Clinical Subgroups and Factors Associated With Progression in Patients With Inclusion Body Myositis

Loss of TDP-43 Splicing Repression Occurs in Myonuclei of Inclusion Body Myositis Patients

OBJECTIVE

Inclusion body myositis (IBM) is an idiopathic inflammatory myopathy with muscle pathology characterized by endomysial inflammation, rimmed vacuoles, and cytoplasmic mislocalization of transactive response DNA-binding protein 43 (TDP-43). We aimed to determine whether loss of TDP-43 splicing repression led to the production of "cryptic peptides" that could be detected in muscle biopsies as a useful biomarker for IBM.

METHODS

We used an antisera against a neoepitope encoded by a TDP-43-dependent cryptic exon within hepatoma-derived growth factor-like protein 2 (HDGFL2) for immunohistochemical analysis on muscle biopsy samples of 122 patients with IBM, 181 disease controls, and 16 healthy controls without abnormal muscle pathology. In situ hybridization was also utilized to detect the localization of cryptic HDGFL2 transcripts.

RESULTS

We found cryptic HDGFL2 peptides localized within myonuclei from muscle biopsies in 79 of 122 patients with IBM (65%), and this staining correlated with TDP-43 depletion. In contrast, cryptic HDGFL2 immunoreactivity was absent in 197 muscle biopsies from a variety of disease controls, except for 2 patients with vacuolar myopathies. Notably, we show that cryptic HDGFL2 transcripts are accompanied by the detection of cryptic HDGFL2 in muscle fibers of IBM without rimmed vacuoles and TDP-43 aggregates.

INTERPRETATION

Together, our findings establish that loss of TDP-43 splicing repression occurs in myonuclei of IBM skeletal muscle and suggest that detection of cryptic peptides in muscle biopsies may be a useful biomarker. We suggest that a therapeutic strategy designed to restore TDP-43 function should be considered to attenuate the degeneration of skeletal muscle in this devastating disease. ANN NEUROL 2025;97:629-641.

Epidemiology of myositis

PURPOSE OF REVIEW

This review aims to synthesize recent developments in the epidemiology of idiopathic inflammatory myopathies (IIMs), focusing on incidence, prevalence, disease classification, and clinical outcomes.

RECENT FINDINGS

IIM is a rare group of autoimmune diseases characterized by muscle weakness and systemic involvement, with incidence rates ranging from 0.2 to 2 cases per 100 000 person-years. The role of myositis-specific autoantibodies (MSAs) in stratifying disease risk and prognosis is increasingly recognized, such as in anti-MDA5 positive DM, which is associated with a high risk of rapidly progressive interstitial lung disease. Furthermore, patients with IIM exhibit elevated risks of comorbidities, including cardiovascular disease and malignancy.

SUMMARY

IIM diseases are complex disorders with significant health impacts, necessitating enhanced awareness and research. Improved classification and understanding of MSAs are crucial for earlier diagnosis and tailored therapeutic strategies. Continued epidemiological research is essential to elucidate underlying mechanisms and inform future interventions, ultimately aiming to enhance the quality of life and clinical outcomes for affected patients.

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.

Inclusion body myositis: an update

PURPOSE OF REVIEW

To review recent advances in our understanding of the epidemiology, pathophysiology, and management of inclusion body myositis (IBM).

RECENT FINDINGS

Recent epidemiologic studies have highlighted the morbidity and mortality associated with IBM, including the impact of dysphagia. Multiomic analyses of IBM tissues have identified new pathogenic pathways and biomarkers for use in clinical trials. New diagnostic criteria and outcome measures have been proposed to improve clinical trial design. Ongoing clinical trials are targeting T cells and autophagy.

SUMMARY

Improvements in our understanding of IBM pathogenesis are identifying new pathways and biomarkers that need validation in larger cohorts. Exercise remains the primary therapeutic modality available, and new treatment targets are needed.

Current biomarkers in inclusion body myositis

Inclusion body myositis (IBM) is an idiopathic muscle disorder primarily affecting adults above the age of 50. IBM is characterized by weakness in the knee extensor and deep finger flexor muscles due to muscle atrophy and fibroadipose replacement. Dynamometry and manual muscle testing are commonly used to assess patient muscle strength, while magnetic resonance imaging and electromyography studies identify the patterns of muscle atrophy and motor unit potentials. Although the underlying pathophysiological mechanisms of IBM are still unknown, common histopathological markers include rimmed vacuoles and inclusions. The immune system is also largely implicated in pathogenesis, as skeletal muscle in IBM overexpresses major histocompatibility complex I (MHC-I), and cluster of differentiation (CD) 8+ T-cells, and features endomysial inflammation. Antibodies to the cytosolic 5'-nucleotidase 1A (cN1A) protein have been associated with IBM but have low sensitivity and specificity. As many classic features of IBM present only in advanced stages of disease, there are substantial challenges to the diagnosis and monitoring of IBM progression in its early stages. Identifying early diagnostic biomarkers and new biomarker signatures associated with IBM disease progression is necessary for clinical trial readiness.

Impact of sex, age at onset, and anti-cN1A antibodies on sporadic inclusion body myositis

BACKGROUND

Inclusion body myositis (IBM) is a progressive myopathy occurring in patients over 45 years of age, with heterogeneous and variable clinical features. This study aimed to determine the influence of autoantibodies, gender, and age of onset on the clinical features of IBM.

