Sensory neuronopathy in patients harbouring recessive polymerase γ mutations

A novel mouse model of mitochondrial disease exhibits juvenile-onset severe neurological impairment due to parvalbumin cell mitochondrial dysfunction

Mitochondrial diseases comprise a common group of neurometabolic disorders resulting from OXPHOS defects, that may manifest with neurological impairments, for which there are currently no disease-modifying therapies. Previous studies suggest inhibitory interneuron susceptibility to mitochondrial impairment, especially of parvalbumin-expressing interneurons (PV+). We have developed a mouse model of mitochondrial dysfunction specifically in PV+ cells via conditional Tfam knockout, that exhibited a juvenile-onset progressive phenotype characterised by cognitive deficits, anxiety-like behaviour, head-nodding, stargazing, ataxia, and reduced lifespan. A brain region-dependent decrease of OXPHOS complexes I and IV in PV+ neurons was detected, with Purkinje neurons being most affected. We validated these findings in a neuropathological study of patients with pathogenic mtDNA and POLG variants showing PV+ interneuron loss and deficiencies in complexes I and IV. This mouse model offers a drug screening platform to propel the discovery of therapeutics to treat severe neurological impairment due to mitochondrial dysfunction.

Astrocytic pathology in Alpers' syndrome

Refractory epilepsy is the main neurological manifestation of Alpers' syndrome, a severe childhood-onset mitochondrial disease caused by bi-allelic pathogenic variants in the mitochondrial DNA (mtDNA) polymerase gamma gene (POLG). The pathophysiological mechanisms underpinning neuronal hyperexcitabilty leading to seizures in Alpers' syndrome remain unknown. However, pathological changes to reactive astrocytes are hypothesised to exacerbate neural dysfunction and seizure-associated cortical activity in POLG-related disease. Therefore, we sought to phenotypically characterise astrocytic pathology in Alpers' syndrome. We performed a detailed quantitative investigation of reactive astrocytes in post-mortem neocortical tissues from thirteen patients with Alpers' syndrome, eight neurologically normal controls and five sudden unexpected death in epilepsy (SUDEP) patients, to control for generalised epilepsy-associated astrocytic pathology. Immunohistochemistry to identify glial fibrillary acidic protein (GFAP)-reactive astrocytes revealed striking reactive astrogliosis localised to the primary visual cortex of Alpers' syndrome tissues, characterised by abnormal-appearing hypertrophic astrocytes. Phenotypic characterisation of individual GFAP-reactive astrocytes demonstrated decreased abundance of mitochondrial oxidative phosphorylation (OXPHOS) proteins and altered expression of key astrocytic proteins including Kir4.1 (subunit of the inwardly rectifying K⁺ ion channel), AQP4 (astrocytic water channel) and glutamine synthetase (enzyme that metabolises glutamate). These phenotypic astrocytic changes were typically different from the pathology observed in SUDEP tissues, suggesting alternative mechanisms of astrocytic dysfunction between these epilepsies. Crucially, our findings provide further evidence of occipital lobe involvement in Alpers' syndrome and support the involvement of reactive astrocytes in the pathogenesis of POLG-related disease.

Mitochondrial encephalomyopathy

Mitochondrial dysfunction, especially perturbation of oxidative phosphorylation and adenosine triphosphate (ATP) generation, disrupts cellular homeostasis and is a surprisingly frequent cause of central and peripheral nervous system pathology. Mitochondrial disease is an umbrella term that encompasses a host of clinical syndromes and features caused by in excess of 300 different genetic defects affecting the mitochondrial and nuclear genomes. Patients with mitochondrial disease can present at any age, ranging from neonatal onset to late adult life, with variable organ involvement and neurological manifestations including neurodevelopmental delay, seizures, stroke-like episodes, movement disorders, optic neuropathy, myopathy, and neuropathy. Until relatively recently, analysis of skeletal muscle biopsy was the focus of diagnostic algorithms, but step-changes in the scope and availability of next-generation sequencing technology and multiomics analysis have revolutionized mitochondrial disease diagnosis. Currently, there is no specific therapy for most types of mitochondrial disease, although clinical trials research in the field is gathering momentum. In that context, active management of epilepsy, stroke-like episodes, dystonia, brainstem dysfunction, and Parkinsonism are all the more important in improving patient quality of life and reducing mortality.

Delineating selective vulnerability of inhibitory interneurons in Alpers’ syndrome

Aims

Methods

Results

Conclusions

Forecasting stroke-like episodes and outcomes in mitochondrial disease

In this retrospective, multicentre, observational cohort study, we sought to determine the clinical, radiological, EEG, genetics and neuropathological characteristics of mitochondrial stroke-like episodes and to identify associated risk predictors. Between January 1998 and June 2018, we identified 111 patients with genetically determined mitochondrial disease who developed stroke-like episodes. Post-mortem cases of mitochondrial disease (n = 26) were identified from Newcastle Brain Tissue Resource. The primary outcome was to interrogate the clinico-radiopathological correlates and prognostic indicators of stroke-like episode in patients with mitochondrial encephalomyopathy, lactic acidosis and stroke-like episodes syndrome (MELAS). The secondary objective was to develop a multivariable prediction model to forecast stroke-like episode risk. The most common genetic cause of stroke-like episodes was the m.3243A>G variant in MT-TL1 (n = 66), followed by recessive pathogenic POLG variants (n = 22), and 11 other rarer pathogenic mitochondrial DNA variants (n = 23). The age of first stroke-like episode was available for 105 patients [mean (SD) age: 31.8 (16.1)]; a total of 35 patients (32%) presented with their first stroke-like episode ≥40 years of age. The median interval (interquartile range) between first and second stroke-like episodes was 1.33 (2.86) years; 43% of patients developed recurrent stroke-like episodes within 12 months. Clinico-radiological, electrophysiological and neuropathological findings of stroke-like episodes were consistent with the hallmarks of medically refractory epilepsy. Patients with POLG-related stroke-like episodes demonstrated more fulminant disease trajectories than cases of m.3243A>G and other mitochondrial DNA pathogenic variants, in terms of the frequency of refractory status epilepticus, rapidity of progression and overall mortality. In multivariate analysis, baseline factors of body mass index, age-adjusted blood m.3243A>G heteroplasmy, sensorineural hearing loss and serum lactate were significantly associated with risk of stroke-like episodes in patients with the m.3243A>G variant. These factors informed the development of a prediction model to assess the risk of developing stroke-like episodes that demonstrated good overall discrimination (area under the curve = 0.87, 95% CI 0.82-0.93; c-statistic = 0.89). Significant radiological and pathological features of neurodegeneration were more evident in patients harbouring pathogenic mtDNA variants compared with POLG: brain atrophy on cranial MRI (90% versus 44%, P < 0.001) and reduced mean brain weight (SD) [1044 g (148) versus 1304 g (142), P = 0.005]. Our findings highlight the often idiosyncratic clinical, radiological and EEG characteristics of mitochondrial stroke-like episodes. Early recognition of seizures and aggressive instigation of treatment may help circumvent or slow neuronal loss and abate increasing disease burden. The risk-prediction model for the m.3243A>G variant can help inform more tailored genetic counselling and prognostication in routine clinical practice.

