Phenotypic spectrum and clinical course of single large-scale mitochondrial DNA deletion disease in the paediatric population: a multicentre study.

BACKGROUND: Large-scale mitochondrial DNA deletions (LMD) are a common genetic cause of mitochondrial disease and give rise to a wide range of clinical features. Lack of longitudinal data means the natural history remains unclear. This study was undertaken to describe the clinical spectrum in a large cohort of patients with paediatric disease onset. METHODS: A retrospective multicentre study was performed in patients with clinical onset <16 years of age, diagnosed and followed in seven European mitochondrial disease centres. RESULTS: A total of 80 patients were included. The average age at disease onset and at last examination was 10 and 31 years, respectively. The median time from disease onset to death was 11.5 years. Pearson syndrome was present in 21%, Kearns-Sayre syndrome spectrum disorder in 50% and progressive external ophthalmoplegia in 29% of patients. Haematological abnormalities were the hallmark of the disease in preschool children, while the most common presentations in older patients were ptosis and external ophthalmoplegia. Skeletal muscle involvement was found in 65% and exercise intolerance in 25% of the patients. Central nervous system involvement was frequent, with variable presence of ataxia (40%), cognitive involvement (36%) and stroke-like episodes (9%). Other common features were pigmentary retinopathy (46%), short stature (42%), hearing impairment (39%), cardiac disease (39%), diabetes mellitus (25%) and renal disease (19%). CONCLUSION: Our study provides new insights into the phenotypic spectrum of childhood-onset, LMD-associated syndromes. We found a wider spectrum of more prevalent multisystem involvement compared with previous studies, most likely related to a longer time of follow-up.

Epidemiology, Survival, and Clinical Characteristics of Inclusion Body Myositis.

OBJECTIVE: We performed a population-based study on inclusion body myositis with the primary aims to define the prevalence, survival rate, and incidence, and to investigate the symptom profiles associated with disease duration and sex over a 33-year period. METHODS: Patients diagnosed between 1985 and 2017 in Region Västra Götaland, Sweden, were identified according to the European Neuromuscular Centre diagnostic criteria from 2011. RESULTS: We identified 128 patients, 89 men and 39 women, with the strict clinicopathological definition of inclusion body myositis. The prevalence was 32 per million inhabitants, 19 per million women and 45 per million men, by December 31, 2017. Mean incidence was 2.5 per million inhabitants and year. Mean age at symptom onset was 64.4 years with quadriceps weakness being the most common presenting symptom followed by finger flexor weakness. Dysphagia was a common presenting symptom being more frequent in women (23%) than men (10%) and was during the disease course reported in 74% of men and 84% of women. Seventy-three patients were deceased, with a mean survival of 14 years from symptom onset. Survival rates from both diagnosis date and symptom onset were decreased compared to the matched population. Twenty-one percent of the patients had an additional autoimmune disease. A cross-sectional analysis of autoantibodies in 50 patients and 28 matched controls showed autoantibodies to cytosolic 5'-nucleotidase 1A in 40% of the patients and 3.6% of controls. INTERPRETATION: Inclusion body myositis is an autoimmune disease with decreased survival rate and with marked sex differences in both prevalence and clinical manifestations. ANN NEUROL 2022;92:201-212.

Proteomic characterisation of polyglucosan bodies in skeletal muscle in RBCK1 deficiency.

AIMS: Several neurodegenerative and neuromuscular disorders are characterised by storage of polyglucosan, consisting of proteins and amylopectin-like polysaccharides, which are less branched than in normal glycogen. Such diseases include Lafora disease, branching enzyme deficiency, glycogenin-1 deficiency, polyglucosan body myopathy type 1 (PGBM1) due to RBCK1 deficiency and others. The protein composition of polyglucosan bodies is largely unknown. METHODS: We combined quantitative mass spectrometry, immunohistochemical and western blot analyses to identify the principal protein components of polyglucosan bodies in PGBM1. Histologically stained tissue sections of skeletal muscle from four patients were used to isolate polyglucosan deposits and control regions by laser microdissection. Prior to mass spectrometry, samples were labelled with tandem mass tags that enable quantitative comparison and multiplexed analysis of dissected samples. To study the distribution and expression of the accumulated proteins, immunohistochemical and western blot analyses were performed. RESULTS: Accumulated proteins were mainly components of glycogen metabolism and protein quality control pathways. The majority of fibres showed depletion of glycogen and redistribution of key enzymes of glycogen metabolism to the polyglucosan bodies. The polyglucosan bodies also showed accumulation of proteins involved in the ubiquitin-proteasome and autophagocytosis systems and protein chaperones. CONCLUSIONS: The sequestration of key enzymes of glycogen metabolism to the polyglucosan bodies may explain the glycogen depletion in the fibres and muscle function impairment. The accumulation of components of the protein quality control systems and other proteins frequently found in protein aggregate disorders indicates that protein aggregation may be an essential part of the pathobiology of polyglucosan storage.

