Prevalence of myocilin mutations in adults with primary open-angle glaucoma in Ghana, West Africa

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.

Stable calcium-free myocilin olfactomedin domain variants reveal challenges in differentiating between benign and glaucoma-causing mutations

Nonsynonymous gene mutations can be beneficial, neutral, or detrimental to the stability, structure, and biological function of the encoded protein, but the effects of these mutations are often not readily predictable. For example, the β-propeller olfactomedin domain of myocilin (mOLF) exhibits a complex interrelationship among structure(s), stability, and aggregation. Numerous mutations within mOLF are linked to glaucoma; the resulting variants are less stable, aggregation-prone, and sequestered intracellularly, causing cytotoxicity. Here, we report the first stable mOLF variants carrying substitutions in the calcium-binding site that exhibit solution characteristics indistinguishable from those of glaucoma variants. Crystal structures of these stable variants at 1.8-2.0-Å resolution revealed features that we could not predict by molecular dynamics simulations, including loss of loop structure, helix unwinding, and a blade shift. Double mutants that combined a stabilizing substitution and a selected glaucoma-causing single-point mutant rescued in vitro folding and stability defects. In the context of full-length myocilin, secretion of stable single variants was indistinguishable from that of the WT protein, and the double mutants were secreted to varying extents. In summary, our finding that mOLF can tolerate particular substitutions that render the protein stable despite a conformational switch emphasizes the complexities in differentiating between benign and glaucoma-causing variants and provides new insight into the possible biological function of myocilin.

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 effects of myocilin expression on functionally relevant trabecular meshwork genes: a mini-review

Myocilin is a secreted glaucoma-associated protein, specifically induced by dexamethasone in human trabecular meshwork cells, where it was discovered. Myocilin is expressed in several tissues of the body, but it causes disease only in the eye. The protein contains two domains: an N-terminal region with significant homologies to nonmuscle myosin, and a C-terminal region, which is similar to the olfactomedin proteins. Forty percent of myocilin undergoes an intracellular endoproteolytic cleavage by calpain II, a calcium-dependent cysteine protease, which releases the 2 domains. The protein is known to interact with intracellular and extracellular matrix proteins, and some is released into the extracellular space associated with exosomes. Myocilin mutations are linked to glaucoma and induce elevated intraocular pressure. Most of the glaucoma-causative mutations map to the olfactomedin domain, which appears to be a critical domain for the function of the protein. Myocilin mutants are misfolded, aggregate in the endoplasmic reticulum, and are not secreted. Overexpression of myocilin and of its mutants in primary human trabecular meshwork cells triggers changes in the expression of numerous genes, many of which have been known to be involved in mechanisms important for the physiology and pathology of the tissue. Here we review recent studies from our laboratory and those of others that deal with trabecular meshwork genes, which are altered by the overexpression of wild-type and glaucoma-causative mutant myocilin genes.

Investigation of known genetic risk factors for primary open angle glaucoma in two populations of African ancestry

PURPOSE

Multiple genes have been associated with primary open angle glaucoma (POAG) in Caucasian populations. We now examine the association of these loci in populations of African ancestry, populations at particularly high risk for POAG.

METHODS

We genotyped DNA samples from two populations: African American (1150 cases and 999 controls) and those from Ghana, West Africa (483 cases and 593 controls). Our analysis included 57 single nucleotide polymorphisms (SNPs) in five loci previously associated with POAG at the genome-wide level, including CDKN2B-AS1, TMCO1, CAV1/CAV2, chromosome 8q22 intergenic region, and SIX1/SIX6. We evaluated association in the full datasets, as well as subgroups with normal pressure glaucoma (NPG, maximum IOP ≤21 mm Hg) and high pressure glaucoma (HPG, IOP >21 mm Hg).

RESULTS

In African Americans, we identified an association of rs10120688 in the CDNK2B-AS1 region with POAG (P = 0.0020). Several other SNPs were nominally associated, but did not survive correction for multiple testing. In the subgroup analyses, significant associations were identified for rs10965245 (P = 0.0005) in the CDKN2B-AS1 region with HPG and rs11849906 in the SIX1/SIX6 region with NPG (P = 0.006). No significant association was identified with any loci in the Ghanaian samples.

CONCLUSIONS

POAG genetic susceptibility alleles associated in Caucasians appear to play a greatly reduced role in populations of African ancestry. Thus, the major genetic components of POAG of African origin remain to be identified. This finding underscores the critical need to pursue large-scale genome-wide association studies in this understudied, yet disproportionately affected population.

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.

