HIV-1-derived lentiviral vectors

Extracellular vesicles as a potential delivery platform for CRISPR-Cas based therapy in epithelial ovarian cancer

Ovarian Cancer (OC) is the most common gynecological malignancy and the eighth most diagnosed cancer in females worldwide. Presently, it ranks as the fifth leading cause of cancer-related mortality among patients globally. Major factors contributing to the lethality of OC worldwide include delayed diagnosis, chemotherapy resistance, high metastatic rates, and the heterogeneity of subtypes. Despite continuous efforts to develop novel targeted therapies and chemotherapeutic agents, challenges persist in the form of OC resistance and recurrence. In the last decade, CRISPR-Cas-based genome editing has emerged as a powerful tool for modifying genetic and epigenetic mechanisms, holding potential for treating numerous diseases. However, a significant challenge for therapeutic applications of CRISPR-Cas technology is the absence of an optimal vehicle for delivering CRISPR molecular machinery into targeted cells or tissues. Recently, extracellular vesicles (EVs) have gained traction as potential delivery vehicles for various therapeutic agents. These heterogeneous, membrane-derived vesicles are released by nearly all cells into extracellular spaces. They carry a molecular cargo of proteins and nucleic acids within their intraluminal space, encased by a cholesterol-rich phospholipid bilayer membrane. EVs actively engage in cell-to-cell communication by delivering cargo to both neighboring and distant cells. Their inherent ability to shield molecular cargo from degradation and cross biological barriers positions them ideally for delivering CRISPR-Cas ribonucleoproteins (RNP) to target cells. Furthermore, they exhibit higher biocompatibility, lower immunogenicity, and reduced toxicity compared to classical delivery platforms such as adeno-associated virus, lentiviruses, and synthetic nanoparticles. This review explores the potential of employing different CRISPR-Cas systems to target specific genes in OC, while also discussing various methods for engineering EVs to load CRISPR components and enhance their targeting capabilities.

Correction of murine SCID-X1 by lentiviral gene therapy using a codon-optimized IL2RG gene and minimal pretransplant conditioning

Clinical trials have demonstrated the potential of ex vivo hematopoietic stem cell gene therapy to treat X-linked severe combined immunodeficiency (SCID-X1) using γ-retroviral vectors, leading to immune system functionality in the majority of treated patients without pretransplant conditioning. The success was tempered by insertional oncogenesis in a proportion of the patients. To reduce the genotoxicity risk, a self-inactivating (SIN) lentiviral vector (LV) with improved expression of a codon optimized human interleukin-2 receptor γ gene (IL2RG) cDNA (coγc), regulated by its 1.1 kb promoter region (γcPr), was compared in efficacy to the viral spleen focus forming virus (SF) and the cellular phosphoglycerate kinase (PGK) promoters. Pretransplant conditioning of Il2rg(-/-) mice resulted in long-term reconstitution of T and B lymphocytes, normalized natural antibody titers, humoral immune responses, ConA/IL-2 stimulated spleen cell proliferation, and polyclonal T-cell receptor gene rearrangements with a clear integration preference of the SF vector for proto-oncogenes, contrary to the PGK and γcPr vectors. We conclude that SIN lentiviral gene therapy using coγc driven by the γcPr or PGK promoter corrects the SCID phenotype, potentially with an improved safety profile, and that low-dose conditioning proved essential for immune competence, allowing for a reduced threshold of cell numbers required.

Gammaretroviral vectors: biology, technology and application

Retroviruses are evolutionary optimized gene carriers that have naturally adapted to their hosts to efficiently deliver their nucleic acids into the target cell chromatin, thereby overcoming natural cellular barriers. Here we will review—starting with a deeper look into retroviral biology—how Murine Leukemia Virus (MLV), a simple gammaretrovirus, can be converted into an efficient vehicle of genetic therapeutics. Furthermore, we will describe how more rational vector backbones can be designed and how these so-called self-inactivating vectors can be pseudotyped and produced. Finally, we will provide an overview on existing clinical trials and how biosafety can be improved.

