Autoantibody Specificities in Myasthenia Gravis; Implications for Improved Diagnostics and Therapeutics

Abstract

Myasthenia gravis (MG) is an autoimmune disease characterized by muscle weakness and fatiguability of skeletal muscles. It is an antibody-mediated disease, caused by autoantibodies targeting neuromuscular junction proteins. In the majority of patients (~85%) antibodies against the muscle acetylcholine receptor (AChR) are detected, while in 6% antibodies against the muscle-specific kinase (MuSK) are detected. In ~10% of MG patients no autoantibodies can be found with the classical diagnostics for AChR and MuSK antibodies (seronegative MG, SN-MG), making the improvement of methods for the detection of known autoantibodies or the discovery of novel antigenic targets imperative. Over the past years, using cell-based assays or improved highly sensitive immunoprecipitation assays, it has been possible to detect autoantibodies in previously SN-MG patients, including the identification of the low-density lipoprotein receptor-related protein 4 (LRP4) as a third MG autoantigen, as well as AChR and MuSK antibodies undetectable by conventional methods. Furthermore, antibodies against other extracellular or intracellular targets, such as titin, the ryanodine receptor, agrin, collagen Q, K_v1.4 potassium channels and cortactin have been found in some MG patients, which can be useful biomarkers. In addition to the improvement of diagnosis, the identification of the patients' autoantibody specificity is important for their stratification into respective subgroups, which can differ in terms of clinical manifestations, prognosis and most importantly their response to therapies. The knowledge of the autoantibody profile of MG patients would allow for a therapeutic strategy tailored to their MG subgroup. This is becoming especially relevant as there is increasing progress toward the development of antigen-specific therapies, targeting only the specific autoantibodies or immune cells involved in the autoimmune response, such as antigen-specific immunoadsorption, which have shown promising results. We will herein review the advances made by us and others toward development of more sensitive detection methods and the identification of new antibody targets in MG, and discuss their significance in MG diagnosis and therapy. Overall, the development of novel autoantibody assays is aiding in the more accurate diagnosis and classification of MG patients, supporting the development of advanced therapeutics and ultimately the improvement of disease management and patient quality of life.

Keywords: LRP4; MuSK; acetylcholine receptor; autoantibody; autoimmunity; diagnosis; myasthenia gravis; therapy.

Figure 1

Schematic representation of the neuromuscular junction and myotube components. Agrin released from the nerve terminal binds to LRP4, which in turn binds to and activates MuSK, causing rapsyn-mediated AChR clustering. Acetylcholine (Ach) released from the nerve terminal binds to AChR causing opening of the receptor channel and triggering muscle contraction. Unbound acetylcholine in the synaptic cleft is broken down into choline and acetic acid by AChE, thus terminating its action. The antigenic targets for autoantibodies in MG known so far are depicted, though not all have been shown to be implicated in pathology. AChR, acetylcholine receptor; MuSK, muscle specific kinase; LRP4, low-density lipoprotein receptor-related protein 4; RyR, ryanodine receptor; ColQ, collagen Q; AChE, acetylcholinesterase; Kv1.4, voltage gated potassium channel 1.4.

Figure 2

Detection of autoantibodies in SNMG by novel assays. We have used CBA and RIPA for screening a large number of MG patients without detectable autoantibodies by the classical assays, as well as several control samples from healthy individuals or patients with other neuroimmune diseases (OND), from 10 to 13 different European countries (44, 80, 99). The numbers above the bars indicate the number of positive samples and the total tested with each assay. The cumulative percentage (black bar) of new positives among the SNMG samples that were positive in more than one assays were taken into account, so as to avoid overestimation of the total new seropositive patients.