Neuropathy, Ataxia, And Retinitis Pigmentosa Research Articles

A novel MT-ATP6 variant associated with complicated ataxia in two unrelated Italian patients: case report and functional studies

BackgroundMT-ATP6 is a mitochondrial gene which encodes for the intramembrane subunit 6 (or A) of the mitochondrial ATP synthase, also known asl complex V, which is involved in the last step of oxidative phosphorylation to produce cellular ATP through aerobic metabolism. Although classically associated with the NARP syndrome, recent evidence highlights an important role of MT-ATP6 pathogenic variants in complicated adult-onset ataxias.MethodsWe describe two unrelated patients with adult-onset cerebellar ataxia associated with severe optic atrophy and mild cognitive impairment. Whole mitochondrial DNA sequencing was performed in both patients. We employed patients’ primary fibroblasts and cytoplasmic hybrids (cybrids), generated from patients-derived cells, to assess the activity of respiratory chain complexes, oxygen consumption rate (OCR), ATP production and mitochondrial membrane potential.ResultsIn both patients, we identified the same novel m.8777 T > C variant in MT-ATP6 with variable heteroplasmy level in different tissues. We identifed an additional heteroplasmic novel variant in MT-ATP6, m.8879G > T, in the patients with the most severe phenotype. A significant reduction in complex V activity, OCR and ATP production was observed in cybrid clones homoplasmic for the m.8777 T > C variant, while no functional defect was detected in m.8879G > T homoplasmic clones. In addition, fibroblasts with high heteroplasmic levelsof m.8777 T > C variant showed hyperpolarization of mitochondrial membranes.ConclusionsWe describe a novel pathogenic mtDNA variant in MT-ATP6 associated with adult-onset ataxia, reinforcing the value of mtDNA screening within the diagnostic workflow of selected patients with late onset ataxias.

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.

Modeling mitochondrial DNA diseases: from base editing to pluripotent stem-cell-derived organoids.

Mitochondrial DNA (mtDNA) diseases are multi-systemic disorders caused by mutations affecting a fraction or the entirety of mtDNA copies. Currently, there are no approved therapies for the majority of mtDNA diseases. Challenges associated with engineering mtDNA have in fact hindered the study of mtDNA defects. Despite these difficulties, it has been possible to develop valuable cellular and animal models of mtDNA diseases. Here, we describe recent advances in base editing of mtDNA and the generation of three-dimensional organoids from patient-derived human-induced pluripotent stem cells (iPSCs). Together with already available modeling tools, the combination of these novel technologies could allow determining the impact of specific mtDNA mutations in distinct human cell types and might help uncover how mtDNA mutation load segregates during tissue organization. iPSC-derived organoids could also represent a platform for the identification of treatment strategies and for probing the in vitro effectiveness of mtDNA gene therapies. These studies have the potential to increase our mechanistic understanding of mtDNA diseases and may open the way to highly needed and personalized therapeutic interventions.

Retinal involvement in mitochondrial disease

AbstractRetinal involvement in mitochondrial disease is a variable feature of mitochondrial dysfunction. The retinal pigment epithelium and photoreceptor layers can both be affected. It manifests with signs ranging from asymptomatic peripheral salt‐and‐pepper retinopathy to classic bone‐spicule pigmentary changes. The consequences can be progressive mild or severe visual field constriction and central visual acuity loss. The pathophysiological mechanisms involved in retinal dysfunction in mitochondrial disease are still being elucidated and may include a range of metabolic and bioenergetic defects.Mitochondrial conditions affecting the eye that display prominent outer retinal involvement include.(a) mitochondrial encephalomyopathy, lactic acidosis and stroke‐like episodes (MELAS) secondary to the m.3243A > G mtDNA mutation in MT‐TL1,(b) neuropathy, ataxia and retinitis pigmentosa (NARP) secondary to the m.8993 T > G mtDNA mutation in MT‐ATP6,(c) chronic progressive external ophthalmoplegia (CPEO), in particular the Kearns‐Sayre syndrome (KSS) secondary to single large‐scale mtDNA deletions, and.(d) some newer entities, such as optic neuropathy and pigmentary retinopathy seen in patients with mutations in the gene SSBP1.The features and diagnostic pathway for these conditions will be illustrated and discussed.

