Stem Cell Biology and Regenerative Medicine in Ophthalmology

Stephen H. Tsang (ed.)Stem Cell Biology and Regenerative MedicineStem Cell Biology and Regenerative Medicine in Ophthalmology201310.1007/978-1-4614-5493-9_1© Springer Science+Business Media New York 2013

1. The Eye as a Target Organ for Stem Cell Therapy

Mark A. Fields¹, John Hwang², Jie Gong¹, Hui Cai³ and Lucian V. Del Priore¹  

(1)

Department of Ophthalmology, Albert Florens Storm Eye Institute, Medical University of South Carolina, Charleston, SC, USA

(2)

Doheny Eye Institute, Keck School of Medicine, University of Southern California, Los Angeles, CA, USA

(3)

Harkness Eye Institute, Columbia University, New York, NY, USA

Lucian V. Del Priore

Email: ldelpriore@yahoo.com

Abstract

Retinal degenerations are a heterogeneous group of disorders that are characterized by progressive cellular dysfunction, cellular disarray, and eventually cell death. Early in the course of disease therapeutic intervention consists of pharmaceutical treatment to prevent cell death or gene therapy to correct the underlying mutation. Due to the nature of pathologies involving these disorders, particularly in late stage of disease, cell replacement therapy or electric stimulation of remaining cells by artificial retinal prosthesis is the only viable option. Stem cell therapies for retinal degenerative diseases such as age-related macular degeneration (AMD) and retinitis pigmentosa (RP) are a promising therapeutic option and will require replacement of lost photoreceptor cells and retinal pigment epithelium (RPE). Current clinical trials are underway to evaluate the potential of stem cell therapy in humans. The use of induced pluripotent stem (iPS) cells hold great promise as a potential reservoir of cells for the treatment of retinal disorders as well as a clinical tool to help understand disease pathology. Advances in stem cell technology will translate these therapies into viable clinical options for the treatment of retinal degenerative diseases and other disorders.

Introduction

Retinal degenerations are a heterogeneous group of disorders that are characterized by progressive cellular dysfunction, cellular disarray, and eventually cell death. Numerous classification systems exist for these disorders, but no one classification system captures the complexity of the disease processes, the diversity of their pathology, and the common themes in treatment that underlie these diseases. Many current classifications distinguish between macular diseases and peripheral retinal degenerations, but this classification system does not represent the complexity of the disease process in a complete fashion. Prior to the discovery of gene mutations that increase the risk profile for age-related macular degeneration (AMD), retinal degenerations were often classified as either hereditary or nonhereditary diseases, but the simplicity of this classification has been called into question based on the observation that certain alleles increased the risk of AMD [1–4]. Thus, for the purpose of this discussion, retinal degenerations will be classified by whether they are Mendelian disorders (e.g., most if not all forms of retinitis pigmentosa (RP), Leber’s congenital amaurosis, and Best’s disease) or non-Mendelian retinal disorders, including AMD.

Because of the complexity of the disease processes, it is possible to dedicate an entire chapter of this book to each disease and still not cover all the details of each condition. However, regardless of the cause of the retinal disorder, it is important to recognize that severe vision loss is typically associated with cellular dysfunction or death. Early in the course of many diseases there is cell dysfunction without cell death. In these early stages, gene therapy, pharmacological treatment to manipulate the cell death pathway, and/or treatment with locally administered growth factors, such as ciliary neurotrophic growth factor, may all prove to be useful. However, late stages of retinal disease, which are usually accompanied by severe vision loss, will require a different approach. For example, in advanced stages of many forms of RP, severe vision loss is due to death of photoreceptors, loss of the native retinal pigment epithelium (RPE) monolayer on Bruch’s membrane, migration of pigmented cells into the retina, and transsynaptic degeneration leading to inner retinal disturbance. In advanced geographic atrophy in AMD, there is loss of RPE and photoreceptors and secondary atrophy of the choriocapillaris. Reversal of vision loss in these late stages of disease, after cell loss has occurred, will likely require cell therapy with transplantation of photoreceptors, RPE and/or choriocapillaris cells; or direct electrical stimulation of the inner neural retina with multi-electrode arrays.

In this review we will discuss the clinical and pathological features of retinal degenerations that are important to their potential treatment with stem cell therapy; the unique combination of eye anatomy and imaging capabilities that makes it an excellent target organ for early stem cell therapy in humans; and the status of human trials.

