Gene therapy holds great promise for the treatment of a wide range of inherited and acquired disorders. The development of viral vector systems to mediate safe and long-lasting expression of therapeutic transgenes in specific target cell populations is continually advancing. Gene therapy for the nervous system is particularly challenging due to the post-mitotic nature of neuronal cells and the restricted accessibility of the brain itself. Viral vectors based on lentiviruses provide particularly attractive vehicles for delivery of therapeutic genes to treat neurological and ocular diseases, since they efficiently transduce non-dividing cells and mediate sustained transgene expression. Furthermore, novel routes of vector delivery to the nervous system have recently been elucidated and these have increased further the scope of lentiviruses for gene therapy application. Several studies have demonstrated convincing therapeutic efficacy of lentiviral-based gene therapies in animal models of severe neurological disorders and the push for progressing such vectors to the clinic is ongoing. This review describes the key features of lentiviral vectors that make them such useful tools for gene therapy to the nervous system and outlines the major breakthroughs in the potential use of such vectors for treating neurodegenerative and ocular diseases.

INTRODUCTION

Viral vectors currently provide the vehicle of choice for gene therapy clinical trials. In the U.K. alone, 74% of all gene therapy trials involve the use of various types of viral vector [1]. Unsurprisingly, the majority of such trials focus on cancer therapy; however, the potential scope for gene therapy far exceeds this field. Adenoviral vectors have been well-utilized in clinical trials to date, primarily because they have been well-characterized, can be generated to high titre and efficiently transduce a wide range of host cell types including non-dividing cells such as neurons [2,3]. However, these vectors have a major drawback in that they have been widely observed to cause immunogenicity in recipient organisms [4,5]. The concerns over toxic effects of adenoviral vector administration were heightened following the death of a clinical trial patient receiving a high dose of a first-generation adenoviral vector used to treat ornithine transcarbamylase deficiency [6]. To address the issue of immunogenicity, ‘gutless’ adenoviral vectors have since been generated in which all non-essential viral genes have been removed [7,8]. However, studies have subsequently demonstrated that an immune response can still be initiated by the adenoviral capsid proteins [9,10]. A further property of adenoviral vectors is the episomal nature of the genome in transduced cells. Although this eliminates the chance of random integration into the host cell genome and possibility of disrupting cellular gene expression, many chronic disease situations, such as progressive neurodegenerative disorders, require a long-term gene therapy approach where expression of the therapeutic gene is sustained.

AAV (adeno-associated viral) vectors also have a broad host tropism and have been demonstrated to efficiently transduce neuronal cell types [1113]. AAV vectors were originally considered a relatively safe gene therapy vehicle in that the wild-type virus itself has not been demonstrated to cause disease in humans and that non-targeted integration of the AAV genome into that of the host cell is inefficient [14]. These vectors are restricted, however, by the limited transgene capacity of 4–5 kb, although this can be overcome to a certain extent by recombination of two AAV vectors in the host cell to provide a larger genome [1517]. Clinical trials in the U.S.A. using AAV vectors for treating haemophilia B have highlighted difficulties in achieving the required level of therapeutic gene expression, and this was compounded by pre-existing immunity to the AAV vectors used [18]. Additionally, studies performed in mice demonstrated that the AAV vector used in these trials integrated with low efficiency into coding regions of genes more frequently than non-coding regions and this has raised questions over the safety of such vectors [19,20]. Two clinical trials using AAV vectors in the CNS (central nervous system) have recently been initiated to treat PD (Parkinson's disease) and the results of such trials are eagerly anticipated [2123].

Vector systems based on HSVs (herpes simplex viruses) exhibit a high efficiency at transducing neuronal cells due to the natural neurotropism of HSVs. These vectors also mediate highly efficient anterograde and retrograde transport within the nervous system. Multiply deleted HSV-based vectors have a large transgene capacity (>50 kb) and the genome remains episomal in the host cell [24]. Problems associated with HSV vectors include the immune response raised against expression of viral proteins encoded in the vector backbone and the limited duration of transgene expression. These factors could limit its use in chronic neurodegenerative disorders. To date the application of HSV vectors to clinical trials has been restricted to cancer-based approaches [25,26].

In terms of long-term expression of a therapeutic transgene, retroviral- and lentiviral-based vectors provide the most promising gene therapy vehicles, since the viral genome is stably integrated into that of the host cell following transduction. Lentiviruses are a complex form of retroviruses that are particularly advantageous for gene therapy approaches to the nervous system, since they efficiently transduce post-mitotic neuronal cell types. Such vectors also have a relatively large transgene capacity (8–10 kb), can be generated to high titre and do not give rise to immunological complications that can compromise transduced cell viability [2729]. One potential risk with all integrating gene therapy vectors is the possibility of insertional mutagenesis, with the potential for disrupting expression of essential genes or the activation of otherwise silent promoter/enhancer regions. This was recently highlighted by the well-documented retroviral-based clinical trial for children with the fatal immunological disorder X-SCID (X-linked severe combined immunodeficiency). Three of the patients treated developed a form of leukaemia that was at least in part caused by the activation of the oncogene, LIM-only2 (LMO2), in two of the patients [30]. It is interesting that a similar clinical trial in the U.K. has had no repeated cases of leukaemia to date. Nevertheless, the risk/benefit associated with any gene therapy approach must be carefully considered.

In this review, we highlight the potential applications of lentiviral vectors for treating neurological diseases including several neurodegenerative and ocular disorders (Table 1) and discuss recent breakthroughs in the advancement of such vectors for clinical application.

Table 1
Candidate neurological disorders for therapeutic intervention using lentiviral-mediated gene therapy
DiseaseApproachGene targetTarget tissue/cell type
AD Gene silencing β- and γ-Secretase, Tau, GSK-3 and Cdk-5 Cortex and hippocampus 
 Overexpression NGF and Nep  
PD Gene silencing α-Synuclein and LRRK2 Substantia nigra 
 Overexpression GDNF and dopamine biosynthesis enzymes  
HD Gene silencing Huntingtin Striatum and cortex 
 Overexpression CNTF  
ALS Gene silencing SOD1 Spinal cord and brain stem motor neurons 
 Overexpression VEGF and IGF-1  
AMD/diabetic retinopathy Gene silencing VEGF Retina 
 Overexpression Endostatin, angiostatin and sFlt1  
RP Overexpression PEDF and PDEβ Photoreceptors 
FED Gene silencing COL8α2 Corneal endothelium 
DiseaseApproachGene targetTarget tissue/cell type
AD Gene silencing β- and γ-Secretase, Tau, GSK-3 and Cdk-5 Cortex and hippocampus 
 Overexpression NGF and Nep  
PD Gene silencing α-Synuclein and LRRK2 Substantia nigra 
 Overexpression GDNF and dopamine biosynthesis enzymes  
HD Gene silencing Huntingtin Striatum and cortex 
 Overexpression CNTF  
ALS Gene silencing SOD1 Spinal cord and brain stem motor neurons 
 Overexpression VEGF and IGF-1  
AMD/diabetic retinopathy Gene silencing VEGF Retina 
 Overexpression Endostatin, angiostatin and sFlt1  
RP Overexpression PEDF and PDEβ Photoreceptors 
FED Gene silencing COL8α2 Corneal endothelium 

LENTIVIRUSES FOR GENE THERAPY IN THE NERVOUS SYSTEM

Lentiviral vectors can be divided into primate-derived, such as HIV and SIV (simian immunodeficiency virus), vectors and non-primate-derived, such as EIAV (equine infectious anaemia virus), FIV (feline immunodeficiency virus) and BIV (bovine immunodeficiency virus), vectors. The first lentiviral vector system to be developed was based on HIV and this exploited the capacity of retroviral particles to be pseudotyped with the VSV-G (vesicular stomatitis virus) envelope, which enabled the generation of high titre vector stocks and conferred a broad tissue tropism to the vector [31]. Subsequent advancements in the generation of lentiviral vectors have resulted in the development of multiply deleted vectors in which the vast majority of the viral genome is removed from the transfer genome. This eliminates the possibilities of side effects due to accessory protein expression and helps to minimize the induction of an immune response in the host. Such ‘minimal’ systems have been developed for HIV [3234], EIAV [35,36], and FIV [37].

