Correction of sickle cell disease in transgenic mouse models by gene therapy

Advances in Gene Therapy for Sickle Cell Disease: From Preclinical Innovations to Clinical Implementation and Access Challenges

Sickle cell disease (SCD) is a hereditary blood disorder caused by a specific mutation in the β-globin gene, leading to the production of hemoglobin S, which deforms red blood cells, causing occlusion in small blood vessels. This results in pain, anemia, organ damage, infections, and increased stroke risk. Treatment options, including disease-modifying therapies and curative hematopoietic stem cell transplants, have limited accessibility. Recently, autologous gene therapy has emerged as a promising curative option, particularly for SCD. Gene editing techniques such as CRISPR, base editing, and prime editing offer potential to correct this mutation. In this review, we discuss recent preclinical studies and clinical trials of gene and cell therapies, focusing on the progress of FDA-approved treatments like Lyfgenia and Casgevy. We also examine the many challenges, including accessibility, safety, and long-term efficacy, which continue to shape the future of SCD gene therapy.

Current and future treatments for sickle cell disease: From hematopoietic stem cell transplantation to in vivo gene therapy

Sickle cell disease (SCD) is a single-gene disorder caused by a point mutation of the β-globin gene, resulting in hemolytic anemia, acute pain, multiorgan damage, and early mortality. Hydroxyurea is a first-line drug therapy that switches sickle-globin to non-pathogenic γ-globin; however, it requires lifelong oral administration. Allogeneic hematopoietic stem cell (HSC) transplantation allows for a one-time cure for SCD, albeit with histocompatibility limitations. Therefore, autologous HSC gene therapy was developed to cure SCD in a single treatment, without HSC donors. Current HSC gene therapy is based on the ex vivo culture of patients' HSCs with lentiviral gene addition and gene editing, followed by autologous transplantation back to the patient. However, the complexity of the treatment process and high costs hinder the universal application of ex vivo gene therapy. Therefore, the development of in vivo HSC gene therapy, where gene therapy tools are directly administered to patients, is desirable to provide a more accessible, cost-effective solution that can cure SCD worldwide. In this review, we discuss current treatments, including drug therapies, HSC transplantation, and ex vivo gene therapy; the development of gene therapy tools; and progress toward curative in vivo gene therapy in SCD.

Ex vivo modification of hematopoietic stem and progenitor cells for gene therapy

The development of viral vectors has been particularly critical for genetic therapies of hematological diseases. Before the development of retrovirus vectors (RVVs), gene transfer into mammalian cells was accomplished by transduction of DNA plasmids by chemical means and later by electroporation. The main limitation of these methods is the inefficiency of transfer of intact sequences, and particularly with electroporation significant cell death of the manipulated cells. The earliest successful human gene therapy trials utilized γ-RVVs and many of the techniques developed in the 1980s. A breakthrough for the field was the exploitation and development of HIV for transfer vectors, termed lentivirus vectors. In this review, we highlight uses of retro- and lentivirus vectors in monogenic diseases in which hematopoietic stem cells are used in the autologous setting to treat immunodeficiencies, hemoglobinopathies and metabolic diseases. The three authors' perspective represent experiences in the field over four decades that encompasses both basic translational research and development and oversight of early and ongoing gene therapy trials utilizing viral vectors.

Gene therapy for β-thalassemia: current and future options

Beta-thalassemia is a severe, hereditary blood disorder characterized by anemia, transfusion dependence, reduced life expectancy, and poor quality of life. Allogeneic transplantation of hematopoietic stem cells (HSCs) is the only curative treatment for transfusion-dependent β-thalassemia, but a lack of compatible donors prevents the use of this approach for most patients. Over the past 20 years, the rise of gene therapy and the development of lentiviral vectors and genome-editing tools has extended curative options to a broader range of patients. Here, we review breakthroughs in gene addition- and genome-editing-based therapies for β-thalassemia, the clinical outcomes enabling approval by regulatory agencies, and perspectives for further development.

Gene therapy in transfusion-dependent non-β0/β0 genotype β-thalassemia: first real-world experience of beti-cel

ABSTRACT

Gene addition and editing strategies for transfusion-dependent β-thalassemia have gained momentum as potentially curative treatment options, with studies showcasing their efficacy and safety. We report, to our knowledge, the first real-world application of betibeglogene autotemcel (beti-cel; Zynteglo) during its period of active license in Europe from January 2020 to March 2022 for patients aged ≥12 years without a β0/β0 genotype and without a HLA-matched sibling donor, before beti-cel marketing authorization was withdrawn by its holder because of nonsafety reasons. Among 15 screened patients, 4 opted out for fertility and safety concerns, 2 were excluded because of marked hepatic siderosis, and 1 had apheresis collection failure. Eight patients received beti-cel after busulfan myeloablative conditioning, all achieving transfusion independence within 8 to 59 days, with posttreatment hemoglobin levels ranging from 11.3 to 19.3 g/dL. No deaths occurred, but acute toxicity mirrored busulfan's known effects. Posttreatment platelet management faced challenges because of HLA-antibodies in 3 patients. Monitoring up to month 24 revealed pituitary-gonadal endocrine dysfunction in all 3 female and in 2 of 5 male patients. Additionally, we observed unexpected posttreatment sequelae: 1 patient developed polycythemia that could not be explained by known genetic or acquired mechanisms, 1 patient developed posttreatment depression and anxiety prohibiting her from returning to work, and 1 patient developed fatigue severely compromising both quality of life and work capacity. This real-world experience corroborates beti-cel's efficacy and safety and provides information on adverse events observed during real-world use of the therapy.

Gene therapy for sickle cell disease: recent advances, clinical trials and future directions

Sickle cell disease (SCD) is the most common inherited blood disorder worldwide, impacting millions and imposing severe healthcare challenges, particularly in resource-limited regions. Current treatments have variable efficacy and require lifelong adherence. Allogeneic Hematopoietic Stem Cell Transplantation can be curative but comes with significant side effects and limited donor availability limits its widespread applicability. Gene therapy, by addressing the root genetic causes, offers a revolutionary alternative. This article discusses the molecular mechanisms of SCD and β-thalassemia and highlights advancements in gene therapy, such as gene addition via lentiviral vectors and gene editing with CRISPR/Cas9 technology. Clinical trials have brought about significant progress but challenges remain, including leukemogenesis, delivery efficiency and cost. Future efforts must focus on enhancing efficiency, reducing costs, developing nongenotoxic conditioning regimens and methods for in vivo application.

Current and emerging drug treatment strategies to tackle sickle cell anemia

INTRODUCTION

Since its discovery in the early 1900s, sickle cell disease (SCD) has contributed significantly to the scientific understanding of hemoglobin and hemoglobinopathies. Despite this, now almost a century later, optimal medical management and even curative options remain limited. Encouragingly, in the last decade, there has been a push toward advancing the care for individuals with SCD and a diversifying interest in options to manage this disorder.

