Gene therapy for primary immunodeficiencies: Part 2

The evolving landscape of gene therapy strategies for the treatment of osteoarthritis

OBJECTIVES

Significant advances have been made in our understanding of osteoarthritis (OA) pathogenesis; however, no disease-modifying therapies have been identified. This review will summarize the gene therapy landscape, its initial successes for OA, and possible challenges using recent studies and examples of gene therapies in clinical trials.

DESIGN

This narrative review has three major sections: 1) vector systems for OA gene therapy, 2) current and emerging targets for OA gene therapy, and 3) considerations and future directions.

RESULTS

Gene therapy is the strategy by which nucleic acids are delivered to treat and reverse disease progression. Specificity and prolonged expression of these nucleic acids are achieved by manipulating promoters, genes, and vector systems. Certain vector systems also allow for the development of combinatorial nucleic acid strategies that can be delivered in a single intraarticular injection - an approach likely required to treat the complexity of OA pathogenesis. Several viral and non-viral vector-based gene therapies are in clinical trials for OA, and many more are being evaluated in the preclinical arena.

CONCLUSIONS

In a post-coronavirus disease 2019 (COVID-19) era, the future of gene therapy for OA is certainly promising; however, the majority of preclinical validation continues to focus heavily on post-traumatic models and changes in only cartilage and subchondral bone. To ensure successful translation, new candidates in the preclinical arena should be examined against all joint tissues as well as pain using diverse models of injury-, obesity-, and age-induced disease. Lastly, consideration must be given to strategies for repeat administration and the cost of treatment owing to the chronic nature of OA.

Improved functionality and potency of next generation BinMLV viral vectors toward safer gene therapy

To develop safer retroviral murine leukemia virus (MLV)-based vectors, we previously mutated and re-engineered the MLV integrase: the W390A mutation abolished the interaction with its cellular tethering factors, BET proteins, and a retargeting peptide (the chromodomain of the CBX1 protein) was fused C-terminally. The resulting BET-independent MLV^W390A-CBX was shown to integrate efficiently and more randomly, away from typical retroviral markers. In this study, we assessed the functionality and stability of expression of the redistributed MLV^W390A-CBX vector in more depth, and evaluated safety using a clinically more relevant vector design encompassing a self-inactivated (SIN) LTR and a weak internal elongation factor 1α short (EFS) promoter. MLV^W390A-CBX-EFS produced like MLV^WT and efficiently transduced laboratory cells and primary human CD34⁺ hematopoetic stem cells (HSC) without transgene silencing over time, while displaying a more preferred, redistributed, and safer integration pattern. In a human mesoangioblast (MAB) stem cell model, the myogenic fusion capacity was hindered following MLV^WT transduction, while this remained unaffected when applying MLV^W390A-CBX. Likewise, smooth muscle cell differentiation of MABs was unaltered by MLV^W390A-CBX-EFS. Taken together, our results underscore the potential of MLV^W390A-CBX-EFS as a clinically relevant viral vector for ex-vivo gene therapy, combining efficient production with a preferable integration site distribution profile and stable expression over time.

Gene therapy in PIDs, hemoglobin, ocular, neurodegenerative, and hemophilia B disorders

A new approach is adopted to treat primary immunodeficiency disorders, such as the severe combined immunodeficiency (SCID; e.g., adenosine deaminase SCID [ADA-SCID] and IL-2 receptor X-linked severe combined immunodeficiency [SCID-X1]). The success, along with the feasibility of gene therapy, is undeniable when considering the benefits recorded for patients with different classes of diseases or disorders needing treatment, including SCID-X1 and ADA-SCID, within the last two decades. β-Thalassemia and sickle cell anemia are two prominent monogenic blood hemoglobin disorders for which a solution has been sought using gene therapy. For instance, transduced autologous CD34+ HSCs via a self-inactivating (SIN)-Lentivirus (LV) coding for a functional copy of the β-globin gene has become a feasible procedure. adeno-associated virus (AAV) vectors have found application in ocular gene transfer in retinal disease gene therapy (e.g., Leber's congenital amaurosis type 2), where no prior treatment existed. In neurodegenerative disorders, successes are now reported for cases involving metachromatic leukodystrophy causing severe cognitive and motor damage. Gene therapy for hemophilia also remains a viable option because of the amount of cell types that are capable of synthesizing biologically active FVIII and FIX following gene transfer using AAV vectors in vivo to correct hemophilia B (FIX deficiency), and it is considered an ideal target, as proven in preclinical studies. Recently, the clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9 gene-editing tool has taken a center stage in gene therapy research and is reported to be efficient and highly precise. The application of gene therapy to these areas has pushed forward the therapeutic clinical application.

