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

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.

Cardiac troponin T and autoimmunity in skeletal muscle aging

Age-related muscle mass and strength decline (sarcopenia) impairs the performance of daily living activities and can lead to mobility disability/limitation in older adults. Biological pathways in muscle that lead to mobility problems have not been fully elucidated. Immunoglobulin G (IgG) infiltration in muscle is a known marker of increased fiber membrane permeability and damage vulnerability, but whether this translates to impaired function is unknown. Here, we report that IgG1 and IgG4 are abundantly present in the skeletal muscle (vastus lateralis) of ~ 50% (11 out of 23) of older adults (> 65 years) examined. Skeletal muscle IgG1 was inversely correlated with physical performance (400 m walk time: r = 0.74, p = 0.005; SPPB score: r =  - 0.73, p = 0.006) and muscle strength (r =  - 0.6, p = 0.05). In a murine model, IgG was found to be higher in both muscle and blood of older, versus younger, C57BL/6 mice. Older mice with a higher level of muscle IgG had lower motor activity. IgG in mouse muscle co-localized with cardiac troponin T (cTnT) and markers of complement activation and apoptosis/necroptosis. Skeletal muscle-inducible cTnT knockin mice also showed elevated IgG in muscle and an accelerated muscle degeneration and motor activity decline with age. Most importantly, anti-cTnT autoantibodies were detected in the blood of cTnT knockin mice, old mice, and older humans. Our findings suggest a novel cTnT-mediated autoimmune response may be an indicator of sarcopenia.

Rippling Muscle Disease with Irregular Toe Jerks and Anti-acetylcholine Receptor Antibodies: Remission after Extended Thymectomy

We herein report a 63-year-old rippling muscle disease (RMD) patient who presented with painless stiffness, muscle hypertrophy and muscle contractions elicited by mechanical stimulation. He also showed irregular toe jerks and a slightly elevated level of anti-acetylcholine receptor antibody (AChR-Ab). Since he had a mediastinal mass mimicking thymoma, which was later revealed to be a bronchial cyst, he underwent extended thymectomy. The irregular toe jerks disappeared within a week after the operation. The other muscle symptoms completely remitted 27 months after the onset. This is the first report of a sporadic case of RMD with irregular toe jerks that resolved after extended thymectomy.

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.

Titin and ryanodine receptor antibodies and neuromuscular involvement in myasthenia gravis

Of sera of patients with myasthenia gravis (MG), 30% contains titin and ryanodine receptor (RyR) antibodies. In early-onset MG (i.e., before 50 years of age), the presence of titin antibodies strongly suggests a thymoma. Late-onset MG comprises one MG group, characterized by a broad antimuscle immune response, including both titin and the acetylcholine receptor (AChR). Another group is preferentially associated with the HLA-A3, B7 and DRw2 antigens, representing a delayed early-onset of the disease that has a selective AChR immune response. The presence of titin and RyR antibodies is associated with more severe disease. Titin antibodies may, in some patients with rippling muscle disease, affect the contractile machinery of myofibers, thereby affecting their mechanical sensitivity. It is not known whether this occurs in MG. RyR antibodies may impair excitation–contraction coupling and contribute to muscle weakness in MG patients. Titin antibodies may serve as tumor markers in early-onset MG patients.