Genetic dysfunction of MT-ATP6 causes axonal Charcot-Marie-Tooth disease

Biallelic variants in COX18 cause a mitochondrial disorder primarily manifesting as peripheral neuropathy

Defects in mitochondrial dynamics are a common cause of Charcot-Marie-Tooth disease (CMT), while primary deficiencies in the mitochondrial respiratory chain (MRC) are rare and atypical for this etiology. This study aims to report COX18 as a novel CMT-causing gene. This gene encodes an assembly factor of mitochondrial Complex IV (CIV) that translocates the C-terminal tail of MTCO2 across the mitochondrial inner membrane. Exome sequencing was performed in four affected individuals. The patients and available family members underwent thorough neurological and electrophysiological assessment. The impact of one of the identified variants on splicing, protein levels, and mitochondrial bioenergetics was investigated in patient-derived lymphoblasts. The functionality of the mutant protein was assessed using a Proteinase K protection assay and immunoblotting. Neuronal relevance of COX18 was assessed in a Drosophila melanogaster knockdown model. Exome sequencing coupled with homozygosity mapping revealed a homozygous splice variant c.435-6A>G in COX18 in two siblings with early-onset progressive axonal sensory-motor peripheral neuropathy. By querying external databases, we identified two additional families with rare deleterious biallelic variants in COX18 . All affected individuals presented with axonal CMT and some patients also exhibited central nervous system symptoms, such as dystonia and spasticity. Functional characterization of the c.435-6A>G variant demonstrated that it leads to the expression of an alternative transcript that lacks exon 2, resulting in a stable but defective COX18 isoform. The mutant protein impairs CIV assembly and activity, leading to a reduction in mitochondrial membrane potential. Downregulation of the COX18 homolog in Drosophila melanogaster displayed signs of neurodegeneration, including locomotor deficit and progressive axonal degeneration of sensory neurons. Our study presents genetic and functional evidence that supports COX18 as a newly identified gene candidate for autosomal recessive axonal CMT with or without central nervous system involvement. These findings emphasize the significance of peripheral neuropathy within the spectrum of primary mitochondrial disorders and the role of mitochondrial CIV in the development of CMT. Our research has important implications for the diagnostic workup of CMT patients.

Clinical practice guidelines for the diagnosis and management of Charcot-Marie-Tooth disease

INTRODUCTION

Charcot-Marie-Tooth (CMT) disease is classified considering the neurophysiological and histological findings, the inheritance pattern and the underlying genetic defect. In recent years, with the advent of next generation sequencing, genetic complexity has increased exponentially, expanding the knowledge about disease pathways, and having an impact in clinical management. The aim of this guide is to offer recommendations for the diagnosis, prognosis, monitoring and treatment of this disease in Spain.

MATERIAL AND METHODS

This consensus guideline has been developed by a multidisciplinary panel encompassing a broad group of professionals including neurologists, neuropediatricians, geneticists, rehabilitators, and orthopaedic surgeons.

RECOMMENDATIONS

The diagnosis is based in the clinical characterization, usually presenting with a common phenotype. It should be followed by an appropriate neurophysiological study that allows for a correct classification, specific recommendations are established for the parameters that should be included. Genetic diagnosis must be approached in sequentially, once the PMP22 duplication has been ruled out if appropriate, a next generation sequencing should be considered taking into account the limitations of the available techniques. To date, there is no pharmacological treatment that modifies the course of the disease, but symptomatic management is important, as are the rehabilitation and orthopaedic considerations. The latter should be initiated early to identify and improve the patient's functional impairments, including individualised exercise guidelines, orthotic adaptation, and assessment of conservative surgeries such as tendon transpositions. The follow-up of patients with CMT is exclusively clinical, ancillary testing are not necessary in routine clinical practice.

Variants in Human ATP Synthase Mitochondrial Genes: Biochemical Dysfunctions, Associated Diseases, and Therapies

Mitochondrial ATP synthase (Complex V) catalyzes the last step of oxidative phosphorylation and provides most of the energy (ATP) required by human cells. The mitochondrial genes MT-ATP6 and MT-ATP8 encode two subunits of the multi-subunit Complex V. Since the discovery of the first MT-ATP6 variant in the year 1990 as the cause of Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP) syndrome, a large and continuously increasing number of inborn variants in the MT-ATP6 and MT-ATP8 genes have been identified as pathogenic. Variants in these genes correlate with various clinical phenotypes, which include several neurodegenerative and multisystemic disorders. In the present review, we report the pathogenic variants in mitochondrial ATP synthase genes and highlight the molecular mechanisms underlying ATP synthase deficiency that promote biochemical dysfunctions. We discuss the possible structural changes induced by the most common variants found in patients by considering the recent cryo-electron microscopy structure of human ATP synthase. Finally, we provide the state-of-the-art of all therapeutic proposals reported in the literature, including drug interventions targeting mitochondrial dysfunctions, allotopic gene expression- and nuclease-based strategies, and discuss their potential translation into clinical trials.

Whole mitogenome sequencing uncovers a relation between mitochondrial heteroplasmy and leprosy severity

BACKGROUND

In recent years, the mitochondria/immune system interaction has been proposed, so that variants of mitochondrial genome and levels of heteroplasmy might deregulate important metabolic processes in fighting infections, such as leprosy.

METHODS

We sequenced the whole mitochondrial genome to investigate variants and heteroplasmy levels, considering patients with different clinical forms of leprosy and household contacts. After sequencing, a specific pipeline was used for preparation and bioinformatics analysis to select heteroplasmic variants.

RESULTS

We found 116 variants in at least two of the subtypes of the case group (Borderline Tuberculoid, Borderline Lepromatous, Lepromatous), suggesting a possible clinical significance to these variants. Notably, 15 variants were exclusively found in these three clinical forms, of which five variants stand out for being missense (m.3791T > C in MT-ND1, m.5317C > A in MT-ND2, m.8545G > A in MT-ATP8, m.9044T > C in MT-ATP6 and m.15837T > C in MT-CYB). In addition, we found 26 variants shared only by leprosy poles, of which two are characterized as missense (m.4248T > C in MT-ND1 and m.8027G > A in MT-CO2).

CONCLUSION

We found a significant number of variants and heteroplasmy levels in the leprosy patients from our cohort, as well as six genes that may influence leprosy susceptibility, suggesting for the first time that the mitogenome might be involved with the leprosy process, distinction of clinical forms and severity. Thus, future studies are needed to help understand the genetic consequences of these variants.

Unveiling the clinical and electrophysiological profile of CMTX6: Insights from two Brazilian families

BACKGROUND AND AIMS

X-linked Charcot-Marie-Tooth disease type 6 (CMTX6) is an extremely rare condition associated with mutations in the PDK3 gene. To date, only three families from different countries have been reported (Australia, South Korea, and Germany). In this study, we sought to provide a comprehensive clinical and electrophysiological characterization of two Brazilian families.

METHODS

We conducted comprehensive clinical assessments, extensive electrophysiological evaluations, and performed whole-exome sequencing in the probands to investigate the genetic basis of the disease.

RESULTS

Males in the family carrying the Arg162His mutation displayed early-onset motor and/or sensory axonal neuropathy, absence of tendon jerks, pes cavus, and frequently reported pain. Females in the same family exhibited a milder phenotype of the disease with later onset and some remained asymptomatic into their 50s. In the unrelated family with a single affected male, the clinical presentation was characterized by severe progressive sensorimotor polyneuropathy accompanied by neuropathic pain.

INTERPRETATION

We report two Brazilian families with CMTX6 including one harboring a previously unpublished variant in the PDK3 gene, which co-segregates with the disease as expected in a X-linked disease. Notably, the clinical presentations across the five families with available descriptions, including our study, share striking similarities. Furthermore, the proximity of the three reported mutations suggests potential functional similarities and common underlying mechanisms. This study contributes to the growing knowledge of CMTX6 and underscores the importance of international collaborations in studying rare genetic disorders.

