Rational development of a HIV-1 gene therapy vector

HIV-1 sequences in lentiviral vector genomes can be substantially reduced without compromising transduction efficiency

Many lentiviral vectors used for gene therapy are derived from HIV-1. An optimal vector genome would include only the viral sequences required for transduction efficiency and gene expression to minimize the amount of foreign sequence inserted into a patient’s genome. However, it remains unclear whether all of the HIV-1 sequence in vector genomes is essential. To determine which viral sequences are required, we performed a systematic deletion analysis, which showed that most of the gag region and over 50% of the env region could be deleted. Because the splicing profile for lentiviral vectors is poorly characterized, we used long-read sequencing to determine canonical and cryptic splice site usage. Deleting specific regions of env sequence reduced the number of splicing events per transcript and increased the proportion of unspliced genomes. Finally, combining a large deletion in gag with repositioning the Rev-response element downstream of the 3’ R to prevent its reverse transcription showed that 1201 nucleotides of HIV-1 sequence can be removed from the integrated vector genome without substantially compromising transduction efficiency. Overall, this allows the creation of lentiviral vector genomes that contain minimal HIV-1 sequence, which could improve safety and transfer less viral sequence into a patient’s DNA.

HIV Rev-isited

The human immunodeficiency virus type 1 (HIV-1) proteome is expressed from alternatively spliced and unspliced genomic RNAs. However, HIV-1 RNAs that are not fully spliced are perceived by the host machinery as defective and are retained in the nucleus. During late infection, HIV-1 bypasses this regulatory mechanism by expression of the Rev protein from a fully spliced mRNA. Once imported into the nucleus, Rev mediates the export of unprocessed HIV-1 RNAs to the cytoplasm, leading to the production of the viral progeny. While regarded as a canonical RNA export factor, Rev has also been linked to HIV-1 RNA translation, stabilization, splicing and packaging. However, Rev's functions beyond RNA export have remained poorly understood. Here, we revisit this paradigmatic protein, reviewing recent data investigating its structure and function. We conclude by asking: what remains unknown about this enigmatic viral protein?

The HIV-1 Rev protein enhances encapsidation of unspliced and spliced, RRE-containing lentiviral vector RNA

Background

During the RNA encapsidation process of human immunodeficiency virus (HIV) viral genomic, unspliced RNA (gRNA) is preferentially incorporated into assembling virions. However, a certain amount of spliced viral transcripts can also be detected in viral particles. Recently, we observed that nuclear export of HIV and lentiviral vector gRNA by Rev is required for efficient encapsidation. Since singly-spliced HIV transcripts also contain the Rev-response element (RRE), we investigated if the encapsidation efficiency of RRE-containing spliced HIV-vector transcripts is also increased by the viral Rev protein.

Findings

Starting with a lentiviral vector imitating the splicing pattern of HIV, we constructed vectors that express an unspliced transcript either identical in sequence to the singly-spliced or the fully-spliced RNA of the parental construct. After transfection of the different lentiviral vectors cytoplasmic and virion-associated RNA levels and vector titers were determined in the presence and absence of Rev. Rev enhanced the infectious titer of vectors containing an RRE 6 to 37-fold. Furthermore, Rev strongly increased encapsidation efficiencies of all RRE-containing transcripts up to 200-fold. However, a good correlation between encapsidation efficiency and lentiviral vector titer could only be observed for the gRNA. The infectious titer of the vector encoding the fully-spliced RNA without RRE as well as the encapsidation efficiency of all transcripts lacking the RRE was not influenced by Rev. Interestingly, the splicing process itself did not seem to interfere with packaging, since the encapsidation efficiencies of the same RNA expressed either by splicing or as an unspliced transcript did not differ significantly.

Conclusions

Rev-mediated nuclear export enhances the encapsidation efficiency of RRE-containing lentiviral vector RNAs independently of whether they have been spliced or not.

