Genetic association of SNPs near ATOH7, CARD10, CDKN2B, CDC7 and SIX1/SIX6 with the endophenotypes of primary open angle glaucoma in Indian population

Gene therapies and gene product-based drug candidates for normalizing and preserving tissue functions in animal models of ocular hypertension and glaucoma

More than 76 million people worldwide are afflicted with the neurodegenerative eye diseases described and grouped together as glaucoma. A common feature amongst the many forms of glaucoma is chronically elevated intraocular pressure (IOP) within the anterior chamber of the eye that physically damages the retina, optic nerve and parts of the brain connected with visual perception. The mediators of the contusing raised IOP responsible for such damage and loss of vision include locally released inflammatory agents, tissue remodeling enzymes and infiltrating immune cells which damage the retinal ganglion cell (RGC) axons and eventually kill a significant number of the RGCs. Additional culprits include genetic defects of the patient that involve aberrations in receptors, enzymes and/or endogenous ligands and possible over- or under-production of the latter. Other genetic abnormalities may include issues with signal transduction machinery within key cells of critical tissues in the front (e.g. trabecular meshwork [TM] and Schlemm's canal [SC]) and back of the eye (e.g. retinal ganglion cells and their axons). Genome-wide associated studies (GWAS) coupled with next generation sequencing have provided powerful linkage of certain gene defects and polymorphic variants to the onset and progression of diseases of the tissues involved in fluid dynamics in the TM and SC, and many retinal elements (lamina cribosa, optic nerve head) at the back of the eye which cause ocular hypertension (OHT) and glaucomatous optic neuropathy (GON), respectively. Despite the availability of some drugs, fluid drainage microshunts and full surgical techniques to lower and control intraocular pressure, the major modifiable biomarker of open-angle and other forms of glaucoma, their side-effect profiles, less than optimum effectiveness and short duration of action present opportunities to clinically manage the glaucomas with next generation of treatments with high therapeutic indices, including gene therapies. Thus, identification, characterization and deployment of genetic data coupled with traditional drug discovery and novel gene replacement, gene editing and genetic engineering technologies may provide some solutions to the aforementioned problems. These aspects will be discussed in this article.

Understanding the complex genetics and molecular mechanisms underlying glaucoma

Glaucoma is the leading cause of irreversible blindness worldwide. Currently the only effective treatment for glaucoma is to reduce the intraocular pressure, which can halt the progression of the disease. Highlighting the importance of identifying individuals at risk of developing glaucoma and those with early-stage glaucoma will help patients receive treatment before sight loss. However, some cases of glaucoma do not have raised intraocular pressure. In fact, glaucoma is caused by a variety of different mechanisms and has a wide range of different subtypes. Understanding other risk factors, the underlying mechanisms, and the pathology of glaucoma might lead to novel treatments and treatment of underlying diseases. In this review we present the latest research into glaucoma including the genetics and molecular basis of the disease.

Elevated Intraocular Pressure and Glaucomatous Optic Neuropathy: Genes to Disease Mechanisms, Therapeutic Drugs, and Gene Therapies

This review article focuses on the pathogenesis of and genetic defects linked with chronic ocular hypertension (cOHT) and glaucoma. The latter ocular disease constitutes a group of ocular degenerative diseases whose hallmark features are damage to the optic nerve, apoptotic demise of retinal ganglion cells, disturbances within the brain regions involved in visual perception and considerable visual impairment that can lead to blindness. Even though a number of pharmaceuticals, surgical and device-based treatments already exist addressing cOHT associated with the most prevalent of the glaucoma types, primary open-angle glaucoma (POAG), they can be improved upon in terms of superior efficacy with reduced side-effects and with longer duration of activity. The linkage of disease pathology to certain genes via genome-wide associated studies are illuminating new approaches to finding novel treatment options for the aforementioned ocular disorders. Gene replacement, gene editing via CRISPR-Cas9, and the use of optogenetic technologies may replace traditional drug-based therapies and/or they may augment existing therapeutics for the treatment of cOHT and POAG in the future.

