Indian Journal of Ophthalmology

SYMPOSIUM
Year
: 2011  |  Volume : 59  |  Issue : 7  |  Page : 31--42

Complex genetic mechanisms in glaucoma: An overview


Kollu N Rao, Srujana Nagireddy, Subhabrata Chakrabarti 
 Kallam Anji Reddy Molecular Genetics Laboratory, Prof. Brien Holden Eye Research Centre, L.V. Prasad Eye Institute, Hyderabad, India

Correspondence Address:
Subhabrata Chakrabarti
Champalimaud Translational Centre, Prof. Brien Holden Eye Research Centre, L.V. Prasad Eye Institute, Banjara Hills, Hyderabad - 500 034
India

Abstract

Glaucomas comprise a group of hereditary optic neuropathies characterized by progressive and irreversible visual field loss and damage to the optic nerve head. It is a complex disease with multiple molecular mechanisms underlying its pathogenesis. Genetic heterogeneity is the hallmark of all glaucomas and multiple chromosomal loci have been linked to the disease, but only a few genes have been characterized, viz. myocilin (MYOC), optineurin (OPTN), WDR36 and neurotrophin-4 (NTF4) in primary open angle glaucoma (POAG) and CYP1B1 and LTBP2 in congenital and developmental glaucomas. Case-control-based association studies on candidate genes involved in different stages of glaucoma pathophysiology have indicated a very limited involvement. The complex mechanisms leading to glaucoma pathogenesis indicate that it could be attributed to multiple genes with varying magnitudes of effect. In this review, we provide an appraisal of the various efforts in unraveling the molecular mystery in glaucoma and also some future directions based on the available scientific knowledge and technological developments.



How to cite this article:
Rao KN, Nagireddy S, Chakrabarti S. Complex genetic mechanisms in glaucoma: An overview.Indian J Ophthalmol 2011;59:31-42


How to cite this URL:
Rao KN, Nagireddy S, Chakrabarti S. Complex genetic mechanisms in glaucoma: An overview. Indian J Ophthalmol [serial online] 2011 [cited 2024 Mar 19 ];59:31-42
Available from: https://journals.lww.com/ijo/pages/default.aspx/text.asp?2011/59/7/31/73685


Full Text

Glaucoma is a complex disease that comprises a group of heterogeneous optic neuropathies characterized by a progressive degeneration of the optic nerve head and visual field defects. [1] It affects 70 million people and is the second leading cause of blindness worldwide. It is estimated that by the year 2020, this number would rise to around 79.6 million. [1] The prevalence of glaucoma varies widely across the different ethnic groups and is significantly higher in blacks (4.7%) than in the white population (1.3%). [2] In India it is estimated that glaucoma affects 12 million people and causes 12.8% of the total blindness in the country. It is considered to be the third most common cause of blindness with a prevalence ranging from 2.6-4.1%. [3],[4],[5]

Elevated intraocular pressure (IOP) is a major risk factor in glaucoma, and experimental elevation of IOP has resulted in glaucoma in animal models. [6] The other common risk factors include age, race, family history, thin cornea, myopia and oxidative stress. [7] Family history of glaucoma is estimated to account for a risk of 1-10 folds among the first-degree relatives of an affected individual. [8]

Glaucomas are categorized into primary and secondary based on their etiology and aqueous humor dynamics. [9] Based on gonioscopy, primary glaucomas are further classified as primary open angle glaucoma (POAG) and primary angle closure glaucoma (PACG). POAG may be associated with or without an elevated IOP and has an adult onset (usually >35 years) or juvenile onset (usually <35 years). [9] Secondary glaucomas are characterized by the involvement of predisposing ocular or systemic diseases such as uveitis, trauma, and diabetes thereby resulting in an alteration of aqueous humor dynamics. These include pseudoexfoliation glaucoma (XFG) and pigmentary glaucoma (PG). [9] The mode of inheritance in adult-onset POAG and PACG is complex in nature. This has limited the identification of large affected families for gene mapping by linkage analysis. On the other hand, the hereditary component in juvenile-onset POAG has facilitated mapping of some candidate loci. [10]

Developmental glaucomas include primary congenital glaucoma (PCG) and glaucoma-associated syndromes (Aniridia and Axenfeld Rieger syndrome). [9] These glaucomas largely follow a Mendelian pattern through autosomal dominant and autosomal recessive modes of inheritance. Thus, co-segregation of candidate gene mutations or the disease-susceptible alleles among the affected subjects in such families are relatively easy to determine. [11]

Among these glaucoma subtypes, primary glaucomas (largely POAG) are the most common form and are attributed to multiple genes with varying magnitudes of effect. [12] Gene mapping in these disorders pose a major challenge but the success of new-generation technologies and high-throughput screening platforms have provided some hope in understanding their underlying molecular mechanisms. Herein, we present an update on the molecular genetics of primary glaucomas, particularly POAG.

 Chromosomal Loci Mapped in POAG



Glaucoma being a complex disorder is attributed to several genes, the majority of which are yet unidentified. Until now, linkage analysis in large affected families has yielded 25 chromosomal loci linked to POAG. Of these, 15 have been designated as GLC1A to GLC1O by the HUGO genome nomenclature committee ( www.gene.ucl.ac.uk/nomenclature ) and mutations in only four genes have been identified in POAG. These include myocilin (MYOC, GLC1A), optineurin (OPTN; GLC1E), WD repeat domain 36 (WDR36, GLC1G) and neurotrophin-4 (NTF4, GLC1O). Further details are provided in [Table 1]. Although these genes harbor POAG-associated mutations, they exhibit a high degree of allelic heterogeneity in different populations. The mutation spectra in these genes do not indicate their overall involvement in the disease pathogenesis. The problem of understanding the underlying molecular mechanism is additionally compounded by the variable penetrance and expressivity of these gene mutations. The other loci that were mapped in POAG are yet to be characterized for the disease-association mutations.{Table 1}

 Candidate Genes in POAG



a) Myocilin (MYOC)

The first evidence of linkage in a large German family with juvenile open angle glaucoma (JOAG) was demonstrated by Sheffield et al, [10] on Chromosome lq21-q31 (GLC1A). Later Myocilin/TIGR (accession numbers: Nucleotide AH006047, Protein NP_000252) on this locus was characterized to be causative for POAG. [33] This gene is also termed as trabecular meshwork-induced glucocorticoid receptor (TIGR) as it overexpresses due to the induction of glucocorticoid to the cells. It consists of three exons that code for an mRNA of about 2.5 kb and encompasses a 57-kDa glycoprotein of 504 amino acids. [34]

Mutations in MYOC have been reported from different populations and account for 2-5% of POAG patients worldwide. [35] Around ~72 mutations have been reported till date and the Gln368Stop mutation is the most common mutation observed across multiple populations except the Japanese. [36] In the Indian scenario, the Gln48His was the predominant mutation observed in cases of JOAG, POAG and PCG. [37] Along with the coding region mutations, a few polymorphisms have been identified, of which the -1000C>G was referred to very frequently with variable degrees of association.[38] The probable disease-causing and benign variations in MYOC are available in the Myocilin allele-specific phenotype database ( http://www.myocilin.com/variants.php ). [39]

Genotype-phenotype correlation has been demonstrated with some MYOC mutations. It has been observed that individuals with the T377M mutation usually have an onset in their fourth decade whereas those with P370L and Y437H mutations were diagnosed in the first and second decades with severe clinical presentations. The predominant mutation Q368X had an average onset in the fifth and sixth decades [40] and the C433R mutation exhibited an onset between 17 and 58 years and was largely associated with a higher IOP and vertical cup/disc ratio compared to those without this mutation. Other mutations have exhibited variable clinical manifestations. [41]

Structure, properties and expression of MYOC

MYOC is a glycoprotein and consists of two major domains: A myosin-like domain near the N terminal region and an olfactomedin-like domain near the C terminal. The N-terminal region of MYOC contains leucine zipper motifs within two coil-coil domains that are important for its interactions with intracellular, extracellular and cell surface proteins, whereas leucine zipper motifs are important in regulating protein function. The olfactomedin-like domain of MYOC has a homology to a family of olfactomedins with a high degree of conservation across species. The presence of >90% glaucoma-associated mutations in this domain indicate its potential functional importance. [40]

