Personalized therapeutic strategies for patients with retinitis pigmentosa

Introduction: Retinitis pigmentosa (RP) encompasses many different hereditary retinal degenerations that are caused by a vast array of different gene mutations and have highly variable disease presentations and severities. This heterogeneity poses a significant therapeutic challenge, although an answer may eventually be found through two recent innovations: induced pluripotent stem cells (iPSCs) and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas genome editing.

Areas covered: This review discusses the wide-ranging applications of iPSCs and CRISPR–including disease modelling, diagnostics and therapeutics – with an ultimate view towards understanding how these two technologies can come together to address disease heterogeneity and orphan genes in a novel personalized medicine platform. An extensive literature search was conducted in PubMed and Google Scholar, with a particular focus on high-impact research published within the last 1 – 2 years and centered broadly on the subjects of retinal gene therapy, iPSC-derived outer retina cells, stem cell transplantation and CRISPR/Cas gene editing.

Expert opinion: For the retinal pigment epithelium, autologous transplantation of gene-corrected grafts derived from iPSCs may well be technically feasible in the near future. Photoreceptor transplantation faces more significant unresolved technical challenges but remains an achievable, if more distant, goal given the rapid pace of advancements in the field.     

Stargardt macular dystrophy and evolving therapies

Introduction: Stargardt macular dystrophy (STGD1) is a hereditary retinal degeneration that lacks effective treatment options. Gene therapy, stem cell therapy, and pharmacotherapy with visual cycle modulators (VCMs) and complement inhibitors are discussed as potential treatments.

Areas covered: Investigational therapies for STGD1 aim to reduce toxic bisretinoids and lipofuscin in the retina and retinal pigment epithelium (RPE). These agents include C20-D3-vitamin A (ALK-001), isotretinoin, VM200, emixustat, and A1120. Avacincaptad pegol is a C5 complement inhibitor that may reduce inflammation-related RPE damage. Animal models of STGD1 show promising data for these treatments, though proof of efficacy in humans is lacking. Fenretinide and emixustat are VCMs for dry AMD and STGD1 that failed to halt geographic atrophy progression or improve vision in trials for AMD. A1120 prevents retinol transport into RPE and may spare side effects typically seen with VCMs (nyctalopia and chromatopsia). Stem cell transplantation suggests potential biologic plausibility in a phase I/II trial. Gene therapy aims to augment the mutated ABCA4 gene, though results of a phase I/II trial are pending.

Expert opinion: Stem cell transplantation, ABCA4 gene therapy, VCMs, and complement inhibitors offer biologically plausible treatment mechanisms for treatment of STGD1. Further trials are warranted to assess efficacy and safety in humans.     

Gene therapy for retinal and choroidal diseases

The eye is a small compartment separated from the systemic circulation by the blood-ocular barriers, providing advantages for intraocular gene transfer – an approach which is being investigated for several types of retinal and choroidal diseases. A compelling application is gene replacement for homozygous loss-of-function mutations in genes differentially expressed in photoreceptors or retinal pigmented epithelial (RPE) cells that result in retinal degeneration. Considerable progress has been made in this area, including demonstration of return of visual function in RPE65^-/- dogs after subretinal injection of adeno-associated viral vectors encoding RPE65, providing groundwork for a clinical trial in patients with Leber’s Congenital Amaurosis. Proof of principle has been provided for intraocular gene transfer of ribozymes for dominantly inherited retinal degenerations. Survival factor gene therapy shows promise for treatments that may be used in multiple retinal degenerations. Transduction of intraocular and/or periocular cells with constructs that encode antiangiogenic proteins provides a new approach for sustained local delivery treatment of retinal and choroidal neovascularisation. While considerable investigation remains to work out critical details, there is substantial evidence suggesting that in the near future, gene therapy-based treatments will be an important addition to what is currently offered to patients with retinal and/or choroidal diseases.     

Voretigene neparvovec-rzyl for treatment of RPE65-mediated inherited retinal diseases: a model for ocular gene therapy development

ABSTRACT

Introduction: Over a decade of research and development culminated in the 2017 United States (US) Food and Drug Administration (FDA) approval of voretigene neparvovec-rzyl (VN) for RPE65 mutation-associated inherited retinal disease (IRD), the first approved gene therapy for a hereditary genetic disease in the US, and the first and only pharmacologic treatment for an IRD.

