Glaucoma is an optic neuropathy that is characterized by the progressive degeneration of the optic nerve, leading to visual impairment. Glaucoma is the main cause of irreversible blindness worldwide, but typically remains asymptomatic until very severe. Open-angle glaucoma comprises the majority of cases in the United States and western Europe, of which, primary open-angle glaucoma (POAG) is the most common type. By contrast, in China and other Asian countries, angle-closure glaucoma is highly prevalent. These two types of glaucoma are characterized based on the anatomic configuration of the aqueous humour outflow pathway. The pathophysiology of POAG is not well understood, but it is an optic neuropathy that is thought to be associated with intraocular pressure (IOP)-related damage to the optic nerve head and resultant loss of retinal ganglion cells (RGCs). POAG is generally diagnosed during routine eye examination, which includes fundoscopic evaluation and visual field assessment (using perimetry). An increase in IOP, measured by tonometry, is not essential for diagnosis. Management of POAG includes topical drug therapies and surgery to reduce IOP, although new therapies targeting neuroprotection of RGCs and axonal regeneration are under development.

Primary open-angle glaucoma.

Population structure, including population stratification and cryptic relatedness, can cause spurious associations in genome-wide association studies (GWAS). Usually, the scaled median or mean test statistic for association calculated from multiple single-nucleotide-polymorphisms across the genome is used to assess such effects, and 'genomic control' can be applied subsequently to adjust test statistics at individual loci by a genomic inflation factor. Published GWAS have clearly shown that there are many loci underlying genetic variation for a wide range of complex diseases and traits, implying that a substantial proportion of the genome should show inflation of the test statistic. Here, we show by theory, simulation and analysis of data that in the absence of population structure and other technical artefacts, but in the presence of polygenic inheritance, substantial genomic inflation is expected. Its magnitude depends on sample size, heritability, linkage disequilibrium structure and the number of causal variants. Our predictions are consistent with empirical observations on height in independent samples of ~4000 and ~133,000 individuals.

Genomic inflation factors under polygenic inheritance

We report a genome-wide association study for open-angle glaucoma (OAG) blindness using a discovery cohort of 590 individuals with severe visual field loss (cases) and 3,956 controls. We identified associated loci at TMCO1 (rs4656461[G] odds ratio (OR) = 1.68, P = 6.1 × 10-10) and CDKN2B-AS1 (rs4977756[A] OR = 1.50, P = 4.7 × 10-9). We replicated these associations in an independent cohort of cases with advanced OAG (rs4656461 P = 0.010; rs4977756 P = 0.042) and two additional cohorts of less severe OAG (rs4656461 combined discovery and replication P = 6.00 × 10-14, OR = 1.51, 95% CI 1.35-1.68; rs4977756 combined P = 1.35 × 10-14, OR = 1.39, 95% CI 1.28-1.51). We show retinal expression of genes at both loci in human ocular tissues. We also show that CDKN2A and CDKN2B are upregulated in the retina of a rat model of glaucoma. © 2011 Nature America, Inc. All rights reserved.

Genome-wide association study identifies susceptibility loci for open angle glaucoma at TMCO1 and CDKN2B-AS1

Primary open-angle glaucoma (POAG) and age-related macular degeneration (AMD) are leading causes of irreversible blindness. Several loci have been mapped using genome-wide association studies. Until very recently, there was no recognized overlap in the genetic contribution to AMD and POAG. At genome-wide significance level, only ABCA1 harbors associations to both diseases. Here, we investigated the genetic architecture of POAG and AMD using genome-wide array data. We estimated the heritability for POAG (h2g = 0.42 ± 0.09) and AMD (h2g = 0.71 ± 0.08). Removing known loci for POAG and AMD decreased the h2g estimates to 0.36 and 0.24, respectively. There was evidence for a positive genetic correlation between POAG and AMD (rg = 0.47 ± 0.25) which remained after removing known loci (rg = 0.64 ± 0.31). We also found that the genetic correlation between sexes for POAG was likely to be less than 1 (rg = 0.33 ± 0.24), suggesting that differences of prevalence among genders may be partly due to heritable factors.

