Secreted Frizzled-related proteins (SFRPs) are modulators of the intermeshing pathways in which signals are transduced by Wnt ligands through Frizzled (Fz) membrane receptors. The Wnt networks influence biological processes ranging from developmental cell fate, cell polarity and adhesion to tumorigenesis and apoptosis. In the five or six years since their discovery, the SFRPs have emerged as dynamically expressed proteins able to bind both Wnts and Fz, with distinctive structural properties in which cysteine-rich domains from Fz- and from netrin-like proteins are juxtaposed. The abundant expression of SFRP genes in the early embryo, altered expression patterns in disease states, and potential significance in the evolution of the vertebrate body plan, make these intriguing molecules relevant to investigations in diverse fields of biology and biomedical sciences.

Summary Background Age-related macular degeneration is the most prevalent form of visual impairment and blindness in developed countries. Genetic studies have made advancements in establishing the molecular cause of this disease, identifying mutations in the complement factor H (CFH) gene and a locus on chromosome 10 encompassing the HTRA1/LOC387715/ARMS2 genes. Variants in complement 3 (C3) and an HLA locus containing both factor B and C2 genes have also been implicated. We aimed to identify further genetic risk factors for this disease. Methods We used a case–control study design in a UK sample of patients with age-related macular degeneration (n=479) and controls (n=479) and undertook a low-density screen of 32 genes using 93 single nucleotide polymorphisms (SNPs). Genes were selected as candidates on the basis of potential functional relevance to age-related macular degeneration. Significant initial findings were confirmed by replication in an independent US cohort of 248 unrelated patients with disease and 252 controls, and by high-density genotyping around association signals. Findings The SNP variant rs2511989, located within intron six of the SERPING1 gene, showed highly significant genotypic association with age-related macular degeneration (uncorrected p=4·0×10−5, corrected p=0·00372). We detected no evidence for association between disease and the other 31 candidate genes. The odds ratio for age-related macular degeneration in rs2511989 G/A heterozygotes compared with wild type G/G homozygotes was 0·63 (95% CI 0·47–0·84). A similar comparison of the A/A homozygotes with the wild type yielded an odds ratio of 0·44 (0·31–0·64). We replicated the observed genotypic association in a US cohort (p=0·008). Furthermore, a secondary high-density genotyping study across the SERPING1 gene region identified five additional SNP variants similarly associated with age-related macular degeneration (rs2244169, rs2511990, rs2509897, rs1005510, and rs2511988). Interpretation Genetic variation in SERPING1 significantly alters susceptibility to age-related macular degeneration. SERPING1 encodes the C1 inhibitor, which has a crucial role in inhibition of complement component 1 (C1) and might implicate the classic pathway of complement activation in this disease. Funding Macula Vision Research Foundation, the Macular Disease Society, the Wellcome Trust, Brian Mercer Trust, the American Health Assistance Foundation, National Institutes of Health, the Howard Hughes Medical Institute.

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