Role of HDACs in optic nerve damage-induced nuclear atrophy of retinal ganglion cells.

Optic neuropathies are characterized by retinal ganglion cell (RGC) death, resulting in the loss of vision. In glaucoma, the most common optic neuropathy, RGC death is initiated by axonal damage, and can be modeled by inducing acute axonal trauma through procedures such as optic nerve crush (ONC) or optic nerve axotomy. One of the early events of RGC death is nuclear atrophy, and is comprised of RGC-specific gene silencing, histone deacetylation, heterochromatin formation, and nuclear shrinkage. These early events appear to be principally regulated by epigenetic mechanisms involving histone deacetylation. Class I histone deacetylases HDACs 1, 2, and 3 are known to play important roles in the process of early nuclear atrophy in RGCs, and studies using both inhibitors and genetic ablation of Hdacs also reveal a critical role in the cell death process. Select inhibitors, such as those being developed for cancer therapy, may also provide a viable secondary treatment option for optic neuropathies.

Matrix metalloproteinases as promising regulators of axonal regrowth in the injured adult zebrafish retinotectal system.

Overcoming the failure of axon regeneration in the mammalian central nervous system (CNS) after injury remains a major challenge, which makes the search for proregenerative molecules essential. Matrix metalloproteinases (MMPs) have been implicated in axonal outgrowth during CNS development and show increased expression levels during vertebrate CNS repair. In mammals, MMPs are believed to alter the suppressive extracellular matrix to become more permissive for axon regrowth. We investigated the role of MMPs in axonal regeneration following optic nerve crush (ONC) in adult zebrafish, which fully recover from such injuries due to a high intrinsic axon growth capacity and a less inhibitory environment. Lowering general retinal MMP activity through intravitreal injections of GM6001 after ONC strongly reduced retinal ganglion cell (RGC) axonal regrowth, without influencing RGC survival. Based on a recently performed transcriptome profiling study, the expression pattern of four MMPs after ONC was determined via combined use of western blotting and immunostainings. Mmp-2 and -13a were increasingly present in RGC somata during axonal regrowth. Moreover, Mmp-2 and -9 became upregulated in regrowing RGC axons and inner plexiform layer (IPL) synapses, respectively. In contrast, after an initial rise in IPL neurites and RGC axons during the injury response, Mmp-14 expression decreased during regeneration. Altogether, a phase-dependent expression pattern for each specific MMP was observed, implicating them in axonal regrowth and inner retina remodeling after injury. In conclusion, these data suggest a novel, neuron-intrinsic function for multiple MMPs in axon regrowth that is distinct from breaking down environmental barriers. J. Comp. Neurol. 524:1472-1493, 2016. © 2015 Wiley Periodicals, Inc.

Spastic paraplegia gene 7 in patients with spasticity and/or optic neuropathy.

Mutations in the spastic paraplegia 7 (SPG7) gene encoding paraplegin are responsible for autosomal recessive hereditary spasticity. We screened 135 unrelated index cases, selected in five different settings: SPG7-positive patients detected during SPG31 analysis using SPG31/SPG7 multiplex ligation-dependent probe amplification (n = 7); previously reported ambiguous SPG7 cases (n = 5); patients carefully selected on the basis of their phenotype (spasticity of the lower limbs with cerebellar signs and/or cerebellar atrophy on magnetic resonance imaging/computer tomography scan and/or optic neuropathy and without other signs) (n = 24); patients with hereditary spastic paraparesis referred consecutively from attending neurologists and the national reference centre in a diagnostic setting (n = 98); and the index case of a four-generation family with autosomal dominant optic neuropathy but no spasticity linked to the SPG7 locus. We identified two SPG7 mutations in 23/134 spastic patients, 21% of the patients selected according to phenotype but only 8% of those referred directly. Our results confirm the pathogenicity of Ala510Val, which was the most frequent mutation in our series (65%) and segregated at the homozygous state with spastic paraparesis in a large family with autosomal recessive inheritance. All SPG7-positive patients tested had optic neuropathy or abnormalities revealed by optical coherence tomography, indicating that abnormalities in optical coherence tomography could be a clinical biomarker for SPG7 testing. In addition, the presence of late-onset very slowly progressive spastic gait (median age 39 years, range 18-52 years) associated with cerebellar ataxia (39%) or cerebellar atrophy (47%) constitute, with abnormal optical coherence tomography, key features pointing towards SPG7-testing. Interestingly, three relatives of patients with heterozygote SPG7 mutations had cerebellar signs and atrophy, or peripheral neuropathy, but no spasticity of the lower limbs, suggesting that SPG7 mutations at the heterozygous state might predispose to late-onset neurodegenerative disorders, mimicking autosomal dominant inheritance. Finally, a novel missense SPG7 mutation at the heterozygous state (Asp411Ala) was identified as the cause of autosomal dominant optic neuropathy in a large family, indicating that some SPG7 mutations can occasionally be dominantly inherited and be an uncommon cause of isolated optic neuropathy. Altogether, these results emphasize the clinical variability associated with SPG7 mutations, ranging from optic neuropathy to spastic paraplegia, and support the view that SPG7 screening should be carried out in both conditions.