Lentiviral vectors: basic to translational

More than two decades have passed since genetically modified HIV was used for gene delivery. Through continuous improvements these early marker gene-carrying HIVs have evolved into safer and more effective lentiviral vectors. Lentiviral vectors offer several attractive properties as gene-delivery vehicles, including: (i) sustained gene delivery through stable vector integration into host genome; (ii) the capability of infecting both dividing and non-dividing cells; (iii) broad tissue tropisms, including important gene- and cell-therapy-target cell types; (iv) no expression of viral proteins after vector transduction; (v) the ability to deliver complex genetic elements, such as polycistronic or intron-containing sequences; (vi) potentially safer integration site profile; and (vii) a relatively easy system for vector manipulation and production. Accordingly, lentivector technologies now have widespread use in basic biology and translational studies for stable transgene overexpression, persistent gene silencing, immunization, in vivo imaging, generating transgenic animals, induction of pluripotent cells, stem cell modification and lineage tracking, or site-directed gene editing. Moreover, in the present high-throughput '-omics' era, the commercial availability of premade lentiviral vectors, which are engineered to express or silence genome-wide genes, accelerates the rapid expansion of this vector technology. In the present review, we assess the advances in lentiviral vector technology, including basic lentivirology, vector designs for improved efficiency and biosafety, protocols for vector production and infection, targeted gene delivery, advanced lentiviral applications and issues associated with the vector system.

Cytoplasmic utilization of human immunodeficiency virus type 1 genomic RNA is not dependent on a nuclear interaction with gag

In some retroviruses, such as Rous sarcoma virus and prototype foamy virus, Gag proteins are known to shuttle between the nucleus and the cytoplasm and are implicated in nuclear export of the viral genomic unspliced RNA (gRNA) for subsequent encapsidation. A similar function has been proposed for human immunodeficiency virus type 1 (HIV-1) Gag based on the identification of nuclear localization and export signals. However, the ability of HIV-1 Gag to transit through the nucleus has never been confirmed. In addition, the lentiviral Rev protein promotes efficient nuclear gRNA export, and previous reports indicate a cytoplasmic interaction between Gag and gRNA. Therefore, functional effects of HIV-1 Gag on gRNA and its usage were explored. Expression of gag in the absence of Rev was not able to increase cytoplasmic gRNA levels of subgenomic, proviral, or lentiviral vector constructs, and gene expression from genomic reporter plasmids could not be induced by Gag provided in trans. Furthermore, Gag lacking the reported nuclear localization and export signals was still able to mediate an efficient packaging process. Although small amounts of Gag were detectable in the nuclei of transfected cells, a Crm1-dependent nuclear export signal in Gag could not be confirmed. Thus, our study does not provide any evidence for a nuclear function of HIV-1 Gag. The encapsidation process of HIV-1 therefore clearly differs from that of Rous sarcoma virus and prototype foamy virus.

DDX1 is an RNA-dependent ATPase involved in HIV-1 Rev function and virus replication

The human immunodeficiency virus type 1 (HIV-1) Rev protein is essential for the virus because it promotes nuclear export of alternatively processed mRNAs, and Rev is also linked to translation of viral mRNAs and genome encapsidation. Previously, the human DEAD-box helicase DDX1 was suggested to be involved in Rev functions, but this relationship is not well understood. Biochemical studies of DDX1 and its interactions with Rev and model RNA oligonucleotides were carried out to investigate the molecular basis for association of these components. A combination of gel-filtration chromatography and circular dichroism spectroscopy demonstrated that recombinant DDX1 expressed in Escherichia coli is a well-behaved folded protein. Binding assays using fluorescently labeled Rev and cell-based immunoprecipitation analysis confirmed a specific RNA-independent DDX1–Rev interaction. Additionally, DDX1 was shown to be an RNA-activated ATPase, wherein Rev-bound RNA was equally effective at stimulating ATPase activity as protein-free RNA. Gel mobility shift assays further demonstrated that DDX1 forms complexes with Rev-bound RNA. RNA silencing of DDX1 provided strong evidence that DDX1 is required for both Rev activity and HIV production from infected cells. Collectively, these studies demonstrate a clear link between DDX1 and HIV-1 Rev in cell-based assays of HIV-1 production and provide the first demonstration that recombinant DDX1 binds Rev and RNA and has RNA-dependent catalytic activity.

The HIV-1 Rev/RRE system is required for HIV-1 5' UTR cis elements to augment encapsidation of heterologous RNA into HIV-1 viral particles

Background

The process of HIV-1 genomic RNA (gRNA) encapsidation is governed by a number of viral encoded components, most notably the Gag protein and gRNA cis elements in the canonical packaging signal (ψ). Also implicated in encapsidation are cis determinants in the R, U5, and PBS (primer binding site) from the 5' untranslated region (UTR). Although conventionally associated with nuclear export of HIV-1 RNA, there is a burgeoning role for the Rev/RRE in the encapsidation process. Pleiotropic effects exhibited by these cis and trans viral components may confound the ability to examine their independent, and combined, impact on encapsidation of RNA into HIV-1 viral particles in their innate viral context. We systematically reconstructed the HIV-1 packaging system in the context of a heterologous murine leukemia virus (MLV) vector RNA to elucidate a mechanism in which the Rev/RRE system is central to achieving efficient and specific encapsidation into HIV-1 viral particles.