Dominantly inherited myosin IIa myopathy caused by aberrant splicing of MYH2.

BACKGROUND: Myosin heavy chain (MyHC) isoforms define the three major muscle fiber types in human extremity muscles. Slow beta/cardiac MyHC (MYH7) is expressed in type 1 muscle fibers. MyHC IIa (MYH2) and MyHC IIx (MYH1) are expressed in type 2A and 2B fibers, respectively. Whereas recessive MyHC IIa myopathy has been described in many cases, myopathy caused by dominant MYH2 variants is rare and has been described with clinical manifestations and muscle pathology in only one family and two sporadic cases. METHODS: We investigated three patients from one family with a dominantly inherited myopathy by clinical investigation, whole-genome sequencing, muscle biopsy, and magnetic resonance imaging (MRI). RESULTS: Three siblings, one woman and two men now 54, 56 and 66 years old, had experienced muscle weakness initially affecting the lower limbs from young adulthood. They have now generalized proximal muscle weakness affecting ambulation, but no ophthalmoplegia. Whole-genome sequencing identified a heterozygous MYH2 variant, segregating with the disease in the three affected individuals: c.5673 + 1G > C. Analysis of cDNA confirmed the predicted splicing defect with skipping of exon 39 and loss of residues 1860-1891 in the distal tail of the MyHC IIa, largely overlapping with the filament assembly region (aa1877-1905). Muscle biopsy in two of the affected individuals showed prominent type 1 muscle fiber predominance with only a few very small, scattered type 2A fibers and no type 2B fibers. The small type 2A fibers were frequently hybrid fibers with either slow MyHC or embryonic MyHC expression. The type 1 fibers showed variation in fiber size, internal nuclei and some structural alterations. There was fatty infiltration, which was also demonstrated by MRI. CONCLUSION: Dominantly inherited MyHC IIa myopathy due to a splice defect causing loss of amino acids 1860-1891 in the distal tail of the MyHC IIa protein including part of the assembly competence domain. The myopathy is manifesting with slowly progressive muscle weakness without overt ophthalmoplegia and markedly reduced number and size of type 2 fibers.

Progressive external ophthalmoplegia associated with novel MT-TN mutations.

OBJECTIVES: To describe two patients with progressive external ophthalmoplegia (PEO) and mitochondrial myopathy associated with mutations in mitochondrial DNA, encoding the tRNAAsn gene (MT-TN), which have not previously been published with clinical descriptions. MATERIALS & METHODS: Two unrelated patients with PEO were clinically examined. Muscle biopsy was performed and investigated by exome sequencing, enzyme histochemistry, and immunohistochemistry. The level of heteroplasmy was investigated in single muscle fibers and in other tissues. RESULTS: Patient 1 was a 52-year-old man with ptosis, PEO, and exercise intolerance since childhood. Muscle biopsy demonstrated mitochondrial myopathy with frequent cytochrome c oxidase (COX)-deficient fibers and a heteroplasmic mutation, m.5669G>A in the MT-TN gene, resulting in a substitution of a highly conserved C to T in the T stem of tRNAAsn . Patient 2 was a 66-year-old woman with ptosis, PEO, and exercise intolerance since many years. Muscle biopsy demonstrated mitochondrial myopathy with frequent COX-deficient fibers. She had a novel m.5702delA mutation in MT-TN, resulting in loss of a highly conserved U in the anticodon stem of tRNAAsn . Single fiber analysis in both cases showed highly significant differences in mutation load between COX-deficient and COX-normal fibers and a high threshold level for COX deficiency. The mutations were not found in blood, urine sediment or buccal cells. CONCLUSION: We describe two MT-TN mutations associated with PEO and mitochondrial myopathy, and their pathogenicity was demonstrated. Together with previous reports, the results indicate that MT-TN is a hot spot for mutations causing sporadic PEO.

