Analysis of myocilin gene mutations in Japanese patients with normal tension glaucoma and primary open-angle glaucoma

Prevalence of MYOC risk variants for glaucoma in different populations

PURPOSE

To assess the clinical relevance of myocilin (MYOC) gene variants as risk factors for glaucoma in literature and to estimate their prevalence in different populations.

METHODS

We reviewed the literature for published MYOC variants in glaucoma patients and estimated their prevalence in general population using gnomAD and BRAVO databases. We used several bioinformatics tools and the criteria of the American College of Medical Genetics and Genomics (ACMG) to assess the pathogenicity of the variants. We evaluated the carrier frequency of the variants in gnomAD, including its subpopulations.

RESULTS

We found 13 missense and 5 loss-of-function (LOF) reported variants in MYOC that were both probable pathogenic or risk variants and listed in gnomAD. Six likely pathogenic missense variants were p.(Cys25Arg), p.(Gln48His), p.(Gly326Ser), p.(Thr353Ile), p.(Thr377Met) and p.(Gly399Val). They were most prevalent in East and South Asia (frequency, 0.92% and 0.81%, respectively). The most common missense variants were p.(Thr353Ile) (0.91% in East Asia) and p.(Gln48His) (0.79% in South Asia). Five LOF variants were p.(Arg46Ter), p.(Arg91Ter), p.(Arg272Ter), p.(Gln368Ter) and p.(Tyr453MetfsTer11). We considered these glaucoma risk variants. They were most prevalent in the East Asian and the Finnish population (0.93% and 0.33%, respectively).

CONCLUSION

Pathogenic MYOC variants appear to be population-associated. Our results highlight allelic heterogeneity of MYOC variants in open-angle glaucoma. Many of the probable pathogenic variants are over-represented in some of the populations causing doubt of their status as monogenic disease-causing variants.

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.

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.

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.

Recurrent Myocilin Asn480Lys glaucoma causative mutation arises de novo in a family of Andean descent

PURPOSE

To search for MYOC mutations in Peruvian primary open angle glaucoma (POAG) families.

PATIENTS AND METHODS

Two patients from each of the 11 POAG Peruvian families were screened for sequence variants in MYOC coding exons by conformational sensitive gel electrophoresis and sequencing was performed on the samples indicating probable sequence changes.

RESULTS

We detected 2 families bearing distortions of conformational sensitive gel electrophoresis indicating mutations. Sequencing of these samples revealed coding sequence changes. A native Andean descent family presented with a MYOC mutation, Asn480Lys (C-->G at nucleotide 1440). This is different from the previously reported C-->A change at nucleotide 1440 that causes Asn480Lys in 2 unrelated French and Dutch families with glaucoma of variable expressivity, and indicates a third independent event. A second family of admixed origin showed the presence of the known Arg76Lys polymorphism.

CONCLUSIONS

In the study of MYOC variants in 11 POAG Peruvian families, we have found a family of ethnically admixed origin with polymorphism Arg76Lys and a family of Andean descent bearing a third event of the Asn480Lys, the MYOC mutation that has been reported in the highest number of POAG patients (>80 cases). Analysis of this family could contribute with information about disease manifestation, progression, and treatment response in the context of a distinct genetic background and also climatic, altitude, and socioeconomical conditions.

Glaucoma phenotype in a large Swiss pedigree with the myocilin Gly367Arg mutation

AIMS

To characterize genotype, phenotype, and age-related penetrance in a Swiss pedigree with juvenile open-angle glaucoma (JOAG).

METHODS

In a large Swiss family with history of glaucoma and 82 living members of four generations, we conducted molecular analysis and a detailed phenotype characterization in 52 family members. Mutation analysis was carried out using single-strand conformation polymorphism and DNA sequence analyses of the suspected candidate gene, myocilin (MYOC).

RESULTS

We detected a Gly367Arg mutation in the MYOC gene of 13 family members. Nine of them (69.2%) had glaucoma: mean IOP 35.3 mm Hg, range 24-50 mm Hg; mean age at diagnosis 34.9 years, range 28-51 years. Two mutation carriers were glaucoma suspects, one (age 15) was unaffected, and one (age 16) not available for clinical examinations. Age-related glaucoma penetrance was 50% at 30 and 78% at 40. Untreated IOP resulted in rapid disease progression, whereas good IOP control, usually only by means of filtration surgery, could stabilize the disease. None of the wild-type members had glaucoma.