As mentioned above, lentiviral vectors have the capacity to be pseudotyped with various envelope proteins conferring an altered tropism or mechanism of transport within the transduced cell. Pseudotyping EIAV vectors with the rabies-G envelope protein for example confers a property of retrograde transport upon the lentiviral vector and has allowed targeting of remote neuronal populations by delivery of the vector to the nerve terminals at distal sites [38,39]. This has proven a particularly useful method for targeting motor neurons of the brain stem and spinal cord by simple injection of the muscle groups innervated by these cells [40]. This property provides novel value of such vectors for therapeutic gene delivery to the nervous system.

Since lentiviral vectors mediate long-term transgene expression in target cells, it would be attractive in terms of safety to have the ability to regulate expression of the therapeutic transgene, both to maintain expression at a therapeutic dose and to silence expression should any undesirable side effects occur. Furthermore, the ability to ‘switch off’ transgene expression in target cells is an attractive option in certain gene therapy applications where the pathological progression of a disorder is acute rather than chronic. Regulatable expression systems are currently under development and several of these have been successfully translated into lentiviral vector systems [41,42]. In particular, the well-characterized Tet-regulatable system demonstrates highly regulatable transgene expression when incorporated into the lentiviral vector setting [43]. It is envisaged that the further development of such regulatable systems will aid the progression of gene-therapy-based approaches to the clinic.

APPLICATION OF LENTIVIRAL VECTORS FOR NEURODEGENERATIVE DISEASE

AD (Alzheimer's disease)

AD is the most commonly occurring neurodegenerative disorder that affects approx. 4.5 million people in the U.S.A. alone. AD has an increasing social and economic relevance in developed countries where populations are aging and it is estimated that its prevalence will triple by 2050 if no therapy intervenes [44]. AD accounts for the majority of dementia cases and is characterized by memory loss and dysfunction of at least one other higher cortical function. The symptoms presented are due to the widespread degeneration of synapses and neurons, particularly in the cholinergic neurons of the basal forebrain.

The major pathological hallmarks of AD are Aβ (amyloid-β) plaques and neurofibrillary tangles containing the microtubule-associated protein tau. The prevailing hypothesis in AD pathogenesis suggests that Aβ accumulation plays a central role in initiating the AD syndrome [4547]. The plaque and vascular amyloid deposits of AD contain the 42- and 40-residue Aβ proteins that are generated constitutively by sequential proteolysis of the APP (Aβ precursor protein) by the β- and γ-secretase enzymes [4850]. Consequently, strategies aimed at decreasing Aβ production, such as inhibition of the respective secretase activity, may be useful therapeutically. Knockout mice for BACE (β-secretase β-site APP-cleaving enzyme)-1 showed reduction in Aβ levels with the preservation of neurological function [51]. Viral vectors have recently been developed to mediate specific and efficient gene silencing through expression of functional RNAi (interfering RNA) molecules in cells of the nervous system in vitro and in vivo [5254]. Lentiviral-mediated delivery of RNAi molecules by stereotaxic injection may therefore provide an alternative therapeutic approach for directly reducing expression of the β- or γ-secretases in vulnerable brain regions. Indeed, functional RNAi target sites have already been identified for BACE [55].

An alternative strategy for reducing Aβ pathology in AD is to increase Aβ clearance from the brain. Nep (neprilysin), a neutral cell-surface-associated zinc metalloendopeptidase, was recently identified as a major extracellular Aβ-degrading enzyme in the brain [56]. Nep knockout mice exhibited a gene dose-dependent increase in cerebral amyloid load [57] and down-regulation of Nep with chemical inhibitors resulted in increased Aβ concentrations in the brain [56], suggesting that Nep plays an important role in Aβ degradation. A lentiviral vector expressing Nep was able to reduce Aβ levels in a CHO (Chinese-hamster ovary) cell line and increase the resistance of primary hippocampal neurons to Aβ-mediated neurotoxicity [58]. Intracerebral or hippocampal injection of this vector into transgenic mouse models of amyloidosis led to an approx. 50% reduction in the number of amyloid plaques and ameliorated neurodegenerative alterations in the frontal cortex and hippocampus [58,59].

Neurotrophic support to degenerating cholinergic neurons can delay disease progression in AD. A candidate for this approach is NGF (nerve growth factor), which is produced and secreted by neurons in the hippocampus and neocortex and binds to specific receptors on cholinergic axon terminals in target regions [6062]. Infusion of NGF protein into the CNS was demonstrated to prevent degeneration of basal forebrain cholinergic neurons in aged rats and primates [63]. In a subsequent clinical trial, intracerebral ventricular infusion of NGF was performed in three AD patients and resulted in a transient improvement in episodic memory and improved physiological parameters. However, despite these improvements the trial was halted due to severe debilitating side effects in other parts of the nervous system [64]. The side effects manifested in this trial highlighted the need for restricted localized delivery of NGF to the basal forebrain regions without spread to other NGF-sensitive neurons in the nervous system. Gene therapy is the ideal tool for this purpose and several groups have studied the use of both ex vivo and in vivo NGF gene therapy approaches for AD [6570]. Indeed, a successful ex vivo Phase 1 study has recently been reported in which autologous fibroblasts from patients with mild AD were genetically modified to express human NGF using a retroviral vector and transplanted back into the forebrain [71].

The current treatments for AD, such as AChEIs (acetylcholinesterase inhibitors), have symptomatic benefits but do little to modify disease. With the unravelling of the neurochemistry of AD, it is likely that more avenues for therapy will be uncovered. For example, GSK-3 (glycogen synthase kinase-3) and Cdk-5 (cyclin-dependent kinase-5) are current targets to reduce abnormal tau phosphorylation in AD pathogenesis while inhibiting the secretases that lead to Aβ accumulation. In either strategy, knockdown of protein activity is desirable and this may be achieved using vector-mediated delivery of specific RNAi molecules. The direct injection of such vectors into the brain will overcome the blood–brain barrier and ensure localized delivery of target molecules to the affected cell populations.

PD

PD is a common progressive neurodegenerative disease with a mean age of onset of 55 years old and the probability of occurrence increasing markedly with age. Up to 50% of individuals over the age of 85 are affected by the disorder. The pathological hallmark of PD is the selective degeneration of the nigrostriatal dopaminergic neurons, resulting in depletion of striatal dopamine levels with subsequent aberrant motor activity causing resting tremor, rigidity and bradykinesia. A further common characteristic of PD is the presence of cytoplasmic inclusions called Lewy bodies, which contain a variety of proteins with α-synuclein as a major component [72]. The vast majority of Parkinson's cases are sporadic, although a few rare cases are caused by several different inherited mutations [7376]. Such inherited cases usually have an earlier age of onset than sporadic forms and often show atypical clinical features.

One potential gene therapy strategy is the stereotaxic delivery of viral vectors designed to express potentially neurotrophic factors in areas of the brain susceptible to degeneration in PD. Such a strategy would be aimed at early-to-mid-stage patients who have a population of intact dopaminergic neurons remaining. GDNF (glial-derived neurotrophic factor) has proven to be particularly potent at protecting against the dopaminergic neuron degeneration observed in PD [77,78]. Several gene therapy approaches using lentiviral vectors generated to express GDNF have successfully demonstrated high transduction efficiencies and subsequent neuroprotection of nigrostriatal dopaminergic neurons and functional improvements in the well-characterized 6-OHDA (6-hydroxydopamine) lesion model of PD in rodents [7982]. Interestingly, this approach was recently demonstrated to be ineffective at preventing neurodegeneration in a transgenic rat model of inherited PD [83]. In a non-human primate model of PD, lentiviral-mediated delivery of GDNF to the striatum and substantia nigra prevented loss of vulnerable dopaminergic neurons and improved motor performance. In addition, the therapy also protected against dopaminergic neuronal death in aged animals [84]. Although these data are promising, safety issues still surround the long-term unregulated expression of neurotrophic factors such as GDNF in the brain, since this has been reported to lead to aberrant neuronal sprouting and down-regulation of TH (tyrosine hydroxylase) expression in dopaminergic neurons [85]. To this end, regulatable expression systems for delivering GDNF are under development [86].