AREAS COVERED

Here, we review the current state of disease modifying therapies for SCD including fetal hemoglobin inducers, monoclonal antibodies, anti-inflammatory modulators, and enzyme activators. We also discuss current curative strategies with specific interest in transformative gene therapies.

EXPERT OPINION

SCD is a chronic, progressive disease that despite a century of clinical description, only now is seeing a growth and advance in therapeutic options to improve the lifespan and quality of life for individuals with SCD. We anticipate newly designed and even repurposed therapies that may work as a single agent or combination agents to tackle the progression of SCD. The vast majority of individuals living with SCD are unlikely to receive gene therapy, therefore improved disease management is critical even for those that may ultimately chose to pursue a potentially curative strategy.

shRNA Targeting Lentiviral Vector Minus-Strand Product Improves the Viral Titer During Viral Packaging

Lentiviral vector (LVV) has been used as one of the common carriers for gene therapy in clinical trials. LVV-mediated clinical trials have being reported in successfully treating hundreds of β-thalassemia cases. These LVVs bear an inversely placed β-hemoglobin (HBB) gene expression cassette for preserving introns during the viral RNA packaging. Consequently, these LVVs often produce a small amount of negatively orientated transcript driven by its internal gene promoter and would lower the viral titer by the minus-strand complemented with the viral backbone. To overcome this problem, we designed shRNAs specifically target the minus-strand RNA driven by the LVV internal promoter that resulted in a notable increase in the viral titer. This report demonstrates a simple and positive mean for increasing the effectiveness for gene therapy with the LVV system.

Sickle Cell Disease Update: New Treatments and Challenging Nutritional Interventions

Sickle cell disease (SCD), a distinctive and often overlooked illness in the 21st century, is a congenital blood disorder characterized by considerable phenotypic diversity. It comprises a group of disorders, with sickle cell anemia (SCA) being the most prevalent and serious genotype. Although there have been some systematic reviews of global data, worldwide statistics regarding SCD prevalence, morbidity, and mortality remain scarce. In developed countries with a lower number of sickle cell patients, cutting-edge technologies have led to the development of new treatments. However, in developing settings where sickle cell disease (SCD) is more prevalent, medical management, rather than a cure, still relies on the use of hydroxyurea, blood transfusions, and analgesics. This is a disease that affects red blood cells, consequently affecting most organs in diverse manners. We discuss its etiology and the advent of new technologies, but the aim of this study is to understand the various types of nutrition-related studies involving individuals suffering from SCD, particularly in Africa. The interplay of the environment, food, gut microbiota, along with their respective genomes collectively known as the gut microbiome, and host metabolism is responsible for mediating host metabolic phenotypes and modulating gut microbiota. In addition, it serves the purpose of providing essential nutrients. Moreover, it engages in direct interactions with host homeostasis and the immune system, as well as indirect interactions via metabolites. Nutrition interventions and nutritional care are mechanisms for addressing increased nutrient expenditures and are important aspects of supportive management for patients with SCD. Underprivileged areas in Sub-Saharan Africa should be accompanied by efforts to define and promote of the nutritional aspects of SCD. Their importance is key to maintaining well-being and quality of life, especially because new technologies and products remain limited, while the use of native medicinal plant resources is acknowledged.

LentiGlobin Administration to Sickle Cell Disease Patients: Effect on Serum Markers and Vaso-Occlusive Crisis

LentiGlobin, an innovative gene therapy, introduces a modified beta-globin gene that yields an anti-sickling hemoglobin variant. It boosts total hemoglobin levels, mitigates hemolysis, curtails inflammation, and addresses iron overload by reducing transfusion requirements. These changes, in turn, provide insights into disease mechanisms and treatment outcomes. Alterations in serum markers, such as hemoglobin levels and inflammatory biomarkers, can illuminate the therapeutic effectiveness of LentiGlobin and its impact on mitigating complications such as vaso-occlusive crises. Therefore, the purpose of this narrative review is to discuss the effects of LentiGlobin administration on diverse serum biomarkers and its correlation with vaso-occlusive crises in individuals with sickle cell disease (SCD).

Hematopoietic Stem Cell Transplantation in Sickle Cell Disease: A Multidimentional Review

While exagamglogene autotemcel (Casgevy) and lovotibeglogene autotemcel (Lyfgenia) have been approved by the US Food and Drug Administration (FDA) as the first cell-based gene therapies for the treatment of patients 12 years of age and older with sickle cell disease (SCD), this treatment is not universally accessible. Allogeneic hematopoietic stem cell transplant (HSCT) has the potential to eradicate the symptoms of patients with SCD, but a significant obstacle in HSCT for SCD is the availability of suitable donors, particularly human leukocyte antigen (HLA)-matched related donors. Furthermore, individuals with SCD face an elevated risk of complications during stem cell transplantation due to SCD-related tissue damage, endothelial activation, and inflammation. Therefore, it is imperative to consider optimal conditioning regimens and investigate HSCT from alternative donors. This review encompasses information on the use of HSCT in patients with SCD, including the indications for HSCT, conditioning regimens, alternative donors, and posttransplant outcomes.

Mouse models of sickle cell disease: Imperfect and yet very informative

The root cause of sickle cell disease (SCD) has been known for nearly a century, however, few therapies to treat the disease are available. Over several decades of work, with advances in gene editing technology and after several iterations of mice with differing genotype/phenotype relationships, researchers have developed humanized SCD mouse models. However, while a large body of preclinical studies has led to huge gains in basic science knowledge about SCD in mice, this knowledge has not led to the development of effective therapies to treat SCD-related complications in humans, thus leading to frustration with the paucity of translational progress in the SCD field. The use of mouse models to study human diseases is based on the genetic and phenotypic similarities between mouse and humans (face validity). The Berkeley and Townes SCD mice express only human globin chains and no mouse hemoglobin. With this genetic composition, these models present many phenotypic similarities, but also significant discrepancies that should be considered when interpreting preclinical studies results. Reviewing genetic and phenotypic similarities and discrepancies and examining studies that have translated to humans and those that have not, offer a better perspective of construct, face, and predictive validities of humanized SCD mouse models.