CONCLUSIONS

This Swiss family is the largest reported Gly367Arg pedigree to date. The exact genotype and phenotype characterization allowed a reliable risk and prognosis assessment and targeted eye-care planning for the family. The study demonstrates the importance of genetic investigations in glaucoma families, carrying the potential of long-term socio-economic benefits.

Coiled–coil targeting of myocilin to intracellular membranes

Mutations in myocilin (MYOC) associate with glaucoma and ocular hypertension. Unfortunately, the specific role of MYOC, a widely expressed protein of unknown function, in ocular hypertension is unknown. Since MYOC localizes both to intracellular membranes and to the cytosol, we tested the hypothesis that MYOC is a cytosolic protein that associates with cellular membranes via its coiled-coil domain. Using green fluorescent protein (GFP) chimeras in expression and metabolic labeling studies, we observed that MYOC's putative signal peptide failed to traffic GFP into the secretory machinery and out of transfected cells. Next, we tested which of MYOC's three folding domains were responsible for targeting. In cell fractionation and immunofluorescence microscopy studies, the coiled-coil, but not the helix-turn-helix or olfactomedin domains, was necessary and sufficient to target GFP chimeras to cell membranes. Interestingly, a vesicular phenotype required sequential addition of the helix-turn-helix and olfactomedin domains to the coiled-coil. Taken together, these data indicate that the coiled-coil domain, not the putative signal sequence, is responsible for the targeting of MYOC to the secretory machinery.

Functional analysis of the glaucoma-causing TIGR/myocilin protein: Integrity of amino-terminal coiled-coil regions and olfactomedin homology domain is essential for extracellular adhesion and secretion

TIGR/MYOC mutations account for 2-4% of the primary open-angle glaucoma (POAG) patients. More than 90% of the known mutations are located within its carboxy-terminus olfactomedin-homology (Olf) domain (amino acids (aa) 245-504). In vitro and in vivo studies showed that several Olf domain mutations prevented myocilin secretion. To investigate if intracellular sequestration was a characteristic feature shared by a majority of the mutations, we analyzed the secretion status of 36 myocilin variants. These encompassed 26 glaucoma-causing mutations and 10 non-disease associated or undefined polymorphisms. As several variants were found to be secreted, we tested for their adhesion to the extracellular matrix (ECM) and/or cell surface. Myocilin variants were generated by site-directed mutagenesis of a vector encoding the human MYOC cDNA. COS-7 or immortalized human trabecular meshwork cells were transfected with wild-type or mutated MYOC constructs. Myocilin levels were estimated by immunoprecipitation and/or immunoblotting. All variants showed identical behaviors in both cell lines; the truncated R46X polypeptide being the only variant which could not be detected in our assays. Of the 35 variants monitored, 20 remained sequestered intracellularly. All of them encoded disease-causing polypeptides carrying Olf domain mutations. Of the 15 variants secreted into the culture medium, six (6) were POAG mutants (of which three (3) located within the Olf domain) while the remaining nine (9) were non-disease causing or undefined polymorphisms. Three (3) of the six (6) secreted mutations caused familial POAG; these were the R126W, T377M and A427T mutants. Both, the T377M and A427T mutants located within the Olf domain. When cells were cultured at 30 degrees C, a process known to facilitate protein folding, 11 of the 20 sequestered mutants were released in the extracellular medium. Out of the 15 secreted variants tested for their adhesion to the ECM and/or cell surface, only the R82C and L95P polypeptides displayed loss of their adhesive properties. Deletion experiments revealed that the coiled-coil (aa 78-105) and leucine zipper (aa 114-183) motifs were essential for adhesion. These experiments demonstrate that intracellular sequestration might be the primary mechanism contributing to myocilin-related POAG as it was associated with more than 80% of the disease-causing mutants tested in our study. A second mechanism may involve abnormal interaction(s) between myocilin and ECM and/or cell surface proteins. Our data further revealed the importance of the olfactomedin-homology domain for myocilin secretion and the significant role of the N-terminal region for its extracellular interactions.

