Clinical applications of retinal gene therapy

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Abstract

Many currently incurable forms of blindness affecting the retina have a genetic etiology and several others, such as those resulting from retinal vascular disturbances, respond to repeated, potentially indefinite administration of molecular based treatments. The recent clinical advances in retinal gene therapy have shown that viral vectors can deliver genes safely to the retina and the promising initial results from a number of clinical trials suggest that certain diseases may potentially be treatable. Gene therapy provides a means of expressing proteins within directly transduced cells with far greater efficacy than might be achieved by traditional systemic pharmacological approaches. Recent developments have demonstrated how vector gene expression may be regulated and further improvements to vector design have limited side effects and improved safety profiles. These recent steps have been most significant in bringing gene therapy into the mainstream of ophthalmology. Nevertheless translating retinal gene therapy from animal research into clinical trials is still a lengthy process, including complexities in human retinal diseases that have been difficult to model in the laboratory. The focus of this review is to summarize the genetic background of the most common retinal diseases, highlight current concepts of gene delivery technology, and relate those technologies to pre-clinical and clinical gene therapy studies.

Introduction

Of all diseases that cause blindness, arguably those with genetic etiology, such as retinitis pigmentosa, are the most devastating, because they affect both eyes and may often lead to total blindness. Frequently, the diagnosis is made early in affected patients providing possible opportunities for treatment, where a therapeutic window exists in which correction of the gene defect before the onset of significant cellular pathology, may serve to prevent cell death and thereby preserve vision. In addition to examining the potential therapeutic applications of gene therapy in the context of various monogenic and complex retinal diseases, this review deals with the challenges in molecular biology and vector technology associated with translation from bench to clinic.

Section snippets

The eye as a target for gene therapy

The eye is a highly specialized organ which has evolved to transduce light stimuli into electrical signals and to relay those signals to the visual cortex. Light sensation and image formation is mediated through the activation of photoreceptor cells located in the outermost layer of the neurosensory retina, where incident light focused by the cornea and lens results in the activation of a signalling cascade and the propagation of an electrical impulse. This photoactivation is initiated by the

Retinal pigment epithelium expressed gene targets

The RPE consists of a monolayer of hexagonally arranged cuboidal or low columnar epithelial cells interposed between the choroid and the neural retina, where the layer's basal aspect forms part of Bruch's membrane, and the apical aspect is highly invaginated and closely associated with the photoreceptors. The RPE forms a selectively permeable barrier between the choroid and the neurosensory retina and is involved in many aspects of photoreceptor maintenance, including phagocytosis of rod and

Photoreceptor expressed gene targets

Photoreceptors are located at the outermost aspect of the neural retina and are the primary light sensitive cells of the retina. Photoreceptors are highly specialized neurons, each consisting of an outer and inner segment separated by connecting cilium, a cell body situated in the outer nuclear layer, and an axonal synaptic terminal (cone pedicle or rod spherule) extending to the outer plexiform layer, through which signalling to second order neurons (bipolar cells) is conducted. A significant

Choroideremia

Choroideremia, first described in 1871 by Mauthner, is a condition leading to degeneration of the choroid, RPE and neural retina that affects an estimated 1 in 50,000 individuals (MacDonald et al., 2010; Mauthner, 1871). The disease follows an X-linked inheritance pattern, with affected males suffering progressive chorioretinal degeneration characterized by night blindness and loss of peripheral visual field. Central vision is usually preserved, but deteriorates later in life. Carrier females

Gene delivery

The success of any gene therapy for the treatment of retinal disease is dependent upon the efficiency with which the therapeutic transgene can be delivered to the appropriate cell type. There are presently two major approaches for the delivery of genetic material: Virally vectored gene delivery and non-viral gene delivery.

Viral

A number of different viral vectors have been shown to have tropism for specific cell types of the eye in animal models and tissue culture, including AAV (Reichel et al., 1998), adenovirus (Campochiaro et al., 2006), herpes simplex virus (Spencer et al., 2000) and lentivirus (Miyoshi et al., 1997). The most commonly used of these vectors both for pre-clinical and clinical gene transfer is adeno-associated virus (AAV).

Lentivirus

Wild-type lentiviruses (family: Retroviridae) are large (∼120 nm) integrating pathogenic viruses with a complex genome and virion structure. Mature virions consist of a conical or rod shape capsid packaging multiple copies of a single stranded positive sense RNA genome which undergoes reverse transcription (Baltimore class VI), surrounded by a matrix and host-derived lipid envelope.

