Identification of autoantibodies associated with rippling muscles and myasthenia gravis that recognize skeletal muscle proteins: possible relationship of antigens and stretch-activated ion channels

Immune-Mediated Rippling Muscle Disease Associated With Thymoma and Anti-MURC/Cavin-4 Autoantibodies

OBJECTIVES

Rippling muscle disease (RMD) is characterized by muscle stiffness, muscle hypertrophy, and rippling muscle induced by stretching or percussion. Hereditary RMD is due to sequence variants in the CAV3 and PTRF/CAVIN1 genes encoding Caveolin-3 or Cavin-1, respectively; a few series of patients with acquired autoimmune forms of RMD (iRMD) associated with AChR antibody-positive myasthenia gravis and/or thymoma have also been described. Recently, MURC/caveolae-associated protein 4 (Cavin-4) autoantibody was identified in 8 of 10 patients without thymoma, highlighting its potential both as a biomarker and as a triggering agent of this pathology. Here, we report the case of a patient with iRMD-AchR antibody negative associated with thymoma.

METHODS

We suspected a paraneoplastic origin and investigated the presence of specific autoantibodies targeting muscle antigens through a combination of Western blotting and affinity purification coupled with mass spectrometry-based proteomic approaches.

RESULTS

We identified circulating MURC/Cavin-4 autoantibodies and found strong similarities between histologic features of the patient's muscle and those commonly reported in caveolinopathies. Strikingly, MURC/Cavin-4 autoantibody titer strongly decreased after tumor resection and immunotherapy correlating with complete disappearance of the rippling phenotype and full patient remission.

DISCUSSION

MURC/Cavin-4 autoantibodies may play a pathogenic role in paraneoplastic iRMD associated with thymoma.

Identification of Caveolae-Associated Protein 4 Autoantibodies as a Biomarker of Immune-Mediated Rippling Muscle Disease in Adults

Question

Is there an autoantibody biomarker of immune-mediated rippling muscle disease (iRMD)?

Findings

In this cohort study, autoantibodies to caveolae-associated protein 4 (cavin-4) were identified and orthogonally validated in 8 of 10 patients with iRMD; results for all healthy and disease-control individuals were seronegative. Immunohistochemical studies demonstrated depletion of cavin-4 expression in biopsied iRMD skeletal muscle.

Meaning

The findings suggest that seropositivity for cavin-4 IgG, the first specific serological biomarker discovered for iRMD, may support an autoimmune pathogenesis for this clinical and immunohistopathologic entity.

Importance

Immune-mediated rippling muscle disease (iRMD) is a rare myopathy characterized by wavelike muscle contractions (rippling) and percussion- or stretch-induced muscle mounding. A serological biomarker of this disease is lacking.

Objective

To describe a novel autoantibody biomarker of iRMD and report associated clinicopathological characteristics.

Design, Setting, and Participants

This retrospective cohort study evaluated archived sera from 10 adult patients at tertiary care centers at the Mayo Clinic, Rochester, Minnesota, and Brigham & Women’s Hospital, Boston, Massachusetts, who were diagnosed with iRMD by neuromuscular specialists in 2000 and 2021, based on the presence of electrically silent percussion- or stretch-induced muscle rippling and percussion-induced rapid muscle contraction with or without muscle mounding and an autoimmune basis. Sera were evaluated for a common biomarker using phage immunoprecipitation sequencing. Myopathology consistent with iRMD was documented in most patients. The median (range) follow-up was 18 (1-30) months.

Exposures

Diagnosis of iRMD.

Main Outcomes and Measures

Detection of a common autoantibody in serum of patients sharing similar clinical and myopathological features.

Results

Seven male individuals and 3 female individuals with iRMD were identified (median [range] age at onset, 60 [18-76] years). An IgG autoantibody specific for caveolae-associated protein 4 (cavin-4) was identified in serum of patients with iRMD using human proteome phage immunoprecipitation sequencing. Immunoassays using recombinant cavin-4 confirmed cavin-4 IgG seropositivity in 8 of 10 patients with iRMD. Results for healthy and disease-control individuals (n = 241, including myasthenia gravis and immune-mediated myopathies) were cavin-4 IgG seronegative. Six of the 8 individuals with cavin-4 IgG were male, and the median (range) age was 60 (18-76) years. Initial symptoms included rippling of lower limb muscles in 5 of 8 individuals or all limb muscles in 2 of 8 sparing bulbar muscles, fatigue in 9 of 10, mild proximal weakness in 3 of 8, and isolated myalgia in 1 of 8, followed by development of diffuse rippling. All patients had percussion-induced muscle rippling and half had percussion- or stretch-induced muscle mounding. Four of the 10 patients had proximal weakness. Plasma creatine kinase was elevated in all but 1 patient. Six of the 10 patients underwent malignancy screening; cancer was detected prospectively in only 1. Muscle biopsy was performed in 7 of the 8 patients with cavin-4 IgG; 6 of 6 specimens analyzed immunohistochemically revealed a mosaic pattern of sarcolemmal cavin-4 immunoreactivity. Three of 6 patients whose results were seropositive and who received immunotherapy had complete resolution of symptoms, 1 had mild improvement, and 2 had no change.

Conclusions and Relevance

The findings indicate that cavin-4 IgG may be the first specific serological autoantibody biomarker identified in iRMD. Depletion of cavin-4 expression in muscle biopsies of patients with iRMD suggests the potential role of this autoantigen in disease pathogenesis.

