Leigh-like encephalopathy complicating Leber's hereditary optic neuropathy

Digenic Leigh syndrome on the background of the m.11778G>A Leber hereditary optic neuropathy variant

Leigh syndrome spectrum (LSS) is a primary mitochondrial disorder defined neuropathologically by a subacute necrotizing encephalomyelopathy and characterized by bilateral basal ganglia and/or brainstem lesions. LSS is associated with variants in several mitochondrial DNA genes and more than 100 nuclear genes, most often related to mitochondrial complex I (CI) dysfunction. Rarely, LSS has been reported in association with primary Leber hereditary optic neuropathy (LHON) variants of the mitochondrial DNA, coding for CI subunits (m.3460G>A in MT-ND1, m.11778G>A in MT-ND4 and m.14484T>C in MT-ND6). The underlying mechanism by which these variants manifest as LSS, a severe neurodegenerative disease, as opposed to the LHON phenotype of isolated optic neuropathy, remains an open question. Here, we analyse the exome sequencing of six probands with LSS carrying primary LHON variants, and report digenic co-occurrence of the m.11778G > A variant with damaging heterozygous variants in nuclear disease genes encoding CI subunits as a plausible explanation. Our findings suggest a digenic mechanism of disease for m.11778G>A-associated LSS, consistent with recent reports of digenic disease in individuals manifesting with LSS due to biallelic variants in the recessive LHON-associated disease gene DNAJC30 in combination with heterozygous variants in CI subunits.

Progress in diagnosis and treatment of Leber's hereditary optic neuropathy

Leber's hereditary optic neuropathy (LHON) is a mitochondrial genetic disease with central vision loss as the main symptom. It is one of the diseases that cause vision loss and optic atrophy in young and middle-aged people. The mutations of these three primary mitochondrial mutations, m.11778G>A, m.14484T>C, and m.3460G>A, are the main molecular basis, but their pathogenesis is also affected by nuclear genes, mitochondrial genetic background, and environmental factors. This article summarizes the research progress on molecular pathogenesis, clinical symptoms, and treatment of LHON in recent years, aiming to summarize the genetic pathogenesis and clinical treatment points of LHON.

Recessive MECR pathogenic variants cause an LHON-like optic neuropathy

BACKGROUND

Leber's hereditary optic neuropathy (LHON) is a mitochondrial disorder characterised by complex I defect leading to sudden degeneration of retinal ganglion cells. Although typically associated with pathogenic variants in mitochondrial DNA, LHON was recently described in patients carrying biallelic variants in nuclear genes DNAJC30, NDUFS2 and MCAT. MCAT is part of mitochondrial fatty acid synthesis (mtFAS), as also MECR, the mitochondrial trans-2-enoyl-CoA reductase. MECR mutations lead to a recessive childhood-onset syndromic disorder with dystonia, optic atrophy and basal ganglia abnormalities.

METHODS

We studied through whole exome sequencing two sisters affected by sudden and painless visual loss at young age, with partial recovery and persistent central scotoma. We modelled the candidate variant in yeast and studied mitochondrial dysfunction in yeast and fibroblasts. We tested protein lipoylation and cell response to oxidative stress in yeast.

RESULTS

Both sisters carried a homozygous pathogenic variant in MECR (p.Arg258Trp). In yeast, the MECR-R258W mutant showed an impaired oxidative growth, 30% reduction in oxygen consumption rate and 80% decrease in protein levels, pointing to structure destabilisation. Fibroblasts confirmed the reduced amount of MECR protein, but failed to reproduce the OXPHOS defect. Respiratory complexes assembly was normal. Finally, the yeast mutant lacked lipoylation of key metabolic enzymes and was more sensitive to H2O2 treatment. Lipoic Acid supplementation partially rescued the growth defect.

CONCLUSION

We report the first family with homozygous MECR variant causing an LHON-like optic neuropathy, which pairs the recent MCAT findings, reinforcing the impairment of mtFAS as novel pathogenic mechanism in LHON.

Leber's hereditary optic neuropathy: Update on current diagnosis and treatment

Leber's hereditary optic neuropathy (LHON) is a fairly prevalent mitochondrial disorder (1:50,000) arising from the dysfunction of the mitochondrial respiratory chain, which eventually leads to apoptosis of retinal ganglion cells. The usual presentation is that of a young male with a sequential reduction in visual acuity. OCT has been used to study the pattern of optic nerve involvement in LHON, showing early thickening of the inferior and superior retinal nerve fibre layer and ganglion cell layer thinning corresponding with the onset of symptoms. Of the three primary mutations for LHON, the m.14484T>C mutation has the best visual prognosis. Recent emerging therapeutic options for LHON include idebenone and the introduction of genetic vector therapy, which is currently in phase III clinical trials. Screening of family members and adequate advice to avoid environmental triggers, such as smoking and alcohol consumption, are also cornerstones in the management of LHON.