Toll-Like Receptors and Relevant Emerging Therapeutics with Reference to Delivery Methods

The built-in innate immunity in the human body combats various diseases and their causative agents. One of the components of this system is Toll-like receptors (TLRs), which recognize structurally conserved molecules derived from microbes and/or endogenous molecules. Nonetheless, under certain conditions, these TLRs become hypofunctional or hyperfunctional, thus leading to a disease-like condition because their normal activity is compromised. In this regard, various small-molecule drugs and recombinant therapeutic proteins have been developed to treat the relevant diseases, such as rheumatoid arthritis, psoriatic arthritis, Crohn’s disease, systemic lupus erythematosus, and allergy. Some drugs for these diseases have been clinically approved; however, their efficacy can be enhanced by conventional or targeted drug delivery systems. Certain delivery vehicles such as liposomes, hydrogels, nanoparticles, dendrimers, or cyclodextrins can be employed to enhance the targeted drug delivery. This review summarizes the TLR signaling pathway, associated diseases and their treatments, and the ways to efficiently deliver the drugs to a target site.

Gene therapy in rare diseases: the benefits and challenges of developing a patient-centric registry for Strimvelis in ADA-SCID

Background

Strimvelis (autologous CD34+ cells transduced to express adenosine deaminase [ADA]) is the first ex vivo stem cell gene therapy approved by the European Medicines Agency (EMA), indicated as a single treatment for patients with ADA-severe combined immunodeficiency (ADA-SCID) who lack a suitable matched related bone marrow donor. Existing primary immunodeficiency registries are tailored to transplantation outcomes and do not capture the breadth of safety and efficacy endpoints required by the EMA for the long-term monitoring of gene therapies. Furthermore, for extended monitoring of Strimvelis, the young age of children treated, small patient numbers, and broad geographic distribution of patients all increase the risk of loss to follow-up before sufficient data have been collected. Establishing individual investigator sites would be impractical and uneconomical owing to the small number of patients from each location receiving Strimvelis.

Results

An observational registry has been established to monitor the safety and effectiveness of Strimvelis in up to 50 patients over a minimum of 15 years. To address the potential challenges highlighted above, data will be collected by a single investigator site at Ospedale San Raffaele (OSR), Milan, Italy, and entered into the registry via a central electronic platform. Patients/families and the patient’s local physician will also be able to submit healthcare information directly to the registry using a uniquely designed electronic platform. Data entry will be monitored by a Gene Therapy Registry Centre (funded by GlaxoSmithKline) who will ensure that necessary information is collected and flows between OSR, the patient/family and the patient’s local healthcare provider.

Conclusion

The Strimvelis registry sets a precedent for the safety monitoring of future gene therapies. A unique, patient-focused design has been implemented to address the challenges of long-term follow-up of patients treated with gene therapy for a rare disease. Strategies to ensure data completeness and patient retention in the registry will help fulfil pharmacovigilance requirements. Collaboration with partners is being sought to expand from a treatment registry into a disease registry. Using practical and cost-efficient approaches, the Strimvelis registry is hoped to encourage further innovation in registry design within orphan drug development.

Gene Therapy for Adenosine Deaminase Deficiency: A Comprehensive Evaluation of Short- and Medium-Term Safety

Loss of adenosine deaminase activity leads to severe combined immunodeficiency (ADA-SCID); production and function of T, B, and natural killer (NK) cells are impaired. Gene therapy (GT) with an autologous CD34⁺-enriched cell fraction that contains CD34⁺ cells transduced with a retroviral vector encoding the human ADA cDNA sequence leads to immune reconstitution in most patients. Here, we report short- and medium-term safety analyses from 18 patients enrolled as part of single-arm, open-label studies or compassionate use programs. Survival was 100% with a median of 6.9 years follow-up (range, 2.3 to 13.4 years). Adverse events were mostly grade 1 or grade 2 and were reported by all 18 patients following GT. Thirty-nine serious adverse events (SAEs) were reported by 15 of 18 patients; no SAEs were considered related to GT. The most common adverse events reported post-GT include upper respiratory tract infection, gastroenteritis, rhinitis, bronchitis, oral candidiasis, cough, neutropenia, diarrhea, and pyrexia. Incidence rates for all of these events were highest during pre-treatment, treatment, and/or 3-month follow-up and then declined over medium-term follow-up. GT did not impact the incidence of neurologic/hearing impairments. No event indicative of leukemic transformation was reported.

