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

A Simple Secretion Assay for Assessing New and Existing Myocilin Variants

PURPOSE

A lack of sufficient functional information exists for appropriately categorizing a large number of myocilin (MYOC) variants and their involvement in primary open angle glaucoma, hindering their clinical significance classification. Most glaucoma-causing MYOC mutations result in protein non-secretion and intracellular insoluble aggregate formation in cultured cells. Herein, we generated a Gaussia luciferase-based MYOC fusion protein to quickly and sensitively track the secretion of MYOC variants and compared these results to the better-established western blotting assay for MYOC.

METHODS

Fourteen clinically-derived MYOC variants with varying degrees of predicted pathogenicity were transfected into HEK-293A cells and analyzed by either a luciferase assay or western blotting.

RESULTS

Eight of the variants (G12R, V53A, T204T, P254L, T325T, D380H, D395_E396insDP, and P481S) had not been biochemically assessed previously. Of these, P254L and D395_E396insDP demonstrated significant secretion defects reminiscent of glaucoma-causing mutations. The luciferase assay results agreed with western blotting for thirteen of the fourteen variants (93%), suggesting a strong concordance.

CONCLUSIONS

These results suggest that the Gaussia luciferase assay may be used as a complementary or standalone assay for quickly assessing MYOC variant behavior and we anticipate that these results will be useful in MYOC variant curation and reclassification.

Penetrance of MYOC gene mutation in primary open-angle glaucoma: A systematic review and meta-analysis

PURPOSE

To investigate the penetrance of MYOC gene mutation in primary open-angle glaucoma (POAG) through systematic review and meta-analysis. To explore the factors affecting the penetrance of MYOC and provide evidence-based medical evidence for clinical work.

METHODS

We searched all studies that reported the penetrance of MYOC mutation in PubMed, Embase, Web of Science, and Chinese databases including Wanfang, CNKI (China National Knowledge Infrastructure), and CBM (China Bio-Med). Random effects meta-analysis was conducted to estimate the penetrance of MYOC mutation in POAG.

RESULTS

Fifty-two studies were included in this analysis after screening. Meta-analysis of the penetrance of MYOC mutation showed that the penetrance of MYOC mutation in POAG was 60% (95% CI: 51.0% to 68.0%) and the penetrance of MYOC mutation in POAG and suspected POAG was 68% (95% CI: 60.0% to 75.0%). The penetrance of MYOC mutation increases with age. Among Caucasians, Asians, and Africans, the penetrance of MYOC mutation in POAG was 55%, 71%, 54%, respectively, and the penetrance of MYOC mutation in POAG and suspected POAG was 64%, 83%, and 57%, respectively. Besides, the penetrance of different MYOC mutation sites was significantly discrepant. The penetrance of MYOC mutation in POAG ranged from 10.3% to 100% depending on the mutation sites. Some MYOC mutation sites have a certain population specificity, which is only pathogenic in Caucasians or Asians.

CONCLUSIONS

The penetrance of MYOC mutation in POAG showed significant differences due to different mutation sites. The penetrance increased with the accrescent of age. Ethnic difference was an important factor affecting the penetrance of MYOC mutation. Knowing the rules and influencing factors of the penetrance of MYOC mutations is significant for the assessment of the risk of POAG in carriers with the MYOC mutation.

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.

A novel single nucleotide polymorphism in exon 3 of MYOC enhances the risk of glaucoma

Genetic polymorphismsof MYOCalter the myocilin protein,which leads to disruption of thenormal regulation of intraocular pressure (IOP) that ultimately causes glaucoma.Theaim of the present study was to identify the polymorphism in exon 3 of the MYOC gene of theglaucoma patients in Lahore, Pakistan. We conducted a case-control study with 100 patients and 100 controls subjects. We extracted DNA from blood samples,amplified the target DNA fragmentby PCR, and identifiedpolymorphisms through sequencing. We observed that the allelic and genotypic frequencies of rs74315341 and rs879255525 were associated with glaucoma in our patient population. The polymorphism atrs74315341 led to the substitutionof serine for arginine,whereas the polymorphism at rs879255525 led to the substitution ofasparagine for lysine. The haplotype TGAAGCCATTTC was associated with disease onset, whereas the haplotype GGAAGCCATTTC was protective against disease development. In conclusion, weidentified MYOC gene polymorphisms in susceptible regions that were associated withglaucoma onset among the Lahore patient population.This is the first report to identify a novel mutation in rs879255525 in exon 3 of the MYOC genethat is associated withglaucoma.

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.

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.

Identification a novel MYOC gene mutation in a Chinese family with juvenile-onset open angle glaucoma

PURPOSE

To describe the clinical and genetic findings in one Chinese family with juvenile-onset open angle glaucoma (JOAG).

METHODS

One family was examined clinically and a follow-up took place 5 years later. After informed consent was obtained, genomic DNA was extracted from the venous blood of all participants. Linkage analysis was performed with three microsatellite markers around the MYOC gene (D1S196, D1S2815, and D1S218) in the family. Mutation screening of all coding exons of MYOC was performed by direct sequencing of PCR-amplified DNA fragments and restriction fragment length polymorphism (RFLP) analysis. Bioinformatics analysis by the Garnier-Osguthorpe-Robson (GOR) method predicted the effects of variants detected on secondary structures of the MYOC protein.