Clinical and Pathological Features of Retinal Degenerations

Retinitis Pigmentosa

Retinitis pigmentosa (RP) is a group of Mendelian hereditary disorders characterized clinically by bilateral progressive loss of peripheral vision, a marked ring-like constriction of the visual field, night blindness, and late loss of central vision. As a group the population prevalence of RP is about 1:4,000, so the estimates are that approximately 100,000 in the USA have this disease. Investigators have identified at least 45 loci for mutations that can cause retinitis pigmentosa, and these genes collectively account for disease in a little over half of all patients [5–7]. Of the cloned genes for retinitis pigmentosa it is estimated that dominant retinitis pigmentosa account for about 50 %, recessive retinitis pigmentosa account for about 40 % and X-linked retinitis pigmentosa account for approximately 80 % of cases, indicating that many genes remain to be identified [6, 8]. Rods are the predominantly affected photoreceptors and dysfunction causes night blindness and peripheral field loss beginning as early as the teenage years [9]. Disease progression leads to central acuity loss and legal blindness in the majority of patients [10]. Classic findings on funduscopic exam include perivascular bony spicule pigmentation, attenuated arterioles, and waxy optic disc pallor, typically associated with vitreous cells and posterior subcapsular cataracts. However, many of these findings may be absent in early stages of disease [11, 12]. Electroretinogram (ERG) testing is important for diagnosis and may provide prognostic information [10]. The genetics of retinitis pigmentosa are extremely complex with diverse modes of inheritance [12]. Potential interventions include vitamin A therapy and carbonic anhydrase inhibitors, but treatment options are extremely limited in the majority of cases with no effective form of therapy. Results evaluating vitamin A efficacy have shown limited benefit but potential risks exist with oral vitamin A supplementation, including the risk of hepatotoxicity [13]. Carbonic anhydrase inhibitors have shown clinical benefit in reducing macular edema and improving visual acuity in some patients with retinitis pigmentosa [14].

Genetics

The genetics of retinitis pigmentosa are extremely complex with diverse modes of inheritance including dominant, recessive, X-linked, mitochondrial, and digenic forms [12]. The disease may manifest solely with visual symptoms or may be accompanied by a constellation of systemic findings in patients with syndromic retinitis pigmentosa. The diversity in genetic transmission and clinical presentation is not entirely surprising given that retinitis pigmentosa constitutes a broad group of diseases that arises from diverse biological pathways.

Retinitis pigmentosa demonstrates multiple modes of segregation [15]. Autosomal dominant transmission occurs most frequently and accounts for 20 % of retinitis pigmentosa cases. Symptoms are generally less severe with adult-onset with variable penetrance of symptoms. Autosomal recessive disease occurs in 13 % of cases and is characterized by earlier onset of symptoms and severe vision loss. X-linked recessive disease accounts for 8 % of cases and has the poorest visual prognosis with early onset and rapid progression of symptoms [12]. Visual deficits typically present within the first decade of life and progress to partial or complete blindness by the third or fourth decade. In approximately 20 % of nonsyndromic cases, the mode of transmission cannot be established because of an unclear family history. These cases are termed simplex retinitis pigmentosa and presumed to arise from autosomal recessive or X-linked transmission. Syndromic retinitis pigmentosa, in which vision loss occurs in the settings of extraocular disease manifestations, constitutes 25 % of cases with Usher (10 %) and Bardet–Biedl (5 %) syndromes occurring most frequently [15].

Mutations in 53 genes are known to cause nonsyndromic retinitis pigmentosa or Leber’s congenital amaurosis (LCA), which may be indistinguishable from early onset retinitis pigmentosa. This includes 25 genes in autosomal recessive retinitis pigmentosa (arRP), 17 genes in autosomal dominant retinitis pigmentosa (adRP), 13 genes in recessive LCA, 2 genes in dominant LCA, and 6 gene mutations in X-linked retinitis pigmentosa (xlRP) [15]. Mutations in a single gene, such as rhodopsin or neural retina-specific leucine zipper (NRL), may result in multiple forms of disease such as adRP and arRP. The proportion of disease caused by mutations in a particular gene is highly variable [15]. The largest proportion of retinitis pigmentosa is caused by mutations in rhodopsin (RHO) in adRP (26.5 %), Usher syndrome 2A (USH2A) in arRP (10.0 %), retinal guanylate cyclase 2D (GUCY2D) in recessive LCA (21.2 %), and retinitis pigmentosa GTPase regulator (RPGR) in xlRP (74.2 %). A significant proportion of the molecular defects underlying retinitis pigmentosa are known to affect the phototransduction cascade, visual cycle, outer segment structure, cilium-mediated protein trafficking, cellular interaction/adhesion, transcription factors, and RNA-intron splicing factors.