CARMA3: A potential therapeutic target in non-cancer diseases

Caspase recruitment domain and membrane-associated guanylate kinase-like protein 3 (CARMA3) is a scaffold protein widely expressed in non-hematopoietic cells. It is encoded by the caspase recruitment domain protein 10 (CARD10) gene. CARMA3 can form a CARMA3-BCL10-MALT1 complex by recruiting B cell lymphoma 10 (BCL10) and mucosa-​associated lymphoid tissue lymphoma translocation protein 1 (MALT1), thereby activating nuclear factor-​κB (NF-κB), a key transcription factor that involves in various biological responses. CARMA3 mediates different receptors-dependent signaling pathways, including G protein-coupled receptors (GPCRs) and receptor tyrosine kinases (RTKs). Inappropriate expression and activation of GPCRs and/or RTKs/CARMA3 signaling lead to the pathogenesis of human diseases. Emerging studies have reported that CARMA3 mediates the development of various types of cancers. Moreover, CARMA3 and its partners participate in human non-cancer diseases, including atherogenesis, abdominal aortic aneurysm, asthma, pulmonary fibrosis, liver fibrosis, insulin resistance, inflammatory bowel disease, and psoriasis. Here we provide a review on its structure, regulation, and molecular function, and further highlight recent findings in human non-cancerous diseases, which will provide a novel therapeutic target.

Genetic association between CDKN2B/CDKN2B-AS1 gene polymorphisms with primary glaucoma in a North Indian cohort: an original study and an updated meta-analysis

Background

Variants in CDKN2B/CDKN2B-AS1 have been reported to modulate glaucoma risk in several GWAS across different populations. CDKN2B/CDKN2A encodes tumor suppressor proteins p16^INK4A/p15^INK4B which influences cell proliferation/senescence in RGCs, the degeneration of which is a risk factor for glaucoma. CDKN2B-AS1 codes a long non-coding RNA in antisense direction and is involved in influencing nearby CDKN2A/CDKN2B via regulatory mechanisms.

Methods

Current study investigated four SNPs (rs2157719, rs3217992, rs4977756, rs1063192) of aforementioned genes in a case–control study in a North Indian cohort. Genotyping was done with Taqman chemistry. In addition, an updated meta-analysis was performed.

Results

Two SNPs, rs3217992 and rs2157719 were found to be significantly associated with the disease. The frequency of ‘T’ allele of rs3217992 was significantly lower in cases (POAG/PACG) [p = 0.045; OR = 0.80(CI = 0.65–0.99) and p = 0.024; OR = 0.73(CI = 0.55–0.96)], respectively than in controls. Genetic model analysis revealed that TT + CT genotype confers 0.73-fold protection against POAG [p = 0.047; OR = 0.73(CI = 0.54–0.99)] and trend assumed additive model gives 0.53 times higher protection against PACG progression. However the association of rs3217992 with POAG and PACG did not remain significant after Bonferroni correction. For rs2157719, the ‘C’ allele was found to be less prevalent among cases (POAG/PACG) with respect to controls. Cochran Armitage trend test assuming additive model revealed 0.77 and 0.64-fold protection against POAG and PACG respectively. Bonferroni correction (p_corr= 0.003) was applied and the association of rs2157719 remained significant in PACG cases but not among POAG cases (p = 0.024). The ‘CC’ genotype also confers protection against primary glaucoma (POAG/PACG) among males and female subjects. The frequency rs1063192 and rs4977756 did not vary significantly among subjects, however the haplotype ‘CATA’ was found to be associated with increased glaucoma risk. An updated meta-analysis conducted on pooled studies on POAG cases and controls revealed significant association between rs1063192, rs2157719, rs4977756 and POAG except rs3217992.

Conclusion

The study concludes significant association between INK4 variants and primary glaucoma in the targeted North Indian Punjabi cohort. We believe that deep-sequencing of INK4 locus may help in identifying novel variants modifying susceptibility to glaucoma. Functional studies can further delineate the role of CDKN2B and CDKN2B-AS1 in primary glaucoma for therapeutic intervention.

