Search

INTRODUCTION/AIMS: Currently, there are no straightforward guidelines for the clinical and diagnostic management of hyperCKemia, a frequent and nonspecific presentation in muscle diseases. Therefore, we aimed to describe our diagnostic workflow for evaluating patients with this condition. METHODS: We selected 83 asymptomatic or minimally symptomatic patients with persistent hyperCKemia for participation in this Italian multicenter study. Patients with facial involvement and distal or congenital myopathies were excluded, as were patients with suspected inflammatory myopathies or predominant respiratory or cardiac involvement. All patients underwent a neurological examination and nerve conduction and electromyography studies. The first step of the investigation included a screening for Pompe disease. We then evaluated the patients for myotonic dystrophy type II-related CCTG expansion and excluded patients with copy number variations in the DMD gene. Subsequently, the undiagnosed patients were investigated using a target gene panel that included 20 genes associated with isolated hyperCKemia. RESULTS: Using this approach, we established a definitive diagnosis in one third of the patients. The detection rate was higher in patients with severe hyperCKemia and abnormal electromyographic findings. DISCUSSION: We have described our diagnostic workflow for isolated hyperCKemia, which is based on electrodiagnostic data, biochemical screening, and first-line genetic investigations, followed by successive targeted sequencing panels. Both clinical signs and electromyographic abnormalities are associated with increased diagnostic yields. This article is protected by copyright. All rights reserved.

Aim Since the immune system plays a role in the pathogenesis of several muscular dystrophies, we aim to characterize several muscular inflammatory features in α- (LGMD R3) and γ-sarcoglycanopathies (LGMD R5). Materials and methods We explored the expression of major histocompatibility complex class I molecules (MHCI), and we analyzed the composition of the immune infiltrates in muscle biopsies from 10 patients with LGMD R3 and 8 patients with LGMD R5, comparing the results to Duchenne muscular dystrophy patients (DMD). Results A consistent involvement of the immune response was observed in sarcoglycanopathies, although it was less evident than in DMD. LGMD R3-R5 and DMD shared an abnormal expression of MHCI, and the composition of the muscular immune cell infiltrate was comparable. Conclusion These findings might serve as a rationale to fine-tune a disease-specific immunomodulatory regimen, particularly relevant in view of the rapid development of gene therapy for sarcoglycanopathies.

Background Syntaxin Binding Protein 1 ( STXBP1 ) mutations determine a central neurotransmission dysfunction through impairment of the synaptic vesicle release, thus causing a spectrum of phenotypes varying from syndromic and non-syndromic epilepsy to intellectual disability of variable degree. Among the antiepileptic drugs, levetiracetam has a unique mechanism of action binding SV2A, a glycoprotein of the synaptic vesicle release machinery. Patient description We report a 1-month-old boy manifesting an epileptic encephalopathy with clonic seizures refractory to phenobarbital, pyridoxine and phenytoin that presented a dramatic response to levetiracetam with full epilepsy control and EEG normalization. Genetic analysis identified a novel de novo heterozygous mutation (c.[922A > T]p.[Lys308 ∗ ]) in the STXBP1 gene that severely affects the protein. Conclusions The observation of a dramatic efficacy of levetiracetam in a case of STXBP1 epileptic encephalopathy refractory to other antiepileptic drugs and considerations regarding the specific mechanism of action of levetiracetam modulating the same system affected by STXBP1 mutations support the hypothesis that this drug may be able to reverse specifically the disease epileptogenic abnormalities. Further clinical observations and laboratory studies are needed to confirm this hypothesis and eventually lead to consider levetiracetam as the first choice treatment of patients with suspected or confirmed STXBP1 -related epilepsies.

Purpose Temporal lobe epilepsy (TLE) is the most common form of focal epilepsy and may be associated with acquired central nervous system lesions or could be genetic. Various susceptibility genes and environmental factors are believed to be involved in the aetiology of TLE, which is considered to be a heterogeneous, polygenic, and complex disorder. Rare point mutations in LGI1 , DEPDC5 , and RELN as well as some copy number variations (CNVs) have been reported in families with TLE patients. Methods We perform a genetic analysis by Array-CGH in a patient with dysmorphic features and temporal lobe epilepsy. Results We report a de novo duplication of the long arm of chromosome 12. Conclusion We confirm that 12q22-q23.3 is a candidate locus for familial temporal lobe epilepsy with febrile seizures and highlight the role of chromosomal rearrangements in patients with epilepsy and intellectual disability.

