-shRNA and H234B-Ery-Lin-shRNA, to probe the functions of these genes in erythroid cells without affecting other hematopoietic lineages. The lineage specificity of these vectors was confirmed by transducing multiple hematopoietic cells to express a fluorescent protein. Unlike the previously reported erythroid lineage RNAi vector, our vectors were designed for cloning the short hairpin RNAs (shRNAs) for any gene, and they also provide superior knockdown of the target gene expression with a single shRNA integration per cell. High-level lineage-specific downregulation of BCL11A and ZBTB7A, two well-characterized transcriptional repressors of HBG in adult erythroid cells, was achieved with substantial induction of fetal hemoglobin with a single-copy lentiviral vector integration. Transduction of primary healthy donor CD34+ cells with these vectors resulted in >80% reduction in the target protein levels and up to 40% elevation in the γ-chain levels in the differentiated erythroid cells. Xenotransplantation of the human CD34+ cells transduced with H23B-Ery-Lin-shBCL11A LV in immunocompromised mice showed ~ 60% reduction in BCL11A protein expression with ~ 40% elevation of γ-chain levels in the erythroid cells derived from the transduced CD34+ cells. Overall, the novel erythroid lineage-specific lentiviral RNAi vectors described in this study provide a high-level knockdown of target gene expression in the erythroid cells, making them suitable for their use in gene therapy for hemoglobinopathies. Additionally, the design of these vectors also makes them ideal for high-throughput RNAi screening for studying normal and pathological erythropoiesis.

Development of a double shmiR lentivirus effectively targeting both BCL11A and ZNF410 for enhanced induction of fetal hemoglobin to treat β-hemoglobinopathies

A promising treatment for β-hemoglobinopathies is the de-repression of γ-globin expression leading to increased fetal hemoglobin (HbF) by targeting BCL11A. Here, we aim to improve a lentivirus vector (LV) containing a single BCL11A shmiR (SS) to further increase γ-globin induction. We engineered a novel LV to express two shmiRs simultaneously targeting BCL11A and the γ-globin repressor ZNF410. Erythroid cells derived from human HSCs transduced with the double shmiR (DS) showed up to a 70% reduction of both BCL11A and ZNF410 proteins. There was a consistent and significant additional 10% increase in HbF compared to targeting BCL11A alone in erythroid cells. Erythrocytes differentiated from SCD HSCs transduced with the DS demonstrated significantly reduced in vitro sickling phenotype compared to the SS. Erythrocytes differentiated from transduced HSCs from β-thalassemia major patients demonstrated improved globin chain balance by increased γ-globin with reduced microcytosis. Reconstitution of DS-transduced cells from Berkeley SCD mice was associated with a statistically larger reduction in peripheral blood hemolysis markers compared with the SS vector. Overall, these results indicate that the DS LV targeting BCL11A and ZNF410 can enhance HbF induction for treating β-hemoglobinopathies and could be used as a model to simultaneously and efficiently target multiple gene products.

Recent advances in lentiviral vectors for gene therapy

Lentiviral vectors (LVs), derived from human immunodeficiency virus, are powerful tools for modifying the genes of eukaryotic cells such as hematopoietic stem cells and neural cells. With the extensive and in-depth studies on this gene therapy vehicle over the past two decades, LVs have been widely used in both research and clinical trials. For instance, third-generation and self-inactive LVs have been used to introduce a gene with therapeutic potential into the host genome and achieve targeted delivery into specific tissue. When LVs are employed in leukemia, the transduced T cells recognize and kill the tumor B cells; in β-thalassemia, the transduced CD34⁺ cells express normal β-globin; in adenosine deaminase-deficient severe combined immunodeficiency, the autologous CD34⁺ cells express adenosine deaminase and realize immune reconstitution. Overall, LVs can perform significant roles in the treatment of primary immunodeficiency diseases, hemoglobinopathies, B cell leukemia, and neurodegenerative diseases. In this review, we discuss the recent developments and therapeutic applications of LVs. The safe and efficient LVs show great promise as a tool for human gene therapy.