Symptoms and Clinical Findings

Retinitis pigmentosa is phenotypically heterogeneous with wide variation in severity, age of onset, and progression. Classically, retinitis pigmentosa manifests with early night blindness (nyctalopia) beginning in teenage years followed by loss of peripheral visual field. The majority of patients are classified as legally blind by age 60 with central visual field diameters less than 20° [9]. Defects in blue–yellow color perception may occur in advanced stages when visual acuity is 20/40 or worse [16].

Syndromic retinitis pigmentosa is a term used to describe cases of retinitis pigmentosa associated with extraocular symptoms. Approximately 25 % of retinitis pigmentosa cases are syndromic and over 30 forms have been identified [17]. Usher syndrome is the most common form and is associated with sensorineural deafness. It accounts for about 10 % of retinitis pigmentosa cases and is divided into three major groups. Type 1 demonstrates profound congenital deafness, vestibular symptoms, and childhood-onset retinopathy [18]. Type 2 manifests with congenital partial, nonprogressive deafness, absence of vestibular symptoms, and mild later-onset retinopathy [19, 20]. Type 3, the least common form, demonstrates progressive deafness beginning in the third decade and adult-onset retinopathy [21]. Bardet–Biedl syndrome is the second most common form of syndromic retinitis pigmentosa and accounts for 5 % of retinitis pigmentosa cases [15]. It is associated with polydactyly, obesity, renal dysfunction, and mental retardation. Other forms of syndromic retinitis pigmentosa account for 10 % of all retinitis pigmentosa cases and include Refsum’s disease, Bassen–Kornzweig syndrome, Kearne–Sayre syndrome, Batten’s disease, and Senior–Loken disease. A complete listing of genes implicated in retinitis pigmentosa can be found on the Retinal Information Network web site http://www.sph.uth.tmc.edu/retnet/.

Retinitis pigmentosa classically leads to fundus changes with accumulation of bony spicule pigmentation. Lesions are generally perivascular and localized to the mid-periphery where rods are concentrated. However, pigment distribution is often variable and may be diffuse, sectoral, or even be absent in certain subtypes of retinitis pigmentosa. Other signs include abnormal retinal pigmentation changes, attenuated arterioles, vitreous cells, waxy optic disc pallor, and blue–yellow color vision deficiency. Vitreous cells and opacities are the most consistent characteristics across all forms of retinitis pigmentosa. Notably, early stages of retinitis pigmentosa may lack appreciable funduscopic findings [5, 11, 12]. Retinitis pigmentosa patients, particularly those over age 40, may demonstrate cystoid macular edema, epiretinal membranes, diffuse retinal vascular leakage, macular preretinal fibrosis, macular RPE defects, and posterior subcapsular cataracts. Other associated findings include myopia and astigmatism [5, 11, 12, 22–24].

Treatment

Treatment options are extremely limited for most retinitis pigmentosa subtypes with no effective approach for prevention, stabilization, or reversal of visual loss.

The efficacy of vitamin A and E supplements on slowing retinitis pigmentosa progression was examined in a randomized, double-masked, prospective study [13]. About 601 patients with non-syndromic retinitis pigmentosa and Usher syndrome (type 2) were randomized into four treatment groups receiving 15,000 IU/d of vitamin A, 400 IU/d of vitamin E, 15,000 IU/d of vitamin A plus 400 IU/d of vitamin E, or trace amounts of both vitamins and followed for 4–6 years. The trial concluded that (1) vitamin A groups demonstrated slower rates of decline in cone ERG amplitudes (2) vitamin A groups were 32 % less likely to have a decline in ERG amplitude of 50 % or more from baseline (3) vitamin E groups were 42 % more likely to have a decline in ERG amplitude of 50 % or more from baseline, and (4) there was no significant difference in visual acuity and field loss. The reduction of ERG decline in patients receiving vitamin A was limited to the 30 Hz and 0.5 Hz flash amplitudes. Significantly, these patients did not demonstrate any improvement in psychophysical visual parameters [25, 26].

Thus, these results suggest that benefits of vitamin A therapy are limited and must be weighed against potential risks such as teratogenic effects in pregnant women, elevated intracranial pressure, hepatomegaly, bone disease in young individuals, and elevated serum lipids [27–29]. Currently many practitioners do not use vitamin A supplementation routinely due to the small treatment effect and the need for monitoring of vitamin A toxicity. In addition, the mixed molecular etiology of retinitis pigmentosa suggests that response to vitamin A may vary across retinitis pigmentosa subtypes. Studies in ABCA4 knockout mice demonstrated increased rates of lipofuscin deposition and photoreceptor degeneration in mice on vitamin A supplementation. These results suggest that if vitamin A supplementation is employed, it should be done so selectively [30, 31] as it may have a deleterious effect on certain subsets of retinitis pigmentosa patients. Because of the small magnitude of the effect on ERG, lack of improvement in psychophysical parameters, concerns about toxicity, and the varied genetics of retinitis pigmentosa, the use of vitamin A supplementation to slow retinitis pigmentosa progression has not been universally adopted. If patients are placed on oral vitamin A therapy, they should undergo periodic liver function testing, osteoporosis screening, and fasting serum vitamin A measurements to avoid toxicity.

