Safety and efficacy of gene replacement therapy for X-linked myotubular myopathy (ASPIRO): a multinational, open-label, dose-escalation trial.

BACKGROUND: X-linked myotubular myopathy is a rare, life-threatening, congenital muscle disease observed mostly in males, which is caused by mutations in MTM1. No therapies are approved for this disease. We aimed to assess the safety and efficacy of resamirigene bilparvovec, which is an adeno-associated viral vector serotype 8 delivering human MTM1. METHODS: ASPIRO is an open-label, dose-escalation trial at seven academic medical centres in Canada, France, Germany, and the USA. We included boys younger than 5 years with X-linked myotubular myopathy who required mechanical ventilator support. The trial was initially in two parts. Part 1 was planned as a safety and dose-escalation phase in which participants were randomly allocated (2:1) to either the first dose level (1·3 × 1014 vector genomes [vg]/kg bodyweight) of resamirigene bilparvovec or delayed treatment, then, for later participants, to either a higher dose (3·5 × 1014 vg/kg bodyweight) of resamirigene bilparvovec or delayed treatment. Part 2 was intended to confirm the dose selected in part 1. Resamirigene bilparvovec was administered as a single intravenous infusion. An untreated control group comprised boys who participated in a run-in study (INCEPTUS; NCT02704273) or those in the delayed treatment cohort who did not receive any dose. The primary efficacy outcome was the change from baseline to week 24 in hours of daily ventilator support. After three unexpected deaths, dosing at the higher dose was stopped and the two-part feature of the study design was eliminated. Because of changes to the study design during its implementation, analyses were done on an as-treated basis and are deemed exploratory. All treated and control participants were included in the safety analysis. The trial is registered with ClinicalTrials.gov, NCT03199469. Outcomes are reported as of Feb 28, 2022. ASPIRO is currently paused while deaths in dosed participants are investigated. FINDINGS: Between Aug 3, 2017 and June 1, 2021, 30 participants were screened for eligibility, of whom 26 were enrolled; six were allocated to the lower dose, 13 to the higher dose, and seven to delayed treatment. Of the seven children whose treatment was delayed, four later received the higher dose (n=17 total in the higher dose cohort), one received the lower dose (n=7 total in the lower dose cohort), and two received no dose and joined the control group (n=14 total, including 12 children from INCEPTUS). Median age at dosing or enrolment was 12·1 months (IQR 10·0-30·9; range 9·5-49·7) in the lower dose cohort, 31·1 months (16·0-64·7; 6·8-72·7) in the higher dose cohort, and 18·7 months (10·1-31·5; 5·9-39·3) in the control cohort. Median follow-up was 46·1 months (IQR 41·0-49·5; range 2·1-54·7) for lower dose participants, 27·6 months (24·6-29·1; 3·4-41·0) for higher dose participants, and 28·3 months (9·7-46·9; 5·7-32·7) for control participants. At week 24, lower dose participants had an estimated 77·7 percentage point (95% CI 40·22 to 115·24) greater reduction in least squares mean hours per day of ventilator support from baseline versus controls (p=0·0002), and higher dose participants had a 22·8 percentage point (6·15 to 39·37) greater reduction from baseline versus controls (p=0·0077). One participant in the lower dose cohort and three in the higher dose cohort died; at the time of death, all children had cholestatic liver failure following gene therapy (immediate causes of death were sepsis; hepatopathy, severe immune dysfunction, and pseudomonal sepsis; gastrointestinal haemorrhage; and septic shock). Three individuals in the control group died (haemorrhage presumed related to hepatic peliosis; aspiration pneumonia; and cardiopulmonary failure). INTERPRETATION: Most children with X-linked myotubular myopathy who received MTM1 gene replacement therapy had important improvements in ventilator dependence and motor function, with more than half of dosed participants achieving ventilator independence and some attaining the ability to walk independently. Investigations into the risk for underlying hepatobiliary disease in X-linked myotubular myopathy, and the need for monitoring of liver function before gene replacement therapy, are ongoing. FUNDING: Astellas Gene Therapies.

