斜方肌肌电图运动单位动作电位成人正常参考值的建立

目的 探讨不同血清抗体重症肌无力(MG)的临床特征.方法 用荧光免疫沉淀法(FIPA)和荧光免疫细胞染色法(CBA)检测119例MG患者血清乙酰胆碱受体抗体(AChR-Ab)和肌肉特异性受体酪氨酸激酶抗体(MuSK-Ab)水平.比较AChR-Ab阳性、MuSK-Ab阳性、血清抗体阴性MG的临床特征.结果 纳入119例患者中,90例AChR-Ab阳性(75.6%),29例阴性:其中5例MuSK-Ab阳性(17.2%),24例血清抗体阴性(82.8%).AChR-Ab阳性、MuSK-Ab阳性和血清抗体阴性MG 3组比较,男女比例和平均发病年龄差异均无统计学意义.3例MuSK-Ab阳性的患者主要表现为延髓肌受累;79.2%(19/24)的血清抗体阴性MG患者表现为美国MG协会(MGFA)Ⅰ型;2例MuSK-Ab阳性的患者MGFA≥Ⅲ型;MuSK-Ab滴度水平与患者病情严重程度相关(r=0.941,P=0.014);MuSK-Ab阳性的患者均未发现有胸腺的异常.结论 MuSK-Ab仅出现在AChR-Ab阴性患者的血清中.MuSK-Ab阳性的患者主要表现为延髓肌受累,病情较重且不伴有胸腺的病变.MuSK-Ab阳性的MG可能是不同于血清AChR-Ab阳性的MG的又一亚型.     

伴肌萎缩重症肌无力4例分析并文献复习

目的探讨伴肌萎缩重症肌无力(myasthenia gravis,MG)患者的临床特征、抗体及电生理特点,提高对此少见疾病的认识。方法回顾性分析作者医院2003—2016年收治的伴肌萎缩MG患者4例的临床资料,分析其临床特征、抗体、神经电生理检查、治疗及预后特点。结果 4例患者病程中出现不同程度的肌肉萎缩,1例为手肌(左侧大鱼际肌、小鱼际肌及第一骨间肌)萎缩,3例为舌肌萎缩(其中1例同时伴面肌萎缩);抗体检查显示乙酰胆碱受体抗体(AChR-Ab)阳性3例,1例为抗骨骼肌特异性受体酪氨酸激酶抗体(MuSK-Ab)阳性。4例患者低频重复神经刺激波幅均可递减,高频刺激波幅未见明显递增。伴手肌萎缩MG患者针极肌电图检查结果提示神经源性损害。结论伴肌肉萎缩MG较罕见,MuSK-Ab、AChR-Ab阳性MG患者均可出现肌肉萎缩,其机制有待进一步明确;早期行神经电生理检查、AChR-Ab及MuSK-Ab测定有助于明确诊断。     

血清乙酰胆碱受体抗体、肌肉特异性酪氨酸激酶抗体浓度与重症肌无力患者临床分型和疾病转归的关系

目的探索血清乙酰胆碱受体抗体(AchR-Ab)、肌肉特异性酪氨酸激酶抗体(MuSK-Ab)浓度与重症肌无力(MG)患者临床分型和疾病转归的关系。方法收集2014年5月-2015年5月在本院神经内科住院治疗的MG患者43例(MG组)和同期在本院健康体检的正常人43例(N组),采用酶联免疫吸附(ELISA)法检测血清AChR-Ab、MuSK-Ab浓度,并计算MG组患者治疗前后的肌无力绝对评分,分析不同临床分型的MG患者治疗前的血清AChR-Ab、MuSK-Ab浓度,并对治疗前后肌无力绝对评分和治疗前后血清AChR-Ab、MuSK-Ab浓度变化进行相关性分析。结果 (1)治疗前,MG组和N组的血清AChR-Ab浓度分别为0.786±0.237ng·mL~(-1)、0.298±0.113ng·mL~(-1),差异有统计学意义(均P0.05),血清Mu SK-Ab浓度分别为0.322±0.128ng·mL~(-1)、0.281±0.123ng·mL~(-1),差异无统计学意义(均P0.05);(2)血清AChR-Ab和MuSK-Ab在不同临床分型MG患者中的浓度均没有明显差异(均P0.05);(3)治疗后肌力绝对评分和血清AChR-Ab浓度均显著下降(均P0.05),而血清MuSK-Ab浓度在治疗前后没有显著变化(P0.05),并且治疗后的血清AChR-Ab浓度和治疗后的肌无力绝对评分呈正相关(r=0.797,P0.05)。结论血清AChR-Ab和MuSK-Ab的浓度与MG的临床分型无明显相关性,血清AChR-Ab对疾病的转归有一定的预示作用。     