Lamotrigine monotherapy does not provide protection against the loss of optic nerve axons in a rat model of ocular hypertension.

Sodium channel blocking agents such as lamotrigine are potent agents for neuroprotection in several animal models of neurodegenerative and neuroinflammatory disease. We therefore explored whether lamotrigine therapy was neuroprotective in a rat model of ocular hypertension characterized by axonal injury and selective loss of retinal ganglion cells. Twenty-seven male Wistar rats were injected subcutaneously twice daily with either lamotrigine (14 mg/kg/day) or vehicle. Two weeks after the first injection, experimental ocular hypertension was induced in one eye by 532 nm trabecular laser treatment. Intraocular pressure (IOP) was monitored by rebound tonometry and four weeks after the elevation of IOP the loss of optic nerve axons was quantified relative to eyes without either IOP elevation or lamotrigine exposure. In other animals with ocular hypertension, the optic nerves were examined by immunohistochemistry for the expression of the inducible form of nitric oxide synthase (iNOS) at 7 and 28 days. Four weeks after initiation of IOP elevation, no significant difference in axonal loss was observed between rats treated with lamotrigine (30.8% ± 10.5%) or vehicle (17.8% ± 5.7%) (P = 0.19, T-test). There was no significant difference in mean IOP, peak IOP and integral IOP exposure. Furthermore, optic nerve axon counts per unit integral IOP exposure were similar in both groups (P = 0.44). The optic nerves were not positive for the expression of iNOS. In conclusion, this study provides no evidence that lamotrigine is neuroprotective for RGC axons after four weeks of experimental ocular hypertension in the rat, in a model where axonal degeneration occurs in the absence of iNOS expression.

Silencing of Fem1cR3 gene expression in the DBA/2J mouse precedes retinal ganglion cell death and is associated with histone deacetylase activity.

PURPOSE: Downregulation of normal gene expression in dying retinal ganglion cells has been documented in both acute and chronic models of optic nerve disease. The authors examined the mechanism and timing of this phenomenon in DBA/2J mice, using genetically modified substrains of this inbred line. METHODS: DBA/2J mice, doubly congenic for the Bax mutant allele and the ganglion cell reporter gene Fem1c(Rosa3) (R3), were evaluated to elucidate the timing of loss of normal gene expression during the apoptotic process. The localization of histone deacetylase 3 (HDAC3) and nuclear histone H4 acetylation were examined by immunofluorescence in dying cells. The role of HDACs in gene silencing during glaucoma was interrogated using the global HDAC inhibitor trichostatin A (TSA). RESULTS: Silencing of the R3 allele occurred in Bax(-/-) ganglion cells, indicating that this process preceded the committed step of the intrinsic apoptotic pathway. Weekly TSA treatment, between the ages of 6 and 10 months, was able to attenuate the loss of R3 expression in the retina, but had no effect on optic nerve degeneration. Dying cells in aging DBA/2J mice exhibited nuclear localization of HDAC3 and a decrease in the level of H4 acetylation. CONCLUSIONS: Retinal ganglion cells exhibit a loss of normal gene expression as an early (pre-BAX involvement) part of their apoptotic program during glaucomatous degeneration. This process can be ameliorated, but not completely blocked, using HDAC inhibitors. Epigenetic changes to active chromatin, such as deacetylation, may be mediated by HDAC3 in dying neurons.

Retinal and optic nerve head pathology in Susac's syndrome.