Differential immune responses mediated by adenovirus- and lentivirus-transduced DCs in a HER-2/neu overexpressing tumor model

Recent investigations have demonstrated that adenoviral and lentiviral vectors encoding HER-2 can be utilized in cancer immunotherapy. However, it is not known whether both viral systems elicit a similar immune response. Here, we compare the immune response in mice induced by dendritic cells (DCs) infected with either recombinant adenovirus or lentivirus encoding rat HER-2 (rHER-2). Both vaccine types yielded similar control of tumor growth, but we found clear differences in their immune responses 10 days after DC immunization. Adenovirus rHER-2-transduced DCs elicited locally and systemically high frequencies of CD4+ and CD8+ T cells, while lentivirus rHER-2-transduced DCs predominantly led to CD4+ T-cell infiltration at the tumor site. Splenocytes from mice immunized with lentivirus rHER-2-transduced DCs secreted higher levels of interferon (IFN)-γ, mainly by CD4+ T cells, following stimulation by RM-1-mHER-2 tumors. In contrast, the adenovirus vaccinated group exhibited CD4+ and CD8+ T cells that both contributed to IFN-γ production. Besides an established cellular immune response, the rHER-2/DC vaccine elicited a significant humoral response that was highest in the adenovirus group. DC subsets and regulatory T cells in the spleen were also differentially modulated in the two vaccine systems. Finally, adoptive transfer of splenocytes from both groups of immunized mice strongly inhibited in vivo tumor growth. Our results suggest that not only the target antigen but also the virus system may determine the nature and magnitude of antitumor immunity by DC vaccination.

Separating lentiviral vector injection and induction of gene expression in time, does not prevent an immune response to rtTA in rats

Background

Lentiviral gene transfer can provide long-term expression of therapeutic genes such as erythropoietin. Because overexpression of erythropoietin can be toxic, regulated expression is needed. Doxycycline inducible vectors can regulate expression of therapeutic transgenes efficiently. However, because they express an immunogenic transactivator (rtTA), their utility for gene therapy is limited. In addition to immunogenic proteins that are expressed from inducible vectors, injection of the vector itself is likely to elicit an immune response because viral capsid proteins will induce “danger signals” that trigger an innate response and recruit inflammatory cells.

Methodology and Principal Findings

We have developed an autoregulatory lentiviral vector in which basal expression of rtTA is very low. This enabled us to temporally separate the injection of virus and the expression of the therapeutic gene and rtTA. Wistar rats were injected with an autoregulatory rat erythropoietin expression vector. Two or six weeks after injection, erythropoietin expression was induced by doxycycline. This resulted in an increase of the hematocrit, irrespective of the timing of the induction. However, most rats only responded once to doxycycline administration. Antibodies against rtTA were detected in the early and late induction groups.

Conclusions

Our results suggest that, even when viral vector capsid proteins have disappeared, expression of foreign proteins in muscle will lead to an immune response.

Reduction of liver macrophage transduction by pseudotyping lentiviral vectors with a fusion envelope from Autographa californica GP64 and Sendai virus F2 domain

Background

Lentiviral vectors are well suited for gene therapy because they can mediate long-term expression in both dividing and nondividing cells. However, lentiviral vectors seem less suitable for liver gene therapy because systemically administered lentiviral vectors are preferentially sequestered by liver macrophages. This results in a reduction of available virus and might also increase the immune response to the vector and vector products.

Reduction of macrophage sequestration is therefore essential for efficient lentiviral liver gene therapy.

Results

Fusions were made of Autographa californica GP64 and the hepatocyte specific Sendai Virus envelope proteins. Lentiviral vectors were produced with either wild type GP64, Sendai-GP64, or both wild type GP64 and Sendai-GP64 and tested in vitro and in vivo for hepatocyte and macrophage gene transfer.

Sendai-GP64 pseudotyped vectors showed specific gene transfer to HepG2 hepatoma cells, with no detectable transduction of HeLa cervical carcinoma cells, and a decreased affinity for RAW mouse macrophages. Co-expression of wild type GP64 and Sendai-GP64 resulted in improved viral titers while retaining increased affinity for HepG2 cells.

In vivo, the Sendai-GP64 vectors also showed decreased transduction of murine liver macrophages.

Conclusion

We demonstrate reduced macrophage transduction in vitro and in vivo with GP64/Sendai chimeric envelope proteins.

Refinement of lentiviral vector for improved RNA processing and reduced rates of self inactivation repair

BACKGROUND

Lentiviral gene therapy vectors are now finding clinical application. In order to fully exploit their potential it is important that vectors are made as efficient and as safe as possible. Accordingly, we have modified a previously reported vector to improve RNA processing, minimise Human Immunodeficiency Virus Type-1 (HIV-1) sequence content and reduce repair of the self inactivating (SIN) deletion.

