In an earlier article, an overview was provided of the emerging world of ‘personalised medicine’ where patient care will increasingly involve the use of information of the genome of individual patients (click here to read the previous article). While this development is taking place across the entire spectrum of patient care, there are already some emerging applications within the various disciplines of ophthalmology. Sadagopan et al1 provide a relevant overview of genetics for the eye care practitioner and offer a useful introduction to this rapidly evolving discipline. In this current article, the key themes identified include those of gene mapping, gene editing and stem cell treatments and details of specific clinical developments within these areas are outlined.
Gene mapping
Retinal diseases such as retinosa pigmentosa have been studied extensively in the context of identification of genes that are involved in specific presenting conditions. Studies in molecular genetics indicate that around 225 genes are involved in various retinal dystrophies and where a spectrum of genes may be involved in a specific diagnosed clinical condition. Daiger et al2 describe the results of genetic screening of a set of 215 patients with autosomal dominant retinitis pigmentosa, where around 58% of patients were identified with defects in known adRP genes as previously described by Sullivan et al.3 This implies that further investigation is required to identify the remaining genes associated with adRP.
Weisschuh et al4 describe the results of genetic profiling of a cohort of patients including 89 independent cases with various subforms of retinal dystrophies. Using a range of sequencing techniques, a detection rate of 61% was identified against 34 known and two novel retinal dystrophy genes. This prompted a review of patients where clinical presentations were not fully consistent with genetic analysis. The authors confirm that retinal dystrophies are associated with a spectrum of mutations in associated genes and, as yet, not all have been identified. Such investigations, however, confirm the inherent complex nature of retinal dystrophies where characteristics of the disease such as age of onset and disease progression will reflect specific genetic characteristics.
Research has been on-going for some time in linking ocular diseases such as glaucoma, cataracts, strabismus, corneal dystrophies and a number of retinal dystrophies with genetic defects/mutations. In the USA, the eyeGENE project,5 initiated in 2003, is an ongoing research project of the National Ophthalmic Disease Genotyping and Phenotyping Network which is a consortium of researchers involved in building a biorepository linking genotypic and phenotypic information. A collection of around 6,000 samples had been obtained by the end of 2015 which marks the end of the first phase of collection of genotype information. A number of Stage 2 research studies investigating specific ophthalmic conditions are currently in progress.
Age-related macular degeneration (AMD) has been specifically targeted for gene mapping since the condition is a leading cause of blindness in the elderly. Fritsche et al6 describe the results of a gene mapping multicentre study (26 centres) undertaken by the International AMD Genomics Consortium (IAMDGC) involving 16,144 late AMD cases and 17,832 controls. The size of the study reflects the scale of data collection which is required to improve knowledge of the specific genes which are involved in the development of AMD. The specific study was able to confirm 18 previous loci7 and identified a further 16 loci associated with disease presentation. This data is identified as highly relevant for the development of rapid test kits for diagnosing AMD and also for development of possible treatments. Analysis of such genetic profiling data also requires highly complex computational tools and an in depth understanding of possible linkage between identified genes and associated significance for disease presentation. Such studies also confirm the necessity for data to be collected on an appropriate scale in order to confirm the significance of specific genes in such studies. This transition in the ability to confirm genetic factors within patients with inherited retinal disease is confirmed by O’Sullivan.8
Stem cell treatments
While gene mapping can be described as an exercise in determination of probabilities of involvement with specific genes for specific clinical presentations, stem cell therapies relate to the use of specific cell lines to effect tissue ‘repair’ at the cellular level. McLaren and Pearson9 initially described in 2006 the challenges of successful stem cell application in treatment of retinal diseases. The regeneration of photoreceptor cells is identified as potentially more achievable than, for example, ganglion cell regeneration due to the reduced number of neural interconnection required for successful cellular regeneration. One limitation, however, of stem cell treatments for AMD is the significant loss of photoreceptor cells which characterise the developing disease stages and where early intervention with stem cells will be potentially more beneficial. The authors also describe the challenges of appropriate ‘programming’ of foetal stem cells and the scope of effectively cloning appropriately differentiated cells.