Amelioration of Leigh syndrome induced by mouse blastocyst complementation with a mutant human mitochondrial ATP synthase 6

AbstractBackgroundMutation of the m. 8993 T > G ATP6 subunit of ATP synthase causes a maternally inherited Leigh Syndrome (LS), a rapidly fatal encephalomyelopathy in childhood called and related Neuropathy, Ataxia and Retinitis Pigmentosa (NARP) in adults. There is no cure, and relevant animal models are few. Here we describe a novel ATP6 mouse model of LS and preclinical efficacy of a gene therapy approach to neutralize the ATP6 mutation by competition with a mitochondrial targeted adeno‐associated virus serotype 9 (AAV9) vector containing wild‐type ATP6.MethodsLS transgenic mice with an ATP6 mutation were developed by microinjecting AAV2 modified by a COX8 mitochondrial targeting sequence (MTS) into mouse blastocysts to deliver the human ATP6 gene with a T > G mutation at position 8993 of the mitochondrial genome, responsible for clinical LS DNA, protein, and metabolic assays confirmed functional gene delivery and in depth physiological/clinical assessments supported germline transmission of an LS phenotype. Rescue of LS in this model was evaluated after intravenous injection of mito‐targeted AAV9 containing wild‐type ATP6 followed by reevaluation of phenotypes.ResultsLS transgenic mice expressed mutant ATP6 protein in multiple relevant tissues including brain, eye, liver. Mutant ATP6 incorporated into Complex V and conferred decreased ATP synthase efficiency. Mice exhibited hallmarks of the human disease with one or multiple systemic symptoms including early death, paralysis, hunching, vision loss and seizures over six generations. Necropsy revealed spongiform encephalopathy, retinal degeneration, and cardiomegaly. Rescue by mitochondrially directed AAV9 containing wild type ATP6 after disease onset conferred prolonged survival and reduced paralysis. When administered prior to disease onset all treated ATP6 mice survived with evidence of globally improved function.ConclusionsThe results are consistent with efficient delivery and expression of foreign genes in mitochondria by AAV vectors directed by MTS and support further development of such strategies via intravenous injection of mitochondrial targeted AAV9 to deliver corrective genes.

Novel Pathogenic Sequence Variation m.5789T>C Causes NARP Syndrome and Promotes Formation of Deletions of the Mitochondrial Genome.

Background and ObjectivesWe report the pathogenic sequence variant m.5789T>C in the anticodon stem of the mitochondrial tRNA for cysteine as a novel cause of neuropathy, ataxia, and retinitis pigmentosa (NARP), which is usually associated with pathogenic variants in the MT-ATP6 gene.MethodsTo address the correlation of oxidative phosphorylation deficiency with mutation loads, we performed genotyping on single laser-dissected skeletal muscle fibers. Stability of the mitochondrial tRNACys was investigated by Northern blotting. Accompanying deletions of the mitochondrial genome were detected by long-range PCR and their breakpoints were determined by sequencing of single-molecule amplicons.ResultsThe sequence variant m.5789T>C, originating from the patient's mother, decreases the stability of the mitochondrial tRNA for cysteine by disrupting the anticodon stem, which subsequently leads to a combined oxidative phosphorylation deficiency. In parallel, we observed a prominent cluster of low-abundance somatic deletions with breakpoints in the immediate vicinity of the m.5789T>C variant. Strikingly, all deletion-carrying mitochondrial DNA (mtDNA) species, in which the corresponding nucleotide position was not removed, harbored the mutant allele, and none carried the wild-type allele.DiscussionIn addition to providing evidence for the novel association of a tRNA sequence alteration with NARP syndrome, our observations support the hypothesis that single nucleotide changes can lead to increased occurrence of site-specific mtDNA deletions through the formation of an imperfect repeat. This finding might be relevant for understanding mechanisms of deletion generation in the human mitochondrial genome.

