Clinical applications of retinal gene therapy
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.
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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.