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.

Recent advances in molecular genetics of glaucoma

Glaucoma represents a heterogeneous group of optic neuropathies, with different genetic bases. It can affect all ages generally with a rise in intra-ocular pressure. Three major types of glaucoma have been reported: primary open angle glaucoma (POAG), primary acute closed angle glaucoma (PACG) and primary congenital glaucoma (PCG), as well as a few others associated with developmental abnormalities. In recent years impressive progress has been made in the molecular genetic studies of POAG and PCG. These include the discovery of three genes--Myocilin, Optineurin and CYP1B1--defects in which results in Mendelian transmission of glaucoma. Identification of single nucleotide polymorphisms in multiple other genes that are associated with glaucoma and alteration of drug sensitivity are enriching our knowledge regarding the complex nature of the disease. This review attempts to present the recent progress made in the molecular genetics of glaucoma.

Genetic analysis of an Indian family with members affected with juvenile-onset primary open-angle glaucoma

PURPOSE

Glaucoma is the second leading cause of blindness. In India, approximately 1.5 million people are blind due to glaucoma. Mutations in the MYOC gene located at the GLC1A locus on chromosome 1q21-q31 have been found in patients with juvenile-onset primary open-angle glaucoma (J-POAG). The purpose of the present study was to identify the genetic cause of glaucoma in a four-generation Indian family affected with J-POAG.

METHODS

Peripheral blood samples were obtained from individuals for genomic DNA isolation. To determine if this family was linked to the GLC1A locus, haplotyping analysis was carried out using microsatellite markers from the GLC1A candidate region. Exon-specific primers from exon 3 of the MYOC gene were used to amplify DNA samples from individuals. Mutation analysis was carried out using PCR-SSCP and DNA sequence analyses.

RESULTS

Pedigree analysis suggested that glaucoma in this family segregated as an autosomal dominant trait. Of six patients, five had J-POAG and one had adult-onset POAG (A-POAG). Haplotype analysis suggested linkage of this family to the GLC1A locus. Mutation and sequence analyses showed a novel missense mutation, c.821C > G (p.P274R), in the C-terminal olfactomedin domain coded by exon 3 of the MYOC gene. One patient was found to be homozygous for this mutation with a severe phenotype.

CONCLUSIONS

This study reports a novel missense mutation in a four-generation Indian family with all but one member affected with J-POAG. The total number of mutations described so far in the MYOC gene, including the one reported here, is 59 with a clustering of 52 mutations in exon 3.

Mutations in the optineurin gene in Japanese patients with primary open-angle glaucoma and normal tension glaucoma

The optineurin gene (OPTN) was identified as a gene that causes primary open-angle glaucoma (POAG) and normal tension glaucoma (NTG). To investigate the frequency of sequence changes in OPTN in Japanese glaucoma patients, single-strand conformation polymorphism analysis and subsequent sequence analysis were performed for genotyping OPTN in 165 unrelated Japanese patients with POAG and 148 patients with NTG, with 196 control subjects without glaucoma as reference subjects. Out of four mutations reported to be associated with risk and to cause disease in Caucasian patients, sequence alterations in 458G > A and 691_692insAG were not detected in any investigated Japanese patients with glaucoma, and alterations in 1944G > A and 603T > A, were present in similar frequencies in glaucoma patients and control subjects. The current results suggest that there may be certain racial differences between Japanese and Caucasians with respect to OPTN genotypes.

Variants of the Myocilin Gene in Japanese Patients with Normal-Tension Glaucoma

Myocilin (MYOC) mutations are associated with juvenile- and adult-onset primary open-angle glaucoma (POAG). The purpose of this study was to determine whether MYOC gene mutations are associated with normal-tension glaucoma (NTG). The prevalence of MYOC mutations was determined in 80 Japanese NTG patients and 100 control subjects. In addition, the expression of mutant MYOC was determined by transforming COS-1 cells with five myocilin variants (R158Q, D208E, I360N, A363T, and I477S) and examining whether myocilin was present in the cultured cells and/or the culture medium by western blotting. Six different nucleotide sequence variants, R46Stop, R76K, R158Q, D208E, A488A, and one in the 3' non-coding region, were identified in 80 NTG patients. Variants in codon 46 (R46Stop), codon 158 (R158Q), and codon 488 (A488A) were not found in the 100 normal controls. The frequency of other sequence changes (R76K, D208E, and 3' non-coding) in NTG patients did not differ significantly from the frequencies in the control subjects. COS-1 cells transfected with the wild type, R158Q, or D208E variants secreted myocilin into the culture medium. On the other hand, the detected myocilin was significantly reduced in the medium of cells transfected with the I360N, A363T, or I477S variants that were previously identified as mutations for POAG. Definitive evidence of MYOC variants associated with NTG was not found.