Insights into the regulatory molecules involved in glaucoma pathogenesis

Glaucoma is an important cause of irreversible blindness, characterized by optic nerve anomalies. Increased intraocular pressure (IOP) and aging are major risk factors. Retinal ganglion cells and trabecular meshwork cells are certainly involved in the etiology of glaucoma. Glaucoma is usually a complex disease, and various genes and functions may contribute to its etiology. Among these may be genes that encode regulatory molecules. In this review, regulatory molecules including 18 transcription factors (TFs), 195 microRNAs (miRNAs), 106 long noncoding RNAs (lncRNAs), and two circular RNAs (circRNAs) that are reasonable candidates for having roles in glaucoma pathogenesis are described. The targets of the regulators are reported. Glaucoma-related features including apoptosis, stress responses, immune functions, ECM properties, IOP, and eye development are affected by the targeted genes. The targeted genes that are frequently targeted by multiple regulators most often affect apoptosis and the related features of cell death and cell survival. BCL2, CDKN1A, and TP53 are among the frequent targets of three types of glaucoma-relevant regulators, TFs, miRNAs, and lncRNAs. TP53 was itself identified as a glaucoma-relevant TF. Several of the glaucoma-relevant TFs are themselves among frequent targets of regulatory molecules, which is consistent with existence of a complex network involved in glaucoma pathogenesis.

Age-dependent regional retinal nerve fibre changes in SIX1/SIX6 polymorphism

SIX1/SIX6 polymorphism has been shown to be associated with glaucoma. Studies have also found that, in older adults, retinal nerve fibre layer (RNFL) thickness is significantly thinned with each copy of the risk allele in SIX1/SIX6. However, it is not known whether these genetic variants exert their effects in younger individuals. Comparing a healthy young adult with an older adult cohort (mean age 20 vs 63 years), both of Northern European descent, we found that there was no significant RNFL thinning in each copy of the risk alleles in SIX1/SIX6 in the eyes of younger individuals. The older cohort showed an unexpectedly thicker RNFL in the nasal sector with each copy of the risk allele for both the SIX1 (rs10483727) and SIX6 (rs33912345) variants. In the temporal sector, thinner RNFL was found with each copy of the risk allele in rs33912345 with a decrease trend observed in rs10483727. Our results suggest that SIX1/SIX6 gene variants exert their influence later in adult life.

Transcriptome analysis of the zebrafish atoh7-/- Mutant, lakritz, highlights Atoh7-dependent genetic networks with potential implications for human eye diseases

Expression of the bHLH transcription protein Atoh7 is a crucial factor conferring competence to retinal progenitor cells for the development of retinal ganglion cells. Several studies have emerged establishing ATOH7 as a retinal disease gene. Remarkably, such studies uncovered ATOH7 variants associated with global eye defects including optic nerve hypoplasia, microphthalmia, retinal vascular disorders, and glaucoma. The complex genetic networks and cellular decisions arising downstream of atoh7 expression, and how their dysregulation cause development of such disease traits remains unknown. To begin to understand such Atoh7-dependent events in vivo, we performed transcriptome analysis of wild-type and atoh7 mutant (lakritz) zebrafish embryos at the onset of retinal ganglion cell differentiation. We investigated in silico interplays of atoh7 and other disease-related genes and pathways. By network reconstruction analysis of differentially expressed genes, we identified gene clusters enriched in retinal development, cell cycle, chromatin remodeling, stress response, and Wnt pathways. By weighted gene coexpression network, we identified coexpression modules affected by the mutation and enriched in retina development genes tightly connected to atoh7. We established the groundwork whereby Atoh7-linked cellular and molecular processes can be investigated in the dynamic multi-tissue environment of the developing normal and diseased vertebrate eye.

The Genetic and Endoplasmic Reticulum-Mediated Molecular Mechanisms of Primary Open-Angle Glaucoma

Glaucoma is a heterogenous, chronic, progressive group of eye diseases, which results in irreversible loss of vision. There are several types of glaucoma, whereas the primary open-angle glaucoma (POAG) constitutes the most common type of glaucoma, accounting for three-quarters of all glaucoma cases. The pathological mechanisms leading to POAG pathogenesis are multifactorial and still poorly understood, but it is commonly known that significantly elevated intraocular pressure (IOP) plays a crucial role in POAG pathogenesis. Besides, genetic predisposition and aggregation of abrogated proteins within the endoplasmic reticulum (ER) lumen and subsequent activation of the protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dependent unfolded protein response (UPR) signaling pathway may also constitute important factors for POAG pathogenesis at the molecular level. Glaucoma is commonly known as a ‘silent thief of sight’, as it remains asymptomatic until later stages, and thus its diagnosis is frequently delayed. Thereby, detailed knowledge about the glaucoma pathophysiology is necessary to develop both biochemical and genetic tests to improve its early diagnosis as well as develop a novel, ground-breaking treatment strategy, as currently used medical therapies against glaucoma are limited and may evoke numerous adverse side-effects in patients.