Epidemiological properties of primary open angle glaucoma in Nigeria

Background. Primary open angle glaucoma (POAG) is progressive chronic optic neuropathy in adults in which intraocular pressure (IOP) and other currently unknown factors contribute to damage. POAG is the second commonest cause of avoidable blindness in Nigeria. Pattern of Presentation. POAG is characterized by late presentation. Absence of pain which is a driving force for seeking medical help, inadequacy of trained eye care personnel, paucity of facilities, misdistribution of resources, lack of awareness, poor education, and poverty may all contribute to this. Medical and surgical treatment options available are challenging and tasking. Screening for Glaucoma. Screening is the presumptive identification of unrecognized disease (POAG) by applying test(s) which can be applied rapidly. Such test(s) should be of high reliability, validity, yield, acceptable, and cost effective. The test should ideally be sensitive, specific, and efficient. It is difficult to select a suitable test that meets these criteria. Intraocular pressure (IOP) appears to be the easiest option. But, high IOP is not diagnostic nor does normal value exclude the disease. Health education is a possible strategy in early case detection and management. Treatment of POAG. Glaucoma treatment can either be medical or surgical (this includes laser). Considering unavailability, potency, cost, and long-term effects of medication, surgery (trabeculectomy) could be a better option. Laser trabeculoplasty is available in a few centers. Viscocanalostomy is not routinely performed. Patient education is vital to success as management is for life. Conclusion. POAG remains a cause of avoidable blindness in Nigeria. There is need for long-term strategy to identify patients early and institute prompt management. Improvement in training of eye care personnel and provision of up to date equipment is essential in achieving this goal.

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.

Cystatin a, a potential common link for mutant myocilin causative glaucoma

Myocilin (MYOC) is a 504 aa secreted glycoprotein induced by stress factors in the trabecular meshwork tissue of the eye, where it was discovered. Mutations in MYOC are linked to glaucoma. The glaucoma phenotype of each of the different MYOC mutation varies, but all of them cause elevated intraocular pressure (IOP). In cells, forty percent of wild-type MYOC is cleaved by calpain II, a cysteine protease. This proteolytic process is inhibited by MYOC mutants. In this study, we investigated the molecular mechanisms by which MYOC mutants cause glaucoma. We constructed adenoviral vectors with variants Q368X, R342K, D380N, K423E, and overexpressed them in human trabecular meshwork cells. We analyzed expression profiles with Affymetrix U133Plus2 GeneChips using wild-type and null viruses as controls. Analysis of trabecular meshwork relevant mechanisms showed that the unfolded protein response (UPR) was the most affected. Search for individual candidate genes revealed that genes that have been historically connected to trabecular meshwork physiology and pathology were altered by the MYOC mutants. Some of those had known MYOC associations (MMP1, PDIA4, CALR, SFPR1) while others did not (EDN1, MGP, IGF1, TAC1). Some, were top-changed in only one mutant (LOXL1, CYP1B1, FBN1), others followed a mutant group pattern. Some of the genes were new (RAB39B, STC1, CXCL12, CSTA). In particular, one selected gene, the cysteine protease inhibitor cystatin A (CSTA), was commonly induced by all mutants and not by the wild-type. Subsequent functional analysis of the selected gene showed that CSTA was able to reduce wild-type MYOC cleavage in primary trabecular meshwork cells while an inactive mutated CSTA was not. These findings provide a new molecular understanding of the mechanisms of MYOC-causative glaucoma and reveal CSTA, a serum biomarker for cancer, as a potential biomarker and drug for the treatment of MYOC-induced glaucoma.

Myocilin and optineurin coding variants in Hispanics of Mexican descent with POAG

Coding variants in both myocilin (MYOC) and optineurin (OPTN) are reported risk factors for primary open-angle glaucoma (POAG) in many populations. This study investigated the contribution of MYOC and OPTN coding variants in Hispanics of Mexican descent with and without POAG. We conducted a case-control study of unrelated POAG cases and nonglaucomatous controls in a population of Hispanics of Mexican descent. Ascertainment criteria for POAG included the presence of glaucomatous optic neuropathy with associated visual field loss and the absence of secondary causes of glaucoma. Controls had normal optic nerves, visual fields and intraocular pressure. All coding exons of MYOC and OPTN were sequenced. The data set consisted of 88 POAG cases and 93 controls. A novel nonsynonymous coding variant (R7H) in the first exon of MYOC was identified. Other identified variants in MYOC and OPTN have been previously described and do not seem to contribute to POAG risk. This is the first comprehensive study of MYOC and OPTN in Hispanics of Mexican descent with POAG. Neither MYOC nor OPTN sequence variants seem to have a major role in the etiology of POAG in this population.

Overexpression of myocilin in the Drosophila eye activates the unfolded protein response: implications for glaucoma

Background

Glaucoma is the world's second leading cause of bilateral blindness with progressive loss of vision due to retinal ganglion cell death. Myocilin has been associated with congenital glaucoma and 2–4% of primary open angle glaucoma (POAG) cases, but the pathogenic mechanisms remain largely unknown. Among several hypotheses, activation of the unfolded protein response (UPR) has emerged as a possible disease mechanism.

Methodology / Principal Findings

We used a transgenic Drosophila model to analyze whole-genome transcriptional profiles in flies that express human wild-type or mutant MYOC in their eyes. The transgenic flies display ocular fluid discharge, reflecting ocular hypertension, and a progressive decline in their behavioral responses to light. Transcriptional analysis shows that genes associated with the UPR, ubiquitination, and proteolysis, as well as metabolism of reactive oxygen species and photoreceptor activity undergo altered transcriptional regulation. Following up on the results from these transcriptional analyses, we used immunoblots to demonstrate the formation of MYOC aggregates and showed that the formation of such aggregates leads to induction of the UPR, as evident from activation of the fluorescent UPR marker, xbp1-EGFP.