PURPOSE: This article describes the first retinal histopathologic findings in a patient with Susac's syndrome (SS). DESIGN: Observational case report. PARTICIPANT: A 51-year-old white woman diagnosed with SS. METHODS: Eyes from a 51-year-old white woman diagnosed with SS were obtained at autopsy. One retina was dissected and processed for adenosine diphosphatase (ADPase) flat embedding. Selected areas were processed further for transmission electron microscopy. MAIN OUTCOME MEASURES: Histopathologic examination using ADPase flat-embedding technique. RESULTS: There were vaso-occlusive changes in the retinal periphery resulting in small areas of capillary dropout. Cross-sections demonstrated serous filled spaces between the retinal blood vessels and the internal limiting membrane. Lumens adjacent to these spaces appeared compressed and sometimes closed, but without thrombosis. Decreased ADPase activity in some peripheral blood vessels suggested endothelial cell dysfunction and vaso-occlusion. In the optic nerve head, numerous corpora amylacea were observed in the vicinity of capillaries with thickened walls and narrow lumens. Transmission electron microscopy demonstrated thickened and amorphous vascular basal lamina and open endothelial cell junctions in some retinal blood vessels. CONCLUSIONS: The serous deposits with compression of retinal vessel lumens observed histologically probably represent the so-called string of pearls described clinically in SS. Chronic extension of these serous deposits along the vessel wall possibly are the cause of retinal arterial wall plaques as described by Gass and other investigators. In the optic nerve head, corpora amylacea are probably a result of microinfarcts resulting from optic nerve head capillary angiopathy. Accumulation of amorphous material in the basal lamina, loss of viable endothelial cells, and capillary dropout suggest that SS may be an endotheliopathy.

Inducible nitric oxide synthase, Nos2, does not mediate optic neuropathy and retinopathy in the DBA/2J glaucoma model.

BACKGROUND: Nitric oxide synthase 2 (NOS2) contributes to neural death in some settings, but its role in glaucoma remains controversial. NOS2 is implicated in retinal ganglion cell degeneration in a rat glaucoma model in which intraocular pressure (IOP) is experimentally elevated by blood vessel cauterization, but not in a rat glaucoma model where IOP was elevated by injection of hypertonic saline. To test the importance of NOS2 for an inherited glaucoma, in this study we both genetically and pharmacologically decreased NOS2 activity in the DBA/2J mouse glaucoma model. METHODS: The expression of Nos2 in the optic nerve head was analyzed at both the RNA and protein levels at different stages of disease pathogenesis. To test the involvement of Nos2 in glaucomatous neurodegeneration, a null allele of Nos2 was backcrossed into DBA/2J mice and the incidence and severity of glaucoma was assessed in mice of each Nos2 genotype. Additionally, DBA/2J mice were treated with the NOS2 inhibitor aminoguanidine and the disease compared to untreated mice. RESULTS: Optic nerve head Nos2 RNA levels varied and increased during moderate but decreased at early and severe stages of disease. Despite the presence of a few NOS2 positive cells in the optic nerve head, NOS2 protein was not substantially increased during the glaucoma. Genetic deficiency of Nos2 or aminoguanidine treatment did not alter the IOP profile of DBA/2J mice. Additionally, neither Nos2 deficiency nor aminoguanidine had any detectable affect on the glaucomatous optic nerve damage. CONCLUSION: Glaucomatous neurodegeneration in DBA/2J mice does not require NOS2 activity. Further experiments involving various models are needed to assess the general importance of Nos2 in glaucoma.

Anterior ischemic optic neuropathy and stroke with use of PDE-5 inhibitors for erectile dysfunction: cause or coincidence?

The PDE-5 inhibitors sildenafil (Viagra) vardenafil (Levitra) and tadalafil (Cialis) have been taken by millions of men for erectile dysfunction. Transient visual symptoms are common but there also have been fourteen cases of nonarteritic anterior ischemic optic neuropathy (NAION) described in patients using these drugs as well as a few other vascular events. NAION is a common optic neuropathy in patients in the age group using these drugs and the question arises whether or not PDE-5 inhibitors are causing NAION. One case of NAION occurred after transient visual symptoms occurred with repeated use and one patient experienced a transient ischemic attack after taking a dose followed by a stroke on using the drug again later. Other than these two cases with strong dechallenge-rechallenge data, the evidence to support PDE-5 inhibitors as a cause of NAION or any vascular event is weak. PDE-5 inhibitors probably are a rare cause of a common ischemic disorder of the optic disc. They should be avoided in men who have already experienced NAION in one eye. Patients should be warned to seek medical attention if they have visual field or acuity loss after using PDE-5 inhibitors. Otherwise there is little basis for modifying the current guidelines for the use of these drugs.