High-throughput transcriptome analyses from ASPIRO, a phase 1/2/3 study of gene replacement therapy for X-linked myotubular myopathy.

X-linked myotubular myopathy (XLMTM) is a severe congenital disease characterized by profound muscle weakness, respiratory failure, and early death. No approved therapy for XLMTM is currently available. Adeno-associated virus (AAV)-mediated gene replacement therapy has shown promise as an investigational therapeutic strategy. We aimed to characterize the transcriptomic changes in muscle biopsies of individuals with XLMTM who received resamirigene bilparvovec (AT132; rAAV8-Des-hMTM1) in the ASPIRO clinical trial and to identify potential biomarkers that correlate with therapeutic outcome. We leveraged RNA-sequencing data from the muscle biopsies of 15 study participants and applied differential expression analysis, gene co-expression analysis, and machine learning to characterize the transcriptomic changes at baseline (pre-dose) and at 24 and 48 weeks after resamirigene bilparvovec dosing. As expected, MTM1 expression levels were significantly increased after dosing (p < 0.0001). Differential expression analysis identified upregulated genes after dosing that were enriched in several pathways, including lipid metabolism and inflammatory response pathways, and downregulated genes were enriched in cell-cell adhesion and muscle development pathways. Genes involved in inflammatory and immune pathways were differentially expressed between participants exhibiting ventilator support reduction of either greater or less than 6 h/day after gene therapy compared to pre-dosing. Co-expression analysis identified similarly regulated genes, which were grouped into modules. Finally, the machine learning model identified five genes, including MTM1, as potential RNA biomarkers to monitor the progress of AAV gene replacement therapy. These findings further extend our understanding of AAV-mediated gene therapy in individuals with XLMTM at the transcriptomic level.

Novel lentiviral vectors for gene therapy of sickle cell disease combining gene addition and gene silencing strategies.

Sickle cell disease (SCD) is due to a mutation in the ß-globin gene causing production of the toxic sickle hemoglobin (HbS; α2ßS 2). Transplantation of autologous hematopoietic stem and progenitor cells (HSPCs) transduced with lentiviral vectors (LVs) expressing an anti-sickling ß-globin (ßAS) is a promising treatment; however, it is only partially effective, and patients still present elevated HbS levels. Here, we developed a bifunctional LV expressing ßAS3-globin and an artificial microRNA (amiRNA) specifically downregulating ßS-globin expression with the aim of reducing HbS levels and favoring ßAS3 incorporation into Hb tetramers. Efficient transduction of SCD HSPCs by the bifunctional LV led to a substantial decrease of ßS-globin transcripts in HSPC-derived erythroid cells, a significant reduction of HbS+ red cells, and effective correction of the sickling phenotype, outperforming ßAS gene addition and BCL11A gene silencing strategies. The bifunctional LV showed a standard integration profile, and neither HSPC viability, engraftment, and multilineage differentiation nor the erythroid transcriptome and miRNAome were affected by the treatment, confirming the safety of this therapeutic strategy. In conclusion, the combination of gene addition and gene silencing strategies can improve the efficacy of current LV-based therapeutic approaches without increasing the mutagenic vector load, thus representing a novel treatment for SCD.

Cerebellar Pathology in an Inducible Mouse Model of Friedreich Ataxia.