Polymorphism analysis of miR182 and CDKN2B genes in Greek patients with primary open angle glaucoma

Glaucoma is a progressive optic neuropathy resulting from retinal ganglion cells death; it represents one of the leading causes of irreversible blindness worldwide. Although, primary open angle glaucoma (POAG) is the most common type of the disease, the pathogenesis of POAG and the genetic factors contributing to disease development remain poorly understood. The aim of this study was to investigate whether the polymorphisms rs76481776 in miR182 gene and rs3217992 in cyclin-dependent kinase inhibitor-2B (CDKN2B) gene are risk factors for POAG in a series of patients of Greek origin. A case-control study was conducted including 120 patients with POAG and 113 unaffected healthy controls of Greek origin, surveyed for polymorphisms with potential correlation to POAG. DNA from each individual was tested for the miR182 rs76481776 and CDKN2B rs3217992 polymorphisms. Regarding the miR182 rs76481776 polymorphism, the T allele occurred with significantly higher frequency in POAG patients compared to controls (OR: 2.62, 95% CI: 1.56-4.39; p = 0.0002). The CDKN2B rs3217992 A allele frequency was found significantly increased in POAG patients compared to healthy individuals (OR: 1.72, 95% CI: 1.18-2.49; p = 0.005). Therefore, both rs76481776 polymorphism in miR182 gene and rs3217992 polymorphism in CDKN2B gene seem to be associated with the development of POAG in a Greek population. The carriers of the T allele of rs76481776 in miR182 and the carriers of the A allele of rs3217992 in CDKN2B have an increased risk of developing POAG.

Family-Based Genome-Wide Association Study of South Indian Pedigrees Supports WNT7B as a Central Corneal Thickness Locus

Purpose

To identify genetic risk factors contributing to central corneal thickness (CCT) in individuals from South India, a population with a high prevalence of ocular disorders.

Methods

One hundred ninety-five individuals from 15 large South Indian pedigrees were genotyped using the Omni2.5 bead array. Family-based association for CCT was conducted using the score test in MERLIN.

Results

Genome-wide association study (GWAS) identified strongest association for single nucleotide polymorphisms (SNPs) in the first intron of WNT7B and CCT (top SNP rs9330813; β = −0.57, 95% confidence interval [CI]: −0.78 to −0.36; P = 1.7 × 10⁻⁷). We further investigated rs9330813 in a Latino cohort and four independent European cohorts. A meta-analysis of these data sets demonstrated statistically significant association between rs9330813 and CCT (β = −3.94, 95% CI: −5.23 to −2.66; P = 1.7 × 10⁻⁹). WNT7B SNPs located in the same genomic region that includes rs9330813 have previously been associated with CCT in Latinos but with other ocular quantitative traits related to myopia (corneal curvature and axial length) in a Japanese population (rs10453441 and rs200329677). To evaluate the specificity of the observed WNT7B association with CCT in the South Indian families, we completed an ocular phenome-wide association study (PheWAS) for the top WNT7B SNPs using 45 ocular traits measured in these same families including corneal curvature and axial length. The ocular PheWAS results indicate that in the South Indian families WNT7B SNPs are primarily associated with CCT.

Conclusions

The results indicate robust evidence for association between WNT7B SNPs and CCT in South Indian pedigrees, and suggest that WNT7B SNPs can have population-specific effects on ocular quantitative traits.

Loss of foxc1 in zebrafish reduces optic nerve size and cell number in the retinal ganglion cell layer

Mutation of FOXC1 causes Axenfeld-Rieger Syndrome (ARS) with early onset or congenital glaucoma. We assessed retinal ganglion cell (RGC) number in zebrafish due to CRISPR-mediated mutation and antisense inhibition of two-forkhead box transcription factors, foxc1a and foxc1b. These genes represent duplicated homologues of human FOXC1. Using a CRISPR induced null mutation in foxc1b, in combination with antisense inhibition of foxc1a, we demonstrate reduced cell number in the retinal ganglion cell layer of developing zebrafish eyes. As early as 5 days post fertilization (dpf), fewer RGCs are found in foxc1b homozygous mutants injected with foxc1a morpholinos, and a thinner optic nerve results. Our data illustrates that foxc1 is required for the expression of atonal homolog 7 (atoh7), a gene that is necessary for RGC differentiation. As markers of differentiated RGCs (pou4f2) are downregulated in foxc1b-/- mutants injected with foxc1a morpholinos and no cell death is observed, our results are consistent with defects in the differentiation of RGCs leading to reduced cell number, as opposed to increased cell death of RGCs or off targets effects of morpholino injection. Our zebrafish model demonstrates that aberrant regulation of RGC number could act in concert with other known glaucoma risk factors to influence the development of congenital and early onset glaucoma due to FOXC1 mutation.

