ORIGINAL COMMUNICATION Anti-LRP4 autoantibodies in AChR- and MuSK-antibody- negative myasthenia gravis Alexandra Pevzner • Benedikt Schoser • Katja Peters • Nicoleta-Carmen Cosma • Andromachi Karakatsani • Berthold Schalke • Arthur Melms • Stephan Kro ¨ger Received: 31 May 2011 / Revised: 27 June 2011 / Accepted: 14 July 2011 Ó Springer-Verlag 2011 Abstract Myasthenia gravis (MG) is an autoimmune dis- order characterized by a defect in synaptic transmission at the neuromuscular junction causing fluctuating muscle weakness with a decremental response to repetitive nerve stimulation or altered jitter in single-fiber electromyography (EMG). Approximately 80% of all myasthenia gravis patients have autoantibodies against the nicotinic acetyl- choline receptor in their serum. Autoantibodies against the tyrosine kinase muscle-specific kinase (MuSK) are respon- sible for 5–10% of all myasthenia gravis cases. The auto- immune target in the remaining cases is unknown. Recently, low-density lipoprotein receptor-related protein 4 (LRP4) has been identified as the agrin receptor. LRP4 interacts with agrin, and the binding of agrin activates MuSK, which leads to the formation of most if not all postsynaptic specializa- tions, including aggregates containing acetylcholine recep- tors (AChRs) in the junctional plasma membrane. In the present study we tested if autoantibodies against LRP4 are detectable in patients with myasthenia gravis. To this end we analyzed 13 sera from patients with generalized myasthenia gravis but without antibodies against AChR or MuSK. The results showed that 12 out of 13 antisera from double-sero- negative MG patients bound to proteins concentrated at the neuromuscular junction of adult mouse skeletal muscle and that approximately 50% of the tested sera specifically bound to HEK293 cells transfected with human LRP4. Moreover, 4 out of these 13 sera inhibited agrin-induced aggregation of AChRs in cultured myotubes by more than 50%, suggesting a pathogenic role regarding the dysfunction of the neuro- muscular endplate. These results indicate that LRP4 is a novel target for autoantibodies and is a diagnostic marker in seronegative MG patients. Keywords Myasthenia gravis Á Low-density lipoprotein receptor-related protein Á LRP4 Á Autoimmune disorder Introduction Myasthenia gravis (MG) is an autoimmune disorder char- acterized by use-dependent muscle weakness due to com- promised synaptic transmission at the neuromuscular junction [1–3]. In approximately 80% of myasthenia gravis patients, the disease is mediated by antibodies against the nicotinic acetylcholine receptor (AChR) [4]. These anti- bodies reduce the number of functional AChRs at the muscle fiber endplate by increasing their degradation and turnover [5], induce complement-mediated damage to the muscle fiber [6], and block acetylcholine binding to the receptor [7]. In up to 50% of MG patients without auto- antibodies to AChR, autoantibodies against the muscle- specific tyrosine kinase MuSK have been detected [8–11]. MuSK is a 110-kDa protein initially isolated from rat skeletal muscle. In skeletal muscle, MuSK is concentrated A. Pevzner Á K. Peters Á N.-C. Cosma Á A. Karakatsani Á S. Kro ¨ger (&) Department of Physiological Genomics, Institute for Physiology, Ludwig-Maximilians University, Pettenkoferstrasse 12, 80336 Munich, Germany e-mail: skroeger@LMU.de B. Schoser Department of Neurology, Friedrich-Baur-Institute, Ludwig-Maximilians University, Munich, Germany B. Schalke Department of Neurology, University of Regensburg, Regensburg, Germany A. Melms Department of Neurology, University of Tu ¨bingen, Tu ¨bingen, Germany 123 J Neurol DOI 10.1007/s00415-011-6194-7