NADPH Oxidase Deficiency: A Multisystem Approach

The immune system is a complex system able to recognize a wide variety of host agents, through different biological processes. For example, controlled changes in the redox state are able to start different pathways in immune cells and are involved in the killing of microbes. The generation and release of ROS in the form of an “oxidative burst” represent the pivotal mechanism by which phagocytic cells are able to destroy pathogens. On the other hand, impaired oxidative balance is also implicated in the pathogenesis of inflammatory complications, which may affect the function of many body systems. NADPH oxidase (NOX) plays a pivotal role in the production of ROS, and the defect of its different subunits leads to the development of chronic granulomatous disease (CGD). The defect of the different NOX subunits in CGD affects different organs. In this context, this review will be focused on the description of the effect of NOX2 deficiency in different body systems. Moreover, we will also focus our attention on the novel insight in the pathogenesis of immunodeficiency and inflammation-related manifestations and on the protective role of NOX2 deficiency against the development of atherosclerosis.

Novel GP64 envelope variants for improved delivery to human airway epithelial cells

Lentiviral vectors pseudotyped with the baculovirus envelope protein GP64 transduce primary cultures of human airway epithelia (HAE) at their apical surface. Our goal in this study was to harness a directed evolution approach to develop a novel envelope glycoprotein with increased transduction properties for HAE. Using error-prone PCR, a library of GP64 mutants was generated and used to prepare a diverse pool of lentiviral virions pseudotyped with GP64 variants. The library was serially passaged on HAE and three GP64 mutations were recovered. Single-, double- and the triple-combination mutant envelope glycoproteins were compared with wild-type GP64 for their ability to transduce HAE. Our results suggest that lentiviral vectors pseudotyped with evolved GP64 transduced HAE with greater efficiency than wild-type GP64. This effect was not observed in primary cultures of porcine airway epithelial cells, suggesting that the directed evolution protocol was species specific. In summary, our studies indicate that serial passage of a GP64 mutant library yielded specific variants with improved HAE cell tropism, yielding tools with the potential to improve the success of gene therapy for airway diseases.

Gene Therapy Approaches to Immunodeficiency

Transfer of gene-corrected autologous hematopoietic stem cells in patients with primary immunodeficiencies has emerged as a new therapeutic approach. Patients with various conditions lacking a suitable donor have been treated with retroviral vectors and a gene-addition strategy. Initial promising results were shadowed by the occurrence of malignancies in some of these patients. Current trials, developed in the last decade, use safer viral vectors to overcome the risk of genotoxicity and have led to improved clinical outcomes. This review reflects the progresses made in specific disorders, including adenosine deaminase deficiency, X-linked severe combined immunodeficiency, chronic granulomatous disease, and Wiskott-Aldrich syndrome.

Viral Vectors: The Road to Reducing Genotoxicity

Viral vector use in gene therapy has highlighted several safety concerns, including genotoxic events. Generally, vector-mediated genotoxicity results from upregulation of cellular proto-oncogenes via promoter insertion, promoter activation, or gene transcript truncation, with enhancer-mediated activation of nearby genes the primary mechanism reported in gene therapy trials. Vector-mediated genotoxicity can be influenced by virus type, integration target site, and target cell type; different vectors have distinct integration profiles which are cell-specific. Non-viral factors, including patient age, disease, and dose can also influence genotoxic potential, thus the choice of test models and clinical trial populations is important to ensure they are indicative of efficacy and safety. Efforts have been made to develop viral vectors with less risk of insertional mutagenesis, including self-inactivating (SIN) vectors, enhancer-blocking insulators, and microRNA targeting of vectors, although insertional mutagenesis is not completely abrogated. Here we provide an overview of the current understanding of viral vector-mediated genotoxicity risk from factors contributing to viral vector-mediated genotoxicity to efforts made to reduce genotoxicity, and testing strategies required to adequately assess the risk of insertional mutagenesis. It is clear that there is not a 'one size fits all' approach to vector modification for reducing genotoxicity, and addressing these challenges will be a key step in the development of therapies such as CRISPR-Cas9 and delivery of future gene-editing technologies.

Recent advances in RNAi-based strategies for therapy and prevention of HIV-1/AIDS

RNA interference (RNAi) provides a powerful tool to silence specific gene expression and has been widely used to suppress host factors such as CCR5 and/or viral genes involved in HIV-1 replication. Newer nuclease-based gene-editing technologies, such as zinc finger nucleases (ZFN), transcription activator-like effector nucleases (TALEN) and the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system, also provide powerful tools to ablate specific genes. Because of differences in co-receptor usage and the high mutability of the HIV-1 genome, a combination of host factors and viral genes needs to be suppressed for effective prevention and treatment of HIV-1 infection. Whereas the continued presence of small interfering/short hairpin RNA (si/shRNA) mediators is needed for RNAi to be effective, the continued expression of nucleases in the gene-editing systems is undesirable. Thus, RNAi provides the only practical way for expression of multiple silencers in infected and uninfected cells, which is needed for effective prevention/treatment of infection. There have been several advances in the RNAi field in terms of si/shRNA design, targeted delivery to HIV-1 susceptible cells, and testing for efficacy in preclinical humanized mouse models. Here, we comprehensively review the latest advances in RNAi technology towards prevention and treatment of HIV-1.

