Whole Exome Sequencing Reveals Novel Candidate Genes in Familial Forms of Glaucomatous Neurodegeneration.

Glaucoma is the largest cause of irreversible blindness with a multifactorial genetic etiology. This study explores novel genes and gene networks in familial forms of primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG) to identify rare mutations with high penetrance. Thirty-one samples from nine MYOC-negative families (five POAG and four PACG) underwent whole-exome sequencing and analysis. A set of prioritized genes and variations were screened in an independent validation cohort of 1536 samples and the whole-exome data from 20 sporadic patients. The expression profiles of the candidate genes were analyzed in 17 publicly available expression datasets from ocular tissues and single cells. Rare, deleterious SNVs in AQP5, SRFBP1, CDH6 and FOXM1 from POAG families and in ACACB, RGL3 and LAMA2 from PACG families were found exclusively in glaucoma cases. AQP5, SRFBP1 and CDH6 also revealed significant altered expression in glaucoma in expression datasets. Single-cell expression analysis revealed enrichment of identified candidate genes in retinal ganglion cells and corneal epithelial cells in POAG; whereas for PACG families, retinal ganglion cells and Schwalbe's Line showed enriched expression. Through an unbiased exome-wide search followed by validation, we identified novel candidate genes for familial cases of POAG and PACG. The SRFBP1 gene found in a POAG family is located within the GLC1M locus on Chr5q. Pathway analysis of candidate genes revealed enrichment of extracellular matrix organization in both POAG and PACG.

Genetic association and stress mediated down-regulation in trabecular meshwork implicates MPP7 as a novel candidate gene in primary open angle glaucoma.

BACKGROUND: Glaucoma is the largest cause of irreversible blindness affecting more than 60 million people globally. The disease is defined as a gradual loss of peripheral vision due to death of Retinal Ganglion Cells (RGC). The RGC death is largely influenced by the rate of aqueous humor production by ciliary processes and its passage through the trabecular meshwork (TM) in the anterior part of the eye. Primary open angle glaucoma (POAG), the most common subtype, is a genetically complex disease. Multiple genes and many loci have been reported to be involved in POAG but taken together they explain less than 10 % of the patients from a genetic perspective warranting more studies in different world populations. The purpose of this study was to perform genome-wide search for common variants associated with POAG in an east-Indian population. METHODS: The study recruited 746 POAG cases and 697 controls distributed into discovery and validation cohorts. In the discovery phase, genome-wide genotype data was generated on Illumina Infinium 660 W-Quad platform and the significant SNPs were genotyped using Illumina GGGT assay in the second phase. Logistic regression was used to test association in the discovery phase to adjust for population sub-structure and chi-square test was used for association analysis in validation phase. Publicly available expression dataset for trabecular meshwork was used to check for expression of the candidate gene under cyclic mechanical stress. Western blot and immunofluorescence experiments were performed in human TM cells and murine eye, respectively to check for expression of the candidate gene. RESULTS: Meta-analysis of discovery and validation phase data revealed the association of rs7916852 in MPP7 gene (p = 5.7x10(-7)) with POAG. We have shown abundant expression of MPP7 in the HTM cells. Expression analysis shows that upon cyclic mechanical stress MPP7 was significantly down-regulated in HTM (Fold change: 2.6; p = 0.018). MPP7 protein expression was also found to be enriched in the ciliary processes of the murine eye. CONCLUSION: Using a genome-wide approach we have identified MPP7 as a novel candidate gene for POAG with evidence of its expression in relevant ocular tissues and dysregulation under mechanical stress possibly mimicking the disease scenario.

Evaluation of genetic association of the INK4 locus with primary open angle glaucoma in East Indian population.