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

PURPOSE

Investigators have noted that primary open-angle glaucoma (POAG) in West Africa has an earlier age of onset and appears to be more clinically severe than in the United States and Europe. Primary open-angle glaucoma patients with mutations in myocilin have a similar phenotype. Therefore, we investigated the role of mutations in myocilin in patients with POAG in a West African population.

MATERIALS AND METHODS

Patients seen at the Emmanuel Eye Clinic in Accra, Ghana, were recruited for this study. Informed consent was obtained from all study patients. Glaucoma specialists from the sponsoring institution (PC, LWH, or RRA) ascertained all POAG and control patients. Age-matched unaffected controls were obtained in patients with an IOP < 22 mm Hg and normal-appearing optic nerves. PCR amplification of each of the three myocilin exons was performed. Denaturing high-performance liquid chromatography (Transgenomics Corp.) was used to detect allelic differences and samples demonstrating a mobility shift were sequenced in both directions.

RESULTS

Ninety unrelated affecteds with POAG and 76 control patients were recruited. Four individuals with severe POAG were found to have novel missense mutations in exon 3. Two exhibit an Asp380Asn mutation and two an Arg342Lys mutation. These changes were not detected in 152 ethnically matched control chromosomes. Fourteen affected individuals and eight controls exhibit a translationally silent polymorphism in codon 325 (Thr325Thr).

CONCLUSIONS

A total of 4.4% of patients with POAG have novel disease-associated mutations in myocilin. Mutations in myocilin appear to play a limited role in the pathogenesis of POAG in this region of West Africa.

Genetic basis of glaucoma

The application of molecular genetic techniques to the study of glaucoma has accelerated greatly during the past few years. In addition to localizing and identifying genes for specific types of glaucoma, researchers have begun to characterize the gene products and investigate molecular mechanisms involved in glaucoma. Much research has been focused on the gene expression, protein processing, and mutations of MYOC/TIGR, which is associated with both juvenile-and adult-onset primary open angle glaucoma. Investigations of other glaucoma-related genes, such as PITX2, FOXC1, and CYP1B1, are enabling a better understanding of anterior segment development and its relation to glaucoma.

Differential occurrence of mutations causative of eye diseases in the Chinese population

Ethnic differences and geographic variations affect the frequencies and nature of human mutations. In the literature, descriptions of causative mutations of eye diseases in the Chinese population are few. In this paper we attempt to reveal molecular information on genetic eye diseases involving Chinese patients from published and unpublished works by us and other groups. Our studies on candidate genes of eye diseases in the Chinese population in Hong Kong include MYOC and TISR for primary open angle glaucoma, RHO and RP1 for retinitis pigmentosa, ABCA4 and APOE for age-related macular degeneration, RB1 for retinoblastoma, APC for familial adenomatous polyposis with congenital hypertrophy of retinal pigment epithelium, BIGH3/TGFBI for corneal dystrophies, PAX6 for aniridia and Reiger syndrome, CRYAA and CRYBB2 for cataracts, and mtDNA for Leber hereditary optic neuropathy. We have revealed novel mutations in most of these genes, and in RHO, RP1, RB1, BIGH3, and PAX6 we have reported mutations that contribute to better understanding of the functions and properties of the respective gene products. We showed absence of MYOC does not necessarily cause glaucoma. No disease causative mutations have been identified in MYOC or ABCA4. There are similarities in the patterns of sequence alterations and phenotype-genotype associations in comparison with other ethnic groups, while the MYOC, RB1, APC, and PAX6 genes have more Chinese-specific sequence alterations. Establishment of a mutation database specific for the Chinese is essential for identification of genetic markers with diagnostic, prognostic, or pharmacological values.