Myasthenia gravis (MG) is an antibody-mediated autoimmune disease of the neuromuscular junction. In approximately 80% of patients, auto-antibodies to the muscle nicotinic acetylcholine receptor (AChR) are present. These antibodies cause loss of AChR numbers and function, and lead to failure of neuromuscular transmission with muscle weakness. The pathogenic mechanisms acting in the 20% of patients with generalized MG who are seronegative for AChR-antibodies (AChR-Ab) have not been elucidated, but there is evidence that they also have an antibody-mediated disorder, with the antibodies directed towards another, previously unidentified muscle-surface-membrane target. Here we show that 70% of AChR-Ab-seronegative MG patients, but not AChR-Ab-seropositive MG patients, have serum auto-antibodies against the muscle-specific receptor tyrosine kinase, MuSK. MuSK mediates the agrin-induced clustering of AChRs during synapse formation, and is also expressed at the mature neuromuscular junction. The MuSK antibodies were specific for the extracellular domains of MuSK expressed in transfected COS7 cells and strongly inhibited MuSK function in cultured myotubes. Our results indicate the involvement of MuSK antibodies in the pathogenesis of AChR-Ab-seronegative MG, thus defining two immunologically distinct forms of the disease. Measurement of MuSK antibodies will substantially aid diagnosis and clinical management.

BackgroundThe aim was to collate all myasthenia gravis (MG) epidemiological studies including AChR MG and MuSK MG specific studies. To synthesize data on incidence rate (IR), prevalence rate (PR) and mortality rate (MR) of the condition and investigate the influence of environmental and technical factors on any trends or variation observed.MethodsStudies were identified using multiple sources and meta-analysis performed to calculate pooled estimates for IR, PR and MR.Results55 studies performed between 1950 and 2007 were included, representing 1.7 billion population-years. For All MG estimated pooled IR (eIR): 5.3 per million person-years (C.I.:4.4, 6.1), range: 1.7 to 21.3; estimated pooled PR: 77.7 per million persons (C.I.:64.0, 94.3), range 15 to 179; MR range 0.1 to 0.9 per millions person-years. AChR MG eIR: 7.3 (C.I.:5.5, 7.8), range: 4.3 to 18.0; MuSK MG IR range: 0.1 to 0.32. However marked variation persisted between populations studied with similar methodology and in similar areas.ConclusionsWe report marked variation in observed frequencies of MG. We show evidence of increasing frequency of MG with year of study and improved study quality. This probably reflects improved case ascertainment. But other factors must also influence disease onset resulting in the observed variation in IR across geographically and genetically similar populations.

Objective: To increase the detection of MuSK-Abs using a CBA and test their pathogenicity. Methods: Sera from 69 MuSK-RIA-positive patients with myasthenia gravis (MG) (Definite MuSK-MG), 169 patients negative for MuSK-RIA and AChR-RIA (seronegative MG, SNMG), 35 healthy individuals (healthy controls, HCs), and 16 NMDA receptor-Ab-positive (NMDAR-Ab) disease controls were tested for binding to MuSK on a CBA using different secondary antibodies. Results: Initially, in addition to 18% of SNMG sera, 11% of HC and 19% of NMDAR-Ab sera showed positive binding to MuSK-transfected cells; this low specificity was due to anti-IgG (H1L) detection of IgM bound nonspecifically to MuSK. Using an IgG Fc gamma-specific secondary antibody, MuSK-Abs were detected by CBA in 68/69 (99%) of Definite MuSK-MG, 0/35 HCs, 0/16 NMDAR-Ab, and 14/169 (8%) of SNMG sera, providing increased sensitivity with high specificity. The RIA-negative, CBA-positive MuSK-IgG sera, but not IgM-MuSK-binding sera, reduced agrin-induced AChR clustering in C2C12 myotubes, qualitatively similar to RIA-positive MuSK-Abs. Conclusions: An IgG-specific MuSK-CBA can reliably detect IgG MuSK-Abs and increase sensitivity. In the MuSK-CBA, IgG specificity is essential. The positive sera demonstrated pathogenic potential in the in vitro AChR-clustering assay, although less effective than Definite MuSK-MG sera, and the patients had less severe clinical disease. Use of IgG-specific secondary antibodies may improve the results of other antibody tests. GLOSSARY AChEI 5 acetylcholinesterase inhibitor; CBA 5 cell-based assay; CI 5 confidence interval; DMEM 5 Dulbecco Modified Eagle Medium; FCS 5 fetal calf serum; HC 5 healthy control; HEK 5 human embryonic kidney; MG 5 myasthenia gravis; MGFA 5 Myasthenia Gravis Foundation of America; PSA 5 Penicillin, Streptomycin, and Amphotericin; RT 5 room temperature ; SNMG 5 seronegative MG. Several methods are available for the detection of antigen-specific antibodies (Abs) in the serum or CSF of patients with autoantibody-mediated CNS and peripheral nervous system diseases. 1 In the case of suspected autoimmune myasthenia gravis (MG), sera are routinely tested by radioimmunoprecipitation assays (RIAs) for Abs to AChR or MuSK. However, indirect immu-nofluorescence on live cells transiently transfected with AChRs and clustered by rapsyn, as they are at the neuromuscular junction, increases the detection of AChR-IgG 2-5 ; CBAs have been found to be sensitive and specific for many antibodies, e.g., for aquaporin-4 (AQP-4) antibodies in patients with neuromyelitis optica. 6 Here, to see whether a CBA might increase sensitivity for From the Nuffield Department of Clinical Neurosciences (

Antibodies to rat muscle specific kinase, MuSK, have recently been identified in some generalized "seronegative" myasthenia gravis (SNMG) patients, who are often females with marked bulbar symptoms. Using immunoprecipitation of (125)I-labelled-human MuSK, 27 of 66 (41%) seronegative patients were positive, but 18 ocular SNMG patients, 105 AChR antibody positive MG patients, and 108 controls were negative. The antibodies are of high affinity (Kds around 100 pM) with titers between 1 and 200 nM. They bind to the extracellular Ig-like domains of soluble or native MuSK. Surprisingly they are predominantly in the IgG4 subclass. MuSK-antibody associated MG may be different in etiological and pathological mechanisms.