Mitochondrial oxodicarboxylate carrier deficiency is associated with mitochondrial DNA depletion and spinal muscular atrophy-like disease.

PURPOSE: To understand the role of the mitochondrial oxodicarboxylate carrier (SLC25A21) in the development of spinal muscular atrophy-like disease. METHODS: We identified a novel pathogenic variant in a patient by whole-exome sequencing. The pathogenicity of the mutation was studied by transport assays, computer modeling, followed by targeted metabolic testing and in vitro studies in human fibroblasts and neurons. RESULTS: The patient carries a homozygous pathogenic variant c.695A>G; p.(Lys232Arg) in the SLC25A21 gene, encoding the mitochondrial oxodicarboxylate carrier, and developed spinal muscular atrophy and mitochondrial myopathy. Transport assays show that the mutation renders SLC25A21 dysfunctional and 2-oxoadipate cannot be imported into the mitochondrial matrix. Computer models of central metabolism predicted that impaired transport of oxodicarboxylate disrupts the pathways of lysine and tryptophan degradation, and causes accumulation of 2-oxoadipate, pipecolic acid, and quinolinic acid, which was confirmed in the patient's urine by targeted metabolomics. Exposure to 2-oxoadipate and quinolinic acid decreased the level of mitochondrial complexes in neuronal cells (SH-SY5Y) and induced apoptosis. CONCLUSION: Mitochondrial oxodicarboxylate carrier deficiency leads to mitochondrial dysfunction and the accumulation of oxoadipate and quinolinic acid, which in turn cause toxicity in spinal motor neurons leading to spinal muscular atrophy-like disease.

The adjunctive application of transcranial direct current stimulation in the management of de novo refractory epilepsia partialis continua in adolescent-onset POLG-related mitochondrial disease.

Focal status epilepticus in POLG-related mitochondrial disease is highly refractory to pharmacological agents, including general anesthesia. We report the challenges in managing a previously healthy teenager who presented with de novo epilepsia partialis continua and metabolic stroke resulting from the homozygous p.Ala467Thr POLG mutation, the most common pathogenic variant identified in the Caucasian population. We applied transcranial direct current stimulation (tDCS; 2 mA; 20 min) daily as an adjunctive therapy because her focal seizures failed to respond to five antiepileptic drugs at maximal doses. The electrical and clinical seizures stopped after 3 days of tDCS. The second course of tDCS was administered for 14 days when the focal seizures re-emerged a month later. The patient tolerated the procedure well. Following 4 months of hospitalization and prolonged community rehabilitation, our patient has now returned to full-time education with support, and there is no report of cognitive deficit. We have demonstrated the safety and efficacy of tDCS in treating refractory focal motor seizures caused by mitochondrial disease.

Clinical and molecular characterization of KCNT1-related severe early-onset epilepsy.

OBJECTIVE: To characterize the phenotypic spectrum, molecular genetic findings, and functional consequences of pathogenic variants in early-onset KCNT1 epilepsy. METHODS: We identified a cohort of 31 patients with epilepsy of infancy with migrating focal seizures (EIMFS) and screened for variants in KCNT1 using direct Sanger sequencing, a multiple-gene next-generation sequencing panel, and whole-exome sequencing. Additional patients with non-EIMFS early-onset epilepsy in whom we identified KCNT1 variants on local diagnostic multiple gene panel testing were also included. When possible, we performed homology modeling to predict the putative effects of variants on protein structure and function. We undertook electrophysiologic assessment of mutant KCNT1 channels in a xenopus oocyte model system. RESULTS: We identified pathogenic variants in KCNT1 in 12 patients, 4 of which are novel. Most variants occurred de novo. Ten patients had a clinical diagnosis of EIMFS, and the other 2 presented with early-onset severe nocturnal frontal lobe seizures. Three patients had a trial of quinidine with good clinical response in 1 patient. Computational modeling analysis implicates abnormal pore function (F346L) and impaired tetramer formation (F502V) as putative disease mechanisms. All evaluated KCNT1 variants resulted in marked gain of function with significantly increased channel amplitude and variable blockade by quinidine. CONCLUSIONS: Gain-of-function KCNT1 pathogenic variants cause a spectrum of severe focal epilepsies with onset in early infancy. Currently, genotype-phenotype correlations are unclear, although clinical outcome is poor for the majority of cases. Further elucidation of disease mechanisms may facilitate the development of targeted treatments, much needed for this pharmacoresistant genetic epilepsy.