Current status of ex vivo gene therapy for hematological disorders: a review of clinical trials in Japan around the world

Gene therapies are classified into two major categories, namely, in vivo and ex vivo. Clinical trials of human gene therapy began with the ex vivo techniques. Based on the initial successes of gene-therapy clinical trials, these approaches have spread worldwide. The number of gene therapy trials approved worldwide increased gradually starting in 1989, reaching 116 protocols per year in 1999, and a total of 2210 protocols had been approved by 2015. Accumulating clinical evidence has demonstrated the safety and benefits of several types of gene therapy, with the exception of serious adverse events in several clinical trials. These painful experiences were translated backward to basic science, resulting in the development of several new technologies that have influenced the recent development of ex vivo gene therapy in this field. To date, six gene therapies have been approved in a limited number of countries worldwide. In Japan, clinical trials of gene therapy have developed under the strong influence of trials in the US and Europe. Since the initial stages, 50 clinical trials have been approved by the Japanese government. In this review, the history and current status of clinical trials of ex vivo gene therapy for hematological disorders are introduced and discussed.

Update on the safety and efficacy of retroviral gene therapy for immunodeficiency due to adenosine deaminase deficiency

Adenosine deaminase (ADA) deficiency is a rare, autosomal-recessive systemic metabolic disease characterized by severe combined immunodeficiency (SCID). The treatment of choice for ADA-deficient SCID (ADA-SCID) is hematopoietic stem cell transplant from an HLA-matched sibling donor, although <25% of patients have such a donor available. Enzyme replacement therapy (ERT) partially and temporarily relieves immunodeficiency. We investigated the medium-term outcome of gene therapy (GT) in 18 patients with ADA-SCID for whom an HLA-identical family donor was not available; most were not responding well to ERT. Patients were treated with an autologous CD34(+)-enriched cell fraction that contained CD34(+) cells transduced with a retroviral vector encoding the human ADA complementary DNA sequence (GSK2696273) as part of single-arm, open-label studies or compassionate use programs. Overall survival was 100% over 2.3 to 13.4 years (median, 6.9 years). Gene-modified cells were stably present in multiple lineages throughout follow up. GT resulted in a sustained reduction in the severe infection rate from 1.17 events per person-year to 0.17 events per person-year (n = 17, patient 1 data not available). Immune reconstitution was demonstrated by normalization of T-cell subsets (CD3(+), CD4(+), and CD8(+)), evidence of thymopoiesis, and sustained T-cell proliferative capacity. B-cell function was evidenced by immunoglobulin production, decreased intravenous immunoglobulin use, and antibody response after vaccination. All 18 patients reported infections as adverse events; infections of respiratory and gastrointestinal tracts were reported most frequently. No events indicative of leukemic transformation were reported. Trial details were registered at www.clinicaltrials.gov as #NCT00598481.

Advances of gene therapy for primary immunodeficiencies

In the recent past, the gene therapy field has witnessed a remarkable series of successes, many of which have involved primary immunodeficiency diseases, such as X-linked severe combined immunodeficiency, adenosine deaminase deficiency, chronic granulomatous disease, and Wiskott-Aldrich syndrome. While such progress has widened the choice of therapeutic options in some specific cases of primary immunodeficiency, much remains to be done to extend the geographical availability of such an advanced approach and to increase the number of diseases that can be targeted. At the same time, emerging technologies are stimulating intensive investigations that may lead to the application of precise genetic editing as the next form of gene therapy for these and other human genetic diseases.

Clinical applications of gene therapy for primary immunodeficiencies

Primary immunodeficiencies (PIDs) have represented a paradigmatic model for successes and pitfalls of hematopoietic stem cells gene therapy. First clinical trials performed with gamma retroviral vectors (γ-RV) for adenosine deaminase severe combined immunodeficiency (ADA-SCID), X-linked SCID (SCID-X1), and Wiskott-Aldrich syndrome (WAS) showed that gene therapy is a valid therapeutic option in patients lacking an HLA-identical donor. No insertional mutagenesis events have been observed in more than 40 ADA-SCID patients treated so far in the context of different clinical trials worldwide, suggesting a favorable risk-benefit ratio for this disease. On the other hand, the occurrence of insertional oncogenesis in SCID-X1, WAS, and chronic granulomatous disease (CGD) RV clinical trials prompted the development of safer vector construct based on self-inactivating (SIN) retroviral or lentiviral vectors (LVs). Here we present the recent results of LV-mediated gene therapy for WAS showing stable multilineage engraftment leading to hematological and immunological improvement, and discuss the differences with respect to the WAS RV trial. We also describe recent clinical results of SCID-X1 gene therapy with SIN γ-RV and the perspectives of targeted genome editing techniques, following early preclinical studies showing promising results in terms of specificity of gene correction. Finally, we provide an overview of the gene therapy approaches for other PIDs and discuss its prospects in relation to the evolving arena of allogeneic transplant.