Novel gene-intergenic fusion involving ubiquitin E3 ligase UBE3C causes distal hereditary motor neuropathy

Distal hereditary motor neuropathies (dHMNs) are a group of inherited diseases involving the progressive, length-dependent axonal degeneration of the lower motor neurons. There are currently 29 reported causative genes and four disease loci implicated in dHMN. Despite the high genetic heterogeneity, mutations in the known genes account for less than 20% of dHMN cases, with the mutations identified predominantly being point mutations or indels. We have expanded the spectrum of dHMN mutations with the identification of a 1.35 Mb complex structural variation (SV) causing a form of autosomal dominant dHMN (DHMN1 OMIM %182906). Given the complex nature of SV mutations and the importance of studying pathogenic mechanisms in a neuronal setting, we generated a patient-derived DHMN1 motor neuron model harbouring the 1.35 Mb complex insertion. The DHMN1 complex insertion creates a duplicated copy of the first 10 exons of the ubiquitin-protein E3 ligase gene (UBE3C) and forms a novel gene-intergenic fusion sense transcript by incorporating a terminal pseudo-exon from intergenic sequence within the DHMN1 locus. The UBE3C intergenic fusion (UBE3C-IF) transcript does not undergo nonsense-mediated decay and results in a significant reduction of wild-type full-length UBE3C (UBE3C-WT) protein levels in DHMN1 iPSC-derived motor neurons. An engineered transgenic Caenorhabditis elegans model expressing the UBE3C-IF transcript in GABA-ergic motor neurons shows neuronal synaptic transmission deficits. Furthermore, the transgenic animals are susceptible to heat stress, which may implicate defective protein homeostasis underlying DHMN1 pathogenesis. Identification of the novel UBE3C-IF gene-intergenic fusion transcript in motor neurons highlights a potential new disease mechanism underlying axonal and motor neuron degeneration. These complementary models serve as a powerful paradigm for studying the DHMN1 complex SV and an invaluable tool for defining therapeutic targets for DHMN1.

Clinical genetics of Charcot-Marie-Tooth disease

Recent research in the field of inherited peripheral neuropathies (IPNs) such as Charcot-Marie-Tooth (CMT) disease has helped identify the causative genes provided better understanding of the pathogenesis, and unraveled potential novel therapeutic targets. Several reports have described the epidemiology, clinical characteristics, molecular pathogenesis, and novel causative genes for CMT/IPNs in Japan. Based on the functions of the causative genes identified so far, the following molecular and cellular mechanisms are believed to be involved in the causation of CMTs/IPNs: myelin assembly, cytoskeletal structure, myelin-specific transcription factor, nuclear related, endosomal sorting and cell signaling, proteasome and protein aggregation, mitochondria-related, motor proteins and axonal transport, tRNA synthetases and RNA metabolism, and ion channel-related mechanisms. In this article, we review the epidemiology, genetic diagnosis, and clinicogenetic characteristics of CMT in Japan. In addition, we discuss the newly identified novel causative genes for CMT/IPNs in Japan, namely MME and COA7. Identification of the new causes of CMT will facilitate in-depth characterization of the underlying molecular mechanisms of CMT, leading to the establishment of therapeutic approaches such as drug development and gene therapy.

Peripheral neuropathy in mitochondrial disease

Mitochondria are essential for the health and viability of both motor and sensory neurons and their axons. Processes that disrupt their normal distribution and transport along axons will likely cause peripheral neuropathies. Similarly, mutations in mtDNA or nuclear encoded genes result in neuropathies that either stand alone or are part of multisystem disorders. This chapter focuses on the more common genetic forms and characteristic clinical phenotypes of "mitochondrial" peripheral neuropathies. We also explain how these various mitochondrial abnormalities cause peripheral neuropathy. In a patient with a neuropathy either due to a mutation in a nuclear or an mtDNA gene, clinical investigations aim to characterize the neuropathy and make an accurate diagnosis. In some patients, this may be relatively straightforward, where a clinical assessment and nerve conduction studies followed by genetic testing is all that is needed. In others, multiple investigations including a muscle biopsy, CNS imaging, CSF analysis, and a wide range of metabolic and genetic tests in blood and muscle may be needed to establish diagnosis.

Gene therapy for primary mitochondrial diseases: experimental advances and clinical challenges

The variable clinical and biochemical manifestations of primary mitochondrial diseases (PMDs), and the complexity of mitochondrial genetics, have proven to be a substantial barrier to the development of effective disease-modifying therapies. Encouraging data from gene therapy trials in patients with Leber hereditary optic neuropathy and advances in DNA editing techniques have raised expectations that successful clinical transition of genetic therapies for PMDs is feasible. However, obstacles to the clinical application of genetic therapies in PMDs remain; the development of innovative, safe and effective genome editing technologies and vectors will be crucial to their future success and clinical approval. In this Perspective, we review progress towards the genetic treatment of nuclear and mitochondrial DNA-related PMDs. We discuss advances in mitochondrial DNA editing technologies alongside the unique challenges to targeting mitochondrial genomes. Last, we consider ongoing trials and regulatory requirements.

MicroRNA and Metabolic Profiling of a Primary Ovarian Neuroendocrine Carcinoma Pulmonary-Type Reveals a High Degree of Similarity with Small Cell Lung Cancer

Small cell neuroendocrine carcinoma is most frequently found in the lung (SCLC), but it has been also reported, albeit with a very low incidence, in the ovary. Here, we analyze a case of primary small cell carcinoma of the ovary of pulmonary type (SCCOPT), a rare and aggressive tumor with poor prognosis, whose biology and molecular features have not yet been thoroughly investigated. The patient affected by SCCOPT had a residual tumor following chemotherapy which displayed pronounced similarity with neuroendocrine tumors and lung cancer in terms of its microRNA expression profile and mTOR-downstream activation. By analyzing the metabolic markers of the neoplastic lesion, we established a likely glycolytic signature. In conclusion, this in-depth characterization of SCCOPT could be useful for future diagnoses, possibly aided by microRNA profiling, allowing clinicians to adopt the most appropriate therapeutic strategy.

Gene Therapy for Mitochondrial Diseases: Current Status and Future Perspective

Mitochondrial diseases (MDs) are a group of severe genetic disorders caused by mutations in the nuclear or mitochondrial genome encoding proteins involved in the oxidative phosphorylation (OXPHOS) system. MDs have a wide range of symptoms, ranging from organ-specific to multisystemic dysfunctions, with different clinical outcomes. The lack of natural history information, the limits of currently available preclinical models, and the wide range of phenotypic presentations seen in MD patients have all hampered the development of effective therapies. The growing number of pre-clinical and clinical trials over the last decade has shown that gene therapy is a viable precision medicine option for treating MD. However, several obstacles must be overcome, including vector design, targeted tissue tropism and efficient delivery, transgene expression, and immunotoxicity. This manuscript offers a comprehensive overview of the state of the art of gene therapy in MD, addressing the main challenges, the most feasible solutions, and the future perspectives of the field.

From the Structural and (Dys)Function of ATP Synthase to Deficiency in Age-Related Diseases

The ATP synthase is a mitochondrial inner membrane complex whose function is essential for cell bioenergy, being responsible for the conversion of ADP into ATP and playing a role in mitochondrial cristae morphology organization. The enzyme is composed of 18 protein subunits, 16 nuclear DNA (nDNA) encoded and two mitochondrial DNA (mtDNA) encoded, organized in two domains, F_O and F₁. Pathogenetic variants in genes encoding structural subunits or assembly factors are responsible for fatal human diseases. Emerging evidence also underlines the role of ATP-synthase in neurodegenerative diseases as Parkinson’s, Alzheimer’s, and motor neuron diseases such as Amyotrophic Lateral Sclerosis. Post-translational modification, epigenetic modulation of ATP gene expression and protein level, and the mechanism of mitochondrial transition pore have been deemed responsible for neuronal cell death in vivo and in vitro models for neurodegenerative diseases. In this review, we will explore ATP synthase assembly and function in physiological and pathological conditions by referring to the recent cryo-EM studies and by exploring human disease models.