Loss of supervillin causes myopathy with myofibrillar disorganization and autophagic vacuoles.

The muscle specific isoform of the supervillin protein (SV2), encoded by the SVIL gene, is a large sarcolemmal myosin II- and F-actin-binding protein. Supervillin (SV2) binds and co-localizes with costameric dystrophin and binds nebulin, potentially attaching the sarcolemma to myofibrillar Z-lines. Despite its important role in muscle cell physiology suggested by various in vitro studies, there are so far no reports of any human disease caused by SVIL mutations. We here report four patients from two unrelated, consanguineous families with a childhood/adolescence onset of a myopathy associated with homozygous loss-of-function mutations in SVIL. Wide neck, anteverted shoulders and prominent trapezius muscles together with variable contractures were characteristic features. All patients showed increased levels of serum creatine kinase but no or minor muscle weakness. Mild cardiac manifestations were observed. Muscle biopsies showed complete loss of large supervillin isoforms in muscle fibres by western blot and immunohistochemical analyses. Light and electron microscopic investigations revealed a structural myopathy with numerous lobulated muscle fibres and considerable myofibrillar alterations with a coarse and irregular intermyofibrillar network. Autophagic vacuoles, as well as frequent and extensive deposits of lipoproteins, including immature lipofuscin, were observed. Several sarcolemma-associated proteins, including dystrophin and sarcoglycans, were partially mis-localized. The results demonstrate the importance of the supervillin (SV2) protein for the structural integrity of muscle fibres in humans and show that recessive loss-of-function mutations in SVIL cause a distinctive and novel myopathy.

Improvements in health status and utility associated with ataluren for the treatment of nonsense mutation Duchenne muscular dystrophy.

INTRODUCTION: Little is known of the impact of ataluren on health status and utility in patients with nonsense mutation Duchenne muscular dystrophy (nmDMD). In this work we sought to investigate the clinical expert consensus of these topics in a Delphi panel study. METHODS: Six Swedish neuromuscular experts participated in this study. Consensus was investigated for responses to the Health Utilities Index (HUI) and a visual analog scale (VAS) for ambulatory and nonambulatory patients treated with ataluren plus best supportive care vs best supportive care alone. RESULTS: Consensus was obtained after three panel rounds across treatments and disease stages, except for HUI question 10 (ability to use hands and fingers) for nonambulatory patients. Utility differences between treatments were 0.31 for ambulatory patients, and 0.15 to 0.18 for nonambulatory patients, respectively. The corresponding VAS differences were 12 and 13. DISCUSSION: The outcomes of this study support the association of ataluren for the treatment of nmDMD with improved health status and utility.

Perceived fatigue in myotonic dystrophy type 1: a case-control study.

BACKGROUND: The aim of this study was to explore perceived fatigue, experienced functional limitations due to fatigue and clinical correlates in patients with Myotonic Dystrophy type 1 (DM1). METHODS: In total, 32 consecutive patients with DM1 (14 women and 18 men) and 30 sex, age and education matched healthy control subjects participated. Perceived fatigue was rated on the Fatigue Impact Scale (FIS). Patients also completed a set of assessments aimed to characterize CTG-repeat size, muscle impairment, depression and cognitive functions. Non-parametric analysis were performed as appropriate, including Mann-Whitney U-test and Spearman correlation test. RESULTS: DM1 patients had higher FIS total score than healthy controls, suggesting higher fatigue levels. More specifically, DM1 patients scored higher on the FIS physical and psychosocial subscales than controls but not on the FIS cognitive scale. Scores on fatigue correlated significantly with muscle impairment and depression. CONCLUSIONS: Perceived fatigue is significantly more common in patients with DM1 than in healthy controls. Higher ratings on depression and muscle impairment were associated with the condition. This indicates that both depression and muscle impairment may contribute to the experience of fatigue in DM1.

