Alternative splicing expands the clinical spectrum of NDUFS6-related mitochondrial disorders.

PURPOSE: We describe 3 families with Charcot-Marie-Tooth neuropathy (CMT), harboring a homozygous NDUFS6 NM_004553.6:c.309+5G>A variant previously linked to fatal Leigh syndrome. We aimed to characterize clinically and molecularly the newly identified patients and understand the mechanism underlying their milder phenotype. METHODS: The patients underwent extensive clinical examinations. Exome sequencing was done in 4 affected individuals. The functional effect of the c.309+5G>A variant was investigated in patient-derived EBV-transformed lymphoblasts at the complementary DNA, protein, and mitochondrial level. Alternative splicing was evaluated using complementary DNA long-read sequencing. RESULTS: All patients presented with early-onset, slowly progressive axonal CMT, and nystagmus; some exhibited additional central nervous system symptoms. The c.309+5G>A substitution caused the expression of aberrantly spliced transcripts and negligible levels of the canonical transcript. Immunoblotting showed reduced levels of mutant isoforms. No detectable defects in mitochondrial complex stability or bioenergetics were found. CONCLUSION: We expand the clinical spectrum of NDUFS6-related mitochondrial disorders to include axonal CMT, emphasizing the clinical and pathophysiologic overlap between these 2 clinical entities. This work demonstrates the critical role that alternative splicing may play in modulating the severity of a genetic disorder, emphasizing the need for careful consideration when interpreting splice variants and their implications on disease prognosis.

Spinal Nerve Roots Abnormalities on MRI in a Child with SURF1 Mitochondrial Disease.

Variants in SURF1, encoding an assembly factor of mitochondrial respiratory chain complex IV, cause Leigh syndrome (LS) and Charcot-Marie-Tooth type 4K in children and young adolescents. Magnetic resonance imaging (MRI) appearance of enlarged nerve roots with postcontrastographic enhancement is a distinctive feature of hypertrophic neuropathy caused by onion-bulb formation and it has rarely been described in mitochondrial diseases (MDs). Spinal nerve roots abnormalities on MRI are novel findings in LS associated with variants in SURF1. Here we report detailed neuroradiological and neurophysiologic findings in a child with LS and demyelinating neuropathy SURF1-related. Our case underlines the potential contributive role of spinal neuroimaging together with neurophysiological examination to identify the full spectrum of patterns in MDs. It remains to elucidate if these observations remain peculiar of SURF1 variants or potentially detectable in other MDs with peripheral nervous system involvement.

Defective mitochondrial protease LonP1 can cause classical mitochondrial disease.

LonP1 is a mitochondrial matrix protease whose selective substrate specificity is essential for maintaining mitochondrial homeostasis. Recessively inherited, pathogenic defects in LonP1 have been previously reported to underlie cerebral, ocular, dental, auricular and skeletal anomalies (CODAS) syndrome, a complex multisystemic and developmental disorder. Intriguingly, although classical mitochondrial disease presentations are well-known to exhibit marked clinical heterogeneity, the skeletal and dental features associated with CODAS syndrome are pathognomonic. We have applied whole exome sequencing to a patient with congenital lactic acidosis, muscle weakness, profound deficiencies in mitochondrial oxidative phosphorylation associated with loss of mtDNA copy number and MRI abnormalities consistent with Leigh syndrome, identifying biallelic variants in the LONP1 (NM_004793.3) gene; c.1693T > C predicting p.(Tyr565His) and c.2197G > A predicting p.(Glu733Lys); no evidence of the classical skeletal or dental defects observed in CODAS syndrome patients were noted in our patient. In vitro experiments confirmed the p.(Tyr565His) LonP1 mutant alone could not bind or degrade a substrate, consistent with the predicted function of Tyr565, whilst a second missense [p.(Glu733Lys)] variant had minimal effect. Mixtures of p.(Tyr565His) mutant and wild-type LonP1 retained partial protease activity but this was severely depleted when the p.(Tyr565His) mutant was mixed with the p.(Glu733Lys) mutant, data consistent with the compound heterozygosity detected in our patient. In summary, we conclude that pathogenic LONP1 variants can lead to a classical mitochondrial disease presentations associated with severe biochemical defects in oxidative phosphorylation in clinically relevant tissues.