The screening by isoelectric focusing of transferrin for the diagnosis of congenital disorders of glycosylation

Background: Congenital Disorders of Glycosylation (CDG) are a group of inherited metabolic disorders caused by the defect in various steps in the biosynthesis of glycoproteins and other glycoconjugates. Material and methods: 40 patients under clinical suspicions for CDG at the Institute of Mother and Child were examined by isoelectric focusing of transferrin (IEFT) in collaboration with RadboudUMC, Netherlands and U.S.A. The spectrum of clinical presentations of these patients was multisystem damage, predominantly neurological manifestations. Results: Most of the patients (55%) had early neurological manifestations from the birth, such as hypotonia, psychomotor disability, cerebral MRI abnormalities, seizures (25%), cutis laxa (17.5%), total alopecia (2.5%), abnormal fat pads (2.5%), myopia (7.5%), nystagmus (5%), strabismus (2.5%), stroke-like episodes (2.5.%), ataxia (7.5%), abnormal coagulation (10%), hepatomegaly (35%) and liver cirrhosis (2.5%). Serum samples analyzed by IEFT showed the results: 37 normal, 2 questionable and 1 abnormal patterns. Two samples questionable belongs to the patients with Galactosemia and Fructosemia, which give the false-positive results. The last positive sample is performed additionally for glycomics profiling. In some cases, with IEFT negative profile was performed genetic test and were diagnosed other diseases, mimicking CDG, such as: NARP syndrome, late diagnosed PKU, GSD, Manosidoses, Prader-Willi Syndrome and chromosomal aberrations. Conclusions: The CDG is a rare metabolic disease with multisystem impairment and variety of symptoms which determine overlapping of phenotype with other genetic disorders. The process of diagnosis is very complex and can take several years.

A Theoretical Model of Mitochondrial ATP Synthase Deficiencies. The Role of Mitochondrial Carriers

The m.8993T>G mutation of the mitochondrial MT-ATP6 gene is associated with NARP syndrome (neuropathy, ataxia and retinitis pigmentosa). The equivalent point mutation introduced in yeast Saccharomyces cerevisiae mitochondrial DNA considerably reduced the activity of ATP synthase and of cytochrome-c-oxidase, preventing yeast growth on oxidative substrates. The overexpression of the mitochondrial oxodicarboxylate carrier (Odc1p) was able to rescue the growth on the oxidative substrate by increasing the substrate-level phosphorylation of ADP coupled to the conversion of α-ketoglutarate (AKG) into succinate with an increase in Complex IV activity. Previous studies showed that equivalent point mutations in ATP synthase behave similarly and can be rescued by Odc1p overexpression and/or the uncoupling of OXPHOS from ATP synthesis. In order to better understand the mechanism of the ATP synthase mutation bypass, we developed a core model of mitochondrial metabolism based on AKG as a respiratory substrate. We describe the different possible metabolite outputs and the ATP/O ratio values as a function of ATP synthase inhibition.

Mitochondrial Retinopathy

To report the retinal phenotype and the associated genetic and systemic findings in patients with mitochondrial disease. Retrospective case series. Twenty-three patients with retinopathy and mitochondrial disease, including chronic progressive external ophthalmoplegia (CPEO), maternally inherited diabetes and deafness (MIDD), mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), Kearns-Sayre syndrome, neuropathy, ataxia, and retinitis pigmentosa (NARP) syndrome, and other systemic manifestations. Review of case notes, retinal imaging, electrophysiologic assessment, molecular genetic testing including protein modeling, and histologic analysis of muscle biopsy. Phenotypic characteristics of mitochondrial retinopathy. Genetic testing identified sporadic large-scale mitochondrial DNA deletions and variants in MT-TL1, MT-ATP6, MT-TK, MT-RNR1, or RRM2B. Based on retinal imaging, 3 phenotypes could be differentiated: type 1 with mild, focal pigmentary abnormalities; type 2 characterized by multifocal white-yellowish subretinal deposits and pigment changes limited to the posterior pole; and type 3 with widespread granular pigment alterations. Advanced type 2 and 3 retinopathy presented with chorioretinal atrophy that typically started in the peripapillary and paracentral areas with foveal sparing. Two patients exhibited a different phenotype: 1 revealed an occult retinopathy, and the patient with RRM2B-associated retinopathy showed no foveal sparing, no severe peripapillary involvement, and substantial photoreceptor atrophy before loss of the retinal pigment epithelium. Two patients with type 1 disease showed additional characteristics of mild macular telangiectasia type 2. Patients with type 1 and mild type 2 or 3 disease demonstrated good visual acuity and no symptoms associated with the retinopathy. In contrast, patients with advanced type 2 or 3 disease often reported vision problems in dim light conditions, reduced visual acuity, or both. Short-wavelength autofluorescence usually revealed a distinct pattern, and near-infrared autofluorescence may be severely reduced in type 3 disease. The retinal phenotype was key to suspecting mitochondrial disease in 11 patients, whereas 12 patients were diagnosed before retinal examination. Different types of mitochondrial retinopathy show characteristic features. Even in absence of visual symptoms, their recognition may facilitate the often challenging and delayed diagnosis of mitochondrial disease, in particular in patients with mild or nebulous multisystem disease.

