Gene Therapy for X-Linked Severe Combined Immunodeficiency: Where Do We Stand?

Comparison and cross-validation of long-read and short-read target-enrichment sequencing methods to assess AAV vector integration into host genome

Evaluation of host integration profiles by adeno-associated virus (AAV) is an important component of de-risking novel AAV gene therapies. Targeted enrichment sequencing (TES) is a cost-effective and comprehensive method for assessing integration. Most published TES datasets have been generated using short-read sequencing, which enables quantitation of integration sites (ISs) and identifies patterns such as hotspots or clonal expansion. Characteristics such as IS length and recombination require longer reads to measure. The present study compared short-read to long-read TES using samples from monkeys treated with AAV and used in vitro lentiviral-treated samples, a stable cell line, and an engineered spike-in as controls. Both methods showed stochastic integration by both AAV and lentivirus, with most vector domains identified in ISs. More ISs were identified by short-read TES, as deeper coverage per base was achieved from a single sequencing run. AAV-treated samples showed minimal evidence of clonal expansion, in contrast to in vitro treated and stably transduced lentiviral cell line samples. Long-read TES revealed vector rearrangement in 4%-40% of ISs in AAV-treated animals. In summary, both methods yielded similar conclusions about relative numbers of ISs and overall patterns. Long-read TES identified fewer ISs but enabled measurement of IS length and recombination patterns.

Assessment of non-myelotoxic agents as a preparatory regimen for hematopoietic stem cell gene therapy

RAG2 deficiency is characterized by a lack of B and T lymphocytes, causing severe lethal infections. Currently, RAG2 deficiency is treated with a Hematopoietic Stem Cell transplantation (HSCT). Most conditioning regimens used before HSCT consist of alkylating myelotoxic agents with or without irradiation and affect growth and development of pediatric patients. Here, we developed a non-myelotoxic regimen using G-CSF, VLA-4I or AMD3100. These agents are known HSC mobilizers or affect bone marrow (BM) permeability and may support the homing of HSCs to the BM, without inducing major side effects. Female Rag2-/- mice were pre-treated with Busulfan (BU), G-CSF, VLA-4I or AMD3100 and transplanted with male BM cells transduced with a lentiviral vector carrying codon optimized human RAG2 (RAG2co). Peripheral blood cell counts increased significantly after G-CSF, VLA-4I and AMD3100 treatment, but not after BU. Reconstitution of PB lymphocytes was comparable for all groups with full immune reconstitution at 6 months post transplantation, despite different methods of conditioning. Survival of mice pre-treated with non-myelotoxic agents was significantly higher than after BU treatment. Here, we show that the non-myelotoxic agents G-CSF, VLA-4I, and AMD3100 are highly effective as conditioning regimen before HSC gene therapy and can be used as an alternative to BU.

Demystifying gene(tic) therapies

This article summarizes the discussion from a session entitled "Demystifying gene therapies" that was held at the joint RE(ACT) congress and IRDiRC conference, 14-15 March 2023 in Berlin, Germany. The focus of this session was to discuss the changing landscape of genetic therapies and whether current resources exist to provide adequate education to stakeholders, such as researchers, clinicians, patient advocates, legislators, as well as the patients and their families. The goal of this article is not to provide a comprehensive overview of the current landscape in genetic therapies, but rather to highlight resources that may be useful to help "demystify" the myriad of genetic therapeutic approaches.

Improving the Assessment of Risk Factors Relevant to Potential Carcinogenicity of Gene Therapies: A Consensus Article

Regulators and industry are actively seeking improvements and alternatives to current models and approaches to evaluate potential carcinogenicity of gene therapies (GTs). A meeting of invited experts was organized by NC3Rs/UKEMS (London, March 2023) to discuss this topic. This article describes the consensus reached among delegates on the definition of vector genotoxicity, sources of uncertainty, suitable toxicological endpoints for genotoxic assessment of GTs, and future research needs. The collected recommendations should inform the further development of regulatory guidelines for the nonclinical toxicological assessment of GT products.

Hematopoietic stem cell gene therapy for the treatment of SYNGAP1-related non-specific intellectual disability

BACKGROUND

Synaptic Ras GTPase activating protein 1 (SYNGAP1)-related non-specific intellectual disability is a neurodevelopmental disorder caused by an insufficient level of SynGAP1 resulting in a dysfunction of neuronal synapses and presenting with a wide array of clinical phenotypes. Hematopoietic stem cell gene therapy has the potential to deliver therapeutic levels of functional SynGAP1 to affected neurons upon transduction of hematopoietic stem and progenitor cells with a lentiviral vector.

METHODS

As a novel approach toward the treatment of SYNGAP1, we have generated a lentiviral vector expressing a modified form of SynGAP1 for transduction of human CD34+ hematopoietic stem and progenitor cells. The gene-modified cells were then transplanted into adult immunodeficient SYNGAP1+/- heterozygous mice and evaluated for improvement of SYNGAP1-related clinical phenotypes. Expression of SynGAP1 was also evaluated in the brain tissue of transplanted mice.

RESULTS

In our proof-of-concept study, we have demonstrated significant improvement of SYNGAP1-related phenotypes including an improvement in motor abilities observed in mice transplanted with the vector transduced cells because they displayed decreased hyperactivity in an open field assay and an increased latency to fall in a rotarod assay. An increased level of SynGAP1 was also detected in the brains of these mice.

CONCLUSIONS

These early-stage results highlight the potential of this stem cell gene therapy approach as a treatment strategy for SYNGAP1.

Molecular Mechanisms in Pathophysiology of Mucopolysaccharidosis and Prospects for Innovative Therapy

Mucopolysaccharidoses (MPSs) are a group of inborn errors of the metabolism caused by a deficiency in the lysosomal enzymes required to break down molecules called glycosaminoglycans (GAGs). These GAGs accumulate over time in various tissues and disrupt multiple biological systems, including catabolism of other substances, autophagy, and mitochondrial function. These pathological changes ultimately increase oxidative stress and activate innate immunity and inflammation. We have described the pathophysiology of MPS and activated inflammation in this paper, starting with accumulating the primary storage materials, GAGs. At the initial stage of GAG accumulation, affected tissues/cells are reversibly affected but progress irreversibly to: (1) disruption of substrate degradation with pathogenic changes in lysosomal function, (2) cellular dysfunction, secondary/tertiary accumulation (toxins such as GM2 or GM3 ganglioside, etc.), and inflammatory process, and (3) progressive tissue/organ damage and cell death (e.g., skeletal dysplasia, CNS impairment, etc.). For current and future treatment, several potential treatments for MPS that can penetrate the blood-brain barrier and bone have been proposed and/or are in clinical trials, including targeting peptides and molecular Trojan horses such as monoclonal antibodies attached to enzymes via receptor-mediated transport. Gene therapy trials with AAV, ex vivo LV, and Sleeping Beauty transposon system for MPS are proposed and/or underway as innovative therapeutic options. In addition, possible immunomodulatory reagents that can suppress MPS symptoms have been summarized in this review.

