Unraveling genes, mutations in inherited retinal diseases

Reviewed by Stephanie Hagstrom, PhD

Investigators are beginning to crack the 200 genes and thousands of mutations that cause inherited retinal dystrophies, characterized by progressive photoreceptor degeneration and vision loss. Some of these more well-known diseases are retinitis pigmentosa (RP), Leber congenital amaurosis, and cone-rod dystrophy.

These diseases differ from each other in the times of onset during patients’ lives and the rates of progression. They result in progressive loss of peripheral vision, temporarily conserved central vision, and ultimately total vision loss.

The broad range of heterogeneity is the challenge in finding treatments for these diseases, according to Stephanie Hagstrom, PhD, an associate professor of ophthalmology in the Department of Ophthalmic Research at the Cole Eye Institute at Cleveland Clinic in Ohio.

“The goals are to not only identify mutations but also identify the functional consequences of these mutations—ie, the disease mechanisms—to target specific pathways to generate therapies to slow or halt the vision loss,” she said.

TULP1 mutations

TULP1 is a photoreceptor-specific protein in the inner segment, connecting the cilium and synaptic terminals. A Tulp1-/- mice model is characterized by early-onset, progressive photoreceptor degeneration that is similar to that in humans with RP. Hagstrom described that the mice show mislocalization of outer segment proteins. The synapses do not have a tight spatial relationship between the ribbon proteins.

In their research, Hagstrom and colleagues generated and studied 2 homozygous knock-in mouse models (D94Y and F491L) that each express a TULP1 missense mutation that causes RP in humans. The difference between this research and the previously described mouse model is that the current models have a missense mutation and are not completely lacking the TULP1 gene, which facilitated a study that could be compared with the knockout model and wild-type mice to determine the mechanism of photoreceptor cell death.

In the first experiment, they found that in the wild-type and mutant mice, TULP1 is localized correctly and is not in the knockout model, as expected. The following study of retinal morphology showed that on postnatal day 17, all mutant murine retina had relatively normal photoreceptors. On postnatal day 60, the F491L model mutant had degenerating retinas, and the inner and outer segments were absent. In the D94Y model, the scenario was similar to the wild-type mice. “These results suggested the disease severity varies with the genotype in these TULP1 models,” Hagstrom said.

In a study of retinal function using electroretinography, on postnatal day 17, the investigators observed an overall reduction in the TULP1 knockout response and in the F491L model compared with the wild type. On postnatal day 60, the a- and b-waves both decreased with increasing age.

The next experiment evaluated the localization of several outer segment-specific proteins, which had been shown to be mistrafficked in the TULP1 knockout retina. Rhodopsin was trafficked correctly in the wild-type and the D94Y retinas and was expressed in the outer segments. However, in the F491L and knockout models, rhodopsin was expressed in different photoreceptor compartments.

Evaluation of the cones showed that cone opsins were correctly trafficked and located in the outer segments of the D94Y and wild-type models but were mislocalized throughout the photoreceptor compartments in the F491L mutant, similar to the knockout model.

In late-stage studies, the D94Y mice were evaluated at 12 and 18 months to determine whether photoreceptor degeneration ultimately developed. The 1-year results showed minimal changes between the D94Y retina compared with the wild-type retinas. Less pigmentation was seen in the retinal pigment epithelial cells. A few dilated structures in the synapses led to evaluation of the electroretinography of the mice, which showed an a-wave comparable with that of the wild-type retinas. However, in the D94Y mice, the b-wave was slightly reduced in dark- and light-adapted conditions, indicating a defect in the photoreceptor to bipolar cell neuronal transmission.

“Evidence indicates these misfolded and mistrafficked proteins initiate a pathological process—the endoplasmic reticulum unfolded protein response that disrupts these pathways—leading to stress and eventual neuronal cell death,” Hagstrom said.

Based on the results, the investigators concluded that the 2 mutations cause variable retinal phenotypes, with each expressing a different mutation, causing RP. Investigation of the mechanism showed activation of the endoplasmic reticulum stress pathway in the F491L model but not the D94Y model, indicating that different mutations in the same gene may cause cell death because of different disease mechanisms and may then require unique therapies.

Stephanie Hagstrom, PhD

E: hagstrs@ccf.org

This article is adapted from Hagstrom’s presentation at the Cleveland Eye Bank Foundation Virtual Vision Symposium. She has no financial interest in this subject matter.