SAT-234 Association Between Hypogonadotrophic Hypogonadism and NARP Due to Mitochondrial Disease

Introduction: Hypogonadotropic hypogonadism is a common disorder encountered in endocrine practice with a prevalence of 1 in 10,000 men (1). We describe a patient with possible mitochondrial mutation causing him to have concurrent central hypogonadism and NARP syndrome (neuropathy, ataxia, retinitis pigmentosa). Case: 65 year old male with central hypogonadism diagnosed at age 17 was on testosterone replacement therapy off and on since then. He was diagnosed with NARP at age 49. On exam, he was OX3, BP 147/83mmHg, HR 63 bpm, wt 185lbs. Pupils were sluggish to light. Unable to assess visual fields as patient is legally blind. No gynecomastia. Cardiac and pulmonary exam were normal. Testicles were soft and the volume was 12mL. Prostate was enlarged, non tender and without nodules. He had ataxic gait, abnormal finger to nose test and strength was normal. LH <0.2 mIU/ml (N 1.2 - 10.6 mIU/ml), FSH < 0.2 mIU/ml (N 0.7 - 10.8mIU), total testosterone as low as 12ng/dL when off and 260ng/dL (N 250-1110ng/dL) while on testosterone IM. CBC, CMP, prolactin, ACTH, cortisol, Igf-I, TSH and PSA wnl. MRI brain showed cerebellar atrophy and normal pituitary. Discussion: NARP is a mitochondrial disorder characterized by neuropathy, ataxia and retinitis pigmentosa primarily affecting the nervous system. NARP results from point mutation at base pair 8993 of the mitochondrial genome in the ATPase 6 gene. Symptoms typically consist of proximal neurogenic muscle weakness, sensory neuropathy, ataxia, and retinitis pigmentosa. According to Al-Gadi et al, hypogonadism has been reported in mitochondrial disease (2). RRM2B is a nuclear-encoded mitochondrial maintenance gene. Mitochondrial DNA depletion due to ar-RRM2B mutations have been reported in association with hypogonadism. A mutation of m.8561C>G in MT-ATP6/8 causing a mitochondrial syndrome with ataxia, peripheral neuropathy, diabetes mellitus, and hypergonadotrophic hypogonadism has also been reported in a case report of two patients (3). Upon literature review, no prior cases of hypogonadotropic hypogonadism in patients with NARP have been reported. The impairment of the hypothalamic-pituitary axis appears to manifest in severe mitochondrial phenotypes. For example, the association of central hypogonadism with Kearns Sayre Syndrome (KSS). Given that both disorders are related to mitochondrial dysfunction, we propose that the hypogonadism may be linked to NARP syndrome. References Rohayem, J.et al. (2016), Causes of hypogonadotropic hypogonadism predict response to gonadotropin substitution in adults. Andrology Al-Gadi.et al. (2018). Endocrine Disorders in Primary Mitochondrial Disease. JES Kytövuori, L. et al. (2016, August 8). A novel mutation m.8561C>G in MT- ATP6/8 causing a mitochondrial syndrome with ataxia, peripheral neuropathy, diabetes mellitus, and hypergonadotropic hypogonadism.

Delineating MT-ATP6-associated disease: From isolated neuropathy to early onset neurodegeneration.

