Mitochondrial DNA variants in inclusion body myositis characterized by deep sequencing

NLRP3 Inflammasome Activation and Altered Mitophagy Are Key Pathways in Inclusion Body Myositis

BACKGROUND

Inclusion body myositis (IBM) is the most prevalent muscle disease in adults for which no current treatment exists. The pathogenesis of IBM remains poorly defined. In this study, we aimed to explore the interplay between inflammation and mitochondrial dysfunction in IBM.

METHODS

The study population consisted of 38 IBM patients and 22 age- and sex-matched controls without a myopathy. Mean age was 62.9 years (SD = 9) in IBM group and 59.7 (10) in controls. Bulk RNA sequencing, Meso Scale Discovery electrochemiluminescence (ECL), western blotting, histochemistry and immunohistochemistry were performed on frozen muscle samples from the study participants.

RESULTS

We demonstrated activation of the NLRP3 inflammasome in IBM muscle samples, with the NLRP3 inflammasome being the most upregulated pathway on RNA sequencing, along with increased expression of NLRP3 and ASC proteins in IBM group. NLRP3 RNA levels most strongly correlated with TLR7 (correlation coefficient ρ = 0.91) and complement activation-related genes, and inversely correlated with several mitochondria-related genes among others. On muscle histopathology, there was increased NRLP3 immunoreactivity in both inflammatory cells and muscle fibres. Mitophagy is critical for removing damaged mitochondria and preventing the formation of a vicious cycle of mitochondrial dysfunction-NLRP3 inflammasome activation. Herein, we showed altered mitophagy, as witnessed by the elevated levels of p-S65-Ubiquitin, a mitophagy marker, in muscle lysates from IBM patients compared to controls (median of 114.3 vs. 81.25 ECL units, p = 0.005). The p-S65-Ubiquitin levels were most significantly elevated in IBM males compared to male controls (136 vs. 83.5 ECL units; p = 0.013), whereas IBM females had milder nonsignificant elevation compared to female controls (97.25 vs. 69 ECL units, p = 0.31). On muscle histopathology, p-S65-Ubiquitin aggregates accumulated in muscle fibres that were mostly Type 2 and devoid of cytochrome-c-oxidase reactivity. NLRP3 RNA levels correlated with p-S65-Ubiquitin levels in both sexes (males: ρ = 0.48, females: ρ = 0.54) but with loss of muscle strength, as reflected by the manual motor test score, only in males (males: ρ = 0.62, females: ρ = -0.14). Lastly, we identified sex-specific molecular pathways in IBM. Females had upregulation of pathways related to response to stress, which could conceivably offset some of the pathomechanisms of IBM, while males had upregulation of pathways related to cell adhesion and migration.

CONCLUSIONS

There is activation of the NLRP3 inflammasome in IBM, along with altered mitophagy, particularly in males, which is of potential therapeutic significance. These findings suggest sex-specific mechanisms in IBM that warrant further investigation.

A window into intracellular events in myositis through subcellular proteomics

OBJECTIVE AND DESIGN

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of inflammatory muscle disorders of unknown etiology. It is postulated that mitochondrial dysfunction and protein aggregation in skeletal muscle contribute to myofiber degeneration. However, molecular pathways that lead to protein aggregation in skeletal muscle are not well defined.

SUBJECTS

Here we have isolated membrane-bound organelles (e.g., nuclei, mitochondria, sarcoplasmic/endoplasmic reticulum, Golgi apparatus, and plasma membrane) from muscle biopsies of normal (n = 3) and muscle disease patients (n = 11). Of the myopathy group, 10 patients displayed mitochondrial abnormalities (IIM (n = 9); mitochondrial myopathy (n = 1)), and one IIM patient did not show mitochondrial abnormalities (polymyositis).

METHODS

Global proteomic analysis was performed using an Orbitrap Fusion mass spectrometer. Upon unsupervised clustering, normal and mitochondrial myopathy muscle samples clustered separately from IIM samples.

