Founder TIGR/myocilin mutations for glaucoma in the Québec population

Novel ancestry-specific primary open-angle glaucoma loci and shared biology with vascular mechanisms and cell proliferation

Primary open-angle glaucoma (POAG), a leading cause of irreversible blindness globally, shows disparity in prevalence and manifestations across ancestries. We perform meta-analysis across 15 biobanks (of the Global Biobank Meta-analysis Initiative) (n = 1,487,441: cases = 26,848) and merge with previous multi-ancestry studies, with the combined dataset representing the largest and most diverse POAG study to date (n = 1,478,037: cases = 46,325) and identify 17 novel significant loci, 5 of which were ancestry specific. Gene-enrichment and transcriptome-wide association analyses implicate vascular and cancer genes, a fifth of which are primary ciliary related. We perform an extensive statistical analysis of SIX6 and CDKN2B-AS1 loci in human GTEx data and across large electronic health records showing interaction between SIX6 gene and causal variants in the chr9p21.3 locus, with expression effect on CDKN2A/B. Our results suggest that some POAG risk variants may be ancestry specific, sex specific, or both, and support the contribution of genes involved in programmed cell death in POAG pathogenesis.

Genetic Basis of Pigment Dispersion Syndrome and Pigmentary Glaucoma: An Update and Functional Insights

Pigment Dispersion Syndrome (PDS) and Pigmentary Glaucoma (PG) comprise a spectrum of ocular disorders characterized by iris pigment dispersion and trabecular meshwork changes, resulting in increased intraocular pressure and potential glaucomatous optic neuropathy. This review summarizes recent progress in PDS/PG genetics including rare pathogenic protein coding alterations (PMEL) and susceptibility loci identified from genome-wide association studies (GSAP and GRM5/TYR). Areas for future research are also identified, especially the development of efficient model systems. While substantial strides have been made in understanding the genetics of PDS/PG, our review identifies key gaps and outlines the future directions necessary for further advancing this important field of ocular genetics.

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.

Quantitative differentiation of benign and misfolded glaucoma-causing myocilin variants on the basis of protein thermal stability

Accurate predictions of the pathogenicity of mutations associated with genetic diseases are key to the success of precision medicine. Inherited missense mutations in the myocilin (MYOC) gene, within its olfactomedin (OLF) domain, constitute the strongest genetic link to primary open-angle glaucoma via a toxic gain of function, and thus MYOC is an attractive precision-medicine target. However, not all mutations in MYOC cause glaucoma, and common variants are expected to be neutral polymorphisms. The Genome Aggregation Database (gnomAD) lists ∼100 missense variants documented within OLF, all of which are relatively rare (allele frequency <0.001%) and nearly all are of unknown pathogenicity. To distinguish disease-causing OLF variants from benign OLF variants, we first characterized the most prevalent population-based variants using a suite of cellular and biophysical assays, and identified two variants with features of aggregation-prone familial disease variants. Next, we considered all available biochemical and clinical data to demonstrate that pathogenic and benign variants can be differentiated statistically based on a single metric: the thermal stability of OLF. Our results motivate genotyping MYOC in patients for clinical monitoring of this widespread, painless and irreversible ocular disease.

PRIMARY OPEN-ANGLE GLAUCOMA DUE TO MUTATIONS IN THE MYOC GENE

AIM

Mutations in the myocilin gene (MYOC) cause trabecular dysfunction and thus are involved in the pathogenesis of primary open-angle glaucoma (POAG). The aim of this study was to characterize and describe the clinical findings in two Czech families with POAG due to pathogenic variants in the MYOC gene.

MATERIAL AND METHODS

Members of the two families affected by POAG underwent complete ophthalmological examination. In the proband from the first family, a direct sequencing of the three most frequent mutations in the MYOC gene was performed, and in the proband from the second family, an exome sequencing was performed. Other family members underwent targeted tests using direct sequencing.

RESULTS

In total, 10 individuals diagnosed with POAG aged 20-70 years (mean 32.2 years, SD ±10,9 years) were examined. Eight of them showed advanced glaucomatous neuropathy with severe changes in the retinal nerve fiber layer. Clinical signs of POAG were present in six individuals in the third decade of life already; another four developed POAG during the fourth decade of life. Eight out of 10 patients had to undergo filtration surgery. Surgery was performed within 1 to 7 years of diagnosis, but mostly was performed within 2 years of glaucoma diagnosis. In the first family, MYOC variant c.1099G&gt;A p.(Gly367Arg) was shown in the affected family members; in the second family MYOC variant c.1440C&gt;A p.(Asn480Lys), both in heterozygous state. The changes were assessed as pathogenic.

CONCLUSION

Our study is the first to describe mutations in the MYOC gene causing POAG in Czech patients. Genetic testing may be recommended for this diagnosis, especially in individuals with early presentation and a positive family history. Carriers of pathogenic variants of the MYOC gene have a lifetime risk of developing POAG of more than 50% and the course of their disease is often more aggressive, requiring surgical intervention to permanently control the intraocular pressure.

Age at Glaucoma Diagnosis in Germline Myocilin Mutation Patients: Associations with Polymorphisms in Protein Stabilities

Glaucoma is the leading cause of irreversible blindness worldwide, with elevated intraocular pressure (IOP) as the only known modifiable risk factor. Trabecular meshwork (TM)-inducible myocilin (the MYOC gene) was the first to be identified and linked to juvenile and primary open-angle glaucoma. It has been suggested that mutations in the MYOC gene and the aggregation of mutant myocilin in the endoplasmic reticulum (ER) of TM may cause ER stress, resulting in a reduced outflow of aqueous humor and an increase in IOP. We selected 20 MYOC mutations with experimentally determined melting temperatures of mutated myocilin proteins. We included 40 published studies with at least one glaucoma patient with one of these 20 MYOC mutations and information on age at glaucoma diagnosis. Based on data from 458 patients, we found that a statistically significant but weak correlation was present between age and melting temperature based on various assumptions for age. We therefore conclude that genetic analysis of MYOC mutations alone cannot be used to accurately predict age at glaucoma diagnosis. However, it might be an important prognostic factor combined with other clinical factors for critical and early detection of glaucoma.

Common and rare myocilin variants: Predicting glaucoma pathogenicity based on genetics, clinical, and laboratory misfolding data

Rare variants of the olfactomedin domain of myocilin are considered causative for inherited, early-onset open-angle glaucoma, with a misfolding toxic gain-of-function pathogenic mechanism detailed by 20 years of laboratory research. Myocilin variants are documented in the scientific literature and identified through large-scale genetic sequencing projects such as those curated in the Genome Aggregation Database (gnomAD). In the absence of key clinical and laboratory information, however, the pathogenicity of any given variant is not clear, because glaucoma is a heterogeneous and prevalent age-onset disease, and common variants are likely benign. In this review, we reevaluate the likelihood of pathogenicity for the ~100 nonsynonymous missense, insertion-deletion, and premature termination of myocilin olfactomedin variants documented in the literature. We integrate available clinical, laboratory cellular, biochemical and biophysical data, the olfactomedin domain structure, and population genetics data from gnomAD. Of the variants inspected, ~50% can be binned based on a preponderance of data, leaving many of uncertain pathogenicity that motivate additional studies. Ultimately, the approach of combining metrics from different disciplines will likely resolve outstanding complexities regarding the role of this misfolding-prone protein within the context of a multifactorial and prevalent ocular disease, and pave the way for new precision medicine therapeutics.