Mitochondrial optic neuropathies

Mitochondrial optic neuropathies have a leading role in the field of mitochondrial medicine ever since 1988, when the first mutation in mitochondrial DNA was associated with Leber's hereditary optic neuropathy (LHON). Autosomal dominant optic atrophy (DOA) was subsequently associated in 2000 with mutations in the nuclear DNA affecting the OPA1 gene. LHON and DOA are both characterized by selective neurodegeneration of retinal ganglion cells (RGCs) triggered by mitochondrial dysfunction. This is centered on respiratory complex I impairment in LHON and defective mitochondrial dynamics in OPA1-related DOA, leading to distinct clinical phenotypes. LHON is a subacute, rapid, severe loss of central vision involving both eyes within weeks or months, with age of onset between 15 and 35 years old. DOA is a more slowly progressive optic neuropathy, usually apparent in early childhood. LHON is characterized by marked incomplete penetrance and a clear male predilection. The introduction of next-generation sequencing has greatly expanded the genetic causes for other rare forms of mitochondrial optic neuropathies, including recessive and X-linked, further emphasizing the exquisite sensitivity of RGCs to compromised mitochondrial function. All forms of mitochondrial optic neuropathies, including LHON and DOA, can manifest either as pure optic atrophy or as a more severe multisystemic syndrome. Mitochondrial optic neuropathies are currently at the forefront of a number of therapeutic programs, including gene therapy, with idebenone being the only approved drug for a mitochondrial disorder.

Leber's hereditary optic neuropathy: Current approaches and future perspectives on Mesenchymal stem cell-mediated rescue

Leber's Hereditary Optic Neuropathy (LHON) is an inherited optic nerve disorder. It is a mitochondrially inherited disease due to point mutation in the MT-ND1, MT-ND4, and MT-ND6 genes of mitochondrial DNA (mtDNA) coding for complex I subunit proteins. These mutations affect the assembly of the mitochondrial complex I and hence the electron transport chain leading to mitochondrial dysfunction and oxidative damage. Optic nerve cells like retinal ganglion cells (RGCs) are more sensitive to mitochondrial loss and oxidative damage which results in the progressive degeneration of RGCs at the axonal region of the optic nerve leading to bilateral vision loss. Currently, gene therapy using Adeno-associated viral vector (AAV) is widely studied for the therapeutics application in LHON. Our review highlights the application of cell-based therapy for LHON. Mesenchymal stem cells (MSCs) are known to rescue cells from the pre-apoptotic stage by transferring healthy mitochondria through tunneling nanotubes (TNT) for cellular oxidative function. Empowering the transfer of healthy mitochondria using MSCs may replace the mitochondria with pathogenic mutation and possibly benefit the cells from progressive damage. This review discusses the ongoing research in LHON and mitochondrial transfer mechanisms to explore its scope in inherited optic neuropathy.

Leber hereditary optic neuropathy and dystonia overlapping mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes due to m.14459G>A mutation

OBJECTIVE

To report a Chinese family with combined m.14459G>A mutation and m.6064A>T mutation of which the female proband presenting unique Leber hereditary optic neuropathy and dystonia (LDYT) overlapping mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes (MELAS) phenotype.

METHODS

Clinical information of the pedigree was collected. We performed muscle biopsy and whole-length mitochondrial DNA (mtDNA) sequencing on the proband. The activity of respiratory chain complexes in immortalized lymphoblasts was determined.

RESULTS

The current 23-year-old proband suffered from vision decline at age 15 and developed seizures and dystonia with bilateral lesions in precentral gyri at age 18. When she was 21, the lesions in bilateral putamen were found with elevated cerebrospinal fluid lactate. Her mother had optic atrophy; one of her brother died at age 4 with respiratory distress; and the other 8-year-old brother was asymptomatic. Muscle biopsy of the proband was unremarkable. The mtDNA sequencing revealed a heteroplasmic m.14459G>A mutation and a previously unreported m.6064A>T mutation. The respiratory chain complex I activity in the proband's immortalized lymphoblasts was 50% less than the normal control; while there was no statistical difference between the proband and the normal control in the activity of complex IV.

CONCLUSIONS

We presented the first case exhibiting LDYT and MELAS phenotype with m.14459G>A mutation, and the decreased complex I activity contributed to the pathogenicity. Our study expanded the clinical spectrum of m.14459G>A mutation.

Blackout in the powerhouse: clinical phenotypes associated with defects in the assembly of OXPHOS complexes and the mitoribosome

Mitochondria produce the bulk of the energy used by almost all eukaryotic cells through oxidative phosphorylation (OXPHOS) which occurs on the four complexes of the respiratory chain and the F₁–F₀ ATPase. Mitochondrial diseases are a heterogenous group of conditions affecting OXPHOS, either directly through mutation of genes encoding subunits of OXPHOS complexes, or indirectly through mutations in genes encoding proteins supporting this process. These include proteins that promote assembly of the OXPHOS complexes, the post-translational modification of subunits, insertion of cofactors or indeed subunit synthesis. The latter is important for all 13 of the proteins encoded by human mitochondrial DNA, which are synthesised on mitochondrial ribosomes. Together the five OXPHOS complexes and the mitochondrial ribosome are comprised of more than 160 subunits and many more proteins support their biogenesis. Mutations in both nuclear and mitochondrial genes encoding these proteins have been reported to cause mitochondrial disease, many leading to defective complex assembly with the severity of the assembly defect reflecting the severity of the disease. This review aims to act as an interface between the clinical and basic research underpinning our knowledge of OXPHOS complex and ribosome assembly, and the dysfunction of this process in mitochondrial disease.

