Background Age-related macular degeneration (AMD) is a heritable, multi-factorial, progressive neurodegenerative disease that leads to loss of central vision through RPE dysfunction and death of photoreceptors. In developed countries, AMD is the leading cause of blindness in individuals over 65 years of age. Genome-wide association studies (GWAS) have identified 20 common susceptibility loci, which explain up to half of the AMD heritability. More recently, targeted sequencing has helped identify penetrant, high-risk rare coding variants at some of the known AMD loci. - Whole exome sequencing To further understand the genetic contribution of rare exonic variants, we undertook whole exome sequencing (WES) in 19 AMD families with multiple affected members. We identified 343 rare segregating variants in these families. Pathway analysis using DAVID (Database for Annotation, Visualization and Integrated Discovery) highlighted extracellular matrix (ECM) genes as the top functional cluster. ECM, along with complement and lipid metabolisms pathways, have been associated with AMD based on GWAS studies. Thus, it was interesting to see this association in exome sequencing analysis. Many of these candidates were highly expressed in human fetal (retina, RPE and choroid), aged and AMD (retina) transcriptome data that are relevant in AMD pathophysiology. Our data shows that exome sequencing in extended AMD families can provide important insights into the disease etiology. As part of our continued efforts to define genetic causes of macular degeneration, we performed WES in a two-generation family with autosomal dominant maculopathy and identified a rare variant p.Glu1144Lys in Fibrillin 2 (FBN2), a glycoprotein of the elastin-rich ECM. Sequencing of 192 maculopathy patients revealed additional rare variants, predicted to disrupt FBN2 function. We then undertook additional studies to explore the relationship of FBN2 to macular disease. We showed that FBN2 localizes to Bruch's membrane and its expression appears reduced in aging and AMD eyes. We detected suggestive association of a common FBN2 non-synonymous variant, rs154001 (p.Val965Ile) with AMD in 10 337 cases and 11 174 controls. We propose that rare and common variants in a single gene-FBN2- can contribute to Mendelian and complex forms of macular degeneration. Our studies provide genetic evidence for a key role of elastin microfibers and Bruch's membrane in maintaining blood-retina homeostasis and establish the importance of studying orphan diseases for understanding more common clinical phenotypes (7). - Gene functional studies Based on expression pattern in the retina, we undertook a study to elucidate the function of the ligand-activated nuclear receptor aryl hydrocarbon receptor (AHR) that is involved in the cellular response to environmental signals. Phenotypic analysis in the retina of Ahr-/- mice revealed subretinal accumulation of microglia and focal RPE atrophy, characteristics observed in AMD. Our study confirms and complements another report on a different Ahr-/- allele and suggests that AHR has a protective role in the retina as an environmental stress sensor. As such, Ahr altered function may contribute to human AMD progression and provide a target for pharmacological intervention (6). - Clinical and GWAS studies One of the benefits of knowing the patient's genetic makeup is the possibility to design targeted recommendations and/or therapies. However, we must be cautious in our interpretation. A recent study suggested that genetic variants in CFH and ARMS2 could predict the response to antioxidant and zinc supplement in AMD patients. To investigate this finding, we collaborated with E. Chew (NEI) and other investigators in the retrospective evaluation of a prospective, randomized, placebo-controlled clinical trial of vitamins and minerals for the treatment of AMD to determine whether genotypes at CFH and ARMS2 could influence the relative benefits of AREDS supplementation. We showed that AREDS supplements reduced the rate of AMD progression across all genotype groups (4). We continue to be involved in large collaborative projects with the extramural and international community. We collaborated with G. Abecasis (U. Michigan) who applied a new method, based on direct analysis of off-target sequence reads, to perform an accurate estimation of individual genetic ancestry. This method improves case-control matching in genetic association studies and reduces the risk of spurious findings due to population structure (8). We participated with Jun Z. (U. Michigan) in the GWAS study and meta-analysis of intraocular pressure (IOP) in >6000 subjects of European ancestry. This study supports the evidence from previous GWAS and heritability estimates that the complex physiological trait of IOP is associated with common DNA variants in multiple loci (2). Finally, our collaborator S. Merbs (Johns Hopkins University) showed that there is no evidence of previously reported differential methylation within the IL17RC gene promoter in peripheral blood lymphocytes between AMD cases and age-matched controls (1).
Showing the most recent 10 out of 30 publications