/pdf/assessment-of-polygenic-effects-links-primary-open-angle-3yn30zym2a.pdf

Assessment of polygenic effects links primary open-angle glaucoma and age-related macular degeneration

Genetic and genomic studies, including genome-wide association studies (GWAS) have accelerated the discovery of genes contributing to glaucoma, the leading cause of irreversible blindness world-wide. Glaucoma can occur at all ages, with Mendelian inheritance typical for the rare early onset disease (before age 40) and complex inheritance evident in common adult-onset forms of disease. Recent studies have suggested possible therapeutic targets for some patients with early-onset glaucoma based on the molecular and cellular events caused by MYOC, OPTN and TBK1 mutations. Diagnostic genetic tests using early-onset glaucoma genes are also proving useful for pre-symptomatic disease detection and genetic counseling. Recent GWAS completed for three types of common adult-onset glaucoma have identified novel loci for POAG (primary-open-angle glaucoma) (ABCA1, AFAP1, GMDS, PMM2, TGFBR3, FNDC3B, ARHGEF12, GAS7, FOXC1, ATXN2, TXNRD2); PACG (primary angle-closure glaucoma (EPDR1, CHAT, GLIS3, FERMT2, DPM2-FAM102); and exfoliation syndrome (XFS) glaucoma (CACNA1A). In total sixteen genomic regions have been associated with POAG (including the normal tension glaucoma (NTG) subgroup), 8 with PACG and 2 with XFS. These studies are defining important biological pathways and processes that contribute to disease pathogenesis.

Genetics of glaucoma.

We performed a genome-wide association study for primary open-angle glaucoma (POAG) in 1,007 cases with high-pressure glaucoma (HPG) and 1,009 controls from southern China. We observed genome-wide significant association at multiple SNPs near ABCA1 at 9q31.1 (rs2487032; P = 1.66 × 10(-8)) and suggestive evidence of association in PMM2 at 16p13.2 (rs3785176; P = 3.18 × 10(-6)). We replicated these findings in a set of 525 HPG cases and 912 controls from Singapore and a further set of 1,374 POAG cases and 4,053 controls from China. We observed genome-wide significant association with more than one SNP at the two loci (P = 2.79 × 10(-19) for rs2487032 representing ABCA1 and P = 5.77 × 10(-10) for rs3785176 representing PMM2). Both ABCA1 and PMM2 are expressed in the trabecular meshwork, optic nerve and other ocular tissues. In addition, ABCA1 is highly expressed in the ganglion cell layer of the retina, a finding consistent with it having a role in the development of glaucoma.

Common variants near ABCA1 and in PMM2 are associated with primary open-angle glaucoma

Intraocular pressure (IOP) is a major risk factor for glaucoma. Genetic determinants of intraocular pressure can provide critical insights into the genetic architecture of glaucoma and, as a result, open new avenues for therapeutic intervention. We performed a genome-wide association study and replication analysis of 8,552 Chinese participants. In the genome-wide association study, we identified 51 loci that surpassed the significance of P<9×10−7, and we formally replicated these loci. A combined discovery and replication meta-analysis identified 21 genome-wide loci that surpassed the genome-wide significance of P<5×10−8, including 4 previously reported loci: rs145063132 (7p21.2, ETV1/DGKB), rs548030386 (7q31.2, ST7 near CAV1/CAV2), rs7047871 (9p24.2, GLIS3), and rs2472494 (9q31.1, ABCA1/SLC44A1). Of the 17 newly identified loci, five were reported to have ocular related phenotypes: PTCH2 (rs7525308 in 1p34.1), LRIF1/DRAM2 (rs1282146 in 1p13.3), COLEC11 (rs201143466 in 2p25.3), SPTBN1 (rs4514918 in 2p16.2), and CRK (rs11078446 in 17p13.3). The genetic loci identified in this study not only increase our understanding of the genes involved in intraocular pressure but also provide important genetic markers to improve future genetic screening and drug discovery for intraocular pressure disorders.

Yilian Cheng

Papers

Common variants near ABCA1 and in PMM2 are associated with primary open-angle glaucoma

Genome-wide analysis identified 17 new loci influencing intraocular pressure in Chinese population

Cholesterol homeostasis regulated by ABCA1 is critical for retinal ganglion cell survival