A mass spectrometry assay for detection of endogenous and lentiviral engineered hemoglobin in cultured cells and sickle cell disease mice

Sickle cell disease (SCD) results from a sequence defect in the β-globin chain of adult hemoglobin (HbA) leading to expression of sickle hemoglobin (HbS). It is traditionally diagnosed by cellulose-acetate hemoglobin electrophoresis or high-performance liquid chromatography. While clinically useful, these methods have both sensitivity and specificity limitations. We developed a novel mass spectrometry (MS) method for the rapid, sensitive and highly quantitative detection of endogenous human β-globin and sickle hβ-globin, as well as lentiviral-encoded therapeutic hβAS3-globin in cultured cells and small quantities of mouse peripheral blood. The MS methods were used to phenotype homozygous HbA (AA), heterozygous HbA-HbS (AS) and homozygous HbS (SS) Townes SCD mice and detect lentiviral vector-encoded hβAS3-globin in transduced mouse erythroid cell cultures and transduced human CD34+ cells after erythroid differentiation. hβAS3-globin was also detected in peripheral blood 6 weeks post-transplant of transduced Townes SS bone marrow cells into syngeneic Townes SS mice and persisted for over 20 weeks post-transplant. As several genome-editing and gene therapy approaches for severe hemoglobin disorders are currently in clinical trials, this MS method will be useful for patient assessment before treatment and during follow-up.

Knowledge of Sickle Cell Disease Among Medical Students at Al-Baha University, Saudi Arabia: A Cross-Sectional Study

BACKGROUND

Sickle cell disease (SCD) presents a major health challenge in Saudi Arabia due to its high prevalence. The important role of medical students as future healthcare leaders necessitates high awareness and knowledge about the disease.

AIM

To assess SCD awareness among Al-Baha University medical students, and to evaluate its relation to gender and academic level.

METHODS

A cross-sectional study was conducted, including 105 medical students from the first to sixth year at Al-Baha University. Data collection utilized an online self-administered questionnaire, covering demographic characteristics and assessing SCD knowledge. Fisher's exact and Pearson Chi-squared tests were employed to analyze associations between gender, academic level, and SCD awareness.

RESULTS

The majority of male participants 52 (89.6%) and all females 47 (100%) demonstrated awareness of SCD. Clinical year enrollment (68.6%) correlated with heightened awareness across various aspects of SCD compared to the preclinical year. Most participants were in clinical years (68.6%), and 94.3% of them had knowledge about SCD. Approximately, 75 (71%) of participants correctly identified features of sickle cell crisis, 83 (79%) reported the accurate cause of SCD, and 75 (71%) cited the appropriate preventive measures. Only 15 (14%) demonstrated knowledge of correct management of SCD. Contrarily, 84 (80%) were aware of SCD complications, 66 (63%) recognized different SCD types, 67 (64%) felt adequately informed about SCD, and 34 (32%) were involved in SCD counseling. Male participants exhibited greater awareness of SCD features than females. Notably, involvement in SCD counseling was more prevalent among students of clinical years.

CONCLUSION

This study underscores the need for targeted educational initiatives, particularly among preclinical year students to enhance SCD awareness among students. We also emphasize the role of clinical education in fostering a comprehensive understanding of SCD, with increased participation in counseling programs.

An α-chain modification rivals the effect of fetal hemoglobin in retarding the rate of sickle cell fiber formation

Adults with sickle cell disease bear a mutation in the β-globin gene, leading to the expression of sickle hemoglobin (HbS; α2βS2). Adults also possess the gene for γ-globin, which is a component of fetal hemoglobin (HbF, α2γ2); however, γ-chain expression normally ceases after birth. As HbF does not form the fibers that cause the disease, pharmacological and gene-modifying interventions have attempted to either reactivate expression of the γ chain or introduce a gene encoding a modified β chain having γ-like character. Here, we show that a single-site modification on the α chain, αPro114Arg, retards fiber formation as effectively as HbF. Because this addition to the repertoire of anti-sickling approaches acts independently of other modifications, it could be coupled with other therapies to significantly enhance their effectiveness.

Gene therapy for sickle cell disease

Sickle cell disease (SCD) is potentially curable after allogeneic hematopoietic stem cell transplantation (HSCT) or autologous HSCT after ex vivo genetic modification. Autologous HSCT with gene therapy has the potential to overcome many of the limitations of allogeneic HSCT that include the lack of suitable donors, graft-versus-host disease, the need for immune suppression, and the potential for graft rejection. Significant progress in gene therapy for SCD has been made over the past several decades, now with a growing number of clinical trials investigating various gene addition and gene editing strategies. Available results from a small number of patients, some with relatively short follow-up, are promising as a potentially curative strategy, with current efforts focused on continuing to improve the efficacy, durability, and safety of gene therapies for the cure of SCD.

Gene addition for beta thalassemia

Individuals with transfusion-dependent beta thalassemia require a high burden of care and experience significant morbidity from the underlying disease and its treatment, which negatively impact the quality of life. Allogeneic hematopoietic stem cell transplantation offers the chance for a cure, but donor availability and transplant-related risks, especially in older patients, limit its use. Gene addition utilizing autologous CD34+ cells is an alternative, potentially curative, treatment option. Several clinical trials have investigated the use of lentiviral vectors containing a functional beta globin gene, including Lentiglobin BB305, GLOBE, and TNS9.3.55. The efficacy and safety data from these ongoing trials are discussed in this review.

Gene-addition/editing therapy in sickle cell disease

Gene therapy is an innovative strategy that offers potential cure for patients with sickle cell disease, and no appropriate donor for transplant consideration. While we await long term data from these clinical trials, we remain optimistic that gene therapy will become a standard of care for curative treatment in sickle cell disease. As gene therapy becomes a standard of treatment in sickle cell disease, we must also acknowledge the potential for financial burden to patients. We also must acknowledge the prevalence of sickle cell disease in low-resource settings. Hopefully, as we learn more about gene therapy, we can assess ways to overcome the financial toxicity that comes with this therapy.

The evolving treatment landscape for children with sickle cell disease

Sickle cell disease is the most common inherited pathological haemoglobinopathy. Over the past 30 years, disease-related morbidity and mortality have improved in high-income countries due to advances in preventive care and treatments. Established disease-modifying therapies, such as hydroxyurea (hydrocarbamide), are continuing to have an important role in the treatment of sickle cell disease, and newer agents also show promise. In the past 5 years, the US Food and Drug Administration approved three additional sickle cell disease-modifying medications, and new gene therapies have been developed as an alternative curative treatment to haematopoietic stem-cell transplantation. In this Review, we discuss the current treatment landscape for paediatric sickle cell disease and emerging innovations in care. We also review the need for close, long-term management for children receiving newer therapies and the importance of ongoing investment in people with sickle cell disease in low-income and middle-income countries.