RESULTS

Clinical examination and pedigree analysis revealed a three- generation family with seven members diagnosed with JOAG, three with ocular hypertension, and five normal individuals. Through genotyping, the pedigree showed a linkage to the MYOC on chromosome 1q24-25. Mutation screening of MYOC in this family revealed an A-->T transition at position 1348 (p. N450Y) of the cDNA sequence. This missense mutation co-segregated with the disease phenotype of the family, but was not found in 100 normal controls. Secondary structure prediction of the p.N450Y by the GOR method revealed the replacement of a coil with a beta sheet at the amino acid 447.

CONCLUSIONS

Early onset JOAG, with incomplete penetrance, is consistent with a novel mutation in MYOC. The finding provides pre-symptomatic molecular diagnosis for the members of this family and is useful for further genetic consultation.

Association of POAG risk factors and the Thr377Met MYOC mutation in an isolated Greek population

PURPOSE

To characterize the MYOC genotype correlation with phenotypes in an isolated Greek population with a high incidence of glaucoma.

METHODS

Five hundred thirty-one villagers were enrolled in the study. Participants underwent a comprehensive ophthalmic examination. All three exons of myocilin were bidirectionally sequenced. Power calculations and measured genotype analysis was conducted using the genetic variance analysis program, SOLAR version 4.2, to account for the relatedness between individuals.

RESULTS

The participants, 376 of whom were linked in a single 11-generation pedigree, ranged in age from 10 to 95 years with a mean age of 49. Sixty-five individuals had POAG, and 27 of those carried the Thr377Met MYOC mutation. Both peak intraocular pressure and vertical cup-to dis- ratio were significantly associated with the MYOC Thr377Met variant (P = 9 x 10(-14) and P = 9 x 10(-8), respectively), whereas central corneal thickness showed no significant association (P < 0.7).

CONCLUSIONS

This village had a high frequency of glaucoma, with 12% of the participants aged 10 to 95 years having the disease. In this cohort, the Thr377Met MYOC mutation was significantly associated with both high intraocular pressures and high vertical cup-to-disc ratios. No association was found with central corneal thickness.

The novel Y371D myocilin mutation causes an aggressive form of juvenile open-angle glaucoma in a Caucasian family from the Middle-East

PURPOSE

To search for the genetic cause of juvenile open-angle glaucoma (JOAG) in a Caucasian family and to perform genotype/phenotype correlation studies in the kindred.

METHODS

Six members of a three-generation family originating from Uzbekistan and now living in the Middle East were recruited from one large clinic in Israel. Ophthalmologic investigations comprised of visual field assessments, intraocular pressure measurements, optic disc evaluation, and gonioscopy. Medical charts were obtained to date the onset of glaucoma and to evaluate aggressivity of the trait. We screened the myocilin gene (MYOC, OMIM 601652) by direct genomic sequencing of its three exons in all family members.

RESULTS

JOAG segregated as an autosomal dominant trait in four members of the family. The proband, a 14-year-old girl, had been diagnosed with juvenile open-angle glaucoma at 12 years old. Her mother, maternal aunt, and maternal grandfather all had JOAG that started at an early age. The disorder progressed rapidly even under optimal medical treatment, and all four patients had to undergo trabeculectomy. One missense mutation, Y371D (1111t-->g, Tyr [Y] 371 Asp [D]), was identified. This mutation cosegregated with the disorder in all affected members and was absent in 200 Caucasian controls. The Y371D MYOC mutation has not been reported before. One cousin of the proband was a silent heterozygotic carrier of the mutation and was still asymptomatic at nine years of age.

CONCLUSIONS

We identified a novel mutation (Y371D) in MYOC from a Caucasian family who presented with an aggressive form of JOAG that required early trabeculectomy. Genetic screening of the MYOC mutation was beneficial in predicting one asymptomatic heterozygotic carrier.

Myocilin variations and familial glaucoma in Taxiarchis, a small Greek village

PURPOSE

To initiate a prospective study of glaucoma in a Greek village reported over 30 years ago to have several large families with primary open-angle glaucoma (POAG).

METHODS

A random group of 126 villagers from Taxiarchis, Greece was examined in the village community center. The detailed evaluation included ophthalmic and general history, measurement of blood pressure, intraocular pressure (IOP), and central corneal thickness (CCT) as well as evaluation of the optic nerve status.

RESULTS

The incidence of glaucoma approached 18% in this small isolated village. Myocilin variants were present in almost half of the individuals screened with Arg76Lys and Thr377Met being the most common finding (25% and 17%, respectively). Over half of the individuals with the Thr377Met mutation were diagnosed with glaucoma. Two of these patients were homozygous for the Thr377Met mutation. Three individuals with the Arg76Lys polymorphism had glaucoma; however, two of these individuals also had the Thr377Met mutation. Only two patients with pseudoexfoliation were identified.

CONCLUSIONS

The incidence of glaucoma and the Thr377Met MYOC mutation in this population is much higher than that reported for other European populations.