Lentiviral Production Platform

Lentiviral-mediated transfection technique is a powerful tool for gene modification in preclinical studies. By using this technique, the desired gene modification can be achieved easily in immune cells, nondividing, and terminally differentiated cells, including hematopoietic stem cells, neurons, and even tumor cells, which other viral vectors cannot do. The main considerations of therapeutic gene delivery using a lentiviral system are the risk of insertional mutagenesis and the immune reaction elicited by infected cells. Although some biosafety concerns need to be addressed before clinical trials in rheumatoid arthritis, the lentiviral system targeting therapeutic targets has been widely used for in vivo gene transfer in animal models. In this chapter, the protocols for production of viral particles and viral concentration are provided. As an alternative utilization, this lentiviral production platform could also be employed to produce a pseudotype severe acute respiratory syndrome-related coronavirus 2 in which the spike glycoprotein of SARS-CoV-2 was incorporated into pseudovirions for viral study.

Salivary Gland Bioengineering

Salivary gland dysfunction affects millions globally, and tissue engineering may provide a promising therapeutic avenue. This review delves into the current state of salivary gland tissue engineering research, starting with a study of normal salivary gland development and function. It discusses the impact of fibrosis and cellular senescence on salivary gland pathologies. A diverse range of cells suitable for tissue engineering including cell lines, primary salivary gland cells, and stem cells are examined. Moreover, the paper explores various supportive biomaterials and scaffold fabrication methodologies that enhance salivary gland cell survival, differentiation, and engraftment. Innovative engineering strategies for the improvement of vascularization, innervation, and engraftment of engineered salivary gland tissue, including bioprinting, microfluidic hydrogels, mesh electronics, and nanoparticles, are also evaluated. This review underscores the promising potential of this research field for the treatment of salivary gland dysfunction and suggests directions for future exploration.

In vivo hematopoietic stem cell modification by mRNA delivery

Hematopoietic stem cells (HSCs) are the source of all blood cells over an individual's lifetime. Diseased HSCs can be replaced with gene-engineered or healthy HSCs through HSC transplantation (HSCT). However, current protocols carry major side effects and have limited access. We developed CD117/LNP-messenger RNA (mRNA), a lipid nanoparticle (LNP) that encapsulates mRNA and is targeted to the stem cell factor receptor (CD117) on HSCs. Delivery of the anti-human CD117/LNP-based editing system yielded near-complete correction of hematopoietic sickle cells. Furthermore, in vivo delivery of pro-apoptotic PUMA (p53 up-regulated modulator of apoptosis) mRNA with CD117/LNP affected HSC function and permitted nongenotoxic conditioning for HSCT. The ability to target HSCs in vivo offers a nongenotoxic conditioning regimen for HSCT, and this platform could be the basis of in vivo genome editing to cure genetic disorders, which would abrogate the need for HSCT.

Multiplex HDR for disease and correction modeling of SCID by CRISPR genome editing in human HSPCs

Severe combined immunodeficiency (SCID) is a group of disorders caused by mutations in genes involved in the process of lymphocyte maturation and function. CRISPR-Cas9 gene editing of the patient's own hematopoietic stem and progenitor cells (HSPCs) ex vivo could provide a therapeutic alternative to allogeneic hematopoietic stem cell transplantation, the current gold standard for treatment of SCID. To eliminate the need for scarce patient samples, we engineered genotypes in healthy donor (HD)-derived CD34+ HSPCs using CRISPR-Cas9/rAAV6 gene-editing, to model both SCID and the therapeutic outcomes of gene-editing therapies for SCID via multiplexed homology-directed repair (HDR). First, we developed a SCID disease model via biallelic knockout of genes critical to the development of lymphocytes; and second, we established a knockin/knockout strategy to develop a proof-of-concept single-allelic gene correction. Based on these results, we performed gene correction of RAG2-SCID patient-derived CD34+ HSPCs that successfully developed into CD3+ T cells with diverse TCR repertoires in an in vitro T cell differentiation platform. In summary, we present a strategy to determine the optimal configuration for CRISPR-Cas9 gene correction of SCID using HD-derived CD34+ HSPCs, and the feasibility of translating this gene correction approach in patient-derived CD34+ HSPCs.

Genetic engineering and genome editing in plants, animals and humans: Facts and myths

Human history is inextricably linked to the introduction of desirable heritable traits in plants and animals. Selective breeding (SB) predates our historical period and has been practiced since the advent of agriculture and farming more than ten thousand years ago. Since the 1970s, methods of direct plant and animal genome manipulation are constantly being developed. These are collectively described as "genetic engineering" (GE). Plant GE aims to improve nutritional value, insect resistance and weed control. Animal GE has focused on livestock improvement and disease control. GE applications also involve medical improvements intended to treat human disease. The scientific consensus built around marketed products of GE organisms (GEOs) is usually well established, noting significant benefits and low risks. GEOs are exhaustively scrutinized in the EU and many non-EU countries for their effects on human health and the environment, but scrutiny should be equally applied to all previously untested organisms derived directly from nature or through selective breeding. In fact, there is no evidence to suggest that natural or selectively bred plants and animals are in principle safer to humans than GEOs. Natural and selectively bred strains evolve over time via genetic mutations that can be as risky to humans and the environment as the mutations found in GEOs. Thus, previously untested plant and animal strains aimed for marketing should be proven useful or harmful to humans only upon comparative testing, regardless of their origin. Highlighting the scientific consensus declaring significant benefits and rather manageable risks provided by equitably accessed GEOs, can mitigate negative predispositions by policy makers and the public. Accordingly, we provide an overview of the underlying technologies and the scientific consensus to help resolve popular myths about the safety and usefulness of GEOs.

