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Inherited retinal diseases (IRDs) are a spectrum of sight-threatening genetic eye disorders caused by genetic mutations in genes such as RPE65, a gene critical for protein synthesis in normal vision. Studying various mutations in genes associated with IRDs is crucial for a better understanding of these rare genetic diseases, especially among Indians who are an underrepresented demographic in global estimates and studies.
Dr. Chitra Kannabiran, a veteran scientist at LVPEI studying the molecular genetics of eye diseases and Dr. Deepak Kumar Bagga, a consultant optometrist, discuss their latest research paper which describes a case series of various mutations in the RPE65 gene observed in patients with Leber congenital amaurosis, early-onset severe retinal dystrophy, and retinitis pigmentosa.
Dr. Kannabiran, what got you interested in the molecular genetics of eye diseases? How has it changed over time?
Dr. Kannabiran: Before I joined LVPEI in 1996, I was working in the field of nuclear immunochemistry at the Indian Institute of Science (IISc), and then, control of gene expression by adenoviral E1A oncoprotein at the Cold Spring Harbor Laboratory, New York. When I heard that LVPEI was going to open a molecular genetics research unit to study eye diseases, I thought it would be a new and interesting field to pursue.
Less than a decade before I was a PhD student at IISc, the advent of molecular cloning (cloning a gene) was the most happening thing in biology. The idea of manipulating genes and putting them into a system of your choice was so big that it gave molecular biology a distinct identity from biochemistry, such as studying proteins or nucleic acids. Back then, such studies drew my interest. Over time, my interests shifted to ocular diseases. I think it is natural for research interests to change with time. It helps keep things fresh.
Dr. Bagga, what are your research interests? How does your area of clinical expertise overlap with your research interests? Is there an interplay between the two?
Dr. Bagga: My research interests are related to the functioning and rehabilitation of people with poor vision, especially in those with retinal dystrophy and IRDs. When examining patients with low vision, the focus is not only on the structural defects and how they negatively impact the functioning of the eye, but also how the disease, and other contextual factors impact the patient’s daily life, including learning, education, their work, and leisure activities. At LVPEI’s institute for vision rehabilitation, we assess vision-related limitations in daily life, explore barriers and enablers for functioning of the individuals, visual functions, and then we try to address affected areas of functioning to improve the patient’s overall quality of life.
We observe patterns in how ocular diseases impact the patients’ lives, what are functions that get affected, and what types of assistive technology are effective and have a high patient preference. In that way, what I observe in the clinic is directly used in my research to draw conclusions and design interventions that can be applied to rehabilitate future patients.
How did this project begin? What were some of the questions that led to it?
Dr. Kannabiran: The retinal pigment epithelium 65 kilodalton protein (RPE65) gene encodes a protein of 65 kilodalton (mass) that is expressed in the retinal pigment epithelium. RPE65 has already been studied in various populations, so it is known that mutations in RPE65 can lead to a spectrum of IRDs. But there is a dearth of information when it comes to RPE65 mutations among Indians. In fact, information about the genetics of ocular diseases among Indians, in general, is scarce. We wanted to know what the genetic scenario for IRDs in Indians is. Moreover, the initial clinical trials on the RPE65 gene therapy were published in 2008. That also piqued our interest, because now there is a treatment option on the horizon.
Dr. Bagga: IRDs have no effective medical treatment as of now, at least in India. So, the only means of managing such cases is low vision assessment and patient rehabilitation. I got involved in this project to understand what kind of change in visual acuity, rehabilitation goals and management were provided to the study cohort over a period. If not, what modifications can be made to their vision rehabilitation?
Why is RPE65 such a crucial gene in vision?
Dr. Kannabiran: In the eye, vitamin A starts off as 11-cis-retinal, a certain isomeric configuration, and gets transformed into another form (all-trans-retinyl) when light hits the retinal photoreceptors. This is necessary for converting light into electrical signals that our brain can interpret. The RPE65 protein is an isomerase enzyme that converts all-trans-retinyl back to 11-cis-retinal. As such, the gene is essential for vision.
Does parental consanguinity play a key role in the manifestation of IRDs? Are there also other factors at play?
Dr. Kannabiran: It does play a role. For a genetically recessive disease to manifest, a child must inherit two copies of that mutant gene—one from each parent. The chances of both parents being a carrier of the mutant gene is much higher if they are related to each other. Consanguinity is quite common in South India and increasing the risk of autosomal recessive disorders manifesting.