Results

We show for the first time that the Rev/RRE system can augment RNA encapsidation independent of all cis elements from the 5' UTR (R, U5, PBS, and ψ). Incorporation of all the 5' UTR cis elements did not enhance RNA encapsidation in the absence of the Rev/RRE system. In fact, we demonstrate that the Rev/RRE system is required for specific and efficient encapsidation commonly associated with the canonical packaging signal. The mechanism of Rev/RRE-mediated encapsidation is not a general phenomenon, since the combination of the Rev/RRE system and 5' UTR cis elements did not enhance encapsidation into MLV-derived viral particles. Lastly, we show that heterologous MLV RNAs conform to transduction properties commonly associated with HIV-1 viral particles, including in vivo transduction of non-dividing cells (i.e. mouse neurons); however, the cDNA forms are episomes predominantly in the 1-LTR circle form.

Conclusions

Premised on encapsidation of a heterologous RNA into HIV-1 viral particles, our findings define a functional HIV-1 packaging system as comprising the 5' UTR cis elements, Gag, and the Rev/RRE system, in which the Rev/RRE system is required to make the RNA amenable to the ensuing interaction between Gag and the canonical packaging signal for subsequent encapsidation.

Lentiviral airway gene transfer in lungs of mice and sheep: successes and challenges

BACKGROUND

Persistent airway gene expression can be achieved in mouse nasal airway using a vesicular stomatitis virus glycoprotein pseudotyped lentiviral (LV) gene vector in combination with lysophosphatidylcholine (LPC) pretreatment. We have now examined the acute in vivo effects of this combination single-dose method for airway LV gene transfer in mouse and sheep lung.

METHODS

Mouse and sheep lungs were exposed to LPC followed 1 h later with the LV vector. Lungs were processed 7 days later using X-gal detection to measure beta-gal gene expression and identify transduced cell types.

RESULTS

In mouse ciliated conducting airways, LPC pretreatment produced extensive gene transfer that extended from the tracheal dosing site into the bronchi and lower airways. Gene expression was present in both terminally differentiated surface cells and in basal cells. Without LPC pretreatment, transduction was limited to the dosing site. In sheep lung, small-volume bronchoscopic instillation delivery produced localized and low-level transduction near the dosing site. Gene expression was again present in surface and basal cells. Neither alterations in LPC dose parameters, nor larger vector volumes increased the level of transduction.

CONCLUSIONS

These findings are the first to confirm the applicability of LPC pretreatment in the production of extensive lentiviral gene transfer in mouse lung airways. However, improved methodologies to increase transduction efficiency are required for adult sheep lung. The results suggest that continued in vivo development of LPC-enhanced lentiviral gene transfer is needed in the lungs of large animals to establish effective lentiviral gene transfer techniques suited to the treatment of airway disease.

Correction of mucopolysaccharidosis type IIIA somatic and central nervous system pathology by lentiviral-mediated gene transfer

BACKGROUND

The hallmark of lysosomal storage disorders (LSDs) is microscopically demonstrable lysosomal distension. In mucopolysaccharidosis type IIIA (MPS IIIA), this occurs as a result of an inherited deficiency of the lysosomal hydrolase sulphamidase. Consequently, heparan sulphate, a highly sulphated glycosaminoglycan, accumulates primarily within the cells of the reticulo-endothelial and monocyte-macrophage systems and, most importantly, neurones. Children affected by MPS IIIA experience a severe, progressive neuropathology that ultimately leads to death at around 15 years of age.

METHODS

MPS IIIA pathology was addressed in a mouse model using two separate methods of therapeutic gene delivery. A lentiviral vector expressing murine sulphamidase was delivered to 6-week-old MPS IIIA affected mice either by intravenous injection, or by intraventricular infusion. Therapeutic outcomes were assessed 7 months after gene transfer.

RESULTS

After intravenous gene delivery, liver sulphamidase was restored to approximately 30% of wild-type levels. The resultant widespread delivery of enzyme secreted from transduced cells to somatic tissues via the peripheral circulation corrected most somatic pathology. However, unlike an earlier study, central nervous system (CNS) pathology remained unchanged. Conversely, intraventricular gene delivery resulted in widespread sulphamidase gene delivery in (and reduced lysosomal storage throughout) the brain. Improvements in behaviour were observed in these mice, and interestingly, pathological urinary retention was prevented.