不同抗体分型的重症肌无力特点分析

目的探讨不同抗体分型重症肌无力(myasthenia gravis,MG)患者的临床特点。方法收集114例临床确诊MG患者的临床资料。使用酶联免疫吸附法检测患者血清中的抗乙酰胆碱受体抗体(acetylcholine receptors antibody,AChR Ab)、肌肉特异性激酶抗体(antibody to muscle-specific kinase, MuSK Ab)、抗联接蛋白抗体(titin antibody, Titin Ab)和抗兰尼碱受体抗体(ryanodine receptors antibody, RyR Ab)。根据致病抗体AChR Ab、MuSK Ab的表达情况将患者分为AChR Ab阳性组,MuSK Ab阳性组和双抗体阴性(double serum negative, DSN)组,对3组的临床表现、实验室检查和胸腺病理进行分析;根据疾病严重程度相关抗体Titin Ab和RyR Ab的表达情况,分为Titin Ab阳性组和Titin Ab阴性组,RyR Ab阳性组和RyR Ab阴性组,并比较两种抗体分型的阳性组和阴性组间的临床表现、实验室检查和胸腺病理,以及Titin Ab阳性MG和RyR Ab阳性MG组的受累肌群。结果 AChR Ab(+)组76例(66.7%)、MuSK Ab(+)组9例(7.9%)、DSN组29例(25.4%),三组间甲状腺功能(100.0%vs. 33.3%vs. 58.6%)和自身抗体异常发生率(63.2%vs. 33.3%vs. 37.9%)以及胸腺病理患者比例(胸腺增生/萎缩48.0%vs.33.3%vs. 71.4%,胸腺瘤52.0%vs.66.7%vs.28.6%)差异均有统计学意义(P0.05);组间两两比较,仅AChR Ab(+)MG甲状腺功能异常率高于MuSK Ab(+)MG及DSN MG组(均P0.05),余差异均无统计学意义(均P0.05)。三组间不同临床分型(眼肌型与全身型)患者比例、球部症状和肌无力危象发生率差异均无统计学意义(均P0.05)。Titin Ab阳性组甲状腺功能异常率高于Titin Ab阴性组(100.0%vs. 33.3%,P0.05),自身抗体、胸腺病理、球部症状及肌无力危象患者比例差异两组间无统计学意义(均P0.05)。RyR Ab阳性组胸腺瘤、肌无力危象发生率高于阴性组,自身抗体异常率低于阴性组(分别为66.7%vs. 32.1%、55.3%vs. 21.4%、30.0%vs. 57.1%,均P0.05);甲状腺功能异常率和球部症状患者比例两组间差异无统计学意义(P0.05)。Titin Ab(+)MG较RyR Ab (+)MG患者易出现眼外肌在受累(100.0%vs. 60.0%,P0.05),而RyR Ab (+)MG更易出现球部肌肉(66.7%vs.25.0%,P0.05)及呼吸肌受累(53.3%vs 20.8%,P0.05)。结论 AChR Ab、MuSK Ab不同表达情况下,AChR Ab(+)MG易出现甲状腺功能异常,MuSK Ab(+)MG易合并胸腺瘤,而双阴性患者病情温和,胸腺病理多呈良性。RyR Ab(+)MG与Titin Ab(+)MG比较,RyR Ab(+)MG易有球部症状和呼吸肌受累,RyR Ab阳性MG病情更严重。     

不同重症肌无力患者人群血清抗体检测的临床意义

目的探讨不同重症肌无力患者人群中血清抗乙酰胆碱受体(AChR)抗体、抗连接素(Titin)抗体、抗Rvanodine受体(RyR)抗体和抗肌肉特异性受体酪氨酸激酶(MuSK)抗体的表达特点。方法采用酶联免疫吸附法(ELISA)检测65例重症肌无力患者血清AChR抗体、Titin抗体、RyR抗体和MuSK抗体水平,分析不同病程、胸腺情况以及美国重症肌无力协会(MGFA)临床分型MG患者各种抗体表达情况。结果患者血清AChR抗体、Titin抗体、RyR抗体和MuSK抗体阳性率分别为47.70%、43.10%、46.20%和12.30%。不同病程MG患者中AChR抗体水平差异无统计学意义(P=0.305),Titin抗体、RyR抗体和MuSK抗体水平差异有统计学意义(分别P=0.000,P=0.001,P=0.004);胸腺瘤组和胸腺正常组间4种血清抗体阳性率和抗体水平差异无统计学意义(均P0.05);MGFA临床分型各亚组中AChR抗体水平差异有统计学意义(P=0.012),重度全身型(Ⅳ型)AChR抗体水平远高于眼肌型(Ⅰ型)和轻度全身型(Ⅱ型)(均P0.05),而Titin抗体、RyR抗体和MuSK抗体水平差异无统计学意义(均P0.05)。结论 MG患者中Titin抗体、RyR抗体和MuSK抗体水平与病程有关,AChR抗体水平病情严重程度有关。     