Correcting inborn errors of immunity: From viral mediated gene addition to gene editing

Allogeneic hematopoietic stem cell transplantation is an effective treatment to cure inborn errors of immunity. Remarkable progress has been achieved thanks to the development and optimization of effective combination of advanced conditioning regimens and use of immunoablative/suppressive agents preventing rejection as well as graft versus host disease. Despite these tremendous advances, autologous hematopoietic stem/progenitor cell therapy based on ex vivo gene addition exploiting integrating γ-retro- or lenti-viral vectors, has demonstrated to be an innovative and safe therapeutic strategy providing proof of correction without the complications of the allogeneic approach. The recent advent of targeted gene editing able to precisely correct genomic variants in an intended locus of the genome, by introducing deletions, insertions, nucleotide substitutions or introducing a corrective cassette, is emerging in the clinical setting, further extending the therapeutic armamentarium and offering a cure to inherited immune defects not approachable by conventional gene addition. In this review, we will analyze the current state-of-the art of conventional gene therapy and innovative protocols of genome editing in various primary immunodeficiencies, describing preclinical models and clinical data obtained from different trials, highlighting potential advantages and limits of gene correction.

Novel suspension retroviral packaging cells generated by transposition using transposase encoding mRNA advance vector yields and enable production in bioreactors

To date, the establishment of high-titer stable viral packaging cells (VPCs) at large scale for gene therapeutic applications is very time- and cost-intensive. Here we report the establishment of three human suspension 293-F-derived ecotropic MLV-based VPCs. The classic stable transfection of an EGFP-expressing transfer vector resulted in a polyclonal VPC pool that facilitated cultivation in shake flasks of 100 mL volumes and yielded high functional titers of more than 1 × 10⁶ transducing units/mL (TU/mL). When the transfer vector was flanked by transposon terminal inverted repeats (TIRs) and upon co-transfection of a plasmid encoding for the transposase, productivities could be slightly elevated to more than 3 × 10⁶ TU/mL. In contrast and using mRNA encoding for the transposase, as a proof of concept, productivities were drastically improved by more than ten-fold exceeding 5 × 10⁷ TU/mL. In addition, these VPC pools were generated within only 3 weeks. The production volume was successfully scaled up to 500 mL employing a stirred-tank bioreactor (STR). We anticipate that the stable transposition of transfer vectors employing transposase transcripts will be of utility for the future establishment of high-yield VPCs producing pseudotype vector particles with a broader host tropism on a large scale.

The Potential of Cell-Penetrating Peptides for mRNA Delivery to Cancer Cells

In vitro transcribed mRNA for the synthesis of any given protein has shown great potential in cancer gene therapy, especially in cancer vaccines for immunotherapy. To overcome physiological barriers, such as rapid degradation by enzymatic attack and poor cellular uptake due to their large size and hydrophilic properties, many delivery carriers for mRNAs are being investigated for improving the bioavailability of mRNA. Recently, cell-penetrating peptides (CPPs) have received attention as promising tools for gene delivery. In terms of their biocompatibility and the ability to target specific cells with the versatility of peptide sequences, they may provide clues to address the challenges of conventional delivery systems for cancer mRNA delivery. In this study, optimal conditions for the CPP/mRNA complexes were identified in terms of complexation capacity and N/P ratio, and protection against RNase was confirmed. When cancer cells were treated at a concentration of 6.8 nM, which could deliver the highest amount of mRNA without toxicity, the amphipathic CPP/mRNA complexes with a size less than 200 nm showed high cellular uptake and protein expression. With advances in our understanding of CPPs, CPPs designed to target tumor tissues will be promising for use in developing a new class of mRNA delivery vehicles in cancer therapy.

Genetics Matters: Voyaging from the Past into the Future of Humanity and Sustainability

The understanding of how genetic information may be inherited through generations was established by Gregor Mendel in the 1860s when he developed the fundamental principles of inheritance. The science of genetics, however, began to flourish only during the mid-1940s when DNA was identified as the carrier of genetic information. The world has since then witnessed rapid development of genetic technologies, with the latest being genome-editing tools, which have revolutionized fields from medicine to agriculture. This review walks through the historical timeline of genetics research and deliberates how this discipline might furnish a sustainable future for humanity.

Advanced Therapy Medicinal Products' Translation in Europe: A Developers' Perspective

Advanced Therapy Medicinal Products (ATMPs) comprising cell, gene, and tissue-engineered therapies have demonstrated enormous therapeutic benefits. However, their development is complex to be managed efficiently within currently existing regulatory frameworks. Legislation and regulation requirements for ATMPs must strike a balance between the patient safety while promoting innovations to optimize exploitation of these novel therapeutics. This paradox highlights the importance of on-going dynamic dialogue between all stakeholders and regulatory science to facilitate the development of pragmatic ATMP regulatory guidelines.

Future of genetic therapies for rare genetic diseases: what to expect for the next 15 years?

Introduction

Rare genetic diseases affect millions of people worldwide. Most of them are caused by defective genes that impair quality of life and can lead to premature death. As genetic therapies aim to fix or replace defective genes, they are considered the most promising treatment for rare genetic diseases. Yet, as these therapies are still under development, it is still unclear whether they will be successful in treating these diseases. This study aims to address this gap by assessing researchers' opinions on the future of genetic therapies for the treatment of rare genetic diseases.

Methods

We conducted a global cross-sectional web-based survey of researchers who recently authored peer-reviewed articles related to rare genetic diseases.

Results

We assessed the opinions of 1430 researchers with high and good knowledge about genetic therapies for the treatment of rare genetic diseases. Overall, the respondents believed that genetic therapies would be the standard of care for rare genetic diseases before 2036, leading to cures after this period. CRISPR-Cas9 was considered the most likely approach to fixing or replacing defective genes in the next 15 years. The respondents with good knowledge believed that genetic therapies would only have long-lasting effects after 2036, while those with high knowledge were divided on this issue. The respondents with good knowledge on the subject believed that non-viral vectors are more likely to be successful in fixing or replacing defective genes in the next 15 years, while most of the respondents with high knowledge believed viral vectors would be more successful.

Conclusion

Overall, the researchers who participated in this study expect that in the future genetic therapies will greatly benefit the treatment of patients with rare genetic diseases.

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.