In humans the MYOC mRNA is expressed in a number of tissues which include both non-ocular and ocular tissues including the trabecular meshwork (TM) that exhibits the highest level of expression, followed by the sclera, ciliary body, choroid, cornea, iris, lamina cribosa, retina, and optic nerve. The non-ocular tissues include mammary gland, small intestine, thymus, prostate, testis, colon, stomach, thyroid, trachea, bone marrow, and brain. [42]

Function of MYOC protein

The normal physiological function of MYOC is still unclear but insights from the knockout animal models indicate that the disease-causing MYOC in humans may act by gain of function. In vitro studies have shown that MYOC is involved in the cytoskeletal organization and extracellular matrix (ECM) remodulation. [43] Mutations in MYOC may not directly affect its expression but it may interfere with protein folding or stability of the folded protein. The misfolded protein may not secrete and accumulate as soluble and insoluble aggregates but may associate with resident proteins of the endoplasmic reticulum (ER). This may lead to the activation of the unfolded protein response finally leading to apoptotic cell death. [44] The higher expression of MYOC in TM cells results in the intracellular accumulation of MYOC aggregates, which is deleterious to TM cells, thereby resulting in deterioration of their function and subsequent elevation of IOP. [44] Studies on the expression of normal and mutant MYOC in cultured ocular and non-ocular cells have suggested that while normal MYOC is secreted from the cultured cells, very little MYOC is secreted from cells expressing different mutations. Thus it can be surmised that glaucoma results either due to insufficient levels of secreted MYOC or compromised TM cell function caused by congestion of the TM secretory pathway. [45] MYOC is also known to be associated with the mitochondrial pathway and it has been shown that overexpression of MYOC carrying P370L mutation results in higher endogenous ROS (reactive oxygen species) production. This further suggests that mutant MYOC may cause mitochondrial defects which may lead to TM cell dysfunction and cell death. [46]

b) Optineurin (OPTN)

Sarfarazi et al., [16] identified the second gene in the GLC1E region (10p15-p14) called optic neuropathy-inducing protein or Optineurin (OPTN). Rezaie et al., [17] initially reported mutations in 16.7% families with hereditary POAG of which most of them had low or normal tension glaucoma (NTG). Subsequently, OPTN was screened across different populations that revealed few mutations implicated in NTG and POAG [Table 2]. Among these mutations, the E50K located in the putative bZIP motif appears to be most strongly associated with NTG. {Table 2}

Structure and expression of OPTN

OPTN contains three non-coding exons in 5'-untranslated region (UTR) and 13 exons that code for 577 amino acids. Alternative splicing in 5'-UTR generates at least three different isoforms, but all have same reading frame (gene accession no: AF420371 to AF420373). The structure of OPTN reveals several motifs, including one bZIP motif, two leucine zippers, coiled-coil motifs and a C-terminal C 2 H 2 -type zinc finger domain. OPTN is expressed in both non-ocular and ocular tissues that include the TM, non-pigmented ciliary epithelium, heart, brain, placenta, skeletal muscle, and kidney. [17]

Functions of OPTN

OPTN serves many cellular functions based on its interaction with a variety of proteins such as Rab8, Huntingtin, Myosin VI, RIP, transcription factor IIIA, metabotropic glutamate receptor and TBK1. Based on its interaction with Myosin VI, its role has been proposed in vesicular trafficking between the golgi and plasma membrane. [47] Overexpression of OPTN protects the cells from H 2 O 2 -induced cell death by inhibiting release of Cytochrome C from mitochondria. The common mutation E50K selectively induces the death of retinal ganglion cells (RGCs) due to TNFα-induced death of RGC. [48] Recently, it has been shown that OPTN negatively regulates TNFα-induced NFκB activation although the exact mechanisms by which these cytokines activate OPTN gene expression are yet unknown. [49] OPTN also plays an important role in the regulation of many genes which include MYOC although the mechanism involved is yet to be elucidated. [50]

c) WDR36

Based on a genomewide scan, Monemi et al., (2005) characterized the WDR36 located on GLC1G locus (5q22.1) to be involved in POAG. They identified four mutations in WDR36 among 17 unrelated POAG subjects, 11 with high-pressure and six with low-pressure glaucoma. [19] The mutations were absent in 200 normal control chromosomes and their residues were conserved between WDR36 orthologs in mouse, rat, dog, chimpanzee and humans. Specific ocular expressions and the observed mutations were consistent with a role for WDR36 in the etiology of both high and low-pressure glaucoma. Analysis of WDR36 sequence revealed that several sequence alterations were exclusive to POAG patients and encoded predicted amino acid substitutions in conserved residues. [19] However, subsequent reports have revealed that the WDR36 may not be directly involved with POAG and may simply act as a modifier gene [Table 3]. {Table 3}

Structure and expression of WDR36

The gene spans about 38.3-kb genomic region and contains 23 exons expressed predominantly as two transcripts (5.9kb and 2.5kb). The full-length of this protein contains 951 aa harboring four conserved domains: (a) nine WD 40 repeat domain; (b) Utp21 domain;(c) AMP-dependent synthetase and ligase domain, and (d) cytochrome cd1-nitrite reductase-like domain. WDR36 is widely expressed in several ocular and non-ocular tissues.

Functions of WDR36

Skarie et al., (2008) studied the functional role of WDR36 in zebra fish and found that it is essential for nucleolar processing of 18s rRNA and is thus required for the biogenesis of ribosomes. [65] In zebrafish, loss of WDR36 function resulted in reduced levels of 18s rRNA and also in ocular dysmorphology leading to activation of p53 stress response pathway. This may indicate the possible role of WDR36 sequence variants in POAG pathogenesis. Since variations in p53 are also known to be involved in POAG, co-inheritance of both P53 and WDR36 variations may thus be involved in the disease progression. [65]

d) CYP1B1

The first genetic locus (GLC3A) for PCG was mapped by Sarfarazi and co-workers in 1995 based on their study of 17 Turkish families comprising 113 individuals that included 79 offsprings, of which 40 were affected with PCG. [11] Stoilov et al., (1997) identified CYP1B1 as a candidate gene at this locus for PCG [66] and subsequently pathogenic mutations in CYP1B1 have been identified in PCG with varying frequencies from 20-100% across different populations. [67],[68],[69],[70],[71],[72]th

CYP1B1 has also been implicated in juvenile and adult onset forms of glaucoma, in various ethnic groups worldwide. Initially, Vincent et al., (2002) showed the involvement of CYP1B1 and MYOC in POAG through a digenic mechanism in a family of East Indian (Guyanese) origin. Based on these observations, it was suggested that PCG and JOAG are allelic variants of CYP1B1. It was also hypothesized that CYP1B1 and MYOC might act through a common biochemical pathway with CYP1B1 acting as a modifier for MYOC. [73] Later the association of CYP1B1 was reported across different populations ranging from 2.2%-23.3% with JOAG, POAG and PACG [Table 4].{Table 4}

Structure and expression of CYP1B1

CYP1B1 encodes a 543 amino acid dioxin inducible member of the cytochrome p450 gene superfamily and consists of three exons that span 8.5 kb of genomic DNA. [73] CYP1B1 is known to express both in ocular and non-ocular tissues. Animal models have shown that CYP1B1 deficiency leads to abnormality in the ocular drainage structures and TM that are similar to those observed in human PCG.