ObjectiveTo delineate the phenotypic and genotypic spectrum in carriers of mitochondrial MT-ATP6 mutations in a large international cohort.MethodsWe analyzed in detail the clinical, genetical, and neuroimaging data from 132 mutation carriers from national registries and local databases from Europe, USA, Japan, and China.ResultsWe identified 113 clinically affected and 19 asymptomatic individuals with a known pathogenic MT-ATP6 mutation. The most frequent mutations were m.8993 T > G (53/132, 40%), m.8993 T > C (30/132, 23%), m.9176 T > C (30/132, 23%), and m.9185 T > C (12/132, 9%). The degree of heteroplasmy was high both in affected (mean 95%, range 20%–100%) and unaffected individuals (mean 73%, range 20%–100%). Age at onset ranged from prenatal to the age of 75 years, but almost half of the patients (49/103, 48%) became symptomatic before their first birthday. In 28 deceased patients, the median age of death was 14 months. The most frequent symptoms were ataxia (81%), cognitive dysfunction (49%), neuropathy (48%), seizures (37%), and retinopathy (14%). A diagnosis of Leigh syndrome was made in 55% of patients, whereas the classic syndrome of neuropathy, ataxia, and retinitis pigmentosa (NARP) was rare (8%).ConclusionsIn this currently largest series of patients with mitochondrial MT-ATP6 mutations, the phenotypic spectrum ranged from asymptomatic to early onset multisystemic neurodegeneration. The degree of mutation heteroplasmy did not reliably predict disease severity. Leigh syndrome was found in more than half of the patients, whereas classic NARP syndrome was rare. Oligosymptomatic presentations were rather frequent in adult-onset patients, indicating the need to include MT-ATP6 mutations in the differential diagnosis of both ataxias and neuropathies.

Heteroplasmy and phenotype spectrum of the mitochondrial tRNALeu (UUR) gene m.3243A>G mutation in seven Han Chinese families

The m.3243A > G mutation in the mitochondrial tRNALeu (UUR) gene is associated with a variety of phenotypic heterogeneity. The clinical spectrum and phenotypic-genotypic correlations in the Chinese patients are poorly understood. In the present study, we reported the clinical and genetic characterization, as well as haplogroups of seven Han Chinese families carrying the m.3243A > G mutation. Of the 39 matrilineal individuals, five suffered from mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes (MELAS), two had life-threatening mitochondrial myopathy (LTMM), and one patient had neuropathy, ataxia, and retinitis pigmentosa (NARP)-like syndrome. The LTMM and NARP like syndromes enriched the phenotypic profile of the m.3243A > G mutation. The heteroplasmy of the m.3243A > G mutation ranged from 16% to 59% in MELAS, 29% to 79% in LTMM, and 57% in a NARP-like syndrome patient. The levels ranged from 0% to 14% in patients that manifested with pure diabetes and pure hearing loss, and 0% to 5% in 13 normal family members. However, we particularly noticed heteroplasmy in four asymptomatic individuals in one LTMM family carried the heteroplasmy mutation ranged from 22% to 78%, implying that there were other modifying factors in this family. The modulation of the phenotype of mtDNA mutations requires further investigation.

Deregulating mitochondrial metabolite and ion transport has beneficial effects in yeast and human cellular models for NARP syndrome.

The m.8993T>G mutation of the mitochondrial MT-ATP6 gene has been associated with numerous cases of neuropathy, ataxia and retinitis pigmentosa and maternally inherited Leigh syndrome, which are diseases known to result from abnormalities affecting mitochondrial energy transduction. We previously reported that an equivalent point mutation severely compromised proton transport through the ATP synthase membrane domain (FO) in Saccharomyces cerevisiae and reduced the content of cytochrome c oxidase (Complex IV or COX) by 80%. Herein, we report that overexpression of the mitochondrial oxodicarboxylate carrier (Odc1p) considerably increases Complex IV abundance and tricarboxylic acid-mediated substrate-level phosphorylation of ADP coupled to conversion of α-ketoglutarate into succinate in m.8993T>G yeast. Consistently in m.8993T>G yeast cells, the retrograde signaling pathway was found to be strongly induced in order to preserve α-ketoglutarate production; when Odc1p was overexpressed, this stress pathway returned to an almost basal activity. Similar beneficial effects were induced by a partial uncoupling of the mitochondrial membrane with the proton ionophore, cyanide m-chlorophenyl hydrazone. This chemical considerably improved the glutamine-based, respiration-dependent growth of human cytoplasmic hybrid cells that are homoplasmic for the m.8993T>G mutation. These findings shed light on the interdependence between ATP synthase and Complex IV biogenesis, which could lay the groundwork for the creation of nutritional or metabolic interventions for attenuating the effects of mtDNA mutations.