Friedreich ataxia (FRDA) is an autosomal recessive neurodegenerative disorder caused by deficiency of the mitochondrial protein frataxin. Lack of frataxin causes neuronal loss in various areas of the CNS and PNS. In particular, cerebellar neuropathology in FRDA patients includes loss of large principal neurons and synaptic terminals in the dentate nucleus (DN), and previous studies have demonstrated early synaptic deficits in the Knockin-Knockout mouse model of FRDA. However, the exact correlation of frataxin deficiency with cerebellar neuropathology remains unclear. Here we report that doxycycline-induced frataxin knockdown in a mouse model of FRDA (FRDAkd) leads to synaptic cerebellar degeneration that can be partially reversed by AAV8-mediated frataxin restoration. Loss of cerebellar Purkinje neurons and large DN principal neurons are observed in the FRDAkd mouse cerebellum. Levels of the climbing fiber-specific glutamatergic synaptic marker VGLUT2 decline starting at 4 weeks after dox induction, whereas levels of the parallel fiber-specific synaptic marker VGLUT1 are reduced by 18-weeks. These findings suggest initial selective degeneration of climbing fiber synapses followed by loss of parallel fiber synapses. The GABAergic synaptic marker GAD65 progressively declined during dox induction in FRDAkd mice, while GAD67 levels remained unaltered, suggesting specific roles for frataxin in maintaining cerebellar synaptic integrity and function during adulthood. Expression of frataxin following AAV8-mediated gene transfer partially restored VGLUT1/2 levels. Taken together, our findings show that frataxin knockdown leads to cerebellar degeneration in the FRDAkd mouse model, suggesting that frataxin helps maintain cerebellar structure and function.

Muscle-directed gene therapy corrects Pompe disease and uncovers species-specific GAA immunogenicity.

Pompe disease is a severe disorder caused by loss of acid α-glucosidase (GAA), leading to glycogen accumulation in tissues and neuromuscular and cardiac dysfunction. Enzyme replacement therapy is the only available treatment. AT845 is an adeno-associated viral vector designed to express human GAA specifically in skeletal muscle and heart. Systemic administration of AT845 in Gaa-/- mice led to a dose-dependent increase in GAA activity, glycogen clearance in muscles and heart, and functional improvement. AT845 was tolerated in cynomolgus macaques at low doses, while high doses caused anti-GAA immune response, inflammation, and cardiac abnormalities resulting in unscheduled euthanasia of two animals. Conversely, a vector expressing the macaque GAA caused no detectable pathology, indicating that the toxicity observed with AT845 was an anti-GAA xenogeneic immune response. Western blot analysis showed abnormal processing of human GAA in cynomolgus muscle, adding to the species-specific effects of enzyme expression. Overall, these studies show that AAV-mediated GAA delivery to muscle is efficacious in Gaa-/- mice and highlight limitations in predicting the toxicity of AAV vectors encoding human proteins in non-human species.

Designing Lentiviral Vectors for Gene Therapy of Genetic Diseases.

Lentiviral vectors are the most frequently used tool to stably transfer and express genes in the context of gene therapy for monogenic diseases. The vast majority of clinical applications involves an ex vivo modality whereby lentiviral vectors are used to transduce autologous somatic cells, obtained from patients and re-delivered to patients after transduction. Examples are hematopoietic stem cells used in gene therapy for hematological or neurometabolic diseases or T cells for immunotherapy of cancer. We review the design and use of lentiviral vectors in gene therapy of monogenic diseases, with a focus on controlling gene expression by transcriptional or post-transcriptional mechanisms in the context of vectors that have already entered a clinical development phase.

Correction of ß-thalassemia by CRISPR/Cas9 editing of the α-globin locus in human hematopoietic stem cells.