A second gene therapy strategy for treating PD is based on dopamine replacement. Such an approach would be suitable for late-stage patients, since it would not require the presence of a surviving population of nigrostriatal dopaminergic neurons. Several viral vectors have been used to deliver critical enzymes in the dopamine biosynthesis pathway to the striatum of animals in models of PD in order to convert striatal cells into dopamine ‘factories’. A single EIAV-based lentiviral vector generated to express TH, AADC (aromatic amino acid decarboxylase) and GTP-CH1 (GTP cyclohydrolase) from a tricistronic cassette has proven to be particularly efficient at transducing neurons in vivo and reversing Parkinsonian phentotypes in a 6-OHDA lesion rat model [87] and a MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) lesion primate model (B. Jarraya, M. Azzouz, C. Jan, J. Miskin, G. S. Ralph, F. Wilkes, L. E. Walmsley, R. D. Barber, J. B. Rohll, X. Drouot, E. Brouillet, F. Condé, S. M. Kingsman, P. Hantraye, K. A. Mitrophanous, N. D. Mazarakis and S. Palfi, unpublished work) following direct intrastriatal delivery. Similar approaches include AAV vectors designed to express enzymes mediating dopamine synthesis from separate viral vectors [8890] and a HSV vector expressing the three critical enzymes described above alongside the dopamine transporter VMAT-2 (vesicular monoamine transporter-2) [91].

HD (Huntington's disease)

HD is a dominantly inherited disease caused by expanded CAG trinucleotide repeats encoding for the amino acid glutamine in the huntingtin gene. The subsequent toxic gain-of-function of the mutant huntingtin protein causes the selective loss of striatal neurons resulting in motor and cognitive deficiencies. The typical age of onset is 35–50 years old and the onset and severity of disease often correlates with the length of the CAG expansion in the individual patient [76]. Gene therapy approaches to HD are not widely reported and, until recently, have been limited to neuroprotection strategies such as viral-vector-mediated delivery of the human CNTF (ciliary neurotrophic factor), GDNF or BDNF (brain-derived neurotrophic factor) in rodent models of HD ([92,93]; for a review of the pharmaceutical, cellular and genetic therapies for HD, see [94]). Since HD is caused by a single autosomal-dominant abnormality, perhaps the most logical therapeutic approach would be to genetically silence expression of the mutant allele, leaving expression of the wild-type huntingtin gene intact [95]. The development of viral vectors to express functional RNAi molecules provides an exciting novel strategy for silencing dominant mutant alleles expressed in such dominantly inherited diseases [53,54,95,96]. Indeed, a recent study has demonstrated improvement in motor function and a reduced neuropathology in a transgenic mouse model of HD by RNAi-mediated silencing of mutant huntingtin expression [97]. The main obstacle that may preclude clinical success with such an approach is a requirement to exclusively silence expression of the mutant gene while maintaining expression of the essential wild-type huntingtin protein. Lentiviral vectors provide ideal candidate vehicles for delivering RNAi to the brains of HD patients due to their low immunogenicity and the ability to mediate stable long-term expression, which would be a requirement for a successful therapy. Furthermore, since the areas of degeneration in HD are localized, stereotaxic injection of viral vectors would provide an ideal delivery route. If the problem of targeting mutant allele expression can be overcome, lentiviral-vector-mediated delivery of RNAi may provide an exciting breakthrough in the treatment of HD.

ALS (amyotrophic lateral sclerosis)

Amyotrophic lateral sclerosis or Lou Gehrig's disease is the most common motor neuron disease typically occurring in mid-adult life. The disease is progressive, causing the selective death of motor neurons in the cortex, brain stem and spinal cord, which leads to paralysis and ultimately death, typically within 3–5 years. Approx. 2% of ALS cases are caused by dominantly inherited mutations in the gene encoding Cu/Zn SOD1 (superoxide dismutase 1; SOD1). Transgenic mice generated to overexpress human forms of mutant SOD1 develop a progressive form of motor neuron disease that closely mimics ALS in humans and provide the best animal models of the disease to date [98100]. Recent studies have demonstrated the remarkable efficacy of gene therapy approaches to ALS in such models. For example, EIAV-vector-mediated delivery of the angiogenic factor VEGF (vascular endothelial growth factor) to motor neuron populations of the well-characterized SOD1G93A transgenic mouse model of ALS [98] resulted in a significant delay in the onset of disease and extended survival of these animals by approx. 30% [40]. In this study, the viral vector was delivered to the motor neuron populations of interest by retrograde transport following a simple intramuscular injection and a high transduction efficiency was achieved [38]. This non-invasive method of targeting transgene expression to vulnerable neuronal populations is particularly attractive in developing such a therapy for the clinic. A similar approach was adopted to deliver the neurotrophic factor IGF-1 (insulin-like growth factor-1) to vulnerable motor neuron populations of SOD1G93A mice using AAV vectors and a similar therapeutic efficacy was observed [101]. Since ALS cases linked to mutations in the SOD-1 gene are dominantly inherited, RNAi provides a potential therapeutic strategy to treat such incidences of the disease. Indeed, recent studies have demonstrated potential therapeutic efficacy in the SOD1G93A mouse using direct intraspinal injection [102] or intramuscular injection (with subsequent retrograde delivery to motor neurons) [54] of lentiviral vectors designed to express functional RNAi molecules directed against SOD1 expression. The latter of these studies used EIAV vectors to specifically target the human mutant form of SOD1 leaving expression of the wild-type mouse gene intact. This resulted in a significant delay in the onset of ALS symptoms in these mice and extended the life span of the animals by approx. 80% [54]. These studies provide great optimism for the future utility of lentiviral delivery of RNAi as a therapy for dominantly inherited diseases.

GENE THERAPY FOR DEGENERATIVE RETINAL DISORDERS

The eye is an ideal target organ for gene delivery. It is accessible and can easily be monitored in vivo by ophthalmoscopy. The development of ocular gene therapy would offer novel approaches to treating eye diseases characterized by chronic retinal degeneration, such as RP (retinitis pigmentosa), chorioiderimea, Best's and Stargardt's disease, or ocular neovascular disorders, such as diabetic retinopathy and AMD (age-related macular degeneration) (Table 1). Gene therapy to treat most ocular disorders would require long-term expression of a therapeutic transgene. Lentiviral vectors provide particularly good candidate delivery vehicles for such disorders as the viral vector genome is integrated into that of the host cell following transduction thereby facilitating long-term gene expression.

Lentiviruses to treat ocular neovascular disorders

Ocular neovascularization is the major cause of blindness in Western society. AMD generally appears in older people over the age of 55 and is caused by choroidal neovascularization that can lead to complete loss of central vision. Diabetic retinopathy is generally associated with younger individuals of so-called ‘working age’, where retinal neovascularization extends beyond the retina into the vitreal space, and these new vessels can bleed and cloud vision [103]. VEGF, a potent angiogenic factor, is a major driving force in these neovascular disorders [104,105]. With this in mind, a transgenic animal model was developed in which VEGF is temporally produced by the photoreceptor cells [106]. Using this model a BIV-based lentiviral vector expressing the potent angiostatic factor endostatin reduced not only retinal neovascularization, but also permeability and retinal detachment, other important hallmarks of the disease [107]. More recently, EIAV-based lentiviral vectors expressing either angiostatin, endostatin or sFlt1 (soluble VEGF receptor) have been shown to reduce neovascularization and permeability in experimental models of choroidal neovascularization (K. Balaggan and K. Binley, unpublished work). In both of these studies, the lentiviral vectors were subretinally delivered to the posterior portion of the eye (Figure 1). This subretinal route of delivery leads preferentially to expression in the RPE (retinal pigment epithelium) with similar results observed with HIV-, SIV-, BIV-, FIV- and EIAV-based vectors, and expression is seen to a lesser extent in photoreceptor cells ([108111]; and K. Balaggan and K. Binley, unpublished work). The RPE lies between the inner retina and the choroid and plays a central role in retinal physiology by forming the outer blood–retinal barrier and supporting the function of the photoreceptors. Lentivirus-mediated transduction and expression in the RPE is very rapid, efficient and stable with expression so far reported up to 16 months for EIAV-based vectors and 24 months for FIV-based vectors ([112,113], and K. Balaggan and K. Binley, unpublished work). Therefore the RPE is an ideal platform for the delivery of an anti-angiogenic gene therapy for the treatment of ocular neovascular disorders.

