Clinical features of Japanese patients with inclusion body myositis

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.

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

BACKGROUND AND OBJECTIVES

Sporadic inclusion body myositis (IBM) is the most common acquired myopathy in individuals older than 50 years. The disorder is slowly progressive, and although many therapies have been investigated, response has generally been poor. Clinical heterogeneity may influence treatment responsiveness; however, data regarding heterogeneity in IBM are limited and often conflicting. We aim to identify clinically distinct subgroups within a large IBM cohort and prognostic factors for disease progression.

METHODS

Clinical, histologic, radiologic, and electrophysiologic data were analyzed for all patients with IBM and other forms of myositis enrolled in a longitudinal cohort from The Johns Hopkins Myositis Center from 2003 to 2018. Patients with IBM were included if they met at least one of the following criteria: Griggs possible, European Neuromuscular Centre 2011 probable, or Lloyd-Greenberg data-derived criteria for IBM. Univariate, multivariate, and graphical analyses were used to identify prognostic factors in patients with IBM. Thus, linear and logistic regressions were used to adjust for potential confounding variables. The evolution of creatine kinase and muscle strength was studied using multilevel linear regression models. Nonmodifiable risk factors (sex, race, disease duration, and age at the onset of first symptoms) were used as adjusting covariates for the regression analyses.

RESULTS

Among the 335 patients meeting the inclusion criteria for IBM, 64% were male with an average age of disease onset of 58.7 years and delay to diagnosis of 5.2 years. Initial misdiagnosis (52%) and immunosuppressant treatment (42%) were common. Less than half (43%) of muscle biopsies demonstrated all 3 pathologic hallmarks: endomysial inflammation, mononuclear cell invasion, and rimmed vacuoles. Black patients had significantly weaker arm abductors, hip flexors, and knee flexors compared with non-Black patients. Female patients had stronger finger flexors and knee extensors compared with their male counterparts. Younger age (<50 years) at onset was not associated with increased weakness.

DISCUSSION

Our study demonstrates that female and Black patients have distinct clinical phenotypes and trajectories within the overarching IBM clinical phenotype. These subgroups may have different responses to therapies, which may influence the design of future clinical trials in IBM.

Muscle-dominant wild-type TDP-43 expression induces myopathological changes featuring tubular aggregates and TDP-43-positive inclusions

Muscle histology of sporadic inclusion body myositis (sIBM) demonstrates inflammatory findings and degenerative features including accumulation of TAR DNA-binding protein of 43 kDa (TDP-43). However, whether sarcoplasmic accumulation of TDP-43 is a primary trigger of muscle degeneration or a secondary event resulting from muscle degeneration in the pathophysiology of sIBM remained unclear. Our study aimed to discover whether muscle-dominant expression of TDP-43 is a primary cause of muscle degeneration. We generated several lines of wild-type TDP-43 transgenic mice driven by a creatine kinase 8 promoter, and analyzed the phenotypes via biochemical, histological, and proteomic techniques. The mice showed increased serum levels of myogenic enzymes. Muscle histology demonstrated myopathic changes including fiber size variation, abundant tubular aggregates, and TDP-43 aggregation with upregulation of endoplasmic reticulum (ER) stress. Proteomic analysis with aggregated materials in degenerative myofibers identified increased sarcoplasmic reticulum (SR)/ER-resident proteins that regulated calcium homeostasis, as well as cytosolic 5'-nucleotidase 1A. Muscle-dominant wild-type TDP-43 expression indeed caused myotoxicity featuring tubular aggregates and TDP-43-positive inclusions. Our observation suggested that TDP-43 aggregates might not be sufficient to trigger the pathogenesis of sIBM although myofiber sarcoplasmic aggregation of TDP-43 led to myofiber degeneration via ER stress and possibly calcium dysregulation, independently of inflammatory process.