ß-thalassemias (ß-thal) are a group of blood disorders caused by mutations in the ß-globin gene (HBB) cluster. ß-globin associates with α-globin to form adult hemoglobin (HbA, α2ß2), the main oxygen-carrier in erythrocytes. When ß-globin chains are absent or limiting, free α-globins precipitate and damage cell membranes, causing hemolysis and ineffective erythropoiesis. Clinical data show that severity of ß-thal correlates with the number of inherited α-globin genes (HBA1 and HBA2), with α-globin gene deletions having a beneficial effect for patients. Here, we describe a novel strategy to treat ß-thal based on genome editing of the α-globin locus in human hematopoietic stem/progenitor cells (HSPCs). Using CRISPR/Cas9, we combined 2 therapeutic approaches: (1) α-globin downregulation, by deleting the HBA2 gene to recreate an α-thalassemia trait, and (2) ß-globin expression, by targeted integration of a ß-globin transgene downstream the HBA2 promoter. First, we optimized the CRISPR/Cas9 strategy and corrected the pathological phenotype in a cellular model of ß-thalassemia (human erythroid progenitor cell [HUDEP-2] ß0). Then, we edited healthy donor HSPCs and demonstrated that they maintained long-term repopulation capacity and multipotency in xenotransplanted mice. To assess the clinical potential of this approach, we next edited ß-thal HSPCs and achieved correction of α/ß globin imbalance in HSPC-derived erythroblasts. As a safer option for clinical translation, we performed editing in HSPCs using Cas9 nickase showing precise editing with no InDels. Overall, we described an innovative CRISPR/Cas9 approach to improve α/ß globin imbalance in thalassemic HSPCs, paving the way for novel therapeutic strategies for ß-thal.

Clinical management of sickle cell liver disease in children and young adults.

Liver involvement in sickle cell disease (SCD) is often referred to as sickle cell hepatopathy (SCH) and is a complication of SCD which may be associated with significant mortality. This review is based on a round-table workshop between paediatric and adult hepatologists and haematologists and review of the literature. The discussion was prompted by the lack of substantial data and guidance in managing these sometimes very challenging cases. This review provides a structured approach for the diagnosis and management of SCH in children and young adults. The term SCH describes any hepatobiliary dysfunction in the context of SCD. Diagnosis and management of biliary complications, acute hepatic crisis, acute hepatic sequestration and other manifestations of SCH are discussed, as well as the role of liver transplantation and haemopoietic stem cell transplantation in the management of SCH.

Editing a γ-globin repressor binding site restores fetal hemoglobin synthesis and corrects the sickle cell disease phenotype.

Sickle cell disease (SCD) is caused by a single amino acid change in the adult hemoglobin (Hb) ß chain that causes Hb polymerization and red blood cell (RBC) sickling. The co-inheritance of mutations causing fetal γ-globin production in adult life hereditary persistence of fetal Hb (HPFH) reduces the clinical severity of SCD. HPFH mutations in the HBG γ-globin promoters disrupt binding sites for the repressors BCL11A and LRF. We used CRISPR-Cas9 to mimic HPFH mutations in the HBG promoters by generating insertions and deletions, leading to disruption of known and putative repressor binding sites. Editing of the LRF-binding site in patient-derived hematopoietic stem/progenitor cells (HSPCs) resulted in γ-globin derepression and correction of the sickling phenotype. Xenotransplantation of HSPCs treated with gRNAs targeting the LRF-binding site showed a high editing efficiency in repopulating HSPCs. This study identifies the LRF-binding site as a potent target for genome-editing treatment of SCD.

Correction: Genomic Analysis of Sleeping Beauty Transposon Integration in Human Somatic Cells.

[This corrects the article DOI: 10.1371/journal.pone.0112712.].

Biosafety Studies of a Clinically Applicable Lentiviral Vector for the Gene Therapy of Artemis-SCID.

Genetic deficiency of the nuclease DCLRE1C/Artemis causes radiosensitive severe combined immunodeficiency (RS-SCID) with lack of peripheral T and B cells and increased sensitivity to ionizing radiations. Gene therapy based on transplanting autologous gene-modified hematopoietic stem cells could significantly improve the health of patients with RS-SCID by correcting their immune system. A lentiviral vector expressing physiological levels of human ARTEMIS mRNA from an EF1a promoter without post-transcriptional regulation was developed as a safe clinically applicable candidate for RS-SCID gene therapy. The vector was purified in GMP-comparable conditions and was not toxic in vitro or in vivo. Long-term engraftment of vector-transduced hematopoietic cells was achieved in irradiated Artemis-deficient mice following primary and secondary transplantation (6 months each). Vector-treated mice displayed T and B lymphopoiesis and polyclonal T cells, had structured lymphoid tissues, and produced immunoglobulins. Benign signs of inflammation were noted following secondary transplants, likely a feature of the model. There was no evidence of transgene toxicity and no induction of hematopoietic malignancy. In vitro, the vector had low genotoxic potential on murine hematopoietic progenitor cells using an immortalization assay. Altogether, these preclinical data show safety and efficacy, and support further development of the vector for the gene therapy of RS-SCID.

