Purpose: To characterise the molecular mechanism underpinning early-onset macular drusen (EOMD), a phenotypically severe sub-type of age-related macular degeneration (AMD), in a sub-group of patients.
Design: Multi-centre case series, in vitro experimentation and retrospective analysis of previously reported variants.
Participants: Seven families with apparently autosomal dominant EOMD.
Methods: Patients underwent comprehensive ophthalmic assessment. Affected individuals from families A, B and E underwent whole exome sequencing. The probands from families C, D, F and G underwent Sanger sequencing analysis of the Complement Factor H (CFH) gene. Mutant recombinant Factor H Like-1 (FHL-1) proteins were expressed in HEK293 cells to assess the impact on FHL-1 expression and function. Previously reported EOMD-causing variants in CFH were reviewed.
Main Outcome Measures: Detailed clinical phenotypes, genomic findings, in vitro characterization of mutation effect on protein function, and postulation of the pathomechanism underpinning EOMD.
Results: All affected participants presented with bilateral drusen. The earliest reported age of onset was 16 years with a median of 46 years). Ultra-rare (MAF ≤0.0001) CFH variants were identified as the cause of disease in each family: CFH c.1243del, p.(Ala415ProfsTer39) het; c.350+1G>T het; c.619+1G>A het, c.380G>A, p.(Arg127His) het; c.694C>T p.(Arg232Ter)het [identified in two unrelated families in this cohort]; and c.1291T>A, p.(Cys431Ser). All mutations affect complement control protein domains (CCP) 2-7, thus are predicted to impact both FHL-1, the predominant isoform in Bruch’s membrane(BrM) of the macula, and FH. In vitro analysis of recombinant proteins FHL-1R127H, FHL-1A415f/s and FHL-1C431S demonstrated that they are not secreted and thus are loss-of-function. Intra-cellular expression of mutant proteins was low, suggesting they may be rapidly degraded due to protein unfolding or instability. Review of 29 previously reported EOMD-causing mutations found that 75.8% (22/29) of impact FHL-1 and FH. In total, 86.2% (25/29) EOMD-associated variants cause haploinsufficiency of FH/FHL-1.
Conclusions: EOMD is an under-recognised, phenotypically severe sub-type of AMD. We propose that haploinsufficiency of FHL-1, the main regulator of the complement pathway in BrM, where drusen develop, is an important mechanism underpinning the development of EOMD in a number of cases. Understanding the molecular basis of EOMD will shed light on AMD pathogenesis given their pathological similarities.