[Analysis of mitochondrial gene mutations in a child with Leigh syndrome].

OBJECTIVE: To explore the genetic basis for a child with Leigh syndrome. METHODS: Clinical features and laboratory test of the patient were analyzed. Sanger sequencing and multiplex ligation-dependent probe amplification (MLPA) of the mitochondrial genome were carried out. Next generation sequencing (NGS) was used to capture and sequence nuclear genes related to mitochondrial structure and function. RESULTS: The child presented with developmental delay, unsteady gait, falling episodes, bilateral upper extremity tremor, muscle hypertonia, convulsions, and mouth angle asymmetry. Serum lactic acid was significantly increased. Cranial MRI showed abnormal signal in bilateral cerebellar hemispheres, bilateral basal ganglia, left thalamus, and corona radiata. Her mother and brother did not show any anomalies. Sanger sequencing revealed the child, her mother and brother all carried the MT-ND3 m.10191 T>C mutation, with heterogeneous rates respectively being 74.34%, 9.73%, and 6.28%. MLPA revealed heterogeneity of (MT-ND6, MTCYB-390nt)] deletion in all three individuals. No significant mutation was found by NGS sequencing of the children, their parents and brother. CONCLUSION: Leigh syndrome can be caused by the simultaneous existence of multiple mitochondrial genes, and multiple mutations may play a synergic role in the occurrence of the disease.

Direct effects of mitochondrial dysfunction on poor bone health in Leigh syndrome.

Mitochondrial diseases are the result of aberrant mitochondrial function caused by mutations in either nuclear or mitochondrial DNA. Poor bone health has recently been suggested as a symptom of mitochondrial diseases; however, a direct link between decreased mitochondrial function and poor bone health in mitochondrial disease has not been demonstrated. In this study, stem cells from human exfoliated deciduous teeth (SHED) were isolated from a child with Leigh syndrome (LS), a mitochondrial disease, and the effects of decreased mitochondrial function on poor bone health were analyzed. Compared with control SHED, LS SHED displayed decreased osteoblastic differentiation and calcium mineralization. The intracellular and mitochondrial calcium levels were lower in LS SHED than in control SHED. Furthermore, the mitochondrial activity of LS SHED was decreased compared with control SHED both with and without osteoblastic differentiation. Our results indicate that decreased osteoblast differentiation potential and osteoblast function contribute to poor bone health in mitochondrial diseases.

Homozygous p.V116* mutation in C12orf65 results in Leigh syndrome.

BACKGROUND: Leigh syndrome (LS) is an early-onset progressive neurodegenerative disorder associated with mitochondrial dysfunction. LS is characterised by elevated lactate and pyruvate and bilateral symmetric hyperintense lesions in the basal ganglia, thalamus, brainstem, cerebral white matter or spinal cord on T2-weighted MRI. LS is a genetically heterogeneous disease, and to date mutations in approximately 40 genes related to mitochondrial function have been linked to the disorder. METHODS: We investigated a pair of female monozygotic twins diagnosed with LS from consanguineous healthy parents of Indian origin. Their common clinical features included optic atrophy, ophthalmoplegia, spastic paraparesis and mild intellectual disability. High-blood lactate and high-intensity signal in the brainstem on T2-weighted MRI were consistent with a clinical diagnosis of LS. To identify the genetic cause of their condition, we performed whole exome sequencing. RESULTS: We identified a homozygous nonsense mutation in C12orf65 (NM_001143905; c.346delG, p.V116*) in the affected twins. Interestingly, the identical mutation was previously reported in an Indian family with Charcot-Marie Tooth disease type 6, which displayed some overlapping clinical features with the twins. CONCLUSIONS: We demonstrate that the identical nonsense mutation in C12orf65 can result in different clinical features, suggesting the involvement of unknown modifiers.

Management of patients with dental disease and mitochondrial disorders.

UNLABELLED: This case report describes dental disease associated with mitochondrial disease (Leigh's disease) in a young adult. Normal preventive dentistry is difficult in these groups of patients and often leads to management required in secondary care. An awareness of the background pathology is needed when managing these groups of patients. Management of dental pathology in this particular patient often required input from other specialties to ensure a successful outcome. CLINICAL RELEVANCE: To raise awareness of the dental pathologies patients with mitochondrial disease may experience as they present to the general dental practitioner, and what treatments may be appropriate.