A snapshot of biologic drug development

Since the approval of insulin as the first recombinant therapeutic protein, the prominence of biologic therapies in drug development has grown significantly. Many modalities beyond traditional biologics are now being developed or explored for various indications with significant unmet medical needs. From early traditional replacement proteins to more recent, highly engineered antibodies, oligonucleotides, fusion proteins, and gene constructs, biologic agents have delivered life-changing therapies, despite often having scientifically and technically challenging development programs. This brief review outlines some of the major biotherapeutic classes and identifies the advantages and challenges with the development of these products.

Lentiviral vectors for the treatment of primary immunodeficiencies

In the last years important progress has been made in the treatment of several primary immunodeficiency disorders (PIDs) with gene therapy. Hematopoietic stem cell (HSC) gene therapy indeed represents a valid alternative to conventional transplantation when a compatible donor is not available and recent success confirmed the great potential of this approach. First clinical trials performed with gamma retroviral vectors were promising and guaranteed clinical benefits to the patients. On the other hand, the outcome of severe adverse events as the development of hematological abnormalities highlighted the necessity to develop a safer platform to deliver the therapeutic gene. Self-inactivating (SIN) lentiviral vectors (LVVs) were studied to overcome this hurdle through their preferable integration pattern into the host genome. In this review, we describe the recent advancements achieved both in vitro and at preclinical level with LVVs for the treatment of Wiskott-Aldrich syndrome (WAS), chronic granulomatous disease (CGD), ADA deficiency (ADA-SCID), Artemis deficiency, RAG1/2 deficiency, X-linked severe combined immunodeficiency (γchain deficiency, SCIDX1), X-linked lymphoproliferative disease (XLP) and immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) syndrome.

Dual-regulated lentiviral vector for gene therapy of X-linked chronic granulomatosis

Regulated transgene expression may improve the safety and efficacy of hematopoietic stem cell (HSC) gene therapy. Clinical trials for X-linked chronic granulomatous disease (X-CGD) employing gammaretroviral vectors were limited by insertional oncogenesis or lack of persistent engraftment. Our novel strategy, based on regulated lentiviral vectors (LV), targets gp91(phox) expression to the differentiated myeloid compartment while sparing HSC, to reduce the risk of genotoxicity and potential perturbation of reactive oxygen species levels. Targeting was obtained by a myeloid-specific promoter (MSP) and posttranscriptional, microRNA-mediated regulation. We optimized both components in human bone marrow (BM) HSC and their differentiated progeny in vitro and in a xenotransplantation model, and generated therapeutic gp91(phox) expressing LVs for CGD gene therapy. All vectors restored gp91(phox) expression and function in human X-CGD myeloid cell lines, primary monocytes, and differentiated myeloid cells. While unregulated LVs ectopically expressed gp91(phox) in CD34(+) cells, transcriptionally and posttranscriptionally regulated LVs substantially reduced this off-target expression. X-CGD mice transplanted with transduced HSC restored gp91(phox) expression, and MSP-driven vectors maintained regulation during BM development. Combining transcriptional (SP146.gp91-driven) and posttranscriptional (miR-126-restricted) targeting, we achieved high levels of myeloid-specific transgene expression, entirely sparing the CD34(+) HSC compartment. This dual-targeted LV construct represents a promising candidate for further clinical development.

Gene correction of human induced pluripotent stem cells repairs the cellular phenotype in pulmonary alveolar proteinosis

RATIONALE

Hereditary pulmonary alveolar proteinosis (hPAP) caused by granulocyte-macrophage colony-stimulating factor (GM-CSF) receptor α-chain (CSF2RA) deficiency is a rare, life-threatening lung disease characterized by accumulation of proteins and phospholipids in the alveolar spaces. The disease is caused by a functional insufficiency of alveolar macrophages, which require GM-CSF signaling for terminal differentiation and effective degradation of alveolar proteins and phospholipids. Therapeutic options are extremely limited, and the pathophysiology underlying the defective protein degradation in hPAP alveolar macrophages remains poorly understood.

OBJECTIVES

To further elucidate the cellular mechanisms underlying hPAP and evaluate novel therapeutic strategies, we here investigated the potential of hPAP patient-derived induced pluripotent stem cell (PAP-iPSCs) derived monocytes and macrophages.

METHODS

Patient-specific PAP-iPSCs were generated from CD34(+) bone marrow cells of a CSF2RA-deficient patient with PAP. We assessed pluripotency, chromosomal integrity, and genetic correction of established iPSC lines. On hematopoietic differentiation, genetically corrected or noncorrected monocytes and macrophages were investigated in GM-CSF-dependent assays.