Evaluation and Designing of Epitopic-Peptide Vaccine Against Bunyamwera orthobunyavirus Using M-Polyprotein Target Sequences

Bunyamwera orthobunyavirus and its serogroup can cause several diseases in humans, cattle, ruminants, and birds. The viral M-polyprotein helps the virus to enter the host body. Therefore, this protein might serve as a potential vaccine target against Bunyamwera orthobunyavirus. The present study applied the immunoinformatics technique to design an epitopic vaccine component that could protect against Bunyamwera infection. Phylogenetic analysis revealed the presence of conserved patterns of M-polyprotein within the viral serogroup. Three epitopes common for both B-cell and T-cell were identified, i.e., YQPTELTRS, YKAHDKEET, and ILGTGTPKF merged with a specific linker peptide to construct an active vaccine component. The low atomic contact energy value of docking complex between human TLR4 (TLR4/MD2 complex) and vaccine construct confirms the elevated protein–protein binding interaction. Molecular dynamic simulation and normal mode analysis illustrate the docking complex’s stability, especially by the higher Eigenvalue. In silico cloning of the vaccine construct was applied to amplify the desired vaccine component. Structural allocation of both the vaccine and epitopes also show the efficacy of the developed vaccine. Hence, the computational research design outcomes support that the peptide-based vaccine construction is a crucial drive target to limit the infection of Bunyamwera orthobunyavirus to an extent.

The Off-Targets of Clustered Regularly Interspaced Short Palindromic Repeats Gene Editing

The repurposing of the CRISPR/Cas bacterial defense system against bacteriophages as simple and flexible molecular tools has revolutionized the field of gene editing. These tools are now widely used in basic research and clinical trials involving human somatic cells. However, a global moratorium on all clinical uses of human germline editing has been proposed because the technology still lacks the required efficacy and safety. Here we focus on the approaches developed since 2013 to decrease the frequency of unwanted mutations (the off-targets) during CRISPR-based gene editing.

mRNA Delivery by a pH-Responsive DNA Nano-Hydrogel

The delivery of mRNA to manipulate protein expression has attracted widespread attention, since that mRNA overcomes the problem of infection and mutation risks in transgenes and can work as drugs for the treatment of diseases. Although there are currently some vehicles that deliver mRNA into cells, they have not yet reached a good balance in terms of expression efficiency and biocompatibility. Here, a DNA nano-hydrogel system for mRNA delivery is developed. The nano-hydrogel is all composed of DNA except the target mRNA, so it has superior biocompatibility compared with those chemical vehicles. In parallel, the nano-hydrogel can be compacted into a nanosphere under the crosslinking by well-designed "X"-shaped DNA scaffolds and DNA linkers, facilitating the delivery into cells through endocytosis. In addition, smart intracellular release of the mRNA is achieved by incorporating a pH-responsive i-motif structure into the nano-hydrogel. Thus, taking the efficient delivery and release together, mRNA can be translated into the corresponding protein with a high efficiency, which is comparable to that of the commercial liposome but with a much better biocompatibility. Due to the excellent biocompatibility and efficiency, this nano-hydrogel system is expected to become a competitive alternative for delivering functional mRNA in vivo.

Economic evaluation of betibeglogene autotemcel (Beti-cel) gene addition therapy in transfusion-dependent β-thalassemia

Background: Standard of care (SoC) for transfusion-dependent β-thalassemia (TDT) requires lifelong, regular blood transfusions as well as chelation to reduce iron accumulation. Objective: This study investigates the cost-effectiveness of betibeglogene autotemcel ('beti-cel'; LentiGlobin for β-thalassemia) one-time, gene addition therapy compared to lifelong SoC for TDT. Study design: Microsimulation model simulated the lifetime course of TDT based on a causal sequence in which transfusion requirements determine tissue iron levels, which in turn determine risk of iron overload complications that increase mortality. Clinical trial data informed beti-cel clinical parameters; effects of SoC on iron levels came from real-world studies; iron overload complication rates and mortality were based on published literature. Setting: USA; commercial payer perspective Participants: TDT patients age 2-50 Interventions: Beti-cel is compared to SoC. Main outcome measure: Incremental cost-effectiveness ratio (ICER) utilizing quality-adjusted life-years (QALYs) Results: The model predicts beti-cel adds 3.8 discounted life years (LYs) or 6.9 QALYs versus SoC. Discounted lifetime costs were $2.28 M for beti-cel ($572,107 if excluding beti-cel cost) and $2.04 M for SoC, with a resulting ICER of $34,833 per QALY gained. Conclusion: Beti-cel is cost-effective for TDT patients compared to SoC. This is due to longer survival and cost offset of lifelong SoC.

Strategies for Targeting Retroviral Integration for Safer Gene Therapy: Advances and Challenges

Retroviruses are obligate intracellular parasites that must integrate a copy of the viral genome into the host DNA. The integration reaction is performed by the viral enzyme integrase in complex with the two ends of the viral cDNA genome and yields an integrated provirus. Retroviral vector particles are attractive gene therapy delivery tools due to their stable integration. However, some retroviral integration events may dysregulate host oncogenes leading to cancer in gene therapy patients. Multiple strategies to target retroviral integration, particularly to genetic safe harbors, have been tested with limited success. Attempts to target integration may be limited by the multimerization of integrase or the presence of host co-factors for integration. Several retroviral integration complexes have evolved a mechanism of tethering to chromatin via a host protein. Integration host co-factors bind chromatin, anchoring the complex and allowing integration. The tethering factor allows for both close proximity to the target DNA and specificity of targeting. Each retrovirus appears to have distinct preferences for DNA sequence and chromatin features at the integration site. Tethering factors determine the preference for chromatin features, but do not affect the subtle sequence preference at the integration site. The sequence preference is likely intrinsic to the integrase protein. New developments may uncouple the requirement for a tethering factor and increase the ability to redirect retroviral integration.

HIV-based lentiviral vectors: origin and sequence differences

Three gene therapy strategies have received US Food and Drug Administration (FDA) approval; one includes HIV-1-based lentiviral vectors. These vectors incorporate features to provide long-term gene transfer and expression while minimizing generation of a replication-competent virus or pathogenicity. Importantly, the coding regions of viral proteins were deleted, and the cis-acting regulatory elements were retained. With the use of representative vectors developed for clinical/commercial applications, we compared the vector backbone sequences to the initial sources of the HIV-1. All vectors included required elements: 5′ long terminal repeat (LTR) through the Ψ packaging signal, central polypurine tract/chain termination sequence (cPPT/CTS), Rev responsive element (RRE), and 3′ LTR, including a poly(A) signal. The Ψ signaling sequence demonstrated the greatest similarity between all vectors with only minor changes. The 3′ LTR was the most divergent sequence with a range of deletions. The RRE length varied between vectors. Phylogenetic analysis of the cPPT/CTS indicated multiple sources, perhaps because of its later inclusion into lentiviral vector systems, whereas other regions revealed node clusters around the HIV-1 reference genomes HXB2 and NL4-3. We examine the function of each region in a lentiviral vector, the molecular differences between vectors, and where optimization may guide development of the lentiviral delivery systems.