Updates on Genes and Genetic Mechanisms Implicated in Primary Angle-Closure Glaucoma

Primary angle-closure glaucoma (PACG) is estimated to affect over 30 million people worldwide by 2040 and is highly prevalent in the Asian population. PACG is more severe and carries three times the higher risk of blindness than primary open-angle glaucoma, thus representing a significant public health concern. High heritability and ethnic-specific predisposition to PACG suggest the involvement of genetic factors in disease development. In the recent past, genetic studies have led to the successful identification of several genes and loci associated with PACG across different ethnicities. The precise cellular and molecular roles of these multiple loci in the development and progression of PACG remains to be elucidated. Nonetheless, these studies have significantly increased our understanding of the emerging cellular processes and biological pathways that might provide more significant insights into the disease’s genetic etiology and may be valuable for future clinical applications. This review aims to summarize and update the current knowledge of PACG genetics analysis research.

Systemic Genotype-Phenotype Analysis of MYOC Variants Based on Exome Sequencing and Literature Review

PURPOSE

This study aims to characterize disease-causing variants in the myocilin gene (MYOC), which is associated with autosomal dominant primary open-angle glaucoma (adPOAG).

DESIGN

Case-control study.

METHODS

MYOC variants were collected from in-house exome sequencing data of 7092 individuals. Genotype-phenotype analysis and bioinformatics evaluation were used to distinguish potential pathogenic variants for POAG from others. MYOC mutations in published works of literature were also systemically analyzed.

RESULTS

In total, 53 variants in MYOC were detected in the 7092 subjects, including 45 rare variants (MAF < 0.01) and 8 polymorphisms (MAF ≥ 0.01), or 48 missense variants and 5 truncation variants. There was no difference in the frequency of the 8 polymorphisms between subjects with and without POAG (P > 0.05). The total number of rare MYOC variants was significantly higher in POAG than that in in-house controls (P = 3.31E-14). The pathogenic/likely pathogenic variants (p.P254T, p.S341P, p.G367R, p.P370L, p.D378G, p.C433Y, and p.L486F) were exclusively present in 8 POAG but absent in in-house controls (P = 2.79E-10). Rare truncation MYOC variants were not enriched in POAG as compared with those in in-house controls (P = 0.28). Further analysis of previously reported MYOC variants suggested that pathogenic/likely pathogenic variants were enriched in the conserved olfactomedin domain. Truncation MYOC variants were scattered in the coding region, where only p.Q368∗ had relatively strong evidence to be causative for adPOAG, whereas most others are questionable.

CONCLUSIONS

Most MYOC variants contributing to adPOAG could be characterized as rare missense variants located in OLF-domain and predicted to be damaging through multiple tools. The effect of other variants, especially for truncation variants (except for p.Q368∗) need further clarification.

The GGLEAM Study: Understanding Glaucoma in the Ohio Amish

Glaucoma leads to millions of cases of visual impairment and blindness around the world. Its susceptibility is shaped by both environmental and genetic risk factors. Although over 120 risk loci have been identified for glaucoma, a large portion of its heritability is still unexplained. Here we describe the foundation of the Genetics of GLaucoma Evaluation in the AMish (GGLEAM) study to investigate the genetic architecture of glaucoma in the Ohio Amish, which exhibits lower genetic and environmental heterogeneity compared to the general population. To date, we have enrolled 81 Amish individuals in our study from Holmes County, Ohio. As a part of our enrollment process, 62 GGLEAM study participants (42 glaucoma-affected and 20 unaffected individuals) received comprehensive eye examinations and glaucoma evaluations. Using the data from the Anabaptist Genealogy Database, we found that 80 of the GGLEAM study participants were related to one another through a large, multigenerational pedigree containing 1586 people. We plan to integrate the health and kinship data obtained for the GGLEAM study to interrogate glaucoma genetics and pathophysiology in this unique population.

Molecular Genetics of Glaucoma: Subtype and Ethnicity Considerations

Glaucoma, the world’s leading cause of irreversible blindness, is a complex disease, with differential presentation as well as ethnic and geographic disparities. The multifactorial nature of glaucoma complicates the study of genetics and genetic involvement in the disease process. This review synthesizes the current literature on glaucoma and genetics, as stratified by glaucoma subtype and ethnicity. Primary open-angle glaucoma (POAG) is the most common cause of glaucoma worldwide, with the only treatable risk factor (RF) being the reduction of intraocular pressure (IOP). Genes associated with elevated IOP or POAG risk include: ABCA1, AFAP1, ARHGEF12, ATXN2, CAV1, CDKN2B-AS1, FOXC1, GAS7, GMDS, SIX1/SIX6, TMCO1, and TXNRD2. However, there are variations in RF and genetic factors based on ethnic and geographic differences; it is clear that unified molecular pathways accounting for POAG pathogenesis remain uncertain, although inflammation and senescence likely play an important role. There are similar ethnic and geographic complexities in primary angle closure glaucoma (PACG), but several genes have been associated with this disorder, including MMP9, HGF, HSP70, MFRP, and eNOS. In exfoliation glaucoma (XFG), genes implicated include LOXL1, CACNA1A, POMP, TMEM136, AGPAT1, RBMS3, and SEMA6A. Despite tremendous progress, major gaps remain in resolving the genetic architecture for the various glaucoma subtypes across ancestries. Large scale carefully designed studies are required to advance understanding of genetic loci as RF in glaucoma pathophysiology and to improve diagnosis and treatment options.

The mutational spectrum of Myocilin gene among familial versus sporadic cases of Juvenile onset open angle glaucoma

PURPOSE

Juvenile onset primary open angle glaucoma (JOAG) is a rare disorder associated with high IOP and progressive optic neuropathy in patients diagnosed before the age of 40 years. While in some populations it has primarily an autosomal dominant pattern of inheritance, in others it occurs in a primarily sporadic form. The main aim of the study was to assess the relative prevalence of Myocilin (MYOC) mutations in familial versus sporadic cases of JOAG.

METHODS

We screened 92 unrelated (sporadic) JOAG patients, and 22 affected families (70 affected members and 36 unaffected) for variations in the MYOC gene. We also analyzed the clinical features associated with these variations.