Schematic diagram of the human eye showing different routes of ocular delivery, including anterior, intravitreal and subretinal

Figure 1
Schematic diagram of the human eye showing different routes of ocular delivery, including anterior, intravitreal and subretinal
Figure 1
Schematic diagram of the human eye showing different routes of ocular delivery, including anterior, intravitreal and subretinal

Lentiviruses to treat inherited retinal degenerative diseases

RP is a heterogeneous group of inherited retinal diseases caused by mutations of various genes. The disease is characterized by progressive degeneration of rod and cone photoreceptor cells and is a major cause of blindness in adults [114]. Delaying the progression of the disease may be of great benefit, since the majority of RP diseases are seen in middle age or later. An SIV-based lentiviral vector was used previously to deliver PEDF (pigment epithelium-derived factor), a potent secreted neurotrophic factor, in the RCS (Royal College of Surgeons) rat, a well-accepted model of RP [115]. The treated eyes showed a significant delay in loss of photoreceptors compared with the untreated eyes, suggesting that a neuroprotective gene therapy could be used to protect retinal degeneration in individuals with RP. In this study, the vector was delivered subretinally and PEDF expression was observed predominantly in the RPE.

An alternative gene therapy approach to prevent retinal degeneration would be to deliver expression of the wild-type gene directly to the photoreceptors (Figure 1). Clearly, this approach would be dependent on highly efficient gene transfer to photoreceptors. Although subretinal delivery of lentiviral vectors leads predominantly to gene expression in the RPE, limited gene expression has been reported in cells of the inner retina, including photoreceptors and ganglion cells, with HIV- and EIAV-based vectors ([108], and K. Balaggan and K. Binley, unpublished work). The expression profile following subretinal delivery appears largely independent of the envelope pseudotype suggesting that the phagocytic nature of the RPE cells may facilitate vector uptake ([116,117], and K. Balaggan and K. Binley, unpublished work). However, it has been shown that the promoter driving transgene expression is a crucial factor for influencing cell-specific expression. For example, an HIV vector carrying the Rho (rhodopsin) promoter not only led to photoreceptor-specific expression, but also to much higher expression levels than with the CMV (cytomegalovirus) promoter [108].

The same Rho promoter was also used to drive expression of the cGMP PDEβ (phosphodiesterase β subunit) delivered subretinally to newborn rd mice by an HIV vector [118]. Mutations in the PDEβ rod photoreceptor gene are found in patients with autosomal-recessive RP and in rd mice. The rd mouse is the best-studied animal model of RP and is characterized by a rapid photoreceptor degeneration after postnatal day 7. HIV-vector-mediated delivery of PDEβ expression showed long-term protection against photoreceptor degeneration compared with untreated eyes, demonstrating that expression levels in the photoreceptors were sufficient to bring about a therapeutic effect. These studies suggest that RP is an excellent candidate for a lentiviral-vector-mediated human gene therapy and that direct access to cells in the inner retina at a therapeutically relevant level is possible by means of subretinal injection.

Lentiviruses to treat other ocular disorders

The subretinal delivery route is primarily used to target structures in the posterior portion of the eye, in particular the retina. However, other delivery routes could be used to target anterior structures of the eye (Figure 1). Intravitreal delivery of lentiviruses generally leads to relatively poor transduction which is mainly visible in the ciliary body [119]. However, following delivery into the anterior chamber, FIV, HIV and EIAV vectors have all been shown to transduce the TM (trabecular meshwork) in murine and feline models and human explants ([120122; and K. Balaggan and K. Binley, unpublished work). The TM is a disease-relevant tissue for the treatment of glaucoma as it is largely responsible for draining the aqueous humour from the eye. The VSV-G tropism of these vectors is likely to be an important factor, since EIAV pseudotyped with rabies-G did not show transduction of the TM (K. Balaggan and K. Binley, unpublished work). In addition to the TM, the corneal endothelium is also efficiently transduced through this delivery route [122,123], leading to the possibility of developing a gene therapy to treat disorders of the cornea such as FED (Fuchs’ endothelial dystrophy), the most common corneal dystrophy characterized by loss of endothelial cells [124].

SUMMARY

There is clearly great scope for the use of lentiviral vectors for the treatment of many neurodegenerative and ocular disorders. Increased understanding of the molecular mechanisms underlying such diseases and the development of exciting new technologies, such as RNAi and regulatable systems, will advance the use of such vectors for therapeutic application. The next few years will hopefully see the translation of some lentiviral-based therapies into the clinical setting, providing patients with novel treatments for currently incurable neurological and ocular diseases.

Abbreviations

     
  • AAV

    adeno-associated viral

  •  
  • AD

    Alzheimer's disease

  •  
  • ALS

    amyotrophic lateral sclerosis

  •  
  • amyloid-β

  •  
  • APP

    Aβ precursor protein

  •  
  • BACE

    β-secretase β-site APP-cleaving enzyme

  •  
  • BIV

    bovine immunodeficiency virus

  •  
  • Cdk-5

    cyclin-dependent kinase-5

  •  
  • CNS

    central nervous system

  •  
  • EIAV

    equine infectious anaemia virus

  •  
  • FED

    Fuchs' endothelial dystrophy

  •  
  • FIV

    feline immunodeficiency virus

  •  
  • GDNF

    glial-derived neurotrophic factor

  •  
  • GSK-3

    glycogen synthase kinase-3

  •  
  • GTP-CH1

    GTP cyclohydrolase

  •  
  • HD

    Huntington's disease

  •  
  • HSV

    herpes simplex virus

  •  
  • IGF-1

    insulin-like growth factor-1

  •  
  • Nep

    neprilysin

  •  
  • NGF

    nerve growth factor

  •  
  • PD

    Parkinson's disease

  •  
  • 6-OHDA

    6-hydroxydopamine

  •  
  • PDEβ

    phosphodiesterase β subunit

  •  
  • PEDF

    pigment epithelium-derived factor

  •  
  • Rho

    rhodopsin

  •  
  • RNAi

    interfering RNA

  •  
  • RP

    retinitis pigmentosa

  •  
  • RPE

    retinal pigment epithelium

  •  
  • SOD1

    superoxide dismutase 1

  •  
  • TH

    tyrosine hydroxylase

  •  
  • TM

    trabecular meshwork

  •  
  • VEGF

    vascular endothelial growth factor

We thank Dr Kam Balaggan who collaborated with the authors to generate the ocular EIAV lentivirus observations cited in this review (this work is currently being prepared for publication).

References

References
1
Relph
 
K.
Harrington
 
K.
Pandha
 
H.
 
Recent developments and current status of gene therapy using viral vectors in the United Kingdom
Br. Med. J.
2004
, vol. 
329
 (pg. 
839
-
842
)
2
Cao
 
H.
Koehler
 
D. R.
Hu
 
J.
 
Adenoviral vectors for gene replacement therapy
Viral Immunol.
2004
, vol. 
17
 (pg. 
327
-
333
)
3
Volpers
 
C.
Kochanek
 
S.
 
Adenoviral vectors for gene transfer and therapy
J. Gene Med.
2004
, vol. 
6
 
Suppl. 1
(pg. 
S164
-
S171
)
4
Schagen
 
F. H.
Ossevoort
 
M.
Toes
 
R. E.
Hoeben
 
R. C.
 
Immune responses against adenoviral vectors and their transgene products: a review of strategies for evasion
Crit. Rev. Oncol. Hematol.
2004
, vol. 
50
 (pg. 
51
-
70
)
5
Thomas
 
C. E.
Birkett
 
D.
Anozie
 
I.
Castro
 
M. G.
Lowenstein
 
P. R.
 
Acute direct adenoviral vector cytotoxicity and chronic, but not acute, inflammatory responses correlate with decreased vector-mediated transgene expression in the brain
Mol. Ther.
2001
, vol. 
3
 (pg. 
36
-
46
)
6
Lehrman
 
S.
 
Virus treatment questioned after gene therapy death
Nature
1999
, vol. 
401
 (pg. 
517
-
518
)
7
Sakhuja
 
K.
Reddy
 
P. S.
Ganesh
 
S.
, et al 
Optimization of the generation and propagation of gutless adenoviral vectors
Hum. Gene Ther.
2003
, vol. 
14
 (pg. 
243
-
254
)
8
Kochanek
 
S.
Schiedner
 
G.
Volpers
 
C.
 
High-capacity ‘gutless’ adenoviral vectors
Curr. Opin. Mol. Ther.
2001
, vol. 
3
 (pg. 
454
-
463
)
9
McKelvey
 
T.
Tang
 
A.
Bett
 
A. J.
Casimiro
 
D. R.
Chastain
 
M.
 