Overview of the current status of gene therapy for primary immune deficiencies (PIDs)

Over 3 decades, gene therapy has advanced from a logical idea to becoming a clinical reality for several of the most severe primary immune deficiencies, as well as other inherited disorders. The first gene therapy medicines have been licensed for marketing and several more are advancing toward that goal to make them widely available, beyond clinical trials. Although common platforms of cells, vectors, or editing reagents are used for these disorders, each individual genetic cause of an immune deficiency requires its own vector or editing tools and a package of preclinical data on efficacy and safety to initiate clinical trials. One-by-one, gene therapy for primary immune deficiencies is being brought to the clinic and hopefully will provide safe and effective therapies.

An appraisal of the Wilson & Jungner criteria in the context of genomic-based newborn screening for inborn errors of immunity

Wilson and Jungner's recommendations for population-based screening have been used to guide decisions regarding candidate disease inclusion in newborn screening programs for the past 50 years. The advent of genomic-based technologies, including next-generation sequencing and its potential application to newborn screening, along with a changing landscape in terms of modern clinical practice and ethical, social, and legal considerations has led to a call for review of these criteria. Inborn errors of immunity (IEI) are a heterogeneous group of more than 450 genetically determined disorders of immunity, which are associated with significant morbidity and mortality, particularly where diagnosis and treatment are delayed. We argue that in addition to screening for severe combined immunodeficiency disease, which has already been initiated in several countries, other clinically significant IEI should be screened for at birth. Because of disease heterogeneity and identifiable genetic targets, a next-generation sequencing-based screening approach would be most suitable. A combination of worldwide experience and technological advances has improved our ability to diagnose and effectively treat patients with IEI. Considering IEI in the context of updated recommendations for population-based screening supports their potential inclusion as disease targets in newborn screening programs.

Immune Reconstitution After Gene Therapy Approaches in Patients With X-Linked Severe Combined Immunodeficiency Disease

X-linked severe immunodeficiency disease (SCID-X1) is an inherited, rare, and life-threating disease. The genetic origin is a defect in the interleukin 2 receptor γ chain (IL2RG) gene and patients are classically characterized by absence of T and NK cells, as well as presence of partially-functional B cells. Without any treatment the disease is usually lethal during the first year of life. The treatment of choice for these patients is hematopoietic stem cell transplantation, with an excellent survival rate (>90%) if an HLA-matched sibling donor is available. However, when alternative donors are used, the success and survival rates are often lower. Gene therapy has been developed as an alternative treatment initially using γ-retroviral vectors to correct the defective γ chain in the absence of pre-conditioning treatment. The results were highly promising in SCID-X1 infants, showing long-term T-cell recovery and clinical benefit, although NK and B cell recovery was less robust. However, some infants developed T-cell acute lymphoblastic leukemia after the gene therapy, due to vector-mediated insertional mutagenesis. Consequently, considerable efforts have been made to develop safer vectors. The most recent clinical trials using lentiviral vectors together with a low-dose pre-conditioning regimen have demonstrated excellent sustained T cell recovery, but also B and NK cells, in both children and adults. This review provides an overview about the different gene therapy approaches used over the last 20 years to treat SCID-X1 patients, particularly focusing on lymphoid immune reconstitution, as well as the developments that have improved the process and outcomes.

Ex vivo generated human T-lymphoid progenitors as a tool to accelerate immune reconstitution after partially HLA compatible hematopoietic stem cell transplantation or after gene therapy

Prolonged T-cell immunodeficiency following HLA- incompatible hematopoietic stem cell transplantation (HSCT) represents a major obstacle hampering the more widespread use of this approach. Strategies to fasten T-cell reconstitution in this setting are highly warranted as opportunistic infections and an increased risk of relapse account for high rates of morbidity and mortality especially during early month following this type of HSCT. We have implemented a feeder free cell system based on the use of the notch ligand DL4 and cytokines allowing for the in vitro differentiation of human T-Lymphoid Progenitor cells (HTLPs) from various sources of CD34+ hematopoietic stem and precursor cells (HSPCs). Co- transplantion of human T-lymphoid progenitors (HTLPs) and non- manipulated HSPCs into immunodeficient mice successfully accelerated the reconstitution of a polyclonal T-cell repertoire. This review summarizes preclinical data on the use of T-cell progenitors for treatment of post- transplantation immunodeficiency and gives insights into the development of GMP based protocols for potential clinical applications including gene therapy approaches. Future clinical trials implementing this protocol will aim at the acceleration of immune reconstitution in different clinical settings such as SCID and leukemia patients undergoing allogeneic transplantation. Apart from pure cell-therapy approaches, the combination of DL-4 culture with gene transduction protocols will open new perspectives in terms of gene therapy applications for primary immunodeficiencies.

Adeno-Associated Virus-Based Gene Therapy for Lifelong Correction of Genetic Disease

The list of successful gene therapy trials using adeno-associated virus (AAV)-based vectors continues to grow and includes a wide range of monogenic diseases. Replication incompetent AAV genomes typically remain episomal and expression dilutes as cells divide and die. Consequently, long-term transgene expression from AAV is best suited for quiescent cell types, such as retinal cells, myocytes, or neurons. For genetic diseases that involve cells with steady turnover, AAV-conferred correction may require routine readministration, where every dose carries the risk of developing an adaptive immune response that renders treatment ineffective. Here, we discuss innovative approaches to permanently modify the host genome using AAV-based platforms, thus potentially requiring only a single dose. Such approaches include using AAV delivery of DNA transposons, homologous recombination templates into safe harbors, and nucleases for targeting integration. In tissues with continual cell turnover, genetic modification of progenitor cell populations will help ensure persistent therapeutic outcomes. Combining the safety profile of AAV-based gene therapy vectors with the ability to integrate a therapeutic transgene creates novel solutions to the challenge of lifelong curative treatments for human genetic diseases.

Successful Preclinical Development of Gene Therapy for Recombinase-Activating Gene-1-Deficient SCID

Recombinase-activating gene-1 (RAG1)-deficient severe combined immunodeficiency (SCID) patients lack B and T lymphocytes due to the inability to rearrange immunoglobulin and T cell receptor genes. Gene therapy is an alternative for those RAG1-SCID patients who lack a suitable bone marrow donor. We designed lentiviral vectors with different internal promoters driving codon-optimized RAG1 to ensure optimal expression. We used Rag1 -/- mice as a preclinical model for RAG1-SCID to assess the efficacy of the various vectors. We observed that B and T cell reconstitution directly correlated with RAG1 expression. Mice with low RAG1 expression showed poor immune reconstitution; however, higher expression resulted in phenotypic and functional lymphocyte reconstitution comparable to mice receiving wild-type stem cells. No signs of genotoxicity were found. Additionally, RAG1-SCID patient CD34+ cells transduced with our clinical RAG1 vector and transplanted into NSG mice led to improved human B and T cell development. Considering this efficacy outcome, together with favorable safety data, these results substantiate the need for a clinical trial for RAG1-SCID.