RESULTS

We have confirmed previously known protein alterations in IIM and identified several new ones. For example, we found differential expression of (i) nuclear proteins that control cell division, transcription, RNA regulation, and stability, (ii) ER and Golgi proteins involved in protein folding, degradation, and protein trafficking in the cytosol, and (iii) mitochondrial proteins involved in energy production/metabolism and alterations in cytoskeletal and contractile machinery of the muscle.

CONCLUSIONS

Our data demonstrates that molecular alterations are not limited to protein aggregations in the cytosol (inclusions) and occur in nuclear, mitochondrial, and membrane compartments of IIM skeletal muscle.

Autoantibodies against a subunit of mitochondrial respiratory chain complex I in inclusion body myositis

BACKGROUND

Autoantibodies are found in up to 80 % of patients with idiopathic inflammatory myopathies (IIM) and are associated with distinct clinical phenotypes. Autoantibodies targeting cytosolic 5'-nucleotidase 1A (anti-NT5C1A) are currently the only known serum biomarker for the subgroup inclusion body myositis (IBM), although detected even in other autoimmune diseases. The aim of the study was to identify new autoimmune targets in IIM.

METHODS

In a first cross-sectional exploratory study, samples from 219 IIM (108 Polymyositis (PM), 80 Dermatomyositis (DM) and 31 IBM) patients, 349 Systemic Lupus Erythematosus (SLE) patients and 306 population controls were screened for IgG reactivity against a panel of 357 proteins using an antigen bead array. All samples were identified in the local biobank of the Rheumatology clinic, Karolinska University Hospital. Positive hits for the IBM subgroup were then validated in an independent larger cohort of 287 patients with IBM followed at nine European rheumatological or neurological centers. IBM serum samples were explored by antigen bead array and results validated by Western blot. As controls, sera from 29 patients with PM and 30 with DM, HLA-matched with the Swedish IBM cohort, were included. Demographics, laboratory, clinical, and muscle biopsy data of the IBM cohort was retrieved.

RESULTS

In the exploratory study, IgG reactivity towards NADH dehydrogenase 1 α subcomplex 11 (NDUFA11), a subunit of the membrane-bound mitochondrial respiratory chain complex I, was discovered with higher frequency in the IBM (9.7 %) than PM (2.8 %) and DM samples (1.3 %), although the difference was not statistically significant. Anti-NDUFA11 IgG was also found in 1.4 % of SLE and 2.0 % of population control samples. In the validation study, anti-NDUFA11 autoantibodies were detected in 10/287 IBM patients (3.5 %), 0/29 p.m. and 0/30 DM patients. Reactivity against NDUFA11 could be confirmed by Western blot. No statistically significant differences were found between patients with and without anti-NDUFA11 antibodies when comparing clinical, laboratory and histological data. However, we observed a trend of higher frequency of distal lower extremity muscle weakness, ragged red fibers and higher CK levels at time of diagnosis in the anti-NDUFA11 positive group. Co-existence of anti-NDUFA11 and anti-NT5C1A antibodies was not observed in any IBM patient.

CONCLUSION

Our results reveal a new autoimmune target in the mitochondrial respiratory chain complex I that might be specifically associated with IBM. This is of particular interest as mitochondrial abnormalities are known histological findings in muscle biopsies of IBM patients.

Lipid storage myopathy associated with sertraline treatment is an acquired mitochondrial disorder with respiratory chain deficiency

Lipid storage myopathies are considered inborn errors of metabolism affecting the fatty acid metabolism and leading to accumulation of lipid droplets in the cytoplasm of muscle fibers. Specific diagnosis is based on investigation of organic aids in urine, acylcarnitines in blood and genetic testing. An acquired lipid storage myopathy in patients treated with the antidepressant drug sertraline, a serotonin reuptake inhibitor, has recently emerged as a new tentative differential diagnosis. We analyzed the muscle biopsy tissue in a group of 11 adult patients with muscle weakness and lipid storage myopathy which developed at a time when they were on sertraline treatment. This group comprise most patients with lipid storage myopathies in western Sweden during the recent nine-year period. By enzyme histochemistry, electron microscopy, quantitative proteomics, immunofluorescence of the respiratory chain subunits, western blot and genetic analyses we demonstrate that muscle tissue in this group of patients exhibit a characteristic morphological and proteomic profile. The patients also showed an acylcarnitine profile in blood suggestive of multiple acyl-coenzyme A dehydrogenase deficiency, but no genetic explanation was found by whole genome or exome sequencing. By proteomic analysis the muscle tissue revealed a profound loss of Complex I subunits from the respiratory chain and to some extent also deficiency of Complex II and IV. Most other components of the respiratory chain as well as the fatty acid oxidation and citric acid cycle were upregulated in accordance with the massive mitochondrial proliferation. The respiratory chain deficiency was verified by immunofluorescence analysis, western blot analysis and enzyme histochemistry. The typical ultrastructural changes of the mitochondria included pleomorphism, dark matrix and frequent round osmiophilic inclusions. Our results show that lipid storage myopathy associated with sertraline treatment is a mitochondrial disorder with respiratory chain deficiency and is an important differential diagnosis with characteristic features.