MEASUREMENTS AND MAIN RESULTS

Although monocytes and macrophages differentiated from noncorrected PAP-iPSCs exhibited distinct defects in GM-CSF-dependent functions, such as perturbed CD11b activation, phagocytic activity, and STAT5 phosphorylation after GM-CSF exposure and lack of GM-CSF uptake, these defects were fully repaired on lentiviral gene transfer of a codon-optimized CSF2RA-cDNA.

CONCLUSIONS

These data establish PAP-iPSC-derived monocytes and macrophages as a valid in vitro disease model of CSF2RA-deficient PAP, and introduce gene-corrected iPSC-derived monocytes and macrophages as a potential autologous cell source for innovative therapeutic strategies. Transplantation of such cells to patients with hPAP could serve as a paradigmatic proof for the potential of iPSC-derived cells in clinical gene therapy.

Gene therapy for rare diseases: summary of a National Institutes of Health workshop, September 13, 2012

Gene therapy has shown clinical efficacy for several rare diseases, using different approaches and vectors. The Gene Therapy for Rare Diseases workshop, sponsored by the National Institutes of Health (NIH) Office of Biotechnology Activities and Office of Rare Diseases Research, brought together investigators from different disciplines to discuss the challenges and opportunities for advancing the field including means for enhancing data sharing for preclinical and clinical studies, development and utilization of available NIH resources, and interactions with the U.S. Food and Drug Administration.

Optimizing retroviral gene expression for effective therapies

With their ability to integrate their genetic material into the target cell genome, retroviral vectors (RV) of both the gamma-retroviral (γ-RV) and lentiviral vector (LV) classes currently remain the most efficient and thus the system of choice for achieving transgene retention and therefore potentially long-term expression and therapeutic benefit. However, γ-RV and LV integration comes at a cost in that transcription units will be present within a native chromatin environment and thus be subject to epigenetic effects (DNA methylation, histone modifications) that can negatively impact on their function. Indeed, highly variable expression and silencing of γ-RV and LV transgenes especially resulting from promoter DNA methylation is well documented and was the cause of the failure of gene therapy in a clinical trial for X-linked chronic granulomatous disease. This review will critically explore the use of different classes of genetic control elements that can in principle reduce vector insertion site position effects and epigenetic-mediated silencing. These transcriptional regulatory elements broadly divide themselves into either those with a chromatin boundary or border function (scaffold/matrix attachment regions, insulators) or those with a dominant chromatin remodeling and transcriptional activating capability (locus control regions,, ubiquitous chromatin opening elements). All these types of elements have their strengths and weaknesses within the constraints of a γ-RV and LV backbone, showing varying degrees of efficacy in improving reproducibility and stability of transgene function. Combinations of boundary and chromatin remodeling; transcriptional activating elements, which do not impede vector production; transduction efficiency; and stability are most likely to meet the requirements within a gene therapy context especially when targeting a stem cell population.

Generation of X-CGD cells for vector evaluation from healthy donor CD34(+) HSCs by shRNA-mediated knock down of gp91(phox)

Innovative approaches for the treatment of rare inherited diseases are hampered by limited availability of patient derived samples for preclinical research. This also applies for the evaluation of novel vector systems for the gene therapy of monogenic hematological diseases like X-linked chronic granulomatous disease (X-CGD), a severe primary immunodeficiency caused by mutations in the gp91^phox subunit of the phagocytic NADPH oxidase. Since current gene therapy protocols involve ex vivo gene modification of autologous CD34⁺ hematopoietic stem cells (HSC), the ideal preclinical model should simulate faithfully this procedure. However, the low availability of patient-derived CD34⁺ cells limits the feasibility of this approach. Here, we describe a straightforward experimental strategy that circumvents this limitation. The knock down of gp91^phox expression upon lentiviral delivery of shRNAs into CD34⁺ cells from healthy donors generates sufficient amounts of X-CGD CD34⁺ cells which subsequently can be used for the evaluation of novel gene therapeutic strategies using a codon-optimized gp91^phox transgene. We have used this strategy to test the potential of a novel gene therapy vector for X-CGD.

Long-term stability and safety of transgenic cultured epidermal stem cells in gene therapy of junctional epidermolysis bullosa

We report a long-term follow-up (6.5 years) of a phase I/II clinical trial envisaging the use of autologous genetically modified cultured epidermal stem cells for gene therapy of junctional epidermolysis bullosa, a devastating genetic skin disease. The critical goals of the trial were to evaluate the safety and long-term persistence of genetically modified epidermis. A normal epidermal-dermal junction was restored and the regenerated transgenic epidermis was found to be fully functional and virtually indistinguishable from a normal control. The epidermis was sustained by a discrete number of long-lasting, self-renewing transgenic epidermal stem cells that maintained the memory of the donor site, whereas the vast majority of transduced transit-amplifying progenitors were lost within the first few months after grafting. These data pave the way for the safe use of epidermal stem cells in combined cell and gene therapy for genetic skin diseases.