Defining global strategies to improve outcomes in sickle cell disease: a Lancet Haematology Commission

All over the world, people with sickle cell disease (an inherited condition) have premature deaths and preventable severe chronic complications, which considerably affect their quality of life, career progression, and financial status. In addition, these people are often affected by stigmatisation or structural racism, which can contribute to stress and poor mental health. Inequalities affecting people with sickle cell disease are also reflected in the distribution of the disease—mainly in sub-Saharan Africa, India, and the Caribbean—whereas interventions, clinical trials, and funding are mostly available in North America, Europe, and the Middle East. Although some of these characteristics also affect people with other genetic diseases, the fate of people with sickle cell disease seems to be particularly unfair. Simple, effective interventions to reduce the mortality and morbidity associated with sickle cell disease are available. The main obstacle preventing better outcomes in this condition, which is a neglected disease, is associated with inequalities impacting the patient populations. The aim of this Commission is to highlight the problems associated with sickle cell disease and to identify achievable goals to improve outcomes both in the short and long term.

The ambition for the management of people with sickle cell disease is that curative treatments become available to every person with the condition. Although this would have seemed unrealistic a decade ago, developments in gene therapy make this potentially achievable, albeit in the distant future. Until these curative technologies are fully developed and become widely available, health-care professionals (with the support of policy makers, funders, etc) should make sure that a minimum standard of care (including screening, prophylaxis against infection, acute medical care, safe blood transfusion, and hydroxyurea) is available to all patients.

In considering what needs to be achieved to reduce the global burden of sickle cell disease and improve the quality of life of patients, this Commission focuses on five key areas: the epidemiology of sickle cell disease (Section 1 ); screening and prevention (Section 2 ); established and emerging treatments for the management of the disease (Section 3 ); cellular therapies with curative potential (Section 4 ); and training and education needs (Section 5 ). As clinicians, researchers, and patients, our objective to reduce the global burden of sickle cell disease aligns with wider public health aims to reduce inequalities, improve health for all, and develop personalised treatment options. We have observed in the past few years some long-awaited momentum following the development of innovative point-of-care testing devices, new approved drugs, and emerging curative options. Reducing the burden of sickle cell disease will require substantial financial and political commitment, but it will impact the lives of millions of patients and families worldwide and the lessons learned in achieving this goal would unarguably benefit society as a whole.

Viral vectors engineered for gene therapy

Gene therapy has seen major progress in recent years. Viral vectors have made a significant contribution through efficient engineering for improved delivery and safety. A large variety of indications such as cancer, cardiovascular, metabolic, hematological, neurological, muscular, ophthalmological, infectious diseases, and immunodeficiency have been targeted. Viral vectors based on adenoviruses, adeno-associated viruses, herpes simplex viruses, retroviruses including lentiviruses, alphaviruses, flaviviruses, measles viruses, rhabdoviruses, Newcastle disease virus, poxviruses, picornaviruses, reoviruses, and polyomaviruses have been used. Proof-of-concept has been demonstrated for different indications in animal models. Therapeutic efficacy has also been achieved in clinical trials. Several viral vector-based drugs have been approved for the treatment of cancer, and hematological, metabolic, and neurological diseases. Moreover, viral vector-based vaccines have been approved against COVID-19 and Ebola virus disease.

Precision Editing as a Therapeutic Approach for β-Hemoglobinopathies

Beta-hemoglobinopathies are the most common genetic disorders worldwide, caused by a wide spectrum of mutations in the β-globin locus, and associated with morbidity and early mortality in case of patient non-adherence to supportive treatment. Allogeneic transplantation of hematopoietic stem cells (allo-HSCT) used to be the only curative option, although the indispensable need for an HLA-matched donor markedly restricted its universal application. The evolution of gene therapy approaches made possible the ex vivo delivery of a therapeutic β- or γ- globin gene into patient-derived hematopoietic stem cells followed by the transplantation of corrected cells into myeloablated patients, having led to high rates of transfusion independence (thalassemia) or complete resolution of painful crises (sickle cell disease-SCD). Hereditary persistence of fetal hemoglobin (HPFH), a syndrome characterized by increased γ-globin levels, when co-inherited with β-thalassemia or SCD, converts hemoglobinopathies to a benign condition with mild clinical phenotype. The rapid development of precise genome editing tools (ZFN, TALENs, CRISPR/Cas9) over the last decade has allowed the targeted introduction of mutations, resulting in disease-modifying outcomes. In this context, genome editing tools have successfully been used for the introduction of HPFH-like mutations both in HBG1/HBG2 promoters or/and in the erythroid enhancer of BCL11A to increase HbF expression as an alternative curative approach for β-hemoglobinopathies. The current investigation of new HbF modulators, such as ZBTB7A, KLF-1, SOX6, and ZNF410, further expands the range of possible genome editing targets. Importantly, genome editing approaches have recently reached clinical translation in trials investigating HbF reactivation in both SCD and thalassemic patients. Showing promising outcomes, these approaches are yet to be confirmed in long-term follow-up studies.

Gene Therapy and Gene Editing for β-Thalassemia

After many years of intensive research, emerging data from clinical trials indicate that gene therapy for transfusion-dependent β-thalassemia is now possible. Strategies for therapeutic manipulation of patient hematopoietic stem cells include lentiviral transduction of a functional erythroid-expressed β-globin gene and genome editing to activate fetal hemoglobin production in patient red blood cells. Gene therapy for β-thalassemia and other blood disorders will invariably improve as experience accumulates over time. The best overall approaches are not known and perhaps not yet established. Gene therapy comes at a high cost, and collaboration between multiple stakeholders is required to ensure that these new medicines are administered equitably.

Gene Therapy for β-Hemoglobinopathies: From Discovery to Clinical Trials

Investigations to understand the function and control of the globin genes have led to some of the most exciting molecular discoveries and biomedical breakthroughs of the 20th and 21st centuries. Extensive characterization of the globin gene locus, accompanied by pioneering work on the utilization of viruses as human gene delivery tools in human hematopoietic stem and progenitor cells (HPSCs), has led to transformative and successful therapies via autologous hematopoietic stem-cell transplant with gene therapy (HSCT-GT). Due to the advanced understanding of the β-globin gene cluster, the first diseases considered for autologous HSCT-GT were two prevalent β-hemoglobinopathies: sickle cell disease and β-thalassemia, both affecting functional β-globin chains and leading to substantial morbidity. Both conditions are suitable for allogeneic HSCT; however, this therapy comes with serious risks and is most effective using an HLA-matched family donor (which is not available for most patients) to obtain optimal therapeutic and safe benefits. Transplants from unrelated or haplo-identical donors carry higher risks, although they are progressively improving. Conversely, HSCT-GT utilizes the patient's own HSPCs, broadening access to more patients. Several gene therapy clinical trials have been reported to have achieved significant disease improvement, and more are underway. Based on the safety and the therapeutic success of autologous HSCT-GT, the U.S. Food and Drug Administration (FDA) in 2022 approved an HSCT-GT for β-thalassemia (Zynteglo™). This review illuminates the β-globin gene research journey, adversities faced, and achievements reached; it highlights important molecular and genetic findings of the β-globin locus, describes the predominant globin vectors, and concludes by describing promising results from clinical trials for both sickle cell disease and β-thalassemia.