Severe Combined Immunodeficiency: A Review for Neonatal Clinicians

The proper development and function of T cells is imperative in the creation of adequate cell-mediated and humoral immunity. Healthy term newborns have baseline immune immaturity, increasing their risk of infections, but significant immunologic consequences can occur, because of abnormal T-cell maturation. Combined immunodeficiencies can result, because B cells and natural killer cells rely on successful interactions with T cells to ensure their proper performance and survival. Severe combined immunodeficiency (SCID) is the most noteworthy of these conditions, leading to considerable early morbidity and often death by the age of 1 year if left untreated. Newborn screening for SCID is effective and allows for early implementation of lifesaving supportive measures, including protective isolation, initiation of prophylactic antimicrobials, caution with blood product transfusions, and avoidance of live vaccinations. Once a definitive diagnosis of SCID has been established, treatment frequently involves bone marrow or stem cell transplantation; however, enzyme replacement and gene therapy are also becoming options in those with SCID due to adenosine deaminase deficiency and other forms of SCID. Neonatal clinicians should understand the screening and diagnostic approach to SCID along with the initial management approaches for these extremely high-risk patients.

Nanoparticle formulated vaccines: opportunities and challenges

Vaccines harness the inherent properties of the immune system to prevent diseases or treat existing ones. Continuous efforts have been devoted to both gaining a mechanistic understanding of how the immune system operates and designing vaccines with high efficacies and effectiveness. Advancements in nanotechnology in recent years have generated unique opportunities to meet the daunting challenges associated with immunology and vaccine development. Firstly, nanoparticle formulated systems provide ideal model systems for studying the operation of the immune system, making it possible to systematically identify key factors and understand their roles in specific immune responses. Also, the versatile compositions/architectures of nanoparticle systems enable new strategies/novel platforms for developing vaccines with high efficacies and effectiveness. In this review, we discuss the advantages of nanoparticles and the challenges faced during vaccine development, through the framework of the immunological mechanisms of vaccination, with the aim of bridging the gap between immunology and materials science, which are both involved in vaccine design. The knowledge obtained provides general guidelines for future vaccine development.

The challenge and prospect of mRNA therapeutics landscape

Messenger RNA (mRNA)-based therapeutics hold the potential to cause a major revolution in the pharmaceutical industry because they can be used for precise and individualized therapy, and enable patients to produce therapeutic proteins in their own bodies without struggling with the comprehensive manufacturing issues associated with recombinant proteins. Compared with the current therapeutics, the production of mRNA is much cost-effective, faster and more flexible because it can be easily produced by in vitro transcription, and the process is independent of mRNA sequence. Moreover, mRNA vaccines allow people to develop personalized medications based on sequencing results and/or personalized conditions rapidly. Along with the great potential from bench to bedside, technical obstacles facing mRNA pharmaceuticals are also obvious. The stability, immunogenicity, translation efficiency, and delivery are all pivotal issues need to be addressed. In the recently published research results, these issues are gradually being overcome by state-of-the-art development technologies. In this review, we describe the structural properties and modification technologies of mRNA, summarize the latest advances in developing mRNA delivery systems, review the preclinical and clinical applications, and put forward our views on the prospect and challenges of developing mRNA into a new class of drug.

Elucidation of the Effects of a Current X-SCID Therapy on Intestinal Lymphoid Organogenesis Using an In Vivo Animal Model

BACKGROUND & AIMS

Organ-level research using an animal model lacking Il2rg, the gene responsible for X-linked severe combined immunodeficiency (X-SCID), is clinically unavailable and would be a powerful tool to gain deeper insights into the symptoms of patients with X-SCID.

METHODS

We used an X-SCID animal model, which was first established in our group by the deletion of Il2rg gene in pigs, to understand the clinical signs from multiple perspectives based on pathology, immunology, microbiology, and nutrition. We also treated the X-SCID pigs with bone marrow transplantation (BMT) for mimicking a current therapeutic treatment for patients with X-SCID and investigated the effect at the organ-level. Moreover, the results were confirmed using serum and fecal samples collected from patients with X-SCID.

RESULTS

We demonstrated that X-SCID pigs completely lacked Peyer's patches (PPs) and IgA production in the small intestine, but possessed some dysfunctional intestinal T and B cells. Another novel discovery was that X-SCID pigs developed a heterogeneous intestinal microflora and possessed abnormal plasma metabolites, indicating that X-SCID could be an immune disorder that affects various in vivo functions. Importantly, the organogenesis of PPs in X-SCID pigs was not promoted by BMT. Although a few isolated lymphoid follicles developed in the small intestine of BMT-treated X-SCID pigs, there was no evidence that they contributed to IgA production and microflora formation. Consistently, most patients with X-SCID who received BMT possessed abnormal intestinal immune and microbial environments regardless of the presence of sufficient serum IgG.

CONCLUSIONS

These results indicate that the current BMT therapies for patients with X-SCID may be insufficient to induce the organogenesis of intestinal lymphoid tissues that are associated with numerous functions in vivo.

Intact Viral Particle Counts Measured by Flow Virometry Provide Insight into the Infectivity and Genome Packaging Efficiency of Moloney Murine Leukemia Virus