Recessive cerebellar and afferent ataxias - clinical challenges and future directions

Cerebellar and afferent ataxias present with a characteristic gait disorder that reflects cerebellar motor dysfunction and sensory loss. These disorders are a diagnostic challenge for clinicians because of the large number of acquired and inherited diseases that cause cerebellar and sensory neuron damage. Among such conditions that are recessively inherited, Friedreich ataxia and RFC1-associated cerebellar ataxia, neuropathy, vestibular areflexia syndrome (CANVAS) include the characteristic clinical, neuropathological and imaging features of ganglionopathies, a distinctive non-length-dependent type of sensory involvement. In this Review, we discuss the typical and atypical phenotypes of Friedreich ataxia and CANVAS, along with the features of other recessive ataxias that present with a ganglionopathy or polyneuropathy, with an emphasis on recently described clinical features, natural history and genotype-phenotype correlations. We review the main developments in understanding the complex pathology that affects the sensory neurons and cerebellum, which seem to be most vulnerable to disorders that affect mitochondrial function and DNA repair mechanisms. Finally, we discuss disease-modifying therapeutic advances in Friedreich ataxia, highlighting the most promising candidate molecules and lessons learned from previous clinical trials.

Bi-allelic loss of function variants in COX20 gene cause autosomal recessive sensory neuronopathy

Sensory neuronopathies are a rare and distinct subgroup of peripheral neuropathies, characterized by degeneration of the dorsal root ganglia neurons. About 50% of sensory neuronopathies are idiopathic and genetic causes remain to be clarified. Through a combination of homozygosity mapping and whole exome sequencing, we linked an autosomal recessive sensory neuronopathy to pathogenic variants in COX20 gene. We identified 8 unrelated families from the eastern China population carrying a founder variant c.41A>G (p. Lys14Arg) within COX20 in either a homozygous or compound heterozygous state. All patients displayed sensory ataxia with non-length-dependent sensory potentials decrease. COX20 encodes a key transmembrane protein implicated in the assembly of mitochondrial complex IV. We showed that COX20 variants lead to reduction of COX20 protein in patient's fibroblasts and transfected cell lines, consistent with a loss-of-function mechanism. Knockdown of COX20 expression in ND7/23 sensory neuron cells resulted in complex IV deficiency and perturbed assembly of complex IV, which subsequently compromised cell spare respiratory capacity and reduced cell proliferation under metabolic stress. Consistent with mitochondrial dysfunction in knockdown cells, reduced complex IV assembly, enzyme activity and oxygen consumption rate were also found in patients' fibroblasts. We speculated that the mechanism of COX20 was similar to other causative genes (e.g. SURF1, COX6A1, COA3 and SCO2) for peripheral neuropathies, all of which were functionally important in the structure and assembly of complex IV. Our study identifies a novel causative gene for the autosomal recessive sensory neuronopathy, whose vital function in complex IV and high expression in the proprioceptive sensory neuron further underlines loss of COX20 contributing to mitochondrial bioenergetic dysfunction as a mechanism in peripheral sensory neuron disease.

Sensory neuronopathies: new genes, new antibodies and new concepts

Degeneration of dorsal root ganglia (DRG) and its central and peripheral projections provokes sensory neuronopathy (SN), a rare disorder with multiple genetic and acquired causes. Clinically, patients with SN usually present with proprioceptive ataxia, patchy and asymmetric sensory abnormalities, widespread areflexia and no weakness. Classic causes of SN include cancer, Sjögren's syndrome, vitamin deficiency, chemotherapy, mitochondrial disorders and Friedreich ataxia. More recently, new genetic and dysimmune disorders associated with SN have been described, including RFC1 gene-linked cerebellar ataxia, neuropathy and vestibular areflexia syndrome (CANVAS) and anti-FGFR3 antibodies. In this review, we detail the pathophysiology of DRG degeneration, and the genetic and acquired causes of SN, with a special focus on the recently described CANVAS and anti-FGFR3 antibodies. We also propose a user-friendly and easily implemented SN diagnostic strategy.

Compound Heterozygosity for a Novel Frameshift Variant Causing Fatal Infantile Liver Failure and Genotype-Phenotype Correlation of POLG c.3286C>T Variant

A variant in the POLG gene is the leading cause of a heterogeneous group of mitochondrial disorders. No definitive treatment is currently available. Prenatal and newborn screening have the potential to improve clinical outcome of patients affected with POLG-related disorders. We reported a 4-month-old infant who presented with developmental delay, fever, and diarrhea. Within two weeks after hospital admission, the patient developed hepatic failure and died. Liver necropsy demonstrated an extensive loss of hepatocytes and bile duct proliferations. Trio-whole exome sequencing identified that the patient was compound heterozygous for a novel frameshift variant c.3102delG (p.Lys1035Serfs*59) and a common variant c.3286C>T (p.Arg1096Cys) in POLG (NM_002693.3) inherited from the mother and father, respectively. The c.3102delG (p.Lys1035Serfs*59) was a null variant and classified as pathogenic according to the American College of Medical Genetics and Genomics Standards and Guidelines. Prenatal genetic screenings using rapid whole exome sequencing successfully detected the heterozygous c.3286C>T variant in the following pregnancy and the normal alleles in the other one. Both children had been healthy. We reviewed all 34 cases identified with the POLG c.3286C>T variant and found that all 15 compound heterozygous cases had two missense variants except our patient who had the truncating variant and showed the earliest disease onset, rapid deterioration, and the youngest death. All homozygous cases had disease onset before age 2 and developed seizure. Here, we report a novel POLG variant expanding the genotypic spectrum, demonstrate the successful use of exome sequencing for prenatal and neonatal screenings of POLG-related disorders, and show the genotype-phenotype correlation of the common c.3286C>T variant.

Mitochondrial neuropathy and neurogenic features in mitochondrial myopathy

Mitochondrial diseases (MIDs) involve multiple organs including peripheral nerves and skeletal muscle. Mitochondrial neuropathy (MN) and mitochondrial myopathy (MM) are commonly associated and linked at the neuromuscular junction (NMJ). Herein we review MN in connection with neurogenic features of MM, and pathological evidence for the involvement of the peripheral nerve and NMJ in MID patients traditionally assumed to have predominantly MM. MN is not uncommon, but still likely under-reported, and muscle biopsies of MM commonly exhibit neurogenic features. Pathological examination remains the gold standard to assess the nerve and muscle changes in patients with MIDs. Ultrastructural studies by electron microscopy are often necessary to fully characterize the pathology of mitochondrial cytopathy in MN and MM.

Ultrasensitive deletion detection links mitochondrial DNA replication, disease, and aging

BACKGROUND

Acquired human mitochondrial genome (mtDNA) deletions are symptoms and drivers of focal mitochondrial respiratory deficiency, a pathological hallmark of aging and late-onset mitochondrial disease.

RESULTS

To decipher connections between these processes, we create LostArc, an ultrasensitive method for quantifying deletions in circular mtDNA molecules. LostArc reveals 35 million deletions (~ 470,000 unique spans) in skeletal muscle from 22 individuals with and 19 individuals without pathogenic variants in POLG. This nuclear gene encodes the catalytic subunit of replicative mitochondrial DNA polymerase γ. Ablation, the deleted mtDNA fraction, suffices to explain skeletal muscle phenotypes of aging and POLG-derived disease. Unsupervised bioinformatic analyses reveal distinct age- and disease-correlated deletion patterns.

