title: Investigation of LGI1 as the antigen in limbic encephalitis previously attributed to potassium channels: a case series.
abstract: BACKGROUND
  Voltage-gated potassium channels are thought to be the target of antibodies associated with limbic encephalitis. However, antibody testing using cells expressing voltage-gated potassium channels is negative; hence, we aimed to identify the real autoantigen associated with limbic encephalitis.
  
  
  METHODS
  We analysed sera and CSF of 57 patients with limbic encephalitis and antibodies attributed to voltage-gated potassium channels and 148 control individuals who had other disorders with or without antibodies against voltage-gated potassium channels. Immunohistochemistry, immunoprecipitation, and mass spectrometry were used to characterise the antigen. An assay with HEK293 cells transfected with leucine-rich, glioma-inactivated 1 (LGI1) and disintegrin and metalloproteinase domain-containing protein 22 (ADAM22) or ADAM23 was used as a serological test. The identity of the autoantigen was confirmed by immunoabsorption studies and immunostaining of Lgi1-null mice.
  
  
  RESULTS
  Immunoprecipitation and mass spectrometry analyses showed that antibodies from patients with limbic encephalitis previously attributed to voltage-gated potassium channels recognise LGI1, a neuronal secreted protein that interacts with presynaptic ADAM23 and postsynaptic ADAM22. Immunostaining of HEK293 cells transfected with LGI1 showed that sera or CSF from patients, but not those from control individuals, recognised LGI1. Co-transfection of LGI1 with its receptors, ADAM22 or ADAM23, changed the pattern of reactivity and improved detection. LGI1 was confirmed as the autoantigen by specific abrogation of reactivity of sera and CSF from patients after immunoabsorption with LGI1-expressing cells and by comparative immunostaining of wild-type and Lgi1-null mice, which showed selective lack of reactivity in brains of Lgi1-null mice. One patient with limbic encephalitis and antibodies against LGI1 also had antibodies against CASPR2, an autoantigen we identified in some patients with encephalitis and seizures, Morvan's syndrome, and neuromyotonia.
  
  
  CONCLUSIONS
  LGI1 is the autoantigen associated with limbic encephalitis previously attributed to voltage-gated potassium channels. The term limbic encephalitis associated with antibodies against voltage-gated potassium channels should be changed to limbic encephalitis associated with LGI1 antibodies, and this disorder should be classed as an autoimmune synaptic encephalopathy.
  
  
  BACKGROUND
  National Institutes of Health, National Cancer Institute, and Euroimmun.
authors.affiliation: Department of Neurology, University of Pennsylvania, School of Medicine, Philadelphia, PA 19104, USA.
authors.forename: Meizan
authors.initials: M
authors.lastname: Lai
authors.forename: Maartje G M
authors.initials: MG
authors.lastname: Huijbers
authors.forename: Eric
authors.initials: E
authors.lastname: Lancaster
authors.forename: Francesc
authors.initials: F
authors.lastname: Graus
authors.forename: Luis
authors.initials: L
authors.lastname: Bataller
authors.forename: Rita
authors.initials: R
authors.lastname: Balice-Gordon
authors.forename: John K
authors.initials: JK
authors.lastname: Cowell
authors.forename: Josep
authors.initials: J
authors.lastname: Dalmau
mesh_terms: D051722:ADAM Proteins / Q000378:metabolism; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000818:Animals; D001323:Autoantibodies; D001324:Autoantigens / Q000235:genetics / Q000276:immunology*; D002460:Cell Line; D004827:Epilepsy / Q000378:metabolism; D005260:Female; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D008024:Ligands; D020363:Limbic Encephalitis / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008875:Middle Aged; D009419:Nerve Tissue Proteins / Q000378:metabolism; D024642:Potassium Channels, Voltage-Gated / Q000172:deficiency / Q000235:genetics / Q000276:immunology*; D011506:Proteins / Q000235:genetics / Q000276:immunology* / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar
chemical_list: D001323:Autoantibodies; D001324:Autoantigens; D047908:Intracellular Signaling Peptides and Proteins; C116499:LGI1 protein, human; D008024:Ligands; D009419:Nerve Tissue Proteins; D024642:Potassium Channels, Voltage-Gated; D011506:Proteins; D051722:ADAM Proteins; C498474:ADAM22 protein, human; C114633:ADAM23 protein, human