Molecular Genetics Overview of Primary Mitochondrial Myopathies

Mitochondrial disorders are the most common inherited conditions, characterized by defects in oxidative phosphorylation and caused by mutations in nuclear or mitochondrial genes. Due to its high energy request, skeletal muscle is typically involved. According to the International Workshop of Experts in Mitochondrial Diseases held in Rome in 2016, the term Primary Mitochondrial Myopathy (PMM) should refer to those mitochondrial disorders affecting principally, but not exclusively, the skeletal muscle. The clinical presentation may include general isolated myopathy with muscle weakness, exercise intolerance, chronic ophthalmoplegia/ophthalmoparesis (cPEO) and eyelids ptosis, or multisystem conditions where there is a coexistence with extramuscular signs and symptoms. In recent years, new therapeutic targets have been identified leading to the launch of some promising clinical trials that have mainly focused on treating muscle symptoms and that require populations with defined genotype. Advantages in next-generation sequencing techniques have substantially improved diagnosis. So far, an increasing number of mutations have been identified as responsible for mitochondrial disorders. In this review, we focused on the principal molecular genetic alterations in PMM. Accordingly, we carried out a comprehensive review of the literature and briefly discussed the possible approaches which could guide the clinician to a genetic diagnosis.

Episodic weakness and axonal sensorimotor neuropathy caused by a mitochondrial MT-ATP6 mutation

Episodic weakness is typically associated with a group of disorders so called periodic paralyses. Their major causes are mutation of ion channels, and have rarely been linked to mitochondrial disorders. We report a 20-year-old man with episodic weakness and axonal sensorimotor neuropathy since the age of 10 years. Analysis of the next generation sequencing data of the entire mitochondrial genome extracted from the blood revealed a homoplasmic m.9185T > C variant in MT-ATP6. Acetazolamide may be responsive for episodic weakness, and supplements with l-carnitine with coenzyme-Q10 seem to be beneficial as well. To the best of our knowledge, this is the first report in Taiwan which reveals episodic weakness and sensorimotor polyneuropathy as a unique phenotype of MT-ATP6 mutations.

Threshold of heteroplasmic truncating MT-ATP6 mutation in reprogramming, Notch hyperactivation and motor neuron metabolism

Mutations in mitochondrial DNA encoded subunit of ATP synthase, MT-ATP6, are frequent causes of neurological mitochondrial diseases with a range of phenotypes from Leigh syndrome and NARP to ataxias and neuropathies. Here we investigated the functional consequences of an unusual heteroplasmic truncating mutation m.9154C>T in MT-ATP6, which caused peripheral neuropathy, ataxia and IgA nephropathy. ATP synthase not only generates cellular ATP, but its dimerization is required for mitochondrial cristae formation. Accordingly, the MT-ATP6 truncating mutation impaired the assembly of ATP synthase and disrupted cristae morphology, supporting our molecular dynamics simulations that predicted destabilized a/c subunit subcomplex. Next, we modeled the effects of the truncating mutation using patient-specific induced pluripotent stem cells. Unexpectedly, depending on mutation heteroplasmy level, the truncation showed multiple threshold effects in cellular reprogramming, neurogenesis and in metabolism of mature motor neurons (MN). Interestingly, MN differentiation beyond progenitor stage was impaired by Notch hyperactivation in the MT-ATP6 mutant, but not by rotenone-induced inhibition of mitochondrial respiration, suggesting that altered mitochondrial morphology contributed to Notch hyperactivation. Finally, we also identified a lower mutation threshold for a metabolic shift in mature MN, affecting lactate utilization, which may be relevant for understanding the mechanisms of mitochondrial involvement in peripheral motor neuropathies. These results establish a critical and disease-relevant role for ATP synthase in human cell fate decisions and neuronal metabolism.

NEUROPATHY, ATAXIA, AND RETINITIS PIGMENTOSA SYNDROME: A MULTIDISCIPLINARY DIAGNOSIS

PURPOSE

To report a case of neuropathy, ataxia, and retinitis pigmentosa syndrome, a rare and undiagnosed disease in ophthalmology due to the need for multidisciplinary evaluation.

METHODS

Multimodal testing was performed, including neurologic, ophthalmologic, and genetic assessments. The neurologic tests comprised electromyogram and muscle biopsy; the ophthalmologic examination consisted of slit-lamp and fundus examinations, optical coherence tomography, visual field testing, and electrophysiology tests such as a full-field electroretinogram and multifocal electroretinogram; and genetic tests were performed for spinocerebellar ataxia. In addition, the patient underwent magnetic resonance imaging, an electrocardiogram, cerebrospinal fluid analysis with lactate levels, and a blood workup including antineuronal, antithyroid peroxidase, antinuclear, antimitochondrial, and antitransglutaminase antibodies and fat-soluble vitamins (A, D, E, K).

RESULTS

The ocular fundus examination showed bone spicules with retinal pigment epithelium alteration, optic nerve pallor, and arterial attenuation. Optical coherence tomography demonstrated macular atrophy. Visual field testing revealed concentric reduction. Electrophysiology examinations showed involvement of rods and cones in both eyes. The muscle biopsy was compatible with mitochondrial disease, and electromyogram demonstrated sensory axonal damage. However, genetic tests for spinocerebellar ataxia were negative. Magnetic resonance imaging showed cerebellar atrophy, whereas the electrocardiogram did not detect any abnormalities. Cerebrospinal fluid lactate levels were above normal but antibody levels in blood were normal.

CONCLUSION

This is the first report of macular atrophy demonstrated by optical coherence tomography in a patient with neuropathy, ataxia, and retinitis pigmentosa syndrome. For the diagnosis, a multidisciplinary team including a neurologist, a geneticist, and an ophthalmologist was essential. Patients with suspected mitochondrial disease could greatly benefit from an ophthalmology examination like that conducted in this case because it was the key factor that led to the suspicion of syndromic disease, and ultimately the diagnosis.

Novel Mutation m.10372A>G in MT-ND3 Causing Sensorimotor Axonal Polyneuropathy

OBJECTIVE

To investigate the pathogenicity of a novel MT-ND3 mutation identified in a patient with adult-onset sensorimotor axonal polyneuropathy and report the clinical, morphologic, and biochemical findings.

METHODS

Clinical assessments and morphologic and biochemical investigations of skeletal muscle and cultured myoblasts from the patient were performed. Whole-genome sequencing (WGS) of DNA from skeletal muscle and Sanger sequencing of mitochondrial DNA (mtDNA) from both skeletal muscle and cultured myoblasts were performed. Heteroplasmic levels of mutated mtDNA in different tissues were quantified by last-cycle hot PCR.

RESULTS

Muscle showed ragged red fibers, paracrystalline inclusions, a significant reduction in complex I (CI) respiratory chain (RC) activity, and decreased adenosine triphosphate (ATP) production for all substrates used by CI. Sanger sequencing of DNA from skeletal muscle detected a unique previously unreported heteroplasmic mutation in mtDNA encoded MT-ND3, coding for a subunit in CI. WGS confirmed the mtDNA mutation but did not detect any other mutation explaining the disease. Cultured myoblasts, however, did not carry the mutation, and RC activity measurements in myoblasts were normal.

CONCLUSIONS

We report a case with adult-onset sensorimotor axonal polyneuropathy caused by a novel mtDNA mutation in MT-ND3. Loss of heteroplasmy in blood, cultured fibroblasts and myoblasts from the patient, and normal measurement of RC activity of the myoblasts support pathogenicity of the mutation. These findings highlight the importance of mitochondrial investigations in patients presenting with seemingly idiopathic polyneuropathy, especially if muscle also is affected.