Functions of CYP1B1

CYP1B1 is involved in the metabolism of steroids, retinol, retinal, arachidonate and melatonin. Although the exact role of CYP1B1 is unknown, their involvement in metabolizing these steroids may contribute to the regulation of IOP. [81]

e) NTF4

NTF4 gene is located on Chromosome 19q13.33 and is translated as pre-pro-neurotrophin and cleaved to release the mature active protein. Lambert et al., (2001) have shown that cells within the lamina cribrosa (LC) express neurotrophins [NTs] and trk receptors. These NTs play an important role in neuronal development, survival and differentiation. [82] NTs constitute a family of polypeptide growth factors that promote development, survival and differentiation of neurons. Animal models have shown that elevated IOP, ischemia impairs the neurotrophins' signaling thereby it leads RGC death. [83] A recent report by Pasutto et al., (2009) has shown that mutations in neurotrophin Factor 4 (NTF4) impairs the neurotrophins' signaling in POAG. Screening of NTF4 revealed 1.7% of POAG patients of European origin carrying mutations in NTF4. Molecular modeling and in vitro studies have shown that the mutations reported in this study reduce the binding affinity of NTF4 to its target receptor TrkB thereby reducing the function of neurotrophins. [28] However, later studies have demonstrated the lack of involvement of NTF4 in Indian and Caucasian populations. [84],[85]

 Genome-Wide Association Studies



Glaucoma being a complex disease, linkage studies were not very successful in identifying the genes responsible for raised IOP and RGC death. This was further compounded by the late onset of symptoms and the unavailability of large affected families. Genome-wide association studies (GWAS) represent a powerful approach for gene mapping in large cohorts using high-density markers like single nucleotide polymorphisms (SNPs) on microarray platforms. These have revealed several loci associated with several complex diseases like diabetes, rheumatoid arthritis and age-related macular degeneration (AMD). [86],[87]

Based on this approach, Thorleifsson et al., (2007) identified three SNPs (rs1048661, rs3825942 and rs2165241) in lysyl oxidase-like protein 1 (LOXL1) gene on Chromosome 15q22 that were significantly associated with XFS/XFG. [88] Due to the association of POAG and XFS, further analysis of these SNPs in POAG in other populations revealed no significant association in POAG, PACG and PG indicating their exclusive involvement with XFS/XFG only. [89]

A recent study by Nakano et al., (2009) demonstrated significant association of six SNPs located on three different chromosomal loci, viz. 1 (ZP4), 10 (PLXDC2) and 12 (DKFZp762A217) in a Japanese population with both high and low-pressure glaucomas. All these SNPs exhibited significant association with combined P values ranging from 1.0X10 -5 to 9.0X 10-5 along with an odds ratio (OR) of 1.33 - 1.49. [90] Replication of these SNPs in an Indian cohort with POAG and PACG indicated that these were not associated with IOP-related glaucomas. [91] The failure of replication may be due to inclusion of cases with high IOP unlike mixture of both high and low-pressure glaucomas in the Japanese cohort.

By using linkage and SNP mapping, Jiao et al., (2009) have identified a locus on Chromosome 2 associated with POAG. [92] The region was characterized on Chromosome 2p by performing linkage analyses in 146 multiplex families from Barbados Family Study of Glaucoma (BFSG). Case-control analysis on independent groups from BFSG participants identified a strong association with rs12994401 and POAG. This region overlapped with previous linkage studies in Chinese and African families, indicating that this locus could be a significant cause of glaucoma in the Chinese and Europeans. [20],[21]

 Candidate Gene- Based Association Studies



Association studies have suggested many genes in single studies while a few of them have been investigated in multiple studies with conflicting results. The variations in association could be due to racial differences, sample size, poorly characterized controls, and clinical heterogeneity between different populations. The POAG-associated genes include ANP, APOE, OPA1, P53, GST, Interleukins, and TNFα. The role of these genes in the etiology of POAG is still controversial [Table 5].{Table 5}

 Interaction of Genes



Glaucoma is a complex disorder in which a single gene may not contribute to disease progression. It has been demonstrated that Apolipoprotein E promoter SNPs previously associated with Alzheimer's disease may also modify POAG phenotype. APOE (-219G) is associated with increased optic nerve damage. The interaction between APOE (−491T) and an SNP in the MYOC promoter, MYOC (−1000G) is associated with increased IOP and with limited effectiveness of IOP-lowering treatments indicating that APOE is modifier gene for the MYOC. [135] The interaction was also observed between TNF-α -863A/C and OPTN 603A/T (or met98Lys) and the carriers of TNF-α /-863A with OPTN /603A (or Lys98) had significantly worse (P = 0.026) visual field scores than those only with OPTN /603A (or Lys98). [110] This interaction was consistent at the molecular level by inducing expression of OPTN by TNFα through NFkB pathway. [48] The interaction between MYOC, OPTN along with APOE has been suggested to contribute towards the progression of POAG. Common polymorphisms in MYOC, OPTN and APOE were identified that might interactively contribute to POAG. [50] Some studies also observed interaction between Noelin 2 (OLFM2/317A) and OPTN (OPTN/412A) and the OLFM2/1281T and OPTN/412A SNPs with OAG with elevated IOP. [142]

 Gene Expression Studies



The TM tissue plays a key role in the regulation of IOP, and altered TM morphology and functions have been observed in POAG. The mechanisms of action in the TM as well as RGC and astrocytes are dependent on the expression of several genes. Gene expression analysis provides the identification of mechanisms which could be involved in the pathophysiology of POAG. Over the past few years several groups across the world have performed expression studies on TM, RGC and astrocytes that revealed several genes in these tissues. These findings support the existence of numerous regulatory mechanisms in the TM as well as in the optic nerve head (ONH). These genes are involved in the expression of ECM and its remodeling cellular metabolism, cell transport and cell defense. [2],[143]

 Factors Involved in The Elevation of IOP



Elevated IOP is the most important risk factor in POAG and all current treatments for glaucoma involve the lowering of IOP. Despite its importance in clinical practice, knowledge of IOP regulation in the human eye is limited. Cells which are present in the outflow pathway play an important role in IOP regulation. Recent reports have shown that there are several factors which are involved in maintaining the TM physiology thereby regulating IOP. Some of the important factors involved in raised IOP are age, abnormal function of genes, alterations of ECM and oxidative stress. Age plays an important role in the regulation of IOP because decreased TM cellularity with age could alter the synthetic and catabolic control of the extracellular environment. As the TM cellularity decreases with age their phagocytic activity is lost and leads to accumulation of several toxic molecules within the drainage channels thereby obstructing outflow. [144] Studies have also shown that genetic factors play an important role in the regulation of IOP. Several in vitro and in vivo studies have shown that mutations in the MYOC are a known cause of the abnormal function of TM causing elevation of IOP. [44] Animal models have shown that BAX, SPARC, bestrophin-2 (Best2) and aquaporin deficiency limits elevation of IOP and these results indicate that multiple pathways regulate IOP. [145],[146],[147],[148] Several other molecules like endothelial leucocyte adhesion molecule-1 (ELAM), endothelins (ET), nitric oxide (NO) and modification of ECM molecules have been altered under oxidative stress and these are involved in raising of IOP. [149] ECM components of the TM play an important role in the regulation of IOP and several ECM molecules have been upregulated in TM of POAG patients. Along with oxidative stress, transforming growth factor beta (TGFβ) plays an important role in modulation of ECM molecules in TM. TGFβ2 is the predominant isoform in the eye and several groups have reported higher levels in the aqueous of POAG patients. Recently, adenoviral gene transfer of active human TGFβ2 has been shown to elevate IOP and reduces outflow facility in rodent eyes, indicating that increased levels of TGFβ2 play a major role in the elevation of IOP in POAG. [150]

 Mechanism of RGC Death



Progressive loss of optic nerve axons and RGCs result in characteristic optic nerve atrophy and visual field defects in glaucoma patients. A number of hypotheses have been proposed to trigger ganglion cell injury and death in glaucoma. These include compromise to blood flow at the optic nerve, mechanical compression due to raised IOP, loss of neurotrophic factors, autoimmune mechanisms, nitric oxide-induced injuries to the optic nerve and glutamate excitotoxicity. A combination of these factors may be involved in causing glaucomatous RGC loss. Increased IOP is a major risk factor and is involved in the formation of mechanical stress on the ONH thereby inducing glaucomatous cell death. The ONH consists of axons which are projected from RGC, which exit the eye through the lamina cribrosa (LC), a collagenous structure with sieve-like openings. Elevated IOP causes mechanical stress on the LC which leads to decreased axonal transport which causes deprivation of neurotrophic factors. Animal models have shown that acute IOP elevation causes blockage of brain-derived neurotrophic factor (BDNF) transport and may contribute to neuronal death. Supplementation of neurotrophins transiently protect the retina from pressure-induced ischemic injury indicating that neurotrophin deprivation is involved in RGC death. [83]

Tissue hypoxia in the ONH and/or retina is thought to develop secondary to or independent from the elevated IOP in glaucomatous eyes and has been proposed to be associated with pathogenic mechanisms underlying optic nerve degeneration in glaucoma. Considerable evidence suggests that tissue hypoxia in the retina may adversely affect the survival of retinal ganglion cells by inducing apoptosis. [151] Brief preconditioning hypoxia induces HIF-1α expression in the retina, which accompanies the expression of adaptive proteins and provides resistance to cell death; however, exposure to hypoxia for a longer period initiates the cell death program. [151]

