Is “seronegative” MG explained by autoantibodies to MuSK?

Muscular weakness in ∼90% of patients with myasthenia gravis (MG) is caused by an antibody-mediated autoimmune response to muscle nicotinic acetylcholine receptors (AChRs).1 Immunization of animals with purified AChRs induces experimental autoimmune MG, in which autoantibodies to AChRs cause muscle weakness by impairing neuromuscular transmission through the same mechanisms found to occur in MG: loss of AChR caused by increased internalization of antibody cross-linked AChRs and destruction of AChRs by complement-mediated focal lysis, disrupted synaptic morphology caused by postsynaptic membrane damage, and (to a limited extent) direct impairment of AChR function by bound autoantibodies.

An explanation for muscular weakness in the 10 to 15% of patients with “seronegative” MG who lack autoantibodies to AChRs had appeared to be autoantibodies to muscle-specific receptor tyrosine kinase (MuSK).2,3⇓ These were found by Liyanagi et al.4 in 38 to 70% of patients with MG seronegative for autoantibodies to AChRs but not in patients with MG with autoantibodies to AChRs. MuSK mediates agrin-induced clustering of AChRs during synapse formation. These autoantibodies to the extracellular domain of MuSK inhibited its function in tissue culture.2

Patients with “seronegative” MG respond to plasma exchange and immunosuppressive therapies.5 Antibodies from five of six patients passively transferred to mice a reduction in miniature endplate potential amplitude.6 In four of six sera transferred, quantal content of the endplate potential was also reduced, suggesting a presynaptic effect and suggesting that seronegative MG could be caused by autoantibodies to several antigens. Fifty percent of seronegative MG sera can inhibit the function of human AChRs expressed in rhabdomyosarcoma lines without actually binding to the AChRs.7 The factor that inhibits AChR function is not an immunoglobulin (Ig) G autoantibody to MuSK but may be an IgM autoantibody to an unknown component.8 Autoantibodies to antigens other than MuSK have also been reported in seronegative MG using an antibody-secreting cell assay.9 In these Chinese patients, cells secreting IgG to AChR were detected in 9 of 13 seronegative patients and in 9 of 12 patients with seropositive MG. IgG antibody to a putative presynaptic β-bungarotoxin binding protein also was detected in four of eight seronegative patients and in six of eight patients with seropositive MG.

There have not been reports of inducing muscular weakness in animals by immunization with purified MuSK. Thus, it has not been demonstrated that autoantibodies to MuSK can actually cause muscle weakness through impairing neuromuscular transmission. The possibility remains that autoantibodies to other presynaptic or postsynaptic proteins could account for some or all “seronegative” MG.

Three articles in this issue of Neurology cast doubt on autoantibodies to MuSK as the explanation for seronegative MG.10–12⇓⇓ In these reports, autoantibodies to MuSK were not found where they were expected,10 were found where they were not expected,11 or were found but do not seem to be the cause of weakness.12

Yeh et al.10 found that only 1 of 26 Taiwanese patients with MG lacking autoantibodies to AChRs actually had autoantibodies to MuSK. This man aged 71 years responded dramatically to plasmapheresis. Most white patients with MG and antibodies to MuSK have been female (58/68). Asian and white patients with MG and antibodies to AChRs differ in age of onset, human leukocyte antigen associations, and other features; therefore, racial differences in features of MG would not be surprising.13 It appears that 96% of seronegative MG in these Chinese patients involves autoantibodies to proteins other than MuSK or other pathologic mechanisms.

Ohta et al.11 found that 7 of 17 (41%) Japanese patients with MG lacking autoantibodies to AChRs had autoantibodies to MuSK. However, 14 of 133 (11%) patients with MG with autoantibodies to AChRs also had antibodies to MuSK. Genetic or sampling differences may account for detection of autoantibodies to MuSK in some of these patients with seropositive MG. These results did not prove that autoantibodies to MuSK caused MG in the patients who had them and indicated that autoantibodies to other antigens or other pathologic mechanisms must account for weakness in at least 59% of these patients with seronegative MG.

Selcen et al.12 report that one patient seronegative for autoantibodies to AChR but seropositive for autoantibodies to MuSK exhibited muscular weakness. However, the cause of weakness could not be explained by substantial binding of autoantibodies to MuSK at endplates or by loss of MuSK or AChRs. Despite detailed electron microscopic and electrophysiologic studies, the cause of muscular weakness in this patient remains unknown. A congenital explanation seems likely; however, no significant mutations were found in MuSK, AChR, or RAPSYN genes. In this particular case, autoantibodies to MuSK appear to be incidental to unknown pathologic mechanisms that cause muscle weakness.

It is known that MuSK is important for neuromuscular development because MuSK knockout mice fail to cluster AChRs or differentiate postsynaptic regions and die perinatally.14 It is now critical to determine whether immunization of rodents with purified MuSK can cause loss of AChRs and muscular weakness. Even if this can be achieved with MuSK, it seems likely that other antigens are also involved in seronegative MG.

Because mutations in many proteins involved in neuromuscular transmission have now been found to cause congenital myasthenic syndromes,15 it should not be surprising that autoantibodies to many presynaptic proteins (e.g., Lambert–Eaton myasthenic syndrome) and postsynaptic proteins (e.g., AChRs and perhaps MuSK and others) should cause myasthenic syndromes.8 The search for autoantigens in “seronegative” MG will go on.