Clinicopathological findings of a mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes/Leigh syndrome overlap patient with a novel m.3482A>G mutation in MT-ND1

We report clinicopathological findings of a patient with mitochondrial encephalopathy, lactic acidosis, and stroke-like episodes/Leigh syndrome (MELAS/LS) associated with a novel m.3482A>G mutation in MT-ND1. A 41-year-old woman had experienced multiple stroke-like episodes since age 16. She developed akinetic mutism two months before admission to our hospital. Neurological examination revealed akinetic mutism, bilateral deafness, and muscular atrophy. Cerebrospinal fluid tests revealed elevated pyruvate and lactate levels. Fluid-attenuated inversion recovery images on magnetic resonance imaging showed hyperintense areas in the right frontal and both sides of temporal and occipital lobes, both sides of the striatum, and the midbrain. Muscle biopsy revealed strongly succinate dehydrogenase-reactive blood vessels. L-arginine therapy improved her consciousness and prevented further stroke-like episodes. However, she died from aspiration pneumonia. Postmortem autopsy revealed scattered infarct-like lesions with cavitation in the cerebral cortex and necrotic lesions in the striatum and midbrain. The patient was pathologically confirmed as having MELAS/LS based on two characteristic clinicopathological findings: presenting MELAS/LS overlap phenotype and effectiveness of L-arginine treatment.

Genotypic and phenotypic characteristics of Korean children with childhood-onset Leber's hereditary optic neuropathy

PURPOSE

We sought to identify the phenotypic and genotypic characteristics of Korean children with genetically confirmed Leber's hereditary optic neuropathy (LHON).

METHODS

The medical records of 64 genetically confirmed LHON patients were reviewed. Seventeen patients aged 13 years or younger with optic atrophy with positive mitochondrial DNA (mtDNA) mutations were considered to demonstrate childhood-onset LHON. The non-childhood-onset group included 47 patients with genetically confirmed LHON who experienced disease onset later than 13 years of age. The type of mtDNA mutation, visual acuity (VA), color vision, fundus photography, retinal nerve fiber layer (RNFL) thickness, and visual field were investigated.

RESULTS

Sequence analysis of the mitochondrial genome revealed five different kinds of LHON-associated mtDNA mutations among our childhood-onset patients, including m.11778G>A (58.8%), m.3496G>T (11.8%), m.3497C>T (5.9%), m.11696G>A (5.9%), and m.14502T>C (5.9%). The mean final best-corrected VA in the childhood-onset group was better than that in the non-childhood-onset group with the value of logMAR 0.29 (0.09-0.75) vs. 0.55 (0.27-1.29) (expressed as median (interquartile range); p = 0.05). Spontaneous visual recovery was observed in 35.3% of the childhood-onset group but in only 12.8% of the non-childhood-onset group (p = 0.04). Eight patients (47.1%) showed interocular asymmetry of the disease, with two presenting true unilateral involvement of the optic nerve and the other six patients demonstrating unilateral subclinical manifestations with bilateral optic atrophy.

CONCLUSION

Involvement of secondary mitochondrial mutations was confirmed in patients with childhood-onset LHON. Characteristic clinical features of childhood-onset LHON included a higher proportion of subacute or insidious onset of symptoms, better VA, higher spontaneous recovery, and asymmetrical ocular involvement.

Personalized pediatric ophthalmology: a case report

The availability of genetic sequencing has given physicians a new tool for diagnosis and treatment of disease, and "personalized medicine" has become an increasingly common term in general but not in pediatric ophthalmology. We present a case of a toddler who developed ataxia, opsoclonus, myoclonus, and developmental regression following anesthesia for a common otolaryngology procedure. The child was found to have a variant in the MT-ND6 gene (m.14484T>C), most commonly associated with Leber hereditary optic neuropathy, despite a phenotype more closely resembling Leigh syndrome. The incongruence of phenotype and genotype prompted whole exome sequencing, which identified an unexpected intronic missense mutation in RB1 (1960+5G>A), with a 90% penetrance for retinoblastoma. Limited evaluation of the posterior pole in clinic did not identify any lesions, and the risks and benefits of examination under anesthesia were discussed among neurology, ophthalmology, and anesthesiology. We report the outcome of these discussions. The value and risks of personalized medicine are discussed.

