Association between the SERPING1 gene and age-related macular degeneration: a two-stage case-control study

Abstract

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=4.0x10(-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.

Figure 1. Association across SERPING1 region with age-related macular degeneration

The x-axis shows the relative positions of the 11 single nucleotide polymorphisms (SNPs) genotyped in the UK sample using the University of California Santa Cruz, March, 2006, reference sequence (NCBI build 36.1). The green diamond represents rs2511989, which was genotyped in both the original scan and the follow-up scan. The blue squares depict SNPs genotyped in the initial candidate gene scan. The red dots depict the SNPs that were typed in the follow-up scan only. The probability threshold for significance Bonferroni corrected for the 93 SNPs genotyped in the initial scan plus the eight additional SNPs genotyped in the follow-up is represented by a dashed green line. The y-axis shows the negative natural log of the p value for association using the Cochrane-Armitage test. The SERPING1 gene is drawn to scale and shows all eight exons. Exon one is untranslated (red bars), whereas exons two and eight are partially translated. Exons three to seven are translated (blue bars).

Figure 2. Expression analysis with RT-PCR showing amplification of complementary DNA for SERPING1 in both neural retina (RET) and RPE-choroidal tissues (RPE/CH)

No product was amplified from either tissue when the reverse transcription step was omitted (–rt).