Inclusion body myositis: Correcting impaired mitochondrial and lysosomal autophagy as a potential therapeutic strategy

Inclusion body myositis (IBM) is a late onset sporadic myopathy with a characteristic clinical presentation, but as yet unknown aetiology or effective treatment. Typical clinical features are early predominant asymmetric weakness of finger flexor and knee extensor muscles. Muscle biopsy shows endomysial inflammatory infiltrate, mitochondrial changes, and protein aggregation. Proteostasis (protein turnover) appears to be impaired, linked to potentially dysregulated chaperone-mediated autophagy and mitophagy (a type of mitochondrial quality control). In this review, we bring together the most recent clinical and biological data describing IBM. We then address the question of diagnosing this pathology and the relevance of the current biological markers that characterize IBM. In these descriptions, we put a particular emphasis on data related to the deregulation of autophagic processes and to the mitochondrial-lysosomal crosstalk. Finally, after a short description of current treatments, an overview is provided pointing towards novel therapeutic targets and emerging regulatory molecules that are being explored for treating IBM. Special attention is paid to autophagy inhibitors that may offer innovative breakthrough therapies for patients with IBM.

Mitochondrial defects in sporadic inclusion body myositis-causes and consequences

Sporadic inclusion body myositis (sIBM) is a distinct subcategory of Idiopathic Inflammatory Myopathies (IIM), characterized by unique pathological features such as muscle inflammation, rimmed vacuoles, and protein aggregation within the myofibers. Although hyperactivation of the immune system is widely believed as the primary cause of IIM, it is debated whether non-immune tissue dysfunction might contribute to the disease's onset as patients with sIBM are refractory to conventional immunosuppressant treatment. Moreover, the findings that mitochondrial dysfunction can elicit non-apoptotic programmed cell death and the subsequent immune response further support this hypothesis. Notably, abnormal mitochondrial structure and activities are more prominent in the muscle of sIBM than in other types of IIM, suggesting the presence of defective mitochondria might represent an overlooked contributor to the disease onset. The large-scale mitochondrial DNA deletion, aberrant protein aggregation, and slowed organelle turnover have provided mechanistic insights into the genesis of impaired mitochondria in sIBM. This article reviews the disease hallmarks of sIBM, the plausible contributors of mitochondrial damage in the sIBM muscle, and the immunological responses associated with mitochondrial perturbations. Additionally, the potential application of mitochondrial-targeted chemicals as a new treatment strategy to sIBM is explored and discussed.

272nd ENMC international workshop: 10 Years of progress - revision of the ENMC 2013 diagnostic criteria for inclusion body myositis and clinical trial readiness. 16-18 June 2023, Hoofddorp, The Netherlands

Since the publication of the 2013 European Neuromuscular Center (ENMC) diagnostic criteria for Inclusion Body Myositis (IBM), several advances have been made regarding IBM epidemiology, pathogenesis, diagnostic tools, and clinical trial readiness. Novel diagnostic tools include muscle imaging techniques such as MRI and ultrasound, and serological testing for cytosolic 5'-nucleotidase-1A antibodies. The 272nd ENMC workshop aimed to develop new diagnostic criteria, discuss clinical outcome measures and clinical trial readiness. The workshop started with patient representatives highlighting several understudied symptoms and the urge for a timely diagnosis. This was followed by presentations from IBM experts highlighting the new developments in the field. This report is composed of two parts, the first part providing new diagnostic criteria on which consensus was achieved. The second part focuses on the use of outcome measures in clinical practice and clinical trials, highlighting current limitations and outlining the goals for future studies.