RESULTS

Three coding sequence variants were identified as mutations causing JOAG. Four families segregated distinct mutations at Gly367Arg, and two families at Gln337Arg, while only two sporadic JOAG cases harbored MYOC mutations (Gly367Arg and Gln48His). The frequency of MYOC mutations in familial cases (27%) was significantly higher than in sporadic JOAG cases (2%); p = 0.001. A 90% penetrance for the Gly367Arg variant was seen by the age of 40 years in our patients. Characteristic allele signatures, indicative of specific founder effects, were not observed for the Gly367Arg mutation that was looked for in 12 patients among 2 geographically close families, which harbored this mutation.

CONCLUSION

Our data demonstrated that genetic screening for MYOC mutations should be focused toward cases with familial rather than sporadically occurring JOAG.

Evaluating correlation between the ocular biometry and genetic variants of MYOC and ABCA1 with primary angle-closure glaucoma in a cohort from northern China

AIM

To investigate whether the gene variants in MYOC and ABCA1 are associated with primary angle-closure glaucoma (PACG) and anterior chamber depth (ACD) and axial length (AL) in samples from northern China.

METHODS

The present case-control association study consisted of 500 PACG patients and 720 unrelated controls. Each participant was genotyped for eleven single nucleotide polymorphisms (SNPs) in MYOC and ABCA1 genes (rs12076134, rs183532, rs235875 and rs235913 in MYOC, rs2422493, rs2487042, rs2472496, rs2472493, rs2487032, rs2472459 and rs2472519 near ABCA1) using an improved multiplex ligation detection reaction (iMLDR) technique. The genetic association analyses were performed by PLINK using a logistic regression model. The association between genotypes and ocular biometric parameters was performed by SPSS using generalized estimation equation. Bonferroni corrections were implemented and the statistical power was calculated by the Power and Sample Size Calculation.

RESULTS

Two SNPs rs183532 and rs235875 as well as a haplotype TTC in MYOC were nominally associated with PACG despite the significance was lost after Bonferroni correction. No association was observed between ABCA1 and PACG, neither did the association between these variants and ACD as well as AL.

CONCLUSION

The present study suggests MYOC and ABCA1 do not play a part in the pathogenesis of PACG as well as the regulation of ocular biometric parameters in a northern Chinese population. Further investigations with large sample size are needed to verify this consequence.

Non-Synonymous variants in premelanosome protein (PMEL) cause ocular pigment dispersion and pigmentary glaucoma

Pigmentary glaucoma (PG) is a common glaucoma subtype that results from release of pigment from the iris, called pigment dispersion syndrome (PDS), and its deposition throughout the anterior chamber of the eye. Although PG has a substantial heritable component, no causative genes have yet been identified. We used whole exome sequencing of two independent pedigrees to identify two premelanosome protein (PMEL) variants associated with heritable PDS/PG. PMEL encodes a key component of the melanosome, the organelle essential for melanin synthesis, storage and transport. Targeted screening of PMEL in three independent cohorts (n = 394) identified seven additional PDS/PG-associated non-synonymous variants. Five of the nine variants exhibited defective processing of the PMEL protein. In addition, analysis of PDS/PG-associated PMEL variants expressed in HeLa cells revealed structural changes to pseudomelanosomes indicating altered amyloid fibril formation in five of the nine variants. Introduction of 11-base pair deletions to the homologous pmela in zebrafish by the clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 method caused profound pigmentation defects and enlarged anterior segments in the eye, further supporting PMEL's role in ocular pigmentation and function. Taken together, these data support a model in which missense PMEL variants represent dominant negative mutations that impair the ability of PMEL to form functional amyloid fibrils. While PMEL mutations have previously been shown to cause pigmentation and ocular defects in animals, this research is the first report of mutations in PMEL causing human disease.

Occurrence of MYOC and CYP1B1 variants in juvenile open angle glaucoma Brazilian patients

BACKGROUND

The purpose of this study was to screen juvenile open angle glaucoma (JOAG) patients from Brazil for variants within the MYOC and CYP1B1 genes.

MATERIAL AND METHODS

In this study, we evaluated the coding regions of MYOC and CYP1B1 genes in 100 non-related patients with JOAG and 200 controls through Sanger sequencing. We also tested the most frequent single nucleotide variants of CYP1B1 for association with JOAG.

RESULTS

Sixteen different sequence variants in the MYOC gene were observed in JOAG patients: eight variants were described as neutral and eight were identified in 34 out of 100 patients with JOAG and no controls, thus being considered damaging. In the CYP1B1 gene, nine neutral variants and two damaging alterations were found among JOAG patients. No association between CYP1B1 variants and JOAG was detected.

CONCLUSION

While MYOC damaging alterations were highly prevalent (34%), CYP1B1 damaging variants were less frequent (2%) in this cohort of Brazilian JOAG patients.

Differential Misfolding Properties of Glaucoma-Associated Olfactomedin Domains from Humans and Mice

Mutations in myocilin, predominantly within its olfactomedin (OLF) domain, are causative for the heritable form of open angle glaucoma in humans. Surprisingly, mice expressing Tyr423His mutant myocilin, corresponding to a severe glaucoma-causing mutation (Tyr437His) in human subjects, exhibit a weak, if any, glaucoma phenotype. To address possible protein-level discrepancies between mouse and human OLFs, which might lead to this outcome, biophysical properties of mouse OLF were characterized for comparison with those of human OLF. The 1.55 Å resolution crystal structure of mouse OLF reveals an asymmetric 5-bladed β-propeller that is nearly indistinguishable from previous structures of human OLF. Wild-type and selected mutant mouse OLFs mirror thermal stabilities of their human OLF counterparts, including characteristic stabilization in the presence of calcium. Mouse OLF forms thioflavin T-positive aggregates with a similar end-point morphology as human OLF, but amyloid aggregation kinetic rates of mouse OLF are faster than human OLF. Simulations and experiments support the interpretation that kinetics of mouse OLF are faster because of a decreased charge repulsion arising from more neutral surface electrostatics. Taken together, phenotypic differences observed in mouse and human studies of mutant myocilin could be a function of aggregation kinetics rates, which would alter the lifetime of putatively toxic protofibrillar intermediates.

Comprehensive sequencing of the myocilin gene in a selected cohort of severe primary open-angle glaucoma patients

Primary open-angle glaucoma (POAG) is the most common form of glaucoma, prevalent in approximately 1–2% of Caucasians in the UK over the age of 40. It is characterised by an open anterior chamber angle, raised intraocular pressure (IOP) and optic nerve damage leading to loss of sight. The myocilin gene (MYOC) is the most common glaucoma-causing gene, accounting for ~2% of British POAG cases. 358 patients were selected for next generation sequencing (NGS) with the following selection criteria: Caucasian ethnicity, intraocular pressure (IOP) 21–40 mm Hg, cup:disc ratio ≥0.6 and visual field mean deviation ≤−3. The entire MYOC gene (17,321 bp) was captured including the promoter, introns, UTRs and coding exons. We identify 12 exonic variants (one stop-gain, five missense and six synonymous variants), two promoter variants, 133 intronic variants, two 3′ UTR variants and 23 intergenic variants. Four known or predicted pathogenic exonic variants (p.R126W, p.K216K, p.Q368* and p.T419A) were identified across 11 patients, which accounts for 3.07% of this POAG cohort. This is the first time that the entire region of MYOC has been sequenced and variants reported for a cohort of POAG patients.

