TULP1 Missense Mutations Cause Variable Retinal Phenotypes

Abstract

Purpose : TULP1 is a photoreceptor-specific member of the Tubby-like protein family. Each member possesses a highly conserved C-terminal tubby domain while the N-terminus is divergent. We determined that mutations in TULP1 were a cause of autosomal recessive retinitis pigmentosa (RP). Interestingly, all but one of the pathogenic missense mutations are located in the C-terminus of the protein. To gain insights into the pathogenicity of the N-terminal missense mutation and a C-terminal missense mutation, we generated two knock-in mouse models each expressing a human RP-causing TULP1 mutation and compared the retinal phenotype to tulp1^-/- and WT mice.

Methods : Tulp1 knock-in homozygous mutant mice were generated using CRISPR-Cas9 gene editing technology. The D94Y mutation was chosen because it is the only known missense mutation located outside of the tubby domain, whereas the F491L mutation was chosen because of its prevalence in patients with RP and location within the tubby domain. Phenotypic analyses of the retinas at several time points were evaluated by histology, immunohistochemistry, electroretinography, and in vivo imaging.

Results : Tulp1 localization in both tulp1^D94Y and tulp1^F491L retinas was expressed in the IS, CC and synapse of the photoreceptors, similar to WT. Characterization of the retinal morphology and function in tulp1^D94Y mice revealed an absence of photoreceptor cell degeneration up to one year of age. However, phenotypic characterization of the retinas of tulp1^F491L mice revealed a rapid and progressive photoreceptor degeneration, similar to the time course seen in tulp1^-/- mice. Expression of endoplasmic reticulum (ER) stress markers in the retinas of tulp1^F491 mice was detected early in the disease time course.

Conclusions : We generated two novel Tulp1 mouse strains. Our data indicate that the D94Y mutation protein does not cause photoreceptor degeneration in mice, questioning whether this variant is truly a pathological human mutation. In contrast, the F491L mutation produced an early-onset, progressive photoreceptor degeneration and triggered activation of ER stress markers. Further studies are ongoing to evaluate the activation of multiple cellular stress response pathways. Knock-in mice, harboring patient-specific mutations, provide additional animal models to study the pathological mechanism of TULP1-related RP and will be invaluable for the development of therapeutic interventions.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.