T-cell response to adenovirus hexon and DNA-binding protein in mice
Gene Ther.
2004
, vol. 
11
 (pg. 
791
-
796
)
10
Liu
 
Q.
Muruve
 
D. A.
 
Molecular basis of the inflammatory response to adenovirus vectors
Gene Ther.
2003
, vol. 
10
 (pg. 
935
-
940
)
11
Tenenbaum
 
L.
Chtarto
 
A.
Lehtonen
 
E.
Velu
 
T.
Brotchi
 
J.
Levivier
 
M.
 
Recombinant AAV-mediated gene delivery to the central nervous system
J Gene Med.
2004
, vol. 
6
 
Suppl. 1
(pg. 
S212
-
S222
)
12
Lu
 
Y.
 
Recombinant adeno-associated virus as delivery vector for gene therapy: a review
Stem Cells Dev.
2004
, vol. 
13
 (pg. 
133
-
145
)
13
Kaplitt
 
M. G.
Leone
 
P.
Samulski
 
R. J.
, et al 
Long-term gene expression and phenotypic correction using adeno-associated virus vectors in the mammalian brain
Nat. Genet.
1994
, vol. 
8
 (pg. 
148
-
154
)
14
McCarty
 
D. M.
Young
 
S. M.
Samulski
 
R. J.
 
Integration of adeno-associated virus (AAV) and recombinant AAV vectors
Annu. Rev. Genet.
2004
, vol. 
38
 (pg. 
819
-
845
)
15
Nakai
 
H.
Storm
 
T. A.
Kay
 
M. A.
 
Increasing the size of rAAV-mediated expression cassettes in vivo by intermolecular joining of two complementary vectors
Nat. Biotechnol.
2000
, vol. 
18
 (pg. 
527
-
532
)
16
Duan
 
D.
Yue
 
Y.
Yan
 
Z.
Engelhardt
 
J. F.
 
A new dual-vector approach to enhance recombinant adeno-associated virus-mediated gene expression through intermolecular cis activation
Nat. Med.
2000
, vol. 
6
 (pg. 
595
-
598
)
17
Sun
 
L.
Li
 
J.
Xiao
 
X.
 
Overcoming adeno-associated virus vector size limitation through viral DNA heterodimerization
Nat. Med.
2000
, vol. 
6
 (pg. 
599
-
602
)
18
Kay
 
M. A.
Manno
 
C. S.
Ragni
 
M. V.
, et al 
Evidence for gene transfer and expression of factor IX in haemophilia B patients treated with an AAV vector
Nat. Genet.
2000
, vol. 
24
 (pg. 
257
-
261
)
19
Nakai
 
H.
Montini
 
E.
Fuess
 
S.
Storm
 
T. A.
Grompe
 
M.
Kay
 
M. A.
 
AAV serotype 2 vectors preferentially integrate into active genes in mice
Nat. Genet.
2003
, vol. 
34
 (pg. 
297
-
302
)
20
Kay
 
M. A.
Nakai
 
H.
 
Looking into the safety of AAV vectors
Nature (London)
2003
, vol. 
424
 pg. 
251
 
21
Luo
 
J.
Kaplitt
 
M. G.
Fitzsimons
 
H. L.
, et al 
Subthalamic GAD gene therapy in a Parkinson's disease rat model
Science
2002
, vol. 
298
 (pg. 
425
-
429
)
22
During
 
M. J.
Kaplitt
 
M. G.
Stern
 
M. B.
Eidelberg
 
D.
 
Subthalamic GAD gene transfer in Parkinson disease patients who are candidates for deep brain stimulation
Hum. Gene Ther.
2001
, vol. 
12
 (pg. 
1589
-
1591
)
23
Betchen
 
S. A.
Kaplitt
 
M.
 
Future and current surgical therapies in Parkinson's disease
Curr. Opin. Neurol.
2003
, vol. 
16
 (pg. 
487
-
493
)
24
Glorioso
 
J. C.
Fink
 
D. J.
 
Herpes vector-mediated gene transfer in treatment of diseases of the nervous system
Annu. Rev. Microbiol.
2004
, vol. 
58
 (pg. 
253
-
271
)
25
Post
 
D. E.
Fulci
 
G.
Chiocca
 
E. A.
Van Meir
 
E. G.
 
Replicative oncolytic herpes simplex viruses in combination cancer therapies
Curr. Gene Ther.
2004
, vol. 
4
 (pg. 
41
-
51
)
26
Shah
 
A. C.
Benos
 
D.
Gillespie
 
G. Y.
Markert
 
J. M.
 
Oncolytic viruses: clinical applications as vectors for the treatment of malignant gliomas
J. Neurooncol.
2003
, vol. 
65
 (pg. 
203
-
226
)
27
Lever
 
A. M.
Strappe
 
P. M.
Zhao
 
J.
 
Lentiviral vectors
J. Biomed. Sci.
2004
, vol. 
11
 (pg. 
439
-
449
)
28
Martin-Rendon
 
E.
Azzouz
 
M.
Mazarakis
 
N. D.
 
Lentiviral vectors for the treatment of neurodegenerative diseases
Curr. Opin. Mol. Ther.
2001
, vol. 
3
 (pg. 
476
-
481
)
29
Azzouz
 
M.
Kingsman
 
S. M.
Mazarakis
 
N. D.
 
Lentiviral vectors for treating and modeling human CNS disorders
J. Gene Med.
2004
, vol. 
6
 (pg. 
951
-
962
)
30
Hacein-Bey-Abina
 
S.
Von Kalle
 
C.
Schmidt
 
M.
, et al 
LMO2-associated clonal T cell proliferation in two patients after gene therapy for SCID-X1
Science
2003
, vol. 
302
 (pg. 
415
-
419
)
31
Naldini
 
L.
Blomer
 
U.
Gallay
 
P.
, et al 
In vivo gene delivery and stable transduction of nondividing cells by a lentiviral vector
Science
1996
, vol. 
272
 (pg. 
263
-
267
)
32
Dull
 
T.
Zufferey
 
R.
Kelly
 
M.
, et al 
A third-generation lentivirus vector with a conditional packaging system
J. Virol.
1998
, vol. 
72
 (pg. 
8463
-
8471
)
33
Kim
 
V. N.
Mitrophanous
 
K.
Kingsman
 
S. M.
Kingsman
 
A. J.
 
Minimal requirement for a lentivirus vector based on human immunodeficiency virus type 1. J
Virol.
1998
, vol. 
72
 (pg. 
811
-
816
)
34
Kotsopoulou
 
E.
Kim
 
V. N.
Kingsman
 
A. J.
Kingsman
 
S. M.
Mitrophanous
 
K. A.
 
A Rev-independent human immunodeficiency virus type 1 (HIV-1)-based vector that exploits a codon-optimized HIV-1 gag-pol gene
J. Virol.
2000
, vol. 
74
 (pg. 
4839
-
4852
)
35
Rohll
 
J. B.
Mitrophanous
 
K. A.
Martin-Rendon
 
E.
, et al 
Design, production, safety, evaluation and clinical applications of nonprimate lentiviral vectors
Methods Enzymol.
2002
, vol. 
346
 (pg. 
466
-
500
)
36
Mitrophanous
 
K.
Yoon
 
S.
Rohll
 
J.
, et al 
Stable gene transfer to the nervous system using a non-primate lentiviral vector
Gene Ther.
1999
, vol. 
6
 (pg. 
1808
-
1818
)
37
Poeschla
 
E. M.
Wong-Staal
 
F.
Looney
 
D. J.
 
Efficient transduction of nondividing human cells by feline immunodeficiency virus lentiviral vectors
Nat. Med.
1998
, vol. 
4
 (pg. 
354
-
357
)
38
Mazarakis
 
N. D.
Azzouz
 
M.
Rohll
 
J. B.
, et al 
Rabies virus glycoprotein pseudotyping of lentiviral vectors enables retrograde axonal transport and access to the nervous system after peripheral delivery
Hum. Mol. Genet.
2001
, vol. 
10
 (pg. 
2109
-
2121
)
39
Wong
 
L. F.
Azzouz
 
M.
Walmsley
 
L. E.
, et al 
Transduction patterns of pseudotyped lentiviral vectors in the nervous system
Mol. Ther.
2004
, vol. 
9
 (pg. 
101
-
111
)
40
Azzouz
 
M.
Ralph
 
G. S.
Storkebaum
 
E.
, et al 
VEGF delivery with retrogradely transported lentivector prolongs survival in a mouse ALS model
Nature (London)
2004
, vol. 
429
 (pg. 
413
-
417
)
41
Sirin
 
O.
Park
 
F.
 