Sporadic Inclusion Body Myositis at the Crossroads between Muscle Degeneration, Inflammation, and Aging

Sporadic inclusion body myositis (sIBM) is the most common muscle disease of older people and is clinically characterized by slowly progressive asymmetrical muscle weakness, predominantly affecting the quadriceps, deep finger flexors, and foot extensors. At present, there are no enduring treatments for this relentless disease that eventually leads to severe disability and wheelchair dependency. Although sIBM is considered a rare muscle disorder, its prevalence is certainly higher as the disease is often undiagnosed or misdiagnosed. The histopathological phenotype of sIBM muscle biopsy includes muscle fiber degeneration and endomysial lymphocytic infiltrates that mainly consist of cytotoxic CD8+ T cells surrounding nonnecrotic muscle fibers expressing MHCI. Muscle fiber degeneration is characterized by vacuolization and the accumulation of congophilic misfolded multi-protein aggregates, mainly in their non-vacuolated cytoplasm. Many players have been identified in sIBM pathogenesis, including environmental factors, autoimmunity, abnormalities of protein transcription and processing, the accumulation of several toxic proteins, the impairment of autophagy and the ubiquitin-proteasome system, oxidative and nitrative stress, endoplasmic reticulum stress, myonuclear degeneration, and mitochondrial dysfunction. Aging has also been proposed as a contributor to the disease. However, the interplay between these processes and the primary event that leads to the coexistence of autoimmune and degenerative changes is still under debate. Here, we outline our current understanding of disease pathogenesis, focusing on degenerative mechanisms, and discuss the possible involvement of aging.

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.

Inclusion body myositis: from genetics to clinical trials

Inclusion body myositis (IBM) belongs to the group of idiopathic inflammatory myopathies and is characterized by a slowly progressive disease course with asymmetric muscle weakness of predominantly the finger flexors and knee extensors. The disease leads to severe disability and most patients lose ambulation due to lack of curative or disease-modifying treatment options. Despite some genes reported to be associated with hereditary IBM (a distinct group of conditions), data on the genetic susceptibility of sporadic IBM are very limited. This review gives an overview of the disease and focuses on the current genetic knowledge and potential therapeutic implications.

Oxidative stress, mitochondrial dysfunction, and respiratory chain enzyme defects in inflammatory myopathies

We investigated the relationship between oxidative stress and inflammatory myopathies. We searched in the current literature the role of mitochondria and respiratory chain defects as sources of oxidative stress and reactive oxygen species production that led to muscle weakness and fatigue. Different molecules and pathways contribute to redox milieu, reactive oxygen species generation, accumulation of misfolded and carbonylated proteins that lose their ability to fulfil cellular activities. Small peptides and physical techniques proved, in mice models, to reduce oxidative stress. We focused on inclusion body myositis, as a major expression of myopathy related to oxidative stress, where mitochondrial abnormalities are causative agents as well. We described the effect of physical exercise in inclusion body myositis that showed to increase strength and to reduce beta amyloid accumulation with subsequent improvement of the mitochondrial functions. We illustrated the influence of epigenetic control on the immune system by non-coding genetic material in the interaction between oxidative stress and inflammatory myopathies.

Respiratory chain dysfunction in perifascicular muscle fibres in patients with dermatomyositis is associated with mitochondrial DNA depletion

AIMS

Patients with dermatomyositis (DM) suffer from reduced aerobic metabolism contributing to impaired muscle function, which has been linked to cytochrome c oxidase (COX) deficiency in muscle tissue. This mitochondrial respiratory chain dysfunction is typically seen in perifascicular regions, which also show the most intense inflammatory reaction along with capillary loss and muscle fibre atrophy. The objective of this study was to investigate the pathobiology of the oxidative phosphorylation deficiency in DM.