Areas covered: VN serves as a model for ocular gene therapy development, while RPE65 mutation-associated IRD serves as an example of a well-suited candidate disorder. This review also discusses development considerations for viral vector gene augmentation, and, studies that led to VN’s FDA approval. Subretinal injection of VN resulted in improved performance on the novel multi-luminance mobility test (MLMT), light sensitivity, and visual fields in patients with RPE65 mutation-associated IRD, which predominantly impairs rod function. Additionally, the dosage, administration technique, pharmacokinetics, and safety data of VN are reviewed.

Expert Opinion: As a model for development, special challenges associated with the introduction of this first ocular gene therapy include limited genetic testing in clinical practice, novel surgical complexity of ocular gene therapy administration, new functional vision endpoints, as well as unique development, launch, and reimbursement considerations associated with orphan therapies and one-time gene therapies.     

Mesenchymal-stem-cell-based experimental and clinical trials: current status and open questions

Introduction: Mesenchymal stem cells (MSCs) possess remarkable self-renewal ability and are able to differentiate into various cell lineages. MSCs can also enhance tissue repair and angiogenesis through a paracrine mechanism. It has been recognized that these cells hold great promise for tissue regeneration and treatment of immune-related diseases.

Areas covered: This review aims at discussing the mechanisms of MSC-mediated immunomodulation and tissue repair and the related clinical trials, with special emphasis on factors that influence the efficiency of MSC-based therapy, including the source of MSCs, cell passage, cell dose, timing and route of administration.

Expert opinion: MSCs may facilitate tissue repair through cell replacement and/or improving the microenvironment by releasing growth factors. Some of these factors also mediate the immunomodulatory effects of MSCs. It is important to establish global guidelines, protocols and standards for production and clinical trials of MSCs, so that MSCs can become a therapeutic agent with a reliable efficacy and good safety.     

BEST1: the Best Target for Gene and Cell Therapies

A retinal pigmented epithelial (RPE) disorder, bestrophinopathy has recently been proven to be amenable to gene and cell-based therapies in preclinical models. RPE disorders and allied retinal degenerations exhibit significant genetic heterogeneity, and diverse mutations can result in similar disease phenotypes. Several RPE disorders have recently become targets for gene therapies in humans. The year 2011 brought a new advance in cell-based therapies, with the Food and Drug Administration approving clinical trials using embryonic stem cells for an RPE disorder known as age-related macular degeneration. Recent studies on induced pluripotent stem (iPS)-RPE generation indicate strong potential for developing patient-specific disease models in vitro, which could eventually enable personalized treatment. This mini-review will briefly highlight the suitability of the retina for gene and cell therapies, the pathophysiology of bestrophinopathy, and the research and treatment opportunities afforded by stem cell and genetic therapies.     

Intravitreal steroid and anti-vascular endothelial growth agents for the management of retinal vein occlusion: evidence from randomized trials

Introduction: Retinal vein occlusion (RVO) is a common retinal vascular disorder, affecting visual acuity and quality of life. Macular edema (ME) and retinal ischemia are the main causes for visual impairment in RVO. Although several modalities have been evaluated for the treatment of ME secondary to RVO in clinical trials, various questions need to be clarified when translating clinical trials into real-world practice.

Areas covered: Intravitreal steroids and anti-VEGF agents are now widely used for the treatment of ME due to RVO. Herein, evidence from randomized controlled trials regarding the use of steroids and anti-VEGF agents in ME related to RVO are presented. In addition, an approach regarding the optimal treatment regimen, the most suitable time for initiating treatment and monitoring patients, as well as the potential role of ischemia in the response to treatment and the impact of treatment on the natural course of the disease was made.

Expert opinion: A comprehensive presentation of randomized clinical trials evaluating intravitreal steroids and anti-VEGF treatment for RVO indicates that both are effective and safe. However, the comparative effectiveness of the various anti-VEGF agents, the most suitable dosing regimen and the effect of these agents on retinal ischemia remains unclear.     

Additive value of adult bone-marrow-derived cell transplantation to conventional revascularization in chronic ischemic heart disease: a systemic review and meta-analysis

Objective: Whether adult bone marrow (BM)-derived cells (BMCs) transplantation benefits patients with chronic ischemic heart disease (IHD) remains controversial. This systemic and meta-analysis study aimed to assess the potential therapeutic effects of BMCs transplantation with revascularization in chronic IHD.