Genetic heterogeneity of motor neuropathies.

OBJECTIVE: To study the prevalence, molecular cause, and clinical presentation of hereditary motor neuropathies in a large cohort of patients from the North of England. METHODS: Detailed neurologic and electrophysiologic assessments and next-generation panel testing or whole exome sequencing were performed in 105 patients with clinical symptoms of distal hereditary motor neuropathy (dHMN, 64 patients), axonal motor neuropathy (motor Charcot-Marie-Tooth disease [CMT2], 16 patients), or complex neurologic disease predominantly affecting the motor nerves (hereditary motor neuropathy plus, 25 patients). RESULTS: The prevalence of dHMN is 2.14 affected individuals per 100,000 inhabitants (95% confidence interval 1.62-2.66) in the North of England. Causative mutations were identified in 26 out of 73 index patients (35.6%). The diagnostic rate in the dHMN subgroup was 32.5%, which is higher than previously reported (20%). We detected a significant defect of neuromuscular transmission in 7 cases and identified potentially causative mutations in 4 patients with multifocal demyelinating motor neuropathy. CONCLUSIONS: Many of the genes were shared between dHMN and motor CMT2, indicating identical disease mechanisms; therefore, we suggest changing the classification and including dHMN also as a subcategory of Charcot-Marie-Tooth disease. Abnormal neuromuscular transmission in some genetic forms provides a treatable target to develop therapies.

FHF1 (FGF12) epileptic encephalopathy.

Voltage-gated sodium channels (Navs) are mainstays of neuronal function, and mutations in the genes encoding CNS Navs (Nav1.1 [SCN1A], Nav1.2 [SCN2A], Nav1.3 [SCN3A], and Nav1.6 [SCN8A]) are causes of some of the most common and severe genetic epilepsies and epileptic encephalopathies (EE).1 Fibroblast-growth-factor homologous factors (FHFs) compose a family of 4 proteins that interact with the C-terminal tails of Navs to modulate the channels' fast, and long-term, inactivations.2FHF2 mutation is a rare cause of generalized epilepsy with febrile seizures plus (GEFS+).3 Recently, a de novo FHF1 mutation (p.R52H) was reported in early-onset EE in 2 siblings.4 We report 3 patients from unrelated families with the same FHF1 p.R52H mutation. The 5 cases together frame the FHF1 R52H EE from infancy to adulthood. As discussed below, this gain-of-function disease may be amenable to personalized therapy.

Mortality in Dravet syndrome.

We measured the mortality rate and the rate of Sudden Unexpected Death in Epilepsy (SUDEP) in Dravet Syndrome (DS). We studied a cohort of 100 consecutively recruited, unrelated patients with DS; 87 had SCN1A mutations. Living cases had a median follow-up of 17 years. Seventeen patients died, at a median age of seven years (inter-quartile range 3-11 years) with causes of death: 10 SUDEP, four status epilepticus, two drowning and one asphyxia. The SUDEP classification included three Definite, one Definite Plus and six Probable. The Dravet-specific mortality rate/1000-person-years was 15.84 (98% CI 9.01-27.85). The Dravet-specific SUDEP rate was 9.32/1000-person-years (98% CI 4.46-19.45). The Dravet-specific SUDEP rate is the only documented syndrome-specific SUDEP rate. SUDEP in DS occurs mainly in childhood. It is also the highest SUDEP rate, considerably higher than the recent 5.1 SUDEP rate/1000-person-years for adults with refractory epilepsy.