Association between muscle strength, histopathology, and magnetic resonance imaging in sporadic inclusion body myositis.

OBJECTIVES: Inclusion body myositis is characterized by inflammatory and degenerative changes, but the temporal relation of these events is unknown. MATERIALS AND METHODS: In nineteen patients with inclusion body myositis, muscle strength was correlated with inflammatory and degenerative findings on magnetic resonance imaging (MRI) and in muscle biopsies in three different muscles (tibialis anterior, vastus lateralis, and biceps brachii). Muscle strength, measured with a handheld dynamometer, was described as percentage of muscle strength in age- and sex-matched normal individuals. The muscles were categorized as the strongest, the intermediate, and the weakest muscle in each individual. T1-weighted sequences on MRI were used to evaluate the degree of fatty infiltration and muscle atrophy and STIR sequences to evaluate edematous changes. RESULTS: The vastus lateralis, which in general was the weakest muscle, was significantly more atrophic compared to the other two muscles and also demonstrated most edema. The biceps brachii had in most cases an intermediate degree of weakness and atrophy but the most pronounced inflammatory cell infiltration on biopsy. Cytochrome c oxidase-negative muscle fibers were significantly more prevalent in the vastus lateralis and biceps brachii muscles than in the tibialis anterior and thus correlated with muscular atrophy, indicating that this is a secondary change. Inflammatory changes as assessed by MRI and muscle biopsy were seen in all muscles irrespective of atrophy and thus appear to be prevalent at all stages of the disease. CONCLUSIONS: Our study could not provide an answer to the question which comes first, the inflammation or the degenerative changes.

Subnormal levels of POLγA cause inefficient initiation of light-strand DNA synthesis and lead to mitochondrial DNA deletions and progressive external ophthalmoplegia [corrected].

The POLG1 gene encodes the catalytic subunit of mitochondrial DNA (mtDNA) polymerase γ (POLγ). We here describe a sibling pair with adult-onset progressive external ophthalmoplegia, cognitive impairment and mitochondrial myopathy characterized by DNA depletion and multiple mtDNA deletions. The phenotype is due to compound heterozygous POLG1 mutations, T914P and the intron mutation c.3104 + 3A > T. The mutant genes produce POLγ isoforms with heterozygous phenotypes that fail to synthesize longer DNA products in vitro. However, exon skipping in the c.3104 + 3A > T mutant is not complete, and the presence of low levels of wild-type POLγ explains patient survival. To better understand the underlying pathogenic mechanisms, we characterized the effects of POLγ depletion in vitro and found that leading-strand DNA synthesis is relatively undisturbed. In contrast, initiation of lagging-strand DNA synthesis is ineffective at lower POLγ concentrations that uncouples leading strand from lagging-strand DNA synthesis. In vivo, this effect leads to prolonged exposure of the heavy strand in its single-stranded conformation that in turn can cause the mtDNA deletions observed in our patients. Our findings, thus, suggest a molecular mechanism explaining how POLγ mutations can cause mtDNA deletions in vivo.

Methods of diagnosis of patients with Pompe disease: Data from the Pompe Registry.

Pompe disease is a rare, autosomal recessive disorder characterized by deficiency of lysosomal acid alpha-glucosidase and accumulation of lysosomal glycogen in many tissues. The variable clinical manifestations, broad phenotypic spectrum, and overlap of signs and symptoms with other neuromuscular diseases make diagnosis challenging. In the past, the diagnosis of Pompe disease was based on enzyme activity assay in skin fibroblasts or muscle tissue. In 2004, methods for measuring acid alpha-glucosidase activity in blood were published. To compare how diagnostic methods changed over time and whether they differed by geographic region and clinical phenotype, we examined diagnostic methods used for 1059 patients enrolled in the Pompe Registry in three onset categories (Group A: onset of signs/symptoms ≤ 12 months of age with cardiomyopathy; Group B: onset ≤ 12 months without cardiomyopathy and onset >1 year to ≤ 12 years; Group C: onset >12 years). Enzyme activity-based assays were used more frequently than other diagnostic methods. Measuring acid alpha-glucosidase activity in blood (leukocytes, lymphocytes, or dried-blood spot) increased over time; use of muscle biopsy decreased. The increased use of blood-based assays for diagnosis may result in a more timely diagnosis in patients across the clinical spectrum of Pompe disease.

