Diagnosis and therapy of myasthenia gravis with antibodies to muscle-specific kinase

Myasthenia gravis (MG) with antibodies to the muscle-specific receptor tyrosine kinase (MuSK-MG) is a rare disease which covers 5–8% of all MG patients. Symptoms are nearly always generalized, though more focal than in MG with anti-acetylcholine receptor antibodies, with predominant involvement of cranial, bulbar and axial muscles; early respiratory crises are frequent. Focal atrophy, mostly of facial, masseter and tongue muscles, occurs in a proportion of patients. Diagnosis is often challenging on account of atypical presentation with little or no symptom fluctuations, lack of response to acetylcholinesterase inhibitors in a high proportion of patients and negative results of electrodiagnostic studies when performed on limb muscles. Immunosuppression is the mainstay of treatment, since the response to acetylcholinesterase inhibitors is generally unsatisfactory and thymectomy does not appear to improve the course of the disease. Although corticosteroids result in marked improvement, disease flares are frequent during prednisone dosage tapering and most patients remain dependent on treatment. Since treatment with rituximab, in uncontrolled studies, induced sustained benefit in patients with refractory disease, B cell depletion is an attractive option for MuSK-MG patients unresponsive to conventional immunosuppressants.     

Pathophysiology of myasthenia gravis with antibodies to the acetylcholine receptor,muscle-specific kinase and low-density lipoprotein receptor-related protein 4

Myasthenia gravis is caused by antibodies to the acetylcholine receptor, muscle-specific kinase, low-density lipoprotein receptor-related protein 4, or possibly yet unidentified antibodies. The mechanisms by which these antibodies interfere with the function of postsynaptic proteins include complement activation, antigenic modulation by crosslinking of the target proteins, competition with ligand binding sites, or steric hindrance which inhibits conformational changes or binding to associated proteins. Screening for auto-antibodies to different postsynaptic targets, and also for low-affinity antibodies, is contributing to a more accurate diagnosis of MG patients. Further studies into the specific pathophysiological pathways of the several MG subforms might help to develop new, more antigen specific, therapies.     

Antibodies against muscle-specific kinase impair both presynaptic and postsynaptic functions in a murine model of myasthenia gravis

Antibodies against acetylcholine receptors (AChRs) cause pathogenicity in myasthenia gravis (MG) patients through complement pathway-mediated destruction of postsynaptic membranes at neuromuscular junctions (NMJs). However, antibodies against muscle-specific kinase (MuSK), which constitute a major subclass of antibodies found in MG patients, do not activate the complement pathway. To investigate the pathophysiology of MuSK-MG and establish an experimental autoimmune MG (EAMG) model, we injected MuSK protein into mice deficient in complement component five (C5). MuSK-injected mice simultaneously developed severe muscle weakness, accompanied by an electromyographic pattern such as is typically observed in MG patients. In addition, we observed morphological and functional defects in the NMJs of EAMG mice, demonstrating that complement activation is not necessary for the onset of MuSK-MG. Furthermore, MuSK-injected mice exhibited acetylcholinesterase (AChE) inhibitor-evoked cholinergic hypersensitivity, as is observed in MuSK-MG patients, and a decrease in both AChE and the AChE-anchoring protein collagen Q at postsynaptic membranes. These findings suggest that MuSK is indispensable for the maintenance of NMJ structure and function, and that disruption of MuSK activity by autoantibodies causes MG. This mouse model of EAMG could be used to develop appropriate medications for the treatment of MuSK-MG in humans.     

Muscle antibodies in myasthenia gravis

The existence of a second type of antibody to the striations of skeletal muscle is confirmed. This antibody, which reacts with the I bands, is found both in subjects with myasthenia gravis and in normal subjects, but is present in higher titres in the former. The relative frequency of the two types of muscle antibody was estimated in a small group of subjects in whom the thymic changes were known; the possible implications of an association between A band antibody and thymic neoplasia are discussed.     

The role of muscle-specific tyrosine kinase (MuSK) and mystery of MuSK myasthenia gravis

MuSK myasthenia gravis is a rare, severe autoimmune disease of the neuromuscular junction, only identified in 2001, with unclear pathogenic mechanisms. In this review we describe the clinical aspects that distinguish MuSK MG from AChR MG, review what is known about the role of MuSK in the development and function of the neuromuscular junction, and discuss the data that address how the antibodies to MuSK lead to neuromuscular transmission failure.     

The reactivity of the antistriational antibodies associated with thymoma and myasthenia gravis.