A highly inbred population—not consanguineous as such, but reproductively isolated—is also at risk of recessive genetic disorders, like IRDs. This can also happen due to strict intra-caste marriages.
Only 3.6% of patients in the study had RPE65 mutations? What are the major factors behind IRDs if RPE65 gene mutation is such a small percentage?
Dr. Kannabiran: In this study, we investigated only RPE65. There are several other genes—most of which are yet to be characterized—whose mutations can lead to IRDs including Leber congenital amaurosis, retinitis pigmentosa (RP), and Early-Onset Severe Retinal Dystrophy (EOSRD). For instance, it is known that mutations in over 50 other genes can lead to LCA.
The Indian population is a huge genetic canvas. Even with access to next-generation sequencing (NGS), where we can sequence multiple genes at once, it is a challenging task to pinpoint every gene responsible. I suspect that no more than 40-50% of patients with IRDs have mutations in the genes that have been studied. The rest could be because of genes whose role in IRDs have yet to be established.
How are IRDs usually diagnosed? What are some typical symptoms associated with them? Was the disease phenotype different in patients with RPE65 mutations?
Dr. Bagga: Patients with IRDs often present with vision problems such as night blindness or progressive loss in their visual field. IRDs are diagnosed after studying clinical images of the retina. We look for the presence of bony spicules, aberrant retinal pigmentation due to photoreceptor cell death, and a waxy pallor in the optic disc, which can indicate damage to the visual pathway.
Dr. Kannabiran: IRDs are progressive. Retinal morphology and the visual parameters change over time. There are no phenotypes that are distinct for RPE65 mutations. Rather, the phenotypes are distinct for each type of IRD. As in, the phenotypes can overlap for mutations in different genes in an IRD.
How can the findings of this study help in the diagnosis and treatment of IRDs?
Dr. Kannabiran: This study is just one step towards understanding the frequency of various genetic mutations—in this case, RPE65—in IRDs and the relative importance of that gene in causing IRDs.
Dr. Bagga: This study gives us longitudinal data—one patient had 11 years of follow-up and two patients had more than 25 years of follow-up—on progressive vision loss in the patients. The data helps us understand how gradual vision loss affects the child’s daily life and functioning, which allows us to adapt rehabilitation goals, and strategies to rehabilitate. Therefore, frequent follow-us with vision rehabilitation services is important to address these changes in visual functioning, rehabilitation goals and suggest if any modification in rehabilitation plan needs to be made.
Do you feel that the treatment will be available in India soon? How effective is it?
Dr. Kannabiran: The RPE65 gene therapy is being marketed as Luxturna™ by Spark Therapeutics. Gene therapy is not available in India yet, but it could be in the future. However, the cost is prohibitive, approximately USD 850,000 for both eyes. Despite the hurdle of affordability, this technology needs to evolve and become applicable to other genes involved in IRDs.
It is also important to understand that gene therapy can only stall the degeneration of photoreceptor cells. It cannot replace the ones already lost. As such, the efficacy of gene therapy depends on when the intervention was made.
What is the next step for this project? What other projects are you working on?
Dr. Kannabiran: We are using NGS to test other genes for their roles in retinal dystrophies. We use targeted panels consisting of multiple genes known to be associated with IRD and screen them for mutations. The process allows us to screen hundreds of genes in parallel, saving time and money.
Dr. Bagga: One of our projects is to assess how the clinical measures of functioning differ in various ocular diseases causing low vision in children. For example, the visual acuity may be similar in patients with different retinal diseases, but their field of vision or contrast sensitivity might be different. How do these differences in functioning affect the patient’s daily functioning? Knowing these details will help us tailor our rehabilitation program to the specific needs of the patient.
Drs. Chitra Kannabiran and Deepak K. Bagga spoke to Sayantan Mitra, Science Writer, LVPEI. Read more about their research here.
Citation
Parameswarappa, D. C., Bagga, D. K., Upadhyaya, A., Balasubramanian, J., Pochaboina, V., Muthineni, V., Jalali, S., & Kannabiran, C. (2024). RPE65 mutations in Leber congenital amaurosis, early-onset severe retinal dystrophy, and retinitis pigmentosa from a tertiary eye care center in India. Ophthalmic Genetics, 1–10. Advance online publication. https://doi.org/10.1080/13816810.2024.2309559