The eye is an organ that is predisposed to great levels of oxidative stress. [152] Oxidative stress evident in glaucomatous tissues is an important factor for the loss of neurons during glaucomatous neurodegeneration. RGC death induced by glaucomatous stimuli involves receptor-mediated caspase cascade, and mitochondria-mediated caspase-dependent and caspase-independent components of cell death cascade. TNF-α and hypoxia are two different stimuli known to preferentially trigger the receptor-mediated or mitochondria-mediated cell death pathways, respectively. Following oxidative modifications of retinal proteins in glaucomatous eyes, reduced ability of cells to cope with the glaucomatous tissue stress may result in impaired cellular homeostasis eventually contributing to neurodegeneration. [153],[154],[155],[156]

Nitric oxide (NO) is another important mediator of glial cell-mediated apoptosis in neuronal cells. Under normal physiological conditions NO plays an important role in performing several physiological functions, including regulation of vascular tone, neurotransmitter release and synaptic plasticity. NO can be synthesized by three enzymes (NOS-1, NOS-2, and NOS-3) and elevated levels of NO have been observed in the AqH and genetic association of iNOS polymorphisms have been reported in glaucoma patients. [157]

Neurotrophins, particularly BDNF, are known to influence RGC survival in vitro, both during retinal development and after lesioning. [158] Loss of physiological neurotrophin levels, particularly BDNF is consistent with known events in the clinical and pathological aspects of glaucoma. Overexpression of BDNF delays RGC death in experimental glaucoma.

 Immune Response



Increasing evidence from both in vivo and in vitro studies over the past few years strongly supports the presence of the immune system in glaucoma pathogenesis. Recent studies have shown increased expression of several autoantibodies against many optic nerve and retinal proteins. These antibodies include hsp60, hsp27, alpha crystallins, vimentin and HSP70. The direct application of Abs against these proteins at similar concentration found to induce RGC death in in vivo and in vitro conditions. Recent reports have also shown that complement cascades have been implicated in glaucomatous neurodegeneration. IOP also modulates the immune system by inducing several complement components like C3, C1q and C3r. Interestingly, it has been shown that complement factor H, a common regulator of the complement system is down regulated, while several other complements components are up-regulated, indicating an abnormal activation of complement system. [159],[160]

 The Future



Glaucoma is a complex disease attributed to multiple genes with varying magnitudes of effect. Newer methods of gene mapping involving GWAS have revealed some interesting results over the last few years. However, many of these studies could not be replicated due to differences in phenotyping, other screening modalities and improper study designs. Several genes implicated through functional studies could not be associated in the genetic studies. Since the effect sizes of these genes would vary, there would be a need for precise characterization at the molecular level using multiple approaches. Whole genome sequencing would be an ideal choice for understanding the unknown genes involved in glaucoma pathogenesis. This would need to be supplemented with functional studies and further validation through proteomic approaches, animal models and replications in different ethnic populations. A collective effort including a multi-disciplinary approach involving the expertise of other branches of science is a must for unraveling the mystery and the underlying molecular mechanisms in glaucoma.

 Acknowledgements



This study was supported in part by the Champalimaud Foundation, Portugal, and through a Program Support grant (BT/01/COE/06/02/10) from the Department of Biotechnology (DBT), Government of India, to SC. KNR acknowledges the Senior Research Fellowships from the Council of Scientific and Industrial Research (CSIR), Government of India.