Etiology of vision loss in ganglioside GM3 synthase deficiency.

PURPOSE: To investigate the cause of vision loss in patients with ganglioside GM3 synthase deficiency, a newly described rare autosomal recessive infantile-onset symptomatic epilepsy syndrome associated with developmental stagnation and blindness. METHODS: We examined four children from two related Amish sibships. Molecular genetic analysis confirmed inheritance of the founder mutation. Electroretinography and fundus photography were obtained in two patients. RESULTS: Despite an initial suspicion of retinal degeneration, retinal function was found to be preserved in both patients and ERG amplitudes were within normal limits. Ophthalmoscopy showed bilateral optic atrophy in all patients. CONCLUSIONS: Vision loss in GM3 synthase deficiency results from central nervous system and optic nerve involvement. Retinal function appears to be otherwise normal into the teenage years.

Proteomics implicates peptidyl arginine deiminase 2 and optic nerve citrullination in glaucoma pathogenesis.

PURPOSE: Proteomic analyses of normal and glaucomatous human optic nerve were pursued for insights into the molecular pathology of primary open-angle glaucoma (POAG). Peptidyl arginine deiminase 2 (PAD2), an enzyme that converts protein arginine to citrulline, was found only in POAG optic nerve and was probed further for a mechanistic role in glaucoma. METHODS: Protein identification used liquid chromatography-tandem mass spectrometry. Northern, Western, and immunohistochemical analyses measured PAD2 expression and/or protein citrullination and arginyl methylation in human and mouse optic nerve and in astrocyte cultures before and after pressure treatment. Proteins were identified after anticitrulline immunoprecipitation. In vitro translation of PAD2 was monitored in polyA RNA depleted optic nerve extracts. PAD2 shRNA transfections were evaluated in pressure-treated astrocytes. RESULTS: Western and immunohistochemical analyses confirmed elevated PAD2 and citrullination in POAG optic nerve and decreased arginyl methylation. PAD2 was also detected in optic nerve from older, glaucomatous DBA/2J mice, but not in younger DBA/2J or control C57BL6J mice. Myelin basic protein was identified as a major citrullinated protein in POAG optic nerve. Pressure-treated astrocytes exhibited elevated PAD2 and citrullination without apparent change in PAD2 mRNA. Addition of exogenous polyA RNA to depleted optic nerve extracts yielded increased PAD2 expression in POAG but not in control extracts. Transfection with shRNA restored PAD2 and citrullination to control levels in pressure-treated astrocytes. CONCLUSIONS: Current results support translational modulation of PAD2 expression and a possible role for the enzyme in POAG optic nerve damage through citrullination and structural disruption of myelination.

Evaluation of inducible nitric oxide synthase in glaucomatous optic neuropathy and pressure-induced optic nerve damage.

PURPOSE: To determine whether inducible nitric oxide synthase (NOS-2) is involved in glaucomatous optic neuropathy. METHODS: Chronic elevation of rat intraocular pressure (IOP) leading to optic nerve damage was induced by episcleral injection of hypertonic saline, which caused sclerosis and blockade of aqueous humor outflow pathways. Expression of NOS-2 in the retina and optic nerve head (ONH) was evaluated by immunohistochemistry, gene array analysis, and quantitative PCR (Q-PCR). Immunohistochemistry was also used to assess the NOS-2 level in the ONH from primary open-angle glaucoma (POAG) and nonglaucomatous human eyes. Finally, an NOS-2 inhibitor, aminoguanidine, administered orally in the drinking water, was tested for its effect on optic nerve injury in rats with ocular hypertension. RESULTS: Chronically elevated IOP in the rat produced optic nerve damage that correlated with pressure change (r(2) = 0.77), but did not increase NOS-2 immunoreactivity in the optic nerve, ONH, or ganglion cell layer. Retinal and ONH NOS-2 mRNA levels did not correlate with either IOP level or severity of optic nerve injury. Similarly, there was no difference in NOS-2 immunoreactivity in the optic nerve or ONH between POAG and nonglaucomatous eyes. Furthermore, aminoguanidine treatment did not affect the development of pressure-induced optic neuropathy in the rat. CONCLUSIONS: As demonstrated by several independent methods, glaucomatous optic neuropathy was not associated with a significant change in the expression of NOS-2 in the retina, ONH, or optic nerve.