Optimization of CRISPR/Cas9 Delivery to Human Hematopoietic Stem and Progenitor Cells for Therapeutic Genomic Rearrangements.

Editing the ß-globin locus in hematopoietic stem cells is an alternative therapeutic approach for gene therapy of ß-thalassemia and sickle cell disease. Using the CRISPR/Cas9 system, we genetically modified human hematopoietic stem and progenitor cells (HSPCs) to mimic the large rearrangements in the ß-globin locus associated with hereditary persistence of fetal hemoglobin (HPFH), a condition that mitigates the clinical phenotype of patients with ß-hemoglobinopathies. We optimized and compared the efficiency of plasmid-, lentiviral vector (LV)-, RNA-, and ribonucleoprotein complex (RNP)-based methods to deliver the CRISPR/Cas9 system into HSPCs. Plasmid delivery of Cas9 and gRNA pairs targeting two HPFH-like regions led to high frequency of genomic rearrangements and HbF reactivation in erythroblasts derived from sorted, Cas9+ HSPCs but was associated with significant cell toxicity. RNA-mediated delivery of CRISPR/Cas9 was similarly toxic but much less efficient in editing the ß-globin locus. Transduction of HSPCs by LVs expressing Cas9 and gRNA pairs was robust and minimally toxic but resulted in poor genome-editing efficiency. Ribonucleoprotein (RNP)-based delivery of CRISPR/Cas9 exhibited a good balance between cytotoxicity and efficiency of genomic rearrangements as compared to the other delivery systems and resulted in HbF upregulation in erythroblasts derived from unselected edited HSPCs.

Pre-clinical Development of a Lentiviral Vector Expressing the Anti-sickling ßAS3 Globin for Gene Therapy for Sickle Cell Disease.

Sickle cell disease (SCD) is caused by a mutation (E6V) in the hemoglobin (Hb) ß-chain that induces polymerization of Hb tetramers, red blood cell deformation, ischemia, anemia, and multiple organ damage. Gene therapy is a potential alternative to human leukocyte antigen (HLA)-matched allogeneic hematopoietic stem cell transplantation, available to a minority of patients. We developed a lentiviral vector expressing a ß-globin carrying three anti-sickling mutations (T87Q, G16D, and E22A) inhibiting axial and lateral contacts in the HbS polymer, under the control of the ß-globin promoter and a reduced version of the ß-globin locus-control region. The vector (GLOBE-AS3) transduced 60%-80% of mobilized CD34+ hematopoietic stem-progenitor cells (HSPCs) and drove ßAS3-globin expression at potentially therapeutic levels in erythrocytes differentiated from transduced HSPCs from SCD patients. Transduced HSPCs were transplanted in NOD.Cg-Prkdcscid Il2rgtm1Wjl/SzJ (NSG)-immunodeficient mice to analyze biodistribution, chimerism, and transduction efficiency in bone marrow (BM), spleen, thymus, and peripheral blood 12-14 weeks after transplantation. Vector integration site analysis, performed in pre-transplant HSPCs and post-transplant BM cells from individual mice, showed a normal lentiviral integration pattern and no evidence of clonal dominance. An in vitro immortalization (IVIM) assay showed the low genotoxic potential of GLOBE-AS3. This study enables a phase I/II clinical trial aimed at correcting the SCD phenotype in juvenile patients by transplantation of autologous hematopoietic stem cells (HSC) transduced by GLOBE-AS3.

Multiple Integrated Non-clinical Studies Predict the Safety of Lentivirus-Mediated Gene Therapy for ß-Thalassemia.