Estimating the age of the p.Cys433Arg variant in the MYOC gene in patients with primary open-angle glaucoma

The aim of this study was to estimate the age of the Cys433Arg (c.1297T>C, p.Cys433Arg) variant by comparing the genotypes of individuals affected and not affected by primary open angle glaucoma juvenile onset (JOAG). Our sample consisted of 35 JOAG-affected individuals from three families, 16 unrelated patients with the MYOC p.Cys433Arg variant and 16 unaffected individuals. Genomic DNA was amplified by PCR; nine short tandem repeats were genotyped through automated electrophoresis and three single nucleotide polymorphisms through Sanger sequencing. The determination of haplotypes was performed using Arlequin software and age estimation was performed using DMLE+ 2.3 and BDMC21 softwares. Four markers constituted the haplotypes associated with the p.Cys433Arg variant. The software DMLE+2.3 predicted an age of 43 generations for this variant with a 95% confidence interval ranging from 28 to 76 generations (560-1520 years) and BDMC21 predicted an age of 59 generations (1180 years) (95% CI: 40 to 100).

Binding of a glaucoma-associated myocilin variant to the αB-crystallin chaperone impedes protein clearance in trabecular meshwork cells

Myocilin (MYOC) was discovered more than 20 years ago and is the gene whose mutations are most commonly observed in individuals with glaucoma. Despite extensive research efforts, the function of WT MYOC has remained elusive, and how mutant MYOC is linked to glaucoma is unclear. Mutant MYOC is believed to be misfolded within the endoplasmic reticulum, and under normal physiological conditions misfolded MYOC should be retro-translocated to the cytoplasm for degradation. To better understand mutant MYOC pathology, we CRISPR-engineered a rat to have a MYOC Y435H substitution that is the equivalent of the pathological human MYOC Y437H mutation. Using this engineered animal model, we discovered that the chaperone αB-crystallin (CRYAB) is a MYOC-binding partner and that co-expression of these two proteins increases protein aggregates. Our results suggest that the misfolded mutant MYOC aggregates with cytoplasmic CRYAB and thereby compromises protein clearance mechanisms in trabecular meshwork cells, and this process represents the primary mode of mutant MYOC pathology. We propose a model by which mutant MYOC causes glaucoma, and we propose that therapeutic treatment of patients having a MYOC mutation may focus on disrupting the MYOC-CRYAB complexes.

Molecular Genetics of Pigment Dispersion Syndrome and Pigmentary Glaucoma: New Insights into Mechanisms

We explore the ideas and advances surrounding the genetic basis of pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG). As PG is the leading cause of nontraumatic blindness in young adults and current tailored interventions have proven ineffective, a better understanding of the underlying causes of PDS, PG, and their relationship is essential. Despite PDS being a subclinical disease, a large proportion of patients progress to PG with associated vision loss. Decades of research have supported a genetic component both for PDS and conversion to PG. We review the body of evidence supporting a genetic basis in humans and animal models and reevaluate classical mechanisms of PDS/PG considering this new evidence.

Adult-Onset Primary Glaucoma and Molecular Genetics: A Review

Purpose

To evaluate recent molecular genetic studies focused on localizing and identifying the genes involved in adult-onset primary glaucoma, characterizing the gene products, and investigating the molecular mechanisms implicated in the pathophysiology of the disease.

METHODS

Several studies have aimed at understanding gene expression and protein processing and attempting to correlate the mutations identified in the involved genes, particularly the TIGR/MYOC gene, with the overall spectrum of the disease, ranging from juvenile glaucoma to typical late-onset primary open-angle glaucoma. Genetic research lemains essential until highly specific and sensitive tests have been developed (plausible disease-causing sequence variations, polymorphisms).

Results

The most effective method for detecting glaucoma clinically is the study of optic nerve and visual field damage, as well as intraocular pressure. In subjects at high risk, in members of families with a strong history of inherited glaucoma, and in families with a MYOC-positive test, the result may represent a marker to assess presymptomatic diagnosis and may be useful as a prognostic marker.

CONCLUSIONS

OPTN seems to have a role confined to the pathogenesis of normotensive glaucoma with a few exceptions. Presently, the introduction of the expensive and time-consuming OPTN gene test in the current diagnosis of familial glaucoma is not justified.

Structure and Misfolding of the Flexible Tripartite Coiled-Coil Domain of Glaucoma-Associated Myocilin

Glaucoma-associated myocilin is a member of the olfactomedins, a protein family involved in neuronal development and human diseases. Molecular studies of the myocilin N-terminal coiled coil demonstrate a unique tripartite architecture: a Y-shaped parallel dimer-of-dimers with distinct tetramer and dimer regions. The structure of the dimeric C-terminal 7-heptad repeats elucidates an unexpected repeat pattern involving inter-strand stabilization by oppositely charged residues. Molecular dynamics simulations reveal an alternate accessible conformation in which the terminal inter-strand disulfide limits the extent of unfolding and results in a kinked configuration. By inference, full-length myocilin is also branched, with two pairs of C-terminal olfactomedin domains. Selected variants within the N-terminal region alter the apparent quaternary structure of myocilin but do so without compromising stability or causing aggregation. In addition to increasing our structural knowledge of naturally occurring extracellular coiled coils and biomedically important olfactomedins, this work broadens the scope of protein misfolding in the pathogenesis of myocilin-associated glaucoma.

Identification of Mutations in Myocilin and Beta-1,4-galactosyltransferase 3 Genes in a Chinese Family with Primary Open-angle Glaucoma

BACKGROUND

Glaucoma is a major cause of irreversible blindness worldwide. There is evidence showing that a subset of the disease is genetically determined. In this study, we screened for mutations in chromosome 1q-linked open-angle glaucoma (GLC1A) in a Chinese family with primary open-angle glaucoma (POAG).

METHODS

A total of 23 members from five generations of a family were enrolled and underwent thorough ophthalmologic examinations. In addition, 200 unrelated healthy Chinese controls were also recruited as normal control. GLC1A gene was amplified by polymerase chain reaction, and DNA sequencing was performed to screen for mutations.