Viral Vectors in Gene Therapy: Where Do We Stand in 2023?

Viral vectors have been used for a broad spectrum of gene therapy for both acute and chronic diseases. In the context of cancer gene therapy, viral vectors expressing anti-tumor, toxic, suicide and immunostimulatory genes, such as cytokines and chemokines, have been applied. Oncolytic viruses, which specifically replicate in and kill tumor cells, have provided tumor eradication, and even cure of cancers in animal models. In a broader meaning, vaccine development against infectious diseases and various cancers has been considered as a type of gene therapy. Especially in the case of COVID-19 vaccines, adenovirus-based vaccines such as ChAdOx1 nCoV-19 and Ad26.COV2.S have demonstrated excellent safety and vaccine efficacy in clinical trials, leading to Emergency Use Authorization in many countries. Viral vectors have shown great promise in the treatment of chronic diseases such as severe combined immunodeficiency (SCID), muscular dystrophy, hemophilia, β-thalassemia, and sickle cell disease (SCD). Proof-of-concept has been established in preclinical studies in various animal models. Clinical gene therapy trials have confirmed good safety, tolerability, and therapeutic efficacy. Viral-based drugs have been approved for cancer, hematological, metabolic, neurological, and ophthalmological diseases as well as for vaccines. For example, the adenovirus-based drug Gendicine® for non-small-cell lung cancer, the reovirus-based drug Reolysin® for ovarian cancer, the oncolytic HSV T-VEC for melanoma, lentivirus-based treatment of ADA-SCID disease, and the rhabdovirus-based vaccine Ervebo against Ebola virus disease have been approved for human use.

CRISPR Gene Therapy: A Promising One-Time Therapeutic Approach for Transfusion-Dependent β-Thalassemia—CRISPR-Cas9 Gene Editing for β-Thalassemia

β-Thalassemia is an inherited hematological disorder that results from genetic changes in the β-globin gene, leading to the reduced or absent synthesis of β-globin. For several decades, the only curative treatment option for β-thalassemia has been allogeneic hematopoietic cell transplantation (allo-HCT). Nonetheless, rapid progress in genome modification technologies holds great potential for treating this disease and will soon change the current standard of care for β-thalassemia. For instance, the emergence of the CRISPR/Cas9 genome editing platform has opened the door for precision gene editing and can serve as an effective molecular treatment for a multitude of genetic diseases. Investigational studies were carried out to treat β-thalassemia patients utilizing CRISPR-based CTX001 therapy targeting the fetal hemoglobin silencer BCL11A to restore γ-globin expression in place of deficient β-globin. The results of recently carried out clinical trials provide hope of CTX001 being a promising one-time therapeutic option to treat β-hemoglobinopathies. This review provides an insight into the key scientific steps that led to the development and application of novel CRISPR/Cas9–based gene therapies as a promising therapeutic platform for transfusion-dependent β-thalassemia (TDT). Despite the resulting ethical, moral, and social challenges, CRISPR provides an excellent treatment option against hemoglobin-associated genetic diseases.

Diverse Approaches to Gene Therapy of Sickle Cell Disease

Sickle cell disease (SCD) results from a single base pair change in the sixth codon of the β-globin chain of hemoglobin, which promotes aggregation of deoxyhemoglobin, increasing rigidity of red blood cells and causing vaso-occlusive and hemolytic complications. Allogeneic transplant of hematopoietic stem cells (HSCs) can eliminate SCD manifestations but is limited by absence of well-matched donors and immune complications. Gene therapy with transplantation of autologous HSCs that are gene-modified may provide similar benefits without the immune complications. Much progress has been made, and patients are realizing significant clinical improvements in multiple trials using different approaches with lentiviral vector-mediated gene addition to inhibit hemoglobin aggregation. Gene editing approaches are under development to provide additional therapeutic opportunities. Gene therapy for SCD has advanced from an attractive concept to clinical reality.

Lovo-cel gene therapy for sickle cell disease: Treatment process evolution and outcomes in the initial groups of the HGB-206 study

lovo-cel (bb1111; LentiGlobin for sickle cell disease [SCD]) gene therapy (GT) comprises autologous transplantation of hematopoietic stem and progenitor cells transduced with the BB305 lentiviral vector encoding a modified β-globin gene (β^A-T87Q ) to produce anti-sickling hemoglobin (HbA^T87Q ). The efficacy and safety of lovo-cel for SCD are being evaluated in the ongoing phase 1/2 HGB-206 study (ClinicalTrials.gov: NCT02140554). The treatment process evolved over time, using learnings from outcomes in the initial patients to optimize lovo-cel's benefit-risk profile. Following modest expression of HbA^T87Q in the initial patients (Group A, n = 7), alterations were made to the treatment process for patients subsequently enrolled in Group B (n = 2, patients B1 and B2), including improvements to cell collection and lovo-cel manufacturing. After 6 months, median Group A peripheral blood vector copy number (≥0.08 c/dg) and HbA^T87Q levels (≥0.46 g/dL) were inadequate for substantial clinical effect but stable and sustained over 5.5 years; both markedly improved in Group B (patient B1: ≥0.53 c/dg and ≥2.69 g/dL; patient B2: ≥2.14 c/dg and ≥6.40 g/dL, respectively) and generated improved biologic and clinical efficacy in Group B, including higher total hemoglobin and decreased hemolysis. The safety of the lovo-cel for SCD treatment regimen largely reflected the known side effects of HSPC collection, busulfan conditioning regimen, and underlying SCD; acute myeloid leukemia was observed in two patients in Group A and deemed unlikely related to insertional oncogenesis. Changes made during development of the lovo-cel treatment process were associated with improved outcomes and provide lessons for future SCD GT studies.

Stem Cell-Based Therapeutic Approaches in Genetic Diseases

Stem cells, which can self-renew and differentiate into different cell types, have become the keystone of regenerative medicine due to these properties. With the achievement of superior clinical results in the therapeutic approaches of different diseases, the applications of these cells in the treatment of genetic diseases have also come to the fore. Foremost, conventional approaches of stem cells to genetic diseases are the first approaches in this manner, and they have brought safety issues due to immune reactions caused by allogeneic transplantation. To eliminate these safety issues and phenotypic abnormalities caused by genetic defects, firstly, basic genetic engineering practices such as vectors or RNA modulators were combined with stem cell-based therapeutic approaches. However, due to challenges such as immune reactions and inability to target cells effectively in these applications, advanced molecular methods have been adopted in ZFN, TALEN, and CRISPR/Cas genome editing nucleases, which allow modular designs in stem cell-based genetic diseases' therapeutic approaches. Current studies in genetic diseases are in the direction of creating permanent treatment regimens by genomic manipulation of stem cells with differentiation potential through genome editing tools. In this chapter, the stem cell-based therapeutic approaches of various vital genetic diseases were addressed wide range from conventional applications to genome editing tools.