Background Nance-Horan syndrome (NHS) is a rare X-linked developmental disorder characterized by congenital cataract, dental anomalies and facial dysmorphisms. Notably, up to 30% of NHS patients have intellectual disability and a few patients have been reported to have congenital cardiac defects. Nance-Horan syndrome is caused by mutations in the NHS gene that is highly expressed in the midbrain, retina, lens, tooth, and is conserved across vertebrate species. Although most pathogenic mutations are nonsense mutations, a few genomic rearrangements involving NHS locus have been reported, suggesting a possible pathogenic role of the flanking genes. Methods Here, we report a microdeletion of 170,6 Kb at Xp22.13 (17.733.948-17.904.576) (GRCh37/hg19), detected by array-based comparative genomic hybridization in an Italian boy with NHS syndrome. Results The microdeletion harbors the NHS, SCLML1, and RAI2 genes and results in a phenotype consistent with NSH syndrome and developmental delay. Conclusion We compare our case with the previous Xp22.13 microdeletions and discuss the possible pathogenetic role of the flanking genes. Birth Defects Research, 2017. © 2017 Wiley Periodicals, Inc.

We present a 13-year-old patient with persistent increase of serum Creatine Kinase (CK) and myalgia after exertion. Skeletal muscle biopsy showed marked reduction of dystrophin expression leading to genetic analysis of DMD gene by MLPA, which detected a single deletion of exon 78. To the best of our knowledge, DMD exon 78 deletion has never been described in literature and, according to prediction, it should lead to loss of reading frame in the dystrophin gene. To further assess the actual effect of exon 78 deletion, we analysed cDNA from muscle mRNA. This analysis confirmed the absence of 32 bp of exon 78. Exclusion of exon 78 changes the open reading frame of exon 79 and generate a downstream stop codon, producing a dystrophin protein of 3703 amino acids instead of 3685 amino acids. Albeit loss of reading frame usually leads to protein degradation and severe phenotype, in this case, we demonstrated that deletion of DMD exon 78 can be associated with a functional protein able to bind DGC complex and a very mild phenotype. This study adds a novel deletion in DMD gene in human and helps to define the compliance between maintaining/disrupting the reading frame and clinical form of the disease.

Objective Alterations of sphingolipid metabolism are implicated in the pathogenesis of many neurodegenerative disorders. Methods We identified a homozygous nonsynonymous mutation in CERS1, the gene encoding ceramide synthase 1, in 4 siblings affected by a progressive disorder with myoclonic epilepsy and dementia. CerS1, a transmembrane protein of the endoplasmic reticulum (ER), catalyzes the biosynthesis of C18-ceramides. Results We demonstrated that the mutation decreases C18-ceramide levels. In addition, we showed that downregulation of CerS1 in a neuroblastoma cell line triggers ER stress response and induces proapoptotic pathways. Interpretation This study demonstrates that impairment of ceramide biosynthesis underlies neurodegeneration in humans. Ann Neurol 2014;76:206–212

The widespread use of Array Comparative Genomic Hybridization (aCGH) technology has enabled the identification of several syndromes associated with copy number variants (CNVs) including the 17q21.31 microdeletion. The 17q21.31 microdeletion syndrome, also known as Koolen-de Vries syndrome, was first described in 2006 in individuals with intellectual disabilities and organ abnormalities. We report the clinical, instrumental, cytogenetic and molecular investigations of a boy admitted for epilepsy and intellectual disabilities. We carried out detailed analysis of the clinical phenotype of this patient and investigated the genetic basis by using aCGH. We identified a de novo microdeletion on chromosome 17q21.31, compatible with Koolen-de Vries syndrome. Our case shares some of the typical characteristics of the syndrome already described by other authors: delayed psychomotor development, primarily affecting the expressive language, dysmorphic facial features, and epilepsy. However the clinical outcome was not severe as the intellectual disabilities were moderate with good adaptive and functional behaviour. Epilepsy was easily controlled by a single drug, and he never needed surgery for organ abnormalities. ! 2016 The Japanese Society of Child Neurology. Published by Elsevier B.V. All rights reserved.