Regulating gene expression using self-inactivating lentiviral vectors containing the mifepristone-inducible system
Gene
2003
, vol. 
323
 (pg. 
67
-
77
)
42
Reiser
 
J.
Lai
 
Z.
Zhang
 
X. Y.
Brady
 
R. O.
 
Development of multigene and regulated lentivirus vectors
J. Virol.
2000
, vol. 
74
 (pg. 
10589
-
10599
)
43
Pluta
 
K.
Luce
 
M. J.
Bao
 
L.
Agha-Mohammadi
 
S.
Reiser
 
J.
 
Tight control of transgene expression by lentivirus vectors containing second-generation tetracycline-responsive promoters
J. Gene Med.
2005
, vol. 
7
 (pg. 
803
-
817
)
44
Hebert
 
L. E.
Scherr
 
P. A.
Bienias
 
J. L.
Bennett
 
D. A.
Evans
 
D. A.
 
State-specific projections through 2025 of Alzheimer disease prevalence
Neurology
2004
, vol. 
62
 pg. 
1645
 
45
Hardy
 
J.
Allsop
 
D.
 
Amyloid deposition as the central event in the aetiology of Alzheimer's disease
Trends Pharmacol. Sci.
1991
, vol. 
12
 (pg. 
383
-
388
)
46
Hardy
 
J.
Selkoe
 
D. J.
 
The amyloid hypothesis of Alzheimer's disease: progress and problems on the road to therapeutics
Science
2002
, vol. 
297
 (pg. 
353
-
356
)
47
Selkoe
 
D. J.
 
The molecular pathology of Alzheimer's disease
Neuron
1991
, vol. 
6
 (pg. 
487
-
498
)
48
Haass
 
C.
Schlossmacher
 
M. G.
Hung
 
A. Y.
, et al 
Amyloid beta-peptide is produced by cultured cells during normal metabolism
Nature (London)
1992
, vol. 
359
 (pg. 
322
-
325
)
49
Glenner
 
G. G.
Wong
 
C. W.
 
Alzheimer's disease: initial report of the purification and characterization of a novel cerebrovascular amyloid protein
Biochem. Biophys. Res. Commun.
1984
, vol. 
120
 (pg. 
885
-
890
)
50
Kang
 
J.
Lemaire
 
H. G.
Unterbeck
 
A.
, et al 
The precursor of Alzheimer's disease amyloid A4 protein resembles a cell-surface receptor
Nature (London)
1987
, vol. 
325
 (pg. 
733
-
736
)
51
Roberds
 
S. L.
Anderson
 
J.
Basi
 
G.
, et al 
BACE knockout mice are healthy despite lacking the primary beta-secretase activity in brain: implications for Alzheimer's disease therapeutics
Hum. Mol. Genet.
2001
, vol. 
10
 (pg. 
1317
-
1324
)
52
Xia
 
H.
Mao
 
Q.
Paulson
 
H. L.
Davidson
 
B. L.
 
siRNA-mediated gene silencing in vitro and in vivo
Nat. Biotechnol.
2002
, vol. 
20
 (pg. 
1006
-
1010
)
53
Miller
 
V. M.
Xia
 
H.
Marrs
 
G. L.
, et al 
Allele-specific silencing of dominant disease genes
Proc. Natl. Acad. Sci. U.S.A.
2003
, vol. 
100
 (pg. 
7195
-
7200
)
54
Ralph
 
G. S.
Radcliffe
 
P. A.
Day
 
D. M.
, et al 
Silencing mutant SOD1 using RNAi protects against neurodegeneration and extends survival in an ALS model
Nat. Med.
2005
, vol. 
11
 (pg. 
429
-
433
)
55
Kao
 
S. C.
Krichevsky
 
A. M.
Kosik
 
K. S.
Tsai
 
L. H.
 
BACE1 suppression by RNA interference in primary cortical neurons
J. Biol. Chem.
2004
, vol. 
279
 (pg. 
1942
-
1949
)
56
Iwata
 
N.
Tsubuki
 
S.
Takaki
 
Y.
, et al 
Identification of the major Aβ1-Aβ42-degrading catabolic pathway in brain parenchyma: suppression leads to biochemical and pathological deposition
Nat. Med.
2000
, vol. 
6
 (pg. 
143
-
150
)
57
Iwata
 
N.
Tsubuki
 
S.
Takaki
 
Y.
, et al 
Metabolic regulation of brain Aβ by neprilysin
Science
2001
, vol. 
292
 (pg. 
1550
-
1552
)
58
Marr
 
R. A.
Guan
 
H.
Rockenstein
 
E.
, et al 
Neprilysin regulates amyloid Beta peptide levels
J. Mol. Neurosci.
2004
, vol. 
22
 (pg. 
5
-
11
)
59
Marr
 
R. A.
Rockenstein
 
E.
Mukherjee
 
A.
, et al 
Neprilysin gene transfer reduces human amyloid pathology in transgenic mice
J. Neurosci.
2003
, vol. 
23
 (pg. 
1992
-
1996
)
60
Dawbarn
 
D.
Allen
 
S. J.
Semenenko
 
F. M.
 
Coexistence of choline acetyltransferase and nerve growth factor receptors in the rat basal forebrain
Neurosci. Lett.
1988
, vol. 
94
 (pg. 
138
-
144
)
61
Holtzman
 
D. M.
Li
 
Y.
Parada
 
L. F.
, et al 
p140trk mRNA marks NGF-responsive forebrain neurons: evidence that trk gene expression is induced by NGF
Neuron
1992
, vol. 
9
 (pg. 
465
-
478
)
62
Kiss
 
J.
McGovern
 
J.
Patel
 
A. J.
 
Immunohistochemical localization of cells containing nerve growth factor receptors in the different regions of the adult rat forebrain
Neuroscience
1988
, vol. 
27
 (pg. 
731
-
748
)
63
Tuszynski
 
M. H.
U
 
H. S.
Alksne
 
J.
, et al 
Growth factor gene therapy for Alzheimer disease
Neurosurg. Focus
2002
, vol. 
13
 pg. 
e5
 
64
Eriksdotter Jonhagen
 
M.
Nordberg
 
A.
Amberla
 
K.
, et al 
Intracerebroventricular infusion of nerve growth factor in three patients with Alzheimer's disease
Dement. Geriatr. Cogn. Disord.
1998
, vol. 
9
 (pg. 
246
-
257
)
65
Rosenberg
 
M. B.
Friedmann
 
T.
Robertson
 
R. C.
, et al 
Grafting genetically modified cells to the damaged brain: restorative effects of NGF expression
Science
1988
, vol. 
242
 (pg. 
1575
-
1578
)
66
Conner
 
J. M.
Darracq
 
M. A.
Roberts
 
J.
Tuszynski
 
M. H.
 
Nontropic actions of neurotrophins: subcortical nerve growth factor gene delivery reverses age-related degeneration of primate cortical cholinergic innervation
Proc. Natl. Acad. Sci. U.S.A.
2001
, vol. 
98
 (pg. 
1941
-
1946
)
67
Smith
 
D. E.
Roberts
 
J.
Gage
 
F. H.
Tuszynski
 
M. H.
 
Age-associated neuronal atrophy occurs in the primate brain and is reversible by growth factor gene therapy
Proc. Natl. Acad. Sci. U.S.A.
1999
, vol. 
96
 (pg. 
10893
-
10898
)
68
Tuszynski
 
M. H.
Roberts
 
J.
Senut
 
M. C.
U
 
H. S.
Gage
 
F. H.
 
Gene therapy in the adult primate brain: intraparenchymal grafts of cells genetically modified to produce nerve growth factor prevent cholinergic neuronal degeneration
Gene Ther.
1996
, vol. 
3
 (pg. 
305
-
314
)
69
Mandel
 
R. J.
Gage
 
F. H.
Clevenger
 
D. G.
Spratt
 
S. K.
Snyder
 
R. O.
Leff
 
S. E.
 
Nerve growth factor expressed in the medial septum following in vivo gene delivery using a recombinant adeno-associated viral vector protects cholinergic neurons from fimbria-fornix lesion-induced degeneration
Exp. Neurol.
1999
, vol. 
155
 (pg. 
59
-
64
)
70
Blomer
 
U.
Kafri
 
T.
Randolph-Moore
 
L.
Verma
 
I. M.
Gage
 
F. H.
 
Bcl-xL protects adult septal cholinergic neurons from axotomized cell death
Proc. Natl. Acad. Sci. U.S.A.
1998
, vol. 
95
 (pg. 
2603
-
2608
)
71
Tuszynski
 