Gene therapy as the new frontier for Sickle Cell Disease

Sickle Cell Disease (SCD) is one of the most common monogenic disorders caused by a point mutation in the β-globin gene. This mutation results in polymerization of hemoglobin (Hb) under reduced oxygenation conditions, causing rigid sickle-shaped RBCs and hemolytic anemia. This clearly defined fundamental molecular mechanism makes SCD a prototypical target for precision therapy. Both the mutant β-globin protein and its downstream pathophysiology are pharmacological targets of intensive research. SCD also is a disease well-suited for biological interventions like gene therapy. Recent advances in hematopoietic stem cell (HSC) transplantation and gene therapy platforms, like Lentiviral vectors and gene editing strategies, expand the potentially curative options for patients with SCD. This review discusses the recent advances in precision therapy for SCD and the preclinical and clinical advances in autologous HSC gene therapy for SCD.

Genetic therapies for the first molecular disease

Sickle cell disease (SCD) is a monogenic disorder characterized by recurrent episodes of severe bone pain, multi-organ failure, and early mortality. Although medical progress over the past several decades has improved clinical outcomes and offered cures for many affected individuals living in high-income countries, most SCD patients still experience substantial morbidity and premature death. Emerging technologies to manipulate somatic cell genomes and insights into the mechanisms of developmental globin gene regulation are generating potentially transformative approaches to cure SCD by autologous hematopoietic stem cell (HSC) transplantation. Key components of current approaches include ethical informed consent, isolation of patient HSCs, in vitro genetic modification of HSCs to correct the SCD mutation or circumvent its damaging effects, and reinfusion of the modified HSCs following myelotoxic bone marrow conditioning. Successful integration of these components into effective therapies requires interdisciplinary collaborations between laboratory researchers, clinical caregivers, and patients. Here we summarize current knowledge and research challenges for each key component, emphasizing that the best approaches have yet to be developed.

Preclinical Evaluation of a Novel Lentiviral Vector Driving Lineage-Specific BCL11A Knockdown for Sickle Cell Gene Therapy

In this work we provide preclinical data to support initiation of a first-in-human trial for sickle cell disease (SCD) using an approach that relies on reversal of the developmental fetal-to-adult hemoglobin switch. Erythroid-specific knockdown of BCL11A via a lentiviral-encoded microRNA-adapted short hairpin RNA (shRNAmiR) leads to reactivation of the gamma-globin gene while simultaneously reducing expression of the pathogenic adult sickle β-globin. We generated a refined lentiviral vector (LVV) BCH-BB694 that was developed to overcome poor vector titers observed in the manufacturing scale-up of the original research-grade LVV. Healthy or sickle cell donor CD34+ cells transduced with Good Manufacturing Practices (GMP)-grade BCH-BB694 LVV achieved high vector copy numbers (VCNs) >5 and gene marking of >80%, resulting in a 3- to 5-fold induction of fetal hemoglobin (HbF) compared with mock-transduced cells without affecting growth, differentiation, and engraftment of gene-modified cells in vitro or in vivo. In vitro immortalization assays, which are designed to measure vector-mediated genotoxicity, showed no increased immortalization compared with mock-transduced cells. Together these data demonstrate that BCH-BB694 LVV is non-toxic and efficacious in preclinical studies, and can be generated at a clinically relevant scale in a GMP setting at high titer to support clinical testing for the treatment of SCD.

Blood rheology biomarkers in sickle cell disease

Sickle cell disease (SCD) is the most common inherited blood disorder, affecting approximately 100,000 patients in the U.S. and millions more worldwide. Patients with SCD experience a wide range of clinical complications, including frequent pain crises, stroke, and early mortality, all originating from a single-point mutation in the β-globin subunit. The RBC changes resulting from the sickle mutation lead to a host of rheological abnormalities that diminish microvascular blood flow, and produce severe anemia due to RBC hemolysis, and ischemia from vaso-occlusion initiated by sticky, rigid sickle RBCs. While the pathophysiology and mechanisms of SCD have been investigated for many years, therapies to treat the disease are limited. In addition to RBC transfusion, there are only two US Food and Drug Administration (FDA)-approved drugs to ameliorate SCD complications: hydroxyurea (HU) and L-glutamine (Endari™). The only curative therapy currently available is allogeneic hematopoietic stem cell transplantation (HSCT), which is generally reserved for individuals with a matched related donor, comprising only 10–15% of the total SCD population. Potentially curative advanced gene therapy approaches for SCD are under investigation in ongoing clinical trials. The ultimate goal of any curative treatment should be to repair the hemorheological abnormalities caused by SCD, and thus normalize blood flow and prevent clinical complications. Our mini-review highlights a set of key hemorheological biomarkers (and the current and emerging technologies used to measure them) that may be used to guide the development of novel curative and palliative therapies for SCD, and functionally assess outcomes.

