The mutation R345W in fibulin-3 (EFEMP1) causes Malattia Leventinese (ML) and Doyne's honeycomb retinal dystrophy (DHRD), two autosomal dominantly inherited macular degenerative diseases with strong similarities to age-related macular degeneration (AMD). Both ML/DHRD and AMD are characterized by drusen. Many factors have been implicated in drusen formation such as complement activation and inflammation. However, it is not clear what triggers these systemic processes to become pathogenic and exert a localized effect in the eye. We found that, in fibulin-3 knock-in (KI) mice carrying the R345W mutation, basal laminar deposit (BLD) develops first, and membranous debris accumulates subsequently within the BLD and in Bruch's membrane to form basal linear deposit (BLinD) before drusen develop. Activated complement components were detected in Bruch's membrane prior to drusen in these mice. Similar findings have been reported by others in AMD. The quantity and site of membranous debris closely correlate with the degree of degeneration in AMD donor eyes. These data indicate that BLD is the initial pathological event, and membranous debris is a critical determinant in drusen formation. BLD is composed of basement membrane material produced by the retinal pigment epithelium (RPE). Membranous debris consists of microvesicles shed from RPE cells. Normally these microvesicles are promptly broken down after their release. Why then do basement membrane material and microvesicles accumulate ultimately leading to drusen in fibulin-3 KI mice and patients with ML/DHRD or AMD? Fibulin-3 is an enzyme inhibitor localized in basement membrane. The R345W mutation does not impair fibulin-3's activity, but makes the protein resistant to degradation leading to its accumulation. Fibulin-3 is also induced by stress. In the KI mice and patients with ML/DHRD or AMD, a high level of fibulin-3 is present in Bruch's membrane and it accumulates in BLD, BLinD, and drusen. However, in fibulin-3 null mice, no BLD or other macular degeneration- associated pathologies were observed. Based on these data, we hypothesize that a high level of fibulin- 3 due to mutation or stress inhibits the turnover of basement membrane material and microvesicles, resulting in BLD and membranous debris accumulation which activates complement and ultimately leads to drusen formation. We will test this hypothesis by determining whether a high level of fibulin-3 or other mechanisms are responsible for BLD/membranous debris accumulation, whether loss of fibulin-3 prevents BLD/membranous debris formation, and when complement activation occurs relative to drusen development and whether it promotes the progression of BLD/membranous debris to drusen. These studies will lead us to a comprehensive understanding of drusen genesis in ML/DHRD and potentially AMD.
These studies will lead us to a comprehensive understanding of drusen genesis and the role of fibulin-3 in inherited and potentially age-related macular degeneration. If our hypothesis is validated; fibulin-3 may be targeted for intervention in drusen formation at the earliest stages in macular degeneration.
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