CONCLUSIONS

These patterns implicate replication by DNA polymerase γ as the deletion driver and suggest little purifying selection against mtDNA deletions by mitophagy in postmitotic muscle fibers. Observed deletion patterns are best modeled as mtDNA deletions initiated by replication fork stalling during strand displacement mtDNA synthesis.

Enhanced axonal response of mitochondria to demyelination offers neuroprotection: implications for multiple sclerosis

Axonal loss is the key pathological substrate of neurological disability in demyelinating disorders, including multiple sclerosis (MS). However, the consequences of demyelination on neuronal and axonal biology are poorly understood. The abundance of mitochondria in demyelinated axons in MS raises the possibility that increased mitochondrial content serves as a compensatory response to demyelination. Here, we show that upon demyelination mitochondria move from the neuronal cell body to the demyelinated axon, increasing axonal mitochondrial content, which we term the axonal response of mitochondria to demyelination (ARMD). However, following demyelination axons degenerate before the homeostatic ARMD reaches its peak. Enhancement of ARMD, by targeting mitochondrial biogenesis and mitochondrial transport from the cell body to axon, protects acutely demyelinated axons from degeneration. To determine the relevance of ARMD to disease state, we examined MS autopsy tissue and found a positive correlation between mitochondrial content in demyelinated dorsal column axons and cytochrome c oxidase (complex IV) deficiency in dorsal root ganglia (DRG) neuronal cell bodies. We experimentally demyelinated DRG neuron-specific complex IV deficient mice, as established disease models do not recapitulate complex IV deficiency in neurons, and found that these mice are able to demonstrate ARMD, despite the mitochondrial perturbation. Enhancement of mitochondrial dynamics in complex IV deficient neurons protects the axon upon demyelination. Consequently, increased mobilisation of mitochondria from the neuronal cell body to the axon is a novel neuroprotective strategy for the vulnerable, acutely demyelinated axon. We propose that promoting ARMD is likely to be a crucial preceding step for implementing potential regenerative strategies for demyelinating disorders.

Sensory neuronopathy as a major clinical feature of mitochondrial trifunctional protein deficiency in adults

INTRODUCTION

Mitochondrial trifunctional protein deficiency (MTPD) is a long-chain fatty acid oxidation disorder characterized by co-existence of rhabdomyolysis episodes and peripheral neuropathy. Two phenotypes are described: generalized mitochondrial trifunctional protein deficiency (gMTPD) and isolated long-chain-3-hydroxyacyl-CoA dehydrogenase deficiency (iLCHADD) that is always associated with the c.1528G>C mutation. Peripheral neuropathy of MTPD is commonly described in children as axonal, length-dependent and sensorimotor.

OBJECTIVES

To report clinical and electrophysiological features of four independent adult MTPD patients with peripheral neuropathy.

RESULTS

Onset of the disease was characterized in all patients by rhabdomyolysis episodes occurring during childhood preceded by severe hypoglycemic episodes in three patients. Peripheral nerve involvement manifesting as sensory ataxia appeared later, during adolescence or adulthood. In all cases, electroneuromyogram showed no length-dependent sensory potentials decrease characteristic of sensory neuronopathy ("ganglionopathy"). All patients harbored at least one c.1528G>C mutation.

DISCUSSION

We describe MTPD as a newly hereditary etiology of sensory neuronopathy in adults, specifically in patients with c.1528G>C mutation. MTPD should be screened for by performing plasma acylcarnitines in patients with chronic sensory neuronopathy and additional suggestive features such as exercise intolerance or retinopathy.

Neuromuscular Junction Abnormalities in Mitochondrial Disease: An Observational Cohort Study

Objective

To determine the prevalence of neuromuscular junction (NMJ) abnormalities in patients with mitochondrial disease.

Methods

Eighty patients with genetically proven mitochondrial disease were recruited from a national center for mitochondrial disease in the United Kingdom. Participants underwent detailed clinical and neurophysiologic testing including single-fiber electromyography.

Results

The overall prevalence of neuromuscular transmission defects was 25.6%. The highest prevalence was in patients with pathogenic dominant RRM2B variants (50%), but abnormalities were found in a wide range of mitochondrial genotypes. The presence of NMJ abnormalities was strongly associated with coexistent myopathy, but not with neuropathy. Furthermore, 15% of patients with NMJ abnormality had no evidence of either myopathy or neuropathy.

Conclusions

NMJ transmission defects are common in mitochondrial disease. In some patients, NMJ dysfunction occurs in the absence of obvious pre- or post-synaptic pathology, suggesting that the NMJ may be specifically affected.

The role of astrocytes in seizure generation: insights from a novel in vitro seizure model based on mitochondrial dysfunction

Approximately one-quarter of patients with mitochondrial disease experience epilepsy. Their epilepsy is often severe and resistant towards conventional antiepileptic drugs. Despite the severity of this epilepsy, there are currently no animal models available to provide a mechanistic understanding of mitochondrial epilepsy. We conducted neuropathological studies on patients with mitochondrial epilepsy and found the involvement of the astrocytic compartment. As a proof of concept, we developed a novel brain slice model of mitochondrial epilepsy by the application of an astrocytic-specific aconitase inhibitor, fluorocitrate, concomitant with mitochondrial respiratory inhibitors, rotenone and potassium cyanide. The model was robust and exhibited both face and predictive validity. We then used the model to assess the role that astrocytes play in seizure generation and demonstrated the involvement of the GABA-glutamate-glutamine cycle. Notably, glutamine appears to be an important intermediary molecule between the neuronal and astrocytic compartment in the regulation of GABAergic inhibitory tone. Finally, we found that a deficiency in glutamine synthetase is an important pathogenic process for seizure generation in both the brain slice model and the human neuropathological study. Our study describes the first model for mitochondrial epilepsy and provides a mechanistic insight into how astrocytes drive seizure generation in mitochondrial epilepsy.

OXA1L mutations cause mitochondrial encephalopathy and a combined oxidative phosphorylation defect

OXA1, the mitochondrial member of the YidC/Alb3/Oxa1 membrane protein insertase family, is required for the assembly of oxidative phosphorylation complexes IV and V in yeast. However, depletion of human OXA1 (OXA1L) was previously reported to impair assembly of complexes I and V only. We report a patient presenting with severe encephalopathy, hypotonia and developmental delay who died at 5 years showing complex IV deficiency in skeletal muscle. Whole exome sequencing identified biallelic OXA1L variants (c.500_507dup, p.(Ser170Glnfs*18) and c.620G>T, p.(Cys207Phe)) that segregated with disease. Patient muscle and fibroblasts showed decreased OXA1L and subunits of complexes IV and V. Crucially, expression of wild‐type human OXA1L in patient fibroblasts rescued the complex IV and V defects. Targeted depletion of OXA1L in human cells or Drosophila melanogaster caused defects in the assembly of complexes I, IV and V, consistent with patient data. Immunoprecipitation of OXA1L revealed the enrichment of mtDNA‐encoded subunits of complexes I, IV and V. Our data verify the pathogenicity of these OXA1L variants and demonstrate that OXA1L is required for the assembly of multiple respiratory chain complexes.