Conclusions / Significance

Our results show that aggregation of MYOC in the endoplasmic reticulum activates the UPR, an evolutionarily conserved stress pathway that culminates in apoptosis. We infer from the Drosophila model that MYOC-associated ocular hypertension in the human eye may result from aggregation of MYOC and induction of the UPR in trabecular meshwork cells. This process could occur at a late age with wild-type MYOC, but might be accelerated by MYOC mutants to account for juvenile onset glaucoma.

The genetics of primary open-angle glaucoma: a review

Glaucoma is the major cause of irreversible blindness worldwide. Primary open-angle glaucoma (POAG), as the most prevalent form of glaucoma, is a complex inherited disorder and affects more than 2 million individuals in the United States. It has become increasingly clear that a host of genetic as well as environmental factors are likely to contribute to the phenotype. A number of chromosomal and genetic associations have been reported for POAG. This review examines what is currently known about the underlying genetic structure, what remains to be learned, and how this may affect our medical management of this major blinding disease.

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.

Novel MYOC gene mutation, Phe369Leu, in Japanese patients with primary open-angle glaucoma detected by denaturing high-performance liquid chromatography

PURPOSE

To screen for mutations in the MYOC gene in Japanese patients with primary open-angle glaucoma (POAG) using denaturing high-performance liquid chromatography (DHPLC).

PATIENTS AND METHODS

Blood samples were collected from 171 patients with POAG and 100 controls from seven institutions in Japan. For high-throughput analysis, seven exonic regions were amplified by polymerase chain reaction using DNA pooled from three patients; each DNA pool was then analyzed chromatographically. For analysis of a small number of samples, 7 exonic regions were amplified separately but simultaneously with annealing at 58 degrees C in each patient and then chromatographed, using 7 wells of the same 96-well plate per sample. When chromatographic patterns were abnormal by either method, the PCR products of the individual samples were sequenced.

RESULTS

Four glaucoma-causing mutations were identified in five POAG patients (2.9%). One missense mutation, Phe369Leu, is new; and three others, Ile360Asn, Ala363Thr, and Thr448Pro, have been reported in Japanese patients. Phe369Leu was associated with adult onset POAG.

CONCLUSIONS

Mutations in the MYOC gene were demonstrated chromatographically in 2.9% of our Japanese POAG patients. The use of pooled DNAs with DHPLC analysis is a time- and labor-saving technique. All mutations detected appear to be specific to Japanese patients.

The dawn of genetic testing for glaucoma

PURPOSE OF REVIEW

To review recent trends in genetic testing in medicine as they apply to newly evolving tests for patients with glaucoma.

RECENT FINDINGS

The utilization of powerful molecular methods for genetic testing is now entering its early stages in the practice of medicine in general, and with testing, many issues, both medical and societal, have been raised. Only recently has testing for a disease risk factor in primary open angle glaucoma (POAG) become available. It is known that some mutations in the gene myocilin are associated with POAG. MYOC.mt1 is a relatively common promoter region variant in the myocilin gene for which there is a commercially available test. Some investigators have found that MYOC.mt1 may increase disease severity in patients with POAG, whereas others have not found this association. There is a need for further testing for the role of the MYOC.mt1 variant in the pathogenesis of POAG.

SUMMARY

Genetic testing for glaucoma holds great promise. Currently available tests for disease-related tests in patients with glaucoma or at risk for this disease remain controversial.

Evaluation of optineurin sequence variations in 1,048 patients with open-angle glaucoma

PURPOSE

To investigate the association of sequence variations in the optineurin (OPTN) gene in patients with open-angle glaucoma.

DESIGN

Prospective case control study.

METHODS

The OPTN gene was screened for sequence variations using a combination of single-strand conformational polymorphism analysis and automated DNA sequencing. A total of 1,299 subjects (1048 glaucoma patients and 251 controls) were screened for variations in the four portions of the gene that had been previously associated with glaucoma. A subset of these subjects (376 patients and 176 controls) was screened for variations in the entire coding sequence. Twenty-four percent of the patients and 35% of the controls were Japanese, whereas the remainder were predominantly Caucasian. Allele frequencies were compared with the Fisher exact test.

RESULTS

The OPTN sequence variations were not significantly associated with any form of high-tension open-angle glaucoma. One proband with familial normal-tension glaucoma was found to harbor the previously reported Glu50Lys variation. Another previously reported change, Met98Lys, was associated with normal-tension glaucoma in Japanese but not in Caucasian patients.

CONCLUSIONS

This study provides some additional evidence for the association of the Glu50Lys OPTN sequence variation with familial normal tension glaucoma. However, because familial normal-tension glaucoma is so rare, this change seems to be responsible for less than 0.1% of all open-angle glaucoma. The Arg545Gln variation is likely to be a nondisease-causing polymorphism. The Met98Lys change may be associated with a fraction of normal-tension glaucoma in patients of Japanese ethnicity.