Homozygous deletion in MICU1 presenting with fatigue and lethargy in childhood.

OBJECTIVE: To define the mechanism responsible for fatigue, lethargy, and weakness in 2 cousins who had a normal muscle biopsy. METHODS: Exome sequencing, long-range PCR, and Sanger sequencing to identify the pathogenic mutation. Functional analysis in the patient fibroblasts included oxygen consumption measurements, extracellular acidification studies, Western blotting, and calcium imaging, followed by overexpression of the wild-type protein. RESULTS: Analysis of the exome sequencing depth revealed a homozygous deletion of exon 1 of MICU1 within a 2,755-base pair deletion. No MICU1 protein was detected in patient fibroblasts, which had impaired mitochondrial calcium uptake that was rescued through the overexpression of the wild-type allele. CONCLUSIONS: MICU1 mutations cause fatigue and lethargy in patients with normal mitochondrial enzyme activities in muscle. The fluctuating clinical course is likely mediated through the mitochondrial calcium uniporter, which is regulated by MICU1.

Exome sequencing in undiagnosed inherited and sporadic ataxias.

Inherited ataxias are clinically and genetically heterogeneous, and a molecular diagnosis is not possible in most patients. Having excluded common sporadic, inherited and metabolic causes, we used an unbiased whole exome sequencing approach in 35 affected individuals, from 22 randomly selected families of white European descent. We defined the likely molecular diagnosis in 14 of 22 families (64%). This revealed de novo dominant mutations, validated disease genes previously described in isolated families, and broadened the clinical phenotype of known disease genes. The diagnostic yield was the same in both young and older-onset patients, including sporadic cases. We have demonstrated the impact of exome sequencing in a group of patients notoriously difficult to diagnose genetically. This has important implications for genetic counselling and diagnostic service provision.

Use of whole-exome sequencing to determine the genetic basis of multiple mitochondrial respiratory chain complex deficiencies.

IMPORTANCE: Mitochondrial disorders have emerged as a common cause of inherited disease, but their diagnosis remains challenging. Multiple respiratory chain complex defects are particularly difficult to diagnose at the molecular level because of the massive number of nuclear genes potentially involved in intramitochondrial protein synthesis, with many not yet linked to human disease. OBJECTIVE: To determine the molecular basis of multiple respiratory chain complex deficiencies. DESIGN, SETTING, AND PARTICIPANTS: We studied 53 patients referred to 2 national centers in the United Kingdom and Germany between 2005 and 2012. All had biochemical evidence of multiple respiratory chain complex defects but no primary pathogenic mitochondrial DNA mutation. Whole-exome sequencing was performed using 62-Mb exome enrichment, followed by variant prioritization using bioinformatic prediction tools, variant validation by Sanger sequencing, and segregation of the variant with the disease phenotype in the family. RESULTS: Presumptive causal variants were identified in 28 patients (53%; 95% CI, 39%-67%) and possible causal variants were identified in 4 (8%; 95% CI, 2%-18%). Together these accounted for 32 patients (60% 95% CI, 46%-74%) and involved 18 different genes. These included recurrent mutations in RMND1, AARS2, and MTO1, each on a haplotype background consistent with a shared founder allele, and potential novel mutations in 4 possible mitochondrial disease genes (VARS2, GARS, FLAD1, and PTCD1). Distinguishing clinical features included deafness and renal involvement associated with RMND1 and cardiomyopathy with AARS2 and MTO1. However, atypical clinical features were present in some patients, including normal liver function and Leigh syndrome (subacute necrotizing encephalomyelopathy) seen in association with TRMU mutations and no cardiomyopathy with founder SCO2 mutations. It was not possible to confidently identify the underlying genetic basis in 21 patients (40%; 95% CI, 26%-54%). CONCLUSIONS AND RELEVANCE: Exome sequencing enhances the ability to identify potential nuclear gene mutations in patients with biochemically defined defects affecting multiple mitochondrial respiratory chain complexes. Additional study is required in independent patient populations to determine the utility of this approach in comparison with traditional diagnostic methods.