M. H.
Thal
 
L.
Pay
 
M.
, et al 
A phase 1 clinical trial of nerve growth factor gene therapy for Alzheimer disease
Nat. Med.
2005
, vol. 
11
 (pg. 
551
-
555
)
72
McKeith
 
I. G.
Mosimann
 
U. P.
 
Dementia with Lewy bodies and Parkinson's disease
Parkinsonism Relat. Disord.
2004
, vol. 
10
 
Suppl. 1
(pg. 
S15
-
S18
)
73
Mouradian
 
M. M.
 
Recent advances in the genetics and pathogenesis of Parkinson disease
Neurology
2002
, vol. 
58
 (pg. 
179
-
185
)
74
Paisan-Ruiz
 
C.
Jain
 
S.
Evans
 
E. W.
, et al 
Cloning of the gene containing mutations that cause PARK8-linked Parkinson's disease
Neuron
2004
, vol. 
44
 (pg. 
595
-
600
)
75
Zimprich
 
A.
Biskup
 
S.
Leitner
 
P.
, et al 
Mutations in LRRK2 cause autosomal-dominant Parkinsonism with pleomorphic pathology
Neuron
2004
, vol. 
44
 (pg. 
601
-
607
)
76
Bossy-Wetzel
 
E.
Schwarzenbacher
 
R.
Lipton
 
S. A.
 
Molecular pathways to neurodegeneration
Nat. Med.
2004
, vol. 
10
 
Suppl.
(pg. 
S2
-
S9
)
77
Kirik
 
D.
Georgievska
 
B.
Bjorklund
 
A.
 
Localized striatal delivery of GDNF as a treatment for Parkinson disease
Nat. Neurosci.
2004
, vol. 
7
 (pg. 
105
-
110
)
78
Gill
 
S. S.
Patel
 
N. K.
Hotton
 
G. R.
, et al 
Direct brain infusion of glial cell line-derived neurotrophic factor in Parkinson disease
Nat. Med.
2003
, vol. 
9
 (pg. 
589
-
595
)
79
Deglon
 
N.
Tseng
 
J. L.
Bensadoun
 
J. C.
, et al 
Self-inactivating lentiviral vectors with enhanced transgene expression as potential gene transfer system in Parkinson's disease
Hum. Gene Ther.
2000
, vol. 
11
 (pg. 
179
-
190
)
80
Bensadoun
 
J. C.
Deglon
 
N.
Tseng
 
J. L.
Ridet
 
J. L.
Zurn
 
A. D.
Aebischer
 
P.
 
Lentiviral vectors as a gene delivery system in the mouse midbrain: cellular and behavioral improvements in a 6-OHDA model of Parkinson's disease using GDNF
Exp. Neurol.
2000
, vol. 
164
 (pg. 
15
-
24
)
81
Azzouz
 
M.
Ralph
 
S.
Wong
 
L. F.
, et al 
Neuroprotection in a rat Parkinson model by GDNF gene therapy using EIAV vector
NeuroReport
2004
, vol. 
15
 (pg. 
985
-
990
)
82
Georgievska
 
B.
Kirik
 
D.
Rosenblad
 
C.
Lundberg
 
C.
Bjorklund
 
A.
 
Neuroprotection in the rat Parkinson model by intrastriatal GDNF gene transfer using a lentiviral vector
NeuroReport
2002
, vol. 
13
 (pg. 
75
-
82
)
83
Lo Bianco
 
C.
Deglon
 
N.
Pralong
 
W.
Aebischer
 
P.
 
Lentiviral nigral delivery of GDNF does not prevent neurodegeneration in a genetic rat model of Parkinson's disease
Neurobiol. Dis.
2004
, vol. 
17
 (pg. 
283
-
289
)
84
Kordower
 
J. H.
Emborg
 
M. E.
Bloch
 
J.
, et al 
Neurodegeneration prevented by lentiviral vector delivery of GDNF in primate models of Parkinson's disease
Science
2000
, vol. 
290
 (pg. 
767
-
773
)
85
Georgievska
 
B.
Kirik
 
D.
Bjorklund
 
A.
 
Aberrant sprouting and downregulation of tyrosine hydroxylase in lesioned nigrostriatal dopamine neurons induced by long-lasting overexpression of glial cell line derived neurotrophic factor in the striatum by lentiviral gene transfer
Exp. Neurol.
2002
, vol. 
177
 (pg. 
461
-
474
)
86
Georgievska
 
B.
Jakobsson
 
J.
Persson
 
E.
Ericson
 
C.
Kirik
 
D.
Lundberg
 
C.
 
Regulated delivery of glial cell line-derived neurotrophic factor into rat striatum, using a tetracycline-dependent lentiviral vector
Hum. Gene Ther.
2004
, vol. 
15
 (pg. 
934
-
944
)
87
Azzouz
 
M.
Martin-Rendon
 
E.
Barber
 
R. D.
, et al 
Multicistronic lentiviral vector-mediated striatal gene transfer of aromatic L-amino acid decarboxylase, tyrosine hydroxylase, and GTP cyclohydrolase I induces sustained transgene expression, dopamine production, and functional improvement in a rat model of Parkinson's disease
J. Neurosci.
2002
, vol. 
22
 (pg. 
10302
-
10312
)
88
Bankiewicz
 
K. S.
Eberling
 
J. L.
Kohutnicka
 
M.
, et al 
Convection-enhanced delivery of AAV vector in parkinsonian monkeys: in vivo detection of gene expression and restoration of dopaminergic function using pro-drug approach
Exp. Neurol.
2000
, vol. 
164
 (pg. 
2
-
14
)
89
Sanchez-Pernaute
 
R.
Harvey-White
 
J.
Cunningham
 
J.
Bankiewicz
 
K. S.
 
Functional effect of adeno-associated virus mediated gene transfer of aromatic L-amino acid decarboxylase into the striatum of 6-OHDA-lesioned rats
Mol. Ther.
2001
, vol. 
4
 (pg. 
324
-
330
)
90
Muramatsu
 
S.
Fujimoto
 
K.
Ikeguchi
 
K.
, et al 
Behavioral recovery in a primate model of Parkinson's disease by triple transduction of striatal cells with adeno-associated viral vectors expressing dopamine-synthesizing enzymes
Hum. Gene Ther.
2002
, vol. 
13
 (pg. 
345
-
354
)
91
Sun
 
M.
Kong
 
L.
Wang
 
X.
, et al 
Coexpression of tyrosine hydroxylase, GTP cyclohydrolase I, aromatic amino acid decarboxylase, and vesicular monoamine transporter 2 from a helper virus-free herpes simplex virus type 1 vector supports high-level, long-term biochemical and behavioral correction of a rat model of Parkinson's disease
Hum. Gene Ther.
2004
, vol. 
15
 (pg. 
1177
-
1196
)
92
de Almeida
 
L. P.
Zala
 
D.
Aebischer
 
P.
Deglon
 
N.
 
Neuroprotective effect of a CNTF-expressing lentiviral vector in the quinolinic acid rat model of Huntington's disease
Neurobiol. Dis.
2001
, vol. 
8
 (pg. 
433
-
446
)
93
Kells
 
A. P.
Fong
 
D. M.
Dragunow
 
M.
During
 
M. J.
Young
 
D.
Connor
 
B.
 
