RNAi-mediated lowering of transthyretin in liver leads to decreased accumulation of ocular A2E in Abca4-/- mice

Abstract

Purpose : Stargardt disease, an autosomal recessive retinal degenerative disease caused by loss of function in the Abca4 gene, is the leading cause of inherited childhood blindness. There are currently no approved therapies. The Abca4 gene encodes a Rim protein that transports all-trans-retinal out of the photoreceptor disc and its loss of function leads to accumulation of toxic bisretinoids, including N-retinylidene-N-retinylethanolamine (A2E). The majority of vitamin A is delivered to the eye bound to retinol binding protein 4 (RBP4) in a complex with transthyretin (TTR). In the absence of TTR, RBP4 is cleared by the kidneys, therefore we evaluated whether RNAi-mediated lowering of TTR alters circulating retinol and ocular A2E in Abca4-/- mice as a model of Stargardt disease.

Methods : All animal procedures were performed at EyeCRO and conformed to recommendations of the American Veterinary Medical Association Panel on Euthanasia, and ARVO. The TTR RNAi was delivered via subcutaneous injection (3 mg/kg, every 21 days) for 16 weeks. Quantification of A2E was performed on dissected posterior eyecups consisting of RPE, choroid, retina, and sclera, and quantified by HPLC-MS/MS. Plasma and liver TTR and RBP4 levels were measured by ELISA and circulating retinol was measured by an LC-MS/MS assay.

Results : Knockdown of TTR via RNAi lowered liver TTR mRNA levels by >95% at week 16 and plasma TTR protein levels by >90% as early as 21 days post-dose. Plasma RBP4 levels were also lowered by 90% relative to baseline over the course of the 16-week study. Less than 10% of plasma retinol levels remained relative to baseline in the TTR RNAi group. A significant reduction of ocular A2E levels was observed after 16 weeks of TTR RNAi treatment compared to no treatment in Abca4-/- mice.

Conclusions : Targeting TTR in the liver could be a valuable approach for impeding retinol delivery to the eye and accumulation of toxic bisretinoids, such as A2E, in Stargardt disease.

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

 

A2E accumulation was significantly decreased with TTR knockdown in Abca4-/- mice after 16 weeks.