Copy-choice recombination during mitochondrial L-strand synthesis causes DNA deletions

Mitochondrial DNA (mtDNA) deletions are associated with mitochondrial disease, and also accumulate during normal human ageing. The mechanisms underlying mtDNA deletions remain unknown although several models have been proposed. Here we use deep sequencing to characterize abundant mtDNA deletions in patients with mutations in mitochondrial DNA replication factors, and show that these have distinct directionality and repeat characteristics. Furthermore, we recreate the deletion formation process in vitro using only purified mitochondrial proteins and defined DNA templates. Based on our in vivo and in vitro findings, we conclude that mtDNA deletion formation involves copy-choice recombination during replication of the mtDNA light strand.

A Targeted Mutation Disrupting Mitochondrial Complex IV Function in Primary Afferent Neurons Leads to Pain Hypersensitivity Through P2Y1 Receptor Activation

As mitochondrial dysfunction is evident in neurodegenerative disorders that are accompanied by pain, we generated inducible mutant mice with disruption of mitochondrial respiratory chain complex IV, by COX10 deletion limited to sensory afferent neurons through the use of an Advillin Cre-reporter. COX10 deletion results in a selective energy-deficiency phenotype with minimal production of reactive oxygen species. Mutant mice showed reduced activity of mitochondrial respiratory chain complex IV in many sensory neurons, increased ADP/ATP ratios in dorsal root ganglia and dorsal spinal cord synaptoneurosomes, as well as impaired mitochondrial membrane potential, in these synaptoneurosome preparations. These changes were accompanied by marked pain hypersensitivity in mechanical and thermal (hot and cold) tests without altered motor function. To address the underlying basis, we measured Ca²⁺ fluorescence responses of dorsal spinal cord synaptoneurosomes to activation of the GluK1 (kainate) receptor, which we showed to be widely expressed in small but not large nociceptive afferents, and is minimally expressed elsewhere in the spinal cord. Synaptoneurosomes from mutant mice showed greatly increased responses to GluK1 agonist. To explore whether altered nucleotide levels may play a part in this hypersensitivity, we pharmacologically interrogated potential roles of AMP-kinase and ADP-sensitive purinergic receptors. The ADP-sensitive P2Y₁ receptor was clearly implicated. Its expression in small nociceptive afferents was increased in mutants, whose in vivo pain hypersensitivity, in mechanical, thermal and cold tests, was reversed by a selective P2Y₁ antagonist. Energy depletion and ADP elevation in sensory afferents, due to mitochondrial respiratory chain complex IV deficiency, appear sufficient to induce pain hypersensitivity, by ADP activation of P2Y₁ receptors.

A complex genomic locus drives mtDNA replicase POLG expression to its disease-related nervous system regions

DNA polymerase gamma (POLG), the mtDNA replicase, is a common cause of mitochondrial neurodegeneration. Why POLG defects especially cause central nervous system (CNS) diseases is unknown. We discovered a complex genomic regulatory locus for POLG, containing three functional CNS‐specific enhancers that drive expression specifically in oculomotor complex and sensory interneurons of the spinal cord, completely overlapping with the regions showing neuronal death in POLG patients. The regulatory locus also expresses two functional RNAs, LINC00925‐RNA and MIR9‐3, which are coexpressed with POLG. The MIR9‐3 targets include NR2E1, a transcription factor maintaining neural stem cells in undifferentiated state, and MTHFD2, the regulatory enzyme of mitochondrial folate cycle, linking POLG expression to stem cell differentiation and folate metabolism. Our evidence suggests that distant genomic non‐coding regions contribute to regulation of genes encoding mitochondrial proteins. Such genomic arrangement of POLG locus, driving expression to CNS regions affected in POLG patients, presents a potential mechanism for CNS‐specific manifestations in POLG disease.

Neurotoxicity of cytarabine (Ara-C) in dorsal root ganglion neurons originates from impediment of mtDNA synthesis and compromise of mitochondrial function

Peripheral Nervous System (PNS) neurotoxicity caused by cancer drugs hinders attainment of chemotherapy goals. Due to leakiness of the blood nerve barrier, circulating chemotherapeutic drugs reach PNS neurons and adversely affect their function. Chemotherapeutic drugs are designed to target dividing cancer cells and mechanisms underlying their toxicity in postmitotic neurons remain to be fully clarified. The objective of this work was to elucidate progression of events triggered by antimitotic drugs in postmitotic neurons. For proof of mechanism study, we chose cytarabine (ara-C), an antimetabolite used in treatment of hematological cancers. Ara-C is a cytosine analog that terminates DNA synthesis. To investigate how ara-C affects postmitotic neurons, which replicate mitochondrial but not genomic DNA, we adapted a model of Dorsal Root Ganglion (DRG) neurons. We showed that DNA polymerase γ, which is responsible for mtDNA synthesis, is inhibited by ara-C and that sublethal ara-C exposure of DRG neurons leads to reduction in mtDNA content, ROS generation, oxidative mtDNA damage formation, compromised mitochondrial respiration and diminution of NADPH and GSH stores, as well as, activation of the DNA damage response. Hence, it is plausible that in ara-C exposed DRG neurons, ROS amplified by the high mitochondrial content shifts from physiologic to pathologic levels signaling stress to the nucleus. Combined, the findings suggest that ara-C neurotoxicity in DRG neurons originates in mitochondria and that continuous mtDNA synthesis and reliance on oxidative phosphorylation for energy needs sensitize the highly metabolic neurons to injury by mtDNA synthesis terminating cancer drugs.

Sensory Neuronopathies

PURPOSE OF REVIEW

The sensory neuronopathies are sensory-predominant polyneuropathies that result from damage to the dorsal root and trigeminal sensory ganglia. This review explores the various causes of acquired sensory neuronopathies, the approach to diagnosis, and treatment.

RECENT FINDINGS

Diagnostic criteria have recently been published and validated to allow differentiation of sensory neuronopathies from other polyneuropathies. On the basis of serial electrodiagnostic studies, the treatment window for the acquired sensory neuronopathies has been identified as approximately 8 months. If treatment is initiated within 2 months of symptom onset, there is a better opportunity for improvement of the patient's condition. Even though sensory neuronopathies are rare, significant progress has been made regarding characterization of their clinical, electrophysiologic, and imaging features. This does not hold true, however, for treatment. There have been no randomized controlled clinical trials to guide management of these diseases, and a standard treatment approach remains undetermined.

Parkin mutation may be associated with serious akinesia in a patient with Parkinson's disease

Acute akinesia (AA) is an unusual motor complication in Parkinson's disease (PD). Reported risk factors for AA include infection, trauma, surgical intervention, and the withdrawal of antiparkinsonian medication. Recently, patients with genetic PD were reported to have a three-fold risk of AA than patients with non-genetic PD. We describe a patient with PD associated with a Parkin mutation in whom serious akinesia developed. A 42-year-old man with exon 2 heterozygous deletion and exon 4 heterozygous deletion in the PARK2 gene showed five unexpected AA for several 12h. At fifth AA, he could not move any part of the body while lying in front of a stove in his house all night. He was admitted to our hospital because a third-degree burn had developed on 16% of the body surface area. Parkin mutation in addition to POLG1 or PINK1 mutation may be associated with serious AA.