Contains fulltext : 168130.pdf (Publisher’s version ) (Closed access) OBJECTIVE: To give a comprehensive overview of the phenotypic and genetic spectrum of STXBP1 encephalopathy (STXBP1-E) by systematically reviewing newly diagnosed and previously reported patients. METHODS: We recruited newly diagnosed patients with STXBP1 mutations through an international network of clinicians and geneticists. Furthermore, we performed a systematic literature search to review the phenotypes of all previously reported patients. RESULTS: We describe the phenotypic features of 147 patients with STXBP1-E including 45 previously unreported patients with 33 novel STXBP1 mutations. All patients have intellectual disability (ID), which is mostly severe to profound (88%). Ninety-five percent of patients have epilepsy. While one-third of patients presented with Ohtahara syndrome (21%) or West syndrome (9.5%), the majority has a nonsyndromic early-onset epilepsy and encephalopathy (53%) with epileptic spasms or tonic seizures as main seizure type. We found no correlation between severity of seizures and severity of ID or between mutation type and seizure characteristics or cognitive outcome. Neurologic comorbidities including autistic features and movement disorders are frequent. We also report 2 previously unreported adult patients with prominent extrapyramidal features. CONCLUSION: De novo STXBP1 mutations are among the most frequent causes of epilepsy and encephalopathy. Most patients have severe to profound ID with little correlation among seizure onset, seizure severity, and the degree of ID. Accordingly, we hypothesize that seizure severity and ID present 2 independent dimensions of the STXBP1-E phenotype. STXBP1-E may be conceptualized as a complex neurodevelopmental disorder rather than a primary epileptic encephalopathy.

Background Female carriers of Duchenne muscular dystrophy (DMD), although usually asymptomatic, develop muscle weakness up to 17% of the time, and a third present cardiac abnormalities or cognitive impairment. Clinical features of DMD carriers during childhood are poorly known. Patients We describe a cohort of pediatric DMD carriers, providing clinical, genetic, and histopathologic features, with a mean follow-up of 7 years. Results Fifteen females with a DMD mutation (age range 5 to 18 years) were included. Seven patients (46%) presented with clinically evident symptoms and signs such as limb girdle weakness, abnormal gait, and exercise intolerance. The other eight patients (53%) were evaluated because of an incidental finding of elevated level of creatine kinase. Creatine kinase level was elevated in all, ranging from 392 to 13,000 U/L. Calf hypertrophy was observed in eight patients (53%). No patient developed respiratory or cardiac involvement. The most frequent complication was scoliosis (46%). Four patients (29%) also presented minor learning disabilities or behavioral problems. We performed electromyography in half of patients, showing myopathic pattern in four (53%). Muscle biopsy revealed a mosaic reduction of dystrophin in nine available cases. DMD gene mutations were mostly deletions (71%), resulting in loss of reading frame in five patients (36%). The three patients who experienced the most severe disease course were affected either by a nonsense or frameshift mutation. Conclusions Our analysis suggests that DMD gene mutations may be suspected in a female child with persistently elevated levels of creatine kinase. Evidence of scoliosis, calf hypertrophy, or myopathic pattern at electromyography may also be helpful, and muscle biopsy is always indicative. DMD carriers should be followed for subtle orthopedic and psychiatric complications during childhood.

Caveolins are the principal protein components of caveolae, invaginations of the plasma membrane involved in cell signaling and trafficking. Caveolin-3 (Cav-3) is the muscle-specific isoform of the caveolin family and mutations in the CAV3 gene lead to a large group of neuromuscular disorders. In unrelated patients, we identified two distinct CAV3 mutations involving the same codon 78. Patient 1, affected by dilated cardiomyopathy and limb girdle muscular dystrophy (LGMD)-1C, shows an autosomal recessive mutation converting threonine to methionine (T78M). Patient 2, affected by isolated familiar hyperCKemia, shows an autosomal dominant mutation converting threonine to lysine (T78K). Cav-3 wild type (WT) and Cav-3 mutations were transiently transfected into Cos-7 cells. Cav-3 WT and Cav-3 T78M mutant localized at the plasma membrane, whereas Cav-3 T78K was retained in a perinuclear compartment. Cav-3 T78K expression was decreased by 87% when compared with Cav-3 WT, whereas Cav-3 T78M protein levels were unchanged. To evaluate whether Cav-3 T78K and Cav-3 T78M mutants behaved with a dominant negative pattern, Cos-7 cells were cotransfected with green fluorescent protein (GFP)-Cav-3 WT in combination with either mutant or WT Cav-3. When cotransfected with Cav-3 WT or Cav-3 T78M, GFP-Cav-3 WT was localized at the plasma membrane, as expected. However, when cotransfected with Cav-3 T78K, GFP-Cav-3 WT was retained in a perinuclear compartment, and its protein levels were reduced by 60%, suggesting a dominant negative action. Accordingly, Cav-3 protein levels in muscles from a biopsy of patient 2 (T78K mutation) were reduced by 80%. In conclusion, CAV3 T78M and T78K mutations lead to distinct disorders showing different clinical features and inheritance, and displaying distinct phenotypes in vitro.