Gene therapy clinical trials require rigorous non-clinical studies in the most relevant models to assess the benefit-to-risk ratio. To support the clinical development of gene therapy for ß-thalassemia, we performed in vitro and in vivo studies for prediction of safety. First we developed newly GLOBE-derived vectors that were tested for their transcriptional activity and potential interference with the expression of surrounding genes. Because these vectors did not show significant advantages, GLOBE lentiviral vector (LV) was elected for further safety characterization. To support the use of hematopoietic stem cells (HSCs) transduced by GLOBE LV for the treatment of ß-thalassemia, we conducted toxicology, tumorigenicity, and biodistribution studies in compliance with the OECD Principles of Good Laboratory Practice. We demonstrated a lack of toxicity and tumorigenic potential associated with GLOBE LV-transduced cells. Vector integration site (IS) studies demonstrated that both murine and human transduced HSCs retain self-renewal capacity and generate new blood cell progeny in the absence of clonal dominance. Moreover, IS analysis showed an absence of enrichment in cancer-related genes, and the genes targeted by GLOBE LV in human HSCs are well known sites of integration, as seen in other lentiviral gene therapy trials, and have not been associated with clonal expansion. Taken together, these integrated studies provide safety data supporting the clinical application of GLOBE-mediated gene therapy for ß-thalassemia.

Ex Vivo COL7A1 Correction for Recessive Dystrophic Epidermolysis Bullosa Using CRISPR/Cas9 and Homology-Directed Repair.

Recessive dystrophic epidermolysis bullosa is a rare and severe genetic skin disease resulting in blistering of the skin and mucosa. Recessive dystrophic epidermolysis bullosa (RDEB) is caused by a wide variety of mutations in COL7A1-encoding type VII collagen, which is essential for dermal-epidermal adhesion. Here we demonstrate the feasibility of ex vivo COL7A1 editing in primary RDEB cells and in grafted 3D skin equivalents through CRISPR/Cas9-mediated homology-directed repair. We designed five guide RNAs to correct a RDEB causative null mutation in exon 2 (c.189delG; p.Leu64Trpfs*40). Among the site-specific guide RNAs tested, one showed significant cleavage activity in primary RDEB keratinocytes and in fibroblasts when delivered as integration-deficient lentivirus. Genetic correction was detected in transduced keratinocytes and fibroblasts by allele-specific highly sensitive TaqMan-droplet digital PCR (ddPCR), resulting in 11% and 15.7% of corrected COL7A1 mRNA expression, respectively, without antibiotic selection. Grafting of genetically corrected 3D skin equivalents onto nude mice showed up to 26% re-expression and normal localization of type VII collagen as well as anchoring fibril formation at the dermal-epidermal junction. Our study provides evidence that precise genome editing in primary RDEB cells is a relevant strategy to genetically correct COL7A1 mutations for the development of future ex vivo clinical applications.

Gene Therapy for Hemoglobinopathies.

Gene therapy for ß-thalassemia and sickle-cell disease is based on transplantation of genetically corrected, autologous hematopoietic stem cells. Preclinical and clinical studies have shown the safety and efficacy of this therapeutic approach, currently based on lentiviral vectors to transfer a ß-globin gene under the transcriptional control of regulatory elements of the ß-globin locus. Nevertheless, a number of factors are still limiting its efficacy, such as limited stem-cell dose and quality, suboptimal gene transfer efficiency and gene expression levels, and toxicity of myeloablative regimens. In addition, the cost and complexity of the current vector and cell manufacturing clearly limits its application to patients living in less favored countries, where hemoglobinopathies may reach endemic proportions. Gene-editing technology may provide a therapeutic alternative overcoming some of these limitations, though proving its safety and efficacy will most likely require extensive clinical investigation.

Gene Therapy for Sickle Cell Disease: A Lentiviral Vector Comparison Study.