AAV-mediated gene delivery of BDNF or GDNF is neuroprotective in a model of Huntington disease
Mol. Ther.
2004
, vol. 
9
 (pg. 
682
-
688
)
94
Handley
 
O. J.
Naji
 
J.
Dunnett
 
S. B.
Rosser
 
A. E.
 
Pharmaceutical, cellular and genetic therapies for Huntington's disease
Clin. Sci.
2006
, vol. 
110
 (pg. 
73
-
88
)
95
Chen
 
Z. J.
Kren
 
B. T.
Wong
 
P. Y.
Low
 
W. C.
Steer
 
C. J.
 
Sleeping Beauty-mediated down-regulation of huntingtin expression by RNA interference
Biochem. Biophys. Res. Commun.
2005
, vol. 
329
 (pg. 
646
-
652
)
96
Xia
 
H.
Mao
 
Q.
Eliason
 
S. L.
, et al 
RNAi suppresses polyglutamine-induced neurodegeneration in a model of spinocerebellar ataxia
Nat. Med.
2004
, vol. 
10
 (pg. 
816
-
820
)
97
Harper
 
S. Q.
Staber
 
P. D.
He
 
X.
, et al 
RNA interference improves motor and neuropathological abnormalities in a Huntington's disease mouse model
Proc. Natl. Acad. Sci. U.S.A.
2005
, vol. 
102
 (pg. 
5820
-
5825
)
98
Gurney
 
M. E.
Pu
 
H.
Chiu
 
A. Y.
, et al 
Motor neuron degeneration in mice that express a human Cu,Zn superoxide dismutase mutation
Science
1994
, vol. 
264
 (pg. 
1772
-
1775
)
99
Bruijn
 
L. I.
Becher
 
M. W.
Lee
 
M. K.
, et al 
ALS-linked SOD1 mutant G85R mediates damage to astrocytes and promotes rapidly progressive disease with SOD1-containing inclusions
Neuron
1997
, vol. 
18
 (pg. 
327
-
338
)
100
Dal Canto
 
M. C.
Gurney
 
M. E.
 
Neuropathological changes in two lines of mice carrying a transgene for mutant human Cu,Zn SOD, and in mice overexpressing wild type human SOD: a model of familial amyotrophic lateral sclerosis (FALS)
Brain Res.
1995
, vol. 
676
 (pg. 
25
-
40
)
101
Kaspar
 
B. K.
Llado
 
J.
Sherkat
 
N.
Rothstein
 
J. D.
Gage
 
F. H.
 
Retrograde viral delivery of IGF-1 prolongs survival in a mouse ALS model
Science
2003
, vol. 
301
 (pg. 
839
-
842
)
102
Raoul
 
C.
Abbas-Terki
 
T.
Bensadoun
 
J. C.
, et al 
Lentiviral-mediated silencing of SOD1 through RNA interference retards disease onset and progression in a mouse model of ALS
Nat. Med.
2005
, vol. 
11
 (pg. 
423
-
428
)
103
Rahmani
 
B.
Tielsch
 
J. M.
Katz
 
J.
, et al 
The cause-specific prevalence of visual impairment in an urban population. The Baltimore Eye Survey
Ophthalmology
1996
, vol. 
103
 (pg. 
1721
-
1726
)
104
Aiello
 
L. P.
Avery
 
R. L.
Arrigg
 
P. G.
, et al 
Vascular endothelial growth factor in ocular fluid of patients with diabetic retinopathy and other retinal disorders
N. Engl. J. Med.
1994
, vol. 
331
 (pg. 
1480
-
1487
)
105
Pe'er
 
J.
Shweiki
 
D.
Itin
 
A.
Hemo
 
I.
Gnessin
 
H.
Keshet
 
E.
 
Hypoxia-induced expression of vascular endothelial growth factor by retinal cells is a common factor in neovascularizing ocular diseases
Lab. Invest.
1995
, vol. 
72
 (pg. 
638
-
645
)
106
Ohno-Matsui
 
K.
Hirose
 
A.
Yamamoto
 
S.
, et al 
Inducible expression of vascular endothelial growth factor in adult mice causes severe proliferative retinopathy and retinal detachment
Am. J. Pathol.
2002
, vol. 
160
 (pg. 
711
-
719
)
107
Takahashi
 
K.
Saishin
 
Y.
Silva
 
R. L.
, et al 
Intraocular expression of endostatin reduces VEGF-induced retinal vascular permeability, neovascularization, and retinal detachment
FASEB J.
2003
, vol. 
17
 (pg. 
896
-
898
)
108
Miyoshi
 
H.
Takahashi
 
M.
Gage
 
F. H.
Verma
 
I. M.
 
Stable and efficient gene transfer into the retina using an HIV-based lentiviral vector
Proc. Natl. Acad. Sci. U.S.A.
1997
, vol. 
94
 (pg. 
10319
-
10323
)
109
Ikeda
 
Y.
Goto
 
Y.
Yonemitsu
 
Y.
, et al 
Simian immunodeficiency virus-based lentivirus vector for retinal gene transfer: a preclinical safety study in adult rats
Gene Ther.
2003
, vol. 
10
 (pg. 
1161
-
1169
)
110
Takahashi
 
K.
Luo
 
T.
Saishin
 
Y.
, et al 
Sustained transduction of ocular cells with a bovine immunodeficiency viral vector
Hum. Gene Ther.
2002
, vol. 
13
 (pg. 
1305
-
1316
)
111
Lotery
 
A. J.
Derksen
 
T. A.
Russell
 
S. R.
, et al 
Gene transfer to the nonhuman primate retina with recombinant feline immunodeficiency virus vectors
Hum. Gene Ther.
2002
, vol. 
13
 (pg. 
689
-
696
)
112
Cheng
 
L.
Toyoguchi
 
M.
Looney
 
D. J.
Lee
 
J.
Davidson
 
M. C.
Freeman
 
W. R.
 
Efficient gene transfer to retinal pigment epithelium cells with long-term expression
Retina
2005
, vol. 
25
 (pg. 
193
-
201
)
113
Loewen
 
N.
Leske
 
D. A.
Cameron
 
J. D.
, et al 
Long-term retinal transgene expression with FIV versus adenoviral vectors
Mol. Vis.
2004
, vol. 
10
 (pg. 
272
-
280
)
114
Pagon
 
R. A.
 
Retinitis pigmentosa
Surv. Ophthalmol.
1988
, vol. 
33
 (pg. 
137
-
177
)
115
Miyazaki
 
M.
Ikeda
 
Y.
Yonemitsu
 
Y.
, et al 
Simian lentiviral vector-mediated retinal gene transfer of pigment epithelium-derived factor protects retinal degeneration and electrical defect in Royal College of Surgeons rats
Gene Ther.
2003
, vol. 
10
 (pg. 
1503
-
1511
)
116
Duisit
 
G.
Conrath
 
H.
Saleun
 
S.
, et al 
Five recombinant simian immunodeficiency virus pseudotypes lead to exclusive transduction of retinal pigmented epithelium in rat
Mol. Ther.
2002
, vol. 
6
 (pg. 
446
-
454
)
117
Auricchio
 
A.
Kobinger
 
G.
Anand
 
V.
, et al 
Exchange of surface proteins impacts on viral vector cellular specificity and transduction characteristics: the retina as a model
Hum. Mol. Genet.
2001
, vol. 
10
 (pg. 
3075
-
3081
)
118
Takahashi
 
M.
Miyoshi
 
H.
Verma
 
I. M.
Gage
 
F. H.
 
Rescue from photoreceptor degeneration in the rd mouse by human immunodeficiency virus vector-mediated gene transfer
J. Virol.
1999
, vol. 
73
 (pg. 
7812
-
7816
)
119
Loewen
 
N.
Leske
 
D. A.
Chen
 
Y.
, et al 
Comparison of wild-type and class I integrase mutant-FIV vectors in retina demonstrates sustained expression of integrated transgenes in retinal pigment epithelium
J. Gene Med.
2003
, vol. 
5
 (pg. 
1009
-
1017
)
120
Loewen
 
N.
Fautsch
 
M. P.
Peretz
 
M.
, et al 
Genetic modification of human trabecular meshwork with lentiviral vectors
Hum. Gene Ther.
2001
, vol. 
12
 (pg. 
2109
-
2119
)
121
Loewen
 
N.
Fautsch
 
M. P.
Teo
 
W. L.
Bahler
 
C. K.
Johnson
 
D. H.
Poeschla
 
E. M.
 
Long-term, targeted genetic modification of the aqueous humor outflow tract coupled with noninvasive imaging of gene expression in vivo
Invest. Ophthalmol. Vis. Sci.
2004
, vol. 
45
 (pg. 
3091
-
3098
)
122
Bainbridge
 
J. W.
Stephens
 
C.
Parsley
 
K.
, et al 
In vivo gene transfer to the mouse eye using an HIV-based lentiviral vector; efficient long-term transduction of corneal endothelium and retinal pigment epithelium
Gene Ther.
2001
, vol. 
8
 (pg. 
1665
-
1668
)
123
Wang
 
X.
Appukuttan
 
B.
Ott
 
S.
, et al 
Efficient and sustained transgene expression in human corneal cells mediated by a lentiviral vector
Gene Ther.
2000
, vol. 
7
 (pg. 
196
-
200
)
124
Wilson
 
S. E.
Bourne
 
W. M.
 
Fuchs' dystrophy
Cornea
1988
, vol. 
7
 (pg. 
2
-
18
)