Gene therapy on the move

The first gene therapy clinical trials were initiated more than two decades ago. In the early days, gene therapy shared the fate of many experimental medicine approaches and was impeded by the occurrence of severe side effects in a few treated patients. The understanding of the molecular and cellular mechanisms leading to treatment- and/or vector-associated setbacks has resulted in the development of highly sophisticated gene transfer tools with improved safety and therapeutic efficacy. Employing these advanced tools, a series of Phase I/II trials were started in the past few years with excellent clinical results and no side effects reported so far. Moreover, highly efficient gene targeting strategies and site-directed gene editing technologies have been developed and applied clinically. With more than 1900 clinical trials to date, gene therapy has moved from a vision to clinical reality. This review focuses on the application of gene therapy for the correction of inherited diseases, the limitations and drawbacks encountered in some of the early clinical trials and the revival of gene therapy as a powerful treatment option for the correction of monogenic disorders.

A possible turning point in the hematopoietic stem cell gene therapy for primary immunodeficiency diseases? Lentiviral vectors could take the place of retroviral vectors

Wiskott-Aldrich syndrome (WAS), an X-linked primary immunodeficiency disease (PID) with unique and characteristic features, had been considered to be a good candidate for gene therapy. In 2010, hematopoietic stem cell (HSC) gene therapy, using a retroviral vector, was performed for WAS patients; however, concerns remain regarding the long-term safety of this therapy as several patients with PID developed myeloproliferative diseases due to insertional mutagenesis related to HSC gene therapy using retroviral vectors. Aiuti et al. first reported HSC gene therapy for WAS using a lentiviral vector and compared the safety and efficacy of the two therapies in the context of the same disease background. They undertook a detailed study of the vector integration sites and concluded that lentiviral HSC gene therapy was safer than retroviral gene therapy.

From bench to bedside: preclinical evaluation of a self-inactivating gammaretroviral vector for the gene therapy of X-linked chronic granulomatous disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency characterized by impaired antimicrobial activity in phagocytic cells. As a monogenic disease affecting the hematopoietic system, CGD is amenable to gene therapy. Indeed in a phase I/II clinical trial, we demonstrated a transient resolution of bacterial and fungal infections. However, the therapeutic benefit was compromised by the occurrence of clonal dominance and malignant transformation demanding alternative vectors with equal efficacy but safety-improved features. In this work we have developed and tested a self-inactivating (SIN) gammaretroviral vector (SINfes.gp91s) containing a codon-optimized transgene (gp91(phox)) under the transcriptional control of a myeloid promoter for the gene therapy of the X-linked form of CGD (X-CGD). Gene-corrected cells protected X-CGD mice from Aspergillus fumigatus challenge at low vector copy numbers. Moreover, the SINfes.gp91s vector generates substantial amounts of superoxide in human cells transplanted into immunodeficient mice. In vitro genotoxicity assays and longitudinal high-throughput integration site analysis in transplanted mice comprising primary and secondary animals for 11 months revealed a safe integration site profile with no signs of clonal dominance.

Primary Immune Deficiency Treatment Consortium (PIDTC) report

The Primary Immune Deficiency Treatment Consortium (PIDTC) is a network of 33 centers in North America that study the treatment of rare and severe primary immunodeficiency diseases. Current protocols address the natural history of patients treated for severe combined immunodeficiency (SCID), Wiskott-Aldrich syndrome, and chronic granulomatous disease through retrospective, prospective, and cross-sectional studies. The PIDTC additionally seeks to encourage training of junior investigators, establish partnerships with European and other International colleagues, work with patient advocacy groups to promote community awareness, and conduct pilot demonstration projects. Future goals include the conduct of prospective treatment studies to determine optimal therapies for primary immunodeficiency diseases. To date, the PIDTC has funded 2 pilot projects: newborn screening for SCID in Navajo Native Americans and B-cell reconstitution in patients with SCID after hematopoietic stem cell transplantation. Ten junior investigators have received grant awards. The PIDTC Annual Scientific Workshop has brought together consortium members, outside speakers, patient advocacy groups, and young investigators and trainees to report progress of the protocols and discuss common interests and goals, including new scientific developments and future directions of clinical research. Here we report the progress of the PIDTC to date, highlights of the first 2 PIDTC workshops, and consideration of future consortium objectives.