RESULTS

HIV-1 sequence in the vector was reduced by substituting the polyadenylation signal with a heterologous signal. Mutation of splice donor sites was undertaken to prevent the majority of splicing within the vector genomic RNA. In addition, a number of other sequences within the vector were deleted. The combination of these modifications was able to significantly reduce the rates of both vector mobilisation and repair of the self inactivating deletion after two rounds of marker rescue.

CONCLUSION

RNA processing can be improved by mutation of the major and minor HIV-1 splice donor sites in the vector. In addition the rate of vector mobilisation and repair of SIN vectors can be successfully reduced by careful vector design that reduces homology between the 5' and 3' long terminal repeats (LTRs) to a minimum.

Cell type-specific targeting with surface-engineered lentiviral vectors co-displaying OKT3 antibody and fusogenic molecule

PURPOSE

The purpose of this study was to investigate the potential of a T-cell-related targeting method using a lentiviral vector-based gene delivery system.

MATERIALS AND METHODS

A lentiviral vector system was constructed by co-incorporating an anti-CD3 antibody (OKT3) and a fusogen into individual viral particles. The incorporation of OKT3 and fusogen was analyzed using confocal microscopy and the in vitro transduction efficiency was evaluated using flow cytometry. Blocking reagents (ammonium chloride (NH(4)Cl) and soluble OKT3 antibody) were added into vector supernatants during transduction to study the mechanism of this two-molecule targeting strategy. To demonstrate the ability of targeted transduction in vivo, Jurkat.CD3 cells were xenografted subcutaneously into the right flank of each mouse and the lentiviral vector was injected subcutaneously on both sides of each mouse 8 h post-injection. Subsequently, the reporter gene (firefly luciferase) expression was monitored using a noninvasive bioluminescence imaging system.

RESULTS

By co-displaying OKT3 and fusogen on the single lentiviral surface, we could achieve targeted delivery of genes to CD3-positive T-cells both in vitro and in vivo.

CONCLUSIONS

These results suggest the potential utility of this engineered lentiviral system as a new tool for cell type-directed gene delivery.

Gene transfer in human vestibular epithelia and the prospects for inner ear gene therapy

Transfer of exogenous genetic material into the mammalian inner ear using viral vectors has been characterized over the last decade. A number of different viral vectors have been shown to transfect the varying cell types of the nonprimate mammalian inner ear. Several routes of delivery have been identified for introduction of vectors into the inner ear while minimizing injury to existing structures and at the same time ensuring widespread distribution of the agent throughout the cochlea and the rest of the inner ear. These studies raise the possibility that gene transfer may be developed as a potential strategy for treating inner ear dysfunction in humans. Furthermore, a recent report showing successful transfection of excised human vestibular epithelia offers proof of principle that viral gene transfer is a viable strategy for introduction and expression of exogenous genetic material to restore function to the inner ear. Human vestibular epithelia were harvested from patients undergoing labyrinthectomy, either for intractable Ménière's disease or vestibular schwannoma resection, and cultured for as long as 5 days. In those experiments, recombinant, multiply-deleted, replication-deficient adenoviral vectors were used to transfect and express a reporter gene as well as the functionally relevant gene, wild-type KCNQ4, a potassium channel gene that when mutated causes the autosomal dominant HL DFNA2.Here, we review the current state of viral-mediated gene transfer in the inner ear and discuss different viral vectors, routes of delivery, and potential applications of gene therapy. Emphasis is placed on experiments demonstrating viral transfection of human inner ear tissue and implications of these findings and for the future of gene therapy in the human inner ear.

