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

A novel LTBP2 gene variant in a Turkish family with juvenile-onset open-angle glaucoma

BACKGROUND

Juvenile-onset open-angle glaucoma (JOAG) is a rare form of primary open-angle glaucoma (POAG) with an early age of onset before 40 years. Latent transforming growth factor-beta binding protein 2 (LTBP-2) is an extracellular matrix protein with a multi-domain structure and homology to fibrillins. LTBP2 gene variants have been associated with JOAG in a small number of patients. Herein, we report a novel missense variant in the LTBP2 gene in a Turkish family with JOAG.

MATERIALS AND METHODS

Blood samples were obtained from three siblings (a 20-year-old woman with JOAG, 26-year-old man with JOAG, and 15-year-old girl with posterior embryotoxon) for genetic analysis. Their father had moderate-severe POAG and the 24-year-old brother had JOAG. The mother and 32-year-old sister were healthy. Although the parents reported no consanguinity, they come from the same village.

RESULTS

Clinical exome sequencing analysis of the two siblings with JOAG revealed a novel c.607C>T p.(R203C) (rs777450651) homozygous LTBP2 variant, while the variant was heterozygous in their 15-year-old sister. There were no mutations in the MYOC, CYP1B1, or FBN1 genes.

CONCLUSION

We documented a novel missense mutation in the LTBP2 gene leading to a severe form of JOAG with refractory IOP and progressive optic nerve damage, which seems to show autosomal recessive inheritance.

Myocilin Mutation N480K Leads to Early Onset Juvenile and Adult-onset Primary Open Angle Glaucoma in a Six Generation Family

PRCIS

A pathogenic autosomal dominant MYOC mutation N480K detected in 6 generations of an Indian family is primarily responsible for juvenile open angle glaucoma (JOAG) and adult-onset primary open angle glaucoma (POAG), emphasizing the importance of screening this mutation at a younger age.

PURPOSE

To screen myocilin mutations in a large South Indian family with early-onset JOAG and adult-onset POAG.

METHODS

In a large South Indian family with 20 members, 8 members diagnosed as JOAG, 7 members as POAG, 4 members as JOAG suspect, and 1 member as POAG suspect were screened for myocilin ( MYOC) mutations using Sanger sequencing. Whole exome sequencing was performed on clinically suspected JOAG/POAG individuals.

RESULTS

Myocilin gene mutation N480K (c.1440C>G) was detected in 20 family members, including proband, of whom 8 were JOAG and 7 were POAG patients, 3 were JOAG suspects, and 2 were unaffected. Among the unaffected carriers, 1 was less than 5 years old, and another was 25 years old. The earliest to develop the disease was a 10-year-old child. The penetrance of the mutation was 95% over 10 years of age. This family had JOAG/POAG suspects with no N480K MYOC mutation, and they were further screened for other mutations using whole-exome sequencing. Polymorphisms CYP1B1 L432V and MYOC R76K were detected in 3 JOAG/POAG suspects, and among these 3, one had another CYP1B1 polymorphic variant R368H. The presence of the CYP1B1 polymorphism along with an MYOC polymorphic variant among the JOAG/POAG suspects needs additional studies to explore their combined role in the onset of glaucoma.

CONCLUSIONS

This study reveals that MYOC mutation is primarily responsible for JOAG and adult-onset POAG in a family, emphasizing the importance of screening for this mutation at a younger age for early treatment.

Serine to proline mutation at position 341 of MYOC impairs trabecular meshwork function by causing autophagy deregulation

Glaucoma is a highly heritable disease, and myocilin was the first identified causal and most common pathogenic gene in glaucoma. Serine-to-proline mutation at position 341 of myocilin (MYOCS341P) is associated with severe glaucoma phenotypes in a five-generation primary open-angle glaucoma family. However, the underlying mechanisms are underexplored. Herein, we established the MYOCS341P transgenic mouse model and characterized the glaucoma phenotypes. Further, we systematically explored the functional differences between wild-type and MYOCS341P through immunoprecipitation, mass spectrometry, and RNA-seq analyses. We found that MYOCS341P transgenic mice exhibit glaucoma phenotypes, characterized by reduced aqueous humor outflow, elevated intraocular pressure, decreased trabecular meshwork (TM) cell number, narrowed Schlemm's canal, retinal ganglion cell loss, and visual impairment. Mechanistically, the secretion of dysfunctional MYOCS341P accumulated in the endoplasmic reticulum (ER), inducing ER stress and dysregulation of autophagy, thereby promoting TM cell death. We describe an effective transgenic model for mechanistic studies and the screening of therapeutic targets. Our data generated from high-throughput analyses help elucidate the mechanism underlying mutant MYOC-related glaucoma.

