Inherited retinal disorders (IRDs) are estimated to affect millions of individuals on a global scale, in many instances leading to significantly reduced vision or blindness. The term “IRD” acts as an umbrella label for many different types of ocular disease and, on a genetic level, it is estimated that almost 200 distinct genes are the causative agent behind one or more inherited retinal pathologies. Retinitis pigmentosa (RP), another large collection of ocular disorders, accounts for approximately 50% of known IRD cases. RP is probably one of the most heterogeneous diseases recorded to date, arising from mutations in more than 50 genes with over 3,000 mutations reported by mid-2013. In addition, many syndromic forms of RP also exhibit genetic heterogeneity which, when combined with incomplete penetrance and clinical heterogeneity, result in a significant medical challenge in terms of devising optimised clincal care strategies. While RP can be inititiated (literally) in thousands of different ways, the course of disease generally progresses through an increasing level of rod photoreceptor cell loss followed by cone photoreceptor degerenation. As such, most cases of RP have a common degenerative pathway of cone cell death following closely on loss of the rod cell population. This common end-stage for the disease has given rise to a therapeutic strategy in which rod cells are “reprogrammed” to direct the cells down a cone cell fate. Directing cells away from rod differentiation and toward cone cell differentiation may, in theory, avoid the molecular pathology arising from mutations in rod-specific genes. To test such a hypothesis researchers at the Washington University School of Medicine carried out just such an experiment on animal models of RP.
Led by Drs. Cynthia Montana and Joseph Corbo, the US-based research reported the rescue of a retinal degeneration by the reprogramming of rods into cones. The study used a well-characterized retinal transcription factor to re-direct rods to a cone cell fate. The principle itself of course is not new. Conversion of one differentiated cell type into another has been carried out in other contexts, including the conversion of pancreatic exocrine cells into β-cells or, auditory endothelial cells into hair cells and fibroblasts into neurons. Of course rod reprogramming is quite different in that the conversion itself would result in a loss of rod function followed by consequent night blindness. However, most individuals might agree that night blindness is an acceptable risk in the context of maintaining a healthy cone cell population and functional photopic vision. The result of the cellular reprogramming in this case reduced rod photoreceptor cell death in a rhodopsin knock-out model of retinitis pigmentosa (RP). As highlighted in the human pathology, the loss of the rod cell population has a deleterious impact on the cones which subsequently degenerate leaving patients with both reduced or absent photopic and scotopic vision. The results of the research suggest that maintaining a rod photoreceptor cell architecture may be sufficient to slow or halt cone cell degeneration.
To achieve their results the research team exploited the biology of the neural retina-specific leucine zipper (Nrl) transcription factor, known from many previous studies to determine photoreceptor cell fate in the retina. Under normal conditions, photoreceptor precursors expressing Nrl become rods whereas those without Nrl progress to a cone photoreceptor lineage. To knock out Nrl in a model of retinitis pigmentosa the research team generated a conditional Nrl knock out controlled through a tamoxifen inducible promoter. A daily injection of 4-hydroxyl-tamoxifen (4-OHT), between postnatal day 42 and 44, caused inactivation of the Nrl transcription factor resulting in the reprogramming of rods to a cone cell fate. Assays of the reprogrammed pseudo-cones demonstrated a significant decrease in the scotopic ERG response suggesting a loss of rod function, while genetic analysis indicated a loss of expression of rod-specific genes, including rhodopsin, with a parallel activation of cone specific genes.
While the reprogramming of rods into cones was only partial and dependent on the timing of Nrl knock-out, the strategy nevertheless rescued the cone cell population within a model of RP. Keeping the rod cells alive, even dysfunctional rods harbouring mutations that cause RP, appeared to be sufficient to maintain the retinal architecture and thereby support cone cell survival. In concluding their research the authors commented that, “apart from providing insights into the plasticity and maintenance of rod photoreceptor identity, th[e] study demonstrated that partial rod-to-cone reprogramming can forestall retinal degeneration in the Rho−/− model of retinitis pigmentosa. Although these cells are not true cones, they exhibit sufficient down-regulation of rod-specific genes to resist the deleterious effects of a rod-specific mutation”. In addition, the research team suggests that the re-programming approach may also be employed to generate a novel cone cell population that could serve to rescue other retinal degenerative conditions, including age-related macular degeneration. While the results indicate the potential for this strategy in the rhodopsin knock-out model, other models of RP, including dominant forms, will need to be tested. Finally, if such a therapeutic approach is ever to find application in a human population then a virally delivered approach to Nrl knockdown will be required. Thankfully, the success of AAV gene delivery to the retina in recent years has been a considerable success providing the necessary platform for an entrepreneurial drive to take the next steps.