Cardiovascular Manifestations of Mitochondrial Disease

Genetic mitochondrial cardiomyopathies are uncommon causes of heart failure that may not be seen by most physicians. However, the prevalence of mitochondrial DNA mutations and somatic mutations affecting mitochondrial function are more common than previously thought. In this review, the pathogenesis of genetic mitochondrial disorders causing cardiovascular disease is reviewed. Treatment options are presently limited to mostly symptomatic support, but preclinical research is starting to reveal novel approaches that may lead to better and more targeted therapies in the future. With better understanding and clinician education, we hope to improve clinician recognition and diagnosis of these rare disorders in order to improve ongoing care of patients with these diseases and advance research towards discovering new therapeutic strategies to help treat these diseases.

Mammalian Mitochondrial Complex I Structure and Disease-Causing Mutations

Complex I has an essential role in ATP production by coupling electron transfer from NADH to quinone with translocation of protons across the inner mitochondrial membrane. Isolated complex I deficiency is a frequent cause of mitochondrial inherited diseases. Complex I has also been implicated in cancer, ageing, and neurodegenerative conditions. Until recently, the understanding of complex I deficiency on the molecular level was limited due to the lack of high-resolution structures of the enzyme. However, due to developments in single particle cryo-electron microscopy (cryo-EM), recent studies have reported nearly atomic resolution maps and models of mitochondrial complex I. These structures significantly add to our understanding of complex I mechanism and assembly. The disease-causing mutations are discussed here in their structural context.

Mechanisms of Mitochondrial DNA Repair in Mammals

Accumulation of mutations in mitochondrial DNA leads to the development of severe, currently untreatable diseases. The contribution of these mutations to aging and progress of neurodegenerative diseases is actively studied. Elucidation of DNA repair mechanisms in mitochondria is necessary for both developing approaches to the therapy of diseases caused by mitochondrial mutations and understanding specific features of mitochondrial genome functioning. Mitochondrial DNA repair systems have become a subject of extensive studies only in the last decade due to development of molecular biology methods. DNA repair systems of mammalian mitochondria appear to be more diverse and effective than it had been thought earlier. Even now, one may speak about the existence of mitochondrial mechanisms for the repair of single- and double-stranded DNA lesions. Homologous recombination also takes place in mammalian mitochondria, although its functional significance and molecular mechanisms remain obscure. In this review, I describe DNA repair systems in mammalian mitochondria, such as base excision repair (BER) and microhomology-mediated end joining (MMEJ) and discuss a possibility of existence of mitochondrial DNA repair mechanisms otherwise typical for the nuclear DNA, e.g., nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination, and classical non-homologous end joining (NHEJ). I also present data on the mechanisms for coordination of the nuclear and mitochondrial DNA repair systems that have been actively studied recently.

Leber Hereditary Optic Neuropathy and Longitudinally Extensive Transverse Myelitis

Leber Hereditary Optic Neuropathy is an inherited optic neuropathy caused by mitochondrial DNA point mutations leading to sudden, painless loss of vision. We report a case of an 8-year-old boy presenting with a radiological phenotype of longitudinally extensive transverse myelitis on a background of severe visual impairment secondary to Leber Hereditary Optic Neuropathy (LHON). He was found to have dual mitochondrial DNA mutations at 14484 (MTND6 gene) and 4160 (MTND1 gene) in a family with a severe form of LHON characterised by not only an unusually high penetrance of optic neuropathy, but also severe extra-ocular neurological complications. The m.14484T>C mutation is a common LHON mutation, but the m.4160T>C mutation is to our knowledge not reported outside this family and appears to drive the neurological manifestations. To our knowledge there have been no previous reports of spinal cord lesions in children with LHON.

The metabolomic signature of Leber's hereditary optic neuropathy reveals endoplasmic reticulum stress

Leber's hereditary optic neuropathy (MIM#535000), the commonest mitochondrial DNA-related disease, is caused by mutations affecting mitochondrial complex I. The clinical expression of the disorder, usually occurring in young adults, is typically characterized by subacute, usually sequential, bilateral visual loss, resulting from the degeneration of retinal ganglion cells. As the precise action of mitochondrial DNA mutations on the overall cell metabolism in Leber's hereditary optic neuropathy is unknown, we investigated the metabolomic profile of the disease. High performance liquid chromatography coupled with tandem mass spectrometry was used to quantify 188 metabolites in fibroblasts from 16 patients with Leber's hereditary optic neuropathy and eight healthy control subjects. Latent variable-based statistical methods were used to identify discriminating metabolites. One hundred and twenty-four of the metabolites were considered to be accurately quantified. A supervised orthogonal partial least squares discriminant analysis model separating patients with Leber's hereditary optic neuropathy from control subjects showed good predictive capability (Q 2cumulated = 0.57). Thirty-eight metabolites appeared to be the most significant variables, defining a Leber's hereditary optic neuropathy metabolic signature that revealed decreased concentrations of all proteinogenic amino acids, spermidine, putrescine, isovaleryl-carnitine, propionyl-carnitine and five sphingomyelin species, together with increased concentrations of 10 phosphatidylcholine species. This signature was not reproduced by the inhibition of complex I with rotenone or piericidin A in control fibroblasts. The importance of sphingomyelins and phosphatidylcholines in the Leber's hereditary optic neuropathy signature, together with the decreased amino acid pool, suggested an involvement of the endoplasmic reticulum. This was confirmed by the significantly increased phosphorylation of PERK and eIF2α, as well as the greater expression of C/EBP homologous protein and the increased XBP1 splicing, in fibroblasts from affected patients, all these changes being reversed by the endoplasmic reticulum stress inhibitor, TUDCA (tauroursodeoxycholic acid). Thus, our metabolomic analysis reveals a pharmacologically-reversible endoplasmic reticulum stress in complex I-related Leber's hereditary optic neuropathy fibroblasts, a finding that may open up new therapeutic perspectives for the treatment of Leber's hereditary optic neuropathy with endoplasmic reticulum-targeting drugs.