Polyglucosan body myopathy caused by defective ubiquitin ligase RBCK1.

Glycogen storage diseases are important causes of myopathy and cardiomyopathy. We describe 10 patients from 8 families with childhood or juvenile onset of myopathy, 8 of whom also had rapidly progressive cardiomyopathy, requiring heart transplant in 4. The patients were homozygous or compound heterozygous for missense or truncating mutations in RBCK1, which encodes for a ubiquitin ligase, and had extensive polyglucosan accumulation in skeletal muscle and in the heart in cases of cardiomyopathy. We conclude that RBCK1 deficiency is a frequent cause of polyglucosan storage myopathy associated with progressive muscle weakness and cardiomyopathy.

Clinical stage and plasma neurofilament concentration in adults with Friedreich ataxia.

Objectives: Friedreich Ataxia (FRDA) is the most common recessive ataxia disorder. Yet, little is known of the prevalence in Sweden. In the future, there may be effective disease-modifying therapies, and use of clinical rating scales as well as possible biomarkers in serum or cerebrospinal fluid may be of importance. We evaluated the axonal protein neurofilament light in plasma (p-NfL) as a possible biomarker for disease severity in FRDA. Materials & methods: We searched for all possible genetically confirmed FRDA cases in the Västra Götaland Region (VGR) of Sweden, and investigated each patient clinically and obtained blood sample for analysis of p-NfL. Results: We found eight patients corresponding to 1/170.000 adults in the VGR, and 5 of these participated in the study. Three out of the five FRDA patients displayed a small or moderate increase in the p-NfL value, compared to the age-adjusted cut-offs for p-NfL established in the Clinical Neurochemistry Laboratory at our hospital. The two others were the oldest and most severely affected, displayed normal values according the cut-off values. The cohort is too small to make any statistically significant correlation between the five p-NfL values with regard to disease severity. Conclusions: FRDA is less prevalent in our region of Sweden than could be assumed. In concordance with previous studies from other authors, we find that p-NfL may be increased in patients with FRDA, but less so in older more clinically affected patients. Thus, we conclude that on an individual basis, p-NFL is of uncertain clinical value as a suitable biomarker.

Neurocognitive disorder in Myotonic dystrophy type 1.

Cognitive deficits and abnormal cognitive aging have been associated with Myotonic dystrophy type 1 (DM1), but the knowledge of the extent and progression of decline is limited. The aim of this study was to examine the prevalence of signs of neurocognitive disorder (mild cognitive impairment and dementia) in adult patients with DM1. A total of 128 patients with childhood, juvenile, adult, and late onset DM1 underwent a screening using the Montreal Cognitive Assessment (MoCA). Demographic and clinical information was collected. The results revealed that signs of neurocognitive disorder were relatively rare among the participants. However, 23.8 % of patients with late onset DM1 (aged over 60 years) scored below MoCA cut-off (=23), and this group also scored significantly worse compared to patients with adult onset. Age at examination were negatively correlated with MoCA scores, although it only explained a small portion of the variation in test results. Other demographic and clinical factors showed no association with MoCA scores. In conclusion, our findings indicate a low prevalence of signs of neurocognitive disorder in adult patients with DM1, suggesting that cognitive deficits rarely progress to severe disorders over time. However, the performance of patients with late onset DM1 suggests that this phenotype warrants further exploration in future studies, including longitudinal and larger sample analyses.

Glycogenin-1 deficiency and inactivated priming of glycogen synthesis.

Glycogen, which serves as a major energy reserve in cells, is a large, branched polymer of glucose molecules. We describe a patient who had muscle weakness, associated with the depletion of glycogen in skeletal muscle, and cardiac arrhythmia, associated with the accumulation of abnormal storage material in the heart. The skeletal muscle showed a marked predominance of slow-twitch, oxidative muscle fibers and mitochondrial proliferation. Western blotting showed the presence of unglucosylated glycogenin-1 in the muscle and heart. Sequencing of the glycogenin-1 gene, GYG1, revealed a nonsense mutation in one allele and a missense mutation, Thr83Met, in the other. The missense mutation resulted in inactivation of the autoglucosylation of glycogenin-1 that is necessary for the priming of glycogen synthesis in muscle.