Inherited mitochondrial genomic instability and chemical exposures

There are approximately 1500 proteins that are needed for mitochondrial structure and function, most of which are encoded in the nuclear genome (Calvo et al., 2006). Each mitochondrion has its own genome (mtDNA), which in humans encodes 13 polypeptides, 22 tRNAs and 2 rRNAs required for oxidative phosphorylation. The mitochondrial genome of humans and most vertebrates is approximately 16.5kbp, double-stranded, circular, with few non-coding bases. Thus, maintaining mtDNA stability, that is, the ability of the cell to maintain adequate levels of mtDNA template for oxidative phosphorylation is essential and can be impacted by the level of mtDNA mutation currently within the cell or mitochondrion, but also from errors made during normal mtDNA replication, defects in mitochondrial quality control mechanisms, and exacerbated by exposures to exogenous and/or endogenous genotoxic agents. In this review, we expand on the origins and consequences of mtDNA instability, the current state of research regarding the mechanisms by which mtDNA instability can be overcome by cellular and chemical interventions, and the future of research and treatments for mtDNA instability.

Clinicopathologic and molecular spectrum of RNASEH1-related mitochondrial disease

OBJECTIVE

Pathologic ribonuclease H1 (RNase H1) causes aberrant mitochondrial DNA (mtDNA) segregation and is associated with multiple mtDNA deletions. We aimed to determine the prevalence of RNase H1 gene (RNASEH1) mutations among patients with mitochondrial disease and establish clinically meaningful genotype-phenotype correlations.

METHODS

RNASEH1 was analyzed in patients with (1) multiple deletions/depletion of muscle mtDNA and (2) mendelian progressive external ophthalmoplegia (PEO) with neuropathologic evidence of mitochondrial dysfunction, but no detectable multiple deletions/depletion of muscle mtDNA. Clinicopathologic and molecular evaluation of the newly identified and previously reported patients harboring RNASEH1 mutations was subsequently undertaken.

RESULTS

Pathogenic c.424G>A p.Val142Ile RNASEH1 mutations were detected in 3 pedigrees among the 74 probands screened. Given that all 3 families had Indian ancestry, RNASEH1 genetic analysis was undertaken in 50 additional Indian probands with variable clinical presentations associated with multiple mtDNA deletions, but no further RNASEH1 mutations were confirmed. RNASEH1-related mitochondrial disease was characterized by PEO (100%), cerebellar ataxia (57%), and dysphagia (50%). The ataxia neuropathy spectrum phenotype was observed in 1 patient. Although the c.424G>A p.Val142Ile mutation underpins all reported RNASEH1-related mitochondrial disease, haplotype analysis suggested an independent origin, rather than a founder event, for the variant in our families.

CONCLUSIONS

In our cohort, RNASEH1 mutations represent the fourth most common cause of adult mendelian PEO associated with multiple mtDNA deletions, following mutations in POLG, RRM2B, and TWNK. RNASEH1 genetic analysis should also be considered in all patients with POLG-negative ataxia neuropathy spectrum. The pathophysiologic mechanisms by which the c.424G>A p.Val142Ile mutation impairs human RNase H1 warrant further investigation.

Peripheral neuropathy in patients with CPEO associated with single and multiple mtDNA deletions

Objective:

To characterize peripheral nerve involvement in patients with chronic progressive external ophthalmoplegia (CPEO) with single and multiple mitochondrial DNA (mtDNA) deletions, based on clinical scores and detailed nerve conduction studies.

Methods:

Peripheral nerve involvement was prospectively investigated in 33 participants with CPEO (single deletions n = 18 and multiple deletions n = 15). Clinically, a modified Total Neuropathy Score (mTNS) and a modified International Cooperative Ataxia Rating Scale (mICARS) were used. Nerve conduction studies included Nn. suralis, superficialis radialis, tibialis, and peroneus mot. Early somatosensory evoked potentials were obtained by N. tibialis stimulation.

Results:

Participants with multiple deletions had higher mTNS and mICARS scores than those with single deletions. Electrophysiologically in both sensory nerves (N. suralis and N. radialis superficialis), compound action potential (CAP) amplitudes and nerve conduction velocities were lower and mostly abnormal in multiple deletions than those in single deletions. Early somatosensory evoked potentials of N. tibialis revealed increased P40 latencies and decreased N35-P40 amplitudes in multiple deletions. Both sensory nerves had higher areas under the receiver operating characteristic curves for the decreased CAP amplitudes than the 2 motor nerves. The N. suralis had the best Youden index, indicating a sensitivity of 93.3% and a specificity of 72.2% to detect multiple deletions.

Conclusions:

Peripheral nerve involvement in participants with multiple mtDNA deletions is an axonal type of predominant sensory neuropathy. This is clinically consistent with higher mTNS and mICARS scores. Sensory nerve involvement in participants with multiple deletions was not correlated with age at onset and duration of disease.

Review: Central nervous system involvement in mitochondrial disease

Mitochondrial respiratory chain defects are an important cause of inherited disorders affecting approximately 1 in 5000 people in the UK population. Collectively these disorders are termed ‘mitochondrial diseases’ and they result from either mitochondrial DNA mutations or defects in nuclear DNA. Although they are frequently multisystem disorders, neurological deficits are particularly common, wide‐ranging and disabling for patients. This review details the manifold neurological impairments associated with mitochondrial disease, and describes the efforts to understand how they arise and progressively worsen in patients with mitochondrial disease. We describe advances in our understanding of disease pathogenesis through detailed neuropathological studies and how this has spurred the development of cellular and animal models of disease. We underscore the importance of continued clinical, molecular genetic, neuropathological and animal model studies to fully characterize mitochondrial diseases and understand mechanisms of neurodegeneration. These studies are instrumental for the next phase of mitochondrial research that has a particular emphasis on finding novel ways to treat mitochondrial disease to improve patient care and quality of life.

Mitochondrial disease: genetics and management

Mitochondrial disease is one of the most common groups of genetic diseases with a minimum prevalence of greater than 1 in 5000 in adults. Whilst multi-system involvement is often evident, neurological manifestation is the principal presentation in most cases. The multiple clinical phenotypes and the involvement of both the mitochondrial and nuclear genome make mitochondrial disease particularly challenging for the clinician. In this review article we cover mitochondrial genetics and common neurological presentations associated with adult mitochondrial disease. In addition, specific and supportive treatments are discussed.

Epilepsy in adults with mitochondrial disease: A cohort study

OBJECTIVE

The aim of this work was to determine the prevalence and progression of epilepsy in adult patients with mitochondrial disease.

METHODS

We prospectively recruited a cohort of 182 consecutive adult patients attending a specialized mitochondrial disease clinic in Newcastle upon Tyne between January 1, 2005 and January 1, 2008. We then followed this cohort over a 7-year period, recording primary outcome measures of occurrence of first seizure, status epilepticus, stroke-like episode, and death.

RESULTS

Overall prevalence of epilepsy in the cohort was 23.1%. Mean age of epilepsy onset was 29.4 years. Prevalence varied widely between genotypes, with several genotypes having no cases of epilepsy, a prevalence of 34.9% in the most common genotype (m.3243A>G mutation), and 92.3% in the m.8344A>G mutation. Among the cohort as a whole, focal seizures, with or without progression to bilateral convulsive seizures, was the most common seizure type. Conversely, all of the patients with the m.8344A>G mutation and epilepsy experienced myoclonic seizures. Patients with the m.3243A>G mutation remain at high risk of developing stroke-like episodes (1.16% per year). However, although the standardized mortality ratio for the entire cohort was high (2.86), this ratio did not differ significantly between patients with epilepsy (2.96) and those without (2.83).

