Inclusion-body myositis and primary Sjögren syndrome: mechanisms for shared etiologies

Potential common molecular mechanisms between Sjögren syndrome and inclusion body myositis: a bioinformatic analysis and in vivo validation

Background

Inclusion body myositis (IBM) is a slowly progressive inflammatory myopathy that typically affects the quadriceps and finger flexors. Sjögren's syndrome (SS), an autoimmune disorder characterized by lymphocytic infiltration of exocrine glands has been reported to share common genetic and autoimmune pathways with IBM. However, the exact mechanism underlying their commonality remains unclear. In this study, we investigated the common pathological mechanisms involved in both SS and IBM using a bioinformatic approach.

Methods

IBM and SS gene expression profiles were obtained from the Gene Expression Omnibus (GEO). SS and IBM coexpression modules were identified using weighted gene coexpression network analysis (WGCNA), and differentially expressed gene (DEG) analysis was applied to identify their shared DEGs. The hidden biological pathways were revealed using Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis. Furthermore, protein-protein interaction (PPI) networks, cluster analyses, and hub shared gene identification were conducted. The expression of hub genes was validated by reverse transcription quantitative polymerase chain reaction (RT-qPCR). We then analyzed immune cell abundance patterns in SS and IBM using single-sample gene set enrichment analysis (ssGSEA) and investigated their association with hub genes. Finally, NetworkAnalyst was used to construct a common transcription factor (TF)-gene network.

Results

Using WGCNA, we found that 172 intersecting genes were closely related to viral infection and antigen processing/presentation. Based on DEG analysis, 29 shared genes were found to be upregulated and enriched in similar biological pathways. By intersecting the top 20 potential hub genes from the WGCNA and DEG sets, three shared hub genes (PSMB9, CD74, and HLA-F) were derived and validated to be active transcripts, which all exhibited diagnostic values for SS and IBM. Furthermore, ssGSEA showed similar infiltration profiles in IBM and SS, and the hub genes were positively correlated with the abundance of immune cells. Ultimately, two TFs (HDGF and WRNIP1) were identified as possible key TFs.

Conclusion

Our study identified that IBM shares common immunologic and transcriptional pathways with SS, such as viral infection and antigen processing/presentation. Furthermore, both IBM and SS have almost identical immune infiltration microenvironments, indicating similar immune responses may contribute to their association.

Significance of Sjögren's syndrome and anti-cN1A antibody in myositis patients

OBJECTIVE

We recently recorded a high prevalence of inclusion body myositis (IBM) in patients with Sjögren's syndrome (SS). Whether myositis patients with SS differ from myositis patients without SS in terms of the characteristics of the myositis is currently unknown. Anti-cytosolic 5'-nucleotidase 1 A (cN1A) has recently been proposed as a biomarker for IBM but is also frequent in SS. Whether anti-cN1A is independently associated with IBM is still an open question. We aimed to assess the significance of SS and anti-cN1A in myositis patients.

METHODS

Cumulative data on all myositis patients (EULAR/ACR 2017 criteria) screened for SS (ACR/EULAR 2016 criteria) in a single center were analyzed. Ninety-nine patients were included, covering the whole spectrum of EULAR/ACR 2017 myositis subgroups and with a median follow-up of 6 years [range 1.0-37.5]. The 34 myositis patients with SS (myositis/SS+) were compared with the 65 myositis patients without SS (myositis/SS-).

RESULTS

IBM was present in 24% of the myositis/SS+ patients vs 6% of the myositis/SS- group (p = 0.020). None of the IBM patients responded to treatment, whether they had SS or not. Anti-cN1A was more frequent in myositis/SS+ patients (38% vs 6%, p = 0.0005), independently of the higher prevalence of IBM in this group (multivariate p-value: 0.02). Anti-cN1A antibody specificity for IBM was 0.96 [95% CI, 0.87-0.99] in the myositis SS- group but dropped to 0.70 [95% CI, 0.48-0.85] in the myositis SS/+ group.

INTERPRETATION

In myositis patients, SS is associated with IBM and with anti-cN1A antibodies, independently of the IBM diagnosis. As a consequence, anti-cN1A has limited specificity for IBM in myositis patients with SS.

Sporadic inclusion body myositis and primary Sjogren's syndrome: an overlooked diagnosis

Sporadic inclusion body myositis (sIBM) has been reported to occur in association with autoimmune diseases and in particular, primary Sjogren's syndrome (pSS). This brief report describes patients identified with a positive SSA antibody and diagnosis of sIBM at a large academic medical center over a 13.5-year period. A cohort identification tool was used to identify patients with positive SSA antibody and a diagnosis of sIBM between January 1, 2006 and June 1, 2019. All cases of sIBM had diagnostic confirmation by a neuromuscular specialist. Demographics, clinical features, autoantibodies, MRI and EMG findings, and muscle biopsy features were reviewed for each identified case. Eight patients were found to carry the diagnosis of pSS and sIBM. Two additional sIBM patients were SSA antibody positive without other pSS features. The mean time from initial symptom onset of muscle weakness to diagnosis was 5.4 years (range 1-15 years). All patients had alternative diagnoses offered for their myopathic symptoms prior to sIBM identification. The NT5c1A antibody was positive in 7 of 8 patients tested. No patient had a durable response to immunosuppressive therapy. The diagnosis of sIBM went unrecognized for over 5 years in our cohort of SSA antibody-positive patients with myopathy. The patients in this cohort were treated with a variety of immunosuppressive agents prior to diagnosis without benefit. Recognizing the clinical features of sIBM in patients with pSS is crucial in instituting appropriate therapy and avoiding unnecessary immunosuppression. Key Points • Sporadic inclusion body myositis (sIBM) can be associated with Sjogren's syndrome. • In this case series, prevalence of the NT5c1A antibody was higher among patients with associated Sjogren's syndrome compared to the cited prevalence of the NT5c1A antibody in patients with isolated sIBM. • It is crucial to consider sIBM in patients with Sjogren's syndrome presenting with motor weakness in order to avoid unnecessary immunosuppression and institute appropriate therapy.