References

1Quigley HA, Broman AT. The number of people with glaucoma worldwide in 2010 and 2020. Br J Ophthalmol 2006;90:262-7.
2Kwon YH, Fingert JH, Kuehn MH, Alward WL. Primary open-angle glaucoma. N Engl J Med 2009;360:1113-24.
3Thylefors B, Negrel AD, Pararajasegaram R, Dadzie KY. Global data on blindness. Bull World Health Organ 1995;73:115-21.
4Ramakrishan R, Nirmalan PK, Krishnadas R, Thulasiraj RD, Tielsch JM, Katz J, et al. Glaucoma in a rural population of southern India, the aravind comprehensive eye survey. Ophthalmology 2003;110:1484-90.
5Dandona L, Dandona R, Srinivas M, Mandal P, John RK, McCarty CA, et al. Open-angle glaucoma in an urban population in southern India, the Andhra Pradesh eye disease study. Ophthalmology 2000;107:1702-9.
6Levkovitch-Verbin H, Quigley HA, Martin KR, Valenta D, Baumrind LA, Pease ME. Translimbal laser photocoagulation to the trabecular meshwork as a model of glaucoma in rats. Invest Ophthalmol Vis Sci 2002;43:402-10.
7Leske C, Connell M, Wu Y, Hyman G, Schachat AP. Risk factors for open-angle glaucoma: The Barbados Eye Study. Arch Ophthalmolol 1995;113:918-24.
8Green CM, Kearns LS, Wu J, Barbour JM, Wilkinson RM, Ring MA, et al. How significant is a family history of glaucoma? Experience from the glaucoma inheritance study in Tasmania. Clin Experiment Ophthalmol 2007;35:793-99.
9Shields MB. Shields' Textbook of Glaucoma. 4 th ed. New York: Lippincott Williams and Wilkins publishers; 2005.
10Sheffield VC, Stone EM, Alward WL, Drack AV, Johnson AT, Streb LM, et al. Genetic linkage of familial open angle glaucoma to chromosome 1q21-q31. Nat Genet 1993;4:47-50.
11Sarfarazi M, Akarsu AN, Hossain A, Turacli ME, Aktan SG, Barsoum-Homsy M, et al. Assignment of a locus (GLC3A) for primary congenital glaucoma (buphthalmos) to 2p21 and evidence for genetic heterogeneity. Genomics 1995;30:171-7.
12Quigley HA. Open-angle glaucoma. N Engl J Med 1993;328:1097-106.
13Stoilova D, Child A, Trifan OC, Crick RP, Coakes RL, Sarfarazi M. Localization of a locus (GLC1B) for adult-onset primary open angle glaucoma to the 2cen-q13 region. Genomics 1996;36:142-50.
14Wirtz MK, Samples JR, Kramer PL, Rust K, Topinka JR, Yount J, et al. Mapping a gene for adult-onset primary open-angle glaucoma to chromosome 3q. Am J Hum Genet 1997;60:296-304.
15Trifan OC, Traboulsi EI, Stoilova D, Alozie I, Nguyen R, Raja S, et al. A third locus (GLC1D) for adult-onset primary open-angle glaucoma maps to the 8q23 region. Am J Ophthalmol 1998;126:17-28.
16Sarfarazi M, Child A, Stoilova D, Brice G, Desai T, Trifan OC, et al. Localization of the fourth locus (GLC1E) for adult-onset primary open-angle glaucoma to the 10p15-p14 region. Am J Hum Genet 1998;62:641-52.
17Rezaie T, Child A, Hitchings R, Brice G, Miller L, Coca-Prados M, et al. Adult-onset primary open-angle glaucoma caused by mutations in optineurin. Science 2002;295:1077-9.
18Wirtz MK, Samples JR, Rust K, Lie J, Nordling L, Schilling K, et al. GLC1F, a new primary open-angle glaucoma locus, maps to 7q35-q36. Arch Ophthalmol 1999;117:237-41.
19Monemi S, Spaeth G, DaSilva A, Popinchalk S, Ilitchev E, Liebmann J et al. Identification of a novel adult-onset primary open-angle glaucoma (POAG) gene on 5q22.1. Hum Mol Genet 2005;14:725-33.
20Lin Y, Liu T, Li J, Yang J, Du Q, Wang J, et al. A genome-wide scan maps a novel autosomal dominant juvenile-onset open-angle glaucoma locus to 2p15-16. Mol Vis 2008;14:739-44.
21Suriyapperuma SP, Child A, Desai T, Brice G, Kerr A, Crick R, et al. A new locus (GLC1H) for adult-onset primary open-angle glaucoma maps to the 2p15-p16 region. Arch Ophthalmol 2007;125:86-92.
22Allingham RR, Wiggs JL, Hauser ER, Larocque-Abramson KR, Santiago-Turla C, Broomer B, et al. Early adult-onset POAG linked to 15q11-13 using ordered subset analysis. Invest Ophthalmol Vis Sci 2005;46:2002-5.
23Wiggs JL, Lynch S, Ynagi G, Maselli M, Auguste J, Del Bono EA, et al. A genomewide scan identifies novel early-onset primary open-angle glaucoma loci on 9q22 and 20p12. Am J Hum Genet 2004;74:1314-20.
24Baird PN, Foote SJ, Mackey DA, Craig J, Speed TP, Bureau A. Evidence for a novel glaucoma locus at chromosome 3p21-22. Hum Genet 2005;117:249-57.
25Pang CP, Fan BJ, Canlas O, Wang DY, Dubois S, Tam PO, et al. A genome-wide scan maps a novel juvenile-onset primary open angle glaucoma locus to chromosome 5q. Mol Vis 2006;12:85-92.
26Wang DY, Fan BJ, Chua JK, Tam PO, Leung CK, Lam DS, et al. A genome-wide scan maps a novel juvenile-onset primary open-angle glaucoma locus to 15q. Invest Ophthalmol Vis Sci 2006;47:5315-21.
27Ip NY, Ibanez CF, Nye SH, McClain J, Jones PF, Gies DR, et al. Mammalian neurotrophin-4: Structure, chromosomal localization, tissue distribution, and receptor specificity. Proc Nat Acad Sci USA 1992;89:3060-4.
28Pasutto F, Matsumoto T, Mardin CY, Sticht H, Brandstätter JH, Michels-Rautenstrauss K, et al. Heterozygous NTF4 mutations impairing neurotrophin-4 signaling in patients with primary open-angle glaucoma. Am J Hum Genet 2009;85:447-56.
29Nemesure B, Jiao X, He Q, Leske MC, Wu SY, Hennis A, et al. A genome-wide scan for primary openangle glaucoma (POAG): The barbados family study of open-angle glaucoma. Hum Genet 2003;112:600-9.
30Wiggs JL, Allingham RR, Hossain A, Kern J, Auguste J, DelBono EA, et al. Genome-wide scan for adult onset primary open angle glaucoma. Hum Mol Genet 2000;9:1109-17.
31Charlesworth JC, Dyer TD, Stankovich JM, Blangero J, Mackey DA, Craig JE, et al. Linkage to 10q22 for maximum intraocular pressure and 1p32 for maximum cup-to-disc ratio in an extended primary open-angle glaucoma pedigree. Invest Ophthalmol Vis Sci 2005;46:3723-9.
32Duggal P, Klein AP, Lee KE, Klein R, Klein BE, Bailey-Wilson JE. Identification of novel genetic loci for intraocular pressure: A genomewide scan of the beaver dam eye study. Arch Ophthalmol 2007;125:74-9.
33Stone EM, Fingert JH, Alward WL, Nguyen TD, Polansky JR, Sunden SL, et al. Identification of a gene that causes primary open angle glaucoma. Science 1997;275:668-70.
34Kubota R, Noda S, Wang Y, Minoshima S, Asakawa S, Kudoh J, et al. A novel myosin-like protein (myocilin) expressed in the connecting cilium of the photoreceptor, molecular cloning, tissue expression, and chromosomal mapping. Genomics 1997;41:360-9.
35Gong G, Kosoko-Lasaki O, Haynatzki GR, Wilson MR. Genetic dissection of myocilin glaucoma. Hum Mol Genet 2004;13:R91-102.
36Fingert JH, Heon E, Liebmann JM, Yamamoto T, Craig JE, Rait J, et al. Analysis of myocilin mutations in 1703 glaucoma patients from five different populations. Hum Mol Genet 1999;8:899-905.
37Chakrabarti S, Kaur K, Komatireddy S, Acharya M, Devi KR, Mukhopadhyay A, et al. Gln48His is the prevalent myocilin mutation in primary open angle and primary congenital glaucoma phenotypes in India. Mol Vis 2005;11:111-3.
38Polansky JR, Juster RP, Spaeth GL. Association of the myocilin mt.1 promoter variant with the worsening of glaucomatous disease over time. Clin Genet 2003;64:18-27.
39Hewitt AW, Mackey DA, Craig JE. Myocilin allele-specific glaucoma phenotype database. Hum Mutat 2008;29:207-11.
40Resch ZT, Fautsch MP. Glaucoma-associated myocilin: A better understanding but much more to learn. Exp Eye Res 2009;88:704-12.
41Povoa CA, Malta RF, Rezende Mde M, de Melo KF, Giannella-Neto D. Correlation between genotype and phenotype in primary open angle glaucoma of Brazilian families with mutations in exon 3 of the TIGR/MYOC gene. Arq Bras Oftalmol 2006;69:289-97.
42Knaupp C, Flügel-Koch C, Goldwich A, Ohlmann A, Tamm ER. The expression of myocilin during murine eye development. Graefes Arch Clin Exp Ophthalmol 2004;242:339-45.