Applying genomic and transcriptomic advances to mitochondrial medicine

Next-generation sequencing (NGS) has increased our understanding of the molecular basis of many primary mitochondrial diseases (PMDs). Despite this progress, many patients with suspected PMD remain without a genetic diagnosis, which restricts their access to in-depth genetic counselling, reproductive options and clinical trials, in addition to hampering efforts to understand the underlying disease mechanisms. Although they represent a considerable improvement over their predecessors, current methods for sequencing the mitochondrial and nuclear genomes have important limitations, and molecular diagnostic techniques are often manual and time consuming. However, recent advances in genomics and transcriptomics offer realistic solutions to these challenges. In this Review, we discuss the current genetic testing approach for PMDs and the opportunities that exist for increased use of whole-genome NGS of nuclear and mitochondrial DNA (mtDNA) in the clinical environment. We consider the possible role for long-read approaches in sequencing of mtDNA and in the identification of novel nuclear genomic causes of PMDs. We examine the expanding applications of RNA sequencing, including the detection of cryptic variants that affect splicing and gene expression and the interpretation of rare and novel mitochondrial transfer RNA variants.

Impaired Mitochondrial Mobility in Charcot-Marie-Tooth Disease

Charcot-Marie-Tooth (CMT) disease is a progressive, peripheral neuropathy and the most commonly inherited neurological disorder. Clinical manifestations of CMT mutations are typically limited to peripheral neurons, the longest cells in the body. Currently, mutations in at least 80 different genes are associated with CMT and new mutations are regularly being discovered. A large portion of the proteins mutated in axonal CMT have documented roles in mitochondrial mobility, suggesting that organelle trafficking defects may be a common underlying disease mechanism. This review will focus on the potential role of altered mitochondrial mobility in the pathogenesis of axonal CMT, highlighting the conceptional challenges and potential experimental and therapeutic opportunities presented by this "impaired mobility" model of the disease.

Therapeutic Approaches to Treat Mitochondrial Diseases: "One-Size-Fits-All" and "Precision Medicine" Strategies

Primary mitochondrial diseases (PMD) refer to a group of severe, often inherited genetic conditions due to mutations in the mitochondrial genome or in the nuclear genes encoding for proteins involved in oxidative phosphorylation (OXPHOS). The mutations hamper the last step of aerobic metabolism, affecting the primary source of cellular ATP synthesis. Mitochondrial diseases are characterized by extremely heterogeneous symptoms, ranging from organ-specific to multisystemic dysfunction with different clinical courses. The limited information of the natural history, the limitations of currently available preclinical models, coupled with the large variability of phenotypical presentations of PMD patients, have strongly penalized the development of effective therapies. However, new therapeutic strategies have been emerging, often with promising preclinical and clinical results. Here we review the state of the art on experimental treatments for mitochondrial diseases, presenting "one-size-fits-all" approaches and precision medicine strategies. Finally, we propose novel perspective therapeutic plans, either based on preclinical studies or currently used for other genetic or metabolic diseases that could be transferred to PMD.

A Mitochondrial tRNA Mutation Causes Axonal CMT in a Large Venezuelan Family

OBJECTIVE

The objective of this study was to identify the genetic cause for progressive peripheral nerve disease in a Venezuelan family. Despite the growing list of genes associated with Charcot-Marie-Tooth disease, many patients with axonal forms lack a genetic diagnosis.

METHODS

A pedigree was constructed, based on family clinical data. Next-generation sequencing of mitochondrial DNA (mtDNA) was performed for 6 affected family members. Muscle biopsies from 4 family members were used for analysis of muscle histology and ultrastructure, mtDNA sequencing, and RNA quantification. Ultrastructural studies were performed on sensory nerve biopsies from 2 affected family members.

RESULTS

Electrodiagnostic testing showed a motor and sensory axonal polyneuropathy. Pedigree analysis revealed inheritance only through the maternal line, consistent with mitochondrial transmission. Sequencing of mtDNA identified a mutation in the mitochondrial tRNA^Val (mt-tRNA^Val ) gene, m.1661A>G, present at nearly 100% heteroplasmy, which disrupts a Watson-Crick base pair in the T-stem-loop. Muscle biopsies showed chronic denervation/reinnervation changes, whereas biochemical analysis of electron transport chain (ETC) enzyme activities showed reduction in multiple ETC complexes. Northern blots from skeletal muscle total RNA showed severe reduction in abundance of mt-tRNA^Val , and mildly increased mt-tRNA^Phe , in subjects compared with unrelated age- and sex-matched controls. Nerve biopsies from 2 affected family members demonstrated ultrastructural mitochondrial abnormalities (hyperplasia, hypertrophy, and crystalline arrays) consistent with a mitochondrial neuropathy.

CONCLUSION

We identify a previously unreported cause of Charcot-Marie-Tooth (CMT) disease, a mutation in the mt-tRNA^Val , in a Venezuelan family. This work expands the list of CMT-associated genes from protein-coding genes to a mitochondrial tRNA gene. ANN NEUROL 2020;88:830-842.

Homoplasmic deleterious MT-ATP6/8 mutations in adult patients

To address the frequency of complex V defects, we systematically sequenced MT-ATP6/8 genes in 512 consecutive patients. We performed functional analysis in muscle or fibroblasts for 12 out of 27 putative homoplasmic mutations and in cybrids for four. Fibroblasts, muscle and cybrids with known deleterious mutations underwent parallel analysis. It included oxidative phosphorylation spectrophotometric assays, western blots, structural analysis, ATP production, glycolysis and cell proliferation evaluation. We demonstrated the deleterious nature of three original mutations. Striking gradation in severity of the mutations consequences and differences between muscle, fibroblasts and cybrids implied a likely under-diagnosis of human complex V defects.

The m.9143T>C Variant: Recurrent Infections and Immunodeficiency as an Extension of the Phenotypic Spectrum in MT-ATP6 Mutations?

Pathogenic variants in the MT-ATP6 are a well-known cause for maternally inherited mitochondrial disorders associated with a wide range of clinical phenotypes. Here, we present a 31- year old female with insulin-dependent diabetes mellitus, recurrent lactic acidosis and ketoacidosis recurrent infections with suspected immunodeficiency with T cell lymphopenia and hypogammaglobulinemia as well as proximal tetraparesis with severe muscle and limb pain and rapid physical exhaustion. Muscle biopsy and respiratory chain activities were normal. Single-exome sequencing revealed a variant in the MT-ATP6 gene: m.9143T>C. Analysis of further specimen of the index and mother (segregation studies) revealed the highest mutation load in muscle (99% level of mtDNA heteroplasmy) of the index patient. Interestingly, acute metabolic and physical decompensation during recurrent illness was documented to be a common clinical feature in patients with MT-ATP6 variants. However, it was not mentioned as a key symptom. Thus, we suggest that the clinical spectrum might be expanded in ATP6-associated diseases.

Mitochondrial disease in children

Mitochondrial disease presenting in childhood is characterized by clinical, biochemical and genetic complexity. Some children are affected by canonical syndromes, but the majority have nonclassical multisystemic disease presentations involving virtually any organ in the body. Each child has a unique constellation of clinical features and disease trajectory, leading to enormous challenges in diagnosis and management of these heterogeneous disorders. This review discusses the classical mitochondrial syndromes presenting most frequently in childhood and then presents an organ-based perspective including systems less frequently linked to mitochondrial disease, such as skin and hair abnormalities and immune dysfunction. An approach to diagnosis is then presented, encompassing clinical evaluation and biochemical, neuroimaging and genetic investigations, and emphasizing the problem of phenocopies. The impact of next-generation sequencing is discussed, together with the importance of functional validation of novel genetic variants never previously linked to mitochondrial disease. The review concludes with a brief discussion of currently available and emerging therapies. The field of mitochondrial medicine has made enormous strides in the last 30 years, with approaching 400 different genes across two genomes now linked to primary mitochondrial disease. However, many important questions remain unanswered, including the reasons for tissue specificity and variability of clinical presentation of individuals sharing identical gene defects, and a lack of disease-modifying therapies and biomarkers to monitor disease progression and/or response to treatment.

