The phenotype of normal tension glaucoma patients with and without OPA1 polymorphisms

Genetics and genetic testing for glaucoma

PURPOSE OF REVIEW

In recent decades, investigators have identified numerous genes and genetic factors that cause or contribute risk for glaucoma. These findings have increased our understanding of disease mechanisms, provided us with new diagnostic tools, and may allow for development of improved therapies for glaucoma. However, genetic testing is most useful when it is reserved for appropriate patients. The purpose of this article is to review key points and recent developments regarding the genetics and genetic testing for glaucoma and to provide recommendations for when genetic testing may be warranted.

RECENT FINDINGS

Large genome-wide association studies have identified multiple new susceptibility loci associated with primary open angle glaucoma and primary angle closure glaucoma.

SUMMARY

Several glaucoma-causing genes and genetic risk factors for glaucoma have been discovered. As a result, there are specific clinical scenarios in which genetic testing is warranted. In select cases (i.e., familial juvenile open angle glaucoma), genetic testing can serve as a powerful tool to improve diagnostic accuracy, efficiency of disease surveillance, and selection of treatment, enabling physicians to better optimize care for their patients.

Myocilin levels in the aqueous humor of open-angle glaucoma patients

Purpose:To investigate the concentration of myocilin in the aqueous humor of open-angle glaucoma (OAG) patients, including correlations with glaucoma subtypes and intraocular pressure (IOP).Patients and Methods:The study comprised 85 patients with OAG. Glaucoma subtypes included 35 cases of high tension glaucoma (HTG), 25 cases of normal tension glaucoma (NTG), and 25 cases of exfoliation glaucoma (ExG). Forty-five patients with senile cataract were included as control. The concentrations of myocilin in the aqueous humor were measured by plotting the densitometry readings of the aqueous humor samples against a recombinant myocilin standard curve. Additionally, the relationships with the glaucoma subtypes, IOP, and glaucoma severity were analyzed.Results:A significantly higher percentage of patients in the glaucoma subgroups were positive for myocilin compared with the cataract group. The mean myocilin concentrations among the glaucoma positive case subgroups were not different (P=0.326). Myocilin levels were significantly higher in human HTG compared with cataract group (P<0.05). There were no significant correlations between the myocilin concentration and the IOP or the severity of glaucoma.Conclusion:Myocilin-positive patients were significant in the glaucoma subgroups than in the cataract group, with a highly significant difference observed for HTG patients.

OPA1 increases the risk of normal but not high tension glaucoma

BACKGROUND

Primary open angle glaucoma is a progressive optic neuropathy characterised by the selective loss of retinal ganglion cells, pathological optic disc cupping and visual field defects. The OPA1 gene encodes an inner mitochondrial membrane protein crucial for normal mitochondrial function, and pathogenic mutations cause autosomal dominant optic atrophy by specifically targeting retinal ganglion cells. This raises the distinct possibility that more subtle genetic variations in OPA1 could alter the risk of developing glaucoma.

METHODS

137 patients with primary open angle glaucoma (67 patients with high-tension glaucoma (HTG), 70 patients with normal-tension glaucoma (NTG)) and 75 controls from the North East of England were studied. Three single-nucleotide polymorphisms in intron 8 (IVS8+4c-->t and IVS8+32t-->c) and exon 4 (c.473A-->G) of the OPA1 gene were genotyped in the study group. In addition, the entire OPA1 coding region was sequenced in 24 individuals with the CT/TT compound genotype using standard BigDye chemistries.

RESULTS

There was no difference in either allele or genotype frequency for the IVS8+32t-->c single-nucleotide polymorphisms between patients and controls, but there was a significant association between the T allele at IVS8+4c-->t and the risk of developing NTG (OR=2.04, 95% CI=1.10 to 3.81, p=0.004), but not HTG. Logistic regression analysis also confirmed a strong association between the CT/TT compound genotype at IVS8+4 and IVS8+32 with NTG (OR=29.75, 95% CI=3.83 to 231.21, p=0.001).