43Fautsch MP, Vrabel AM, Johnson DH. The identification of myocilin-associated proteins in the human trabecular meshwork. Exp Eye Res 2006;82:1046-52.
44Yam GH, Gaplovska-Kysela K, Zuber C, Roth J. Aggregated myocilin induces russell bodies and causes apoptosis: implications for the pathogenesis of myocilin-caused primary open angle glaucoma. Am J Pathol 2007;170:100-9.
45Jacobson N, Andresws M, Shepard AR, Nishimura D, Searby C, Fingert JH, et al. Non-secretion of mutant proteins of the glaucoma gene Myocilin in cultured trabecular meshwork cells and in aqueous humor. Hum Mol Genet 2001;10:117-25.
46He Y, Leung KW, Zhuo YH, Ge J. Pro370Leu mutant myocilin impairs mitochondrial functions in human trabecular meshwork cells. Mol Vis 2009;15:815-25.
47Chalasani ML, Balasubramanian D, Swarup G. Focus on molecules: Optineurin. Exp Eye Res 2008;87:1-2.
48Chalasani ML, Radha V, Gupta V, Agarwal N, Balasubramanian D, Swarup G. A glaucoma-associated mutant of optineurin selectively induces death of retinal ganglion cells which is inhibited by antioxidants. Invest Ophthalmol Vis Sci 2007;48:1607-14.
49Sudhakar C, Nagabhushana A, Jain N, Swarup G. NF-kappaB mediates tumor necrosis factor alpha-induced expression of optineurin, a negative regulator of NF-kappaB. PLoS One 2009;4:e5114.
50Park BC, Tibudan M, Samaraweera M, Shen X, Yue BY. Interaction between two glaucoma genes, optineurin and myocilin. Genes Cells 2007;12:969-79.
51Xiao Z, Meng Q, Tsai JC, Yuan H, Xu N, Li Y. A novel optineurin genetic mutation associated with open-angle glaucoma in a Chinese family. Mol Vis 2009;15:1649-54.
52Leung YF, Fan BJ, Lam DS, Lee WS, Tam PO, Chua JK, et al. Different optineurin mutation pattern in primary open-angle glaucoma. Invest Ophthalmol Vis Sci 2003;44:3880-4.
53Weisschuh N, Neumann D, Wolf C, Wissinger B, Gramer E. Prevalence of myocilin and optineurin sequence variants in German normal tension glaucoma patients. Mol Vis 2005;11:284-7.
54Sripriya S, Nirmaladevi J, George R, Hemamalini A, Baskaran M, Prema R, et al. OPTN gene: Profile of patients with glaucoma from India. Mol Vis 2006;12:816-20.
55Mukhopadhyay A, Komatireddy S, Acharya M, Bhattacharjee A, Mandal AK, Thakur SK, et al. Evaluation of optineurin as a candidate gene in Indian patients with primary open angle glaucoma. Mol Vis 2005;11:792-7.
56Alward WL, Kwon YH, Kawase K, Craig JE, Hayreh SS, Johnson AT, et al. Evaluation of optineurin sequence variations in 1,048 patients with open-angle glaucoma. Am J Ophthalmol 2003;136:904-10.
57Fuse N, Takahashi K, Akiyama H, Nakazawa T, Seimiya M, Kuwahara S, et al. Molecular genetic analysis of optineurin gene for primary open-angle and normal tension glaucoma in the Japanese population. J Glaucoma 2004;13:299-303.
58Funayama T. Variants in optineurin gene and their association with tumor necrosis factor-alpha polymorphisms in Japanese patients with glaucoma. Invest Ophthalmol Vis Sci 2004;45:4359-67.
59Aung T, Ebenezer ND, Brice G, Child AH, Prescott Q, Lehmann OJ, et al. Prevalence of optineurin sequence variants in adult primary open angle glaucoma: Implications for diagnostic testing. J Med Genet 2003;40:e101.
60Fan BJ, Wang DY, Cheng CY, Ko WC, Lam SC, Pang CP. Different WDR36 mutation pattern in Chinese patients with primary open-angle glaucoma. Mol Vis 2009;15:646-53.
61Pasutto F, Mardin CY, Michels-Rautenstrauss K, Weber BH, Sticht H, Chavarria-Soley G, et al. Profiling of WDR36 missense variants in German patients with glaucoma. Invest Ophthalmol Vis Sci 2008;49:270-4.
62Weisschuh N, Wolf C, Wissinger B, Gramer E. Variations in the WDR36 gene in German patients with normal tension glaucoma. Mol Vis 2007;13:724-9.
63Miyazawa A, Fuse N, Mengkegale M, Ryu M, Seimiya M, Wada Y, et al. Association between primary open-angle glaucoma and WDR36 DNA sequence variants in Japanese. Mol Vis 2007;13:1912-9.
64Hauser MA, Allingham RR, Linkroum K, Wang J, LaRocque-Abramson K, Figueiredo D, et al. Distribution of WDR36 DNA sequence variants in patients with primary open-angle glaucoma. Invest Ophthalmol Vis Sci 2006;47:2542-6.
65Skarie JM, Link BA. The primary open-angle glaucoma gene WDR36 functions in ribosomal RNA processing and interacts with the p53 stress-response pathway. Hum Mol Genet 2008;17:2474-85.
66Stoilov I, Akarsu AN, Sarfarazi M. Identification of three different truncating mutations in cytochrome P4501B1 (CYP1B1) as the principal cause of primary congenital glaucoma (Buphthalmos) in families linked to the GLC3A locus on chromosome 2p21. Hum Mol Genet 1997;6:641-7.
67Mashima Y, Suzuki Y, Sergeev Y, Ohtake Y, Tanino T, Kimura I, et al. Novel cytochrome P4501B1 (CYP1B1) gene mutations in Japanese patients with primary congenital glaucoma. Invest Ophthalmol Vis Sci 2001;42:2211-6.
68Sitorus R, Ardjo SM, Lorenz B, Preising M. CYP1B1 gene analysis in primary congenital glaucoma in Indonesian and European patients. J Med Genet 2003;40:e9.
69Reddy AB, Kaur K, Mandal AK, Panicker SG, Thomas R, Hasnain SE, et al. Mutation spectrum of the CYP1B1 gene in Indian primary congenital glaucoma patients. Mol Vis 2004;30:696-702.
70Stoilov IR, Costa VP, Vasconcellos JP, Melo MB, Betinjane AJ, Carani JC, et al. Molecular genetics of primary congenital glaucoma in Brazil. Invest Ophthalmol Vis Sci 2002;43:1820-7.
71Bejjani BA, Stockton DW, Lewis RA, Tomey KF, Dueker DK, Jabak M, et al. Multiple CYP1B1 mutations and incomplete penetrance in an inbred population segregating primary congenital glaucoma suggest frequent de novo events and a dominant modifier locus. Hum Mol Genet 2000;12:367-74.
72Plásilová M, Stoilov I, Sarfarazi M, Kádasi L, Feráková E, Ferák V. Identification of a single ancestral CYP1B1 mutation in Slovak Gypsies (Roms) affected with primary congenital glaucoma. J Med Genet 1999;36:290-4.
73Vincent AL, Billingsley G, Buys Y, Levin AV, Priston M, Trope G, et al. Digenic inheritance of early-onset glaucoma: CYP1B1, a potential modifier gene. Am J Hum Genet 2002;70:448-60.
74Melki R, Colomb E, Lefort N, Brezin AP, Garchon HJ. CYP1B1 mutations in French patients with early-onset primary open-angle glaucoma. J Med Genet 2004; 41:647-51.
75Pasutto F, Chavarria-Soley G, Mardin CY, Michels-Rautenstrauss K, Ingelman-Sundberg M, Fernández-Martínez L, et al. Heterozygous loss of function variants in CYP1B1 predispose to primary open angle glaucoma. Invest Ophthalmol Vis Sci 2010;51:249-54.
76Acharya M, Mookherjee S, Bhattacharjee A, Bandyopadhyay AK, Daulat Thakur SK, Bhaduri G, et al. Primary role of CYP1B1 in Indian juvenile-onset POAG patients. Mol Vis 2006;12:399-404.
77Kumar A, Basavaraj MG, Gupta SK, Qamar I, Ali AM, Bajaj V, et al. Role of CYP1B1, MYOC, OPTN, and OPTC genes in adult-onset primary open-angle glaucoma: Predominance of CYP1B1 mutations in Indian patients. Mol Vis 2007;13:667-76.
78Chakrabarti S, Devi KR, Komatireddy S, Kaur K, Parikh RS, Mandal AK et al. Glaucoma-associated CYP1B1 mutations share similar haplotype backgrounds in POAG and PACG phenotypes. Invest Ophthalmol Vis Sci 2007;48:5439-44.
79Suri F, Kalhor R, Zargar SJ, Nilforooshan N, Yazdani S, Nezari H, et al. Screening of common CYP1B1 mutations in Iranian POAG patients using a microarray-based PrASE protocol. Mol Vis 2008;14:2349-56.
80López-Garrido MP, Sánchez-Sánchez F, López-Martínez F, Aroca-Aguilar JD, Blanco-Marchite C, Coca-Prados M, et al. Heterozygous CYP1B1 gene mutations in Spanish patients with primary open-angle glaucoma. Mol Vis 2006;12:748-55.
81Vasiliou V, Gonzalez FJ. Role of CYP1B1 in glaucoma. Annu Rev Pharmacol Toxicol 2008;48:333-58.