Functional analysis of various promoters in lentiviral vectors at different stages of in vitro differentiation of mouse embryonic stem cells

Given the therapeutic potential offered by embryonic stem (ES) cells, it is critical to optimize stable gene delivery and expression at different developmental stages of ES cell differentiation. Here, we systematically analyzed lentiviral vectors containing the following promoters: the human elongation factor 1alpha (EF1alpha) promoter, the human cytomegalovirus (CMV) immediate early region enhancer-promoter, the composite CAG promoter (consisting of the CMV immediate early enhancer and the chicken beta-actin promoter), the human phosphoglycerate kinase 1 (PGK) promoter, the murine stem cell virus (MSCV) long terminal repeat (LTR), or the gibbon ape leukemia virus (GALV) LTR. Our results show that the EF1alpha promoter directed robust transgene expression at every stage of mouse ES cell differentiation, whereas the CMV promoter drove transgene expression only during late stages. Similarly, the CAG and PGK promoters drove transgene expression at a significant level only during late stages. The MSCV LTR and the GALV LTR exhibited much lower promoter activities at all stages. Interestingly, mouse ES cells transduced with the EF1alpha promoter-containing lentiviral vector lost most of their transgene expression during in vitro differentiation to neural precursors and neuronal cells. Our results demonstrate that different cellular and viral promoters exhibit very distinct and dynamic properties not only in terms of promoter strength but also with respect to differentiation stage-specific activity.

Complete knockdown of CCR5 by lentiviral vector-expressed siRNAs and protection of transgenic macrophages against HIV-1 infection

The CCR5 co-receptor is necessary for cellular entry by R5 tropic viral strains involved in primary HIV infection, but is dispensable for normal human physiology. Owing to its crucial role in HIV-1 infection, the CCR5 co-receptor has been the subject of many therapeutic approaches, including gene therapy. siRNA targeting was shown to be effective in downregulating CCR5 expression and conferring significant protection against HIV-1 in susceptible cells. However, complete knockdown of CCR5 expression has not been achieved and thus remains an elusive goal. In these studies, we identified new CCR5 siRNAs capable of achieving complete knockdown of the co-receptor expression. Our transfection studies have shown that longer 28-mer short hairpin siRNAs are very effective in gene downregulation as assessed by fluorescence-activated cell sorting and transcript quantitation by quantitative real-time polymerase chain reaction. These siRNAs conferred strong antiviral protection during viral challenge. To obtain stable expression, highly potent siRNA expression cassettes were introduced into lentiviral vectors. Similar high levels of CCR5 downregulation were observed in stably transduced cells with concomitant viral protection in cultured cell lines. To translate these results to a stem cell gene therapy setting, CD34 hematopoietic progenitor cells were transduced with lentiviral vectors to derive transgenic macrophages. The transgenic cells also exhibited high levels of CCR5 downregulation and viral resistance. With regard to Pol-III promoter-mediated siRNA expression, higher efficacies were obtained with U6-driven CCR5 siRNAs. However, in contrast to previous reports, no apparent cytotoxicities were observed in transgenic cells containing U6-driven siRNA constructs. Thus the above anti-CCR5 siRNAs are among the most effective demonstrated to date and are very promising candidates for clinical applications.

Vector design for expression of O6-methylguanine-DNA methyltransferase in hematopoietic cells

Enhancing DNA repair activity of hematopoietic cells by stably integrating gene vectors that express O(6)-methylguanine-DNA-methyltransferase (MGMT) is of major interest for innovative approaches in tumor chemotherapy and for the control of hematopoietic chimerism in the treatment of multiple other acquired or inherited disorders. Crucial determinants of this selection principle are the stringency of treatment with O(6)-alkylating agents and the level of transgenic MGMT expression. Attempts to generate clinically useful MGMT vectors focus on the design of potent expression cassettes, an important component of which is formed by enhancer sequences that are active in primitive as well as more differentiated hematopoietic cells. However, recent studies have revealed that vectors harboring strong enhancer sequences are more likely to induce adverse events related to insertional mutagenesis. Safety-improved vectors that maintain high levels of MGMT expression may be constructed based on the following principles: choice of enhancer-promoter sequences with relatively mild long-distance effects despite a high transcription rate, improved RNA processing (export, stability and translation), and protein design. The need for optimizing MGMT protein design is supported by recent observations suggesting that the P140K mutant of MGMT, developed to be resistant to inhibitors such as O(6)-benzylguanine, may confer a selective disadvantage when expressed at high levels. Here, we provide a review of the literature exploring MGMT expression vectors for bone marrow chemoprotection, and describe experimental evidence suggesting that high expression of MGMT P140K induces a selective disadvantage in the absence of alkylating agents. We conclude that the appropriate design of expression vectors and MGMT protein features will be crucial for the long-term prospects of this promising selection principle.