Association of the SIX6 locus with primary open angle glaucoma in southern Chinese and Japanese

The purpose of the study was to evaluate the association profiles of the SIX6 locus with primary open-angle glaucoma (POAG) in southern Chinese and Japanese. In this study, we tested single marker and haplotype-based associations of 11 tagging single nucleotide polymorphisms (SNPs) covering the SIX6 locus with POAG in a Hong Kong Chinese cohort (N = 1402). A novel SNP (i.e., rs12436579) and two SNPs (i.e., rs33912345 and rs10483727) from previous genome-wide association studies were further tested in a Chinese cohort from Shantou (N = 888) and a Japanese cohort from Osaka (N = 463). Results from the three cohorts were meta-analysed using a random-effect model. We found rs12436579, which has not been previously reported, was associated with POAG in Hong Kong and Shantou Chinese (P_combined = 4.3 × 10^-5, OR = 0.72, I² = 0). Additionally, we replicated the association of one known SNP, rs33912345 (P_combined = 0.0061, OR = 0.69, I² = 45%), with POAG in the Chinese cohorts but not in the Japanese cohort (P > 0.6). Another known SNP, rs10483727, was nominally associated with POAG in the two Chinese cohorts (P_combined = 0.017, OR = 0.70, I² = 53%). All these three SNPs were significantly associated with POAG when the three cohorts were combined in meta-analysis (P_combined<0.005). Furthermore, two haplotypes, C-C (P_combined = 1.13 × 10^-5, OR = 1.41, I² = 0) and A-A (P_combined = 0.045, OR = 0.68, I² = 70%), defined by rs33912345-rs12436579 were associated with POAG in Chinese but not in Japanese. In conclusion, this study confirmed the association between two GWAS SNPs in SIX6 (rs33912345 and rs10483727) and POAG. Also, a SNP, rs12436579, not associated with POAG before, was found to be associated with POAG in Chinese. Further studies are warranted to elucidate the role of this novel SNP in POAG.

The polymorphisms of ATOH 7, ET-1 and ACE in non-arteritic anterior ischemic optic neuropathy

Non-arteritic anterior ischemic optic neuropathy (NAION) is a common cause of acute optic neuropathy in the elderly. The role of the genetic polymorphisms of Atonal Homolog 7 (ATOH7), Endothelin-1 (ET-1) and Angiotensin Converting Enzyme (ACE) in NAION and the combined effects of the gene-gene and gene-medical comorbidities on NAION were not clear. We conducted a perspective, case-control study. 71 NAION patients and 142 age and sex-matched healthy controls were enrolled. Single nucleotide polymorphisms of ATOH7 (rs1900004), ET-1 (rs5370) and ACE (rs1799752) were identified by polymerase chain reaction (PCR) method and all PCR products were screened with Sanger sequencing. The prevalence of genetic factors in NAION patients were compared to normal people, and assessed in conditional logistic regression models. The modified effects of gene-gene or gene-medical comorbidities on NAION development were assessed with a multiplicative model. A significant high risk was found in the T allele of ATOH7 in NAION, with an odds ratio (OR) of 1.55 (P = 0.04). Conditional logistic regression analysis, including diabetes and hypertension, revealed that ATOH7 TT genotype carriers conferred a significantly increased risk of NAION (TT/CC + CT, OR = 3.32, 95% confidence interval (CI) = 1.16-9.53, P = 0.03). Interaction analysis showed that ET-1 (P = 0.01), ACE (P = 0.046) and hypertension (P = 0.02) have modified effects on NAION development. Our results showed that the polymorphism of optic disc associated gene-ATOH7 conferred a significant risk of NAION. Combination of ATOH7 and ET-1, ATOH7 and ACE, as well as ATOH7 and hypertension, increased the susceptibility of NAION. Our data may be useful for NAION predicting.