Evaluation of Myocilin Variant Protein Structures Modeled by AlphaFold2

Deep neural network-based programs can be applied to protein structure modeling by inputting amino acid sequences. Here, we aimed to evaluate the AlphaFold2-modeled myocilin wild-type and variant protein structures and compare to the experimentally determined protein structures. Molecular dynamic and ligand binding properties of the experimentally determined and AlphaFold2-modeled protein structures were also analyzed. AlphaFold2-modeled myocilin variant protein structures showed high similarities in overall structure to the experimentally determined mutant protein structures, but the orientations and geometries of amino acid side chains were slightly different. The olfactomedin-like domain of the modeled missense variant protein structures showed fewer folding changes than the nonsense variant when compared to the predicted wild-type protein structure. Differences were also observed in molecular dynamics and ligand binding sites between the AlphaFold2-modeled and experimentally determined structures as well as between the wild-type and variant structures. In summary, the folding of the AlphaFold2-modeled MYOC variant protein structures could be similar to that determined by the experiments but with differences in amino acid side chain orientations and geometries. Careful comparisons with experimentally determined structures are needed before the applications of the in silico modeled variant protein structures.

A rare optineurin mutation in an Indian family with coexistence of JOAG and PCG

Purpose

This study focused on the genetic screening of Myocilin (MYOC), Cytochrome P450 family 1 subfamily B member 1 (CYP1B1), Optineurin (OPTN), and SIX homeobox 6 (SIX6) genes in a family with coexistence of primary congenital glaucoma (PCG) and juvenile open-angle glaucoma (JOAG).

Methods

Sanger sequencing was used to examine the coding region of all four genes. Six different online available algorithms were used for the pathogenicity prediction of missense variant. Structural analysis was done using Garnier-Osguthorpe-Robson (GOR), PyMol, ChimeraX, and Molecular Dynamic (MD) Simulations (using Graphics Processing Unit (GPU)-enabled Desmond module of Schrödinger).

Results

There were a total of three sequence variants within the family. All seven algorithms determined that a single mutation, G538E, in the OPTN gene is pathogenic. The loops connecting the strands became more flexible, as predicted structurally and functionally by pathogenic mutations. Mutations create perturbations and conformational rearrangements in proteins, hence impairing their functioning.

Conclusion

In this study, we describe a North Indian family in which members were having JOAG and PCG due to a rare homozygous/heterozygous mutation in OPTN. The coexistence of two types of glaucoma within a single pedigree suggests that certain OPTN mutations may be responsible for the onset of different glaucoma phenotypes.

A novel heterozygous c.733 T > G MYOC mutation associated with juvenile-onset open-angle glaucoma in a Chinese family

AIMS

To detect mutations in juvenile-onset open-angle glaucoma in a Chinese family and to describe the characteristic ophthalmic phenotypes of this pedigree.

METHODS

There were 14 individuals in this four-generation pedigree. All living members of the family underwent comprehensive ophthalmic examinations. Five patients presented with elevated intraocular pressures. All of them shared early-onset disease, with a mean onset age of 14.4 years and continuing aggressive damage to their optic nerves. Hyperpigmentation in the trabecular meshwork and sometimes-broad iris processes were noted in this family using gonioscopy. All exons of candidate genes (MYOC, OPTN, CYP1B1) were amplified using the polymerase chain reaction, and analysed with an ABI 3700XL Genetic Analyser.

RESULTS

A heterozygous missense mutation in exon 3 (c.733 T > G) of the MYOC gene was found in the five JOAG patients and one 7-year-old boy with no ophthalmic manifestation of glaucoma, but it was absent in other members of the family and in the controls. This mutation resulted in a transversion of cysteine to glycine (Cys245Gly).

CONCLUSIONS

We concluded the novel MYOC c.733 T > G mutation found in a Chinese family with JOAG caused a severe type of JOAG exhibiting early onset, high IOP, and severe optic nerve damage. Interestingly, unlike other reported MYOC mutation families, our patients exhibited marked angle pigmentation and iris processes.

Juvenile open angle glaucoma: current diagnosis and management

PURPOSE OF REVIEW

The aim of this article is to summarize up-to-date research on the diagnosis and management of juvenile open-angle glaucoma (JOAG).

RECENT FINDINGS

JOAG can be subclassified into four clinical phenotypes, and faster myopic shift is a risk factor for disease progression. Vessel density is associated with structural damage and worsening visual acuity in JOAG and can be monitored with optical coherence tomography angiography. Genetic studies have revealed molecular causes of JOAG including variants in CPAMD8, MYOC, and CYP1B1. Tube shunt surgeries as well as gonioscopy-assisted transluminal trabeculotomy have been shown to be successful in JOAG.

SUMMARY

Although genetic advances may improve future screening, intraocular pressure monitoring and fundoscopic exam remain the current mainstay of diagnosis. Medical treatment alone for JOAG is typically insufficient with patients requiring surgical management. Selective laser trabeculoplasty may delay or decrease the need for surgery. Trabeculectomy has traditionally been shown to be effective in JOAG, but tube shunt surgery and microinvasive glaucoma surgery are effective alternatives.

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.

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>A p.(Gly367Arg) was shown in the affected family members; in the second family MYOC variant c.1440C>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.