Development of gene therapy for blood disorders: an update

This review addresses the current status of gene therapy for immunodeficiencies, chronic granulomatous disease, suicide gene therapy for graft-versus-host disease, viral infections, malignant hematologic disorders, hemophilia, and the hemoglobin disorders. New developments in vector design have fostered improved expression as well as enhanced safety, particularly of integrating retroviral vectors. Several immunodeficiencies have been treated successfully by stem cell-targeted, retroviral-mediated gene transfer with reconstitution of the immune system following infusion of the transduced cells. In a trial for hemophilia B, long-term expression of human FIX has been observed following adeno-associated viral vector-mediated gene transfer into the liver. This approach should be successful in treating any disorder in which liver production of a specific protein is therapeutic.

Lentiviral hematopoietic stem cell gene therapy in patients with Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an inherited immunodeficiency caused by mutations in the gene encoding WASP, a protein regulating the cytoskeleton. Hematopoietic stem/progenitor cell (HSPC) transplants can be curative, but, when matched donors are unavailable, infusion of autologous HSPCs modified ex vivo by gene therapy is an alternative approach. We used a lentiviral vector encoding functional WASP to genetically correct HSPCs from three WAS patients and reinfused the cells after a reduced-intensity conditioning regimen. All three patients showed stable engraftment of WASP-expressing cells and improvements in platelet counts, immune functions, and clinical scores. Vector integration analyses revealed highly polyclonal and multilineage haematopoiesis resulting from the gene-corrected HSPCs. Lentiviral gene therapy did not induce selection of integrations near oncogenes, and no aberrant clonal expansion was observed after 20 to 32 months. Although extended clinical observation is required to establish long-term safety, lentiviral gene therapy represents a promising treatment for WAS.

Controversies in IgG replacement therapy in patients with antibody deficiency diseases

This Current perspectives article will review and highlight the importance of accurate diagnosis of patients who have failed to produce specific antibodies to naturally encountered foreign proteins or polysaccharides or after vaccination and the appropriate institution of immunoglobulin replacement therapy. The field of primary immunodeficiency disease (PIDD) has expanded remarkably since the early descriptions 6 decades ago. With greater recognition and advanced cellular and molecular diagnostic technology, new entities and single-gene defects in patients with PIDD are rapidly being defined. This, combined with treatment advances and newborn screening for severe combined immunodeficiency, has resulted in improved outcomes and survival and even permanent cures. Awareness of PIDD has also increased, but the guidelines for recognition remain to be validated. The zeal for registering and enrolling patients has potentially created a large body of "patients" treated with immunoglobulin replacement unnecessarily. The complexity, diversity, and availability of laboratory testing have brought awareness of PIDD to the forefront, but because of an absence of standardization of certain assays, concerns about the correct diagnosis and appropriate treatment have increased. We hope to refocus the discussion on identifying clear laboratory and clinical guidelines for the establishment of an accurate diagnosis of antibody deficiency, its rationale, and, where indicated, institution of safe treatment.

In Wiskott-Aldrich syndrome, platelet count matters

In a retrospective analysis of the French Registry of patients with Wiskott-Aldrich Syndrome (WAS), Mahlaoui et al have identified severe refractory thrombocytopenia (SRT) early in life as a major risk factor for poor outcome.

Gene therapy for PIDs: progress, pitfalls and prospects

Substantial progress has been made in the past decade in treating several primary immunodeficiency disorders (PIDs) with gene therapy. Current approaches are based on ex-vivo transfer of therapeutic transgene via viral vectors to patient-derived autologous hematopoietic stem cells (HSCs) followed by transplantation back to the patient with or without conditioning. The overall outcome from all the clinical trials targeting different PIDs has been extremely encouraging but not without caveats. Malignant outcomes from insertional mutagenesis have featured prominently in the adverse events associated with these trials and have warranted intense pre-clinical investigation into defining the tendencies of different viral vectors for genomic integration. Coupled with issues pertaining to transgene expression, the therapeutic landscape has undergone a paradigm shift in determining safety, stability and efficacy of gene therapy approaches. In this review, we aim to summarize the progress made in the gene therapy trials targeting ADA-SCID, SCID-X1, CGD and WAS, review the pitfalls, and outline the recent advancements which are expected to further enhance favourable risk benefit ratios for gene therapeutic approaches in the future.

Evaluating risks of insertional mutagenesis by DNA transposons in gene therapy

Investigational therapy can be successfully undertaken using viral- and nonviral-mediated ex vivo gene transfer. Indeed, recent clinical trials have established the potential for genetically modified T cells to improve and restore health. Recently, the Sleeping Beauty (SB) transposon/transposase system has been applied in clinical trials to stably insert a chimeric antigen receptor (CAR) to redirect T-cell specificity. We discuss the context in which the SB system can be harnessed for gene therapy and describe the human application of SB-modified CAR(+) T cells. We have focused on theoretical issues relating to insertional mutagenesis in the context of human genomes that are naturally subjected to remobilization of transposons and the experimental evidence over the last decade of employing SB transposons for defining genes that induce cancer. These findings are put into the context of the use of SB transposons in the treatment of human disease.