Non-viral

Therapeutic gene delivery with viral vectors presents issues with regard to both packaging capacity, which can be limiting as in the case of AAV, and immunogenicity, which can lead to adverse patient outcomes. Non-viral delivery of exogenous nucleotides, normally DNA, is an alternative approach which, in theory at least, allows the delivery of large nucleotide fragments in a form unlikely to trigger an immunogenic reaction.

Autosomal recessive and X-linked disease

As discussed in previous sections, treatment of autosomal recessive and X-linked conditions is typically approached through gene replacement therapy. In most cases recessive and X-linked mutations (with the exception of RPGR ORF15) cause an absence of protein, or production of functionally null protein, and consequently the expression of wild-type protein is likely to significantly ameliorate the disease phenotype. Recessive genotypes where gene replacement would be appropriate include, but is

Animal models of retinal disease and pre-clinical studies

There are now many natural and experimentally-engineered animal models that enable the assessment of genetic and environmental factors on visual defects. However, species differences must be accounted for in vector design and this is becoming increasingly important as research moves into the clinical phase. Of particular note is that non-primate mammals, specifically laboratory rodents, do not possess a macula. Caution must therefore be applied when extrapolating the results from mice. However,

Conclusions and future outlook

The use of AAV derived vectors has led to significant advances in retinal gene transfer in both pre-clinical and clinical research. These advances make it increasingly conceivable that gene therapy may become a commonly used tool for the treatment of retinal disease, although it is likely to take many years before gene therapy becomes a part of mainstream medicine. A greater understanding of the mutations and mechanisms that cause visual defects is essential to the future development of

Conflict of interest

The authors have no conflict or commercial interest to disclose.

Funding

Fight for Sight; the Wellcome Trust; Health Foundation; Medical Research Council; Royal College of Surgeons of Edinburgh; Oxford Stem Cell Institute; NIHR Ophthalmology and Oxford Biomedical Research Centres.

References (284)

  • A.I. den Hollander et al.

    Mutations in the CEP290 (NPHP6) gene are a frequent cause of Leber congenital amaurosis

    Am. J. Hum. Genet.

    (2006)
  • A.I. den Hollander et al.

    Leber congenital amaurosis: genes, proteins and disease mechanisms

    Prog. Retin. Eye Res.

    (2008)
  • L. Dudus et al.

    Persistent transgene product in retina, optic nerve and brain after intraocular injection of rAAV

    Vis. Res.

    (1999)
  • J. Francois et al.

    Neovascularization after argon laser photocoagulation of macular lesions

    Am. J. Ophthalmol.

    (1975)
  • R.J. Goody et al.

    Optimization of laser-induced choroidal neovascularization in African green monkeys

    Exp. Eye Res.

    (2011)
  • M. Gorbatyuk et al.

    Preservation of photoreceptor morphology and function in P23H rats using an allele independent ribozyme

    Exp. Eye Res.

    (2007)
  • C. Grimm et al.

    Neuroprotection by hypoxic preconditioning: HIF-1 and erythropoietin protect from retinal degeneration

    Semin. Cell. Dev. Biol.

    (2005)
  • M.O. Hall et al.

    Outer segment phagocytosis by cultured retinal pigment epithelial cells requires Gas6

    Exp. Eye Res.

    (2001)
  • M.O. Hall et al.

    Both protein S and Gas6 stimulate outer segment phagocytosis by cultured rat retinal pigment epithelial cells

    Exp. Eye Res.

    (2005)
  • D.T. Hartong et al.

    Retinitis pigmentosa

    Lancet

    (2006)
  • M.L. Hirsch et al.

    Little vector, big gene transduction: fragmented genome reassembly of adeno-associated virus

    Mol. Ther.

    (2010)
  • G.M. Acland et al.

    Gene therapy restores vision in a canine model of childhood blindness

    Nat. Genet.

    (2001)
  • G.D. Aguirre et al.

    Congenital stationary night blindness in the dog: common mutation in the RPE65 gene indicates founder effect

    Mol. Vis.

    (1998)
  • B. Akache et al.

    The 37/67-kilodalton laminin receptor is a receptor for adeno-associated virus serotypes 8, 2, 3, and 9

    J. Virol.

    (2006)
  • J.J. Alexander et al.

