Clinical aspects of MuSK antibody positive seronegative MG

Abstract

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 (

Myasthenia gravis

“…Many case series have begun to elucidate key clinical features, and important distinctions in response to therapy in patients with MuSK vs patients with other generalized MG. One of the more striking differences is the female predominance of patients with MuSK, ranging between series from 78% to 100% women. [10][11][12] Three relatively distinct clinical patterns have emerged: oculopharyngeal weakness, with occasional profound tongue and facial atrophy; neck, shoulder, and respiratory weakness without ocular weakness; and a phenotype indistinguishable from AChR-positive MG. One large series of 53 MuSK-positive patients form 9 US neuromuscular centers found facial and bulbar weakness in 96%, neck weakness in 92%, limb weakness in 81%, ocular weakness in 72%, and breathing difficulties in 60% of patients. 10 The high prevalence of respiratory involvement is important when considering diagnosis and therapeutic options.…”

Anti-MuSK autoantibodies block binding of collagen Q to MuSK

“…10,11 MuSK-MG patients respond favorably to immunotherapy, but usually do not respond to, or are even worsened by, cholinesterase inhibitors. [12][13][14][15] Anti-AChR antibodies comprise IgG1 and IgG3 moieties that bind complement whereas anti-MuSK antibodies are largely IgG4 that do not activate complement, and complement deposits at the NMJ are sparse. 16 -18 However, the exact target of MuSK-IgG remains elusive.…”

Análise do jitter com agulha concêntrica em pacientes com miastenia gravis autoimune adquirida

“…Na primeira descrição desse anticorpo na MG (Hoch et al, 2001) (Stickler et al, 2005;Guptill et al, 2011;Nikolic et al, 2014). Pacientes com essa forma da doença são caracteristicamente mulheres adultas jovens, com fraqueza focal oculobulbar grave, na face e/ou músculos do pescoço (Evoli et al, 2003;Sanders et al, 2003), crises frequentes de insuficiência respiratória, além de reposta clínica limitada ao uso de piridostigmina e imunossupressão (Scuderi et al, 2002;Evoli et al, 2003;Guptill et al, 2011). Entretanto, nem sempre esses pacientes têm fraqueza grave e resposta pobre ao tratamento, mas podem ter apresentação clínica indistinguível daquela de pacientes positivos para ac-AChR (Sanders et al, 2003), conforme os pacientes vistos no presente estudo.…”

“…Clinicamente, esses pacientes podem ser indistinguíveis dos pacientes com MG soropositivos para ac-AChR; podem apresentar fraqueza de predomínio oculobulbar, com oftalmoparesia e atrofia de língua, ou ainda, crises frequentes de insuficiência respiratória e fraqueza predominante nos músculos da cintura escapular (Sanders et al, 2003;Evoli, 2006;Padua et al, 2006 repetitiva normal e nas formas leves da doença (Stålberg, 1980;Oh et al, 1992;Sanders, Stålberg, 1996;Sanders, 2002;Howard, 2013).…”

“…Estima-se que 15% a 20% dos pacientes com MG são soronegativos para ac-AChR. (Mossman et al, 1986;Vincent et al, 2001;Padua et al, 2006;Chan et al, 2007) Nesse subgrupo de pacientes, aproximadamente 40% apresentam dosagem sérica positiva para ac-MuSK, uma proteína de membrana essencial para o desenvolvimento da JNM e integração estável dos receptores de acetilcolina (Sanders et al, 2003;Padua et al, 2006). A utilização atual do termo "MG soronegativa" refere-se aos pacientes com dosagens negativas para ac-AChR e ac-MuSK (Zisimopoulou et al, 2013).…”