Impact statement

Severe impairment of blood rheology is the hallmark of SCD pathophysiology, and one of the key factors predisposing SCD patients to pain crises, organ damage, and early mortality. As novel therapies emerge to treat or cure SCD, it is crucial that these treatments are functionally evaluated for their effect on blood rheology. This review describes a comprehensive panel of rheological biomarkers, their clinical uses, and the technologies used to obtain them. The described technologies can produce highly sensitive measurements of the ability of current treatments to improve blood rheology of SCD patients. The goal of curative therapies should be to achieve blood rheology biomarkers measurements in the range of sickle cell trait individuals (HbAS). The use of the panel of rheological biomarkers proposed in this review could significantly accelerate the development, optimization, and clinical translation of novel therapies for SCD.

Low-Dose Busulfan Reduces Human CD34+ Cell Doses Required for Engraftment in c-kit Mutant Immunodeficient Mice

Humanized animal models are central to efforts aimed at improving hematopoietic stem cell (HSC) transplantation with or without genetic modification. Human cell engraftment is feasible in immunodeficient mice; however, high HSC doses and conditioning limit broad use of xenograft models. We assessed human CD45⁺ chimerism after transplanting varying doses of human CD34⁺ HSCs (2 × 10⁵ to 2 × 10⁶ cells/mouse) with or without busulfan (BU) pretransplant conditioning in c-kit mutant mice that do not require conditioning (non-obese diabetic [NOD]/B6/severe combined immunodeficiency [SCID]/ interleukin-2 receptor gamma chain null (IL-2rγ^-/-) Kit^W41/W41 [NBSGW]). We then tested a range of BU (5-37.5 mg/kg) using 2 × 10⁵ human CD34⁺ cells. Glycophorin-A erythrocyte chimerism was assessed after murine macrophage depletion using clodronate liposomes. We demonstrated successful long-term engraftment of human CD34⁺ cells at all cell doses in this model, and equivalent engraftment using 10-fold less CD34⁺ cells with the addition of BU conditioning. Low-dose BU (10 mg/kg) was sufficient to allow human engraftment using 2 × 10⁵ CD34⁺ cells, whereas higher doses (≥37.5 mg/kg) were toxic. NBSGW mice support human erythropoiesis in the bone marrow; however, murine macrophage depletion provided only minimal and transient increases in peripheral blood human erythrocytes. Our xenograft model is therefore useful in HSC gene therapy and genome-editing studies, especially for modeling in disorders, such as sickle cell disease, where access to HSCs is limited.

Gene Therapy for Beta-Hemoglobinopathies: Milestones, New Therapies and Challenges

Inherited monogenic disorders such as beta-hemoglobinopathies (BH) are fitting candidates for treatment via gene therapy by gene transfer or gene editing. The reported safety and efficacy of lentiviral vectors in preclinical studies have led to the development of several clinical trials for the addition of a functional beta-globin gene. Across trials, dozens of transfusion-dependent patients with sickle cell disease (SCD) and transfusion-dependent beta-thalassemia (TDT) have been treated via gene therapy and have achieved reduced transfusion requirements. While overall results are encouraging, the outcomes appear to be strongly influenced by the level of lentiviral integration in transduced cells after engraftment, as well as the underlying genotype resulting in thalassemia. In addition, the method of procurement of hematopoietic stem cells can affect their quality and thus the outcome of gene therapy both in SCD and TDT. This suggests that new studies aimed at maximizing the number of corrected cells with long-term self-renewal potential are crucial to ensure successful treatment for every patient. Recent advancements in gene transfer and bone marrow transplantation have improved the success of this approach, and the results obtained by using these strategies demonstrated significant improvement of gene transfer outcome in patients. The advent of new gene-editing technologies has suggested additional therapeutic options. These are primarily focused on correcting the defective beta-globin gene or editing the expression of genes or genomic segments that regulate fetal hemoglobin synthesis. In this review, we aim to establish the potential benefits of gene therapy for BH, to summarize the status of the ongoing trials, and to discuss the possible improvement or direction for future treatments.