CONCLUSIONS

The CT/TT compound genotype at IVS8+4 and IVS8+32 is a strong genetic risk determinant for NTG but not HTG.

Elevated hydrostatic pressure triggers release of OPA1 and cytochrome C, and induces apoptotic cell death in differentiated RGC-5 cells

PURPOSE

This study was conducted to determine whether elevated hydrostatic pressure alters mitochondrial structure, triggers release of the dynamin-related guanosine triphosphatase (GTPase) optic atrophy type 1 (OPA1) or cytochrome C from mitochondria, alters OPA1 gene expression, and can directly induce apoptotic cell death in cultured retinal ganglion cell (RGC)-5 cells.

METHODS

Differentiated RGC-5 cells were exposed to 30 mmHg for three days in a pressurized incubator. As a control, differentiated RGC-5 cell cultures were incubated simultaneously in a conventional incubator. Live RGC-5 cells were then labeled with MitoTracker Red and mitochondrial morphology was assessed by fluorescence microscopy. Mitochondrial structural changes were also assessed by electron microscopy and three-dimensional (3D) electron microscope tomography. OPA1 mRNA was measured by Taqman quantitative PCR. The cellular distribution of OPA1 protein and cytochrome C was assessed by immunocytochemistry and western blot. Caspase-3 activation was examined by western blot. Apoptotic cell death was evaluated by the terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) method.

RESULTS

Mitochondrial fission, characterized by the conversion of tubular fused mitochondria into isolated small organelles, was triggered after three days exposure to elevated hydrostatic pressure. Electron microscopy confirmed the fission and noted no changes to mitochondrial architecture, nor outer membrane rupture. Electron microscope tomography showed that elevated pressure depleted mitochondrial cristae content by fourfold. Elevated hydrostatic pressure increased OPA1 gene expression by 35+/-14% on day 2, but reduced expression by 36+/-4% on day 3. Total OPA1 protein content was not changed on day 2 or 3. However, pressure treatment induced release of OPA1 and cytochrome C from mitochondria to the cytoplasm. Elevated pressure also activated caspase-3 and induced apoptotic cell death.

CONCLUSIONS

Elevated hydrostatic pressure triggered mitochondrial changes including mitochondrial fission and abnormal cristae depletion, alteration of OPA1 gene expression, and release of OPA1 and cytochrome C into the cytoplasm before the onset of apoptotic cell death in differentiated RGC-5 cells. These results suggest that sustained moderate pressure elevation may directly damage RGC integrity by injuring mitochondria.

Myocilin protein levels in the aqueous humor of the glaucomas in selected canine breeds

OBJECTIVE

To compare aqueous humor myocilin protein levels in dogs with the primary glaucomas to those with the secondary glaucomas, primary cataracts, and diabetic cataracts.

MATERIALS AND METHODS

Four groups were selected, based on diagnosis by the attending veterinary ophthalmologists and included: primary glaucoma (primary open-angle glaucoma (POAG) and primary closed angle glaucoma (PCAG); n = 155); secondary glaucoma (n = 94); primary (presumed inherited) cataract (n = 142), and diabetic cataract (n = 83). A total of 474 samples (187 males, 263 females, 24 unreported) with average ages of 117 months for the males and 101 months for the females were analyzed. Myocilin protein was measured using the Coomassie staining and Western blot methods relative to a myocilin control.

RESULTS

Differences were seen between nonglaucomatous (cataractous) and glaucomatous dogs with myocilin levels in glaucomatous eyes being many times higher than those in the cataractous dogs. Primary glaucomatous dogs were found to have an aqueous humor myocilin protein level of 17.30 +/- 1.03 units. Secondary glaucomas had the highest level of myocilin in the aqueous humor with 19.27 +/- 1.41 units. Diabetic cataractous dogs had the lowest levels of myocilin reported with 6.60 +/- 0.88 (mean +/- SEM) units. Normal (cataractous) dogs had a myocilin level in the aqueous humor of 8.05 +/- 0.86 units.