Validation of a mitochondrial RNA therapeutic strategy using fibroblasts from a Leigh syndrome patient with a mutation in the mitochondrial ND3 gene.

We report on the validation of a mitochondrial gene therapeutic strategy using fibroblasts from a Leigh syndrome patient by the mitochondrial delivery of therapeutic mRNA. The treatment involves delivering normal ND3 protein-encoding mRNA as a therapeutic RNA to mitochondria of the fibroblasts from a patient with a T10158C mutation in the mtDNA coding the ND3 protein, a component of the mitochondrial respiratory chain complex I. The treatment involved the use of a liposome-based carrier (a MITO-Porter) for delivering therapeutic RNA to mitochondria via membrane fusion. The results confirmed that the mitochondrial transfection of therapeutic RNA by the MITO-Porter system resulted in a decrease in the levels of mutant RNA in mitochondria of diseased cells based on reverse transcription quantitative PCR. An evaluation of mitochondrial respiratory activity by respirometry also showed that transfection using the MITO-Porter resulted in an increase in maximal mitochondrial respiratory activity in the diseased cells.

Facial dysmorphism in Leigh syndrome with SURF-1 mutation and COX deficiency.

Leigh syndrome is an inherited, progressive neurodegenerative disorder of infancy and childhood. Mutations in the nuclear SURF-1 gene are specifically associated with cytochrome C oxidase-deficient Leigh syndrome. This report describes two patients with similar facial features. One of them was a 2(1/2)-year-old male, and the other was a 3-year-old male with a mutation in SURF-1 gene and facial dysmorphism including frontal bossing, brachycephaly, hypertrichosis, lateral displacement of inner canthi, esotropia, maxillary hypoplasia, hypertrophic gums, irregularly placed teeth, upturned nostril, low-set big ears, and retrognathi. The first patient's magnetic resonance imaging at 15 months of age indicated mild symmetric T2 prolongation involving the subthalamic nuclei. His second magnetic resonance imaging at 2 years old revealed a symmetric T2 prolongation involving the subthalamic nuclei, substantia nigra, and medulla lesions. In the second child, at the age of 2 the first magnetic resonance imaging documented heavy brainstem and subthalamic nuclei involvement. A second magnetic resonance imaging, performed when he was 3 years old, revealed diffuse involvement of the substantia nigra and hyperintense lesions of the central tegmental tract in addition to previous lesions. Facial dysmorphism and magnetic resonance imaging findings, observed in these cases, can be specific findings in Leigh syndrome patients with cytochrome C oxidase deficiency. SURF-1 gene mutations must be particularly reviewed in such patients.

Outpatient anesthesia for oral surgery in a juvenile with Leigh disease.

We report a case of anesthesia for elective outpatient third molar extraction in a juvenile with Leigh disease, a progressive neurodegenerative disorder related to respiratory chain deficiency. This syndrome usually presents in infancy and is characterized by nervous system dysfunction and respiratory abnormalities. Anesthesia has been reported to aggravate respiratory symptoms and frequently precipitate respiratory failure. Preoperative swallowing difficulty or respiratory symptoms should be carefully diagnosed, because they can be a warning sign of postoperative complications or mortality. Adverse effects of anesthesia may quickly lead into metabolic acidosis. Anesthetics should be carefully chosen that do not interfere with mitochondrial respiration, which can lead to lactic acidosis.

Association of the mitochondrial 8344 MERRF mutation with maternally inherited spinocerebellar degeneration and Leigh disease.

We report previously undescribed or atypical clinical and biochemical manifestations of the mitochondrial DNA MERRF mutation at nucleotide 8344 in members of a multigenerational family with maternally inherited, highly variable neurodegenerative disorder. The more profound neurologic abnormalities include Leigh disease, spinocerebellar degeneration, and atypical Charcot-Marie-Tooth disease.

Leigh disease with mitochondrial DNA A8344G mutation: case report and brief review.