METHODS

Muscle biopsy specimens with perifascicular COX deficiency from five juveniles and seven adults with DM were investigated. We combined immunohistochemical analyses of subunits in the respiratory chain including complex I (subunit NDUFB8), complex II (succinate dehydrogenase, subunit SDHB) and complex IV (COX, subunit MTCO1) with in situ hybridisation, next generation deep sequencing and quantitative polymerase chain reaction (PCR).

RESULTS

There was a profound deficiency of complexes I and IV in the perifascicular regions with enzyme histochemical COX deficiency, whereas succinate dehydrogenase activity and complex II were preserved. In situ hybridisation of mitochondrial RNA showed depletion of mitochondrial DNA (mtDNA) transcripts in the perifascicular regions. Analysis of mtDNA by next generation deep sequencing and quantitative PCR in affected muscle regions showed an overall reduction of mtDNA copy number particularly in the perifascicular regions.

CONCLUSION

The respiratory chain dysfunction in DM muscle is associated with mtDNA depletion causing deficiency of complexes I and IV, which are partially encoded by mtDNA, whereas complex II, which is entirely encoded by nuclear DNA, is preserved. The depletion of mtDNA indicates a perturbed replication of mtDNA explaining the muscle pathology and the disturbed aerobic metabolism.

Inclusion body myositis: evolving concepts

PURPOSE OF REVIEW

To discuss recent developments in our understanding of epidemiology, diagnostics, biomarkers, pathology, pathogenesis, outcome measures, and therapeutics in inclusion body myositis (IBM).

RECENT FINDINGS

Recent epidemiology data confirms a relatively higher prevalence in the population aged above 50 years and the reduced life expectancy. Association with cancer and other systemic disorders is better defined. The role of magnetic resonance imaging (MRI) and ultrasound in diagnosis as well as in following disease progression has been elucidated. There are new blood and imaging biomarkers that show tremendous promise for diagnosis and as outcome measures in therapeutic trials. Improved understanding of the pathogenesis of the disease will lead to better therapeutic interventions, but also highlights the importance to have sensitive and responsive outcome measures that accurately quantitate change.

SUMMARY

There are exciting new developments in our understanding of IBM which should lead to improved management and therapeutic options.

Inclusion body myositis: The interplay between ageing, muscle degeneration and autoimmunity

Inclusion body myositis (IBM) is a slowly progressive muscle disease affecting ageing individuals. IBM presents with a distinctive pattern of weakness involving the quadriceps and finger flexor muscles, although other muscles including pharyngeal muscles become affected over time. Pathological hallmarks of IBM include autoimmune features, including endomysial infiltration by highly differentiated T cells, as well as degenerative features marked by intramyofibre protein aggregates organised into inclusion bodies. Despite some progress in understanding the cellular pathways involved in IBM, it remains untreatable, and the progression of the disease leads to progressive weakness, disability, wheelchair dependency and loss of independence. Therefore, there is an urgent need to improve our understanding of the underlying mechanisms and pathways involved in this disease to identify new treatment targets. Here, we discuss the current understanding of aetiopathogenesis, the interrelationship between autoimmunity and degeneration, and how ageing is a major influencer of both these features.

Inclusion body myositis and associated diseases: an argument for shared immune pathologies

Inclusion body myositis (IBM) is the most prevalent idiopathic inflammatory myopathy (IIM) affecting older adults. The pathogenic hallmark of IBM is chronic inflammation of skeletal muscle. At present, we do not classify IBM into different sub-entities, with the exception perhaps being the presence or absence of the anti-cN-1A-antibody. In contrast to other IIM, IBM is characterized by a chronic and progressive disease course. Here, we discuss the pathophysiological framework of IBM and highlight the seemingly prototypical situations where IBM occurs in the context of other diseases. In this context, understanding common immune pathways might provide insight into the pathogenesis of IBM. Indeed, IBM is associated with a distinct set of conditions, such as human immunodeficiency virus (HIV) or hepatitis C-two conditions associated with premature immune cell exhaustion. Further, the pathomorphology of IBM is reminiscent of other muscle diseases, notably HIV-associated myositis or granulomatous myositis. Distinct immune pathways are likely to drive these commonalities and senescence of the CD8+ T cell compartment is discussed as a possible mechanism of pathogenesis. Future effort directed at understanding the co-occurrence of IBM and associated diseases could prove valuable to better understand the enigmatic IBM pathophysiology.