Charcot-Marie-Tooth disease and related disorders: an evolving landscape

PURPOSE OF REVIEW

Charcot-Marie-Tooth (CMT) disease and related disorders are the commonest group of inherited neuromuscular diseases and represent a heterogeneous group of disorders. This review will cover recent advances in genetic diagnosis and the evolving genetic and phenotype landscape of this disease group. We will review recent evidence of the increasingly recognized phenotypic overlap with other neurodegenerative conditions including hereditary spastic paraplegia, hereditary ataxias and mitochondrial diseases and highlight the importance of deep phenotyping to inform genetic diagnosis and prognosis.

RECENT FINDINGS

Through whole exome sequencing and multicentre collaboration new genes are being identified as causal for CMT expanding the genetic heterogeneity of this condition. In addition, an increasing number of variants have been identified in genes known to cause complex inherited diseases in which the peripheral neuropathy is part of the disorder and may be the presenting feature. The recent discovery of a repeat expansion in the RFC1 gene in cerebellar ataxia, neuropathy, vestibular areflexia syndrome highlights the prevalence of late-onset recessive conditions which have historically been considered to cause early-onset disease.

SUMMARY

CMT is an evolving field with considerable phenotypic and genetic heterogeneity and deep phenotyping remains a cornerstone in contemporary CMT diagnostics.

Expanding the molecular and phenotypic spectrum of truncating MT-ATP6 mutations

Objective

To describe the clinical and functional consequences of 1 novel and 1 previously reported truncating MT-ATP6 mutation.

Methods

Three unrelated probands with mitochondrial encephalomyopathy harboring truncating MT-ATP6 mutations are reported. Transmitochondrial cybrid cell studies were used to confirm pathogenicity of 1 novel variant, and the effects of all 3 mutations on ATPase 6 and complex V structure and function were investigated.

Results

Patient 1 presented with adult-onset cerebellar ataxia, chronic kidney disease, and diabetes, whereas patient 2 had myoclonic epilepsy and cerebellar ataxia; both harbored the novel m.8782G>A; p.(Gly86*) mutation. Patient 3 exhibited cognitive decline, with posterior white matter abnormalities on brain MRI, and severely impaired renal function requiring transplantation. The m.8618dup; p.(Thr33Hisfs*32) mutation, previously associated with neurogenic muscle weakness, ataxia, and retinitis pigmentosa, was identified. All 3 probands demonstrated a broad range of heteroplasmy across different tissue types. Blue-native gel electrophoresis of cultured fibroblasts and skeletal muscle tissue confirmed multiple bands, suggestive of impaired complex V assembly. Microscale oxygraphy showed reduced basal respiration and adenosine triphosphate synthesis, while reactive oxygen species generation was increased. Transmitochondrial cybrid cell lines studies confirmed the deleterious effects of the novel m.8782 G>A; p.(Gly86*) mutation.

Conclusions

We expand the clinical and molecular spectrum of MT-ATP6-related mitochondrial disorders to include leukodystrophy, renal disease, and myoclonic epilepsy with cerebellar ataxia. Truncating MT-ATP6 mutations may exhibit highly variable mutant levels across different tissue types, an important consideration during genetic counseling.

Early infantile-onset Leigh syndrome complicated with infantile spasms associated with the m.9185 T > C variant in the MT-ATP6 gene: Expanding the clinical spectrum

BACKGROUND

The mitochondrial DNA MT-ATP6 gene encodes the ATP6 subunit of the mitochondrial ATP synthase. The m.9185 T > C variant in MT-ATP6 has been reported to cause various neurological disorders including late-onset Leigh syndrome (LS). To our knowledge, there has been no reported case of infantile-onset LS associated with the m.9185 T > C variant. Herein, we report a patient with early-onset LS complicated with infantile spasms who exhibited profound developmental delay.

CASE REPORT

A 3-month-old Japanese girl presented with focal seizures. Brain magnetic resonance imaging (MRI) revealed bilateral lesions in the basal ganglia and cerebral peduncle. Laboratory evaluation demonstrated marked elevations of lactate and pyruvate in both venous blood and cerebrospinal fluid. At 6 months, she developed infantile spasms, which were ceased by adrenocorticotropic hormone therapy. At 2 years of age, she was bedridden due to hypotonic quadriplegia and was unable to make eye contact. Whole-exome sequencing identified apparently de novo homoplasmic m.9185 T > C variant in her blood.

CONCLUSION

This is the first case report describing early infantile-onset LS associated with the m.9185 T > C variant, and thereby broadens the phenotypic spectrum of m.9185 T > C-related disorders.

Next-generation sequencing in Charcot-Marie-Tooth disease: opportunities and challenges

Charcot-Marie-Tooth disease and the related disorders hereditary motor neuropathy and hereditary sensory neuropathy, collectively termed CMT, are the commonest group of inherited neuromuscular diseases, and they exhibit wide phenotypic and genetic heterogeneity. CMT is usually characterized by distal muscle atrophy, often with foot deformity, weakness and sensory loss. In the past decade, next-generation sequencing (NGS) technologies have revolutionized genomic medicine and, as these technologies are being applied to clinical practice, they are changing our diagnostic approach to CMT. In this Review, we discuss the application of NGS technologies, including disease-specific gene panels, whole-exome sequencing, whole-genome sequencing (WGS), mitochondrial sequencing and high-throughput transcriptome sequencing, to the diagnosis of CMT. We discuss the growing challenge of variant interpretation and consider how the clinical phenotype can be combined with genetic, bioinformatic and functional evidence to assess the pathogenicity of genetic variants in patients with CMT. WGS has several advantages over the other techniques that we discuss, which include unparalleled coverage of coding, non-coding and intergenic areas of both nuclear and mitochondrial genomes, the ability to identify structural variants and the opportunity to perform genome-wide dense homozygosity mapping. We propose an algorithm for incorporating WGS into the CMT diagnostic pathway.

Pathogenic variants in MT-ATP6: A United Kingdom-based mitochondrial disease cohort study

Distinct clinical syndromes have been associated with pathogenic MT-ATP6 variants. In this cohort study, we identified 125 individuals (60 families) including 88 clinically affected individuals and 37 asymptomatic carriers. Thirty-one individuals presented with Leigh syndrome and 7 with neuropathy ataxia retinitis pigmentosa. The remaining 50 patients presented with variable nonsyndromic features including ataxia, neuropathy, and learning disability. We confirmed maternal inheritance in 39 families and demonstrated that tissue segregation patterns and phenotypic threshold are variant dependent. Our findings suggest that MT-ATP6-related mitochondrial DNA disease is best conceptualized as a mitochondrial disease spectrum disorder and should be routinely included in genetic ataxia and neuropathy gene panels. ANN NEUROL 2019;86:310-315.

The 7q11.23 Protein DNAJC30 Interacts with ATP Synthase and Links Mitochondria to Brain Development

Despite the known causality of copy-number variations (CNVs) to human neurodevelopmental disorders, the mechanisms behind each gene's contribution to the constellation of neural phenotypes remain elusive. Here, we investigated the 7q11.23 CNV, whose hemideletion causes Williams syndrome (WS), and uncovered that mitochondrial dysfunction participates in WS pathogenesis. Dysfunction is facilitated in part by the 7q11.23 protein DNAJC30, which interacts with mitochondrial ATP-synthase machinery. Removal of Dnajc30 in mice resulted in hypofunctional mitochondria, diminished morphological features of neocortical pyramidal neurons, and altered behaviors reminiscent of WS. The mitochondrial features are consistent with our observations of decreased integrity of oxidative phosphorylation supercomplexes and ATP-synthase dimers in WS. Thus, we identify DNAJC30 as an auxiliary component of ATP-synthase machinery and reveal mitochondrial maladies as underlying certain defects in brain development and function associated with WS.