Retinal abiotrophy in mitochondrial pathology: NARP sindrome (a clinical case)

Purpose: to present a clinical case of retinal abiotrophy in mitochondrial pathology (NARP syndrome) caused by the mutation m.8993T&gt;G in the ATPase gene type 6 in order to improve the diagnosis of hereditary abiotrophies. Material and methods. The results of a clinical molecular genetic examination of the patient’s family, undertaken in order to clarify the diagnosis and determine the genetic risk, are presented. The family was found to have an isolated pathology of the eye. Results. DNA studies by MLPA method and the analysis of clinical data in the family revealed a hereditary syndromic pathology which caused changes in the eyes. The inheritance type was found to be maternal. Conclusion. NARP syndrome is a syndrome with the maternal type of inheritance in which retinal abiotrophy is primarily associated with the mutation m.8993T&gt;G mtDNA and can be considered as the main diagnostic feature among other clinical manifestations. The case demonstrates the difficulties of diagnosing hereditary syndromes accompanied by eye pathology.

Muscle pain in mitochondrial diseases: a picture from the Italian network.

Muscle pain may be part of many neuromuscular disorders including myopathies, peripheral neuropathies and lower motor neuron diseases. Although it has been reported also in mitochondrial diseases (MD), no extensive studies in this group of diseases have been performed so far. We reviewed clinical data from 1398 patients affected with mitochondrial diseases listed in the database of the "Nation-wide Italian Collaborative Network of Mitochondrial Diseases", to assess muscle pain and its features. Muscle pain was present in 164 patients (11.7%). It was commonly observed in subjects with chronic progressive external ophthalmoplegia (cPEO) and with primary myopathy without cPEO, but also-although less frequently-in multisystem phenotypes such as MELAS, MERFF, Kearns Sayre syndrome, NARP, MNGIE and Leigh syndrome. Patients mainly complain of diffuse exercise-related muscle pain, but focal/multifocal and at rest myalgia were often also reported. Muscle pain was more commonly detected in patients with mitochondrial DNA mutations (67.8%) than with nuclear DNA changes (32.2%). Only 34% of the patients showed a good response to drug therapy. Interestingly, patients with nuclear DNA mutations tend to have a better therapeutic response than patients with mtDNA mutations. Muscle pain is present in a significant number of patients with MD, being one of the most common symptoms. Although patients with a myopathic phenotype are more prone to develop muscle pain, this is also observed in patients with a multi system involvement, representing an important and disabling symptom having poor response to current therapy.

Sleep and circadian defects in a Drosophila model of mitochondrial encephalomyopathy

Mitochondrial encephalomyopathies (ME) are complex, incurable diseases characterized by severe bioenergetic distress that can affect the function of all major organ systems but is especially taxing to neuromuscular tissues. Animal models of MEs are rare, but the Drosophila ATP61 mutant is a stable, well-characterized genetic line that accurately models progressive human mitochondrial diseases such as Maternally-Inherited Leigh Syndrome (MILS), Neuropathy, Ataxia, and Retinitis Pigmentosa (NARP), and Familial Bilateral Striatal Necrosis (FBSN). While it is established that this model exhibits important hallmarks of ME, including excess cellular and mitochondrial reactive oxygen species, shortened lifespan, muscle degeneration, and stress-induced seizures, it is unknown whether it exhibits defects in sleep or circadian function. This is a clinically relevant question, as many neurological and neurodegenerative diseases are characterized by such disturbances, which can exacerbate other symptoms and worsen quality of life. Since Drosophila is highly amenable to sleep and circadian studies, we asked whether we could detect disease phenotypes in the circadian behaviors of ATP61. Indeed, we found that day-time and night-time activity and sleep are altered through disease progression, and that circadian patterns are disrupted at both the behavioral and neuronal levels. These results establish ATP61 as an important model of sleep and circadian disruption in ME that can be studied mechanistically at the molecular, cellular, and behavioral level to uncover underlying pathophysiology and test novel therapies.