INTERPRETATION

Epilepsy is a common manifestation of mitochondrial disease. It develops early in the disease and, in the case of the m.3243A>G mutation, often presents in the context of a stroke-like episode or status epilepticus. However, epilepsy does not itself appear to contribute to the increased mortality in mitochondrial disease.

Magnetic resonance imaging correlates of genetically characterized patients with mitochondrial disorders: A study from south India

BACKGROUND

Large studies analyzing magnetic resonance imaging correlates in different genotypes of mitochondrial disorders are far and few. This study sought to analyze the pattern of magnetic resonance imaging findings in a cohort of genetically characterized patients with mitochondrial disorders.

METHODS

The study cohort included 33 patients (age range 18 months-50 years, M:F - 0.9:1) with definite mitochondrial disorders seen over a period of 8 yrs. (2006-2013). Their MR imaging findings were analyzed retrospectively.

RESULTS

The patients were classified into three groups according to the genotype, Mitochondrial point mutations and deletions (n=21), SURF1 mutations (n=7) and POLG1 (n=5). The major findings included cerebellar atrophy (51.4%), cerebral atrophy (24.2%), signal changes in basal ganglia (45.7%), brainstem (34.2%) & white matter (18.1%) and stroke like lesions (25.7%). Spinal cord imaging showed signal changes in 4/6 patients. Analysis of the special sequences revealed, basal ganglia mineralization (7/22), lactate peak on magnetic resonance spectrometry (10/15), and diffusion restriction (6/22). Follow-up images in six patients showed that the findings are dynamic. Comparison of the magnetic resonance imaging findings in the three groups showed that cerebral atrophy and cerebellar atrophy, cortical signal changes and basal ganglia mineralization were seen mostly in patients with mitochondrial mutation. Brainstem signal changes with or without striatal lesions were characteristically noted in SURF1 group. There was no consistent imaging pattern in POLG1 group.

CONCLUSION

Magnetic resonance imaging findings in mitochondrial disorders are heterogeneous. Definite differences were noted in the frequency of anatomical involvement in the three groups. Familiarity with the imaging findings in different genotypes of mitochondrial disorders along with careful analysis of the family history, clinical presentation, biochemical findings, histochemical and structural analysis will help the physician for targeted metabolic and genetic testing.

Sudden adult death syndrome in m.3243A>G-related mitochondrial disease: an unrecognized clinical entity in young, asymptomatic adults

Aims

To provide insight into the mechanism of sudden adult death syndrome (SADS) and to give new clinical guidelines for the cardiac management of patients with the most common mitochondrial DNA mutation, m.3243A>G. These studies were initiated after two young, asymptomatic adults harbouring the m.3243A>G mutation died suddenly and unexpectedly. The m.3243A>G mutation is present in ∼1 in 400 of the population, although the recognized incidence of mitochondrial DNA (mtDNA) disease is ∼1 in 5000.

Methods and results

Pathological studies including histochemistry and molecular genetic analyses performed on various post-mortem samples including cardiac tissues (atrium and ventricles) showed marked respiratory chain deficiency and high levels of the m.3243A>G mutation. Systematic review of cause of death in our m.3243A>G patient cohort showed the person-time incidence rate of sudden adult death is 2.4 per 1000 person-years. A further six cases of sudden death among extended family members have been identified from interrogation of family pedigrees.

Conclusion

Our findings suggest that SADS is an important cause of death in patients with m.3243A>G and likely to be due to widespread respiratory chain deficiency in cardiac muscle. The involvement of asymptomatic relatives highlights the importance of family tracing in patients with m.3243A>G and the need for specific cardiac arrhythmia surveillance in the management of this common genetic disease. In addition, these findings have prompted the derivation of cardiac guidelines specific to patients with m.3243A>G-related mitochondrial disease. Finally, due to the prevalence of this mtDNA point mutation, we recommend inclusion of testing for m.3243A>G mutations in the genetic autopsy of all unexplained cases of SADS.

SANDO syndrome in a cohort of 107 patients with CPEO and mitochondrial DNA deletions

OBJECTIVE

The sensory ataxic neuropathy with dysarthria and ophthalmoparesis (SANDO) syndrome is a subgroup of mitochondrial chronic progressive external ophthalmoplegia (CPEO)-plus disorders associated with multiple mitochondrial DNA (mtDNA) deletions. There is no systematic survey on SANDO in patients with CPEO with either single or multiple large-scale mtDNA deletions.

METHODS

In this retrospective analysis, we characterised the frequency, the genetic and clinical phenotype of 107 index patients with mitochondrial CPEO (n=66 patients with single and n=41 patients with multiple mtDNA deletions) and assessed these for clinical evidence of a SANDO phenotype. Patients with multiple mtDNA deletions were additionally screened for mutations in the nuclear-encoded POLG, SLC25A4, PEO1 and RRM2B genes. The clinical, histological and genetic data of 11 patients with SANDO were further analysed.

RESULTS

None of the 66 patients with single, large-scale mtDNA deletions fulfilled the clinical criteria of SANDO syndrome. In contrast, 9 of 41 patients (22%) with multiple mtDNA deletions and two additional family members fulfilled the clinical criteria for SANDO. Within this subgroup, multiple mtDNA deletions were associated with the following nuclear mutations: POLG (n=6), PEO1 (n=2), unidentified (n=2). The combination of sensory ataxic neuropathy with ophthalmoparesis (SANO) was observed in 70% of patients with multiple mtDNA deletions but only in 4% with single deletions. The combination of CPEO and sensory ataxic neuropathy (SANO, incomplete SANDO) was found in 43% of patients with multiple mtDNA deletions but not in patients with single deletions.

CONCLUSION

The SANDO syndrome seems to indicate a cluster of symptoms within the wide range of multisystemic symptoms associated with mitochondrial CPEO. SANO seems to be the most frequent phenotype associated with multiple mtDNA deletions in our cohort but not or is rarely associated with single, large-scale mtDNA deletions.

A diagnostic flow chart for POLG-related diseases based on signs sensitivity and specificity

OBJECTIVE

Diseases due to mutations of POLG gene, encoding the mitochondrial DNA polymerase, are reputed to have very diverse clinical presentations and have been proposed to cause up to 25% adult mitochondrial diseases. Our objective was the evaluation of the specificity and sensitivity of the signs encountered with POLG mutations.

DESIGN

Forty-four patients out of 154 with sequenced POLG gene had mutations affecting either one (POLG(+/-) group) or two POLG alleles (POLG(+/+) group). Phenotyping included clinical signs, electroneuromyography and brain imaging while mitochondrial investigations encompassed muscle histochemistry, respiratory chain assays and search for multiple mitochondrial deletions. The specificity and sensitivity of the signs associated with POLG mutations were analysed by comparison between POLG(+/+) and patients without POLG mutation.

RESULTS

High sensitivity but low specificity was observed with single signs such as axonal sensory neuropathy, cerebellar syndrome, movement disorders and weakness involving ocular, pharyngeal, axial and/or limb muscles. Specificity was increased with combination of previous signs plus psychiatric symptoms, cognitive impairment and epilepsy. High specificity and sensitivity was only obtained with sensory neuronopathy associated with one of the following signs: weakness of ocular, pharyngeal, axial and/or limb muscles. Mitochondrial investigations did not suffice for diagnosis. The widespread neuromuscular signs were often present since disease onset and were the rule above 50 years of age leading to a very low probability of POLG mutations in patients with less than three signs and absent sensory neuropathy.