CONCLUSIONS

The CNS remains the last major barrier to effective therapy for children affected by LSDs. The blood-brain barrier (BBB) limits the uptake of lysosomal enzymes from the peripheral circulation into the CNS, making direct gene delivery to the brain a reasonable, albeit more challenging, therapeutic option. Future work will further assess the relative advantages of directly targeting the brain with somatic gene delivery with sulphamidase modified to increase the efficiency of transport across the BBB.

Comparison of ventricular and intravenous lentiviral-mediated gene therapy for murine MPS VII

Mucopolysaccharidosis type VII (MPS VII) is caused by the deficiency of the lysosomal hydrolase β-glucuronidase. Symptoms include intellectual impairment, growth retardation, visual and hearing deficits and organ malfunction. The MPS VII mouse displays most of the symptoms variously associated with the MPS disorders, and has been widely used as a developmental paradigm for gene therapy. In this study, a lentiviral vector expressing murine β-glucuronidase was delivered to 6-week-old MPS VII affected mice, either by intravenous injection, or by ventricular infusion. Therapeutic outcomes were assessed 7 months after gene transfer. Intravenous vector delivery restored liver β-glucuronidase to normal levels. Consequently, most somatic pathology was corrected, and brain pathology was reduced. In mice that received ventricular vector most brain regions appeared biochemically and histologically normal. These animals showed significantly improved behavioural performance within the open-field test. An additional positive outcome of ventricular vector delivery was the significant reduction of lysosomal storage within the eye. The blood-brain barrier is not completely impervious to lysosomal enzymes, therefore, therapeutic enzyme can be distributed widely throughout the brain via the extensive cerebral vasculature. However, improvements in somatic gene delivery and expression are required for this to be completely successful. Ventricular vector delivery cleared lysosomal storage within the CNS making this a reasonable, albeit more challenging, therapeutic option for the MPS. The best therapeutic outcomes, with possible synergistic effects within the CNS, might be expected to occur when vector delivery to the brain is used in combination with somatic gene transfer.

OCT-1 function varies with cell lineage but is not influenced by BCR-ABL

BACKGROUND

Despite the excellent responses to imatinib therapy observed in patients with chronic phase chronic myeloid leukemia, approximately 25% of patients display primary resistance or suboptimal response. The OCT-1 activity in mononuclear cells reflects the efficiency of active influx of imatinib. OCT-1 activity in mononuclear cells is highly variable between patients and significantly correlates with a patient's molecular response to imatinib treatment and overall survival. The present study examined whether cell lineage and BCR-ABL expression influenced OCT-1 activity.

DESIGN AND METHODS

The OCT-1 activity and OCT-1 mRNA expression was assessed in pure populations of neutrophils, monocytes and lymphocytes recovered from chronic myeloid leukemia patients at diagnosis, in cytogenetic remission and normal individuals. The role of BCR-ABL on OCT-1 activity and differentiation was examined in a cell line model of ectopic BCR-ABL expression.

RESULTS

The OCT-1 activity and OCT-1 mRNA expression was highest in the neutrophil population and lowest in lymphocytes (P<0.05). This was observed for patients at diagnosis, in cytogenetic remission and normal individuals. Interestingly, neutrophil OCT-1 activity was not significantly different between patients at diagnosis, in remission and normal donors. This was also observed for monocytes and lymphocytes. Furthermore, OCT-1 activity in mononuclear cells was significantly correlated with the OCT-1 activity in neutrophils (P=0.001). In a cell line model in which BCR-ABL was ectopically expressed, we found no evidence that BCR-ABL directly affected OCT-1 expression and function. However, BCR-ABL stimulated granulocyte differentiation which, in turn, led to significantly increased OCT-1 activity (P=0.024).

CONCLUSIONS

These studies suggest that the predictive OCT-1 activity in patient mononuclear cells is strongly related to cell lineage, particularly the presence of neutrophils in the peripheral blood. Furthermore, BCR-ABL expression is unlikely to directly influence OCT-1 activity but may have an indirect role by enhancing granulocyte differentiation.

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.