Murine leukemia viruses (MLVs) have long been used as a research model to further our understanding of retroviruses. These simple gammaretroviruses have been studied extensively in various facets of science for nearly half a century, yet we have surprisingly little quantitative information about some of the basic features of these viral particles. These include parameters such as the genome packaging efficiency and the number of particles required for a productive infection. The reason for this knowledge gap relies primarily on the technical challenge of accurately measuring intact viral particles from infected cell supernatants. Virus-infected cells are well known to release soluble viral proteins, defective viruses, and extracellular vesicles (EVs) harboring viral proteins that may mimic viruses, all of which can skew virus titer quantifications. Flow virometry, also known as nanoscale flow cytometry or simply small-particle flow cytometry, is an emerging analytical method enabling high-throughput single-virus phenotypic characterizations. By utilizing the viral envelope glycoprotein (Env) and monodisperse light scattering characteristics as discerning parameters of intact virus particles, here, we analyzed the basic properties of Moloney MLV (M-MLV). We show that <24% of the total p30 capsid protein measured in infected cell supernatants is associated with intact viruses. We calculate that about one in five M-MLV particles contains a viral RNA genome pair and that individual intact particle infectivity is about 0.4%. These findings provide new insights into the characteristics of an extensively studied prototypical retrovirus while highlighting the benefits of flow virometry for the field of virology.IMPORTANCE Gammaretroviruses, or, more specifically, murine leukemia viruses (MLVs), have been a longstanding model for studying retroviruses. Although being extensively analyzed and dissected for decades, several facets of MLV biology are still poorly understood. One of the primary challenges has been enumerating total intact virus particles in a sample. While several analytical methods can precisely measure virus protein amounts, MLVs are known to induce the secretion of soluble and vesicle-associated viral proteins that can skew these measurements. With recent technological advances in flow cytometry, it is now possible to analyze viruses down to 90 nm in diameter with an approach called flow virometry. The technique has the added benefit of being able to discriminate viruses from extracellular vesicles and free viral proteins in order to confidently provide an intact viral particle count. Here, we used flow virometry to provide new insights into the basic characteristics of Moloney MLV.

Emerging gene therapies for cystic fibrosis

Introduction: Cystic fibrosis (CF) remains a life-threatening genetic disease, with few clinically effective treatment options. Gene therapy and gene editing strategies offer the potential for a one-time CF cure, irrespective of the CFTR mutation class. Areas covered: We review emerging gene therapies and gene delivery strategies for the treatment of CF particularly viral and non-viral approaches with potential to treat CF. Expert opinion: It was initially anticipated that the challenge of developing a gene therapy for CF lung disease would be met relatively easily. Following early proof-of-concept clinical studies, CF gene therapy has entered a new era with innovative vector designs, approaches to subvert the humoral immune system and increase gene delivery and gene correction efficiencies. Developments include integrating adenoviral vectors, rapamycin-loaded nanoparticles, and lung-tropic lentiviral vectors. The characterization of novel cell types in the lung epithelium, including pulmonary ionocytes, may also encourage cell type-specific targeting for CF correction. We anticipate preclinical studies to further validate these strategies, which should pave the way for clinical trials. We also expect gene editing efficiencies to improve to clinically translatable levels, given advancements in viral and non-viral vectors. Overall, gene delivery technologies look more convincing in producing an effective CF gene therapy.

The study of genes and signal transduction pathways involved in mustard lung injury: A gene therapy approach

Sulfur mustard (SM) is a destructive and harmful chemical agent for the eyes, skin and lungs that causes short-term and long-term lesions and was widely used in Iraq war against Iran (1980-1988). SM causes DNA damages, oxidative stress, and Inflammation. Considering the similarities between SM and COPD (Chronic Obstructive Pulmonary Disease) pathogens and limited available treatments, a novel therapeutic approach is not developed. Gene therapy is a novel therapeutic approach that uses genetic engineering science in treatment of most diseases including chronic obstructive pulmonary disease. In this review, attempts to presenting a comprehensive study of mustard lung and introducing the genes therapy involved in chronic obstructive pulmonary disease and emphasizing the pathways and genes involved in the pathology and pathogenesis of sulfur Mustard. It seems that, given the high potential of gene therapy and the fact that this experimental technique is a candidate for the treatment of pulmonary diseases, further study of genes, vectors and gene transfer systems can draw a very positive perspective of gene therapy in near future.

Generation of High-Titer Self-Inactivated γ-Retroviral Vector Producer Cells

The γ-retroviral vector is a gene delivery vehicle that is commonly used in gene therapy. Despite its efficacy, its strong enhancers contributed to malignant transformations in some hematopoietic stem cell (HSC) gene therapy trials. A safer version without viral enhancers (SIN) is available, but its production is cumbersome, as high titers can only be obtained in transient transfection. Our aim was to develop a system that could easily generate high-titer SIN vectors from stable producer cells. The use of the cytomegalovirus enhancer-promoter sequence to generate the full-length genomic RNA combined to sequences that decrease transcriptional readthrough (WPRE and strong polyadenylation sequences) led to 6 × 10⁶ infectious units (IU)/mL of a SIN GFP vector in transient transfection. The incorporation of a blasticidin selection cassette to the retroviral plasmid allowed the generation of stable clones in the 293Vec packaging cells that release 2 × 10⁷ IU/mL and 1.4 × 10⁷ IU/mL of a SIN GFP and a SIN PIGA vector, respectively. A titer of 1.8 × 10⁶ IU/mL was obtained with a SIN vector containing the long 8.9-kb COL7A1 cDNA. Thus, an efficient process was established for the generation of stable 293Vec-derived retrovirus producer cells that release high-titer SIN vectors.

Peptide-mediated delivery of therapeutic mRNA in ovarian cancer

Ovarian cancer is the most lethal gynecological malignancy in the developed world. In spite of intensive research, the mortality has hardly decreased over the past twenty years. This necessitates the exploration of novel therapeutic modalities. Transient protein expression through delivery of mRNA is emerging as a highly promising option. In contrast to gene therapy there is no risk of integration into the genome. Here, we explore the expression of mRNA in models of ovarian cancer of increasing complexity. The cell-penetrating peptide (CPP) PepFect 14 (PF14) was used to formulate CPP-mRNA nanoparticles. Efficient expression of a reporter protein was achieved in two-dimensional tissue cultures and in three-dimensional cancer cell spheroids. PF14 nanoparticles greatly outperformed a lipid-based transfection agent in vivo, leading to expression in various cell types of tumor associated tissue. Protein expression was restricted to the peritoneal cavity. Messenger RNA expression across different cell types was confirmed in primary ovarian cancer explants. As ovarian cancer is confined to the peritoneal cavity in most cases, the results create the basis for applications in which the tumor microenvironment is transiently modified through protein expression.

Gene correction for SCID-X1 in long-term hematopoietic stem cells

Gene correction in human long-term hematopoietic stem cells (LT-HSCs) could be an effective therapy for monogenic diseases of the blood and immune system. Here we describe an approach for X-linked sSevere cCombined iImmunodeficiency (SCID-X1) using targeted integration of a cDNA into the endogenous start codon to functionally correct disease-causing mutations throughout the gene. Using a CRISPR-Cas9/AAV6 based strategy, we achieve up to 20% targeted integration frequencies in LT-HSCs. As measures of the lack of toxicity we observe no evidence of abnormal hematopoiesis following transplantation and no evidence of off-target mutations using a high-fidelity Cas9 as a ribonucleoprotein complex. We achieve high levels of targeting frequencies (median 45%) in CD34+ HSPCs from six SCID-X1 patients and demonstrate rescue of lymphopoietic defect in a patient derived HSPC population in vitro and in vivo. In sum, our study provides specificity, toxicity and efficacy data supportive of clinical development of genome editing to treat SCID-Xl.

