Complement factor B is necessary for sub-RPE deposit formation in Efemp1^R345W/R345W knock-in mice

Abstract

Purpose : The EFEMP1 R345W mutation in humans causes Doyne honeycomb retinal dystrophy (DHRD), a disease with similar pathology to AMD. Efemp1^R345W/R345W knock-in mice (Efemp1 mice) develop deposits on the basal side of retinal pigment epithelial (RPE) cells, which are dependent on the presence of complement component C3.¹ The role of the alternative complement pathway component factor B (Cfb) was investigated in Efemp1 mice.

Methods : Efemp1 mice were crossed with Cfb^-/- mice. Eighteen month old wild type, Efemp1, Cfb^-/-, and Efemp1:Cfb^-/- mice were characterized for complement activation via Western blots, sub-RPE deposit formation via transmission electron microscopy, and tandem mass tag-based proteomic profiling (IQ Proteomics). Ten month-old female Efemp1 mice were dosed orally daily with a complement factor B inhibitor (compound 1) at 60 mg/kg for two months. The area of basal deposit was evaluated by quantifying electron microscopy images using ImageJ.

Results : C3 and FB breakdown products iC3b and Ba were elevated two-fold (p<0.05) in the eyes of Efemp1 mice, while plasma complement levels are unchanged. Sub-RPE deposits detected in Efemp1 mice were not observed in Efemp1:Cfb^-/- mice. Proteomics analysis of eye lysate identified increased levels of proteins in vesicle transport, endoplasmic reticulum (ER) protein processing, and extracellular matrix (ECM) organization pathways and reduced levels of proteins in mitochondria/metabolism pathways in Efemp1 mice compared to wild-type littermate controls. In Efemp1:Cfb^-/- mice, these pathways are more similar to wild-type mice. Treatment of Efemp1 mice with the FB inhibitor compound 1 from 10 to 12 months of age reduced the amount of sub-RPE deposits by 65% (p=0.051).

Conclusions : Efemp1 mice have increased ocular complement activation, altered ER protein processing, reduced mitochondria metabolism, ECM dysregulation and basal RPE deposits, confirming some of previous observations.¹ Basal RPE deposits are early features of AMD and may be a consequence of the pathways identified in this proteomic study. Inhibition or deletion of FB reduces deposit formation and reversed multiple protein changes in the eyes of Efemp1 mice implicating a broad therapeutic effect from reducing alternative complement pathway activation. Inhibition of the alternative pathway is an attractive strategy to treat early AMD and DHRD.

1. Garland DL,et al. Hum Mol Genet 2014;23:52-68.

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