Sickle cell disease (SCD) is an inherited blood disorder caused by a single amino acid substitution in the ß-globin chain of hemoglobin. Gene therapy is a promising therapeutic alternative, particularly in patients lacking an allogeneic bone marrow (BM) donor. One of the major challenges for an effective gene therapy approach is the design of an efficient vector that combines high-level and long-term ß-globin expression with high infectivity in primary CD34+ cells. Two lentiviral vectors carrying an anti-sickling ß-globin transgene (AS3) were directly compared: the Lenti/ßAS3-FB, and Globe-AS3 with and without the FB insulator. The comparison was performed initially in human BM CD34+ cells derived from SCD patients in an in vitro model of erythroid differentiation. Additionally, the comparison was carried out in two in vivo models: First, an NOD SCID gamma mouse model was used to compare transduction efficiency and ß-globin expression in human BM CD34+ cells after transplant. Second, a sickle mouse model was used to analyze ß-globin expression produced from the vectors tested, as well as hematologic correction of the sickle phenotype. While minor differences were found in the vectors in the in vitro study (2.4-fold higher vector copy number in CD34+ cells when using Globe-AS3), no differences were noted in the overall correction of the SCD phenotype in the in vivo mouse model. This study provides a comprehensive in vitro and in vivo analysis of two globin lentiviral vectors, which is useful for determining the optimal candidate for SCD gene therapy.

An Optimized Lentiviral Vector Efficiently Corrects the Human Sickle Cell Disease Phenotype.

Autologous transplantation of hematopoietic stem cells transduced with a lentiviral vector (LV) expressing an anti-sickling HBB variant is a potential treatment for sickle cell disease (SCD). With a clinical trial as our ultimate goal, we generated LV constructs containing an anti-sickling HBB transgene (HBBAS3), a minimal HBB promoter, and different combinations of DNase I hypersensitive sites (HSs) from the locus control region (LCR). Hematopoietic stem progenitor cells (HSPCs) from SCD patients were transduced with LVs containing either HS2 and HS3 (ß-AS3) or HS2, HS3, and HS4 (ß-AS3 HS4). The inclusion of the HS4 element drastically reduced vector titer and infectivity in HSPCs, with negligible improvement of transgene expression. Conversely, the LV containing only HS2 and HS3 was able to efficiently transduce SCD bone marrow and Plerixafor-mobilized HSPCs, with anti-sickling HBB representing up to ∼60% of the total HBB-like chains. The expression of the anti-sickling HBB and the reduced incorporation of the ßS-chain in hemoglobin tetramers allowed up to 50% reduction in the frequency of RBC sickling under hypoxic conditions. Together, these results demonstrate the ability of a high-titer LV to express elevated levels of a potent anti-sickling HBB transgene ameliorating the SCD cell phenotype.

Preclinical Development of a Lentiviral Vector for Gene Therapy of X-Linked Severe Combined Immunodeficiency.

X-linked severe combined immunodeficiency (SCID-X1) is caused by mutations in the interleukin-2 receptor γ chain gene (IL2RG), and it is characterized by profound defects in T, B, and natural killer (NK) cell functions. Transplantation of hematopoietic stem/progenitor cells (HSPCs) genetically corrected with early murine leukemia retrovirus (MLV)-derived gammaretroviral vectors showed restoration of T cell immunity in patients, but it resulted in vector-induced insertional oncogenesis. We developed a self-inactivating (SIN) lentiviral vector carrying a codon-optimized human IL2RG cDNA driven by the EF1α short promoter (EFS-IL2RG), and we tested its efficacy and safety in vivo by transplanting transduced Il2rg-deficient Lin- HSPCs in an Il2rg-/-/Rag2-/- mouse model. The study showed restoration of T, B, and NK cell counts in bone marrow and peripheral blood and normalization of thymus and spleen cellularity and architecture. High-definition insertion site analysis defined the EFS-IL2RG genomic integration profile, and it showed no sign of vector-induced clonal selection or skewing in primarily and secondarily transplanted animals. The study enables a phase I/II clinical trial aimed at restoring both T and B cell immunity in SCID-X1 children upon non-myeloablative conditioning.