Research design and methods: Randomized controlled trials of BMCs in combination with coronary artery bypass grafting (CABG) or percutaneous coronary intervention (PCI) for chronic IHD were identified by searching Medline, Embase, the Cochrane Controlled Trials Register, the Cochrane Library, and the Web of Science. We conducted a random-effects meta-analysis across eligible studies measuring the same outcomes.

Results: Ten randomized controlled trials including 422 participants were reviewed. In the trials with six months of follow-up, BMCs transplantation improved left ventricular (LV) ejection fraction (LVEF) by 4.02% and reduced LV end-systolic and end-diastolic volumes. Subgroup analysis revealed a statistically significant difference in LVEF associated with primary intervention, route of cell delivery, cell type, and baseline LVEF, but not with cell dose or storage duration.

Conclusions: Selected-BMCs transplantation through myocardial injection after surgical revascularization may benefit patients with chronic IHD and severely impaired LV function. Due to the limitation of patient number, RCT with larger sample size and long follow-up are required for future research.     

Airway tissue engineering

Introduction: Prosthetic replacements, autologous tissue transfer and allografts have so far failed to offer functional solutions for the treatment of long circumferential tracheal defects and loss of a functioning larynx. Interest has therefore turned increasingly to the field of tissue-engineering which applies the principles and methods of bioengineering, material science, cell transplantation and life sciences in an effort to develop in vitro biological substitutes able to restore, maintain or improve tissue and organ function.

Areas covered: This article gives an overview of the tissue-engineering approach to airway replacement and will describe the encouraging results obtained so far in tracheal regeneration. The recent advances in the field of tissue-engineering have provided a new attractive approach towards the concept of functional substitutes and may represent an alternative to the shortage of suitable grafts for reconstructive airway surgery. We summarize fundamental questions, as well as future directions in airway tissue engineering.

Expert opinion: The replacement of active movement, as would be necessary to replace an entire larynx introduces another order of magnitude of complexity, although progress in this area is starting to bear fruit. In addition, the stem cell field is advancing rapidly, opening new avenues for this type of therapy.     

The role of α-smooth muscle actin in myogenic differentiation of human glandular stem cells and their potential for smooth muscle cell replacement therapies

Importance of the field: Cellular replacement therapies in vascular and urogenital organ disorders require an abundant source of smooth muscle cells. A promising approach would be the directed myogenic differentiation (characterized by the expression of α-smooth muscle actin (α-SMA)) into a sufficient amount of smooth muscle cells through easily obtainable adult stem cells, for example from the sweat gland.

Areas covered in this review: We present novel multipotent adult stem cell populations derived from glandular tissues like pancreas, salivary gland and sweat gland and assess their myogenic potential. Their possible application in cell replacement therapies is discussed, with regard to numerous scaffold-based approaches in the course of the last decade.

What the reader will gain: Multipotent glandular stem cells can be manipulated by different means to express a predominant smooth muscle-like phenotype. Possible promising applications of myogenic differentiated stem cells were evaluated, since several studies revealed the beneficial effect of somatic stem cells in replacement therapies for blood vessels, bladder reconstructions, etc.

Take home message: Glandular stem cells, especially sweat-gland-derived cells, provide an easily accessible and efficient source for autologous smooth muscle tissue, which might be used to replace vascular tissue in case of organ failure or disorder.     

Biomaterials and stem cells for tissue engineering

Introduction: Organ failure and tissue loss are challenging health issues due to widespread injury, the lack of organs for transplantation and limitations of conventional artificial implants. The field of tissue engineering aims to provide alternative living substitutes that restore, maintain or improve tissue function.

Areas covered: In this paper, a wide range of porous scaffolds are reviewed, with an emphasis on phase-separation techniques that generate advantageous nanofibrous 3D scaffolds for stem cell-based tissue engineering applications. In addition, methods for presentation and delivery of bioactive molecules to mimic the properties of stem cell niches are summarized. Recent progress in using these bioinstructive scaffolds to support stem cell differentiation and tissue regeneration is also presented.

Expert opinion: Stem cells have great clinical potential because of their capability to differentiate into multiple cell types. Biomaterials have served as artificial extracellular environments to regulate stem cell behavior. Biomaterials with various physical, mechanical and chemical properties can be designed to control stem cell development for regeneration.

Conclusion: The research at the interface of stem cell biology and biomaterials has made and will continue to make exciting advances in tissue engineering.     