Other therapies have also been advocated as potentially effective in retinitis pigmentosa. To date, however, there is no evidence of clinical visual improvement with lutein supplements [32], docosahexaenoic acid supplements [33–35], light deprivation [36], therapeutic bee stings [37], vasodilators [38], or placental tissue injections [39]. Interestingly, repeat intravitreal injections and/or pars plana vitrectomy are not currently used to treat patients with retinal degenerations, despite the fact that there is a well-known rescue effect of vitreous and subretinal surgery on retinal degeneration. Subretinal insertion of a dry needle results in a degree of photoreceptor rescue similar to that of intravitreal or subretinal basic fibroblast growth factor injection in the Royal College of Surgeons rat [40]. Anterior chamber injection of placebo and brain-derived neurotrophic growth factor produces similar rescue effects in axotomized rat ganglion cells [41]. Lensectomy and vitrectomy alone rescue degenerating photoreceptors in the P347L transgenic pig, which contains a rhodopsin mutation known to cause retinitis pigmentosa in humans [42]. Subretinal saline injection produces a rescue effect in the Royal College of Surgeons rat [43]. These studies demonstrate clearly that vitreous and subretinal surgery alone may produce some rescue effect in retinal degenerations, but long-term demonstration of their efficacy awaits additional preclinical and clinical trials.

There is some therapeutic benefit of dietary modifications and nutritional supplements for two rare forms of syndromic retinitis pigmentosa. Phytanic acid oxidase deficiency (Refsum’s disease) arises from failure of phytanic acid degradation and consequent elevation of serum phytanic acid. Clinical manifestations include ataxia, peripheral neuropathy, deafness, and cardiac conduction defects [44–46]. Dietary restriction of phytanic acid may halt or reduce progression of retinitis pigmentosa. Abetalipoproteinemia (Bassen–Kornzweig syndrome) is characterized by low serum levels of apolipoprotein B, resulting in fat malabsorption and low plasma concentrations of fat-soluble vitamins. Systemic signs generally manifest in childhood and include diarrhea, cerebellar ataxia, and acanthocytosis. Therapy with high doses of vitamin A may allow rapid restoration of visual function in early stages of disease [47–49]. Laboratory studies of serum phytanic acid levels and serum lipoprotein electrophoresis can assist in the diagnosis of Refsum’s disease and Bassen–Kornzweig syndrome, respectively.

Visual function in retinitis pigmentosa may be improved by monitoring for and treating associated conditions such as cystoid macular edema, posterior subcapsular cataract, and epiretinal membranes. In addition, referral to low vision clinics can help optimize remaining visual function.

The Argus II Retinal Prosthesis System developed by Second Sight Medical Products, Inc. is intended to provide electrical stimulation of the retina to elicit visual perception in blind subjects with retinitis pigmentosa [50]. The technology is currently being evaluated in a clinical study conducted in the USA and recently received a CE (European Conformity) mark in Europe which is a key indicator of a product’s compliance with European Union legislation. The device consists of a surgically implanted 60-electrode stimulating microelectrode array consisting of 200 μm diameter disc electrodes, an inductive coil link used to transmit power and data to the internal portion of the implant, an external belt-worn video processing unit and a miniature camera mounted on a pair of glasses [51, 52]. The video camera is designed to capture a portion of the visual field and relay the information to the video processing unit. The video processing unit then digitizes the signal in real time, applies a series of image processing filters, down-samples the image to a 6 × 10 pixelized grid, and creates a series of stimulus pulses based on pixel brightness values and look-up tables customized for each subject [51]. The stimulus pulses are delivered to the microelectrode array via application-specific circuitry and a superior-temporally placed inductive radio frequency coil link allowing for wireless forward and reverse telemetry between intra and extra-ocular portions of the system [51]. The prosthesis is expected to generate limited amounts of vision in patients with severe to profound vision loss in the range of hand motions or light perception vision.