Identification and genotype phenotype correlation of novel mutations in SIX6 gene in primary open angle glaucoma

BACKGROUND

Recently SIX1 and SIX6 genes have been associated with primary open angle glaucoma (POAG). This study was planned to do mutation screening in SIX1 and SIX6 genes in North Indian POAG patients and correlate with clinical phenotypes.

MATERIALS AND METHODS

SIX1 and SIX6 genes were amplified by PCR and sequenced in 115 POAG cases and 105 controls. Four pathogenecity prediction tools (MutationTaster, PolyPhen-2 HumDiv, PolyPhen-2 HumVar and SIFT) were used to predict the pathogenicity of the missense mutations. Protein modeling studies were done to predict the effect of the missense mutations on the protein structure and function.

RESULTS

Two novel mutations p.R116G and p.R116E were observed in the SIX6 gene of patients with POAG. The mutations p.R116G and p.R116E were predicted to be pathogenic and replacement of R116 by G or E might lead to loss of interaction between DNA and R116 of wild type SIX6 protein. The patients with the mutation p.R116E had significantly more visual field damage (MD) and early age of onset of the disease. No sequence variations were observed in the SIX1 gene.

CONCLUSION

These results expand the mutation spectrum of SIX6 gene and suggest that SIX6 gene plays an important role in POAG pathogenesis.

Polymorphism rs10483727 in the SIX1/SIX6 Gene Locus Is a Risk Factor for Primary Open Angle Glaucoma in a Saudi Cohort

AIMS

Variant rs10483727 in the SIX1/SIX6 locus has been significantly associated with primary open angle glaucoma (POAG) in multiple ethnic groups. We conducted a case-control study to investigate the association between this variant and POAG in a Saudi cohort.

MATERIALS AND METHODS

Polymorphism rs10483727 was genotyped by using a TaqMan^® assay in 186 subjects comprising 92 unrelated POAG cases and 94 controls all of Saudi origin.

RESULTS

The "C" allele frequency was 0.33 and 0.45 among POAG cases and controls, respectively (odds ratio [OR] = 0.58, 95% confidence interval [CI] = 0.38-0.89; p = 0.013), suggesting a protective effect; and the "T" allele was associated with increased susceptibility to POAG (OR = 1.7, 95% CI = 1.11-2.58; p = 0.013). Genotype distribution was also significantly associated with POAG (χ² = 6.41, df = 2, p = 0.041). Endophenotype traits such as intraocular pressure and cup/disk ratio did not show any significant genotype distribution in POAG cases. A binary logistic regression analysis used to evaluate the effects of age, gender, and genotype on the likelihood of having POAG showed that genotype distribution (p = 0.012) significantly affected the disease outcome as compared with age (p = 0.055) and sex (p = 0.432).

CONCLUSION

The "T" allele of the rs10483727 polymorphism is an independent significant risk factor for POAG in the Saudi population.

CDKN2B gene rs1063192 polymorphism decreases the risk of glaucoma

The aim of this meta-analysis was to evaluate the association between cyclin-dependent kinase Inhibitor-2B (CDKN2B) gene rs1063192 polymorphism and glaucoma risk. We searched the databases of PubMed, and Embase. Pooled odds ratios (ORs) and 95% confidence intervals (CIs) were calculated by using fixed-effect or random-effect models. A total of 14 case-control studies involving 11,316 cases and 24,055 controls were included. Meta-analysis showed that CDKN2B gene rs1063192 polymorphism was associated with a decreased risk of glaucoma. Stratification analysis of ethnicity indicated that rs1063192 polymorphism decreased the risk of glaucoma among Caucasians and Asians. Stratification analysis by type of glaucoma revealed that rs1063192 polymorphism conferred a protective factor of primary open-angle glaucoma (POAG) and non-POAG. Stratification by source of controls uncovered an association between rs1063192 polymorphism and glaucoma in groups of population-based controls. In conclusion, this meta-analysis indicates that CDKN2B gene rs1063192 polymorphism is significantly associated with a decreased risk of glaucoma.