Cell-culture assays reveal the importance of retroviral vector design for insertional genotoxicity

Retroviral vectors with long terminal repeats (LTRs), which contain strong enhancer/promoter sequences at both ends of their genome, are widely used for stable gene transfer into hematopoietic cells. However, recent clinical data and mouse models point to insertional activation of cellular proto-oncogenes as a dose-limiting side effect of retroviral gene delivery that potentially induces leukemia. Self-inactivating (SIN) retroviral vectors do not contain the terminal repetition of the enhancer/promoter, theoretically attenuating the interaction with neighboring cellular genes. With a new assay based on in vitro expansion of primary murine hematopoietic cells and selection in limiting dilution, we showed that SIN vectors using a strong internal retroviral enhancer/promoter may also transform cells by insertional mutagenesis. Most transformed clones, including those obtained after dose escalation of SIN vectors, showed insertions upstream of the third exon of Evi1 and in reverse orientation to its transcriptional orientation. Normalizing for the vector copy number, we found the transforming capacity of SIN vectors to be significantly reduced when compared with corresponding LTR vectors. Additional modifications of SIN vectors may further increase safety. Improved cell-culture assays will likely play an important role in the evaluation of insertional mutagenesis.

Mobilization and mechanism of transcription of integrated self-inactivating lentiviral vectors

Permanent genetic modification of replicating primitive hematopoietic cells by an integrated vector has many potential therapeutic applications. Both oncoretroviral and lentiviral vectors have a predilection for integration into transcriptionally active genes, creating the potential for promoter activation or gene disruption. The use of self-inactivating (SIN) vectors in which a deletion of the enhancer and promoter sequences from the 3' long terminal repeat (LTR) is copied over into the 5' LTR during vector integration is designed to improve safety by reducing the risk of mobilization of the vector genome and the influence of the LTR on nearby cellular promoters. Our results indicate that SIN vectors are mobilized in cells expressing lentiviral proteins, with the frequency of mobilization influenced by features of the vector design. The mechanism of transcription of integrated vector genomes was evaluated using a promoter trap design with a vector encoding tat but lacking an upstream promoter in a cell line in which drug resistance depended on tat expression. In six clones studied, all transcripts originated from cryptic promoters either upstream or within the vector genome. We estimate that approximately 1 in 3,000 integrated vector genomes is transcribed, leading to the inference that activation of cryptic promoters must depend on local features of chromatin structure and the constellation of nearby regulatory elements as well as the nature of the regulatory elements within the vector.

CXCR4 and CCR5 shRNA transgenic CD34+ cell derived macrophages are functionally normal and resist HIV-1 infection

BACKGROUND

Stable simultaneous knock down of the HIV-1 coreceptors CCR5 and CXCR4 is a promising strategy to protect cells from both R5 macrophage tropic and X4 T cell tropic as well as dual tropic viral infections. The potency of shRNAs in targeted gene silencing qualifies them as powerful tools for long term HIV gene therapy. Our previous work with a bispecific lentiviral vector containing CXCR4 and CCR5 shRNAs showed efficacy in down regulating both coreceptors and conferring viral resistance to both X4 and R5-tropic strains of HIV-1 in cultured cell lines. To extend these results to a stem cell gene therapy setting, here we show transduction of primary CD34+ hematopoietic progenitor cells to derive normal end stage cells that are resistant to HIV-1 infection.

RESULTS

The bispecific XHR lentiviral vector harboring CXCR4 and CCR5 shRNA expression cassettes was efficient in transducing CD34+ cells. The transduced cells gave rise to morphologically normal transgenic macrophages when cultured in cytokine media. There was a marked down regulation of both coreceptors in the stably transduced macrophages which showed resistance to both R5 and X4 HIV-1 strains upon in vitro challenge. Since off target effects by some shRNAs may have adverse effects on transgenic cells, the stably transduced macrophages were further analyzed to determine if they are phenotypically and functionally normal. FACS evaluation showed normal levels of the characteristic surface markers CD14, CD4, MHC class II, and B7.1. Phagocytic functions were also normal. The transgenic macrophages demonstrated normal abilities in up-regulating the costimulatory molecule B7.1 upon LPS stimulation. Furthermore, IL-1 and TNFalpha cytokine secretion in response to LPS stimulation was also normal. Thus, the transgenic macrophages appear to be phenotypically and functionally normal.