Leigh disease, subacute necrotizing encephalomyelopathy, is a neurodegenerative disorder often seen in infancy or childhood but rarely reported in adults. Genetic heterogeneity is well recognized, and the associated etiologies include both mitochondrial and nuclear DNA defects. We describe an infant presenting with developmental delay and then progressive multisystem disorder and neuroradiologic features of Leigh disease. He and his maternal relatives all have the A8344G mitochondrial DNA mutation. However, only minor clinical features are seen in his maternal relatives, with migraine being the most common problem. Additionally the A8344G mitochondrial DNA mutation is associated with spinocerebellar degeneration, other nonspecific mitochondrial encephalomyopathies, atypical Charcot-Marie-Tooth disease, and progressive external ophthalmoplegia. The A8344G mitochondrial DNA mutation may present with Leigh disease or other different atypical clinical features without myoclonic epilepsy and ragged red fibers.

Defining the impact on yeast ATP synthase of two pathogenic human mitochondrial DNA mutations, T9185C and T9191C.

Mutations in the human mitochondrial ATP6 gene encoding ATP synthase subunit a/6 (referred to as Atp6p in yeast) are at the base of neurodegenerative disorders like Neurogenic Ataxia and Retinitis Pigmentosa (NARP), Leigh syndrome (LS), Charcot-Marie-Tooth (CMT), and ataxia telangiectasia. In previous studies, using the yeast Saccharomyces cerevisiae as a model we were able to better define how several of these mutations impact the ATP synthase. Here we report the construction of yeast models of two other ATP6 pathogenic mutations, T9185C and T9191C. The first one was reported as conferring a mild, sometimes reversible, CMT clinical phenotype; the second one has been described in a patient presenting with severe LS. We found that an equivalent of the T9185C mutation partially impaired the functioning of yeast ATP synthase, with only a 30% deficit in mitochondrial ATP production. An equivalent of the mutation T9191C had much more severe effects, with a nearly complete block in yeast Atp6p assembly and an >95% drop in the rate of ATP synthesis. These findings provide a molecular basis for the relative severities of the diseases induced by T9185C and T9191C.

Naltrexone therapy of apnea in children with elevated cerebrospinal fluid beta-endorphin.

Previous studies have indicated increased immunoreactivity of the endogenous opioid peptide beta-endorphin in the cerebrospinal fluid (CSF) of infants under 2 years of age with apnea. To assess the role of endogenous opioids in the pathogenesis of apnea in children, the effect of oral treatment with the opioid antagonist naltrexone was studied in apneic infants, as well as in older apneic children, with demonstrated increases in CSF immunoreactive beta-endorphin (i-BE). In the 8 apneic infants with elevated i-BE in lumbar CSF (range, 55-155 pg/ml; normal, 17-52 pg/ml), no further apnea occurred during naltrexone therapy (1 mg/kg/day, by mouth). Five children (2-8 years old) with apnea of unknown cause had elevated CSF i-BE (range, 74-276 pg/ml) compared to 6 age-matched nonapneic children (range, 15-48 pg/ml). No apneic events occurred during naltrexone therapy, except in 1 child during stressful events, but apnea recurred in some patients after attempts to discontinue naltrexone treatment. Adverse effects of naltrexone included complaints of headaches in 2 children and symptoms of a narcotic withdrawal syndrome during the first 3 days of treatment in 1 child. Three children with Leigh's syndrome had elevated CSF i-BE (range, 104-291 pg/ml) and their apnea also responded to naltrexone. We conclude that elevated endogenous opioids contribute to the pathogenesis of apnea in children and may even result in physical dependence.

Self-inflicted orodental injury in a child with Leigh disease.

Leigh disease is an inherited progressive mitochondrial neurodegenerative disease that affects the neurological, respiratory and cardiovascular systems and is associated with retardation of the intellectual and physical development. This report describes the case of a 4-year-old boy with Leigh disease who presented with self-inflicted traumatic injury to the teeth, alveolar bone, lips and tongue during repeated episodes of intense orofacial spasms. Conservative management of the injury included repositioning the fractured alveolar bone, splinting the traumatized teeth and planning for a mouthguard. However, after a second incident of severe self-induced injury to the teeth and alveolar bone, extraction of the anterior teeth became inevitable to protect the child from further self-mutilation and to allow healing of the injured tissues.