Retinal cells can be repaired with stem cell therapy
More recently Ng et al10 describe the use of mesenchymal stem cells for the treatment of a range of neurodegenerative diseases and where these cells are commonly derived from the patient’s bone marrow. Such cells have the characteristic of migrating to areas where there is a requirement for cell regeneration and where appropriate cell differentiation spontaneously occurs within the target cell environment. Stem cells in such treatments have also been associated with neuroprotective effects and immunomodulatory properties. This mode of stem cell treatment uses the patient’s own cells, without genetic manipulation, thus removing moral and ethical challenges associated with genetic manipulation and safety of cell lines.
A number of studies have already reported positive outcomes from introduction of mesenchymal stem cells into the eye. Weiss et al,11 for example, recently describe a specific case within the Stem Cell Ophthalmology Treatment Study (SCOTS) where the initial diagnosis of the patient was idiopathic bilateral optic neuritis leading to bilateral optic neuropathy. Prior to the stem cell procedure, the best-corrected visual acuity was 6/240 right eye and 6/1200 left eye. Following treatment four months later, the central visual acuity was observed to improve to 6/30 right and 6/12 left. Bone marrow for the procedure was extracted from the patient’s posterior iliac crest. Of specific relevance will be the long term outcomes from such interventions. Weiss et al11 also indicate that the visual improvement can be due to a range of mechanisms and further research is required to determine the precise nature of such cellular interactions.
Schwartz et al12 report initial findings of studies of safety, tolerability and involving the implantation of human embryonic stem cell derived retinal pigment epithelium in nine patients with Stargardt’s macular dystrophy and nine with atrophic age-related macular degeneration. With treatments undertaken in single eyes, outcomes were identified as being essentially safe and well tolerated and with measurable improvements in vision within the two groups. This, however, represents a more complex clinical intervention where immunosuppression is required to prevent rejection of the implanted stem cells.
Gene therapies
Specific eye diseases are associated with identified genetic defects. One of the first such diseases to be investigated for gene therapy was Leber’s congenital amaurosis where the production of RPE65, retinoid isomerohydrolase, in the retina is impaired through a gene mutation. A range of investigators have successfully introduced the corrected gene using recombinant adeno-associated virus (AAV) technique and where the altered genetic material has typically been introduced without adverse reaction. Studies such as those undertaken initially by Maguire et al14 and more recently by Weleber et al15 have demonstrated improvement in vision of patients receiving such treatments though there is the perception that episodes of visual improvement occur within a long term phase of cellular degeneration.
Genome sequencing is now a cost-effective reality
In this context, an advantage is identified in delivering treatments before the extensive loss of photoreceptor cells occurs. While researchers comment on the lack of adverse reaction to the altered genetic material, the very act of injection of the genetic material into the eye presents an ongoing element of trauma and risk. Hung et al13 provides an overview of the technical challenges of application of Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR) gene editing techniques within ophthalmology and also the associated ethical issues raised by their use.
Discussion
The genetic investigation of eye disease presents a new perspective in relation to the diagnosis and treatment of such conditions. There is also, however, a distinct learning curve in understanding of the scientific and clinical framework of such activities and the potential clinical benefits. An overview of the clinical implications/opportunities made possible by ‘personalised medicine’ in ophthalmology is described by Porter and Black.16 The driving of healthcare through genetic ‘individualisation’ also provides the potential for more appropriate follow up of patients with specific ophthalmic conditions, making possible enhanced surveillance with the potential provision and development of early and anticipatory treatments.
With the NHS currently in a state of funding crisis, there is opportunity for the private sector to offer an ‘enhanced diagnostic’ service for potential undiagnosed clinical conditions through, for example, identification of genetic markers. What is less obvious, however, is how such increased demand for treatment identified through genetic profiling will be resourced. What presents specifically as a challenge within ophthalmology, however, is now a challenge across the whole of healthcare.
Postcript
In an era where whole genome sequencing is about to play a key role across many areas of medicine, it is worth reflecting that Illumina, the USA company that dominates the technology of genome sequencing, obtained its technology when in 2007 it bought out Solexa, a technology offshoot company of Cambridge University in the UK. The key innovations of Cambridge scientists Shankar Balasubramanian and David Klenerman who founded Solexa have made cost-effective genomic sequencing a reality.
Dr Douglas Clarkson is development and quality manager at the department of clinical physics and bio-engineering, Coventry and Warwickshire University Hospital Trust
References
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