82Lambert W, Agarwal R, Howe W, Clark AF, Wordinger RJ. Neurotrophin and neurotrophin receptor expression by cells of the human lamina cribrosa. Invest Ophthalmol Vis Sci 2001;42:2315-23.
83Pease ME, McKinnon SJ, Quigley HA, Kerrigan-Baumrind LA, Zack DJ. Obstructed axonal transport of BDNF and its receptor TrkB in experimental glaucoma. Invest Ophthalmol Vis Sci 2000;41:764-74.
84Rao KN, Kaur I, Parikh RS, Mandal AK, Chandrasekhar G, Thomas R, et al. Variations in NTF4, VAV2, and VAV3 genes are not involved with primary open-angle and primary angle-closure glaucomas in an indian population. Invest Ophthalmol Vis Sci 2010;51:4937-41.
85Liu Y, Liu W, Crooks K, Schmidt S, Allingham RR, Hauser MA. No evidence of association of heterozygous NTF4 mutations in patients with primary open-angle glaucoma. Am J Hum Genet 2010;86:498-9.
86Hirschhorn JN, Daly MJ. Genome-wide association studies for common diseases and complex traits. Nat Rev Genet 2005;6:95-108.
87Klein RJ, Zeiss C, Chew EY, Tsai JY, Sackler RS, Haynes C, et al. Complement factor H polymorphism in age-related macular degeneration. Science 2005;308:385-9.
88Thorleifsson G, Magnuson KP, Sulem P, Walters GB, Gudbjartsson DF, Stefansson H, et al. Common sequence variants in the LOXL1 gene confer susceptibility to exfoliation glaucoma. Science 2007;317:1397-400.
89Chakrabarti S, Rao KN, Kaur I, Parikh RS, Mandal AK, Chandrasekhar G, et al. The LOXL1 gene variations are not associated with primary open-angle and primary angle-closure glaucomas. Invest Ophthalmol Vis Sci 2008;49:2343-7.
90Nakano M, Ikeda Y, Taniguchi T, Yagi T, Fuwa M, Omi N, et al. Three susceptible loci associated with primary open-angle glaucoma identified by genome-wide association study in a Japanese population. Proc Natl Acad Sci USA 2009;106:12838-42.
91Rao KN, Kaur I, Chakrabarti S. Lack of association of three primary open-angle glaucoma-susceptible loci with primary glaucomas in an Indian population. Proc Natl Acad Sci USA 2009;106:E125-6.
92Jiao X, Yang Z, Yang X, Chen Y, Tong Z, Zhao C, et al. Common variants on chromosome 2 and risk of primary open-angle glaucoma in the Afro-Caribbean population of Barbados. Proc Natl Acad Sci, USA 2009;106:17105-10.
93Tunny TJ, Richardson KA, Clark CV, Gordon RD. The atrial natriuretic peptide gene in patients with familial primary open-angle glaucoma. Biochem Biophys Res Commun 1996;223:221-5.
94Richardson KA, Tunny TJ, Clark CV. PCR-SSCP analysis of the glucocorticoid-responsive element of the atrial natriuretic peptide gene in familial primary open-angle glaucoma. Clin Exp Pharmacol Physiol 1997;24:427-9.
95Jeoung JW, Kim DM, Ko HS, Park SS, Kim JY, Kim SY, et al. Investigation of the association between normal-tension glaucoma and single nucleotide polymorphisms in natriuretic peptide gene. Korean J Ophthalmol 2007;21:33-8.
96Hashizume K, Mashima Y, Fumayama T, Ohtake Y, Kimura I, Yoshida K, et al. Genetic polymorphisms in the angiotensin II receptor gene and their association with open-angle glaucoma in a Japanese population. Invest Ophthalmol Vis Sci 2005;46:1993-2001.
97Ozkur M, Erbagci I, Gungor K, Nacak M, Aynacioglu S, Bekir NA. Angiotensin-converting enzyme insertion-deletion polymorphism in primary open-angle glaucoma. Ophthalmologica 2004;218:415-8.
98Tsai FJ, Lin HJ, Chen WC, Tsai CH, Tsai SW. A codon 31ser-arg polymorphism of the WAF-1/CIP-1/p21/tumour suppressor gene in Chinese primary open-angle glaucoma. Acta Ophthalmol Scand 2004;82:76-80.
99Shibuya E, Meguro A, Ota M, Kashiwagi K, Mabuchi F, Iijima H, et al. Association of Toll-like receptor 4 gene polymorphisms with normal tension Glaucoma. Invest Ophthalmol Vis Sci 2008;49:4453-57.
100Guven M, Unal M, Batar B, Eroðlu E, Devaroglu K, Tamçelik N, et al. Polymorphisms of DNA repair genes XRCC1 and XPD and risk of primary open angle glaucoma (POAG). Mol Vis 2007;13:12-7.
101Mossbock G, Weger M, Faschinger C, Schmut O, Renner W. Plasminogen activator inhibitor-1 4G/5G gene polymorphism and primary open-angle glaucoma. Mol Vis 2008;14:1240-4.
102Wang IJ, Chiang TH, Shih YF, Lu SC, Lin LL, Shieh JW, et al. The association of single nucleotide polymorphisms in the MMP-9 genes with susceptibility to acute primary angle closure glaucoma in Taiwanese patients. Mol Vis 2006;12:1223-32.
103Aung T, Yong VH, Lim MC, Venkataraman D, Toh JY, Chew PT, et al. Lack of association between the rs2664538 polymorphism in the MMP-9 gene and primary angle closure glaucoma in Singaporean subjects. J Glaucoma 2008;17:257-8.
104Wang IJ, Lin S, Chiang TH, Chen ZT, Lin LL, Hung PT, et al. The association of membrane frizzled-related protein (MFRP) gene with acute angle-closure glaucoma--a pilot study. Mol Vis 2008;14:1673-9.
105Tosaka K, Mashima Y, Funayama T, Ohtake Y, Kimura I. Association between open-angle glaucoma and gene polymorphism for heat-shock protein 70-1. Jpn J Ophthalmol 2007;51:417-23.
106Fourgeux C, Martine L, Björkhem I, Diczfalusy U, Joffre C, Acar N, et al. Primary open-angle glaucoma: Association with cholesterol 24S-hydroxylase (CYP46A1) gene polymorphism and plasma 24-hydroxycholesterol levels. Invest Ophthalmol Vis Sci 2009;50:5712-7.
107Gungör K, Ozkur M, Cascorbi I, Brockmöller J, Bekir N, Roots I, et al. Beta 2-adrenergic receptor polymorphism and susceptibility to primary congenital and primary open angle glaucoma. Eur J Clin Pharmacol 2003;597:527-31.
108Lin HJ, Tsai FJ, Chen WC, Shi YR, Hsu Y, Tsai SW. Association of tumour necrosis factor alpha -308 gene polymorphism with primary open angle glaucoma in Chinese. Eye 2003;17:31-4.
109Mossbock G, Weger M, Moray M, Renner W, Haller-Schober EM, Mattes D, et al. TNF-alpha promoter polymorphisms and primary open-angle glaucoma. Eye 2006;20:1040-3.
110Funayama T, Ishikawa K, Ohtake Y, Tanino T, Kurosaka D, Kimura I, et al. Variants in optineurin gene and their association with tumor necrosis factor-alpha polymorphisms in Japanese patients with glaucoma. Invest Ophthalmol Vis Sci 2004;45:4359-67.
111Ressiniotis T, Griffiths PG, Birch M, Keers S, Chinnery PF. Primary open angle glaucoma is associated with a specific p53 gene haplotype. J Med Genet 2004;41:296-8.
112Acharya M, Mitra S, Mukhopadhyay A, Khan M, Roychoudhury S, Ray K. Distribution of p53 codon 72 polymorphism in Indian primary open angle glaucoma patients. Mol Vis 2002;8:367-71.
113Dimasi DP, Hewitt AW, Green CM, Mackey DA, Craig JE. Lack of association of p53 polymorphisms and haplotypes in high and normal tension open angle glaucoma. J Med Genet 2005;42:e55.
114Silva RE, Arruda JT, Rodrigues FW, Moura KK. Primary open angle glaucoma was not found to be associated with p53 codon 72 polymorphism in a Brazilian cohort. Genet Mol Res 2009;8:268-72.
115Lin HJ, Chen WC, Tsai FJ, Tsai SW. Distributions of p53 codon 72 polymorphism in primary open angle glaucoma. Br J Ophthalmol 2002;86:767-70.
116Mabuchi F, Sakurada Y, Kashiwagi K, Yamagata Z, Iijima H, Tsukahara S. Lack of association between p53 gene polymorphisms and primary open angle glaucoma in the Japanese population. Mol Vis 2009;15:1045-9.
117Saglar E, Yucel D, Bozkurt B, Ozgul RK, Irkec M, Ogus A. Association of polymorphisms in APOE, p53, and p21 with primary open-angle glaucoma in Turkish patients. Mol Vis 2009;15:1270-6.
118Powell B, Toomes C, Scott S, Yeung A, Marchbank N, Spry P, et al. Polymorphisms in OPA1 are associated with normal tension glaucoma. Mol Vis 2003;9:460-4.
119Mabuchi F, Tang S, Kashiwagi K, Yamagata Z, Iijima H, Tsukahara S. The OPA1 gene polymorphism is associated with normal tension and high tension glaucoma. Am J Ophthalmol 2007;143:125-30.