RESULTS

Six members were diagnosed as POAG, with severe clinical manifestations, and history of high intraocular pressures. The mean age of disease onset was 26.3 years. However, the others were asymptomatic. In six affected and three asymptomatic members, gene sequencing revealed a mutation c.C1456T in exon 3 of myocilin gene (MYOC). Furthermore, we also identified a novel mutation c.G322A in beta-1,4-galactosyltransferase 3 (B4GALT3) gene in all six affected and three asymptomatic members, which was not reported previously in POAG patients. The two newly identified variants were absent in other family members as well as controls.

CONCLUSION

The mutations c.1456C < T (p.L486F) in MYOC and c.322G < A (p.V108I) in B4GALT3 are likely responsible for the pathogenesis of POAG in this family.

Haplotype analysis of association of the MYOC gene with primary angle-closure glaucoma in a Han Chinese population

PURPOSE

The aim of this study is to examine whether or not myocilin (MYOC) genetic variations are associated with susceptibility to primary angle-closure glaucoma (PACG) in the Han Chinese population.

METHODS

Four single-nucleotide polymorphisms (SNPs)-rs235913, rs183532, rs12076134, and rs235875-in the MYOC gene were genotyped in 212 adult patients with PACG and 255 age-, sex-, and ethnic-matched healthy controls by using a polymerase chain reaction-restriction fragment length polymorphism assay. Data were analyzed by chi-square analysis.

RESULTS

The four SNPs in the MYOC gene were in the Hardy-Weinberg equilibrium in all the subjects. The frequencies of A allele rs183532 were significantly different between the PACG patients and the controls (0.238 vs. 0.169, p=0.008; OR=1.541; 95% CI: 1.117-2.127). The frequencies of the AA genotype and A allele of rs235913 were increased in PACG patients compared with controls, but the difference was not significant (p=0.037, p=0.017, respectively). A comparison of the distributions of the genotypes and alleles of rs12076134 and rs235875 showed no statistically significant differences between the PACG patients and the controls (p>0.05). Haplotype analysis indicated that the frequency of the AATG and AATA haplotypes was significantly higher for PACG patients than for control subjects (both p<0.001). However, the frequency of CGGA and CGTG haplotypes was lower for PACG patients than for control subjects (p<0.001).

CONCLUSIONS

Our study suggests that rs183532 is associated with an increased risk of PACG in the Chinese Han population.

Analyses of Sequence Variants in the MYOC Gene and of Single Nucleotide Polymorphisms in the NR3C1 and FKBP5 Genes in Corticosteroid-Induced Ocular Hypertension

BACKGROUND

To perform an independent replication study to determine whether genetic variants in MYOC, NR3C1 and FKBP5 are involved in steroid-induced ocular hypertension.

MATERIALS AND METHODS

A retrospective case-control study was peformed on native Dutch patients who were treated with 4.0 mg intravitreal triamcinolone acetonide (IVTA). The patients were divided into an intraocular hypertension group (intraocular pressure >21 mmHg within a year after IVTA) and a non-intraocular hypertension group. The cohort was genotyped for 31 single-nucleotide polymorphisms (SNPs): 21 in NR3C1 and 10 in FKBP5. In addition, the open reading frame of MYOC was sequenced.

RESULTS

A total of 102 patients were included in this study: 58 steroid responders and 44 non-responders. No significant associations were found for the studied SNPs in NR3C1 and FKBP5. Heterozygous amino acid variants were detected in the MYOC gene in two patients of the non-intraocular hypertension group.

CONCLUSIONS

This study does not confirm a role for genetic variants in the MYOC, NR3C1 and FKBP5 genes in the pathogenesis of corticosteroid-induced ocular hypertension.

Targeting the ER-autophagy system in the trabecular meshwork to treat glaucoma

A major drainage network involved in aqueous humor dynamics is the conventional outflow pathway, which is gated by the trabecular meshwork (TM). The TM acts as a molecular sieve, providing resistance to aqueous outflow, which is responsible for regulating intraocular pressure (IOP). If the TM is damaged, aqueous outflow is impaired, IOP increases and glaucoma can manifest. Mutations in the MYOC gene cause hereditary primary open-angle glaucoma (POAG) by promoting the abnormal amyloidosis of the myocilin protein in the endoplasmic reticulum (ER), leading to ER stress-induced TM cell death. Myocilin accumulation is observed in approximately 70-80% of all glaucoma cases suggesting that environmental or other genetic factors may also promote myocilin toxicity. For example, simply preventing myocilin glycosylation is sufficient to promote its abnormal accretion. These myocilin amyloids are unique as there are no other known pathogenic proteins that accumulate within the ER of TM cells and cause toxicity. Moreover, this pathogenic accumulation only kills TM cells, despite expression of this protein in other cell types, suggesting that another modifier exclusive to the TM participates in the proteotoxicity of myocilin. ER autophagy (reticulophagy) is one of the pathways essential for myocilin clearance that can be impacted dramatically by aging and other environmental factors such as nutrition. This review will discuss the link between myocilin and autophagy, evaluating the role of this degradation pathway in glaucoma as well as its potential as a therapeutic target.

The genetic mechanisms of primary angle closure glaucoma

Primary Angle Closure Glaucoma (PACG) is one of the most common types of glaucoma affecting over 15 million individuals worldwide. Family history and ethnicity are strongly associated with the development of the disease, suggesting that one or more genetic factors contribute to PACG. Although strictly heritable disease-causing mutations have not been identified, a number of recent association studies have pointed out genetic factors that appear to contribute to an individual's risk to develop PACG. In addition, genetic factors have been identified that modify PACG endophenotypes for example, axial length. Herein we review the current literature on this important topic.

Identification of a novel MYOC mutation in a Chinese family with primary open-angle glaucoma

PURPOSE

The myocilin (MYOC) gene has been shown to be related to primary open-angle glaucoma (POAG). This study was aimed to detect the mutations in MYOC in a Chinese family with POAG.

METHODS

A family with four members, the parents, a son and a daughter, was enrolled in this study. All members of the family underwent the complete ophthalmologic examinations. Genomic DNA was collected from peripheral blood of all the participants. The coding sequence of MYOC was amplified by polymerase chain reaction (PCR), followed by direct DNA sequencing.

RESULTS

The son, who was the proband of this family, was diagnosed as early-onset POAG in both eyes. His mother was diagnosed as POAG ten years ago. A novel heterozygous missense mutation c.761C<G (p.P254R) in the MYOC gene, was identified as being co-segregated with the phenotype of this family. This mutation was detected in the two affected patients, but not present in the other normal family members or 384 normal controls.

CONCLUSIONS

This study provides a mutation spectrum of MYOC resulting in POAG development in a Chinese population, which may help to better understand the molecular pathogenesis and clinical diagnosis of MYOC-associated POAG.