Comparisons of Schlemm's canal and trabecular meshwork morphologies between juvenile and primary open angle glaucoma

The histologic differences in Schlemm's canal (SC) and trabecular meshwork (TM), obtained from the trabeculectomy specimens of different age-group glaucoma patients, were compared. This study involved 44 trabeculectomy specimens of 37 juvenile-onset open-angle glaucoma (JOAG) patients (Group A) and 24 trabeculectomy specimens of 24 elderly-onset primary OAG (POAG) patients (age range: 70-79 years, Group B) with no familial history of POAG. Clinical parameters of gender, maximum intraocular pressure (IOP), and the number of glaucoma medications used prior to trabeculectomy were investigated and compared between the two groups. From light microscopy photographs of hematoxylin-eosin, and immunohistochemical staining of markers for SC endothelium (SCE), the total SC length (TSC), comprised of the opened-SC length (OSC) and the closed-SC length (CSC), the percentage of CSC in TSC (%CSC), the percentage of positive SCE marker in CSC (%PinCSC), and the percentage of negative SCE marker in OSC (%NinOSC) were analyzed. Moreover, podoplanin staining patterns in the TM were investigated and compared between the two groups. Among the clinical parameters, the mean maximum IOP in Group A (33.41 ± 9.24 mmHg) was the only significant parameter when compared to that in Group B (22.96 ± 7.17 mmHg, P = 0.000003). TSC in Group A was significantly shorter than that in Group B (P = 0.00092), and %CSC (P = 0.00004) and %PinCSC (P = 0.00342) in Group B were significantly higher than those in Group A. No statistically significant difference in %NinOSC was found between Group A and Group B (P = 0.76060). Juxtacanalicular tissue (JCT) in Group A showed compact and weak staining with podoplanin, while the JCT and closed-SC area in Group B showed intense staining. In the Group A subjects, TSC (P = 0.04819) and OSC (P = 0.02867) were significantly shorter in the non-familial cases than in the familial cases. Platelet coagulations 10-37 μm in size at the defect of the SCE in the inner wall of the SC were observed in 8 eyes (18%) and 4 eyes (17%) in Group A and Group B, respectively. The platelets appeared to repair the SCE damage for maintaining the blood aqueous barrier in both groups of POAG eyes. Smaller SC diameters and accompanying TM abnormality were features observed in the young-onset JOAG patients, thus suggesting developmental abnormalities in the outflow routes. The collapse of SC lumen, presumably due to aging, was the feature observed in the elderly-onset POAG patients. In Group A, the significantly higher IOP, despite of no significant number of topical medications used prior to trabeculectomy, also suggested that JOAG eyes can be categorized as a distinct type of POAG from the eyes of elder-aged POAG patients. The SCE drop out observed in the glaucomatous eyes of the different age groups suggested that worsening of IOP control may possibly occur equally in both groups.

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.

Molecular Insights into Myocilin and Its Glaucoma-Causing Misfolded Olfactomedin Domain Variants

Numerous human disorders arise due to the inability of a particular protein to adopt its correct three-dimensional structure in the context of the cell, leading to aggregation. A new addition to the list of such protein conformational disorders is the inherited subtype of glaucoma. Different and rare coding mutations in myocilin, found in families throughout the world, are causal for early onset ocular hypertension, a key glaucoma risk factor. Myocilin is expressed at high levels in the trabecular meshwork (TM) extracellular matrix. The TM is the anatomical region of the eye that regulates intraocular pressure, and its dysfunction is associated with most forms of glaucoma. Disease variants, distributed across the 30 kDa olfactomedin domain (mOLF), cause myocilin to be sequestered intracellularly instead of being secreted to the TM extracellular matrix. The working hypothesis is that the intracellular aggregates cause a toxic gain of function: TM cell death is thought to lead to TM matrix dysfunction, hastening elevated intraocular pressure and subsequent vision loss.Our lab has provided molecular underpinnings for myocilin structure and misfolding, placing myocilin-associated glaucoma within the context of amyloid diseases like Alzheimer and diabetes. We have dissected complexities of the modular wild-type (WT) myocilin structure and associated misfolded states. Our data support the model that full-length WT myocilin adopts a Y-shaped dimer-of-dimers conferred by two different coiled-coil regions, generating new hypotheses regarding its mysterious function. The mOLF β-propellers are paired at each tip of the Y. Disease-associated variants aggregate because mOLFs are less stable, leading to facile aggregation under physiological conditions (37 °C, pH 7.2). Mutant myocilin aggregates exhibit numerous characteristics of amyloid in vitro and in cells, and aggregation proceeds from a partially folded state accessed preferentially by disease variants at physiological conditions. Interestingly, destabilization is not a universal consequence of mutation. We identified counterintuitive, stabilizing point variants that adopt a non-native structure and do not aggregate; however, these variants have not been identified in glaucoma patients. An ongoing effort is predicting the consequence of any given mutation. This effort is relevant to interpreting data from large-scale sequencing projects where clinical and family history data are not available. Finally, our work suggests avenues to develop disease-modifying precision medicines for myocilin-associated glaucoma.