CONCLUSION

Aqueous humor protein levels were elevated, relative to the myocilin control, in both the primary and secondary glaucoma groups compared to the cataract and diabetic cataract groups. Like in the Beagle POAG, aqueous humor myocilin protein levels are increased. Further studies are indicated to investigate the exact role of the aqueous humor myocilin protein in the genesis in increased IOP in these primary glaucomatous breeds.

Nutritional optic neuropathies

Nutritional deficiency may be the cause of a genuine optic neuropathy, sometimes associated with involvement of the peripheral nervous system. Nutritional optic neuropathies are usually bilateral, painless, chronic, insidious and slowly progressive. Most often, they present as a non-specific retrobulbar optic neuropathy. The differential diagnosis with other causes of optic nerve involvement, in particular of toxic origin, may be particularly difficult. Nutritional deficits are often associated with toxic effects from alcohol and tobacco; therefore, the separation of the nutritional and toxic components is often illusory and artificial. The pathophysiological mechanisms involved in nutritional -- and toxic -- optic neuropathies affect biochemical pathways involved in cell energetic production, correction of oxidative stress and quenching of free radicals. The recognition of these mechanisms could provide future therapeutic alternatives. Currently, the treatment is limited to the intensive use of vitamins with variable results in individual cases, and to the implementation of preventive measures, when feasible.

The OPA1 gene polymorphism is associated with normal tension and high tension glaucoma

PURPOSE

To assess whether genetic polymorphisms of optic atrophy 1 (OPA1) are associated with primary open-angle glaucoma (POAG).

DESIGN

Prospective case control association study.

METHODS

Japanese patients with normal tension glaucoma (NTG, n = 194), and high tension glaucoma (HTG, n = 191), and 185 control subjects were analyzed for the OPA1 intervening sequence (IVS) 8+4 cystosine thymine (C/T) and IVS 8+32 thymine cystosine (T/C) polymorphisms using pyrosequencing technique.

RESULTS

There was a significant difference in the OPA1 IVS 8 +32 T/C genotype frequencies between the NTG patients and control subjects (P = .0074), and the frequency of the cystosine (C) allele was significantly higher in the NTG patients compared with the control subjects (19.3% vs 11.6%, P = .0036). Adjusted for age, gender, refractive error, and intraocular pressure, an almost two-fold increased risk of NTG (P = .004, odds ratio 2.27, 95% confidence interval 1.30 to 3.97) was found with the OPA1 IVS 8 +32 C allele. Although there was no significant difference in the OPA1 IVS 8 +32 T/C genotype frequencies between the HTG patients and control subjects (P = .24), the age at the time of diagnosis (53 +/- 11.0 years, median value +/- median absolute deviation) in the HTG patients with the OPA1 IVS 8 +32 C allele was significantly younger than that (57 +/- 12.0 years) in the HTG patients without C allele (P = .048).

CONCLUSIONS

The OPA1 IVS 8 +32 T/C polymorphism is associated with NTG, and may be used as a marker for this disease association. This polymorphism also influences the phenotypic feature in patients with HTG and should be considered to be a genetic risk factor not only for NTG, but also for HTG.

Short wavelength-automated perimetry compared with standard achromatic perimetry in autosomal dominant optic atrophy

BACKGROUND

Autosomal dominant optic atrophy (ADOA, Kjer-type) is a heterogeneous, non-inflammatory degeneration of retinal ganglion cells. The diagnosis of ADOA can be challenging owing to its insidious onset and large variability in phenotypic expression, both within and between individual pedigrees. The earliest literature reports relatively mild centrocaecal scatomas to white targets in ADOA, but extensive and dense peripheral field loss to coloured targets, especially blue, with Bjerrum perimetry. The phrase "inverted peripheral visual fields to coloured targets" has been used to describe this phenomenon.

METHODS

Humphrey standard achromatic perimetry (SAP) and short wavelength-automated perimetry (SWAP) were carried out on five patients with ADOA.

RESULTS

Regardless of wide variations in patient age, visual acuity, disc appearance and colour vision, the SWAP mean deviation (MD) was between 10 and 20 times more depressed than the SAP MD. The actual differences ranged from 9.38 to 13.78 dB.