Lentiviral Vectors for the Treatment and Prevention of Cystic Fibrosis Lung Disease

Despite the continued development of cystic fibrosis transmembrane conductance regulator (CFTR) modulator drugs for the treatment of cystic fibrosis (CF), the need for mutation agnostic treatments remains. In a sub-group of CF individuals with mutations that may not respond to modulators, such as those with nonsense mutations, CFTR gene transfer to airway epithelia offers the potential for an effective treatment. Lentiviral vectors are well-suited for this purpose because they transduce nondividing cells, and provide long-term transgene expression. Studies in primary cultures of human CF airway epithelia and CF animal models demonstrate the long-term correction of CF phenotypes and low immunogenicity using lentiviral vectors. Further development of CF gene therapy requires the investigation of optimal CFTR expression in the airways. Lentiviral vectors with improved safety features have minimized insertional mutagenesis safety concerns raised in early clinical trials for severe combined immunodeficiency using γ-retroviral vectors. Recent clinical trials using improved lentiviral vectors support the feasibility and safety of lentiviral gene therapy for monogenetic diseases. While work remains to be done before CF gene therapy reaches the bedside, recent advances in lentiviral vector development reviewed here are encouraging and suggest it could be tested in clinical studies in the near future.

Ex Vivo Expansion of Hematopoietic Stem Cells for Therapeutic Purposes: Lessons from Development and the Niche

Expansion of hematopoietic stem cells (HSCs) for therapeutic purposes has been a “holy grail” in the field for many years. Ex vivo expansion of HSCs can help to overcome material shortage for transplantation purposes and genetic modification protocols. In this review, we summarize improved understanding in blood development, the effect of niche and conservative signaling pathways on HSCs in mice and humans, and also advances in ex vivo culturing protocols of human HSCs with cytokines or small molecule compounds. Different expansion protocols have been tested in clinical trials. However, an optimal condition for ex vivo expansion of human HSCs still has not been found yet. Translating and implementing new findings from basic research (for instance by using genetic modification of human HSCs) into clinical protocols is crucial to improve ex vivo expansion and eventually boost stem cell gene therapy.

Delivering CRISPR: a review of the challenges and approaches

Gene therapy has long held promise to correct a variety of human diseases and defects. Discovery of the Clustered Regularly-Interspaced Short Palindromic Repeats (CRISPR), the mechanism of the CRISPR-based prokaryotic adaptive immune system (CRISPR-associated system, Cas), and its repurposing into a potent gene editing tool has revolutionized the field of molecular biology and generated excitement for new and improved gene therapies. Additionally, the simplicity and flexibility of the CRISPR/Cas9 site-specific nuclease system has led to its widespread use in many biological research areas including development of model cell lines, discovering mechanisms of disease, identifying disease targets, development of transgene animals and plants, and transcriptional modulation. In this review, we present the brief history and basic mechanisms of the CRISPR/Cas9 system and its predecessors (ZFNs and TALENs), lessons learned from past human gene therapy efforts, and recent modifications of CRISPR/Cas9 to provide functions beyond gene editing. We introduce several factors that influence CRISPR/Cas9 efficacy which must be addressed before effective in vivo human gene therapy can be realized. The focus then turns to the most difficult barrier to potential in vivo use of CRISPR/Cas9, delivery. We detail the various cargos and delivery vehicles reported for CRISPR/Cas9, including physical delivery methods (e.g. microinjection; electroporation), viral delivery methods (e.g. adeno-associated virus (AAV); full-sized adenovirus and lentivirus), and non-viral delivery methods (e.g. liposomes; polyplexes; gold particles), and discuss their relative merits. We also examine several technologies that, while not currently reported for CRISPR/Cas9 delivery, appear to have promise in this field. The therapeutic potential of CRISPR/Cas9 is vast and will only increase as the technology and its delivery improves.

Gene therapy for hemophilia: what does the future hold?

Recent phase I/II adeno-associated viral vector-mediated gene therapy clinical trials have reported remarkable success in ameliorating disease phenotype in hemophilia A and B. These trials, which highlight the challenges overcome through decades of preclinical and first in human clinical studies, have generated considerable excitement for patients and caregivers alike. Optimization of vector and transgene expression has significantly improved the ability to achieve therapeutic factor levels in these subjects. Long-term follow-up studies will guide standardization of the approach with respect to the combination of serotype, promoter, dose, and manufacturing processes and inform safety for inclusion of young patients. Certain limitations preclude universal applicability of gene therapy, including transient liver transaminase elevations due to the immune responses to vector capsids or as yet undefined mechanisms, underlying liver disease from iatrogenic viral hepatitis, and neutralizing antibodies to clotting factors. Integrating vectors show promising preclinical results, but manufacturing and safety concerns still remain. The prospect of gene editing for correction of the underlying mutation is on the horizon with considerable potential. Herein, we review the advances and limitations that have resulted in these recent successful clinical trials and outline avenues that will allow for broader applicability of gene therapy.

Safety of intra-articular transplantation of lentivirally transduced mesenchymal stromal cells for haemophilic arthropathy in a non-human primate

Joint bleeding and resultant arthropathy are major determinants of quality of life in haemophilia patients. We previously developed a mesenchymal stromal cell (MSC)-based treatment approach for haemophilic arthropathy in a mouse model of haemophilia A. Here, we evaluated the long-term safety of intra-articular injection of lentivirally transduced autologous MSCs in non-human primates. Autologous bone-marrow-derived MSCs transduced with a lentiviral vector expressing coagulation factor VIII (FVIII) were injected into the left knee joint of cynomolgus monkeys. We first conducted codon optimization to increase FVIII production in the cells. Lentiviral transduction of autologous MSCs resulted in a significant increase of FVIII in the culture supernatant before transplantation. We did not find any tumour generation around the knee structure at 11-16 months after injection by magnetic resonance imaging. The proviral sequence of the simian immunodeficiency virus lentiviral vector was not detected in the heart, lungs, spleen, liver, testis, or bone marrow by real-time quantitative PCR. We confirmed the long-term safety of intra-articular injection of transduced MSCs in a non-human primate. The procedure may be an attractive therapeutic approach for joint diseases in haemophilia patients.