Activation of the proteasome pathway is one of the secondary processes of cell damage, which ultimately lead to muscle degeneration and necrosis in Duchenne muscular dystrophy (DMD). In mdx mice, the proteasome inhibitor bortezomib up-regulates the membrane expression of members of the dystrophin complex and reduces the inflammatory reaction. However, chronic inhibition of the 26S proteasome may be toxic, as indicated by the systemic side-effects caused by this drug. Therefore, we sought to determine the components of the ubiquitin-proteasome pathway that are specifically activated in human dystrophin-deficient muscles. The analysis of a cohort of patients with genetically determined DMD or Becker muscular dystrophy (BMD) unveiled a selective up-regulation of the ubiquitin ligase tripartite motif-containing protein 32 (TRIM32). The induction of TRIM32 was due to a transcriptional effect and it correlated with disease severity in BMD patients. In contrast, atrogin1 and muscle RING-finger protein-1 (MuRF-1), which are strongly increased in distinct types of muscular atrophy, were not affected by the DMD dystrophic process. Knock-out models showed that TRIM32 is involved in ubiquitination of muscle cytoskeletal proteins as well as of protein inhibitor of activated STAT protein gamma (Piasγ) and N-myc downstream-regulated gene, two inhibitors of satellite cell proliferation and differentiation. Accordingly, we showed that in DMD/BMD muscle tissue, TRIM32 induction was more pronounced in regenerating myofibers rather than in necrotic muscle cells, thus pointing out a role of this protein in the regulation of human myoblast cell fate. This finding highlights TRIM32 as a possible therapeutic target to favor skeletal muscle regeneration in DMD patients.

Chromodomain helicase DNA-binding protein 2 (CHD2) gene mutations have been reported in patients with myoclonic-atonic epilepsy (MAE), as well as in patients with Lennox-Gastaut, Dravet, and Jeavons syndromes and other epileptic encephalopathies featuring generalized epilepsy and intellectual disability. The aim of this study was to assess the impact of CHD2 mutations in a series of patients with MAE. Twenty patients affected by MAE were included in the study. We analyzed antecedents, age at onset, seizure semiology and frequency, EEG, treatment, and neuropsychological outcome. We sequenced the CHD2 gene with Sanger technology. We identified a CHD2 frameshift mutation in one patient (c.4256del19). He was a 17-year-old boy with no familial history for epilepsy and normal development before epilepsy onset. Epilepsy onset was at 3years and 5months: he presented with myoclonic-atonic seizures, head drops, myoclonic jerks, and absences. Interictal EEGs revealed slow background activity associated with generalized epileptiform abnormalities and photoparoxysmal response. His seizures were highly responsive to valproic acid, and an attempt to withdraw it led to seizure recurrence. Neuropsychological evaluation revealed moderate intellectual disability. Chromodomain-helicase-DNA-binding protein 2 is not the major gene associated with MAE. Conversely, CHD2 could be responsible for a proper phenotype characterized by infantile-onset generalized epilepsy, intellectual disability, and photosensitivity, which might overlap with MAE, Lennox-Gastaut, Dravet, and Jeavons syndromes.