120Yu-Wai-Man P, Stewart JD, Hudson G, Andrews RM, Griffiths PG, Birch MK, et al. OPA1 increases the risk of normal but not high tension glaucoma. J Med Genet 2010;47:120-5.
121Woo SJ, Kim DM, Kim JY, Park SS, Ko HS, Yoo T. Investigation of the association between OPA1 polymorphisms and normal-tension glaucoma in Korea. J Glaucoma 2004;13:492-5.
122Yao WL, Jiao XD, Hejtmancik J, Leske M, Hennis A, Nemesure B. Evaluation of the association between OPA1 polymorphisms and primary open-angle glaucoma in Barbados families. Mol Vis 2006;12:649-54.
123Junemann AG, von Ahsen N, Reulbach U, Roedl J, Bönsch D, Kornhuber J, et al. C677T variant in the methylentetrahydrofolate reductase gene is a genetic risk factor for primary open-angle glaucoma. Am J Ophthalmol 2005;139:721-3.
124Mabuchi F, Tang S, Kashiwagi K, Yamagata Z, Iijima H, Tsukahara S. Methylenetetrahydrofolate reductase gene polymorphisms c.677C/T and c.1298A/C are not associated with open angle glaucoma. Mol Vis 2006;12:735-9.
125Micheal S, Qamar R, Akhtar F, Khan MI, Khan WA, Ahmed A. MTHFR gene C677T and A1298C polymorphisms and homocysteine levels in primary open angle and primary closed angle glaucoma. Mol Vis 2009;15:2268-78.
126Woo SJ, Kim JY, Kim DM, Park SS, Ko HS, Yoo T. Investigation of the association between 677C>T and 1298A>C 5, 10-methylenetetra-hydrofolate reductase gene polymorphisms and normal-tension glaucoma. Eye 2009;23:17-24.
127Lin HJ, Tsai SC, Tsai FJ, Chen WC, Tsai JJ, Hsu CD. Association of interleukin 1beta and receptor antagonist gene polymorphisms with primary open-angle glaucoma. Ophthalmologica 2003;217:358-64.
128Wang CY, Shen YC, Lo FY, Su CH, Lee SH, Lin KH, et al. Polymorphism in the IL-1alpha (-889) locus associated with elevated risk of primary open angle glaucoma. Mol Vis 2006;12:1380-5.
129Wang CY, Shen YC, Su CH, Lo FY, Lee SH, Tsai HY, et al. Investigation of the association between interleukin-1beta polymorphism and normal tension glaucoma. Mol Vis 2007;13:719-23.
130How AC, Aung T, Chew X, Yong VH, Lim MC, Lee KY, et al. Lack of association between interleukin-1 gene cluster polymorphisms and glaucoma in Chinese subjects. Invest Ophthalmol Vis Sci 2007;48:2123-6.
131Juronen E, Tasa G, Veromann S, Parts L, Tiidla A, Pulges R, et al. Polymorphic glutathione S-transferase M1 is a risk factor of primary openangle glaucoma among Estonians. Exp Eye Res 2000;71:447-52.
132Abu-Amero KK, Morales J, Mohamed GH, Osman MN, Bosley TM. Glutathione S-transferase M1 and T1 polymorphisms in Arab glaucoma patients. Mol Vis 2008;14:425-30.
133Unal M, Guven M, Devranoglu K, Ozaydin A, Batar B, Tamcelik N, et al. Glutathione S transferase M1 and T1 genetic polymorphisms are related to the risk of primary open-angle glaucoma: A study in a Turkish population. Br J Ophthalmol 2007;91:527-30.
134Jansson M, Rada A, Tomic L, Larsson LI, Wadelius C. Analysis of the Glutathione S-transferase M1 gene using pyrosequencing and multiplex PCR-no evidence of association to glaucoma. Exp Eye Res 2003;77:239-43.
135Copin B, Brézin AP, Valtot F, Dascotte JC, Béchetoille A, Garchon HJ. Apolipoprotein E promoter singlenucleotide polymorphisms affect the phenotype of primary open angle glaucoma and demonstrate interaction with the myocilin gene. Am J Hum Genet 2002;70:1575-81.
136Vickers JC, Craig JE, Stankovich J, McCormack GH, West AK, Dickinson JL, et al. The apolipoprotein 4 gene is associated with elevated risk of normal-tension glaucoma. Mol Vis 2002;8:389-93.
137Lake S, Liverani D, Desai M, Casson R, James B, Clark A, Salmon JF. Normal tension glaucoma is not associated with the common apolipoprotein E gene polymorphisms. Br J Ophthalmol 2004;88:491-3.
138Mabuchi F, Tang S, Ando D, Yamakita M, Wang J, Kashiwagi K, et al. The apolipoprotein E gene polymorphism is associated with open angle glaucoma in the Japanese population. Mol Vis 2005;11:609-12.
139Al-Dabbagh NM, Al-Dohayan N, Arfin M, Tariq M. Apolipoprotein E polymorphisms and primary glaucoma in Saudis. Mol Vis 2009;15:912-9.
140Ressiniotis T, Griffiths PG, Birch M, Keers SM, Chinnery PF. Apolipoprotein E promoter polymorphisms do not have a major influence on the risk of developing primary open-angle glaucoma. Mol Vis 2004;10:805-7.
141Tsai FJ, Lin HJ, Chen WC, Chen HY, Fan SS. Insulin-like growth factor-II gene polymorphism is associated with primary open angle glaucoma. J Clin Lab Anal 2003;17:259-63.
142Funayama T, Mashima Y, Ohtake Y, Ishikawa K, Fuse N, Yasuda N, et al. SNPs and interaction analyses of noelin 2, myocilin, and optineurin genes in Japanese patients with open-angle glaucoma. Invest Ophthalmol Vis Sci 2006;47:5368-75.
143Tamm ER. The trabecular meshwork outflow pathways: Structural and functional aspects. Exp Eye Res 2009;88:648-55.
144Tschumper RC, Johnson DH. Trabecular meshwork cellularity: Differences between fellow eyes. Invest Ophthalmol Vis Sci 1990;31:1327-31.
145Libby RT, Li Y, Savinova OV, Barter J, Smith RS, Nickells RW, et al. Susceptibility to neurodegeneration in a glaucoma is modified by Bax gene dosage. PLoS Genet 2005;1:17-26.
146Haddadin RI, Oh DJ, Kang MH, Filippopoulos T, Gupta M, Hart L, et al. SPARC-null mice exhibit lower intraocular pressures. Invest Ophthalmol Vis Sci 2009;50:3771-7.
147Zhang Y, Davidson BR, Stamer WD, Barton JK, Marmorstein LY, Marmorstein AD. Enhanced inflow and outflow rates despite lower IOP in bestrophin-2-deficient mice. Invest Ophthalmol Vis Sci 2009;50:765-70.
148Verkman AS. Role of aquaporin water channels in eye function. Exp Eye Res 2003;76:137-43.
149Sacca SC, Izzotti A, Rossi P, Traverso C. Glaucomatous outflow pathway and oxidative stress. Exp Eye Res 2007;84:389-99.
150Shepard AR, Millar JC, Pang IH, Jacobson N, Wang WH, Clark AF. Adenoviral gene transfer of active human transforming growth factor-{beta}2 elevates intraocular pressure and reduces outflow facility in rodent eyes. Invest Ophthalmol Vis Sci 2010;51:2067-76.
151Tezel G, Wax MB. Hypoxia-inducible factor 1alpha in the glaucomatous retina and optic nerve head. Arch Ophthalmol 2004;122:1348-56.
152Mozaffarieh M, Grieshaber MC, Flammer J. Oxygen and blood flow: Players in the pathogenesis of glaucoma. Mol Vis 2008;14:224-33.
153Nakazawa T, Nakazawa C, Matsubara A, Noda K, Hisatomi T, She H, et al. Tumor necrosis factor-alpha mediates oligodendrocyte death and delayed retinal ganglion cell loss in a mouse model of glaucoma. J Neurosci 2006;26:12633-41.
154Tezel G. Oxidative stress in glaucomatous neurodegeneration: Mechanisms and consequences. Prog Retin Eye Res 2006;25:490-513.
155Vorwerk CK, Gorla MS, Dreyer EB. An experimental basis for implicating excitotoxicity in glaucomatous optic neuropathy. Surv Ophthalmol 1999;43:S142-50.
156Mali RS, Cheng M, Chintala SK. Plasminogen activators promote excitotoxicity-induced retinal damage. FASEB J 2005;19:1280-9.
157Motallebipour M, Rada-Iglesias A, Jansson M, Wadelius C. The promoter of inducible nitric oxide synthase implicated in glaucoma based on genetic analysis and nuclear factor binding. Mol Vis 2005;11:950-7.
158Loeliger MM, Briscoe T, Rees SM. BDNF increases survival of retinal dopaminergic neurons after prenatal compromise. Invest Ophthalmol Vis Sci 2008;49:1282-9.
159Khalyfa A, Chlon T, Qiang H, Agarwal N, Cooper NG. Microarray reveals complement components are regulated in the serum-deprived rat retinal ganglion cell line. Mol Vis 2007;13:293-308.
160Stasi K, Nagel D, Yang X, Wang RF, Ren L, Podos SM, et al. Complement component 1Q (C1Q) upregulation in retina of murine, primate, and human glaucomatous eyes. Invest Ophthalmol Vis Sci 2006;47:1024-9.