Mutations in COA7 cause spinocerebellar ataxia with axonal neuropathy

Several genes related to mitochondrial functions have been identified as causative genes of neuropathy or ataxia. Cytochrome c oxidase assembly factor 7 (COA7) may have a role in assembling mitochondrial respiratory chain complexes that function in oxidative phosphorylation. Here we identified four unrelated patients with recessive mutations in COA7 among a Japanese case series of 1396 patients with Charcot-Marie-Tooth disease (CMT) or other inherited peripheral neuropathies, including complex forms of CMT. We also found that all four patients had characteristic neurological features of peripheral neuropathy and ataxia with cerebellar atrophy, and some patients showed leukoencephalopathy or spinal cord atrophy on MRI scans. Validated mutations were located at highly conserved residues among different species and segregated with the disease in each family. Nerve conduction studies showed axonal sensorimotor neuropathy. Sural nerve biopsies showed chronic axonal degeneration with a marked loss of large and medium myelinated fibres. An immunohistochemical assay with an anti-COA7 antibody in the sural nerve from the control patient showed the positive expression of COA7 in the cytoplasm of Schwann cells. We also observed mildly elevated serum creatine kinase levels in all patients and the presence of a few ragged-red fibres and some cytochrome c oxidase-negative fibres in a muscle biopsy obtained from one patient, which was suggestive of subclinical mitochondrial myopathy. Mitochondrial respiratory chain enzyme assay in skin fibroblasts from the three patients showed a definitive decrease in complex I or complex IV. Immunocytochemical analysis of subcellular localization in HeLa cells indicated that mutant COA7 proteins as well as wild-type COA7 were localized in mitochondria, which suggests that mutant COA7 does not affect the mitochondrial recruitment and may affect the stability or localization of COA7 interaction partners in the mitochondria. In addition, Drosophila COA7 (dCOA7) knockdown models showed rough eye phenotype, reduced lifespan, impaired locomotive ability and shortened synaptic branches of motor neurons. Our results suggest that loss-of-function COA7 mutation is responsible for the phenotype of the presented patients, and this new entity of disease would be referred to as spinocerebellar ataxia with axonal neuropathy type 3.

Disorders of mitochondrial dynamics in peripheral neuropathy: Clues from hereditary neuropathy and diabetes

Peripheral neuropathy is a common and debilitating complication of diabetes and prediabetes. Recent clinical studies have identified an association between the development of neuropathy and dyslipidemia in prediabetic and diabetic patients. Despite the prevalence of this complication, studies identifying molecular mechanisms that underlie neuropathy progression in prediabetes or diabetes are limited. However, dysfunctional mitochondrial pathways in hereditary neuropathy provide feasible molecular targets for assessing mitochondrial dysfunction in neuropathy associated with prediabetes or diabetes. Recent studies suggest that elevated levels of dietary saturated fatty acids (SFAs) associated with dyslipidemia impair mitochondrial dynamics in sensory neurons by inducing mitochondrial depolarization, compromising mitochondrial bioenergetics, and impairing axonal mitochondrial transport. This causes lower neuronal ATP and apoptosis. Conversely, monounsaturated fatty acids (MUFAs) restore nerve and sensory mitochondrial function. Understanding the mitochondrial pathways that contribute to neuropathy progression in prediabetes and diabetes may provide therapeutic targets for the treatment of this debilitating complication.

MT-ATP6 mitochondrial disease variants: Phenotypic and biochemical features analysis in 218 published cases and cohort of 14 new cases

Mitochondrial complex V (CV) generates cellular energy as adenosine triphosphate (ATP). Mitochondrial disease caused by the m.8993T>G pathogenic variant in the CV subunit gene MT-ATP6 was among the first described human mitochondrial DNA diseases. Due to a lack of clinically available functional assays, validating the definitive pathogenicity of additional MT-ATP6 variants remains challenging. We reviewed all reportedMT-ATP6 disease cases ( n = 218) to date, to assess for MT-ATP6 variants, heteroplasmy levels, and inheritance correlation with clinical presentation and biochemical findings. We further describe the clinical and biochemical features of a new cohort of 14 kindreds with MT-ATP6 variants of uncertain significance. Despite extensive overlap in the heteroplasmy levels of MT-ATP6 variant carriers with and without a wide range of clinical symptoms, previously reported symptomatic subjects had significantly higher heteroplasmy load (p = 2.2 x 10-16 ). Pathogenic MT-ATP6 variants resulted in diverse biochemical features. The most common findings were reduced ATP synthesis rate, preserved ATP hydrolysis capacity, and abnormally increased mitochondrial membrane potential. However, no single biochemical feature was universally observed. Extensive heterogeneity exists among both clinical and biochemical features of distinct MT-ATP6 variants. Improved mechanistic understanding and development of consistent biochemical diagnostic analyses are needed to permit accurate pathogenicity assessment of variants of uncertain significance in MT-ATP6.

Photophobia as the Presenting Symptom of Internal Carotid Artery Dissection

A 53-year-old man developed right eye photophobia after a hypertensive crisis, followed by right-sided headache and retro-orbital pain. On examination, a right Horner syndrome was evident and an urgent brain and neck Computed Tomography confirmed the suspicion of an internal carotid artery (ICA) dissection. While photophobia may occur in many neurological disorders, to our knowledge, it has never been described when the ICA is involved. Possible hypotheses of photophobia caused by carotid artery dissection are discussed.

Adult-onset Leigh syndrome linked to the novel stop codon mutation m.6579G>A in MT-CO1

Adult-onset Leigh syndrome is a rare but important manifestation of mitochondrial disease. We report a 17 year old female who presented with subacute encephalopathy, brainstem and extrapyramidal signs, raised CSF lactate, and symmetrical hyperintensities in the basal ganglia on T2-weighted cerebral MRI. The presence of cytochrome c oxidase deficient fibres in muscle tissue prompted sequencing of the entire mitochondrial genome which revealed the novel stop codon mutation m.6579G>A; p.Gly226X in MT-CO1. Here we present the case and review the clinicopathological and molecular spectrum of previously reported MT-CO1 truncating mutations.

Serum vitamin D deficiency in subjects with severe acquired brain injury and relationship with functional severity

BACKGROUND

Vitamin D may be important for the development and function of the nervous system. Low serum vitamin D levels have been detected in several neurological diseases.

OBJECTIVE

To ascertain the relationship between 25(OH)D serum level and disability in subjects with severe acquired brain injury (sABI).

DESIGN

Prospective cross-sectional study Methods: Consecutive subjects with sABI admitted to neuro-rehabilitation were enrolled. A sample of subjects from the neurological ward was considered the control group. Vitamin D serum levels and blood parameters were measured at admission. Disability Rating Scale (DRS), Glasgow Outcome Scale (GOS), and Level of Cognitive Functioning (LCF) were used in assessing disability.

RESULTS

A total of 104 subjects (34 F, 70 M; mean age 53.9 ± 15.2 years) were enrolled: 54 (19 F, 35 M) with sABI and 50 (15 F, 35 M) subjects as control group. Deficient mean serum levels of vitamin D (19.2 ± 9.4 ng/mL) were detected in the subjects with sABI and a significant inverse correlation between vitamin D serum levels and DRS score was detected (p = 0.04).

CONCLUSION

Subjects with sABI showed vitamin D deficiency that might correlate to disability severity. The reason is unclear and might represent a secondary phenomenon resulting from the inflammatory process.