Leber Hereditary Optic Neuropathy: Bringing the Lab to the Clinic

Leber hereditary optic neuropathy (LHON) was the first clinically characterized mitochondrial disorder. Since its first description in 1871, much has been discovered regarding the genetics and pathophysiology of the disease. This has enabled the development of in vitro cell and animal models that can be used to try to determine not only the effects of the genetic mutation upon the clinical phenotype but to also test potential novel therapies. Treatments for LHON have ranged from vitamins and minerals to immunosuppressants and, more recently, targeted gene therapy. This article reviews the pathophysiology and clinical features of LHON with a focus on translational research.

Childhood-Onset Progressive Dystonia With Mitochondrial DNA G14459A Mutation: Efficacy of Long-Term Sodium Succinate Treatment

This article reports the case of an 11-year-old boy with progressive dystonia caused by the homoplasmic G14459A mitochondrial DNA mutation. The patient presented with focal dystonia in the right upper limb at 3 years of age, which progressed over 4 years to exhibit dystonia in both the upper and lower limbs. At 7 years of age, high signal intensity lesions in the bilateral striata and the midbrain were observed on fluid-attenuated inversion recovery images. It was observed on diffusion-weighted images that with time, these high signal intensity lesions migrated from the putamen to the caudate nuclei, which closely correlated with disease progression. Because his symptoms and abnormal magnetic resonance imaging findings progressed despite treatment with coenzyme Q10 and l-carnitine, at 7 years of age he was then started on sodium succinate, hoping to improve his complex I deficiency. After treatment, progression of MRI abnormalities appeared to have been suppressed for 4 years, although no improvement was observed in dystonia.

Uncommon Leber "plus" disease associated with mitochondrial mutation m.11778G>A in a premature child

Leber hereditary optic neuropathy is a well-known mitochondrial disorder that leads to bilateral subacute visual failure. Although visual impairment is often the sole clinical feature, additional and severe neurologic abnormalities also have been documented for this disease. We report on a 13-year-old boy who has presented with severe visual failure since early childhood in a context of prematurity. In the first years of his life, clinical features included delayed psychomotor development and ataxia. The clinical presentation, which was initially attributed to prematurity, worsened thereafter, and the child developed acute neurologic degradation with the typical radiological findings of Leigh syndrome. The mitochondrial DNA point mutation 11778G>A was identified in the ND4 gene. The probable influence of environmental background on clinical expression of Leber optic neuropathy, particularly those of prematurity and oxygen therapy, is discussed in our manuscript.

Association of the mtDNA m.4171C>A/MT-ND1 mutation with both optic neuropathy and bilateral brainstem lesions

Background

An increasing number of mitochondrial DNA (mtDNA) mutations, mainly in complex I genes, have been associated with variably overlapping phenotypes of Leber’s hereditary optic neuropathy (LHON), mitochondrial encephalomyopathy with stroke-like episodes (MELAS) and Leigh syndrome (LS). We here describe the first case in which the m.4171C>A/MT-ND1 mutation, previously reported only in association with LHON, leads also to a Leigh-like phenotype.

Case presentation

A 16-year-old male suffered subacute visual loss and recurrent vomiting and vertigo associated with bilateral brainstem lesions affecting the vestibular nuclei. His mother and one sister also presented subacute visual loss compatible with LHON. Sequencing of the entire mtDNA revealed the homoplasmic m.4171C>A/MT-ND1 mutation, previously associated with pure LHON, on a haplogroup H background. Three additional non-synonymous homoplasmic transitions affecting ND2 (m.4705T>C/MT-ND2 and m.5263C>T/MT-ND2) and ND6 (m.14180T>C/MT-ND6) subunits, well recognized as polymorphisms in other mtDNA haplogroups but never found on the haplogroup H background, were also present.

Conclusion

This case widens the phenotypic expression of the rare m.4171C>A/MT-ND1 LHON mutation, which may also lead to Leigh-like brainstem lesions, and indicates that the co-occurrence of other ND non-synonymous variants, found outside of their usual mtDNA backgrounds, may have increased the pathogenic potential of the primary LHON mutation.