INK4 locus at chromosome 9p21 has been reported to be associated with primary open angle glaucoma (POAG) and its subtypes along with the associated optic disc parameters across the populations of European, Japanese and African ancestries. The locus encodes three tumor suppressor genes namely CDKN2A, ARF, CDKN2B and a long non-coding RNA CDKN2B-AS1 (also known as ANRIL). Here, we report association study of 34 SNPs from INK4 locus with POAG in a population of Indo-European ancestry from the eastern part of India (350 patients and 354 controls). With 81% power to detect genetic association we observed only nominal association of rs1011970 (uncorrected p = 0.048) with POAG and rs10120688 (uncorrected p = 0.048) in patients without a high intra-ocular pressure (IOP<21 mm of Hg) compared to controls. This study, in contrast to the previous reports, suggests lack of significant genetic association of INK4 locus with POAG in East Indian population which needs to be replicated in larger studies in diverse world populations.

Gene-rich large deletions are overrepresented in POAG patients of Indian and Caucasian origins.

PURPOSE: Large copy number variations (CNV) can contribute to increased burden for neurodegenerative diseases. In this study, we analyzed the genome-wide burden of large CNVs > 100 kb in primary open angle glaucoma (POAG), a neurodegenerative disease of the eye that is the largest cause of irreversible blindness. METHODS: Genome-wide analysis of CNVs > 100 kb were analyzed in a total of 1720 individuals, including an Indian cohort (347 POAG cases and 345 controls) and a Caucasian cohort (624 cases and 404 controls). All the CNV data were obtained from experiments performed on Illumina 660W-Quad (infinium) arrays. RESULTS: We observed that for both the populations CNVs > 1 Mb was significantly enriched for gene-rich regions unique to the POAG cases (P < 10(-11)). In the Indian cohort CNVs > 1 Mb (39 calls) in patients influenced 125 genes while in controls 31 such CNVs influenced only 5 genes with no overlap. In both cohorts we observed 1.9-fold gene enrichment in patients for deletions compared to duplications, while such a bias was not observed in controls (0.3-fold). Overall duplications > 1 Mb were more than deletions (Del/Dup = 0.82) confirming that the enrichment of gene-rich deletions in patients was associated with the disease. Of the 39 CNVs > 1 Mb from Indian patients, 28 (72%) also were implicated in other neurodegenerative disorders, like autism, schizophrenia, sensorineural hearing loss, and so forth. We found one large duplication encompassing CNTN4 gene in Indian and Caucasian POAG patients that was absent in the controls. CONCLUSIONS: To our knowledge, our study is the first report on large CNV bias for gene-rich regions in glaucomatous neurodegeneration, implicating its impact across populations of contrasting ethnicities. We identified CNTN4 as a novel candidate gene for POAG.

Mitochondrial genome analysis of primary open angle glaucoma patients.

Primary open angle glaucoma (POAG) is a multi-factorial optic disc neuropathy characterized by accelerating damage of the retinal ganglion cells and atrophy of the optic nerve head. The vulnerability of the optic nerve damage leading to POAG has been postulated to result from oxidative stress and mitochondrial dysfunction. In this study, we investigated the possible involvement of the mitochondrial genomic variants in 101 patients and 71 controls by direct sequencing of the entire mitochondrial genome. The number of variable positions in the mtDNA with respect to the revised Cambridge Reference Sequence (rCRS), have been designated "Segregating Sites". The segregating sites present only in the patients or controls have been designated "Unique Segregating Sites (USS)". The population mutation rate (θ = 4Neµ) as estimated by Watterson's θ (θw), considering only the USS, was significantly higher among the patients (p = 9.8 × 10(-15)) compared to controls. The difference in θw and the number of USS were more pronounced when restricted to the coding region (p<1.31 × 10(-21) and p = 0.006607, respectively). Further analysis of the region revealed non-synonymous variations were significantly higher in Complex I among the patients (p = 0.0053). Similar trends were retained when USS was considered only within complex I (frequency 0.49 vs 0.31 with p<0.0001 and mutation rate p-value <1.49×10(-43)) and ND5 within its gene cluster (frequency 0.47 vs 0.23 with p<0.0001 and mutation rate p-value <4.42×10(-47)). ND5 is involved in the proton pumping mechanism. Incidentally, glaucomatous trabecular meshwork cells have been reported to be more sensitive to inhibition of complex I activity. Thus mutations in ND5, expected to inhibit complex I activity, could lead to generation of oxidative stress and favor glaucomatous condition.