Structural basis for misfolding in myocilin-associated glaucoma

Olfactomedin (OLF) domain-containing proteins play roles in fundamental cellular processes and have been implicated in disorders ranging from glaucoma, cancers and inflammatory bowel disorder, to attention deficit disorder and childhood obesity. We solved crystal structures of the OLF domain of myocilin (myoc-OLF), the best studied such domain to date. Mutations in myoc-OLF are causative in the autosomal dominant inherited form of the prevalent ocular disorder glaucoma. The structures reveal a new addition to the small family of five-bladed β-propellers. Propellers are most well known for their ability to act as hubs for protein-protein interactions, a function that seems most likely for myoc-OLF, but they can also act as enzymes. A calcium ion, sodium ion and glycerol molecule were identified within a central hydrophilic cavity that is accessible via movements of surface loop residues. By mapping familial glaucoma-associated lesions onto the myoc-OLF structure, three regions sensitive to aggregation have been identified, with direct applicability to differentiating between neutral and disease-causing non-synonymous mutations documented in the human population worldwide. Evolutionary analysis mapped onto the myoc-OLF structure reveals conserved and divergent regions for possible overlapping and distinctive functional protein-protein or protein-ligand interactions across the broader OLF domain family. While deciphering the specific normal biological functions, ligands and binding partners for OLF domains will likely continue to be a challenging long-term experimental pursuit, atomic detail structural knowledge of myoc-OLF is a valuable guide for understanding the implications of glaucoma-associated mutations and will help focus future studies of this biomedically important domain family.

A molecular mechanism for glaucoma: endoplasmic reticulum stress and the unfolded protein response

Primary open angle glaucoma (POAG) is a common late-onset neurodegenerative disease. Ocular hypertension represents a major risk factor, but POAG etiology remains poorly understood. Some cases of early-onset congenital glaucoma and adult POAG are linked to mutations in myocilin, a secreted protein of poorly defined function. Transgenic overexpression of myocilin in Drosophila and experiments in mice and human populations implicate the unfolded protein response (UPR) in the pathogenesis of glaucoma. We postulate that compromised ability of the UPR to eliminate misfolded mutant or damaged proteins, including myocilin, causes endoplasmic reticulum stress, resulting in functional impairment of trabecular meshwork cells that regulate intraocular pressure. This mechanism of POAG is reminiscent of other age-dependent neurodegenerative diseases that involve accumulation of protein aggregates.

Single nucleotide polymorphism of MYOC affected the severity of primary open angle glaucoma

AIM

To detect the mutations in two candidate genes, myocilin (MYOC) and cytochrome P450 1B1 (CYP1B1), in a Chinese family with primary open angle glaucoma (POAG).

METHODS

The family was composed of three members, the parents and a daughter. All members of the family underwent complete ophthalmologic examinations. Exons of MYOC and CYP1B1 genes were screened for sequence alterations by polymerase chain reaction (PCR) and direct DNA sequencing.

RESULTS

The mother was the proband, she was diagnosed as POAG in both eyes. Her daughter was diagnosed as juvenile-onset POAG. The father was asymptomatic. One MYOC heterozygous mutation c.1150 G>A (D384N) in exon 3 was identified in the mother, another MYOC heterozygous variation c.1058 C>T (T353I) in exon 3 was identified in the father, and the daughter inherited both of the variations. Meanwhile, three single nucleotide polymorphisms (SNPs) in CYP1B1 gene were found in the family.

CONCLUSION

The D384N mutation of MYOC has been reported as one of disease-causing mutations in POAG, whereas T353I variation of MYOC was thought as a high risk factor for POAG. The two variations of MYOC were first reported in one juvenile-onset POAG patient who presented with more severe clinical manifestations, suggesting that T353I polymorphism of MYOC may be associated with the severity of POAG.

Higher prevalence of myocilin mutations in advanced glaucoma in comparison with less advanced disease in an Australasian disease registry

OBJECTIVES

To determine the proportion of all Myocilin coding mutations responsible for advanced primary open-angle glaucoma (POAG) in early-age-at-onset individuals and to investigate the prevalence of exon 3 Myocilin mutations in advanced POAG at any age at onset in a large Australasian cohort.

DESIGN

Cross-sectional study using a national disease registry.

PARTICIPANTS

One thousand sixty individuals with advanced POAG (103 with age at onset of 40 years or younger) and 320 with nonadvanced POAG all recruited by the Australian and New Zealand Registry of Advanced Glaucoma.

METHODS

Participants were examined and referred by their eye practitioner, and Myocilin genetic testing was performed by direct sequencing. Cascade genetic testing was made available for relatives of participants found to carry a Myocilin mutation.

MAIN OUTCOME MEASURES

Advanced glaucoma diagnosis based on strict visual field entry criteria. Prevalence and spectrum of Myocilin mutations in individuals with advanced and nonadvanced POAG.

RESULTS

This is the first study to report Myocilin mutations in an advanced POAG cohort. No pathogenic Myocilin mutations were identified in exons 1 and 2 in early-age-at-onset advanced POAG cases. Exon 3 Myocilin mutations were identified in 45 advanced POAG patients (4.2%), which is significantly higher (P = 0.02) compared with nonadvanced POAG patients (1.6%). A novel mutation (Trp373X) and a new variant of uncertain pathogenicity (Ala447Thr) also were reported. The prevalence of Myocilin mutations rose from 16% to 40% in selected advanced POAG subgroups based on different thresholds of maximum recorded intraocular pressure, age at diagnosis, and the presence and strength of positive family history. Twenty-six individuals with Myocilin mutations were identified through cascade genetic testing of first-degree relatives of affected mutation carriers.

CONCLUSIONS

The prevalence of Myocilin mutations in glaucoma cases with severe visual field loss is significantly greater than in nonadvanced glaucoma patients. Myocilin screening in phenotypically selected cases can have a much higher yield than in previous unselected series. Identifying individuals who have Myocilin mutations provides an opportunity to screen at-risk clinically unaffected relatives and to reduce glaucoma blindness through early management and intervention.

FINANCIAL DISCLOSURE(S)

The author(s) have no proprietary or commercial interest in any materials discussed in this article.

The genetics of pigment dispersion syndrome and pigmentary glaucoma

We review the inheritance patterns and recent genetic advances in the study of pigment dispersion syndrome (PDS) and pigmentary glaucoma (PG). Both conditions may result from combinations of mutations in more than one gene or from common variants in many genes, each contributing small effects. We discuss the currently known genetic loci that may be related with PDS/PG in humans, the role of animal models in expanding our understanding of the genetic basis of PDS, the genetic factors underlying the risk for conversion from PDS to PG and the relationship between genetic and environmental--as well as anatomical--risk factors.

Myocilin polymorphisms and primary open-angle glaucoma: a systematic review and meta-analysis

Background

Glaucoma is the leading cause of irreversible blindness in the world. Recent evidence indicates a role for genetic susceptibility to primary open-angle glaucoma (POAG). The relation between myocilin polymorphisms and POAG susceptibility has been studied in different populations.