Behr's Syndrome is Typically Associated with Disturbed Mitochondrial Translation and Mutations in the C12orf65 Gene.

BACKGROUND: Behr's syndrome is a classical phenotypic description of childhood-onset optic atrophy combined with various neurological symptoms, including ophthalmoparesis, nystagmus, spastic paraparesis, ataxia, peripheral neuropathy and learning difficulties. OBJECTIVE: Here we describe 4 patients with the classical Behr's syndrome phenotype from 3 unrelated families who carry homozygous nonsense mutations in the C12orf65 gene encoding a protein involved in mitochondrial translation. METHODS: Whole exome sequencing was performed in genomic DNA and oxygen consumption was measured in patient cell lines. RESULTS: We detected 2 different homozygous C12orf65 nonsense mutations in 4 patients with a homogeneous clinical presentation matching the historical description of Behr's syndrome. The first symptom in all patients was childhood-onset optic atrophy, followed by spastic paraparesis, distal weakness, motor neuropathy and ophthalmoparesis. CONCLUSIONS: We think that C12orf65 mutations are more frequent than previously suggested and screening of this gene should be considered not only in patients with mitochondrial respiratory chain deficiencies, but also in inherited peripheral neuropathies, spastic paraplegias and ataxias, especially with pre-existing optic atrophy.

Neurological features of epilepsy, ataxia, sensorineural deafness, tubulopathy syndrome.

AIM: Recently, we reported a previously unrecognized symptom constellation comprising epilepsy, ataxia, sensorineural deafness, and tubulopathy (EAST syndrome) associated with recessive mutations in the KCNJ10 gene. Here, we provide a detailed characterization of the clinical features of the syndrome to aid patient management with respect to diagnosis, prognostic counselling, and identification of best treatment modalities. METHOD: We conducted a retrospective review of the detailed neurological and neuroradiological features of nine children (four females, five males; age range at last examination 6-20y) with genetically proven EAST syndrome. RESULTS: All children presented with tonic-clonic seizures in infancy. Later, non-progressive, cerebellar ataxia and hearing loss were noted. Whilst seizures mostly responded well to treatment, ataxia proved to be the most debilitating feature, with three patients non-ambulant. All available magnetic resonance imaging (MRI) revealed subtle symmetrical signal changes in the cerebellar dentate nuclei. Moreover, four patients had a small corpus callosum and brainstem hypoplasia, and three had a small spinal cord. Regional quantitative volumetric analysis of the images confirmed the corpus callosum and brainstem hypoplasia and showed further patterns of variation from the norm. INTERPRETATION: The neurological features of EAST syndrome appear to be non-progressive, which is important for prognostic counselling. The spectrum of EAST syndrome includes consistent abnormalities on brain MRI, which may aid diagnosis. Further longitudinal documentation is required to determine the true natural history of the disorder.

TUBA1A mutation-associated lissencephaly: case report and review of the literature.

Lissencephaly is a disorder of neuronal migration resulting in abnormal cerebral cortical sulcation and gyration. Affected children present with microcephaly, developmental delay, and early-onset epileptic seizures. Recently, de novo missense mutations in the tubulin α-1A (TUBA1A) gene were identified as causing a distinctive radiologic phenotype comprising of posteriorly predominant lissencephaly with dysgenetic corpus callosum, cerebellar and brainstem hypoplasia, and more recently, polymicrogyria. We describe a 14-month-old girl with TUBA1A mutation-associated lissencephaly, and summarize the clinical and neuroradiologic findings of 19 cases in the literature.

Intracerebral large artery disease in Aicardi-Goutières syndrome implicates SAMHD1 in vascular homeostasis.