全身型重症肌无力的临床和电生理特点分析

目的 探讨不同亚型全身型重症肌无力(Myasthenia gravis,MG)临床和电生理特点之间的区别,并分析电生理结果与疾病严重程度的相关性。方法 回顾性分析2016年7月至2020年7月在香港大学深圳医院神经内科就诊或住院的101例全身型MG患者的临床和电生理资料,按照其临床特点,分为早发乙酰胆碱受体(Acetylcholine receptor,AchR)抗体阳性全身型MG组(52例)、晚发AchR抗体阳性全身型MG组(23例)、胸腺瘤型MG组(22例)和肌肉特异性受体酪氨酸激酶(Muscle-specific receptor tyrosine kinase,MuSK)抗体阳性型MG组(4例),比较不同亚组MG患者的人口学特点、重症肌无力评分(Quantitative Myasthenia Gravis score,QMGs)、美国重症肌无力协会(MG Foundation America,MGFA)临床分级、重复神经电刺激(Repetitive nerve stimulation,RNS)结果,分析RNS结果与QMGs之间的相关性。结果 非MuSK抗体阳性全身型MG中,早发AchR抗体阳性全身型MG女性患者所占比率最高(71. 2%vs 65. 2%vs 36. 4%,P 0. 05),胸腺瘤型MG呼吸肌/球部肌肉受累为主患者所占比率最高(28. 8%vs 43. 5%vs 63. 6%,P 0. 05),早发AchR抗体阳性全身型MG被检肌肉RNS均为阳性患者所占比率最高(44. 2%vs 17. 4%vs 22. 7%,P 0. 05),QMGs与RNS最大波幅下降比率、RNS总波幅下降比率呈明显正相关关系(P 0. 01); MuSK抗体阳性全身型MG,其中重度受累患者所占比率、呼吸肌/球部肌肉受累为主患者所占比率均较高(均为75%),而被检肌肉RNS均为阳性患者所占比率较低(0%)。结论 不同亚型全身型MG的人群分布、性别占比、主要受累肌肉、受累肌肉受累程度及分布范围均具有显著的差别,这在全身型MG的分型诊断方面具有很大的参考价值;其RNS结果与疾病严重程度具有高度相关性,提示RNS在全身型MG的病情评估方面具有重要临床价值。     

Osserman V型重症肌无力5例临床分析及文献复习

目的对Osserman V型重症肌无力(MG)的临床特点及诊治体会进行回顾性总结分析,以提高对此少见疾病中少见类型的认识。方法对作者医院2004-11—2006-11期间收治的491例MG患者中所有OssermanV型患者的临床特点和血清学检测结果进行回顾性分析。结果491例MG患者中5例为Osserman V型,在病程中先后出现不同程度舌肌、颞肌或上肢远端小肌肉萎缩。其中4例肌电图检查重频电刺激低频递减,血清乙酰胆碱受体抗体(AChR-Ab)阳性,另1例抗骨骼肌特异性受体酪氨酸激酶抗体(MuSK-Ab)阳性。5例患者均对溴吡斯的明对症治疗反应差,但对糖皮质激素和静脉丙种球蛋白(IVIG)治疗反应较好。半年后随访肌肉萎缩有不同程度改善。结论Osserman V型MG很罕见,肌肉萎缩的发病机制尚不清楚,部分患者可能与MuSK-Ab有关。血清学检测有助于对Osserman V型MG确诊、避免误诊及改善预后。     