Using immunofluorescence forty-nine sera giving striational staining of skeletal and cardiac muscle have been tested on glycerinated myofibrils. By comparing immunofluorescence and phase contrast it has been found that these antibodies are rather heterogeneous but at least three different staining patterns can be identified. Some patterns may be associated with the presence of a thymoma or may be induced by penicillamine treatment of patients with rheumatoid arthritis. Routine screening for antistriational antibodies may lead to the recognition of undiagnosed myasthenia gravis and thymoma. Antistriational autoantibodies may be useful in the further study of the structure and function of the sarcomere.     

Are spontaneous anti-idiotypic antibodies against anti-acetylcholine receptor antibodies present in myasthenia gravis?

The presence of anti-acetylcholine receptor anti-idiotypic antibodies in sera from 102 myasthenia gravis patients and from 33 first-degree relatives was investigated by: (a) Enzyme linked immunosorbent assay (ELISA) using monoclonal antibodies raised against human acetylcholine receptor, (b) immunoprecipitation of 125I-monoclonal anti-acetylcholine receptor antibodies; (c) inhibition of anti-acetylcholine receptor monoclonal antibody binding to the receptor and/or (d) inhibition of autologous and heterologous anti-acetylcholine receptor antibody binding to the receptor. No clear evidence for the presence of abnormal levels of spontaneous anti-idiotypic antibodies to anti-acetylcholine receptor antibodies was found.     

Heart muscle antibodies in myasthenia gravis.

Myasthenia gravis (MG) may in some patients, especially in those with a thymoma, involve the heart. Using an ELISA, we measured heart muscle antibodies in sera from 75 MG patients and in 173 control sera. Heart muscle antibodies were detected in 29/30 thymoma MG patients, in none of 30 young onset MG patients with thymic hyperplasia and in 7/15 late onset MG patients with thymic atrophy. Among the controls, four sera had a very low concentration of heart muscle antibodies. Absorption studies showed that the heart muscle antibodies cross-react with skeletal muscle, but are unrelated to acetylcholine receptor antibodies.     

Plasmaexchange in the treatment of myasthenia gravis associated with thymoma

BACKGROUND: All patients with thymomatous Myasthenia Gravis (MG) should undergo early and total thymectomy, but controversy abounds in the choice of chronic immunosuppressive agents. The value of plasmaexchange (PE) in MG has been clearly established in preoperative preparation and treatment of myasthenic crisis. Whether PE may be used in the chronic long-term therapy of patients with thymomatous MG in addition to conventional immunosuppressive agents and cholinesterase inhibitors is yet to be answered. CASE HISTORY: We present a 40-year old woman with an 11 year history of MG. Thymectomy was done during the first year of the disease and the histopathologic finding was thymoma. To sustain clinical remission after thymectomy she continued with immunosuppression with methylprednisolone and cyclosporin A (or azathioprine) in addition to cholinesterase inhibitors. Despite the almost continuous immunosuppression, the disease course continued with fluctuating myasthenic weakness which few times progressed to myasthenic crisis requiring mechanical ventilation. During myasthenic crisis we performed 6-8 plasmapheresis at 2-3 day intervals in addition to conventional immunosuppressive therapy. The disease rapidly worsened in January 2000 and we started with intermittent plasmapheresis (3-6 procedures at 2-3 day intervals, every 6-8 weeks) in order to sustain remission. With this therapeutic protocol, during 20 months follow-up we managed to prevent myasthenic crisis and to avoid ventilatory support. CONCLUSIONS: Plasmaexchange could be used as a successful and safe therapeutic tool in chronic long-term therapy in addition to conventional immunosuppressive agents to sustain remission in patients with MG. This is particularly important in the treatment of patients with thymomatous MG because they have an increased frequency of myasthenic crisis and often respond poorly an to immunosuppression with steroids or other immunosuppressants.     

An enzyme-linked immunosorbent assay for antibodies against saline-soluble muscle components in myasthenia gravis

An enzyme-linked immunosorbent assay (ELISA) system has been developed for measuring antibodies against rat skeletal muscle components solubilized with phosphate-buffered saline. With this assay, 53.8% (50/93) of sera from patients with myasthenia gravis (MG) was positive (the values over the mean plus 3 SD of 256 healthy individuals were considered significant). No sera from patients with neurological disorders other than MG gave positive values (0%; 0/60). No correlation between titers of antibody against the muscle components and those of anti-acetylcholine receptor antibody (r = 0.01) was found. These results indicate that our ELISA system is useful for diagnosing myasthenia gravis.     

Autoimmunity against the ryanodine receptor in myasthenia gravis

Some myasthenia gravis (MG) patients have antibodies against skeletal muscle antigens in addition to the acetylcholine receptor (AChR). A major antigen for these antibodies is the Ca²⁺ release channel of the sarcoplasmic reticulum the ryanodine receptor (RyR). These antibodies are found mainly in MG patients with a thymoma MG and correlate with severe MG symptoms. The antibodies recognize a region near the N‐terminus on the RyR, which seems to be of importance for RyR regulation. The antibodies cause allosteric inhibition of RyR function in vitro, inhibiting Ca²⁺ release from sarcoplasmic reticulum.     

