Description
Introduction: Retinitis Pigmentosa (RP) is one of the most common forms of inherited retinal dystrophy (IRD) and a leading cause of blindness in the working-age population. Disease progression can be exacerbated by environmental light exposure. Variants in the RHO gene encoding rhodopsin, the light-sensitive G protein-coupled receptor that initiates phototransduction, are a major cause of autosomal dominant RP. The RHOM39R variant is one of the most common in the UK and is associated with sector RP, where degeneration affects regions of the retina most exposed to light. While RhoM39R mouse models confirm strong light-induced degeneration, the lack of appropriate human models has limited mechanistic insight and therapeutic development.
Methods: Human retinal organoids were generated from induced pluripotent stem cells carrying the RHOM39R variants, including patient-derived heterozygous, CRISPR-corrected isogenic controls and gene-edited heterozygous and homozygous lines. iPSC were differentiated to retinal organoids and characterised by immunohistochemistry (IHC). A light damage assay was established using four conditions: ambient light ± 9-cis retinal and bright light ± 9-cis retinal. Cell death was assessed by TUNEL reactivity.
Results: Both heterozygous and homozygous RHOM39R organoids showed phenotypic differences compared to isogenic controls, including altered brush border organisation. Homozygous RHOM39R organoids exhibited rhodopsin mislocalisation, with reduced levels of rhodopsin in the outer segment and retention in the inner segment and outer nuclear layer. Bright light exposure in the presence of 9-cis retinal significantly increased TUNEL-positive nuclei in both patient-derived and gene-edited heterozygous organoids across media conditions, compared to all other conditions. No significant differences were observed in control or homozygous organoids.
Conclusions: These findings demonstrate that human retinal organoids can model key features of RHOM39R associated light-induced degeneration. The degeneration observed in heterozygous organoids highlights the role of light in driving disease pathology. In contrast, the absence of a light damage phenotype in homozygous organoids is consistent with rhodopsin mislocalisation limiting light activation. This model provides a platform to study human photoreceptors responses to light-induced stress and to support the development of strategies to protect photoreceptors.
Lay Abstract
Inherited retinal dystrophies (IRDs) are a group of genetic eye diseases and a leading cause of blindness in working-age individuals. In some forms, such as retinitis pigmentosa (RP), vision loss can be worsened by exposure to bright light. Photoreceptors are the cells in the retina responsible for detecting light. They rely on rhodopsin, a light-sensitive protein, to function properly and genetic changes in rhodopsin cause RP.
In this project, we use human stem cells to generate retinal organoids, often referred to as “retina-in-a-dish” models. These allow us to study how human photoreceptors behave in both healthy and disease conditions. We developed a controlled light exposure system to investigate how different light conditions affect these cells.
Our results show that retinal organoids carrying a disease-causing rhodopsin variant RHOM39R, are more vulnerable to bright light, with increased levels of death compared to healthy organoids.
This human model provides a valuable platform to study how light contributes to retinal degeneration. Future work will use advanced molecular approaches to identify the key biological pathways involved, with the aim of discovering new therapeutic targets to protect photoreceptors and preserve vision.
| Lay Title | Studying how light influences inherited retinal disease |
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| Role | Research Assistant |