The Genetics of Autoimmune Myositis

The idiopathic inflammatory myopathies (IIM) are rare, heterogeneous systemic autoimmune disorders, characterized by inflammation of skeletal muscle and multi-organ involvement. Studies to identify genetic risk factors and dysregulated gene expression in IIM aim to increase our understanding of disease pathogenesis. Genome-wide association studies have confirmed the HLA region as the most strongly associated region in IIM, with different associations between clinically-defined subgroups. Associated genes are involved in both the innate and adaptive immune response, while identification of variants reported in other autoimmune disorders suggests shared biological pathways. Targeted imputation analysis has identified key associated amino acid residues within HLA molecules that may influence antigen recognition. These amino acids increase risk for specific clinical phenotypes and autoantibody subgroups, and suggest that serology-defined subgroups may be more homogeneous. Recent data support the contribution of rare genetic variation to disease susceptibility in IIM, including mitochondrial DNA variation in sporadic inclusion body myositis and somatic mutations and loss of heterozygosity in cancer-associated myositis. Gene expression studies in skeletal muscle, blood and skin from individuals with IIM has confirmed the role of interferon signalling and other dysregulated pathways, and identified cell-type specific signatures. These dysregulated genes differentiate IIM subgroups and identify potential biomarkers. Here, we review recent genetic studies in IIM, and how these inform our understanding of disease pathogenesis and provide mechanistic insights into biological pathways.

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 tRNA^Glu (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.

Revisiting the Role of GSK3, A Modulator of Innate Immunity, in Idiopathic Inclusion Body Myositis

Idiopathic or sporadic inclusion body myositis (IBM) is the leading age-related (onset >50 years of age) autoimmune muscular pathology, resulting in significant debilitation in affected individuals. Once viewed as primarily a degenerative disorder, it is now evident that much like several other neuro-muscular degenerative disorders, IBM has a major autoinflammatory component resulting in chronic inflammation-induced muscle destruction. Thus, IBM is now considered primarily an inflammatory pathology. To date, there is no effective treatment for sporadic inclusion body myositis, and little is understood about the pathology at the molecular level, which would offer the best hopes of at least slowing down the degenerative process. Among the previously examined potential molecular players in IBM is glycogen synthase kinase (GSK)-3, whose role in promoting TAU phosphorylation and inclusion bodies in Alzheimer’s disease is well known. This review looks to re-examine the role of GSK3 in IBM, not strictly as a promoter of TAU and Abeta inclusions, but as a novel player in the innate immune system, discussing some of the recent roles discovered for this well-studied kinase in inflammatory-mediated pathology.

Primary mitochondrial myopathies in childhood

Primary mitochondrial myopathies are genetic metabolic disorders of mitochondrial dysfunction affecting mainly, but not exclusively, skeletal muscle. Although individually rare, they are the most common inherited metabolic disorders in childhood. They can be similar to other childhood muscle diseases such as congenital myopathies, dystrophies, myasthenic syndromes or metabolic myopathies and a muscle biopsy and genetic testing are important in the differential diagnosis. Mitochondrial myopathies can present at any age but typically childhood onset myopathies have more significant muscle involvement and are caused by genes encoded in the nuclear DNA. Mitochondrial myopathy in infants presents with hypotonia, muscle weakness and difficulty feeding. In toddlers and older children delayed motor development, exercise intolerance and premature fatigue are common. A number of nuclear DNA and mitochondrial DNA encoded genes are known to cause isolated myopathy in childhood and they are important in a range of mitochondrial functions such as oxidative phosphorylation, mitochondrial transcription/translation and mitochondrial fusion/fission. A rare cause of isolated myopathy in children, reversible infantile respiratory chain deficiency myopathy, is non-progressive and typically associated with spontaneous full recovery. Promising targeted treatments have been reported for a number or mitochondrial myopathies including riboflavin in ACAD9 and ETFDH-myopathies and deoxynucleoside for TK2-related disease.