TULP1 Mutations Cause Variable Retinal Phenotypes and ER-UPR Activation

Abstract

Purpose : Mutations in TULP1 are associated with early-onset retinitis pigmentosa (RP). To better recapitulate RP patients’ gene dosage and spatiotemporal degeneration, and to determine the pathologic disease mechanism, we generated two novel knock-in mouse models each expressing a human RP-causing missense TULP1 mutation. We compared their retinal phenotype to tulp1^-/- and WT mice.

Methods : Tulp1 knock-in mice were generated by CRISPR-Cas9 gene editing. The tulp1^F491L mouse contains a mutation affecting a conserved amino acid in the tubby domain, whereas the tulp1^D94Y mouse contains the only known mutation located outside the tubby domain. Phenotypic analyses of the retinas were evaluated by histology, IHC, Western blot and ERG.

Results : In both mutant retinas, Tulp1 levels and distribution were comparable to WT. Retinal morphology was preserved in tulp1^D94Y mice up to 1 year, although we noted less RPE pigmentation, dilated structures in the OPL, and a slight reduction in ERG b-wave amplitude. In contrast, tulp1^F491L mice displayed rapid photoreceptor degeneration, similar to tulp1^-/- mice. Rhodopsin and cone opsins, but not peripherin, were mislocalized in tulp1^F491L retinas. In addition, tulp1^F491L retinas displayed punctate Ribeye staining and abnormal shaped ribbons. Building on prior in vitro observations that Tulp1 mutations activate the endoplasmic reticulum (ER)-unfolded protein response (UPR) complex, we noted activation of the PERK-eIF2α branch of the ER-UPR pathway in tulp1^−/− and tulp1^F491L retinas.

Conclusions : Our results show that Tulp1 mutations lead to variable retinal phenotypes in mice. The conserved tubby domain of Tulp is involved in ciliary trafficking of integral membrane proteins in multiple cell types. Our results support this interpretation in photoreceptors by showing early-onset, photoreceptor degeneration and mistrafficking of OS proteins in tulp1^F491L mice. In contrast, tulp1^D94Y mice, carrying a mutation outside of the tubby domain, display a late-onset, mild phenotype primarily affecting the photoreceptor synapse, thereby suggesting a novel function for this domain. Importantly, our in vivo results demonstrate that the ER-UPR pathway, a known mechanism of apoptosis, is involved in Tulp1-associated photoreceptor degeneration, suggesting a potential therapeutic target for this form of RP.

This abstract was presented at the 2023 ARVO Annual Meeting, held in New Orleans, LA, April 23-27, 2023.