Lentiviral-mediated correction of MPS VI cells and gene transfer to joint tissues

Joint disease in mucopolysaccharidosis type VI (MPS VI) remains difficult to treat despite the success of enzyme replacement therapy in treating other symptoms. In this study, the efficacy of a lentiviral vector to transduce joint tissues and express N-acetylgalactosamine-4-sulphatase (4S), the enzyme deficient in MPS VI, was evaluated in vitro and the expression of beta-galactosidase was used to evaluate transduction in vivo. High viral copy number was achieved in MPS VI fibroblasts and 4-sulphatase activity reached 12 times the normal level. Storage of accumulated glycosaminoglycan was reduced in a dose dependent manner in both MPS VI skin fibroblasts and chondrocytes. Enzyme expression was maintained in skin fibroblasts for up to 41 days. Comparison of two promoters; the murine phosphoglycerate kinase gene promoter (pgk) and the myeloproliferative sarcoma virus long terminal repeat promoter (mpsv), demonstrated a higher level of marker gene expression driven by the mpsv promoter in both chondrocytes and synoviocytes in vitro. When injected into the rat knee, the expression of beta-galactosidase from the mpsv promoter was widespread across the synovial membrane and the fascia covering the cruciate ligaments and meniscus. No transduction of chondrocytes or ligament cells was observed. Transduction was maintained for at least 8 weeks after injection. These results indicate that the lentiviral vector can be used to deliver 4S to a range of joint tissues in vitro and efficiently transduce synovial cells and express beta-galactosidase in vivo.

Rev: beyond nuclear export

Rev remains a hot topic. In this review, we revisit the insights that have been gained into the control of gene expression by the retroviral protein Rev and speculate on where current research is leading. We outline what is known about the role of Rev in translation and encapsidation and how these are linked to its more traditional role of nuclear export, underlining the multifaceted nature of this small viral protein. We discuss what more is to be learned in these fields and why continuing research on these 116 amino acids and understanding their function is still important in devising methods to combat AIDS.

Gene delivery to airway epithelial cells in vivo: a direct comparison of apical and basolateral transduction strategies using pseudotyped lentivirus vectors

Lentivirus vectors are being investigated as gene delivery vehicles for cystic fibrosis airway gene therapy. Vesicular stomatitis virus G glycoprotein (VSV-G)-pseudotyped vectors transduce airway epithelia via receptors that are located predominantly on the basolateral surface of the airway epithelium. Effective transduction with VSV-G-pseudotyped vectors requires the use of a pre-treatment that disrupts epithelial tight junctions, allowing access to these basolateral receptors. In contrast, it has been reported that apically targeted lentiviral vectors allow efficient gene transfer in the absence of any pre-treatment. In a direct comparison of transduction by a VSV-G-pseudotyped vector, in combination with a pre-treatment with lysophosphatidylcholine (LPC), and the same vector pseudotyped with the apically targeted baculovirus GP64 envelope (without any pre-treatment), the GP64 vector was found to be significantly less efficient. However, when a pre-treatment with LPC was used the level of transduction with the GP64-pseudotyped lentiviral vector was not significantly different to that resulting from the VSV-G-pseudotyped vector. The cell types transduced with each vector were essentially the same, with the majority of cells transduced being respiratory (ciliated cells). However, unlike the VSV-G-pseudotyped vector, which results in persisting gene expression, transduction with the GP64-pseudotyped vector resulted in gene expression that declined to undetectable levels over six months, whether or not an LPC pre-treatment was used.

Rev proteins of human and simian immunodeficiency virus enhance RNA encapsidation

The main function attributed to the Rev proteins of immunodeficiency viruses is the shuttling of viral RNAs containing the Rev responsive element (RRE) via the CRM-1 export pathway from the nucleus to the cytoplasm. This restricts expression of structural proteins to the late phase of the lentiviral replication cycle. Using Rev-independent gag-pol expression plasmids of HIV-1 and simian immunodeficiency virus and lentiviral vector constructs, we have observed that HIV-1 and simian immunodeficiency virus Rev enhanced RNA encapsidation 20- to 70-fold, correlating well with the effect of Rev on vector titers. In contrast, cytoplasmic vector RNA levels were only marginally affected by Rev. Binding of Rev to the RRE or to a heterologous RNA element was required for Rev-mediated enhancement of RNA encapsidation. In addition to specific interactions of nucleocapsid with the packaging signal at the 5′ end of the genome, the Rev/RRE system provides a second mechanism contributing to preferential encapsidation of genomic lentiviral RNA.

The AIDS pandemic is still an important public health problem, particularly in developing countries. A comprehensive understanding of the HIV replication cycle might allow development of new therapeutics. Despite 20 years of extensive research, the intracellular fate of the different RNAs produced during virus replication is not fully understood. It is known that the viral regulatory protein Rev binds to large viral RNAs and shuttles them from the nucleus to the cytoplasm by a cellular export pathway. We now provide evidence for a more far-reaching role of Rev. We observed that Rev enhances packaging of viral RNA into viral particles to a much larger extent than its effect on viral RNA levels in the cytoplasm. Thus, an early nuclear event (binding of Rev to the viral RNA) seems to be intimately linked to RNA encapsidation occurring at a late step of the viral replication cycle. Since Rev is not part of the viral particles, Rev seems to act indirectly, possibly by targeting the viral RNA to a cytoplasmic compartment favourable for RNA encapsidation. Thus, further studies on the function of Rev might also advance our understanding of cytoplasmic RNA trafficking and subcytoplasmic compartmentalization.

