Pluripotent stem cell-derived models of retinal disease: Elucidating pathogenesis, evaluating novel treatments, and estimating toxicity.

Blindness poses a growing global challenge, with approximately 26% of cases attributed to degenerative retinal diseases. While gene therapy, optogenetic tools, photosensitive switches, and retinal prostheses offer hope for vision restoration, these high-cost therapies will benefit few patients. Understanding retinal diseases is therefore key to advance effective treatments, requiring in vitro models replicating pathology and allowing quantitative assessments for drug discovery. Pluripotent stem cells (PSCs) provide a unique solution given their limitless supply and ability to differentiate into light-responsive retinal tissues encompassing all cell types. This review focuses on the history and current state of photoreceptor and retinal pigment epithelium (RPE) cell generation from PSCs. We explore the applications of this technology in disease modelling, experimental therapy testing, biomarker identification, and toxicity studies. We consider challenges in scalability, standardisation, and reproducibility, and stress the importance of incorporating vasculature and immune cells into retinal organoids. We advocate for high-throughput automation in data acquisition and analyses and underscore the value of advanced micro-physiological systems that fully capture the interactions between the neural retina, RPE, and choriocapillaris.

Clonal expansion of secondary mitochondrial DNA deletions associated with spinocerebellar ataxia type 28.

IMPORTANCE: Progressive external ophthalmoplegia (PEO) is a common feature in adults with mitochondrial (mt) DNA maintenance disorders associated with somatic mtDNA deletions in muscle, yet the causal genetic defect in many patients remains undetermined. OBSERVATIONS: Whole-exome sequencing identified a novel, heterozygous p.(Gly671Trp) mutation in the AFG3L2 gene encoding an mt protease--previously associated with dominant spinocerebellar ataxia type 28 disease--in a patient with indolent ataxia and PEO. Targeted analysis of a larger, genetically undetermined cohort of patients with PEO with suspected mtDNA maintenance abnormalities identified a second unrelated patient with a similar phenotype and a novel, heterozygous p.(Tyr689His) AFG3L2 mutation. Analysis of patient fibroblasts revealed mt fragmentation and decreased AFG3L2 transcript expression. Western blotting of patient fibroblast and muscle showed decreased AFG3L2 protein levels. CONCLUSIONS AND RELEVANCE: Our observations suggest that AFG3L2 mutations are another important cause, albeit rare, of a late-onset ataxic PEO phenotype due to a disturbance of mtDNA maintenance.