Immune-mediated rippling muscle disease and myasthenia gravis

Cases of acquired rippling muscle disease in association with myasthenia gravis have been reported. We present three patients with iRMD (immune-mediated rippling muscle disease) and AChR-antibody positive myasthenia gravis. None of them had thymus pathology. They presented exercise-induced muscle rippling combined with generalized myasthenia gravis. One of them had muscle biopsy showing a myopathic pattern and a patchy immunostaining with caveolin antibodies. They were successfully treated steroids and azathioprine. The immune nature of this association is supported by the response to immunotherapies and the positivity of AChR-antibodies.

Characterization of the in vitro expressed autoimmune rippling muscle disease immunogenic domain of human titin encoded by TTN exons 248-249

Autoimmune rippling muscle disease (ARMD) is an autoimmune neuromuscular disease associated with myasthenia gravis (MG). Past studies in our laboratory recognized a very high molecular weight skeletal muscle protein antigen identified by ARMD patient antisera as the titin isoform. These past studies used antisera from ARMD and MG patients as probes to screen a human skeletal muscle cDNA library and several pBluescript clones revealed supporting expression of immunoreactive peptides. This study characterizes the products of subcloning the titin immunoreactive domain into pGEX-3X and the subsequent fusion protein. Sequence analysis of the fusion gene indicates the cloned titin domain (GenBank ID: EU428784) is in frame and is derived from a sequence of N2-A spanning the exons 248-250 an area that encodes the fibronectin III domain. PCR and EcoR1 restriction mapping studies have demonstrated that the inserted cDNA is of a size that is predicted by bioinformatics analysis of the subclone. Expression of the fusion protein result in the isolation of a polypeptide of 52 kDa consistent with the predicted inferred amino acid sequence. Immunoblot experiments of the fusion protein, using rippling muscle/myasthenia gravis antisera, demonstrate that only the titin domain is immunoreactive.

Identification of skeletal muscle autoantigens by expression library screening using sera from autoimmune rippling muscle disease (ARMD) patients

Novel forms of contractile regulation observed in skeletal muscle are evident in neuromuscular diseases like rippling muscle disease (RMD). Previous studies of an autoimmune form of RMD (ARMD) identified a very high molecular weight skeletal muscle protein antigen recognized by ARMD patient antisera. This study utilized ARMD and myasthenia gravis (MG) patient antisera, to screen a human skeletal muscle cDNA library that subsequently identified proteins that could play a role in ARMD. Based on nucleotide sequence analysis, three distinct ARMD antigens were identified: titin Isoform N2A, ATP synthase 6, and PPP1R3 (protein phosphatase 1 regulatory subunit 3). The region of titin identified by ARMD antisera is distinct from the main immunogenic region (MIR) recognized by classical MG antibodies. Sera from classical MG patient identifies an expressed sequence corresponding to the titin MIR. Although the mechanism of antibody penetration is not known, previous studies have shown that rippling muscle antibodies affect the contractile machinery of myofibers resulting in mechanical sensitivity. Titin's role as a modulator of muscle contractility makes it a potential target in understanding muscle mechanosensitive regulation.

Rippling muscle disease may be caused by ?silent? action potentials in the tubular system of skeletal muscle fibers

Rippling muscle disease (RMD) is a generally benign, myotonic-like myopathy associated with rapid rolling contractions and percussion-induced contractions. These contractions are electrically silent in electromyographic recordings, which is taken as evidence that action potentials are not involved in the phenomena. The pathophysiological mechanisms underlying the symptoms have not been elucidated. Many cases of RMD are caused by mutations in caveolin-3, and aberrations in the tubular system are commonly observed. Here, recent data are discussed showing that action potentials can travel over substantial distances entirely within the transverse and longitudinal tubular systems of a muscle fiber and that stretch can induce such action potentials. Action potentials travelling in the tubular system in most circumstances probably cannot excite the sarcolemma and hence would not be detected. It is suggested that the distinctive contractions in RMD may be due to stretch-induced generation of action potentials within the tubular system.

Ion channel and striational antibodies define a continuum of autoimmune neuromuscular hyperexcitability

Neuromuscular hyperexcitability is a characteristic of Isaacs' syndrome. Autoantibodies specific for voltage-gated potassium channels (VGKC) or ganglionic nicotinic acetylcholine receptors (AChR) are markers of this disorder. To determine the frequency of these ion channel antibodies and of related neuron- and muscle-specific antibodies in patients with acquired neuromuscular hyperexcitability, we tested serum specimens from 77 affected patients (35 neuromyotonia, 32 cramp-fasciculation syndrome, 5 rippling muscle syndrome, and 5 focal neuromuscular hyperexcitability) and 85 control subjects. Among study patients, 14% had coexisting myasthenia gravis, and 16% had an associated neoplasm. We found that 35% had VGKC antibodies, 12% ganglionic AChR antibodies, 16% muscle AChR antibodies, and 10% striational antibodies. Overall, 55% had serological evidence of neurological autoimmunity compared to 2% of control subjects. Patients with neuromyotonia were more frequently seropositive (71%) than patients with cramp-fasciculation syndrome (31%). We conclude that acquired neuromuscular hyperexcitability consists of a continuum of clinical disorders with a common autoimmune pathogenesis.

Rippling muscle disease: a review

Rippling muscle disease (RMD) is a benign myopathy with symptoms and signs of muscular hyperexcitability. The typical finding is electrically silent muscle contractions provoked by mechanical stimuli and stretch. After the first description in 1975, there have been several publications on this disorder. Although RMD most often is reported with autosomal dominant inheritance, some sporadic cases are found, and an association with other diseases such as myasthenia gravis has also been reported. The pathophysiological mechanism is still not clarified. Abnormalities in calcium homeostasis in the sarcoplasmic reticulum have been proposed as the most probable causes. However, recent genetic studies make a primary channelopathy unlikely. In this article, a review of this curious disease is presented.