Efficient and error-free correction of sickle mutation in human erythroid cells using prime editor-2

Sickle cell anaemia (SCA) is one of the common autosomal recessive monogenic disorders, caused by a transverse point mutation (GAG > GTG) at the sixth codon of the beta-globin gene, which results in haemolytic anaemia due to the fragile RBCs. Recent progress in genome editing has gained attention for the therapeutic cure for SCA. Direct correction of SCA mutation by homology-directed repair relies on a double-strand break (DSB) at the target site and carries the risk of generating beta-thalassaemic mutations if the editing is not error-free. On the other hand, base editors cannot correct the pathogenic SCA mutation resulting from A > T base transversion. Prime editor (PE), the recently described CRISPR/Cas 9 based gene editing tool that enables precise gene manipulations without DSB and unintended nucleotide changes, is a viable approach for the treatment of SCA. However, the major limitation with the use of prime editing is the lower efficiency especially in human erythroid cell lines and primary cells. To overcome these limitations, we developed a modular lenti-viral based prime editor system and demonstrated its use for the precise modelling of SCA mutation and its subsequent correction in human erythroid cell lines. We achieved highly efficient installation of SCA mutation (up to 72%) and its subsequent correction in human erythroid cells. For the first time, we demonstrated the functional restoration of adult haemoglobin without any unintended nucleotide changes or indel formations using the PE2 system. We also validated that the off-target effects mediated by the PE2 system is very minimal even with very efficient on-target conversion, making it a safe therapeutic option. Taken together, the modular lenti-viral prime editor system developed in this study not only expands the range of cell lines targetable by prime editor but also improves the efficiency considerably, enabling the use of prime editor for myriad molecular, genetic, and translational studies.

Phenotypic screening of the ReFRAME drug repurposing library to discover new drugs for treating sickle cell disease

Stem cell transplantation and genetic therapies offer potential cures for patients with sickle cell disease (SCD), but these options require advanced medical facilities and are expensive. Consequently, these treatments will not be available for many years to the majority of patients suffering from this disease. What is urgently needed now is an inexpensive oral drug in addition to hydroxyurea, the only drug approved by the FDA that inhibits sickle-hemoglobin polymerization. Here, we report the results of the first phase of our phenotypic screen of the 12,657 compounds of the Scripps ReFRAME drug repurposing library using a recently developed high-throughput assay to measure sickling times following deoxygenation to 0% oxygen of red cells from sickle trait individuals. The ReFRAME library is a very important collection because the compounds are either FDA-approved drugs or have been tested in clinical trials. From dose-response measurements, 106 of the 12,657 compounds exhibit statistically significant antisickling at concentrations ranging from 31 nM to 10 μM. Compounds that inhibit sickling of trait cells are also effective with SCD cells. As many as 21 of the 106 antisickling compounds emerge as potential drugs. This estimate is based on a comparison of inhibitory concentrations with free concentrations of oral drugs in human serum. Moreover, the expected therapeutic potential for each level of inhibition can be predicted from measurements of sickling times for cells from individuals with sickle syndromes of varying severity. Our results should motivate others to develop one or more of these 106 compounds into drugs for treating SCD.

Evolution of Gene Therapy, Historical Perspective

The earliest conceptual history of gene therapy began with the recognition of DNA as the transforming substance capable of changing the phenotypic character of a bacterium and then as the carrier of the genomic code. Early studies of oncogenic viruses that could insert into the mammalian genome led to the concept that these same viruses might be engineered to carry new genetic material into mammalian cells, including human hematopoietic stem cells (HSC). In addition to properly engineered vectors capable of efficient safe transduction of HSC, successful gene therapy required the development of efficient materials, methods, and equipment to procure, purify, and culture HSC. Increased understanding of the preparative conditioning of patients was needed to optimize the engraftment of genetically modified HSC. Testing concepts in pivotal clinical trials to assess the efficacy and determine the cause of adverse events has advanced the efficiency and safety of gene therapy. This article is a historical overview of the separate threads of discovery that joined together to comprise our current state of gene therapy targeting HSC.

Limitations of mouse models for sickle cell disease conferred by their human globin transgene configurations

We characterized the human β-like globin transgenes in two mouse models of sickle cell disease (SCD) and tested a genome-editing strategy to induce red blood cell fetal hemoglobin (HbF; α₂γ₂). Berkeley SCD mice contain four to 22 randomly arranged, fragmented copies of three human transgenes (HBA1, HBG2-HBG1-HBD-HBB^S and a mini-locus control region) integrated into a single site of mouse chromosome 1. Cas9 disruption of the BCL11A repressor binding motif in the γ-globin gene (HBG1 and HBG2; HBG) promoters of Berkeley mouse hematopoietic stem cells (HSCs) caused extensive death from multiple double-strand DNA breaks. Long-range sequencing of Townes SCD mice verified that the endogenous Hbb genes were replaced by single-copy segments of human HBG1 and HBB^S including proximal but not some distal gene-regulatory elements. Townes mouse HSCs were viable after Cas9 disruption of the HBG1 BCL11A binding motif but failed to induce HbF to therapeutic levels, contrasting with human HSCs. Our findings provide practical information on the genomic structures of two common mouse SCD models, illustrate their limitations for analyzing therapies to induce HbF and confirm the importance of distal DNA elements in human globin regulation.

This article has an associated First Person interview with the first author of the paper.

Editor's choice: This study describes the genomic structures of two common sickle cell disease mouse models, illustrates their limitations for analyzing some genetic therapies and confirms the importance of distal DNA elements in human globin gene regulation.

Globin vector regulatory elements are active in early hematopoietic progenitor cells

The globin genes are archetypal tissue-specific genes that are silent in most tissues but for late-stage erythroblasts upon terminal erythroid differentiation. The transcriptional activation of the β-globin gene is under the control of proximal and distal regulatory elements located on chromosome 11p15.4, including the β-globin locus control region (LCR). The incorporation of selected LCR elements in lentiviral vectors encoding β and β-like globin genes has enabled successful genetic treatment of the β-thalassemias and sickle cell disease. However, recent occurrences of benign clonal expansions in thalassemic patients and myelodysplastic syndrome in patients with sickle cell disease call attention to the non-erythroid functions of these powerful vectors. Here we demonstrate that lentivirally encoded LCR elements, in particular HS1 and HS2, can be activated in early hematopoietic cells including hematopoietic stem cells and myeloid progenitors. This activity is position-dependent and results in the transcriptional activation of a nearby reporter gene in these progenitor cell populations. We further show that flanking a globin vector with an insulator can effectively restrain this non-erythroid activity without impairing therapeutic globin expression. Globin lentiviral vectors harboring powerful LCR HS elements may thus expose to the risk of trans-activating cancer-related genes, which can be mitigated by a suitable insulator.