Evaluation of the role of LRRK2 gene in Parkinson's disease in an East Indian cohort.

Leucine rich repeat kinase 2 (LRRK2) gene defects cause Parkinson's disease (PD). Recently, LRRK2 has also been shown by genome wide association (GWA) studies to be a susceptibility gene for the disease. In India mutations in LRRK2 is a rare cause of PD. We, therefore, genotyped 64 SNPs across LRRK2 in 161 control samples and finally studied 6 haplotype tagging SNPs for association-based study on 300 cases and 446 ethnically matched controls to explore the potential role of LRRK2 as a susceptibility gene in PD for East Indians. We did not find any significant allele/ genotype or haplotype associations with PD suggesting that common genetic variants within LRRK2 play limited role in modulating PD among East Indians. In addition, we also screened for the common mutations (viz. p.R1441C, p.R1441G, p.R1441H, p.Y1699C, p.G2019S), and a risk variant common among Asians (p.G2385R) but did not observe any of the above mentioned variants in our cohort. Our study, therefore, strongly suggests that LRRK2 has minimal role as a candidate and susceptibility gene in PD pathogenesis among East Indians.

WDR36 variants in East Indian primary open-angle glaucoma patients.

PURPOSE: Glaucoma is a heterogeneous group of optic neuropathies with a complex genetic basis. To date, only the following four genes have been identified: viz. myocilin (MYOC), optineurin (OPTN), WD repeat domain 36 (WDR36), and neurotrophin 4 (NTF4). However, there are conflicting reports regarding the involvement of WDR36 in the pathogenesis of primary open-angle glaucoma (POAG). In the Asian population, mutations in WDR36 appear to play a minor role in POAG pathogenesis but polymorphic variants have been found to be associated with POAG, especially in patients with high tension glaucoma (HTG). The purpose of this study is to determine the role of WDR36 in East Indian POAG patients. To date, no other studies have yet examined this role. METHODS: Ten single nucleotide polymorphisms (SNPs; rs1971050, rs1993465, rs13153937, rs10038177, rs11241095, rs10043631, rs10038058, rs10491424, rs17553936, and rs13186912) spanning almost the entire WDR36 gene were selected and their association with eastern Indian POAG patients was evaluated. Our study pool consisted of 323 POAG patients. Of these 116 were patients who had HTG with intraocular perssure (IOP) >21mmHg and 207 were found to be non-HTG patients (presenting IOP<21mmHg). The study also included 303 participants as controls. The polymorphisms were genotyped in both the patients and the controls using the PCR-RFLP method. Moreover, the SNP that showed significant association was validated by DNA sequencing. The haplotypes were obtained using Haploview 4.1 software. The allele and haplotype frequencies were compared between the patient group and the control group using Pearson's χ(2) test. RESULTS: First, we genotyped the selected SNPs in the 323 POAG patients and 119 of the participants in the control group, in which only rs10038177 (c.710+30C>T) was found to be strongly associated with the HTG cases (OR=2.186; 95% CI=1.458-3.277; p=1.4×10(-4)). To increase the significance of the study, the SNP was genotyped in an additional 184 of the participants in the control group and it was observed that the SNP retained the association (OR=1.216; 95% CI=1.064-2.306; p=0.002). However, no haplotype was found to have any sustainable association with POAG. Based on the LD pattern and location of rs10038177, exon 5 of WDR36 was sequenced but no suspected disease-causing variant was detected. CONCLUSIONS: Our study suggests a possible association between WDR36 SNP in a cohort of eastern Indian POAG patients who also have high intraocular pressure (IOP). This study needs to be further validated in a larger patient cohort.