Background Idiopathic inflammatory myositis (IIM) and inclusion body myositis (IBM) remain poorly understood. IIM represents a group of inflammatory muscle conditions that are well characterised clinically and serologically. Anti-Jo-1 is the most common antibody identified in IIM and is associated with anti-synthetase syndrome (ASS)- a clinical condition comprising myositis, interstitial lung disease and skin fissuring on the hands. Unlike ASS, onset of symptoms in IBM is typically insidious. It often affects distal musculature in contrast to proximal involvement seen in ASS, and usually has no internal organ involvement. Despite advances in animal disease models and improving serological classification of both IIM and IBM, their pathogenesis is poorly understood.

Next generation sequencing (NGS) enables determination of the complete transcriptome from cells and tissues. By comparing RNA expression in muscle of anti-Jo 1 positive IIM, IBM, and control individuals, we can observe changes in the differences in signaling pathways and unravel the pathogenesis of these conditions.

Objectives To investigate the differential expression of RNA in muscle biopsies between Jo-1 positive IIM, IBM and controls

Methods Existing muscle biopsies from patients with Jo-1 antibody positive IIM and IBM were identified (snap frozen, stored at -80°C). Control muscle biopsies were obtained from patients biopsied to investigate weakness, but with no histological features of inflammation or other pathology identified.

RNA was extracted from 5 muscle biopsies in each of the anti-Jo 1 IIM and control group, and 2 in the IBM group using commercially available kits (QIAshredder and Qiagen RNeasy columns). This was pooled into 3 samples (anti-Jo 1 IIM, IBM and control) for NGS. The muscle transcriptome was determined using NGS (Illumina Hi-Seq 2000, Poly A+ extraction). Data was mapped onto the human genome and changes in expression analysed.

Results We observed widespread changes in the expression of mRNAs and long non-coding RNAs. We demonstrated a strong upregulation of CD74 (part of MHC Class II) in anti-Jo 1 positive IIM and IBM (absolute change 222.47 FPKM/2.88 log2 fold change, and 174.78 FPKM/2.12 log2 fold change, respectively). RIG-1 was also identified as being strongly expressed in both anti-Jo 1 IIM and IBM compared to controls (absolute change 52.01 FPKM/induced gene, and 26.76 FPKM/3.12 log2 fold change respectively). B2 microglobulin (a subunit of MHC Class 1) was highly transcribed in anti-Jo 1 IIM (absolute change 1314.72 FPKM/2.79 log2 fold change) compared to controls.

Conclusions These data correspond with previous studies identifying MHC class 1 and 2 upregulation in IIM and IBM. Our data also show that RIG-1 is markedly upregulated in anti-Jo 1 positive IIM and IBM. This is of particular interest as RIG-1 acts as an intracellular viral RNA sensor in the innate immune response. RIG-1 is also closely related to melanoma differentiation associated protein 5 (MDA 5; a RIG-I-like receptor), a recognised autoantigen associated with clinically amyopathic dermatomyositis, and suggests the innate immune response may play a role in these conditions. In addition, we have also identified for the first time, changes in the expression of long non-coding RNAs.

Acknowledgements Professor Seth Love, Department of Neuropathology, Frenchay Hospital, Bristol

Bath Institute for Rheumatic Diseases Charitable Funds, Bath

Royal National Hospital for Rheumatic Diseases Charitable Funds, Bath

Disclosure of Interest None Declared