AIM: To describe a spectrum of intracerebral large artery disease in Aicardi-Goutières syndrome (AGS) associated with mutations in the AGS5 gene SAMHD1. METHOD: We used clinical and radiological description and molecular analysis. RESULTS: Five individuals (three males, two females) were identified as having biallelic mutations in SAMHD1 and a cerebral arteriopathy in association with peripheral vessel involvement resulting in chilblains and ischaemic ulceration. The cerebral vasculopathy was primarily occlusive in three patients (with terminal carotid occlusion and basal collaterals reminiscent of moyamoya syndrome) and aneurysmal in two. Three of the five patients experienced intracerebral haemorrhage, which was fatal in two individuals. Post-mortem examination of one patient suggested that the arteriopathy was inflammatory in origin. INTERPRETATION: Mutations in SAMHD1 are associated with a cerebral vasculopathy which is likely to have an inflammatory aetiology. A similar disease has not been observed in patients with mutations in AGS1 to AGS4, suggesting a particular role for SAMHD1 in vascular homeostasis. Our report raises important questions about the management of patients with mutations in SAMHD1.

Treatment of primary angiitis of the central nervous system in childhood with mycophenolate mofetil.

OBJECTIVE: To assess the use of mycophenolate mofetil (MMF) in the treatment of refractory primary angiitis of the CNS in childhood (cPACNS). METHODS: A retrospective chart review was performed in patients with cPACNS who were treated with MMF following failure of a combination of corticosteroids and another immunosuppressant. RESULTS: Three patients from two centres were included in this study. The age of onset of disease was 5, 6 and 9 years. All the patients improved when treated with MMF, such that the dose of corticosteroids could be weaned or stopped. CONCLUSIONS: MMF should be considered for maintenance treatment in the management of patients with cPACNS refractory to the combination of corticosteroids and first-line immunosuppressive agents.

Coma blisters in 2 children on anticonvulsant medication.

Blister formation and eccrine sweat gland necrosis have been recognized to occur in states of impaired consciousness and were first reported following barbiturate intoxication. Their etiology is complex and cannot simply be explained by pressure effects. Now that barbiturates are less frequently used, clinicians are likely to be less aware of the phenomenon of coma blister formation; however, newer drugs have also been associated with the occurrence of coma blisters. We describe 2 new associations of coma blisters and anticonvulsants in children. In the first child, blisters recurred on multiple occasions along with obtundation and edema. Our aims are to alert clinicians to the occurrence of coma blisters in children sedated on anticonvulsant medications and to report the new finding of recurrent coma blisters.

Outcome of hematopoietic stem cell transplantation in severe combined immune deficiency with central nervous system viral infection.

BACKGROUND: Patients with severe combined immunodeficiency and preexisting viral pneumonitis formally had a poor outcome from hematopoietic stem cell transplantation. With inhaled steroid and antitumor necrosis factor alpha antibody treatment, results improved. The poor outcome of patients with viral central nervous system infection prompted this retrospective single center review. RESULTS: Eight of 71 patients with severe combined immunodeficiency transplanted since 1987 were identified with viral central nervous system infection (adenovirus [1], cytomegalovirus [2], Epstein-Barr virus [2], parvovirus [1], varicella zoster virus [1], human herpesvirus 6 [1]). Nonspecific neurologic symptoms included drowsiness, irritability, head lag, fisting and floppiness. Later symptoms included unresponsiveness, apnea, posturing, hypotonia, hyperreflexia and seizures. All had neuroradiologic investigations. Only one had an initially normal computed tomography scan. Magnetic resonance image abnormalities included cerebral atrophy, basal ganglia changes, diffuse leukoencephalopathy, and multifocal mass lesions. Five patients had virus identified from cerebrospinal fluid by polymerase chain reaction and brain tissue examination from 3 patients identified human herpesvirus 6, adenovirus type 41 and varicella zoster virus. Three children remain alive, 2 received replete marrow and one remains untransplanted. Others who received T cell depleted marrow died of neurologic sequelae. CONCLUSION: Outcome of viral central nervous system infection after hematopoietic stem cell transplantation for severe combined immunodeficiency is poor, particularly associated with T cell depleted marrow.