Characterizing two novel mouse models of Ctrp5/C1qtnf5 to understand disease mechanism in late-onset retinal degeneration.

Abstract

Purpose : Late Onset Retinal Degeneration (L-ORD) is an autosomal dominant macular dystrophy which results from mutations in Ctrp5/C1qtnf5. A mouse model (Ctrp5^+/-) for the most common S163R mutation has previously been described. It is still unclear whether L-ORD results from a gain or loss of function. We seek to resolve this by studying two novel mouse models with homozygous knock-in (S163R) (Ctrp5^+/+) and homozygous Ctrp5 knock-out (Ctrp5^-/-) genotypes.

Methods : The Ctrp5^+/+ mice were bred from the Ctrp5^+/- mice whilst the Ctrp5^-/- mice were created using Cre mediated homologous recombination. Their genotypes were confirmed by sequencing. Both models were rd1 and rd8 mutation free. Autofluorescence scanning laser ophthalmoscopy, spectral domain optical coherence tomography and electroretinography (ERG) were used for phenotyping at 20 months (mo) of age (n=6). Retinal phenotypes were studied using light microscopic and ultrastructural analysis at 4.5, 6, 11 and 18.5 mo (n=3) in Ctrp5^-/- mice and at 13, 16 and 20 mo in Ctrp5^+/+ mice (n=3). We performed histological analysis for cone opsin counts and CTRP5 and studied deposits using lipid and cholesterol markers Oil red O, Plin2 and Filipin staining using 18-20 mo eyecups (n=3 for each). The findings were compared with age-matched wild-type (WT) and Ctrp5^+/- mice eyecups.

Results : Autofluorescent fundus spots were significantly greater in Ctrp5^-/- and Ctrp5^+/+ mice than in WT (P<0.0001). ERG also demonstrated a significant reduction in photopic and scotopic response in both new models compared with WT. Light microscopy showed increased RPE stress with vacuolization and thinning as early as 6 mo in Ctrp5^-/- mice and 13 mo in Ctrp5^+/+ mice. Ultrastructural analysis revealed the presence of sub-RPE deposits in both new models but histology failed to show any significant differences in lipid or cholesterol markers when compared with WT and Ctrp5^+/- mice. Additionally, a sub-RPE accumulation of CTRP5 was noted only in Ctrp5^+/- mice.

Conclusions : In theses studies we found that Ctrp5^+/+ mice shared retinal pathology with Ctrp5^-/- mice. Additionally, the electrophysiological and ultrastructural findings in Ctrp5^-/- mice are similar to those previously reported in Ctrp5^+/- mice. This suggests that pathology in L-ORD mouse model (Ctrp5^+/-) results from a loss of CTRP5 function.

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