CONCLUSIONS

These data are consistent with the original reports suggesting that, early in this disease process, the blue-target deficits are typically peripheral and that this difference between SAP and SWAP perimetry may be a robust indicator of ADOA in both early and late stages of this disease.

Glaucoma screening: current trends, economic issues, technology, and challenges

PURPOSE OF REVIEW

Glaucoma is one of the leading causes of irreversible blindness worldwide. Early glaucoma detection and treatment are currently the only known methods for preventing blindness and low vision resulting from this frequently asymptomatic disease.

RECENT FINDINGS

New technologies for detecting early glaucomatous damage are important in diagnosing optic nerve disease, not only in community screening settings but also in clinics. Imaging of the optic nerve head and macula and retinal nerve fiber layer analysis can provide quick, automated, and quantitative measurements in agreement with clinical estimates of optic disc structure and visual function. In the area of perimetry, frequency-doubling technology is a promising and feasible mass-screening method with reasonable sensitivity for detecting visual field loss. Central corneal thickness has emerged as a new risk factor for the development and progression of glaucoma, thereby complicating the role of tonometry and measurement of intraocular pressure as screening parameters for glaucoma. Along with technological advances, strides are also being made with public policy and legislative efforts to bring glaucoma onto the national and global health care agenda. These initiatives incorporate vision-screening goals into national disease prevention programs emphasizing the need for early glaucoma detection and treatment.

SUMMARY

Glaucoma awareness needs to be increased through better education, and compliance with follow-up care needs to be improved to decrease the economic and social costs from glaucoma. In addition, screening models need to be developed that will be effective in developing countries where the risk of blindness from glaucoma is highest.

Gene mapping for primary open angle glaucoma

Primary open angle glaucoma (POAG) is a leading cause of visual impairment and blindness worldwide. To date, at least 20 genetic loci for POAG have been reported. Only 3 causative genes are identified from these loci: myocilin (MYOC), optineurin (OPTN) and WD repeat domain 36 (WDR36), which together account for less than 10% of POAG. Only a portion of POAG follows Mendelian inheritance, and a considerable fraction results from a large number of variants in several genes, each contributing small effects. Over the past 10 years, there has been vigorous research on mapping the POAG genes. The main technological approaches are functional cloning, family linkage analysis, genome-wide scan, case-control association study, and microarray analysis. Association studies found 16 genes related to POAG, but reports on glaucoma-causing effects of these genes are conflicting. Ten microarray gene expression studies related to POAG have been published. A number of genes potentially related to POAG have been identified, and they provide a good resource to select candidate genes for mutation analysis in association studies. While linkage studies remain a mainstay, the current trend is to use genome-wide association studies to map genes for POAG. This review gives an overview of the efforts in the past decade to identify the POAG genes through linkage studies, genome-wide scans, case-control association studies and microarray studies. In the near future such comprehensive studies are expected to greatly advance our understanding of the genetic basis of POAG and provide information for effective glaucoma therapy.

New thoughts on normal tension glaucoma

Normal tension glaucoma (NTG) is considered a primary optic neuropathy within the glaucoma family. Whereas NTG shares many of the same pathologic characteristics as primary open angle glaucoma (POAG), there are several differences such as pattern of visual field defect and incidence of disc hemorrhaging. The most notable difference is that NTG does not manifest a recordable intraocular pressure (IOP) greater than 21 mmHg. Debate has existed as to whether NTG is a pressure-sensitive neuropathy and if pressure reduction affects the course of the disease. Likely, there are aspects in the pathophysiology of NTG that are both pressure sensitive and insensitive, and the etiology of NTG has never been adequately explained.

Hereditary optic neuropathies: from the mitochondria to the optic nerve

PURPOSE

To review our current knowledge of inherited optic neuropathies.

DESIGN

Perspective.

METHODS

Literature review.