Association of mitochondrial genetic variation with carotid atherosclerosis

In human pathology, several diseases are associated with somatic mutations in the mitochondrial genome (mtDNA). Even though mitochondrial dysfunction leads to increased oxidative stress, the role of mitochondrial mutations in atherosclerosis has not received much attention so far. In this study we analyzed the association of mitochondrial genetic variation with the severity of carotid atherosclerosis, as assessed by carotid intima-media thickness (cIMT) and the presence of coronary heart disease (CHD) in 190 subjects from Moscow, Russia, a population with high CHD occurrence. cIMT was measured by high-resolution B-mode ultrasonography and mtDNA heteroplasmies by a pyrosequencing-based method. We found that heteroplasmies for several mutations in the mtDNA in leukocytes, including C3256T, T3336C, G12315A, G13513A, G14459A, G14846A, and G15059A mutations, were significantly (p<0.001) associated with both the severity of carotid atherosclerosis and the presence of CHD. These findings indicate that somatic mitochondrial mutations have a role in the development of atherosclerosis.

Cybrid studies establish the causal link between the mtDNA m.3890G>A/MT-ND1 mutation and optic atrophy with bilateral brainstem lesions

Complex I (CI) deficiency is a frequent cause of mitochondrial disorders and, in most cases, is due to mutations in CI subunit genes encoded by mitochondrial DNA (mtDNA). In this study, we establish the pathogenic role of the heteroplasmic mtDNA m.3890G>A/MT-ND1 (p.R195Q) mutation, which affects an extremely conserved amino acid position in ND1 subunit of CI. This mutation was found in a young-adult male with optic atrophy resembling Leber's hereditary optic neuropathy (LHON) and bilateral brainstem lesions. The only previously reported case with this mutation was a girl with fatal infantile Leigh syndrome with bilateral brainstem lesions. Transfer of the mutant mtDNA in the cybrid cell system resulted in a marked reduction of CI activity and CI-dependent ATP synthesis in the presence of a normally assembled enzyme.

These findings establish the pathogenicity of the m.3890G>A/MT-ND1 mutation and remark the link between CI mutations affecting the mtDNA-encoded ND subunits and LHON-like optic atrophy, which may be complicated by bilateral and symmetric lesions affecting the central nervous system. Peculiar to this mutation is the distribution of the brainstem lesions, with sparing of the striatum in both patients.

Vertical supranuclear gaze palsy in Niemann-Pick type C disease

Vertical supranuclear gaze palsy (VSGP) is a key clinical feature in patients with Niemann-Pick type C disease (NP-C), a rare, autosomal recessive, neuro-visceral disorder caused by mutations in either the NPC1 or NPC2 gene. VSGP is present in approximately 65 % of the cases and is, with gelastic cataplexy, an important risk indicator for NP-C. VSGP in NP-C is characterized by a paralysis of vertical saccades, especially downward, with the slow vertical eye movement systems (smooth pursuit and the vestibulo-ocular reflex) spared in the early phase of the disease. This dissociation is caused by a selective vulnerability of the neurons in the rostral interstitial nuclei of the medial longitudinal fasciculus (riMLF) in NP-C. Here we discuss VSGP in NP-C and how clinicians can best elicit this sign.

[Leber's hereditary optic neuropathy]

Leber's hereditary optic neuropathy (LHON) is a rare disease primarily affecting the retinal ganglion cells. In most cases patients with LHON develop permanent visual loss with a large central scotoma in the visual field of both eyes. The optic disc becomes partially or completely pale. At the onset of the disease many patients are considered to suffer from an optic neuritis and are treated under the diagnostic and therapeutic regimen of optic neuritis. LHON is mostly only considered when high dose cortisone therapy fails to be effective or the second eye is affected. Thereafter, molecular genetic analysis will prove LHON in these cases. Detailed anamnesis including pedigree analysis in combination with observance of the peripapillary microangiopathic alterations at the fundus will help to speed up the diagnosis of LHON, but even after exact clinical and molecular genetic diagnosis of LHON some aspects of the disease still remain a mystery today.

Cockayne syndrome group B protein prevents the accumulation of damaged mitochondria by promoting mitochondrial autophagy

Cells from Cockayne syndrome patients and a mouse model of the disease show increased metabolism as a result of impaired autophagy-mediated removal of damaged mitochondria.

Cockayne syndrome (CS) is a devastating autosomal recessive disease characterized by neurodegeneration, cachexia, and accelerated aging. 80% of the cases are caused by mutations in the CS complementation group B (CSB) gene known to be involved in DNA repair and transcription. Recent evidence indicates that CSB is present in mitochondria, where it associates with mitochondrial DNA (mtDNA). We report an increase in metabolism in the CSB^m/m mouse model and CSB-deficient cells. Mitochondrial content is increased in CSB-deficient cells, whereas autophagy is down-regulated, presumably as a result of defects in the recruitment of P62 and mitochondrial ubiquitination. CSB-deficient cells show increased free radical production and an accumulation of damaged mitochondria. Accordingly, treatment with the autophagic stimulators lithium chloride or rapamycin reverses the bioenergetic phenotype of CSB-deficient cells. Our data imply that CSB acts as an mtDNA damage sensor, inducing mitochondrial autophagy in response to stress, and that pharmacological modulators of autophagy are potential treatment options for this accelerated aging phenotype.

