Objective:

To investigate the pathophysiologic mechanisms underlying immune-mediated Rippling Muscle Disease (iRMD).

Background:

Rippling Muscle Disease (RMD) is a rare skeletal myopathy characterized by abnormal muscular excitability manifesting with wave-like muscle contractions and stretch-induced muscle mounding. Hereditary RMD is associated with caveolin-3 or cavin-1 mutations. Recently we discovered anti-cavin 4 autoantibodies as a biomarker of iRMD, but the underlying disease-mechanisms are poorly understood.

Design/Methods:

Transcriptomic studies were performed on muscle biopsies of 8 patients (5 males; 3 females; ages 26-to-80) with iRMD.  Subsequent pathway analysis compared iRMD to human non-disease control and disease-control (dermatomyositis) muscle samples. Confirmatory immunohistochemistry for protein level quantification was performed.

Results:

Transcriptomic studies demonstrated changes in key pathways of muscle homeostasis, including muscle development, protein quality control, and autoimmunity. All iRMD samples had significantly upregulated cavin-4 expression compared to controls, likely compensatory for autoantibody-mediated protein degradation. Excitation-Contraction coupling genes (including SERCA1, PMCA, PLN and CACNA1S) were significantly upregulated in iRMD compared to controls. These changes were validated with immunofluorescent staining for SERCA1, PMCA, PLN and CACNA1S which confirmed increased protein levels except for CACNA1S. Comparison of iRMD to dermatomyositis transcriptomics demonstrated significant overlap in autoimmunity pathways including MHC2 antigen presentation and innate immune response.

Conclusions:

This study represents the first muscle transcriptomic analysis of iRMD patients and elucidates the underlying disease mechanisms. Increases of sarcolemmal and cellular calcium channels as well as PLN, an inhibitor of the SERCA pump for calcium into the sarcoplasm, likely alter the calcium dynamics in iRMD. These changes in crucial components of Excitation-Contraction coupling and muscle relaxation pathways may underlie rippling by altering calcium flux. We propose a model in which autoantibody mediated cavin-4 destruction leads to RMD through alterations in key genes regulating Excitation-Contraction coupling.