Timing of diagnosis of patients with Pompe disease: data from the Pompe registry.

Diagnostic delays in Pompe disease are common. The diagnostic gap (the time from the onset of symptoms to the diagnosis of Pompe disease) and factors associated with diagnostic delays were examined among Pompe Registry patients in three onset categories: Group A, onset ≤12 months of age with cardiomyopathy; Group B, onset ≤12 months without cardiomyopathy and onset >12 months to ≤12 years; and Group C, onset >12 years. Of 1,003 patients, 647 were available for analysis. In all groups, musculoskeletal signs and symptoms were among the most frequent presenting signs and symptoms, in addition to cardiomyopathy in Group A, which was part of the group's definition. Diagnostic gaps existed in all three groups. Patients presenting with respiratory and musculoskeletal signs and symptoms concurrently had the shortest diagnostic gap, while those presenting with neither respiratory nor musculoskeletal signs and symptoms had the longest. Independent factors influencing the probability of a long diagnostic gap included presenting signs and symptoms (all three groups) and year of diagnosis and age at symptom onset (Groups B and C). Group B, which represents the infantile patients without cardiomyopathy and juvenile Pompe cases, had the longest median gap (12.6 years). Diagnostic testing methods used also were reviewed. Despite the availability of blood-based assays that can be used to quickly and accurately diagnose Pompe disease, diagnostic gaps in Pompe patients across the disease spectrum continue.

Hereditary myopathy with early respiratory failure associated with a mutation in A-band titin.

Hereditary myopathy with early respiratory failure and extensive myofibrillar lesions has been described in sporadic and familial cases and linked to various chromosomal regions. The mutated gene is unknown in most cases. We studied eight individuals, from three apparently unrelated families, with clinical and pathological features of hereditary myopathy with early respiratory failure. The investigations included clinical examination, muscle histopathology and genetic analysis by whole exome sequencing and single nucleotide polymorphism arrays. All patients had adult onset muscle weakness in the pelvic girdle, neck flexors, respiratory and trunk muscles, and the majority had prominent calf hypertrophy. Examination of pulmonary function showed decreased vital capacity. No signs of cardiac muscle involvement were found. Muscle histopathological features included marked muscle fibre size variation, fibre splitting, numerous internal nuclei and fatty infiltration. Frequent groups of fibres showed eosinophilic inclusions and deposits. At the ultrastructural level, there were extensive myofibrillar lesions with marked Z-disc alterations. Whole exome sequencing in four individuals from one family revealed a missense mutation, g.274375T>C; p.Cys30071Arg, in the titin gene (TTN). The mutation, which changes a highly conserved residue in the myosin binding A-band titin, was demonstrated to segregate with the disease in all three families. High density single nucleotide polymorphism arrays covering the entire genome demonstrated sharing of a 6.99 Mb haplotype, located in chromosome region 2q31 including TTN, indicating common ancestry. Our results demonstrate a novel and the first disease-causing mutation in A-band titin associated with hereditary myopathy with early respiratory failure. The typical histopathological features with prominent myofibrillar lesions and inclusions in muscle and respiratory failure early in the clinical course should be incentives for analysis of TTN mutations.

Inclusion body myositis with early onset: a population-based study.

INTRODUCTION: Inclusion body myositis (IBM), an inflammatory myopathy with progressive weakness without efficient treatment, typically presents after 45 years of age and younger patients are sparsely studied. METHODS: In a population-based study during a 33-year period, 142 patients with IBM were identified in western Sweden. Six patients fell outside the European Neuromuscular Centre 2011 criteria for IBM due to young age at symptom onset, verified by a muscle biopsy < 50 years of age. These were defined as early-onset IBM and included in this study. Medical records, muscle strength, comorbidities, muscle biopsies, and nuclear- and mitochondrial DNA were examined and compared with patients with IBM and age matched controls from the same population. RESULTS: The median age at symptom onset was 36 (range 34-45) years and at diagnosis 43 (range 38-58) years. Four patients were deceased at a median age of 59 (range 50-75) years. The median survival from diagnosis was 14 (range 10-18) years. The prevalence December 31 2017 was 1.2 per million inhabitants and the mean incidence 0.12 patients per million inhabitants and year. The mean decline in quadriceps strength ± 1 standard deviation was 1.21 ± 0.2 Newton or 0.91 ± 0.2% per month and correlated to time from diagnosis (p < 0.001). Five patients had swallowing difficulties. All patients displayed mitochondrial changes in muscle including cytochrome c oxidase deficiency and the mitochondrial DNA mutation load was high. CONCLUSIONS: Early-onset IBM is a severe disease, causing progressive muscle weakness, high muscle mitochondrial DNA mutation load and a reduced cumulative survival in young and middle-aged individuals.