Stable gene expression occurs from a minority of integrated HIV-1-based vectors: transcriptional silencing is present in the majority

Human immunodeficiency virus (HIV)-based vectors are being increasingly used in vitro for gene transfer and in vivo for gene therapy. The proportion of integrated retroviral vectors that are silenced or remain transcriptionally active, and the stability of gene expression in the latter remains poorly explored. To study this, T cells were infected with an HIV-1-based vector construct containing a long terminal repeat-driven reporter gene. Only a small percentage of detectable integrated vector expressed gene product. In clones derived from cells with transcriptionally active vector, gene expression was remarkably stable with more than 80% continuing to express for greater than 18 months. Failure to continue expressing the vector was associated with epigenetic changes. Our data suggest that there are two forms of vector silencing: one occurring immediately after integration affecting the majority of the vectors, and one occurring in the much longer term affecting a small minority of vectors which had previously established expression.

Lentivirus-mediated gene transfer to the rat, ovine and human cornea

Gene therapy of the cornea shows promise for modulating corneal transplant rejection but the most appropriate vector for gene transfer has yet to be determined. We investigated a lentiviral vector (LV) for its ability to transduce corneal endothelium. A lentivector expressing enhanced yellow fluorescent protein (eYFP) under the control of the Simian virus type 40 early promoter (LV-SV40-eYFP) transduced 80-90% of rat, ovine and human corneal endothelial cells as detected by fluorescence microscopy. The kinetics of gene expression varied among species, with ovine corneal endothelium showing a relative delay in detectable reporter gene expression compared with the rat or human corneal endothelium. Vectors containing the myeloproliferative sarcoma virus promoter or the phosphoglycerate kinase promoter were not significantly more effective than LV-SV40-eYFP. The stability of eYFP expression in rat and ovine corneas following ex vivo transduction of the donor cornea was assessed following orthotopic corneal transplantation. Following transduction ex vivo, eYFP expression was maintained in corneal endothelial cells for at least 28 days after corneal transplantation in the sheep and >60 days in the rat. Thus, rat, ovine and human corneal endothelial cells were efficiently transduced by the LV, and gene expression appeared stable over weeks in vivo.

Lentiviral-mediated gene correction of mucopolysaccharidosis type IIIA

BACKGROUND

Mucopolysaccharidosis type IIIA (MPS IIIA) is the most common of the mucopolysaccharidoses. The disease is caused by a deficiency of the lysosomal enzyme sulphamidase and results in the storage of the glycosaminoglycan (GAG), heparan sulphate. MPS IIIA is characterised by widespread storage and urinary excretion of heparan sulphate, and a progressive and eventually profound neurological course. Gene therapy is one of the few avenues of treatment that hold promise of a sustainable treatment for this disorder.

METHODS

The murine sulphamidase gene cDNA was cloned into a lentiviral vector and high-titre virus produced. Human MPS IIIA fibroblast cultures were transduced with the sulphamidase vector and analysed using molecular, enzymatic and metabolic assays. High-titre virus was intravenously injected into six 5-week old MPS IIIA mice. Three of these mice were pre-treated with hyperosmotic mannitol. The weight of animals was monitored and GAG content in urine samples was analysed by polyacrylamide gel electrophoresis.

RESULTS

Transduction of cultured MPS IIIA fibroblasts with the sulphamidase gene corrected both the enzymatic and metabolic defects. Sulphamidase secreted by gene-corrected cells was able to cross correct untransduced MPS IIIA cells. Urinary GAG was found to be greatly reduced in samples from mice receiving the vector compared to untreated MPS IIIA controls. In addition, the weight of treated mice became progressively normalised over the 6-months post-treatment.

CONCLUSION

Lentiviral vectors appear promising vehicles for the development of gene therapy for MPS IIIA.