Major review: Molecular genetics of primary open-angle glaucoma

Glaucoma is a leading cause of irreversible blindness worldwide. Primary open-angle glaucoma (POAG), the most common type, is a complex inherited disorder that is characterized by progressive retinal ganglion cell death, optic nerve head excavation, and visual field loss. The discovery of a large, and growing, number of genetic and chromosomal loci has been shown to contribute to POAG risk, which carry implications for disease pathogenesis. Differential gene expression analyses in glaucoma-affected tissues as well as animal models of POAG are enhancing our mechanistic understanding in this common, blinding disorder. In this review we summarize recent developments in POAG genetics and molecular genetics research.

Identification and characterization of variants and a novel 4 bp deletion in the regulatory region of SIX6, a risk factor for primary open-angle glaucoma

Background

Primary open‐angle glaucoma (POAG) is a complex disease of multigenic inheritance and the most common subtype of glaucoma. SIX6 encodes a transcription factor involved in retina, optic nerve, and pituitary development. Previous studies showed a genetic association between the SIX6 locus and POAG, identifying risk alleles. Whether these alleles are present also in the south Indian population is unclear.

Methods

To address this question, the SIX6 gene and an already characterized and highly conserved SIX6 enhancer (Ch14:60974427‐60974430) were sequenced in two south Indian cohorts, respectively, composed of 65/65 and 200/200 POAG cases/age‐matched controls. We next used Taqman‐based allelic discrimination assay to genotype a common variant (rs33912345: c.421A>C) and the rs1048372 SNP in two cohorts, respectively, composed of 557/387 and 590/448 POAG cases/age‐matched controls. An additional cohort of 153 POAG cases was subsequently recruited to assess the association of the rs33912345:c.421A>C and rs10483727 variants with more prominent changes in two POAG diagnostic parameters: retinal nerve fiber layer thickness and vertical cup/disc ratio, using spectral domain optical coherence tomography. The activity of the newly identified enhancer variants was assessed by transgenesis in zebrafish and luciferase assays.

Results

We identified two known rare and two common variants in the SIX6 locus and a novel 4 bp deletion in the analyzed enhancer. Contrary to previous studies, we could not establish a significant association between the rs10483727 and rs33912345:c.421A>C variants and PAOG in the south Indian ethnicity but patients carrying the corresponding C or T risk alleles exhibited a dose‐dependent reduction of the thickness of the retinal nerve fiber layer and a significant increase in the vertical cup/disc ratio. Transgenesis in zebrafish and luciferase assays demonstrated that the newly identified 4 bp deletion significantly reduced reporter expression in cells of the retinal ganglion and amacrine layers, where human SIX6 is expressed.

Conclusion

Altogether, our data further support the implication of SIX6 variants as POAG risk factors and implicates SIX6 haploinsufficiency in POAG pathogenesis.

Rare variants in optic disc area gene CARD10 enriched in primary open-angle glaucoma

BACKGROUND

Genome-wide association studies (GWAS) have identified association of common alleles with primary open-angle glaucoma (POAG) and its quantitative endophenotypes near numerous genes. This study aims to determine whether rare pathogenic variants in these disease-associated genes contribute to POAG.

METHODS

Participants fulfilled strict inclusion criteria of advanced POAG at a young age of diagnosis. Myocilin mutation carriers were excluded using direct sequencing. Whole exome sequencing was performed on 187 glaucoma cases and 103 local screened nonglaucoma controls then joint-called with exomes of 993 previously sequenced Australian controls. GWAS-associated genes were assessed for enrichment of rare predicted pathogenic variants in POAG. Significantly enriched genes were compared against Exome Aggregation Consortium (ExAC) public control.

RESULTS

Eighty-six GWAS disease or trait-associated glaucoma genes were captured and sequenced. CARD10 showed enrichment after Bonferroni correction for rare variants in glaucoma cases (OR = 13.2, P = 6.94 × 10^-5) with mutations identified in 4.28% of our POAG cohort compared to 0.27% in controls. CARD10 was significantly associated with optic disc parameters in previous GWAS. The whole GWAS gene set showed no enrichment in POAG overall (OR = 1.12, P = 0.51).

CONCLUSION

We report here an enrichment of rare predicted pathogenic coding variants within a GWAS-associated locus in POAG (CARD10). These findings indicate that both common and rare pathogenic coding variants in CARD10 may contribute to POAG pathogenesis.

Analysis of Polymorphism rs1900004 in Atonal bHLH Transcription Factor 7 in Saudi Patients with Primary Open Angle Glaucoma

AIMS

To investigate the association between the rs1900004 polymorphism in the atonal bHLH transcription factor 7 (ATOH7) gene and primary open angle glaucoma (POAG) in Saudi patients.

