Project description:Stargardt disease (STGD1) is an inherited retinopathy affecting approximately 1:8,000 individuals. It is characterised by biallelic mutations in ABCA4, and encodes a vital protein for the recycling of retinaldehyde in the retina. Despite its prevalence and impact, there are currently no treatments available for this condition. Furthermore, 35% of STGD1 cases remain genetically unsolved. Efforts have been directed towards comprehending the fundamental disease mechanisms and identifying all disease-causing variants in the extensive 150kb gene, to better aid with the development of effective therapeutic strategies. In this study, we generated iPSC lines from two monoallelic (PT1 & PT2), late onset STGD1 cases with the heterozygous complex allele - c.[5461-10T>C;5603A>T]. We differentiated these cells alongside a biallelic affected control (AC) - c.4892T>C, and c.4539+2001G>A, to retinal organoids (ROs) allowing us to investigate cellular and molecular characteristics associated with STGD1. We hypothesized that the missing inheritance in our monoallelic cases is due to an RNA defect. Consequently, we utilised a myriad of sequencing strategies including WGS, single-cell RNA sequencing (scRNAseq) and long-read RNA sequencing (LRS) to address this. ROs were grown for 230 days and developed all key retinal neurons with photoreceptor outer segments capable of ABCA4 expression. We observed patient-specific disruption to lamination with OPN1MW/LW+ cone photoreceptor retention in the RO centre during differentiation. Retention was more severe in the AC case affecting both cones and rods, suggesting a genotype/phenotype correlation. scRNAseq suggests retention may be due to the induction of apoptosis in photoreceptors. WGS successfully identified the missing alleles in both cases; PT1 reported c. 5603A>T in homozygous state and PT2 uncovered a rare hypomorph - c.4865T>C. Furthermore, ROs were able to recapitulate the retina-specific splicing defect in PT1 as shown by LRS data. Collectively, these results highlight the suitability of ROs in STGD1 modelling. Their ability to display genotype-phenotype correlations enhances their utility as a platform for therapeutic development. Importantly, both PT1 and PT2 cases were genetically resolved in this study, providing two more individuals their confirmed genetic diagnosis.
Project description:Stargardt retinopathy is an inherited form of macular degeneration caused by mutations in gene ABCA4 and characterized by the accumulation of lipid-rich deposits in the retinal pigment epithelium (RPE), RPE atrophy, and photoreceptor cell death. Inadequate mechanistic insights into pathophysiological changes occurring in Stargardt RPE have contributed to its lacking treatments. Here we show that ABCA4 knockout or Stargardt patient’s induced pluripotent stem cells-derived RPE (STGD1-iRPE) differentiate normally but display intracellular lipid and ceramide deposits reminiscent of the disease phenotype. STGD1-iRPE also shows defective photoreceptor outer segment (POS) processing and reduced cathepsin B activity, indicating higher lysosomal pH. Lipid deposits in STGD1-iRPE are reduced by increasing the activity of ABCA1, a lipid transporter, and ABCA4 ortholog. Overall, our work suggests that ABCA4 is involved in POS and lipid handling in RPE cells and provides guidance for ongoing gene therapy approaches to target both RPE and photoreceptor cells for an effective treatment.
Project description:The intestinal epithelium has a remarkably high turnover in homeostasis. It remains unresolved how turnover is orchestrated at the cellular level and how the behaviour of stem and progenitor cells ensures tissue maintenance. To address this, we combined quantitative fate mapping in three complementary mouse models with mathematical modelling and single-cell RNA sequencing. Our integrated approach generated a spatially and temporally defined model of crypt maintenance that is based on two cycling populations: crypt-based columnar (CBC) and transit amplifying (TA) cells. Validation experiments substantiated the predictions from the model revealing TA cells as the major contributor to the absorptive lineage, while balanced CBC cell fate choices controlled the numbers of cells in the secretory lineage. By unravelling these mechanisms, we gain insights into the process of tissue turnover and provide direct evidence to support the notion of CBC cells as the major driver of the intestinal epithelium replenishment.