Gene transfer system derived from the caprine arthritis-encephalitis lentivirus

Lentiviruses are attractive candidates for therapeutic vectors, because of their ability to infect non-dividing target cells. Vectors based on HIV-1 efficiently transfer gene expression to a variety of dividing or quiescent cells, but are subject to reservations on safety grounds. Caprine arthritis encephalitis virus (CAEV) is a lentivirus inducing only minor pathology in its natural host and in related species after cross-species transmission. To test the CAEV potential as vector for gene transfer, a cassette expressing the green fluorescent protein (GFP) under control of a CMV promoter was inserted into the CAEV genome, producing the pK2EGFPH vector. When pseudotyped with vesicular stomatitis virus (VSV)-G envelope protein, this vector allowed efficient transfer of GFP expression in human cells (up to 86% of GFP-expressing cells into the TE671 cell line). Three vectors carrying different parts of the viral gag, pol and env genes were then developed, together with a CAEV packaging system. These vectors allowed delimitation of the minimal CAEV sequences necessary for an improvement of vector production compared to the previously described CAEV-based vectors [Mselli-Lakhal et al., 1998. Defect in RNA transport and packaging are responsible for low transduction efficiency of CAEV-based vectors. Arc. Virol. 143, 681-695]. While our previous vectors were produced in a helper/vector system, the present vectors are produced in a helper/free system. However, these vector titers remain lower than those obtained with other lentiviral vectors carrying equivalent packaging sequences. We discuss on possible reasons of such differences and possible improvements.

Optimisation of a multipartite human immunodeficiency virus based vector system; control of virus infectivity and large-scale production

BACKGROUND

We have previously described a five-plasmid HIV-1 vector system that utilises a codon-optimised gagpol gene. While this system was shown to be safer than systems using proviral type helpers, the titre of virus produced was relatively low. Therefore, a process of optimising all aspects of virus production was initiated.

METHODS

A systematic approach was taken to the optimisation of virus production by transient expression using a five-plasmid packaging system. Codon-manipulation was used to reduce homology between helper and vector constructs. Ultrafiltration and ultracentrifugation were used for large-scale virus production.

RESULTS

We describe codon-optimised reading frames for Tat and Rev and the optimisation of virus production. The optimisation process resulted in an increase in virus titre of 7- to 8-fold. Several other approaches to increasing viral titre described by others proved ineffective in our system after it had been optimised. In addition, we show that by varying the ratio of the GagPol helper construct to vector, the infectivity of the virus could be controlled. The use of a novel codon-optimised HIV-1 GagPol expression construct with reduced homology to vector sequences significantly reduced transfer of gagpol sequences to transduced cells. Virus could be collected in serum-free medium without a significant loss of titre, which facilitated subsequent processing. Processing using a combination of ultrafiltration and ultracentrifugation allowed efficient and rapid processing of litre volumes of virus supernatant.

CONCLUSIONS

By taking a systematic approach to optimising all aspects of our five-plasmid lentiviral vector system we improved titre, safety, large-scale production, and demonstrated that infectivity could be specifically controlled.

Human mesenchymal stem cells ectopically expressing full-length dystrophin can complement Duchenne muscular dystrophy myotubes by cell fusion

Duchenne muscular dystrophy (DMD) is the most prevalent inheritable muscle disease. It is caused by mutations in the approximately 2.5-megabase dystrophin (Dys) encoding gene. Therapeutic attempts at DMD have relied on injection of allogeneic Dys-positive myoblasts. The immune rejection of these cells and their limited availability have prompted the search for alternative therapies and sources of myogenic cells. Stem cell-based gene therapy aims to restore tissue function by the transplantation of gene-corrected autologous cells. It depends on (i) the capacity of stem cells to participate in tissue regeneration and (ii) the efficient genetic correction of defective autologous stem cells. We explored the potential of bone marrow-derived human mesenchymal stem cells (hMSCs) genetically modified with the full-length Dys-coding sequence to engage in myogenesis. By tagging hMSCs with enhanced green fluorescent protein (EGFP) or the membrane dye PKH26, we demonstrated that they could participate in myotube formation when cultured together with differentiating human myoblasts. Experiments performed with EGFP-marked hMSCs and DsRed-labeled DMD myoblasts revealed that the EGFP-positive DMD myotubes were also DsRed-positive indicating that hMSCs participate in human myogenesis through cellular fusion. Finally, we showed that hMSCs transduced with a tropism-modified high-capacity hybrid viral vector encoding full-length Dys could complement the genetic defect of DMD myotubes.