RESULTS

The hereditary optic neuropathies consist of a group of disorders in which optic nerve dysfunction figures solely or prominently and direct inheritance is clinically or genetically proven. The most common of these disorders are autosomal dominant optic atrophy (Kjers' disease) and maternally-inherited Leber's hereditary optic neuropathy. Other inherited neurologic and systemic syndromic diseases will frequently manifest optic neuropathy. A selective vulnerability of the optic nerve to perturbations in mitochondrial function may underlie a final common pathway among these disorders.

CONCLUSIONS

The ophthalmologist should be familiar with the clinical characteristics and diagnosis of the hereditary optic neuropathies. Recent advances in our understanding of the underlying pathophysiology of the inherited optic neuropathies may provide insight into their treatment and the treatment of acquired optic nerve disorders.

Hereditary optic neuropathies

AIMS

To provide a clinical update on the hereditary optic neuropathies.

METHODS

Review of the literature.

RESULTS

The hereditary optic neuropathies comprise a group of disorders in which the cause of optic nerve dysfunction appears to be hereditable, based on familial expression or genetic analysis. In some hereditary optic neuropathies, optic nerve dysfunction is typically the only manifestation of the disease. In others, various neurologic and systemic abnormalities are regularly observed.

CONCLUSION

The most common hereditary optic neuropathies are autosomal dominant optic atrophy (Kjer's disease) and maternally inherited Leber's hereditary optic neuropathy. We review the clinical phenotypes of these and other inherited disorders with optic nerve involvement.

Mitochondrial dysfunction as a cause of optic neuropathies

Mitochondria are increasingly recognized as central players in the life and death of cells and especially of neurons. The energy-dependence of retinal ganglion cells (RGC) and their axons, which form the optic nerve, is singularly skewed. In fact, while mitochondria are very abundant in the initial, unmyelinated part of the axons anterior to the lamina cribrosa, their number suddenly decreases as the myelin sheath begins more posteriorly. The vascular system also presents different blood-brain barrier properties anterior and posterior to the lamina, possibly reflecting the different metabolic needs of the optic nerve head (unmyelinated) and of the retrobulbar optic nerve (myelinated). Mitochondrial biogenesis occurs within the cellular somata of RGC in the retina. It needs the coordinated interaction of nuclear and mitochondrial genomes. Mitochondria are then transported down the axons and distributed where they are needed. These locations are along the unmyelinated portion of the nerve, under the nodes of Ranvier in the retrobulbar nerve, and at the synaptic terminals. Efficient transportation of mitochondria depends on multiple factors, including their own energy production, the integrity of the cytoskeleton and its protein components (tubulin, etc.), and adequate myelination of the axons. Any dysfunction of these systems may be of pathological relevance for optic neuropathies with primary or secondary involvement of mitochondria. Leber's hereditary optic neuropathy (LHON) is the paradigm of mitochondrial optic neuropathies where a primary role for mitochondrial dysfunction is certified by maternal inheritance and association with specific mutations in the mitochondrial DNA (mtDNA). Clinical phenocopies of this pathology are represented by the wide array of optic neuropathies associated with vitamin depletion, toxic exposures, alcohol and tobacco abuse, and use of certain drugs. Moreover, the recent identification of mutations in the nuclear gene OPA1 as the causative factor in dominant optic atrophy (DOA, Kjer's type) brought the unexpected finding that this gene encodes for a mitochondrial protein, suggesting that DOA and LHON may be linked by similar pathogenesis. Polymorphisms in this very same gene may be associated with normal tension glaucoma (NTG), which might be considered a genetically determined optic neuropathy that again shows similarities with both LHON and DOA. Exciting new developments come from first examples of mitochondrial optic neuropathies in animal models that are genetically determined or are the result of ingenious engineering of mitochondrial gene expression, or from biochemical manipulations of the respiratory complexes. Even more exciting is the first successful attempt to correct the LHON-related complex I dysfunction by the allotopic nuclear expression of the recoded mitochondrial gene. There is hope that the genetic complexities, biochemical dysfunctions, and integrated anatomical-physiological cellular relationships will soon be precisely delineated and that promising therapeutic and prophylactic strategies will be proposed.