Estimation of kidney function in patients with primary neuromuscular diseases: is serum cystatin C a better marker of kidney function than creatinine?

BACKGROUND: Using serum creatinine leads to an overestimation of kidney function in patients with primary neuromuscular disorders, and reduced kidney function may remain undetected. Cystatin C (CysC) could provide a better estimation. AIM: To evaluate the precision, accuracy, and bias of two creatinine-, one cystatin C-based and one combined equation to estimate glomerular filtration rate (eGFR) in patients with primary neuromuscular disease. PATIENTS AND METHODS: Of the 418 patients initially identified at the out-patient clinic, data on kidney function was obtained for 145 adult patients (age 46 ± 14 years, BMI 26 ± 6 kg/m2) with primary neuromuscular disease. Kidney function was measured by iohexol clearance, and blood samples for serum creatinine and CysC were drawn simultaneously. Bias was defined as the mean difference between eGFR and measured iohexol clearance, and accuracy as the proportion of eGFRs within ± 10% (P10) of measured clearance. RESULTS: Kidney function (iohexol clearance) was 81 ± 19 (38-134) ml/min/1.73m2. All equations overestimated kidney function by 22-60 ml/min/1.73m2. eGFR CysC had the lowest bias overall 22 (95% CI 20-26) ml/min/1.73m2 also at all levels of kidney function we evaluated (at 30-59 ml/min/1.73m2 bias was 27 (95% CI 21-35), at 60-89 it was 25 (95% CI 20-28) and at ≥ 90 it was 12 (95% CI 7-22)). eGFR CysC also had the best accuracy in patients with reduced kidney function (P10 was 5.9% at 30-59 ml/min/1.73m2). CONCLUSIONS: Cystatin C-based estimations of kidney function performed better than creatinine-based ones in patients with primary neuromuscular disease, but most importantly, all evaluated equations overestimated kidney function, especially in patients with reduced kidney function. Therefore, kidney function should be measured by gold-standard methods when precision and accuracy are needed.

Expression pattern of mitochondrial respiratory chain enzymes in skeletal muscle of patients with mitochondrial myopathy associated with the homoplasmic m.14674T>C variant.

Two homoplasmic variants in tRNAGlu (m.14674T>C/G) are associated with reversible infantile respiratory chain deficiency. This study sought to further characterize the expression of the individual mitochondrial respiratory chain complexes and to describe the natural history of the disease. Seven patients from four families with mitochondrial myopathy associated with the homoplasmic m.14674T>C variant were investigated. All patients underwent skeletal muscle biopsy and mtDNA sequencing. Whole-genome sequencing was performed in one family. Western blot and immunohistochemical analyses were used to characterize the expression of the individual respiratory chain complexes. Patients presented with hypotonia and feeding difficulties within the first weeks or months of life, except for one patient who first showed symptoms at 4 years of age. Histopathological findings in muscle included lipid accumulation, numerous COX-deficient fibers, and mitochondrial proliferation. Ultrastructural abnormalities included enlarged mitochondria with concentric cristae and dense mitochondrial matrix. The m.14674T>C variant in MT-TE was identified in all patients. Immunohistochemistry and immunoblotting demonstrated pronounced deficiency of the complex I subunit NDUFB8. The expression of MTCO1, a complex IV subunit, was also decreased, but not to the same extent as NDUFB8. Longitudinal follow-up data demonstrated that not all features of the disorder are entirely transient, that the disease may be progressive, and that signs and symptoms of myopathy may develop during childhood. This study sheds new light on the involvement of complex I in reversible infantile respiratory chain deficiency, it shows that the disorder may be progressive, and that myopathy can develop without an infantile episode.