Methods

A meta-analysis of 32 published genetic association case-control studies, which examined the relation between POAG and the R46X, R76K, Y347Y, T353I, and Q368X polymorphisms of the myocilin gene, was carried out.

Results

In meta-analysis, significant associations were observed between POAG risk and two myocilin polymorphisms with summarized odds ratio of 4.68 (95%CI, 2.02–10.85) for Q368X and 2.17 (95% CI, 1.32–3.57) for T353I. Both Q368X and T353I were significantly associated with high-tension glaucoma, with summarized odds ratio of 4.26 (1.69, 10.73) and 2.26 (1.37–3.72). In Westerners, significant association was observed for Q368X mutation (odds ratio, 5.17; 95% CI, 2.16–12.40). However, in Asians it was for T353I (odds ratio, 2.17; 95% CI, 1.32–3.57).

Conclusions

There is strong evidence that myocilin polymorphisms are associated with POAG susceptibility, and the prevalence of myocilin mutations might be ethnicity-dependent in Caucasians for Q368X and in Asians for T353I.

Keeping an eye on myocilin: a complex molecule associated with primary open-angle glaucoma susceptibility

MYOC encodes a secretary glycoprotein of 504 amino acids named myocilin. MYOC is the first gene to be linked to juvenile open-angle glaucoma (JOAG) and some forms of adult-onset primary open-angle glaucoma (POAG). The gene was identified as an up-regulated molecule in cultured trabecular meshwork (TM) cells after treatment with dexamethasone and was originally referred to as trabecular meshwork-inducible glucocorticoid response (TIGR). Elevated intraocular pressure (IOP), due to decreased aqueous outflow, is the strongest known risk factor for POAG. Increasing evidence showed that the modulation of the wild-type (wt) myocilin protein expression is not causative of glaucoma while some misfolded and self-assembly aggregates of mutated myocilin may be associated with POAG in related or unrelated populations. The etiology of the disease remains unclear. Consequently, a better understanding of the molecular mechanisms underlyingPOAG is required to obtain early diagnosis, avoid potential disease progression, and develop new therapeutic strategies. In the present study, we review and discuss the most relevant studies regarding structural characterizations, expressions, molecular interactions, putative functions of MYOC gene and/or its corresponding protein in POAG etiology.

The stability of myocilin olfactomedin domain variants provides new insight into glaucoma as a protein misfolding disorder

Myocilin variants, localized to the olfactomedin (OLF) domain, are linked to early-onset, inherited forms of open-angle glaucoma. Disease-causing myocilin variants accumulate within trabecular meshwork cells instead of being secreted to the trabecular extracellular matrix of the eye. We hypothesize that, like in other diseases of protein misfolding, aggregation and downstream pathogenesis originate from the compromised thermal stability of mutant myocilins. In an expansion of our pilot study of four mutants, we compare 21 additional purified OLF variants by using a fluorescence stability assay and investigate the secondary structure of the most stable variants by circular dichroism. Variants with lower melting temperatures are correlated with earlier glaucoma diagnoses. The chemical chaperone trimethylamine N-oxide is capable of restoring the stability of most, but not all, variants to wild-type (WT) levels. Interestingly, three reported OLF disease variants, A427T, G246R, and A445V, exhibited properties indistinguishable from those of WT OLF, but an increased apparent aggregation propensity in vitro relative to that of WT OLF suggests that biophysical factors other than thermal stability, such as kinetics and unfolding pathways, may also be involved in myocilin glaucoma pathogenesis. Similarly, no changes from WT OLF stability and secondary structure were detected for three annotated single-nucleotide polymorphism variants. Our work provides the first quantitative demonstration of compromised stability among many identified OLF variants and places myocilin glaucoma in the context of other diseases of protein misfolding.

Primary open angle glaucoma due to T377M MYOC: Population mapping of a Greek founder mutation in Northwestern Greece

Background:

Mutations in the MYOC gene have been shown to explain 5% of unrelated primary open angle glaucoma (POAG) in different populations. In particular, the T377M MYOC mutation has arisen at least three separate times in history, in Great Britain, India, and Greece. The purpose of this study is to investigate the distribution of the mutation among different population groups in the northwestern region of Greece.

Materials and methods:

We explored the distribution of the “Greek” T377M founder mutation in the Epirus region in Northwestern Greece, which could be its origin. Genotyping was performed in POAG cases and controls by PCR amplification of the MYOC gene, followed by digestion with restriction enzyme. Statistical analyses were performed by an exact test, the Kaplan–Meier method and the t-test.

Results:

In the isolated Chrysovitsa village in the Pindus Mountains, a large POAG family demonstrated the T377M mutation in 20 of 66 family members while no controls from the Epirus region (n = 124) carried this mutation (P < 0.001). Among other POAG cases from Epirus, 2 out of 14 familial cases and 1 out of 80 sporadic cases showed the mutation (P = 0.057). The probability of POAG diagnosis with advancing age among mutation carriers was 23% at age 40, and reached 100% at age 75. POAG patients with the T377M mutation were diagnosed at a mean age of 51 years (SD ± 13.9), which is younger than the sporadic or familial POAG cases: 63.1 (SD ± 11) and 66.8 (SD ± 9.8) years, respectively.

Conclusions:

The T377M mutation was found in high proportion in members of the Chrysovitsa family (30.3%), in lower proportion in familial POAG cases (14.2%) and seems rare in sporadic POAG cases (1.2%), while no controls (0%) from the Epirus region carried the mutation. Historical and geographical data may explain the distribution of this mutation within Greece and worldwide.

Variable clinical spectrum of the myocilin Gln368X mutation in a Dutch family with primary open angle glaucoma

PURPOSE

To describe the clinical phenotype in a family with primary open angle glaucoma harboring a p.Gln368X mutation in MYOC.

MATERIALS AND METHODS

We identified a proband with primary open angle glaucoma and the p.Gln368X MYOC mutation. She and her six siblings were examined clinically, including Heidelberg Retina Tomography II, and venous blood samples were screened for other variants in MYOC, WDR36, OPTN, and CYP1B1.

RESULTS

Four individuals showed the p.Gln368X MYOC mutation, no other genetic variations were assessed. Two of these four siblings had glaucomatous optic disc changes with corresponding visual field losses and abnormal Heidelberg Retina Tomography results by the Moorfields regression analysis, one had abnormal results by the Moorfields regression analysis but no visual field loss, and one showed no glaucomatous signs or symptoms at all. These findings did not correlate with the age of the affected individuals.

CONCLUSION

In the primary open angle glaucoma family described here, we documented a wide range in clinical symptoms, demonstrating a highly variable penetrance of the MYOC p.Gln368X mutation.