Optic neuropathy induced by reductions in mitochondrial superoxide dismutase.

PURPOSE: Reactive oxygen species (ROS) are suspected to play a pivotal role in the pathogenesis of Leber hereditary optic neuropathy (LHON), caused by mutated complex I subunit genes. It seems surprising that optic neuropathy has not been described in animals with a knockout of genes encoding critical anti-ROS defenses. If ROS have a role in the optic nerve injury of LHON, then increasing mitochondrial levels of ROS should induce optic neuropathy. METHODS: To develop an animal model system for study of oxidative injury to the optic nerve, mitochondrial defenses were decreased against ROS by designing hammerhead ribozymes to degrade SOD2 mRNA. Several potential ribozymes were analyzed in vitro. The one with the best kinetic characteristics was cloned into a recombinant adeno-associated virus (rAAV) vector for delivery and testing in cells and animals. The effects of the AAV-expressing ribozyme on murine cell growth, SOD2 mRNA and protein, cellular ROS levels, and apoptosis were evaluated by RNase protection assay, immunoblot analysis, and ROS- and apoptosis-activated fluorescent probes. The rAAV-ribozyme was then injected into the eyes of DBA/1J mice, and the effect on the optic nerve was evaluated by ocular histopathologic examination. RESULTS: The AAV-expressing ribozyme decreased SOD2 mRNA and protein levels by as much as 85%, increased cellular superoxide, reduced mitochondrial membrane potential, and culminated in the death of infected cell lines by apoptosis without significantly altering complex I and III activity, somewhat spared in the most common LHON mutation (G11778A), although adenosine triphosphate (ATP) synthesis is markedly reduced. When inoculated into the eyes of mice, the AAV-expressing ribozyme led to loss of axons and myelin in the optic nerve and ganglion cells in the retina, the hallmarks of optic nerves examined at autopsy of patients with LHON. CONCLUSIONS: The striking similarity of the optic neuropathy to the histopathology of LHON is powerful evidence supporting ROS as a key factor in the pathogenesis of LHON.

Suppression of complex I gene expression induces optic neuropathy.

Optic nerve degeneration is a feature common to diseases with mutations in genes that encode complex I of the respiratory chain. Vulnerability of this central nervous system tract is a mystery, because of the paucity of animal models used to investigate effects of the mutated DNA in tissues rather than isolated in cultured cells. Using a ribozyme designed to degrade the mRNA encoding a critical nuclear-encoded subunit gene of complex I (NDUFA1), we tested whether oxidative phosphorylation deficiency can recapitulate the optic neuropathy of mitochondrial disease. Injection of adenoassociated virus expressing this ribozyme led to axonal destruction and demyelination, the hallmarks of Leber hereditary optic neuropathy.

Biochemical studies of patients with Cuban epidemic neuropathy.

In 1992-1994, a disorder known as the epidemic neuropathy afflicted more than 50,000 Cubans. Three different forms of the illness were identified: epidemic optic neuropathy, peripheral neuropathy and mixed optic and peripheral neuropathy. The causes are still unknown. Skeletal muscle biopsy samples were analyzed by standard histological techniques and by biochemical assays. Elevated activities of citrate synthase, a non-respiratory-chain mitochondrial matrix enzyme, suggested possible mitochondrial proliferation in 7 of the 8 patients. Nicotinamide adenine dinucleotide phosphate (NADP(+)) levels were higher in the patients than in the controls (p = 0.04). Levels of nicotinamide adenine dinucleotide (NAD) and the reduced compounds NADH and NADPH were comparable in patients and controls. Elevations of succinate dehydrogenase and citrate synthase activities and high NADP(+) levels suggest that alterations of mitochondrial functions may be associated with this disorder.

Ocular pathology in mitochondrial superoxide dismutase (Sod2)-deficient mice.