CONCLUSIONS

Phenotypes associated with POLG mutations follow a reproducible pattern, which allows establishing a diagnostic flow chart.

Extensive respiratory chain defects in inhibitory interneurones in patients with mitochondrial disease

Aims

Mitochondrial disorders are among the most frequently inherited cause of neurological disease and arise due to mutations in mitochondrial or nuclear DNA. Currently, we do not understand the specific involvement of certain brain regions or selective neuronal vulnerability in mitochondrial disease. Recent studies suggest γ‐aminobutyric acid (GABA)‐ergic interneurones are particularly susceptible to respiratory chain dysfunction. In this neuropathological study, we assess the impact of mitochondrial DNA defects on inhibitory interneurones in patients with mitochondrial disease.

Methods

Histochemical, immunohistochemical and immunofluorescent assays were performed on post‐mortem brain tissue from 10 patients and 10 age‐matched control individuals. We applied a quantitative immunofluorescent method to interrogate complex I and IV protein expression in mitochondria within GABAergic interneurone populations in the frontal, temporal and occipital cortices. We also evaluated the density of inhibitory interneurones in serial sections to determine if cell loss was occurring.

Results

We observed significant, global reductions in complex I expression within GABAergic interneurones in frontal, temporal and occipital cortices in the majority of patients. While complex IV expression is more variable, there is reduced expression in patients harbouring m.8344A>G point mutations and POLG mutations. In addition to the severe respiratory chain deficiencies observed in remaining interneurones, quantification of GABAergic cell density showed a dramatic reduction in cell density suggesting interneurone loss.

Conclusions

We propose that the combined loss of interneurones and severe respiratory deficiency in remaining interneurones contributes to impaired neuronal network oscillations and could underlie development of neurological deficits, such as cognitive impairment and epilepsy, in mitochondrial disease.

[Familial progressive external opthalmoplegia, parkinsonism and polyneuropathy associated with POLG1 mutation]

Multiple mitochondrial DNA (mtDNA) deletions usually occur secondarily to a mutation in one of the enzymes involved in mtDNA maintenance, such as polymerase γ, which is encoded by the nuclear polymerase γ1 gene (POLG1) and POLG2. Patients with multiple mtDNA deletion disorders show clinical heterogeneity of symptoms, in addition to usually seen progressive external ophthalmoplegia (PEO). We conducted clinical, histological and genetic analyses of two affected sisters in a family with the autosomal dominant inheritance pattern of PEO. A 73-year-old woman (patient 1) with congenital hypogonadism and PEO developed L-dopa responsive parkinsonism about the age of 60. Neurological examination revealed mild proximal muscle weakness and polyneuropathy too. Her 69-year-old sister (patient 2) also showed PEO, parkinsonism and polyneuropathy. Histopathological studies of biopsied muscle specimens from patient 1 revealed numerous ragged red fibers as well as fibers with increased succinate dehydrogenase activity and decreased cytochrome c oxidase activity. Multiple mtDNA deletions were detected, both by Southern blot and long-range PCR assays of total DNA from the biopsied muscle specimens. A systemic mutational analysis in both sisters revealed a heterozygous p.Y955C (c.2864A>G) mutation in POLG1. This is the first Japanese family identified with this mutation. We reviewed cases with this mutation highlighting a wide phenotypic spectrum of this disorder.

Peripheral neuropathy predicts nuclear gene defect in patients with mitochondrial ophthalmoplegia

Mitochondrial ophthalmoplegia is a genetically heterogeneous disorder. Horga et al. investigate whether peripheral neuropathy can predict the underlying genetic defect in patients with progressive external ophthalmoplegia. Results indicate that neuropathy is highly predictive of a nuclear DNA defect and that it is rarely associated with single mitochondrial DNA deletions.

Progressive external ophthalmoplegia is a common clinical feature in mitochondrial disease caused by nuclear DNA defects and single, large-scale mitochondrial DNA deletions and is less frequently associated with point mutations of mitochondrial DNA. Peripheral neuropathy is also a frequent manifestation of mitochondrial disease, although its prevalence and characteristics varies considerably among the different syndromes and genetic aetiologies. Based on clinical observations, we systematically investigated whether the presence of peripheral neuropathy could predict the underlying genetic defect in patients with progressive external ophthalmoplegia. We analysed detailed demographic, clinical and neurophysiological data from 116 patients with genetically-defined mitochondrial disease and progressive external ophthalmoplegia. Seventy-eight patients (67%) had a single mitochondrial DNA deletion, 12 (10%) had a point mutation of mitochondrial DNA and 26 (22%) had mutations in either POLG, C10orf2 or RRM2B, or had multiple mitochondrial DNA deletions in muscle without an identified nuclear gene defect. Seventy-seven patients had neurophysiological studies; of these, 16 patients (21%) had a large-fibre peripheral neuropathy. The prevalence of peripheral neuropathy was significantly lower in patients with a single mitochondrial DNA deletion (2%) as compared to those with a point mutation of mitochondrial DNA or with a nuclear DNA defect (44% and 52%, respectively; P < 0.001). Univariate analyses revealed significant differences in the distribution of other clinical features between genotypes, including age at disease onset, gender, family history, progressive external ophthalmoplegia at clinical presentation, hearing loss, pigmentary retinopathy and extrapyramidal features. However, binomial logistic regression analysis identified peripheral neuropathy as the only independent predictor associated with a nuclear DNA defect (P = 0.002; odds ratio 8.43, 95% confidence interval 2.24–31.76). Multinomial logistic regression analysis identified peripheral neuropathy, family history and hearing loss as significant predictors of the genotype, and the same three variables showed the highest performance in genotype classification in a decision tree analysis. Of these variables, peripheral neuropathy had the highest specificity (91%), negative predictive value (83%) and positive likelihood ratio (5.87) for the diagnosis of a nuclear DNA defect. These results indicate that peripheral neuropathy is a rare finding in patients with single mitochondrial DNA deletions but that it is highly predictive of an underlying nuclear DNA defect. This observation may facilitate the development of diagnostic algorithms. We suggest that nuclear gene testing may enable a more rapid diagnosis and avoid muscle biopsy in patients with progressive external ophthalmoplegia and peripheral neuropathy.

Dissecting the mechanisms underlying the accumulation of mitochondrial DNA deletions in human skeletal muscle

Large-scale mitochondrial DNA (mtDNA) deletions are an important cause of mitochondrial disease, while somatic mtDNA deletions cause focal respiratory chain deficiency associated with ageing and neurodegenerative disorders. As mtDNA deletions only cause cellular pathology at high levels of mtDNA heteroplasmy, an mtDNA deletion must accumulate to levels which can result in biochemical dysfunction-a process known as clonal expansion. A number of hypotheses have been proposed for clonal expansion of mtDNA deletions, including a replicative advantage for deleted mitochondrial genomes inferred by their smaller size--implying that the largest mtDNA deletions would also display a replicative advantage over smaller mtDNA deletions. We proposed that in muscle fibres from patients with mtDNA maintenance disorders, which lead to the accumulation of multiple mtDNA deletions, we would observe the largest mtDNA deletions spreading the furthest longitudinally through individual muscle fibres by means of a greater rate of clonal expansion. We characterized mtDNA deletions in patients with mtDNA maintenance disorders from a range of 'large' and 'small' cytochrome c oxidase (COX)-deficient regions in skeletal muscle fibres. We measured the size of clonally expanded deletions in 62 small and 60 large individual COX-deficient f regions. No significant difference was observed in individual patients or in the total dataset (small fibre regions mean 6.59 kb--large fibre regions mean 6.51 kb). Thus no difference existed in the rate of clonal expansion throughout muscle fibres between mtDNA deletions of different sizes; smaller mitochondrial genomes therefore do not appear to have an inherent replicative advantage in human muscle.