Stem cell-derived neurotrophic support for the neuromuscular junction in spinal muscular atrophy

Importance of the field: Spinal muscular atrophy (SMA) is a neurodegenerative disease characterized by specific degeneration of α-motor neurons in the spinal cord. The use of cell transplantation to restore lost function through cell replacement or prevent further degeneration of motor neurons and synapses through neurotrophic support heralds tremendous hope in the SMA field.

Areas covered in this review: Much research has been carried out in the last decade on the use of embryonic stem cells in cell replacement strategies for various neurodegenerative diseases. Cell replacement is contingent on the ability of transplanted cells to integrate and form new functional connections with host cells. In the case of SMA, cell replacement is a tall order in that axons of transplanted cells would be required to grow over long distances from the spinal cord through growth-averse terrain to synapse with muscles in the periphery. The efficacy of neurotrophic support is contingent on the ability of transplanted cells to secrete neurotrophins appropriate for degenerating motor neurons in the spinal cord or development/stability of the neuromuscular junction (NMJ) in the periphery.

What the reader will gain: The reader will gain an understanding of the potential of neurotrophins to promote development of the NMJ in a diseased or injured environment.

Take home message: Neurotrophins play a major role in NMJ development and thus may be a key factor in the pathogenesis of NMJs in SMA. Further research into the signaling mechanisms involved in NMJ maturation may identify additional mechanisms by which transplanted cells may be of therapeutic benefit.     

A Bruch's membrane substitute fabricated from silk fibroin supports the function of retinal pigment epithelial cells in vitro

Silk fibroin provides a promising biomaterial for ocular tissue reconstruction, including the damaged outer blood–retinal barrier of patients afflicted with age‐related macular degeneration (AMD). The aim of the present study was to evaluate the function of retinal pigment epithelial (RPE) cells in vitro, when grown on fibroin membranes manufactured to a thickness similar to that of Bruch's membrane (3 µm). Confluent cultures of RPE cells (ARPE‐19) were established on fibroin membranes and maintained under conditions designed to promote maturation over 4 months. Control cultures were grown on polyester cell culture well inserts (Transwell^®). Cultures established on either material developed a cobblestone morphology, with partial pigmentation, within 12 weeks. Immunocytochemistry at 16 weeks revealed a similar distribution pattern between cultures for F‐actin, ZO‐1, ezrin, cytokeratin pair 8/18, RPE‐65 and Na⁺/K⁺‐ATPase. Electron microscopy revealed that cultures grown on fibroin displayed a rounder apical surface with a more dense distribution of microvilli. Both cultures avidly ingested fluorescent microspheres coated with vitronectin and bovine serum albumin (BSA), but not controls coated with BSA alone. VEGF and PEDF were detected in the conditioned media collected from above and below the two membrane types. Levels of PEDF were significantly higher than for VEGF on both membranes and a trend was observed towards larger amounts of PEDF in apical compartments. These findings demonstrated that RPE cell functions on fibroin membranes are equivalent to those observed for standard test materials (polyester membranes). As such, these studies support advancement to studies of RPE cell implantation on fibroin membranes in a preclinical model. Copyright © 2015 John Wiley & Sons, Ltd.     

The potential of amniotic fluid stem cells for cellular therapy and tissue engineering

Introduction: Foetal cells present in amniotic fluid (AF) have been used for many years to perform prenatal genetic screening. Recent reports suggested that these cells might have additional benefits. AF contains, in addition to committed and differentiated cells, a subpopulation with stem cell characteristics. AF-derived stem cells (AFS) have functions found in mesenchymal stem cells, but in addition, exhibit a potent expansion capacity and plasticity. AFS are able to undergo multi-lineage differentiation and produce progeny indicative of all three germ layers.

Areas covered: The experimental approaches available to isolate AFS and their potential for tissue engineering, the repair of organs through cell replacement and tissue regeneration.

Expert opinion: The deployment of AFS for tissue regeneration offers advantages over the use of embryonic or adult stem cells: i) AF represents a convenient and non-contested source for obtaining stem cells; ii) their derivation is relatively simple and rapid; iii) no feeder layers are required for their cultivation; iv) they display no spontaneous differentiation in culture; and v) their stem cell phenotype is not affected by long-term storage. The application of AFS for tissue replacement therapies in vivo is at a very early stage, but existing studies indicate great potential for clinical use.