Age-Related Macular Degeneration

Age-related macular degeneration (AMD) affects 30–50 million elderly people worldwide and is the leading cause of blindness in individuals over the age of 50 in the Western world [53, 54]. It is estimated that approximately 30 % of adults over the age of 75 have some signs of AMD and that at least 10 % develop the advanced or late stage of disease [55, 56]. AMD as a disease entity primarily exists in two forms, nonexudative (atrophic or dry) AMD and exudative (neovascular or wet) AMD. Although the vast majority of patients with AMD are of the nonexudative type, approximately 90 % of significant vision loss due to AMD is secondary to central vision deterioration from the exudative type [56, 57]. Early in the course of disease there is cellular dysfunctional without cell death. In late-stage disease, AMD is characterized by extensive cell death, as with late-stage RP.

Genetics of AMD

Age-related macular degeneration is a complex disease that results from a combination of genetic and environmental factors. Many of these factors have been identified, but some remain unknown. Because AMD occurs late in life, it has been very difficult to elucidate the genetic factors correlated with the disease. AMD’s heterogenicity in phenotypes presents a challenge as well [58]. It also may be discovered that each individual’s susceptibility is due to multiple genetic and environmental effects and interactions [58–62].

Symptoms and Clinical Findings

Patients with advanced AMD typically present with blurry central vision, metamorphopsia, and reduced vision. These symptoms can then evolve to a central scotoma and severe loss of vision [63]. Ophthalmoscopic examination of the fundus at late stages of disease demonstrates patchy chorioretinal atrophy in the dry type and exudation in the wet variety, often manifested by the presence of retinal hemorrhages and lipid exudate in and around the macula [63].

One of the earliest clinical findings associated with AMD is the presence of drusen, which represent accumulation of extracellular material beneath the RPE [64]. In the case of dry AMD, loss of vision develops due to loss of the RPE, photoreceptors, and/or choriocapillaris; this can lead to patches of atrophy which are manifest clinically by central and paracentral scotomas [64]. In the case of wet AMD overexpression or loss of normal apical–basal polarity in the expression of angiogenic factors such as vascular endothelial growth factor (VEGF) can cause neovascularization to arise from the neural retina (retinal angiomatous proliferation) or choriocapillaris. In early stages of the disease patients experience minimal vision loss but some symptoms may occur such as blurred vision, visual scotomas, decreased contrast sensitivity, abnormal dark adaptation, and the need for bright light or magnification to decipher images [64]. In the late stages of advanced non-neovascular disease, patients typically present with a gradual loss of vision that becomes more severe and affects central or pericentral vision [64]. This form usually progresses and leads to irreversible vision loss. In patients with neovascular disease, loss of vision can be much more sudden with loss occurring within days to weeks due to subretinal hemorrhage or fluid accumulation secondary to choroidal neovascularization [64, 65].

Treatment

While the last decade has brought about a revolution in the treatment of exudative AMD, there are currently no approved therapies for geographic atrophy. Numerous investigational therapies are in various stages of clinical trials. These include ciliary neurotrophic factor, complement inhibitors, weekly vaccination with glatiramer acetate, fenretinide and OT-551 [66–70]. These therapies are promising, but none have progressed beyond clinical trials, leaving a large void in the current therapy of geographic atrophy.

Ninety percent of AMD patients who experience severe vision loss do so as a result of choroidal or intraretinal neovascularization [71]. Choroidal neovascularization represents growth of neovascular tissue from the choriocapillaris, within Bruch’s membrane, and eventually in the subretinal pigment epithelium and/or subretinal space. Retinal angiomatous proliferation is a form of wet AMD in which the abnormal vessels arise from the neural retina [72, 73]. Developing new treatments that prevent or reverse vision loss in AMD are of paramount importance due to the severe visual deficits that occur with this condition and the knowledge that disease prevalence will increase with shifting demographics of an aging western population.

Treatments for the wet form of this disease involve intravitreal antiangiogenic therapy, photocoagulation and photodynamic therapy, and vitreoretinal surgery. Intravitreal antiangiogenic treatment is currently the primary therapy for wet AMD and delivered directly to the vitreous. Treatment with intravitreal injection of anti-VEGF agents improves vision in patients with wet AMD but maintenance of the therapeutic effect requires continued administration of intravitreal agents, and this can be associated with potentially serious side effects such as endophthalmitis, retinal detachment, intraocular hemorrhage, increased intraocular pressure, and, in some cases, retinal detachment [74]. Photodynamic therapy uses light sensitive medicine that identifies abnormal vessel growth under the macula. Laser light then activates the light sensitive dye which can then decrease exudation from the neovascularization.

Despite these significant advances in the management of exudative AMD, there is a large unmet need for many patients