Clinical and molecular features of an infant patient affected by Leigh Disease associated to m.14459G>A mitochondrial DNA mutation: a case report

Background

Leigh Syndrome (LS) is a severe neurodegenerative disorder characterized by bilateral symmetrical necrotic lesions in the basal ganglia and brainstem. Onset is in early infancy and prognosis is poor. Causative mutations have been disclosed in mitochondrial DNA and nuclear genes affecting respiratory chain subunits and assembly factors.

Case presentation

Here we report the clinical and molecular features of a 15-month-old female LS patient. Direct sequencing of her muscle-derived mtDNA revealed the presence of two apparently homoplasmic variants: the novel m.14792C > G and the already known m.14459G > A resulting in p.His16Asp change in cytochrome b (MT-CYB) and p.Ala72Val substitution in ND6 subunit, respectively. The m.14459G > A was heteroplasmic in the mother's blood-derived DNA.

Conclusions

The m.14459G > A might lead to LS, complicated LS or Leber Optic Hereditary Neuropathy. A comprehensive re-evaluation of previously described 14459G > A-mutated patients does not explain this large clinical heterogeneity.

Atypical presentation of Leigh syndrome associated with a Leber hereditary optic neuropathy primary mitochondrial DNA mutation

Leber hereditary optic neuropathy (LHON) is caused by point mutations in mitochondrial DNA (mtDNA), and is characterized by bilateral, painless sub-acute visual loss that develops during the second decade of life. Here we report the case of a five year old girl who presented with clinical and neuroradiological findings reminiscent of Leigh syndrome but carried a mtDNA mutation m.11778G>A (p.R340H) in the MTND4 gene usually observed in patients with LHON. This case is unusual for age of onset, gender, associated neurological findings and evolution, further expanding the clinical spectrum associated with primary LHON mtDNA mutations.

[Hereditary optic atrophies]

INTRODUCTION

Hereditary optic neuropathies, resulting from retinal ganglion cell degeneration, are a heterogeneous group of diseases ranging from asymptomatic forms to legal blindness.

STATE OF KNOWLEDGE

Two most frequent phenotypes are Kjer's disease, an autosomal dominant optic atrophy caused by OPA1 gene mutations, and Leber's disease due to maternally inherited mitochondrial DNA mutations.

PROSPECTS AND CONCLUSION

Both optic neuropathies usually isolated are sometimes associated with extraocular symptoms, especially neurological symptoms, thus justifying a systematic neurological evaluation and brain imaging.

Pediatric-onset dystonia associated with bilateral striatal necrosis and G14459A mutation in a Korean family: a case report

We describe a Korean family presenting with pediatric-onset, progressive, generalized dystonia with bilateral striatal necrosis and the homoplasmic G14459A mutation in the mitochondrial ND6 gene. The G14459A mutation has been reported in families presenting with Leber hereditary optic neuropathy (LHON) alone, LHON plus dystonia, or pediatric-onset dystonia. The proband had shown dysarthria, progressive generalized dystonia, and spasticity at 5 yr. Brain MRI demonstrated bilateral striatal necrosis. Additional investigation of family members revealed the presence of homoplasmic G14459A mutation in asymptomatic individuals. The clinical manifestation of the homoplasmic G14459A mtDNA mutation within the same family showed asymptomatic or pediatric-onset dystonia, without optic neuropathy. This study reemphasizes that the G14459A mutation is a candidate mutation for maternally inherited dystonia, regardless of optic neuropathy, and supports the hypothesis that nuclear genes may play a role in modifying the clinical expression of mitochondrial disease.

Leigh and Leigh-like syndrome in children and adults

Leigh syndrome (also termed subacute, necrotizing encephalopathy) is a devastating neurodegenerative disorder, characterized by almost identical brain changes, e.g., focal, bilaterally symmetric lesions, particularly in the basal ganglia, thalamus, and brainstem, but with considerable clinical and genetic heterogeneity. Clinically, Leigh syndrome is characterized by a wide variety of abnormalities, from severe neurologic problems to a near absence of abnormalities. Most frequently the central nervous system is affected, with psychomotor retardation, seizures, nystagmus, ophthalmoparesis, optic atrophy, ataxia, dystonia, or respiratory failure. Some patients also present with peripheral nervous system involvement, including polyneuropathy or myopathy, or non-neurologic abnormalities, e.g., diabetes, short stature, hypertrichosis, cardiomyopathy, anemia, renal failure, vomiting, or diarrhea (Leigh-like syndrome). In the majority of cases, onset is in early childhood, but in a small number of cases, adults are affected. In the majority of cases, dysfunction of the respiratory chain (particularly complexes I, II, IV, or V), of coenzyme Q, or of the pyruvate dehydrogenase complex are responsible for the disease. Associated mutations affect genes of the mitochondrial or nuclear genome. Leigh syndrome and Leigh-like syndrome are the mitochondrial disorders with the largest genetic heterogeneity.