Biologic and Clinical Efficacy of LentiGlobin for Sickle Cell Disease

BACKGROUND

Sickle cell disease is characterized by the painful recurrence of vaso-occlusive events. Gene therapy with the use of LentiGlobin for sickle cell disease (bb1111; lovotibeglogene autotemcel) consists of autologous transplantation of hematopoietic stem and progenitor cells transduced with the BB305 lentiviral vector encoding a modified β-globin gene, which produces an antisickling hemoglobin, HbA^T87Q.

METHODS

In this ongoing phase 1-2 study, we optimized the treatment process in the initial 7 patients in Group A and 2 patients in Group B with sickle cell disease. Group C was established for the pivotal evaluation of LentiGlobin for sickle cell disease, and we adopted a more stringent inclusion criterion that required a minimum of four severe vaso-occlusive events in the 24 months before enrollment. In this unprespecified interim analysis, we evaluated the safety and efficacy of LentiGlobin in 35 patients enrolled in Group C. Included in this analysis was the number of severe vaso-occlusive events after LentiGlobin infusion among patients with at least four vaso-occlusive events in the 24 months before enrollment and with at least 6 months of follow-up.

RESULTS

As of February 2021, cell collection had been initiated in 43 patients in Group C; 35 received a LentiGlobin infusion, with a median follow-up of 17.3 months (range, 3.7 to 37.6). Engraftment occurred in all 35 patients. The median total hemoglobin level increased from 8.5 g per deciliter at baseline to 11 g or more per deciliter from 6 months through 36 months after infusion. HbA^T87Q contributed at least 40% of total hemoglobin and was distributed across a mean (±SD) of 85±8% of red cells. Hemolysis markers were reduced. Among the 25 patients who could be evaluated, all had resolution of severe vaso-occlusive events, as compared with a median of 3.5 events per year (range, 2.0 to 13.5) in the 24 months before enrollment. Three patients had a nonserious adverse event related or possibly related to LentiGlobin that resolved within 1 week after onset. No cases of hematologic cancer were observed during up to 37.6 months of follow-up.

CONCLUSIONS

One-time treatment with LentiGlobin resulted in sustained production of HbA^T87Q in most red cells, leading to reduced hemolysis and complete resolution of severe vaso-occlusive events. (Funded by Bluebird Bio; HGB-206 ClinicalTrials.gov number, NCT02140554.).

Long-term outcomes of lentiviral gene therapy for the β-hemoglobinopathies: the HGB-205 trial

Sickle cell disease (SCD) and transfusion-dependent β-thalassemia (TDT) are the most prevalent monogenic disorders worldwide. Trial HGB-205 ( NCT02151526 ) aimed at evaluating gene therapy by autologous CD34⁺ cells transduced ex vivo with lentiviral vector BB305 that encodes the anti-sickling β^A-T87Q-globin expressed in the erythroid lineage. HGB-205 is a phase 1/2, open-label, single-arm, non-randomized interventional study of 2-year duration at a single center, followed by observation in long-term follow-up studies LTF-303 ( NCT02633943 ) and LTF-307 ( NCT04628585 ) for TDT and SCD, respectively. Inclusion and exclusion criteria were similar to those for allogeneic transplantation but restricted to patients lacking geno-identical, histocompatible donors. Four patients with TDT and three patients with SCD, ages 13-21 years, were treated after busulfan myeloablation 4.6-7.9 years ago, with a median follow-up of 4.5 years. Key primary endpoints included mortality, engraftment, replication-competent lentivirus and clonal dominance. No adverse events related to the drug product were observed. Clinical remission and remediation of biological hallmarks of the disease have been sustained in two of the three patients with SCD, and frequency of transfusions was reduced in the third. The patients with TDT are all transfusion free with improvement of dyserythropoiesis and iron overload.

Development and clinical translation of ex vivo gene therapy

Retroviral gene therapy has emerged as a promising therapeutic modality for multiple inherited and acquired human diseases. The capability of delivering curative treatment or mediating therapeutic benefits for a long-term period following a single application fundamentally distinguishes this medical intervention from traditional medicine and various lentiviral/γ-retroviral vector-mediated gene therapy products have been approved for clinical use. Continued advances in retroviral vector engineering, genomic editing, synthetic biology and immunology will broaden the medical applications of gene therapy and improve the efficacy and safety of the treatments based on genetic correction and alteration. This review will summarize the advent and clinical translation of ex vivo gene therapy, with the focus on the milestones during the exploitation of genetically engineered hematopoietic stem cells (HSCs) tackling a variety of pathological conditions which led to marketing approval. Finally, current statue and future prospects of gene editing as an alternative therapeutic approach are also discussed.

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 (Hb) disorder results from a single point mutation. Advances in genomic sequencing have increased the understanding of Hb 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 past few decades to increase expression of normal or antisickling globins as strategies to ameliorate SCD. Many hurdles had to be addressed before clinical translation, including collecting 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 that cells must be harvested from and chemotherapy administered 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 have to be portable, safe, and efficient, to make an in vivo-based approach attractive. In addition, adequate resources for SCD screening and access to standardized care are critically important for gene therapy to be a viable treatment option for SCD.

Inclusion of a shRNA targeting BCL11A into a β-globin expressing vector allows concurrent synthesis of curative adult and fetal hemoglobin

Not available.

Advances in Sickle Cell Disease Treatments

Sickle Cell Disease (SCD) is an inherited disorder of red blood cells that is caused by a single mutation in the β -globin gene. The disease, which afflicts millions of patients worldwide mainly in low income countries, is characterized by high morbidity, mortality and low life expectancy. The new pharmacological and non-pharmacological strategies for SCD is urgent in order to promote treatments able to reduce patient's suffering and improve their quality of life. Since the FDA approval of HU in 1998, there have been few advances in discovering new drugs; however, in the last three years voxelotor, crizanlizumab, and glutamine have been approved as new therapeutic alternatives. In addition, new promising compounds have been described to treat the main SCD symptoms. Herein, focusing on drug discovery, we discuss new strategies to treat SCD that have been carried out in the last ten years to discover new, safe, and effective treatments. Moreover, non-pharmacological approaches, including red blood cell exchange, gene therapy and hematopoietic stem cell transplantation will be presented.

