Basis of physiopathology in a novel murine model of autosomal dominant Retinitis Pigmentosa type 10 by mutations in Inosine Monophosphate Dehydrogenase 1.

Abstract

Purpose : The mechanisms underlying the physiopathology of autosomal dominant retinitis pigmentosa type 10 (adRP10) caused by mutations in Inosine Monophosphate Dehydrogenase 1 (IMPDH1) remain unknown, and there is no current treatment for this disease. We established a mouse model of the disease to test the hypothesis that retinal neurodegeneration is due to constitutive activity of the enzyme due to the loss of GDP/GTP allosteric inhibition.

Methods : We introduced the D226N mutation in the endogenous murine Impdh1 gene by CRISPR/Cas9. We performed a time-course analysis of retinal morphology and visual function by electroretinography (ERG) of mutant and control mice. We assessed IMPDH1 cytoophidia formation by IMPDH1 immunostaining in retinal sections and by IMPDH1 immunoblot of retinal homogenates fractionated by size exclusion chromatography. We determined the total levels of GDP and GTP in retinal homogenates by high pressure liquid chromatography (HPLC).

Results : Mice expressing the D226N mutation in heterozygosis showed a progressive retinal degeneration that reduced the number of photoreceptor cells by 35% at postnatal day 40, and by >50% by seven months of age (n=7). Homozygous mice showed a loss of 35% of photoreceptor cells at p25 and of > 60% at p40 (n=7). The rate of retinal degeneration correlated with the loss of visual function by ERG. Irreversible cytoophidia formation in D226N/IMPDH1-expressing retinas was noticeable in heterozygous mice (n=7) and obvious in homozygous mice (n=7), independently of the dark/light-adaptation conditions; while control wild type mice only showed reversible cytoophidia after long periods of bright light exposure (n=3). The D226N mutation shifted the IMPDH1 protein to higher molecular weight fractions in gel filtration analysis of retinal homogenates (n=3). D226N/IMPDH1 expression led to substantially increased GDP and GTP levels in retinal homogenates, compared to wild type mice (n=9).

Conclusions : We have established a murine model of adRP10 that mimics the disease in patients. Our results suggest that Impdh1 mutations at the Bateman domain result in cytoophidia formation and constitutive enzymatic activity in vivo, due to a reduction of the GDP/GTP allosteric inhibition. This mouse model will allow us to test novel therapeutic approaches for phenotypic rescue, using methods transferable to patients.

This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.