HIV-1 Gag-RNA interaction occurs at a perinuclear/centrosomal site; analysis by confocal microscopy and FRET

The Gag polyprotein is the major structural protein of human immunodeficiency virus-1 (HIV-1) constituting the viral core. Between translation on cytoplasmic polysomes and assembly into viral particles at the plasma membrane, it specifically captures the RNA genome of the virus through binding RNA structural motifs (packaging signals -Psi) in the RNA. RNA is believed to be a structural facilitator of Gag assembly. Using a combined approach of immunofluorescence detection of Gag protein and in situ hybridisation detection of viral genomic RNA, we demonstrate that Gag protein colocalises early after expression with Psi+ RNA in the perinuclear region and also colocalises with centrioles. Colocalised RNA and protein subsequently traffic through the cytoplasm to the plasma membrane of the cell. Gag expressed from Psi- RNA diffuses throughout the cell. It is not found at centrioles and shows delayed cytoplasmic colocalisation with the RNA genome. RNA capture through Psi does not influence binding of Gag to microfilaments. Gag does not bind to tubulin during export. The presence of the packaging signal may coordinate capture of Psi+ RNA by Gag protein at the centrosome followed by their combined transport to the site of budding. HIV-1 Psi thus acts as a subcellular localisation signal as well as a high-affinity-binding site for Gag.

Reduced mobilization of Rev-responsive element-deficient lentiviral vectors

Infection of cells transduced with a lentiviral vector by human immunodeficiency virus (HIV) could lead to packaging of the lentiviral vector RNA into HIV particles and unintended transfer of the vector. To prevent this, the Rev-responsive element (RRE) of an HIV-1 vector was functionally replaced by a heterologous RNA element (MS2). Providing Rev fused to an MS2 binding protein allowed efficient vector production. Mobilization of the vector from infected target cells was below the level of detection and at least 10(3)- to 10(4)-fold lower than for the RRE-containing vector. Thus, RRE-deficient lentiviral vectors provide a novel approach to reduce the risk of vector mobilization.

The use of retroviral vectors for gene therapy-what are the risks? A review of retroviral pathogenesis and its relevance to retroviral vector-mediated gene delivery

Retroviral vector-mediated gene transfer has been central to the development of gene therapy. Retroviruses have several distinct advantages over other vectors, especially when permanent gene transfer is the preferred outcome. The most important advantage that retroviral vectors offer is their ability to transform their single stranded RNA genome into a double stranded DNA molecule that stably integrates into the target cell genome. This means that retroviral vectors can be used to permanently modify the host cell nuclear genome. Recently, retroviral vector-mediated gene transfer, as well as the broader gene therapy field, has been re-invigorated with the development of a new class of retroviral vectors which are derived from lentiviruses. These have the unique ability amongst retroviruses of being able to infect non-cycling cells. Vectors derived from lentiviruses have provided a quantum leap in technology and seemingly offer the means to achieve significant levels of gene transfer in vivo.The ability of retroviruses to integrate into the host cell chromosome also raises the possibility of insertional mutagenesis and oncogene activation. Both these phenomena are well known in the interactions of certain types of wild-type retroviruses with their hosts. However, until recently they had not been observed in replication defective retroviral vector-mediated gene transfer, either in animal models or in clinical trials. This has meant the potential disadvantages of retroviral mediated gene therapy have, until recently, been seen as largely, if not entirely, hypothetical. The recent clinical trial of gammac mediated gene therapy for X-linked severe combined immunodeficiency (X-SCID) has proven the potential of retroviral mediated gene transfer for the treatment of inherited metabolic disease. However, it has also illustrated the potential dangers involved, with 2 out of 10 patients developing T cell leukemia as a consequence of the treatment. A considered review of retroviral induced pathogenesis suggests these events were qualitatively, if not quantitatively, predictable. In addition, it is clear that the probability of such events can be greatly reduced by relatively simple vector modifications, such as the use of self-inactivating vectors and vectors derived from non-oncogenic retroviruses. However, these approaches remain to be fully developed and validated. This review also suggests that, in all likelihood, there are no other major retroviral pathogenetic mechanisms that are of general relevance to replication defective retroviral vectors. These are important conclusions as they suggest that, by careful design and engineering of retroviral vectors, we can continue to use this gene transfer technology with confidence.

Lentiviral vectors

Vectors based on lentiviruses have reached a state of development such that clinical studies using these agents as gene delivery vehicles have now begun. They have particular advantages for certain in vitro and in vivo applications especially the unique capability of integrating genetic material into the genome of non-dividing cells. Their rapid progress into clinical use reflects in part the huge body of knowledge which has accumulated about HIV in the last 20 years. Despite this, many aspects of viral assembly on which the success of these vectors depends are rather poorly understood. Sufficient is known however to be able to produce a safe and reproducible high titre vector preparation for effective transduction of growth-arrested tissues such as neural tissue, muscle and liver.