Genetic diagnosis of Charcot-Marie-Tooth disease in a population by next-generation sequencing

Charcot-Marie-Tooth (CMT) disease is the most prevalent inherited neuropathy. Today more than 40 CMT genes have been identified. Diagnosing heterogeneous diseases by conventional Sanger sequencing is time consuming and expensive. Thus, more efficient and less costly methods are needed in clinical diagnostics. We included a population based sample of 81 CMT families. Gene mutations had previously been identified in 22 families; the remaining 59 families were analysed by next-generation sequencing. Thirty-two CMT genes and 19 genes causing other inherited neuropathies were included in a custom panel. Variants were classified into five pathogenicity classes by genotype-phenotype correlations and bioinformatics tools. Gene mutations, classified certainly or likely pathogenic, were identified in 37 (46%) of the 81 families. Point mutations in known CMT genes were identified in 21 families (26%), whereas four families (5%) had point mutations in other neuropathy genes, ARHGEF10, POLG, SETX, and SOD1. Eleven families (14%) carried the PMP22 duplication and one family carried a MPZ duplication (1%). Most mutations were identified not only in known CMT genes but also in other neuropathy genes, emphasising that genetic analysis should not be restricted to CMT genes only. Next-generation sequencing is a cost-effective tool in diagnosis of CMT improving diagnostic precision and time efficiency.

Molecular pathogenesis of polymerase γ-related neurodegeneration

Objective

Polymerase gamma (POLG) mutations are a common cause of mitochondrial disease and have also been linked to neurodegeneration and aging. We studied the molecular mechanisms underlying POLG-related neurodegeneration using postmortem tissue from a large number of patients.

Methods

Clinical information was available from all subjects. Formalin-fixed and frozen brain tissue from 15 patients and 23 controls was studied employing a combination of histopathology, immunohistochemistry, and molecular studies of microdissected neurons.

Results

The primary consequence of POLG mutation in neurons is mitochondrial DNA depletion. This was already present in infants with little evidence of neuronal loss or mitochondrial dysfunction. With longer disease duration, we found an additional, progressive accumulation of mitochondrial DNA deletions and point mutations accompanied by increasing numbers of complex I–deficient neurons. Progressive neurodegeneration primarily affected the cerebellar systems and dopaminergic cells of the substantia nigra. Superimposed on this chronic process were acute, focal cortical lesions that correlated with epileptogenic foci and that showed massive neuronal loss.

Interpretation

POLG mutations appear to compromise neuronal respiration via a combination of early and stable depletion and a progressive somatic mutagenesis of the mitochondrial genome. This leads to 2 distinct but overlapping biological processes: a chronic neurodegeneration reflected clinically by progressive ataxia and cognitive impairment, and an acute focal neuronal necrosis that appears to be related to the presence of epileptic seizures. Our findings offer an explanation of the acute-on-chronic clinical course of this common mitochondrial encephalopathy. ANN NEUROL 2014;76:66–81

The impact of pathogenic mitochondrial DNA mutations on substantia nigra neurons

Mitochondrial defects within substantia nigra (SN) neurons are implicated in the pathogenesis of Parkinson's disease. SN neurons show increased mitochondrial defects, mitochondrial DNA deletion levels, and susceptibility to such dysfunction, although the role of mitochondria in neuronal degeneration remains uncertain. In this study, we addressed this important question by exploring changes within the mitochondria of SN neurons from patients with primary mitochondrial diseases to determine whether mitochondrial dysfunction leads directly to neuronal cell loss. We counted the pigmented neurons and quantified mitochondrial respiratory activity, deficiencies in mitochondrial proteins, and the percentage of pathogenic mutations in single neurons. We found evidence of defects of both complex I and complex IV of the respiratory chain in all patients. We found that marked neuronal cell loss was only observed in a few patients with mitochondrial disease and that all these patients had mutations in polymerase gamma (POLG), which leads to the formation of multiple mitochondrial DNA deletions over time, similar to aging and Parkinson's disease. Interestingly, we detected α-synuclein pathology in two mitochondrial patients with POLG mutations. Our observations highlight the complex relationship between mitochondrial dysfunction and the susceptibility of SN neurons to degeneration and α-synuclein pathology. Our finding that the loss of SN neurons was only severe in patients with POLG mutations suggests that acquired mitochondrial defects may be less well tolerated by SN neurons than by inherited ones.

Mitochondrial disorders: aetiologies, models systems, and candidate therapies

It has become evident that many human disorders are characterised by mitochondrial dysfunction either at a primary level, due to mutations in genes whose encoded products are involved in oxidative phosphorylation, or at a secondary level, due to the accumulation of mitochondrial DNA (mtDNA) mutations. This has prompted keen interest in the development of cell and animal models and in exploring innovative therapeutic strategies to modulate the mitochondrial deficiencies observed in these diseases. Key advances in these areas are outlined in this review, with a focus on Leber hereditary optic neuropathy (LHON). This exciting field is set to grow exponentially and yield many candidate therapies to treat this class of disease.

Severe nigrostriatal degeneration without clinical parkinsonism in patients with polymerase gamma mutations

The role of mitochondria in the pathogenesis of neurodegeneration is an area of intense study. It is known that defects in proteins involved in mitochondrial quality control can cause Parkinson's disease, and there is increasing evidence linking mitochondrial dysfunction, and particularly mitochondrial DNA abnormalities, to neuronal loss in the substantia nigra. Mutations in the catalytic subunit of polymerase gamma are among the most common causes of mitochondrial disease and owing to its role in mitochondrial DNA homeostasis, polymerase gamma defects are often considered a paradigm for mitochondrial diseases generally. Yet, despite this, parkinsonism is uncommon with polymerase gamma defects. In this study, we investigated structural and functional changes in the substantia nigra of 11 patients with polymerase gamma encephalopathy. We characterized the mitochondrial DNA abnormalities and examined the respiratory chain in neurons of the substantia nigra. We also investigated nigrostriatal integrity and function using a combination of post-mortem and in vivo functional studies with dopamine transporter imaging and positron emission tomography. At the cellular level, dopaminergic nigral neurons of patients with polymerase gamma encephalopathy contained a significantly lower copy number of mitochondrial DNA (depletion) and higher levels of deletions than normal control subjects. A selective and progressive complex I deficiency was seen and this was associated with a severe and progressive loss of the dopaminergic neurons of the pars compacta. Dopamine transporter imaging and positron emission tomography showed that the degree of nigral neuronal loss and nigrostriatal depletion were severe and appeared greater even than that seen in idiopathic Parkinson's disease. Despite this, however, none of our patients showed any signs of parkinsonism. The additional presence of both thalamic and cerebellar dysfunction in our patients suggested that these may play a role in counteracting the effects of basal ganglia dysfunction and prevent the development of clinical parkinsonism.