Functional investigation of an universally conserved leucine residue in subunit a of ATP synthase targeted by the pathogenic m.9176 T>G mutation

Protons are transported from the mitochondrial matrix to the intermembrane space of mitochondria during the transfer of electrons to oxygen and shuttled back to the matrix by the a subunit and a ring of identical c subunits across the membrane domain (FO) of ATP synthase, which is coupled to ATP synthesis. A mutation (m.9176 T > G) of the mitochondrial ATP6 gene that replaces an universally conserved leucine residue into arginine at amino acid position 217 of human subunit a (aL217R) has been associated to NARP (Neuropathy, Ataxia and Retinitis Pigmentosa) and MILS (Maternally Inherited Leigh's Syndrome) diseases. We previously showed that an equivalent thereof in Saccharomyces cerevisiae (aL237R) severely impairs subunit a assembly/stability and decreases by >90% the rate of mitochondrial ATP synthesis. Herein we identified three spontaneous first-site intragenic suppressors (aR237M, aR237T and aR237S) that fully restore ATP synthase assembly. However, mitochondrial ATP synthesis rate was only partially recovered (40–50% vs wild type yeast). In light of recently described high-resolution yeast ATP synthase structures, the detrimental consequences of the aL237R change can be explained by steric and electrostatic hindrance with the universally conserved subunit a arginine residue (aR176) that is essential to FO activity. aL237 together with three other nearby hydrophobic residues have been proposed to prevent ion shortage between two physically separated hydrophilic pockets within the FO. Our results suggest that aL237 favors subunit c-ring rotation by optimizing electrostatic interaction between aR176 and an acidic residue in subunit c (cE59) known to be essential also to the activity of FO.

Defective mitochondrial ATPase due to rare mtDNA m.8969G>A mutation-causing lactic acidosis, intellectual disability, and poor growth.

Mutations in mitochondrial ATP synthase 6 (MT-ATP6) are a frequent cause of NARP (neurogenic muscle weakness, ataxia, and retinitis pigmentosa) or Leigh syndromes, especially a point mutation at nucleotide position 8993. M.8969G>A is a rare MT-ATP6 mutation, previously reported only in three individuals, causing multisystem disorders with mitochondrial myopathy, lactic acidosis, and sideroblastic anemia or IgA nephropathy. We present two siblings with the m.8969G>A mutation and a novel, substantially milder phenotype with lactic acidosis, poor growth, and intellectual disability. Our findings expand the phenotypic spectrum and show that mtDNA mutations should be taken account also with milder, stable phenotypes.

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.

Near-complete elimination of mutant mtDNA by iterative or dynamic dose-controlled treatment with mtZFNs.

Mitochondrial diseases are frequently associated with mutations in mitochondrial DNA (mtDNA). In most cases, mutant and wild-type mtDNAs coexist, resulting in heteroplasmy. The selective elimination of mutant mtDNA, and consequent enrichment of wild-type mtDNA, can rescue pathological phenotypes in heteroplasmic cells. Use of the mitochondrially targeted zinc finger-nuclease (mtZFN) results in degradation of mutant mtDNA through site-specific DNA cleavage. Here, we describe a substantial enhancement of our previous mtZFN-based approaches to targeting mtDNA, allowing near-complete directional shifts of mtDNA heteroplasmy, either by iterative treatment or through finely controlled expression of mtZFN, which limits off-target catalysis and undesired mtDNA copy number depletion. To demonstrate the utility of this improved approach, we generated an isogenic distribution of heteroplasmic cells with variable mtDNA mutant level from the same parental source without clonal selection. Analysis of these populations demonstrated an altered metabolic signature in cells harbouring decreased levels of mutant m.8993T>G mtDNA, associated with neuropathy, ataxia, and retinitis pigmentosa (NARP). We conclude that mtZFN-based approaches offer means for mtDNA heteroplasmy manipulation in basic research, and may provide a strategy for therapeutic intervention in selected mitochondrial diseases.