Isaac's syndrome associated with myasthenia gravis and thymoma

A 65-year-old male developed fatigable weakness of ocular and bulbar muscle and positive anti-acetyl cholinesterase antibodies suggesting the diagnosis of myasthenia gravis. His condition responded to anticholinesterase and immunotherapy. However, 18 months later, he developed painful paresthesiae, muscle cramps with hyperhiderosis, and was diagnosed as having Isaac's syndrome (neuromyotonia, continuous muscle fibre activity). Computed tomography of the chest revealed a thymic mass, which was confirmed after surgery and histopathology as thymic cell carcinoma. The co-occurrence of myasthenia gravis and continuous muscle fiber activity should prompt the consideration of the occurrence of these disorders as one of the paraneoplastic manifestations, most often due to a thymic neoplasm. Both these conditions respond to treatment of underlying thymoma. This case is a very rare presentation worth reporting.     

Complement associated pathogenic mechanisms in myasthenia gravis

The complement system is profoundly involved in the pathogenesis of acetylcholine receptor (AChR) antibody (Ab) related myasthenia gravis (MG) and its animal model experimental autoimmune myasthenia gravis (EAMG). The most characteristic finding of muscle pathology in both MG and EAMG is the abundance of IgG and complement deposits at the nerve–muscle junction (NMJ), suggesting that AChR-Ab induces muscle weakness by complement pathway activation and consequent membrane attack complex (MAC) formation. This assumption has been supported with EAMG resistance of complement factor C3 knockout (KO), C4 KO and C5 deficient mice and amelioration of EAMG symptoms following treatment with complement inhibitors such as cobra venom factor, soluble complement receptor 1, anti-C1q, anti-C5 and anti-C6 Abs. Moreover, the complement inhibitor decay accelerating factor (DAF) KO mice exhibit increased susceptibility to EAMG. These findings have brought forward improvisation of novel therapy methods based on inhibition of classical and common complement pathways in MG treatment.     

Relation of HLA-DRB1 to IgG4 autoantibody and cytokine production in muscle-specific tyrosine kinase myasthenia gravis (MuSK-MG)

A small subset of myasthenia gravis (MG) patients develop autoantibodies against muscle-specific kinase (MuSK), which are predominantly of the immunoglobulin (Ig)G4 isotype. MuSK-MG is strongly associated with HLA-DRB1*14, HLA-DRB1*16 and HLA-DQB1*05. In this study, the possible effects of these HLA associations on MuSK IgG autoantibody or cytokine production were investigated. Samples from 80 MG patients with MuSK antibodies were studied. The disease-associated HLA types were screened in the DNA samples. The IgG1, IgG2, IgG3 and IgG4 titres of the MuSK antibodies and the levels of interleukin (IL)-4, IL-6, IL-17A and IL-10 were measured in the sera. Comparisons were made among the groups with or without HLA-DRB1*14, HLA-DRB1*16 or HLA-DQB1*05. The IgG4 titres of the MuSK antibodies were higher than those of the IgG1, IgG2 and IgG3 isotypes among the whole group of patients. DRB1*14 (+) DRB1*16 (–) patients had higher levels of IgG4 antibodies than those of DRB1*14 (–) DRB1*16 (+) patients. DRB1*14 (+) DRB1*16 (+) patients also had higher levels of IgG4 antibodies than those of DRB1*14 (–) DRB1*16 (+) and DRB1*14 (–) DRB1*16 (–) patients. Higher IL-10 and lower IL-17A levels were measured in DRB1*14 (+) DRB1*16 (–) patients than in DRB1*14 (–) DRB1*16 (–) patients. The higher IgG4 titres of MuSK autoantibodies in patients carrying HLA-DRB1*14 than those in the other patients suggest a role for HLA in the production of the antibodies. The differences in IL-10 and IL-17A support the role of DRB1 in the etiopathogenesis of this autoimmune response.     

Autoimmunity against the ryanodine receptor in myasthenia gravis.

Some myasthenia gravis (MG) patients have antibodies against skeletal muscle antigens in addition to the acetylcholine receptor (AChR). A major antigen for these antibodies is the Ca2+ release channel of the sarcoplasmic reticulum the ryanodine receptor (RyR). These antibodies are found mainly in MG patients with a thymoma MG and correlate with severe MG symptoms. The antibodies recognize a region near the N-terminus on the RyR, which seems to be of importance for RyR regulation. The antibodies cause allosteric inhibition of RyR function in vitro, inhibiting Ca2+ release from sarcoplasmic reticulum.