ATP Synthase Diseases of Mitochondrial Genetic Origin

Devastating human neuromuscular disorders have been associated to defects in the ATP synthase. This enzyme is found in the inner mitochondrial membrane and catalyzes the last step in oxidative phosphorylation, which provides aerobic eukaryotes with ATP. With the advent of structures of complete ATP synthases, and the availability of genetically approachable systems such as the yeast Saccharomyces cerevisiae, we can begin to understand these molecular machines and their associated defects at the molecular level. In this review, we describe what is known about the clinical syndromes induced by 58 different mutations found in the mitochondrial genes encoding membrane subunits 8 and a of ATP synthase, and evaluate their functional consequences with respect to recently described cryo-EM structures.

Leigh syndrome in individuals bearing m.9185T>C MTATP6 variant. Is hyperventilation a factor which starts its development?

Leigh syndrome (LS), subacute necrotizing encephalomyelopathy is caused by various genetic defects, including m.9185T>C MTATP6 variant. Mechanism of LS development remains unknown. We report on the acid-base status of three patients with m.9185T>C related LS. At the onset, it showed respiratory alkalosis, reflecting excessive respiration effort (hyperventilation with low pCO₂). In patient 1, the deterioration occurred in temporal relation to passive oxygen therapy. To the contrary, on the recovery, she demonstrated a relatively low respiratory drive, suggesting that a "hypoventilation" might be beneficial for m.9185T>C carriers. As long as circumstances of the development of LS have not been fully explained, we recommend to counteract hyperventilation and carefully dose oxygen in patients with m.9185T>C related LS.

Periodic paralysis

The periodic paralyses are a group of skeletal muscle channelopathies characterizeed by intermittent attacks of muscle weakness often associated with altered serum potassium levels. The underlying genetic defects include mutations in genes encoding the skeletal muscle calcium channel Ca_v1.1, sodium channel Na_v1.4, and potassium channels K_ir2.1, K_ir3.4, and possibly K_ir2.6. Our increasing knowledge of how mutant channels affect muscle excitability has resulted in better understanding of many clinical phenomena which have been known for decades and sheds light on some of the factors that trigger attacks. Insights into the pathophysiology are also leading to new therapeutic approaches.

A 2 bp deletion in the mitochondrial ATP 6 gene responsible for the NARP (neuropathy, ataxia, and retinitis pigmentosa) syndrome

Mitochondrial (mt) DNA-associated NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) syndrome is due to mutation in the MT-ATP6 gene. We report the case of a 18-year-old man who presented with deafness, a myoclonic epilepsy, muscle weakness since the age of 10 and further developed a retinitis pigmentosa and ataxia. The whole mtDNA analysis by next-generation sequencing revealed the presence of the 2 bp microdeletion m.9127-9128 del AT in the ATP6 gene at 82% heteroplasmy in muscle and to a lower load in blood (10-20%) and fibroblasts (50%). Using the patient's fibroblasts, we demonstrated a 60% reduction of the oligomycin-sensitive ATPase hydrolytic activity, a 40% decrease in the ATP synthesis and determination of the mitochondrial membrane potential using the fluorescent probe tetramethylrhodamine, ethyl ester indicated a significant reduction in oligomycin sensitivity. In conclusion, we demonstrated that this novel AT deletion in the ATP6 gene is pathogenic and responsible for the NARP syndrome.

Molecular Epidemiology of Charcot-Marie-Tooth Disease in Northern Ostrobothnia, Finland: A Population-Based Study

BACKGROUND

Charcot-Marie-Tooth (CMT) disease is the most common hereditary neuromuscular disorder with a population prevalence of 9.7-82.3/100,000. In this study, we have estimated the prevalence of CMT and its subtypes in Finland and examined the frequency of molecular etiologies.

METHODS

A population-based survey included adult patients with peripheral neuropathy from the province of Northern Ostrobothnia, Finland. Secondary causes of peripheral polyneuropathy were excluded and patients with clinical and neurophysiological features pertinent with CMT were included. Molecular diagnostics was carried out when DNA was available.

RESULTS

We found 107 subjects with CMT yielding a prevalence 34.6/100,000 in Northern Ostrobothnia. The heterozygous point mutation p.His123Arg in ganglioside induced differentiation associated protein 1 (GDAP1) was found in 31.5% and peripheral myelin protein 22 (PMP22) duplication in 16.9% of the affected. Point mutations in myelin protein zero, mitofusin 2, and gap junction protein beta 1 accounted for 6.7% of the cases. In addition, 18 persons had hereditary neuropathy with liability to pressure palsies and 15 of them carried the PMP22 deletion.

CONCLUSIONS

The prevalence of CMT in Northern Ostrobothnia, Finland, seems to be slightly higher than those in previous studies in European populations. Founder mutation in the GDAP1 gene accounts for a large part of the genetically defined CMT2 in Finland.

Episodic weakness and Charcot-marie-tooth disease due to a mitochondrial MT-ATP6 mutation

INTRODUCTION

Episodic muscle weakness is the hallmark of a heterogeneous group of disorders known as periodic paralysis. A majority are due to single nucleotide mutations causing membrane depolarization.

METHODS

We report 2 family members with chronic, slowly progressive, distal axonal neuropathy, or Charcot-Marie-Tooth disease type 2 (CMT2) and episodic weakness resembling periodic paralysis.

RESULTS

Next generation sequencing (NGS) identified a mitochondrial MT-ATP6 mutation m.9185T>C (p.Leu220Pro) in both patients, consistent with a previous report of an association with this phenotype. The episodic weakness has been responsive to acetazolamide therapy for a few decades. By contrast, the underlying axonal neuropathy is quite progressive despite treatment with acetazolamide.

CONCLUSIONS

Mitochondrial DNA mutations should be considered in patients with a history of episodic weakness and axonal inherited neuropathy (CMT2). The episodic weakness is responsive to acetazolamide therapy, and electrophysiological testing for periodic paralysis with a long exercise protocol is negative in these cases. Muscle Nerve 55: 922-927, 2017.

Human iPSC-Derived Neural Progenitors Are an Effective Drug Discovery Model for Neurological mtDNA Disorders

Mitochondrial DNA (mtDNA) mutations frequently cause neurological diseases. Modeling of these defects has been difficult because of the challenges associated with engineering mtDNA. We show here that neural progenitor cells (NPCs) derived from human induced pluripotent stem cells (iPSCs) retain the parental mtDNA profile and exhibit a metabolic switch toward oxidative phosphorylation. NPCs derived in this way from patients carrying a deleterious homoplasmic mutation in the mitochondrial gene MT-ATP6 (m.9185T>C) showed defective ATP production and abnormally high mitochondrial membrane potential (MMP), plus altered calcium homeostasis, which represents a potential cause of neural impairment. High-content screening of FDA-approved drugs using the MMP phenotype highlighted avanafil, which we found was able to partially rescue the calcium defect in patient NPCs and differentiated neurons. Overall, our results show that iPSC-derived NPCs provide an effective model for drug screening to target mtDNA disorders that affect the nervous system.

Neuromuscular Manifestations in Mitochondrial Diseases in Children

Mitochondrial diseases exhibit significant clinical and genetic heterogeneity. Mitochondria are highly dynamic organelles that are the major contributor of adenosine triphosphate, through oxidative phosphorylation. These disorders may be developed at any age, with isolated or multiple system involvement, and in any pattern of inheritance. Defects in the mitochondrial respiratory chain impair energy production and almost invariably involve skeletal muscle and peripheral nerves, causing exercise intolerance, cramps, recurrent myoglobinuria, or fixed weakness, which often affects extraocular muscles and results in droopy eyelids (ptosis), progressive external ophthalmoplegia, peripheral ataxia, and peripheral polyneuropathy. This review describes the main neuromuscular symptomatology through different syndromes reported in the literature and from our experience. We want to highlight the importance of searching for the "clue clinical signs" associated with inheritance pattern as key elements to guide the complex diagnosis process and genetic studies in mitochondrial diseases.