METHODS

Eighty-seven unrelated POAG cases and 94 unrelated control subjects of Saudi origin were genotyped utilizing a TaqMan^® assay. The association between mutant genotypes and POAG and its related clinical indices was investigated.

RESULTS

The genotype and allele frequencies of the polymorphism in ATOH7 did not show any statistically significant association with POAG compared to controls. The minor allele frequency was 0.32 in both cases and controls. None of the demographic, systemic diseases nor glaucoma-specific clinical indices such as intraocular pressure (IOP), cup/disc ratio, and number of antiglaucoma medication, showed any significant association with genotypes. Binary logistic regression analysis (adjusted for age and gender) showed that age was a marginally significant risk factor for the development of glaucoma (adjusted odds ratio = 1.1; 95% confidence interval = 1.079-1.158; p < 0.0001).

CONCLUSIONS

The study did not detect any direct link between genotype/allele frequency of rs1900004 in ATOH7 and POAG or its related clinical indices such as IOP and cup/disc ratio indicating that this polymorphism is not a risk factor for POAG in a Saudi cohort.

Analysis of Cyclin-Dependent Kinase Inhibitor-2B rs1063192 Polymorphism in Saudi Patients with Primary Open-Angle Glaucoma

AIMS

To investigate whether the polymorphism rs1063192 (A>G) in the cyclin-dependent kinase Inhibitor-2B (CDKN2B) gene is a risk factor for primary open-angle glaucoma (POAG).

METHOD

A case-control study was conducted wherein we genotyped 87 unrelated POAG cases and 94 control subjects from Saudi Arabia using the Taq-Man^® assay.

RESULTS

The minor allele frequency was 0.20 in POAG cases and 0.21 in controls. Both the genotype and allele frequencies were not significantly different between cases and controls. No significant association was found between genotypes and glaucoma clinical indices, except that the mutant homozygous genotype (G/G) was associated with the family history of glaucoma (p = 0.024).

CONCLUSION

Polymorphism rs1063192 in CDKN2B is not a risk factor for POAG in Saudi cohort.

Strategies for Gene Mapping in Inherited Ophthalmic Diseases

Gene mapping of inherited ophthalmic diseases such as congenital cataracts, retinal degeneration, glaucoma, age-related macular degeneration, myopia, optic atrophy, and eye malformations has shed more light on the disease pathology, identified targets for research on therapeutics, earlier detection, and treatment options for disease management and patient care. This article details the different approaches to gene identification for both Mendelian and complex eye disorders.

Association of three single nucleotide polymorphisms at the SIX1-SIX6 locus with primary open angle glaucoma in the Chinese population

This study investigated the genetic association of three single nucleotide polymorphisms (SNPs; rs10483727, rs33912345, and rs146737847) at the SIX1-SIX6 locus with primary open angle glaucoma (POAG) in the Chinese population. A total of 866 subjects with POAG (685 high-tension glaucoma (HTG) and 181 normal-tension glaucoma (NTG)) and 266 control individuals were included. Significant genetic association was identified for rs10483727 in HTG (P=0.02; odds ratio (OR)=1.31), NTG (P=7.41×10(-6); OR=2.71), and POAG (i.e., HTG and NTG combined; P=0.001; OR=1.44). rs33912345 was also significantly associated with HTG (P=0.008; OR=1.36), NTG(P=2.72×10(-6); OR=2.27), and POAG (P=3.84×10(-4); OR=1.49). The rare SIX6 mutation, rs146737847, was not found in the subjects enrolled in this study. Stratification by patient age identified that both rs10483727 and rs33912345 were significantly associated with NTG in patients aged above 40 years (P=2.08×10(-5); OR=2.28), whereas in patients aged between 20-40 years, rs33912345 was significantly associated with NTG (P=0.017; OR=2.06). In HTG, the genetic associations for both rs10483727 and rs33912345 were significant in patients aged between 20-40 years (P=0.006; OR=1.56) but not in those aged above 40 years (P=0.118, OR=1.21 and P=0.042, OR=1.29, respectively). This study replicated the association of POAG with two SNPs at the SIX1-SIX6 locus and demonstrated that SNPs, rs10483727 and rs33912345, are significantly associated with POAG, especially with NTG in patients aged above 40 years.