Molecular genetic and clinical aspects of mitochondrial disorders in childhood

Mitochondrial OXPHOS disorders are caused by mutations in mitochondrial or nuclear genes, which directly or indirectly affect mitochondrial oxidative phosphorylation (OXPHOS). Primary mtDNA abnormalities in children are due to rearrangements (deletions or duplications) and point mutations or insertions. Mutations in the nuclear-encoded polypeptide subunits of OXPHOS result in complex I and II deficiency, whereas mutations in the nuclear proteins involved in the assembly of OXPHOS subunits cause defects in complexes I, III, IV, and V. Here, we review recent progress in the identification of mitochondrial and nuclear gene defects and the associated clinical manifestations of these disorders in childhood.

Approche diagnostique des maladies mitochondriales à présentation neurologique

Mitochondrial respiratory chain abnormalities are a cause of neuromuscular diseases. They present with very diverse clinical presentations,involving either the central nervous system, the peripheral nervous system, or skeletal muscle, and may be due to mutations either in mitochondrial or nuclear genome. The aim of this review is to familiarise the clinician with these diseases, to evoke main syndromes, and to give guidelines for their diagnostic investigation.

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.

Biochemical and molecular diagnosis of mitochondrial respiratory chain disorders

Biochemical diagnosis of mitochondrial respiratory chain disorders requires caution to avoid misdiagnosis of secondary enzyme defects, and can be improved by the use of conservative diagnostic criteria. Pathogenic mutations causing mitochondrial disorders have now been identified in more than 30 mitochondrial DNA (mtDNA) genes encoding respiratory chain subunits, ribosomal- and t-RNAs. mtDNA mutations appear to be responsible for most adult patients with mitochondrial disease and approximately a quarter of paediatric patients. A family history suggesting maternal inheritance is the exception rather than the norm for children with mtDNA mutations, many of whom have de novo mutations. Prenatal diagnosis and pre-implantation genetic diagnosis can be offered to some women at risk of transmitting a mtDNA mutation, particularly those at lower recurrence risk. Mutations in more than 30 nuclear genes, including those encoding for respiratory chain subunits and assembly factors, have now been shown to cause mitochondrial disorders, creating difficulties in prioritising which genes should be studied by mutation analysis in individual patients. A number of approaches offer promise to guide the choice of candidate genes, including Blue Native-PAGE immunoblotting and microarray expression analysis.

White matter involvement in mitochondrial diseases

White matter involvement is recently being realized as a common finding in mitochondrial disorders. It is considered an inherent part of the classical mitochondrial syndromes which are usually associated with alterations in the mitochondrial DNA such as: Leigh disease, Kearns-Sayre syndrome, mitochondrial encephalomyopathy lactic acidosis, and stroke like episodes, mitochondrial neuro-gastro-intestinal encephalomyopathy and Leber's hereditary optic neuropathy. White matter involvement is also described in mitochondrial disorders due to mutations in the nuclear DNA which are transmitted in an autosomal pattern. MRI findings suggestive of a mitochondrial disease are: small cyst-like lesions in abnormal white matter, involvement of both cerebral and cerebellar white matter, and a combination of a leukoencephalopathy with bilateral basal ganglia lesions. The clinical manifestations may be disproportionate to the extent of white matter involvement. Other organs may frequently be involved. The onset is often in infancy with a neurodegenerative course. The finding of a leukoencephalopathy in a patient with a complex neurologic picture and multisystem involvement should prompt a thorough mitochondrial evaluation.

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.

Mitochondrial disorders

We present here a discussion on the most relevant recent publications on mitochondrial disease. In addition to many papers concerning the description of the genotype-to-phenotype correlations in mitochondrial DNA-related disorders, this very broad area of neurogenetics includes a number of novel observations on the basic aspects of mitochondrial biogenesis that can be relevant in explaining the molecular mechanisms of mitochondrial abnormalities. The completion of the human genome project and the wealth of knowledge gained on the genetics of oxidative phosphorylation in yeast have promoted a substantial acceleration in the discovery of a remarkable number of nuclear genes associated with specific mitochondrial disorders. A further development of these contributions has been the generation of several cellular and animal models of disease that can now be exploited for testing both pathogenetic hypotheses and therapeutic strategies. Most of the latter are based on the use of chemical compounds aimed at reducing the negative impact of mitochondrial defects on both energy production and generation of reactive oxygen species. The first successful attempts for gene therapy of some mitochondrial diseases have recently been achieved and will hopefully increase in the near future.

Bilateral striatal necrosis associated with a novel mutation in the mitochondrial ND6 gene

We report the molecular findings in two independent patients presenting with progressive generalized dystonia and bilateral striatal necrosis in whom we have identified a mutation (T14487C) in the mitochondrial ND6 gene. The mutation is heteroplasmic in all samples analyzed, and it fulfills all accepted criteria of pathogenicity. Transmitochondrial cell lines harboring 100% mutant mitochondrial DNA showed a marked decrease in the activity of complex I of the respiratory chain supporting the pathogenic role of T14487C.