TBC1D32 variants disrupt retinal ciliogenesis and cause retinitis pigmentosa

Abstract

Retinitis pigmentosa (RP) is the most common inherited retinal disease (IRD) and is characterized by photoreceptor degeneration and progressive vision loss. We report 4 patients presenting with RP from 3 unrelated families with variants in TBC1D32 , which to date has never been associated with an IRD. To validate TBC1D32 as a putative RP causative gene, we combined Xenopus in vivo approaches and human induced pluripotent stem cell–derived (iPSC-derived) retinal models. Our data showed that TBC1D32 was expressed during retinal development and that it played an important role in retinal pigment epithelium (RPE) differentiation. Furthermore, we identified a role for TBC1D32 in ciliogenesis of the RPE. We demonstrated elongated ciliary defects that resulted in disrupted apical tight junctions, loss of functionality (delayed retinoid cycling and altered secretion balance), and the onset of an epithelial-mesenchymal transition–like phenotype. Last, our results suggested photoreceptor differentiation defects, including connecting cilium anomalies, that resulted in impaired trafficking to the outer segment in cones and rods in TBC1D32 iPSC-derived retinal organoids. Overall, our data highlight a critical role for TBC1D32 in the retina and demonstrate that TBC1D32 mutations lead to RP. We thus identify TBC1D32 as an IRD-causative gene.

Dual CRALBP isoforms unveiled: iPSC-derived retinal modelling and AAV2/5-RLBP1 gene transfer raise considerations for effective therapy

“…In control iPSC-derived RPE, CRALBP staining was diffused throughout the cytosol but highly concentrated on the inner side of the plasma membrane (Fig. 3C), consistent with our previous reports (28, 29). Unexpectedly, in RPA1 RPE, CRALBP expression was faint and partially mis-localised; the staining was mainly diffused in the cytosol and unevenly distributed near the plasma membrane (Fig.…”

“…This is consistent with prior localisation studies in mouse and rat RPE (54). Interestingly, in the RPE of patient RPA1 (BD), the plasma membrane localisation was lost and CRALBP appeared more cytosolic, which was similar to our previous observations in an iPSC-derived RPE model of another IRD due to TBC1D32 deficiency that also showed a disruption of the visual cycle (28). By contrast, in the RPE of patient RPA2 (RPA), presenting with the mildest clinical form, the dual CRALBP localisation pattern was preserved but CRALBP staining was less intense; no CRALBP expression, irrespective of isoform, was detected in RPA3, consistent with the severe NFRCD phenotype.…”

“…Lastly, we previously showed retinosome accumulation in association with defective polarised secretion in iPSC-derived RPE from TBC1D32-associated IRD (28). Hence, we assayed vascular endothelial growth factor (VEGF), which is primarily secreted basally, and pigment epithelium-derived factor (PEDF), which is primarily secreted apically (33), from control and RLBP1 RPE.…”

A Missense Variant in HACE1 Is Associated with Intellectual Disability, Epilepsy, Spasticity, and Psychomotor Impairment in a Pakistani Kindred

“…In the brain, scRNA data analysis revealed the higher expression overlap of HACE1 with TRRAP, TBC1D32, HTT, RAC1, RAB4A, and PRNP in cortical glia, excitatory neurons, and intermediate progenitor cells. TRRAP is a transformation/transcription domainassociated protein kinase with epigenetic-based transcription activity and acts as a checkpoint in cell division to control chromatin remodeling and repair DNA breaks [18], while TBC1D32 is known to participate in the Hedgehog signaling pathway and regulates the structure of the primary cilium in the neural tube [19]. The HTT huntingtin protein involve in neural development has been shown to dysregulate cell migration, reduce proliferation, and increase cell death in the neocortex of murine Htt knockouts [20,21].…”

Editorial: Novel strategies to target cell death signaling in cancer and neurodegenerative diseases: new findings and mechanistic studies

“…Retinitis pigmentosa (RP) is a group of inherited retinal dystrophies characterized by the progressive loss of rod photoreceptors and the subsequent degeneration of cone photoreceptors ( Bocquet et al, 2023 ; Nguyen et al, 2023 ). The article by Huang et al provided a study of genetic genealogy to identify causative variants in Han-Chinese people with autosomal recessive RP.…”