A novel mutation m.8561C>G in MT-ATP6/8 causing a mitochondrial syndrome with ataxia, peripheral neuropathy, diabetes mellitus, and hypergonadotropic hypogonadism

Defects in the respiratory chain or mitochondrial ATP synthase (complex V) result in mitochondrial dysfunction that is an important cause of inherited neurological disease. Two of the subunits of complex V are encoded by MT-ATP6 and MT-ATP8 in the mitochondrial genome. Pathogenic mutations in MT-ATP6 are associated with the Leigh syndrome, the syndrome of neuropathy, ataxia, and retinitis pigmentosa (NARP), as well as with non-classical phenotypes, while MT-ATP8 is less frequently mutated in patients with mitochondrial disease. We investigated two adult siblings presenting with features of cerebellar ataxia, peripheral neuropathy, diabetes mellitus, sensorineural hearing impairment, and hypergonadotropic hypogonadism. As the phenotype was suggestive of mitochondrial disease, mitochondrial DNA was sequenced and a novel heteroplasmic mutation m.8561C>G in the overlapping region of the MT-ATP6 and MT-ATP8 was found. The mutation changed amino acids in both subunits. Mutation heteroplasmy correlated with the disease phenotype in five family members. An additional assembly intermediate of complex V and increased amount of subcomplex F₁ were observed in myoblasts of the two patients, but the total amount of complex V was unaffected. Furthermore, intracellular ATP concentration was lower in patient myoblasts indicating defective energy production. We suggest that the m.8561C>G mutation in MT-ATP6/8 is pathogenic, leads biochemically to impaired assembly and decreased ATP production of complex V, and results clinically in a phenotype with the core features of cerebellar ataxia, peripheral neuropathy, diabetes mellitus, and hypergonadotropic hypogonadism.

Neurophysiological profile of peripheral neuropathy associated with childhood mitochondrial disease

INTRODUCTION

Peripheral nerve involvement is common in mitochondrial disease but often unrecognised due to the prominent central nervous system features. Identification of the underlying neuropathy may assist syndrome classification, targeted genetic testing and rehabilitative interventions.

METHODS

Clinical data and the results of nerve conduction studies were obtained retrospectively from the records of four tertiary children's hospital metabolic disease, neuromuscular or neurophysiology services. Nerve conductions studies were also performed prospectively on children attending a tertiary metabolic disease service. Results were classified and analysed according to the underlying genetic cause.

RESULTS

Nerve conduction studies from 27 children with mitochondrial disease were included in the study (mitochondrial DNA (mtDNA) - 7, POLG - 7, SURF1 - 10, PDHc deficiency - 3). Four children with mtDNA mutations had a normal study while three had mild abnormalities in the form of an axonal sensorimotor neuropathy when not acutely unwell. One child with MELAS had a severe acute axonal motor neuropathy during an acute stroke-like episode that resolved over 12months. Five children with POLG mutations and disease onset beyond infancy had a sensory ataxic neuropathy with an onset in the second decade of life, while the two infants with POLG mutations had a demyelinating neuropathy. Seven of the 10 children with SURF1 mutations had a demyelinating neuropathy. All three children with PDHc deficiency had an axonal sensorimotor neuropathy. Unlike CMT, the neuropathy associated with mitochondrial disease was not length-dependent.

CONCLUSIONS

This is the largest study to date of peripheral neuropathy in genetically- classified childhood mitochondrial disease. Characterising the underlying neuropathy may assist with the diagnosis of the mitochondrial syndrome and should be an integral part of the assessment of children with suspected mitochondrial disease.

Pathogenic mechanisms underlying X-linked Charcot-Marie-Tooth neuropathy (CMTX6) in patients with a pyruvate dehydrogenase kinase 3 mutation

Charcot-Marie-Tooth disease (CMT) is the most common inherited peripheral neuropathy. An X-linked form of CMT (CMTX6) is caused by a missense mutation (R158H) in the pyruvate dehydrogenase kinase isoenzyme 3 (PDK3) gene. PDK3 is one of 4 isoenzymes that negatively regulate the activity of the pyruvate dehydrogenase complex (PDC) by reversible phosphorylation of its first catalytic component pyruvate dehydrogenase (designated as E1). Mitochondrial PDC catalyses the oxidative decarboxylation of pyruvate to acetyl CoA and links glycolysis to the energy-producing Krebs cycle. We have previously shown the R158H mutation confers PDK3 enzyme hyperactivity. In this study we demonstrate that the increased PDK3 activity in patient fibroblasts (PDK3(R158H)) leads to the attenuation of PDC through hyper-phosphorylation of E1 at selected serine residues. This hyper-phosphorylation can be reversed by treating the PDK3(R158H) fibroblasts with the PDK inhibitor dichloroacetate (DCA). In the patient cells, down-regulation of PDC leads to increased lactate, decreased ATP and alteration of the mitochondrial network. Our findings highlight the potential to develop specific drug targeting of the mutant PDK3 as a therapeutic approach to treating CMTX6.

New perspective in diagnostics of mitochondrial disorders: two years' experience with whole-exome sequencing at a national paediatric centre

Background

Whole-exome sequencing (WES) has led to an exponential increase in identification of causative variants in mitochondrial disorders (MD).

Methods

We performed WES in 113 MD suspected patients from Polish paediatric reference centre, in whom routine testing failed to identify a molecular defect. WES was performed using TruSeqExome enrichment, followed by variant prioritization, validation by Sanger sequencing, and segregation with the disease phenotype in the family.

Results

Likely causative mutations were identified in 67 (59.3 %) patients; these included variants in mtDNA (6 patients) and nDNA: X-linked (9 patients), autosomal dominant (5 patients), and autosomal recessive (47 patients, 11 homozygotes). Novel variants accounted for 50.5 % (50/99) of all detected changes. In 47 patients, changes in 31 MD-related genes (ACAD9, ADCK3, AIFM1, CLPB, COX10, DLD, EARS2, FBXL4, MTATP6, MTFMT, MTND1, MTND3, MTND5, NAXE, NDUFS6, NDUFS7, NDUFV1, OPA1, PARS2, PC, PDHA1, POLG, RARS2, RRM2B, SCO2, SERAC1, SLC19A3, SLC25A12, TAZ, TMEM126B, VARS2) were identified. The ACAD9, CLPB, FBXL4, PDHA1 genes recurred more than twice suggesting higher general/ethnic prevalence. In 19 cases, variants in 18 non-MD related genes (ADAR, CACNA1A, CDKL5, CLN3, CPS1, DMD, DYSF, GBE1, GFAP, HSD17B4, MECP2, MYBPC3, PEX5, PGAP2, PIGN, PRF1, SBDS, SCN2A) were found. The percentage of positive WES results rose gradually with increasing probability of MD according to the Mitochondrial Disease Criteria (MDC) scale (from 36 to 90 % for low and high probability, respectively). The percentage of detected MD-related genes compared with non MD-related genes also grew with the increasing MD likelihood (from 20 to 97 %). Molecular diagnosis was established in 30/47 (63.8 %) neonates and in 17/28 (60.7 %) patients with basal ganglia involvement. Mutations in CLPB, SERAC1, TAZ genes were identified in neonates with 3-methylglutaconic aciduria (3-MGA) as a discriminative feature. New MD-related candidate gene (NDUFB8) is under verification.

Conclusions

We suggest WES rather than targeted NGS as the method of choice in diagnostics of MD in children, including neonates with 3-MGA aciduria, who died without determination of disease cause and with limited availability of laboratory data. There is a strong correlation between the degree of MD diagnosis by WES and MD likelihood expressed by the MDC scale.

Electronic supplementary material

The online version of this article (doi:10.1186/s12967-016-0930-9) contains supplementary material, which is available to authorized users.