    Restoration of cone vision in a mouse model of achromatopsia

    Nat. Med.

    (2007)
  • R.R. Ali et al.

    Gene transfer into the mouse retina mediated by an adeno-associated viral vector

    Hum. Mol. Genet.

    (1996)
  • R.R. Ali et al.

    Adeno-associated virus gene transfer to mouse retina

    Hum. Gene Ther.

    (1998)
  • M. Allocca et al.

    Serotype-dependent packaging of large genes in adeno-associated viral vectors results in effective gene delivery in mice

    J. Clin. Invest.

    (2008)
  • J.S. Andersen et al.

    Proteomic characterization of the human centrosome by protein correlation profiling

    Nature

    (2003)
  • C. Andrieu-Soler et al.

    Enhanced oligonucleotide delivery to mouse retinal cells using iontophoresis

    Mol. Vis.

    (2006)
  • E. Ayuso et al.

    High AAV vector purity results in serotype- and tissue-independent enhancement of transduction efficiency

    Gene Ther.

    (2010)
  • J.W. Bainbridge et al.

    Inhibition of retinal neovascularisation by gene transfer of soluble VEGF receptor sFlt-1

    Gene Ther.

    (2002)
  • K.S. Balaggan et al.

    Stable and efficient intraocular gene transfer using pseudotyped EIAV lentiviral vectors

    J. Gene Med.

    (2006)
  • K.S. Balaggan et al.

    EIAV vector-mediated delivery of endostatin or angiostatin inhibits angiogenesis and vascular hyperpermeability in experimental CNV

    Gene Ther.

    (2006)
  • J.S. Bartlett et al.

    Infectious entry pathway of adeno-associated virus and adeno-associated virus vectors

    J. Virol.

    (2000)
  • A.P. Bemelmans et al.

    Lentiviral vectors containing a retinal pigment epithelium specific promoter for leber congenital amaurosis gene therapy. Lentiviral gene therapy for LCA

    Adv. Exp. Med. Biol.

    (2006)
  • J. Bennett et al.

    Adenovirus vector-mediated in vivo gene transfer into adult murine retina

    Invest. Ophthalmol. Vis. Sci.

    (1994)
  • J. Bennett et al.

    Real-time, noninvasive in vivo assessment of adeno-associated virus-mediated retinal transduction

    Invest. Ophthalmol. Vis. Sci.

    (1997)
  • J. Bennett et al.

    Stable transgene expression in rod photoreceptors after recombinant adeno-associated virus-mediated gene transfer to monkey retina

    Proc. Natl. Acad. Sci. U. S. A.

    (1999)
  • J. Bennett

    Immune response following intraocular delivery of recombinant viral vectors

    Gene Ther.

    (2003)
  • I. Bentwich et al.

    Identification of hundreds of conserved and nonconserved human microRNAs

    Nat. Genet.

    (2005)
  • A.J. Berk

    Adenoviridae: the Viruses and Their Replication

    (2006)
  • K.I. Berns et al.

    Use of recombinant angiostatin to prevent retinal neovascularization

    Trans. Am. Clin. Climatol. Assoc.

    (2001)
  • K.I. Berns

    Paroviridae: the viruses and their replication

  • M.M. Bilak et al.

    Pigment epithelium-derived factor (PEDF) protects motor neurons from chronic glutamate-mediated neurodegeneration

    J. Neuropathol. Exp. Neurol.

    (1999)
  • P. Blacharski

    Fundus flacimaculatus

  • M. Bornens

    The centrosome in cells and organisms

    Science

    (2012)
  • S.E. Boye et al.

    Functional and behavioral restoration of vision by gene therapy in the guanylate cyclase-1 (GC1) knockout mouse

    PLoS One

    (2010)
  • K. Bujakowska et al.

    CRB1 mutations in inherited retinal dystrophies

    Hum. Mutat.

    (2012)
  • T.R. Burke et al.

    Allelic and phenotypic heterogeneity in ABCA4 mutations

    Ophthalmic Genet.

    (2011)
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    Percentage of work contributed by each author in the production of the manuscript is as follows: Initial draft manuscript prepared by M. Thake; Subsequent manuscripts written and edited by D.M. Lipinski; Corrections and review by R.E. MacLaren; Figures: Daniel M. Lipinski; Abstract: Robert E. MacLaren.

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