PURPOSE: To characterize the pathologic features in retina, optic nerve, and extraocular muscle of mitochondrial superoxide dismutase (Sod2)-deficient mice, a model of increased mitochondrial production of reactive oxygen species. METHODS: Morphometric and ultrastructural study of eyes of 43 homozygous sod2(tm1Cje-/-) mice and wild-type control animals. For retinal morphometric analysis, 32 manganese 5,10,15,20-tetrakis (4-benzoic acid) porphyrin (MnTBAP)-treated animals aged either 9 to 10 days or 20 to 21 days were studied. Ultrastructural examination was performed on tissue from the treated animals, and 11 additional untreated mutant and control animals. RESULTS: In treated Sod2-deficient animals, the photoreceptor layer was thinner centrally at 9 to 10 days than in control animals (mean 8.8 vs. 14.7 microm). By 20 to 21 days, all retinal layers apart from the outer nuclear layer and retinal pigment epithelium (RPE) were thinner centrally in mutant animals (total retinal thickness, 233.2 vs. 272.6 microm; combined nerve fiber layer, ganglion cell layer, and inner plexiform layer, 86.2 vs. 103.4 microm; inner nuclear layer, 51.8 vs. 60.3 microm; photoreceptors, 26.7 vs. 35.6 microm). Optic nerve cross-sectional area was less in 20- to 21-day-old treated Sod2-deficient animals than in control animals. Mitochondrial morphologic abnormalities (swelling, pale matrix, and disorganized cristae) were found predominantly in older mutant animals' (16 and 20 to 21 days) RPE and in extraocular muscle of a 16-day-old untreated mutant. CONCLUSIONS: In sod2(tm1Cje-/-) mice, there is relative progressive retinal thinning, with particular involvement of the inner retinal layers and an early effect on the photoreceptor layer, as well as mitochondrial morphologic abnormalities, all consistent with mitochondrial disease.

Acid phosphatase localization in accumulated membranous organelles of optic nerve axons following acute elevation of intraocular pressure.

Acid phosphatase localization in accumulated membranous organelles of optic nerve axons of guinea pigs following acute elevation of intraocular pressure (IOP) was determined, employing light and electron microscopic enzymic cytochemistry with beta-glycerophosphate as a substrate. Positive reaction products appeared to accumulate in the region of the lamina cribrosa, as revealed with light microscopic enzyme cytochemistry. Electron microscopic enzyme cytochemistry also demonstrated that such reaction products mainly localized on multivesicular or multilamellar bodies and myelin-like structures in the unmyelinated optic nerve axons. Following an acutely elevated IOP, retrograde-moving membranous organelles in the optic nerve axons were found to contain AcPase, suggesting that these organelles could be degraded in the axons through the lysosomal pathway.

Effect of aminoguanidine on optic nerve involvement in experimental diabetic rats.

The effect of aminoguanidine (AG) on structural abnormalities in optic nerve fibers was evaluated in streptozotocin-induced diabetic rats. Morphometry of myelinated optic nerve fibers showed a partial amelioration on the reduction of the axon size but complete preservation of myelin size with low dose (25 mg kg-1 body weight) AG treatment. High dose (100 mg kg-1 body weight) AG completely prevented myelinated nerve fiber atrophy. These findings indicated that nonenzymatic glycation contributes to the development of optic nerve changes in diabetic rats.

Compensatory elevation of complex II activity in Leber's hereditary optic neuropathy.

AIMS: To evaluate the mitochondrial respiratory enzyme activities in blood cells of Leber's hereditary optic neuropathy (LHON) with 11778 point mutation of mitochondrial DNA. METHODS: Assays for the activities of NADH-cytochrome c reductase (complex I+complex III), succinate-cytochrome c reductase (complex II+complex III), and cytochrome c oxidase (complex IV) on blood cell mitochondria of seven LHON patients and 15 normal controls. RESULTS: There was no statistically significant difference in NADH-cytochrome c reductase and cytochrome c oxidase activities between LHON patients and controls, but activities of succinate-cytochrome c reductase in LHON patients was significantly elevated compared with normal controls. CONCLUSION: The observations that the activity of NADH-cytochrome c reductase is normal but that of succinate-cytochrome c reductase is increased in LHON patients with 11778 point mutation of mitochondrial DNA indicate an elevation of complex II activity, which may be due to a nuclear compensatory effect for defects of the respiratory function of mitochondria.

Deficiency of thiosulphate sulphurtransferase (rhodanese) in Leber's hereditary optic neuropathy.

Leber's hereditary optic neuropathy is a rare cause of progressive visual failure. Its cause is unknown, but one hypothesis is that patients have a defect in the detoxication of cyanide. One of the enzymes used in this detoxication is thiosulphate sulphurtransferase (rhodanese). The activity of this enzyme was measured in the rectal mucosa of a group of subjects with Leber's hereditary optic neuropathy, and it was found to be considerably reduced compared with that in a group of controls (p less than 0.001). This finding supports the hypothesis of an inborn error of cyanide detoxication in this condition.