New mutation in the MYOC gene and its association with primary open-angle glaucoma in a Chinese family

Myocilin (MYOC) gene is expressed in many ocular tissues, including the trabecular meshwork, a specialized eye tissue essential in regulating intraocular pressure. Many mutations in MYOC have been detected in primary open-angle glaucoma (POAG). We investigated whether MYOC mutations contributed to the susceptibility to POAG in a Chinese family. In a four-generation family affected with POAG, ocular examinations were performed on all members of the pedigree to determine their disease status, and 200 healthy matched controls were recruited. PCR-restriction fragment length polymorphism (PCR-RFLP) analysis and DNA sequencing were used to determine the mutations in MYOC. Biological software was used to analyze the corresponding proteins for missense mutations. The c.1084G>- was found, for the first time, in four of eight affected patients and in one of two patients with suspected POAG. The c.1006C>T mutation was found in two of eight patients and in one of 19 subjects who were asymptomatic. The frequencies of c.1084G>- and c.1006C>T were 12.82 and 7.69%, respectively, in patients but not in the controls. These data provide additional clues to the pathogenesis of POAG because no other mutation was detected in either group. Our results suggest that the MYOC c.1084G>- may contribute to a genetic predisposition to POAG.

Genetic isolates in ophthalmic diseases

In recent years, noteworthy gains have been made in unravelling the genetic contribution to some complex ocular diseases, principally age-related macular degeneration. Yet, a relatively poor understanding of the genetic aetiology for many other heritable blinding diseases, such as glaucoma, keratoconus and myopia, remains. Genetic isolates, populations with varying degrees of geographical or cultural seclusion, provide an effective means for investigating the molecular mechanisms involved in human diseases. This is particularly true for rare diseases in which founded alleles can be rapidly driven to a high frequency due to restriction of gene flow in the population. Recent success in complex gene mapping has resulted from the widened linkage disequilibrium (LD) in the genome of genetically isolated populations. An improved understanding of the predisposing genetic risk factors allows for enhanced screening modalities and paves the foundations for the translation of genomic technology into the clinic. This review focuses on the role population isolates have had in the investigation of genes underlying complex eye diseases and discusses their likely usefulness given the expansion of large-scale case-control association studies.

Angle-closure: risk factors, diagnosis and treatment

INTRODUCTION

Primary angle-closure glaucoma (PACG) is the leading cause of blindness in East Asia. The disease can be classified into primary angle-closure suspect, primary angle closure (PAC), and PACG. Pupil-block, anterior nonpupil-block (plateau iris and peripheral iris crowding), lens related and retrolenticular mechanisms have been suggested as the four main mechanisms of angle closure.

RISK FACTORS

The risk factors for PAC are female gender, increasing age, Inuit or East Asian ethnicity, shallow anterior chamber, shorter axial length, and genetic factors.

DIAGNOSIS

The diagnosis of acute PAC is mainly clinical. Diagnosis can be made with careful slit lamp examination, including intraocular pressure (IOP) measurement and gonioscopy. The diagnosis of chronic PAC and chronic PACG also require a careful history to assess risk factors, slit lamp examination including IOP and gonioscopy. Further investigations may also be required including visual fields, ultrasound biomicroscopy, and other imaging methods.

MANAGEMENT

In acute PAC, rapid control of the IOP needs to be achieved to limit optic-nerve damage. This can be carried out medically, and/or by laser iridoplasty. Both the affected and fellow eye should undergo laser peripheral iridotomy (PI). The aim of treating chronic PAC is to eliminate the underlying pathophysiological mechanism and to reduce IOP. This can be done by carrying out laser PI, iridoplasty, medical therapy, or surgery (trabeculectomy, lens extraction, combined lens extraction with trabeculectomy and goniosynechialysis).

CONCLUSION

Angle-closure glaucoma is usually an aggressive, visually destructive disease. By assessing the risk factors and diagnosing the mechanism involved in a patient's condition, the management of that patient can be tailored appropriately.

Clinical features associated with an Asp380His Myocilin mutation in a US family with primary open-angle glaucoma

PURPOSE

To determine the glaucoma phenotype of an American pedigree with the myocilin Asp380His.

DESIGN

An observational case series study.

METHODS

An observational case series study was used to examine a family in which an Asp380His myocilin mutation was segregating. Thirteen family members were examined and medical records were obtained on the remaining two individuals. Blood samples were collected from all 15 participants following the tenets of the Helsinki declaration under the auspices of the Oregon Health & Sciences University Institutional Review Board and screened for myocilin variants by denaturing high-performance liquid chromatography (dHPLC). Any DNA samples with dHPLC data different from the control sample were sequenced for base pair analysis.

RESULTS

An Asp380His myocilin mutation was identified in eight members, seven of whom had primary open-angle glaucoma (POAG). The eighth individual had high intraocular pressures (IOPs). The disease presents in this family with extremely high IOPs requiring trabeculectomies to control the pressure. The age at diagnosis ranged from 30 to 45.

CONCLUSIONS

This family with an Asp380His myocilin mutation presents with an intermediate phenotype between juvenile- and adult-onset glaucoma. The Asp380 amino acid residue appears to be important in myocilin function based on the finding that substitution of this amino acid with four different amino acids (His, Ala, Asn, or Gly) all result in a similar presentation of POAG that is intermediate between the more severe clinical presentations observed in individuals with the Pro370Leu or Lys423Glu variant and the milder findings in patients with the Gln368Stop mutation.

Aggregated myocilin induces russell bodies and causes apoptosis: implications for the pathogenesis of myocilin-caused primary open-angle glaucoma

Primary open-angle glaucoma with elevated intraocular pressure is a leading cause of blindness worldwide. Mutations of myocilin are known to play a critical role in the manifestation of the disease. Misfolded mutant myocilin forms secretion-incompetent intracellular aggregates. The block of myocilin secretion was proposed to alter the extracellular matrix environment of the trabecular meshwork, with subsequent impediment of aqueous humor outflow leading to elevated intraocular pressure. However, the molecular pathogenesis of myocilin-caused glaucoma is poorly defined. In this study, we show that heteromeric complexes composed of wild-type and mutant myocilin were retained in the rough endoplasmic reticulum, aggregating to form inclusion bodies typical of Russell bodies. The presence of myocilin aggregates induced the unfolded protein response proteins BiP and phosphorylated endoplasmic reticulum-localized eukaryotic initiation factor-2alpha kinase (PERK) with the subsequent activation of caspases 12 and 3 and expression of C/EBP homologous protein (CHOP)/GADD153, leading to apoptosis. Our findings identify endoplasmic reticulum stress-induced apoptosis as a pathway to explain the reduction of trabecular meshwork cells in patients with myocilin-caused glaucoma. As a consequence, the phagocytotic capacity of the remaining trabecular meshwork cell population would be insufficient for effective cleaning of aqueous humor, constituting a major pathogenetic factor for the development of increased intraocular pressure in primary open-angle glaucoma.