Mutations in the Caveolin-3 gene ( CAV3 ) have been associated with heterogeneous overlapping phenotypes, including: Limb Girdle Muscular Dystrophy type 1C (LGMD-1C), Isolated Elevated Serum Creatine Kinase (HyperCKemia), Distal Myopathy (DM), Rippling Muscle Disease (RMD), Hypertrophic Cardiomyopathy (HCM) and Congenital Long-QT Syndrome (LQTS).1 2 Hereditary RMD is a rare muscle disorder that is characterised by muscle stiffness, exercise-induced myalgias, cramp-like sensations and self-propagating rippling of muscles induced by stretch or percussion. Muscle hypertrophy and increased serum CK levels are also typical features. RMD is usually transmitted in an autosomal-dominant manner, but one homozygous missense mutation (A92T) was recently identified in two patients with a recessive form of RMD.3 Caveolin-3 (Cav-3) is a 22 kDa protein that is composed of 150 amino-acid residues, which are encoded by the CAV3 gene (MIM #601253) on chromosome 3p25.1 Here, we report the clinical, morphological and molecular analysis of a patient with autosomal-recessive RMD carrying two novel compound heterozygous CAV3 mutations that lead to a severe protein truncation. At the age of 34 years, a woman was referred to our attention for generalised muscle weakness, exercise-related cramps and myalgias, and hyperCKemia, symptoms that were present since her childhood. Physical examination revealed muscular weakness (particularly severe in the lower limbs), …

15q.13.3 microdeletion has been described in a variety of neurodevelopmental disorders. Epilepsy appears to be a common feature and, specifically, the 15q13.3 microdeletion is found in about 1% of patients with idiopathic generalized epilepsy. Recently, absence seizures with intellectual disability (ID) have been reported in patients carrying this mutation. We describe two families in which several affected members carry a 15q13.3 microdeletion in a pattern suggestive of autosomal dominant inheritance. Their phenotype includes mainly absence epilepsy and mild ID, suggesting only similarities with genetic/idiopathic generalized epilepsies but not typical features. The importance of studying such families is crucial to broaden the phenotype and understand the long-term outcome of patients with this condition.

Alternating hemiplegia of childhood is a rare, predominantly sporadic disorder. Diagnosis is clinical, and little is known about genetics. Glucose transporter 1 deficiency syndrome shares with alternating hemiplegia of childhood paroxysmal and nonparoxysmal symptoms. The aim of the study was to investigate glucose transporter 1 mutations in 30 Italian patients. Genetic material was analyzed by DNA amplification and glucose transporter 1 region sequencing. Mutational analysis findings of the SLC2A1 gene were negative in all patients. The pattern of movement disorders was reviewed. Interictal dystonia and multiple paroxysmal events were typical of alternating hemiplegia of childhood. In conclusion, alternating hemiplegia of childhood is a heterogeneous clinical condition, and although glucose transporter 1 deficiency can represent an undiagnosed cause of this disorder, mutational analysis is not routinely recommended. Alternatively, a careful clinical analysis and the 3-O-methyl-D-glucose uptake test can allow prompt identification of a subgroup of patients with alternating hemiplegia of childhood treatable with a ketogenic diet.

Objective To perform an extensive search for genomic rearrangements by microarray-based comparative genomic hybridization in patients with epilepsy. Design Prospective cohort study. Setting Epilepsy centers in Italy. Patients Two hundred seventy-nine patients with unexplained epilepsy, 265 individuals with nonsyndromic mental retardation but no epilepsy, and 246 healthy control subjects were screened by microarray-based comparative genomic hybridization. Main Outcomes Measures Identification of copy number variations (CNVs) and gene enrichment. Results Rare CNVs occurred in 26 patients (9.3%) and 16 healthy control subjects (6.5%) (P = .26). The CNVs identified in patients were larger (P = .03) and showed higher gene content (P = .02) than those in control subjects. The CNVs larger than 1 megabase (P = .002) and including more than 10 genes (P = .005) occurred more frequently in patients than in control subjects. Nine patients (34.6%) among those harboring rare CNVs showed rearrangements associated with emerging microdeletion or microduplication syndromes. Mental retardation and neuropsychiatric features were associated with rare CNVs (P = .004), whereas epilepsy type was not. The CNV rate in patients with epilepsy and mental retardation or neuropsychiatric features is not different from that observed in patients with mental retardation only. Moreover, significant enrichment of genes involved in ion transport was observed within CNVs identified in patients with epilepsy. Conclusions Patients with epilepsy show a significantly increased burden of large, rare, gene-rich CNVs, particularly when associated with mental retardation and neuropsychiatric features. The limited overlap between CNVs observed in the epilepsy group and those observed in the group with mental retardation only as well as the involvement of specific (ion channel) genes indicate a specific association between the identified CNVs and epilepsy. Screening for CNVs should be performed for diagnostic purposes preferentially in patients with epilepsy and mental retardation or neuropsychiatric features.