Hematopoietic-Stem-Cell-Targeted Gene-Addition and Gene-Editing Strategies for β-hemoglobinopathies

Sickle cell disease (SCD) is caused by a well-defined point mutation in the β-globin gene and therefore is an optimal target for hematopoietic stem cell (HSC) gene-addition/editing therapy. In HSC gene-addition therapy, a therapeutic β-globin gene is integrated into patient HSCs via lentiviral transduction, resulting in long-term phenotypic correction. State-of-the-art gene-editing technology has made it possible to repair the β-globin mutation in patient HSCs or target genetic loci associated with reactivation of endogenous γ-globin expression. With both approaches showing signs of therapeutic efficacy in patients, we discuss current genetic treatments, challenges, and technical advances in this field.

Coordinated β-globin expression and α2-globin reduction in a multiplex lentiviral gene therapy vector for β-thalassemia

A primary challenge in lentiviral gene therapy of β-hemoglobinopathies is to maintain low vector copy numbers to avoid genotoxicity while being reliably therapeutic for all genotypes. We designed a high-titer lentiviral vector, LVβ-shα2, that allows coordinated expression of the therapeutic β^A-T87Q-globin gene and of an intron-embedded miR-30-based short hairpin RNA (shRNA) selectively targeting the α2-globin mRNA. Our approach was guided by the knowledge that moderate reduction of α-globin chain synthesis ameliorates disease severity in β-thalassemia. We demonstrate that LVβ-shα2 reduces α2-globin mRNA expression in erythroid cells while keeping α1-globin mRNA levels unchanged and β^A-T87Q-globin gene expression identical to the parent vector. Compared with the first β^A-T87Q-globin lentiviral vector that has received conditional marketing authorization, BB305, LVβ-shα2 shows 1.7-fold greater potency to improve α/β ratios. It may thus result in greater therapeutic efficacy and reliability for the most severe types of β-thalassemia and provide an improved benefit/risk ratio regardless of the β-thalassemia genotype.

Restoration of β-Globin Expression with Optimally Designed Lentiviral Vector for β-Thalassemia Treatment in Chinese Patients

β-Thalassemia is one of the most prevalent genetic diseases worldwide. The current treatment for β-thalassemia is allogeneic hematopoietic stem cell transplantation, which is limited due to lack of matched donors. Gene therapy has been developed as an alternative therapeutic option for transfusion-dependent β-thalassemia (TDT). However, successful gene therapy for β-thalassemia patients in China has not been reported. Here, we present the results of preclinical studies of an optimally designed lentiviral vector (LV) named LentiHBB^T87Q in hematopoietic stem and progenitor cells (HSPCs) derived from Chinese TDT patients. LentiHBB^T87Q was selected from a series of LVs with optimized backbone and de novo cloning strategy. It contains an exogenous T87Q β-globin gene (HBB^T87Q) driven by a specific reconstituted locus control region, and efficiently expresses HBB mRNA and HBB protein in erythroblasts derived from cord blood HSPCs. To facilitate clinical transformation, we manufactured clinical-grade LentiHBB^T87Q (cLentiHBB^T87Q) and optimized its transduction procedure. Importantly, transduction of cLentiHBB^T87Q restored expression of HBB monomer and adult hemoglobin tetramer to relatively normal level in erythroblasts from bone marrow HSPCs of Chinese TDT patients that carry the most common mutation types and cover various genotypes, including β0/β0. Furthermore, viral integration sites (VISs) of cLentiHBB^T87Q were similar to other LVs safely used in previous clinical trials, and gene-ontology (term) analysis of VIS targeted genes suggests that no tumor-associated pathways were enriched in treated samples. Taken together, we have engineered the cLentiHBB^T87Q that can restore β-globin expression in the HSPCs-derived erythroblasts of Chinese TDT patients with minimal risk of tumorigenesis, providing a favorable starting point for future clinical application.

A Small Key for a Heavy Door: Genetic Therapies for the Treatment of Hemoglobinopathies

Throughout the past decades, the search for a treatment for severe hemoglobinopathies has gained increased interest within the scientific community. The discovery that ɤ-globin expression from intact HBG alleles complements defective HBB alleles underlying β-thalassemia and sickle cell disease, has provided a promising opening for research directed at relieving ɤ-globin repression mechanisms and, thereby, improve clinical outcomes for patients. Various gene editing strategies aim to reverse the fetal-to-adult hemoglobin switch to up-regulate ɤ-globin expression through disabling either HBG repressor genes or repressor binding sites in the HBG promoter regions. In addition to these HBB mutation-independent strategies involving fetal hemoglobin (HbF) synthesis de-repression, the expanding genome editing toolkit is providing increased accuracy to HBB mutation-specific strategies encompassing adult hemoglobin (HbA) restoration for a personalized treatment of hemoglobinopathies. Moreover, besides genome editing, more conventional gene addition strategies continue under investigation to restore HbA expression. Together, this research makes hemoglobinopathies a fertile ground for testing various innovative genetic therapies with high translational potential. Indeed, the progressive understanding of the molecular clockwork underlying the hemoglobin switch together with the ongoing optimization of genome editing tools heightens the prospect for the development of effective and safe treatments for hemoglobinopathies. In this context, clinical genetics plays an equally crucial role by shedding light on the complexity of the disease and the role of ameliorating genetic modifiers. Here, we cover the most recent insights on the molecular mechanisms underlying hemoglobin biology and hemoglobinopathies while providing an overview of state-of-the-art gene editing platforms. Additionally, current genetic therapies under development, are equally discussed.

Lentiviral vector ALS20 yields high hemoglobin levels with low genomic integrations for treatment of beta-globinopathies

Ongoing clinical trials for treatment of beta-globinopathies by gene therapy involve the transfer of the beta-globin gene, which requires integration of three to four copies per genome in most target cells. This high proviral load may increase genome toxicity, potentially limiting the safety of this therapy and relegating its use to total body myeloablation. We hypothesized that introducing an additional hypersensitive site from the locus control region, the complete sequence of the second intron of the beta-globin gene, and the ankyrin insulator may enhance beta-globin expression. We identified a construct, ALS20, that synthesized significantly higher adult hemoglobin levels than those of other constructs currently used in clinical trials. These findings were confirmed in erythroblastic cell lines and in primary cells isolated from sickle cell disease patients. Bone marrow transplantation studies in beta-thalassemia mice revealed that ALS20 was curative at less than one copy per genome. Injection of human CD34⁺ cells transduced with ALS20 led to safe, long-term, and high polyclonal engraftment in xenograft experiments. Successful treatment of beta-globinopathies with ALS20 could potentially be achieved at less than two copies per genome, minimizing the risk of cytotoxic events and lowering the intensity of myeloablation.