METHODS

Medical records and muscle histology findings of 570 participants with suspected IBM were reviewed. Various characteristics of patients who met the 2011 ENMC IBM diagnostic criteria were compared based on the presence of anti-cytosolic 5'-nucleotidase 1 A (cN1A) autoantibodies, gender, age of onset, and disease duration.

RESULTS

Of the 353 patients who met the criteria, 41.6% were female. The mean age at onset was 64.6 ± 9.3 years, and the mean duration from onset to diagnosis was 5.7 ± 4.7 years. 196 of the 353 patients (55.5%) were positive for anti-cN1A autoantibodies and 157 were negative. Logistic regression showed that patients with anti-cN1A autoantibodies had a higher frequency of finger flexion weakness. Multiple regression showed that patients with later age of onset had shorter disease duration, lower BMI, and lower serum CK levels. Male patients had a higher frequency of onset with finger weakness and female patients had a lower BMI.

CONCLUSION

Autoantibodies, gender, age of onset, and disease duration may influence the clinical presentation of IBM, highlighting the need for a precision medicine approach that considers these factors along with the underlying mechanisms of the disease.

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. Inflammation and mitochondrial dysfunction are the most common histopathological findings. In this study, we aimed to explore the interplay between inflammation and mitochondrial dysfunction in IBM patients, highlighting sex differences.

Methods

We included 38 IBM patients and 22 age- and sex-matched controls without myopathy. Bulk RNA sequencing, Meso Scale Discovery ELISA, 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 pathway being the most upregulated. On muscle histopathology, there is increased NRLP3 immunoreactivity in both inflammatory cells and muscle fibers. Mitophagy is critical for removing damaged mitochondria and preventing the formation of a vicious cycle of mitochondrial dysfunction-NLRP3 activation. In the IBM muscle samples, we showed altered mitophagy, most significantly in males, with elevated levels of p-S65-Ubiquitin, a mitophagy marker. Furthermore, p-S65-Ubiquitin aggregates accumulated in muscle fibers that were mostly type 2 and devoid of cytochrome-c-oxidase reactivity. Type 2 muscle fibers are known to be more prone to mitochondrial dysfunction. NLRP3 RNA levels correlated with p-S65-Ubiquitin levels in both sexes but with loss of in muscle strength only in males. Finally, we identified sex-specific molecular pathways in IBM, with females having activation of pathways that could offset some of the pathomechanisms of IBM.

Conclusions

NLRP3 inflammasome is activated 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.

Effects of sporadic inclusion body myositis on skeletal muscle fibre type specific morphology and markers of regeneration and inflammation

Sporadic inclusion body myositis (sIBM) is a subgroup of idiopathic inflammatory myopathies characterised by progressive muscle weakness and skeletal muscle inflammation. Quantitative data on the myofibre morphology in sIBM remains scarce. Further, no previous study has examined fibre type association of satellite cells (SC), myonuclei number, macrophages, capillaries, and myonuclear domain (MD) in sIBM patients. Muscle biopsies from sIBM patients (n = 18) obtained previously (NCT02317094) were included in the analysis for fibre type-specific myofibre cross-sectional area (mCSA), SCs, myonuclei and macrophages, myonuclear domain, and capillarisation. mCSA (p < 0.001), peripheral myonuclei (p < 0.001) and MD (p = 0.005) were higher in association with type 1 (slow-twitch) than type 2 (fast-twitch) fibres. Conversely, quiescent SCs (p < 0.001), centrally placed myonuclei (p = 0.03), M1 macrophages (p < 0.002), M2 macrophages (p = 0.013) and capillaries (p < 0.001) were higher at type 2 fibres compared to type 1 fibres. In contrast, proliferating (Pax7+/Ki67+) SCs (p = 0.68) were similarly associated with each fibre type. Type 2 myofibres of late-phase sIBM patients showed marked signs of muscle atrophy (i.e. reduced mCSA) accompanied by higher numbers of associated quiescent SCs, centrally placed myonuclei, macrophages and capillaries compared to type 1 fibres. In contrast, type 1 fibres were suffering from pathological enlargement with larger MDs as well as fewer nuclei and capillaries per area when compared with type 2 fibres. More research is needed to examine to which extent different therapeutic interventions including targeted exercise might alleviate these fibre type-specific characteristics and countermeasure their consequences in impaired functional performance.

Autoantibody evaluation in idiopathic inflammatory myopathies

Idiopathic inflammatory myopathies (IIM), generally referred to as myositis is a heterogeneous group of diseases characterized by muscle inflammation and/or skin involvement, diverse extramuscular manifestations with variable risk for malignancy and response to treatment. Contemporary clinico-serologic categorization identifies 5 main clinical groups which can be further stratified based on age, specific clinical manifestations and/or risk for cancer. The serological biomarkers for this classification are generally known as myositis-specific (MSAs) and myositis-associated antibodies. Based on the use of these antibodies, IIM patients are classified into anti-synthetase syndrome, dermatomyositis, immune-mediated necrotizing myopathy, inclusion body myositis, and overlap myositis. The current classification criteria for IIM requires clinical findings, laboratory measurements, and histological findings of the muscles. However, the use MSAs and myositis-associated autoantibodies as an adjunct for disease evaluation is thought to provide a cost-effective personalized approach that may not only guide diagnosis but aid in stratification and/or prognosis of patients. This review provides a comprehensive overview of contemporary autoantibodies that are specific or associated myositis. In addition, it highlights possible pathways for the detection and interpretation of these antibodies with limitations for routine clinical use.

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.

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.