Summary: Purpose: Benign familial infantile seizures (BFIS) is a genetically heterogeneous condition characterized by partial seizures, onset age from 3 to 9 months, and favorable outcome. BFIS loci were identified on chromosomes 19q12-13.1 and 16p12-q12, allelic to infantile convulsions and choreathetosis. The identification of SCN2A mutations in families with only infantile seizures indicated that BFNIS and BFIS may show overlapping clinical features. Infantile seizures also were in a family with familial hemiplegic migraine and mutations in the ATP1A2 gene. We have examined the heterogeneous genetics of BFIS by means of linkage analysis. Methods: Sixteen families were examined. Probands underwent neurologic examination, at least one EEG recording, and, when possible, brain CT and MRI. Clinical information about relatives was collected. Families with SCN2A or ATP1A2 mutations were excluded from the study. Chromosome 16p and 19q loci were examined by linkage analysis using two models that differed in penetrance rate. Genetic heterogeneity was evaluated with both models. Results: Clinical information was available for 124 members of affected families. BFIS was diagnosed in 69 subjects. One patient without BFIS had a single febrile seizure, and another had rare episodes of paroxysmal dystonia. Evidence of linkage was obtained only for chromosome 16. Moreover, the high penetrance allowed the identification of genetic heterogeneity. Conclusions: Our data confirm the relevance of the chromosome 16 locus in BFIS and suggest the presence of an additional locus. This study shows that the genetic model used affects the outcome of linkage analysis.

Genetic testing of Duchenne and Becker muscular dystrophies (DMD/BMD) is a difficult task due to the occurrence of deletions or duplications within dystrophin (DMD) gene that requires dose sensitive tests. We developed three multiplex quantitative real-time PCR assays for dystrophin exon 5, 45, and 51 within two major hotspots of deletion/duplication. Each exon was co-amplified with a reference X-linked gene and the copy number of the target fragment was calculated by comparative threshold cycle method (delta deltaC(t)). We compared the performance of this method with previously described end-point PCR fluorescent analysis (EPFA) by studying 24 subjects carrying DMD deletions or duplications. We showed that Q-PCR is an accurate and sensitive technique for the identification of deletions and duplications in DMD/BMD. Q-PCR is a valuable tool for independent confirmation of EPFA screening, particularly when deletions/duplications of single exons occur or for rapid identification of known mutations in at risk carriers.

In this report, we have developed a novel method to identify compounds that rescue the dystrophin-glycoprotein complex (DGC) in patients with Duchenne or Becker muscular dystrophy. Briefly, freshly isolated skeletal muscle biopsies (termed skeletal muscle explants) from patients with Duchenne or Becker muscular dystrophy were maintained under defined cell culture conditions for a 24-h period in the absence or presence of a specific candidate compound. Using this approach, we have demonstrated that treatment with a well-characterized proteasome inhibitor, MG-132, is sufficient to rescue the expression of dystrophin, β-dystroglycan, and α-sarcoglycan in skeletal muscle explants from patients with Duchenne or Becker muscular dystrophy. These data are consistent with our previous findings regarding systemic treatment with MG-132 in a dystrophin-deficient mdx mouse model (Bonuccelli G, Sotgia F, Schubert W, Park D, Frank PG, Woodman SE, Insabato L, Cammer M, Minetti C, and Lisanti MP. Am J Pathol 163: 1663–1675, 2003). Our present results may have important new implications for the possible pharmacological treatment of Duchenne or Becker muscular dystrophy in humans.

We report a novel nonsense mitochondrial cytochrome b mutation (G15170A) in a 40-year-old woman with progressive exercise intolerance and lactic acidosis. Muscle biopsy showed several cytochrome c oxidase-positive ragged-red fibers, and reduced activities of respiratory chain complexes I and III. This mutation, resulting in the loss of 228 amino acids of the protein, was very abundant in the patient's muscle, but undetectable in lymphocytes and fibroblasts. Clinical and laboratory data indicate that this defect is the primary cause of the disease, thus adding a new mutation in the cytochrome b gene among the growing number of patients with exercise intolerance and lactic acidosis.