CONCLUSION

These studies have demonstrated for the first time that a bispecific lentiviral vector could be used to stably deliver shRNAs targeted to both CCR5 and CXCR4 coreceptors into CD34+ hematopoietic progenitor cells and derive transgenic macrophages. Transgenic macrophages with down regulated coreceptors were resistant to both R5 and X4 tropic HIV-1 infections. The differentiated cells were also phenotypically and functionally normal indicating no adverse effects of shRNAs on lineage specific differentiation of stem cells. It is now possible to construct gene therapeutic lentiviral vectors incorporating multiple shRNAs targeted to cellular molecules that aid in HIV-1 infection. Use of these vectors in a stem cell setting shows great promise for sustained HIV/AIDS gene therapy.

Comparison of single regulated lentiviral vectors with rtTA expression driven by an autoregulatory loop or a constitutive promoter

Regulated expression of a therapeutic gene is crucial for safe and efficacious gene therapy. Many inducible regulatory systems use a constitutive promoter to express a regulatory protein, such as rtTA in the Tet-On system, which may restrict their use because of cytotoxicity and immunogenicity. Autoregulatory expression of rtTA provides extremely low levels of rtTA when transgene expression is off, with rapid transgene induction upon addition of doxycycline. Lentiviral vectors efficiently transfer genes to dividing and non-dividing cells with long-term gene expression both in vitro and in vivo. We compared regulatory function in a single lentiviral vector where rtTA was either expressed from a constitutive promoter or placed in an autoregulatory loop. Autoregulatory expression of rtTA was superior to constitutive promoter expression, resulting in higher viral titers, undetectable levels of both rtTA and transgene expression in the absence of doxycycline, improved induction kinetics and increased induction levels in all cells tested. We further expanded the utility of the autoregulatory vector by using an improved rtTA variant with an increased sensitivity to doxycycline. This lentiviral vector with doxycycline-regulated transgene expression may be useful for gene therapy applications and in experimental settings where strict temporal expression of a transgene is required.

Lentivirus-based genetic manipulations of cortical neurons and their optical and electrophysiological monitoring in vivo

It is becoming increasingly clear that single cortical neurons encode complex and behaviorally relevant signals, but efficient means to study gene functions in small networks and single neurons in vivo are still lacking. Here, we establish a method for genetic manipulation and subsequent phenotypic analysis of individual cortical neurons in vivo. First, lentiviral vectors are used for neuron-specific gene delivery from alpha-calcium/calmodulin-dependent protein kinase II or Synapsin I promoters, optionally in combination with gene knockdown by means of U6 promoter-driven expression of short-interfering RNAs. Second, the phenotypic analysis at the level of single cortical cells is carried out by using two-photon microscopy-based techniques: high-resolution two-photon time-lapse imaging is used to monitor structural dynamics of dendritic spines and axonal projections, whereas cellular response properties are analyzed electrophysiologically by two-photon microscopy directed whole-cell recordings. This approach is ideally suited for analysis of gene functions in individual neurons in the intact brain.

High-level beta-globin expression and preferred intragenic integration after lentiviral transduction of human cord blood stem cells

Transplantation of genetically corrected autologous hematopoietic stem cells is an attractive approach for the cure of sickle-cell disease and beta-thalassemia. Here, we infected human cord blood cells with a self-inactivating lentiviral vector encoding an anti-sickling betaA-T87Q-globin transgene and analyzed the transduced progeny produced over a 6-month period after transplantation of the infected cells directly into sublethally irradiated NOD/LtSz-scid/scid mice. Approximately half of the human erythroid and myeloid progenitors regenerated in the mice containing the transgene, and erythroid cells derived in vitro from these in vivo-regenerated cells produced high levels of betaA-T87Q-globin protein. Linker-mediated PCR analysis identified multiple transgene-positive clones in all mice analyzed with 2.1 +/- 0.1 integrated proviral copies per cell. Genomic sequencing of vector-containing fragments showed that 86% of the proviral inserts had occurred within genes, including several genes implicated in human leukemia. These findings indicate effective transduction of very primitive human cord blood cells with a candidate therapeutic lentiviral vector resulting in the long-term and robust, erythroid-specific production of therapeutically relevant levels of beta-globin protein. However, the frequency of proviral integration within genes that regulate hematopoiesis points to a need for additional safety modifications.