The ERCC6 gene and age-related macular degeneration

Abstract

Background: Age-related macular degeneration (AMD) is the leading cause of irreversible visual loss in the developed countries and is caused by both environmental and genetic factors. A recent study (Tuo et al., PNAS) reported an association between AMD and a single nucleotide polymorphism (SNP) (rs3793784) in the ERCC6 (NM_000124) gene. The risk allele also increased ERCC6 expression. ERCC6 is involved in DNA repair and mutations in ERCC6 cause Cockayne syndrome (CS). Amongst others, photosensitivity and pigmentary retinopathy are hallmarks of CS.

Methodology/principal findings: Separate and combined data from three large AMD case-control studies and a prospective population-based study (The Rotterdam Study) were used to analyse the genetic association between ERCC6 and AMD (2682 AMD cases and 3152 controls). We also measured ERCC6 mRNA levels in retinal pigment epithelium (RPE) cells of healthy and early AMD affected human donor eyes. Rs3793784 conferred a small increase in risk for late AMD in the Dutch population (The Rotterdam and AMRO-NL study), but this was not replicated in two non-European studies (AREDS, Columbia University). In addition, the AMRO-NL study revealed no significant association for 9 other variants spanning ERCC6. Finally, we determined that ERCC6 expression in the human RPE did not depend on rs3793784 genotype, but, interestingly, on AMD status: Early AMD-affected donor eyes had a 50% lower ERCC6 expression than healthy donor eyes (P = 0.018).

Conclusions/significance: Our meta-analysis of four Caucasian cohorts does not replicate the reported association between SNPs in ERCC6 and AMD. Nevertheless, our findings on ERCC6 expression in the RPE suggest that ERCC6 may be functionally involved in AMD. Combining our data with those of the literature, we hypothesize that the AMD-related reduced transcriptional activity of ERCC6 may be caused by diverse, small and heterogeneous genetic and/or environmental determinants.

Figure 1. Linkage disequilibrium (LD) display in Haploview of SNPs encompassing the ERCC6 gene.

SNP selection was based on criteria like functional relevance, minor allele frequency (MAF)>10%, coverage of the main linkage disequilibrium (LD) blocks and tagging of the most common haplotypes. Tag SNPs were selected by use of Tagger, an option of Haploview (all SNPs were captured with a LD tagging criteria of r²>0.8). Figure 1 displays the 5 distinct haplotype blocks and all SNPs that were tested in the AMRO-NL study population (A). LD scores (D' and R²) between markers genotyped. Note D' above the diagonal and R² scores below the diagonal (B).

Figure 2. ERCC6 expression levels in relation to genotype and disease status.

Mean ERCC6 expression level in human donor eyes in relation to rs3793784 genotype (C/C, N = 8; C/G, N = 11and G/G, N = 4). (A). Mean ERCC6 expression level in human donor eyes in relation to “status” =  (old) healthy or early AMD (N  = 14 donor eyes with early AMD and 9 old healthy donor eyes) (B). Quantitative RT-PCR analysis, normalized to the geometric mean of three housekeeping genes (RPLP0, PPIA, and EEF1a1) , . Two way ANOVA was used to test the independent effect of status and genotype on the mean expression, as well as the interaction between these variables. Abbreviations: AMD =  age-related macular degeneration.