联合检测重症肌无力的三种血清抗体诊断

目的对重症肌无力(MG)患者的乙酰胆碱受体抗体(Ach R-Ab)、连接素抗体(Titin-Ab)和肌肉特异性酪氨酸激酶抗体(Mu SK-Ab)评价该3种血清抗体联合检测对MG的诊断价值。方法选择MG组患者112例、其他疾病对照组53例和健康对照组56例,采用放射免疫法测定Ach R-Ab,ELISA法分别测定Titin-Ab和Mu SK-Ab,并对结果进行分析。结果 MG患者Ach R-Ab、Titin-Ab和Mu SK-Ab的阳性率分别为68.8%、54.5%和5.4%,明显高于对照组(P<0.05)。三种抗体联合检测,其敏感性和特异性分别为91.1%和89%。其中,Mu SK-Ab仅见于Ach R-Ab阴性的MG患者,主要表现为延髓肌受累,病情较重且不伴有胸腺病变。Titin-Ab多见于MG合并胸腺瘤(MGT)患者,其MGT组的阳性率高于未合并胸腺瘤的MG(NTMG)患者(P<0.05)。将MG患者根据Osserman评分进行分型,Titin-Ab阳性率与病情严重程度有关(P<0.05)。结论 Ach R-Ab、Titin-Ab、Mu SK-Ab三种抗体是MG的诊断性标记抗体,联合检测可以避免因单项检测出现的漏诊和误诊,此外,Titin-Ab对MG合并胸腺瘤具有显著相关性,还可作为判断MG病情严重程度的指标。     

Osserman Ⅴ型重症肌无力5例临床分析及文献复习

目的 对Osserman Ⅴ型重症肌无力（MG）的临床特点及诊治体会进行回顾性总结分析，以提高对此少见疾病中少见类型的认识。方法 对作者医院2004-11-2006-11期间收治的491例MG患者中所有Osserman Ⅴ型患者的临床特点和血清学检测结果进行回顾性分析。结果 491例MG患者中5例为Osserman Ⅴ型，在病程中先后出现不同程度舌肌、颞肌或上肢远端小肌肉萎缩。其中4例肌电图检查重频电刺激低频递减，血清乙酰胆碱受体抗体（AChR-Ab）阳性，另1例抗骨骼肌特异性受体酪氨酸激酶抗体（MuSK-Ab）阳性。5例患者均对溴吡斯的明对症治疗反应差，但对糖皮质激素和静脉丙种球蛋白（IVIG）治疗反应较好。半年后随访肌肉萎缩有不同程度改善。结论 Osserman Ⅴ型MG很罕见，肌肉萎缩的发病机制尚不清楚，部分患者可能与MuSK-Ab有关。血清学检测有助于对Osserman Ⅴ型MG确诊、避免误诊及改善预后。     

Tfh细胞在重症肌无力患者胸腺中的表达及作用机制

目的探讨Tfh细胞在重症肌无力(MG)患者胸腺中乙酰胆碱受体抗体(AChR-Ab)产生中的作用及机制。方法收集28例MG患者、9例无胸腺异常的先天性心脏病患者和9例未合并MG的胸腺增生患者。采用流式细胞术检测胸腺中滤泡辅助T(Tfh)细胞和B细胞的比例,分别通过RT-PCR和Western-Blotting技术检测胸腺中Th1、Th2和Tfh细胞特异性转录因子T-bet、GATA-3、Bcl-6 mRNA水平和细胞因子干扰素γ(IFN-γ)、白细胞介素4(IL-4)、IL-21表达;采用HE和多标免疫荧光染色观察胸腺中生发中心以及Tfh细胞和B细胞之间的组织结构关系;采用放射免疫沉淀技术(RIA)检测MG患者胸腺和血清中AChR-Ab滴度;免疫磁珠提取胸腺中Tfh细胞和B细胞共培养,检测培养上清中IL-21和AChR-Ab滴度水平。结果 MG患者胸腺中Tfh细胞和B细胞比例及Tfh细胞相关转录因子Bcl-6mRNA水平和细胞因子IL-21表达均较对照组增高(P0.01);MG患者胸腺中抗AChR-Ab滴度较对照组增高(P0.01);MG患者胸腺中存在异位生发中心,且在生发中心内Tfh细胞和B细胞存在共定位;MG患者胸腺Tfh和B细胞共培养上清中IL-21和AChR-Ab水平较对照组增多(P0.01),且AChR-Ab产生可以被IL-21中和抗体阻断。结论 Tfh细胞通过在MG患者胸腺中辅助B细胞形成异位生发中心及促进抗AChR-Ab产生参与MG的发生和发展。     

Neurophysiological and mitochondrial abnormalities in MuSK antibody seropositive myasthenia gravis compared to other immunological subtypes.

OBJECTIVE: To compare the electrophysiological and histopathological features of immunological myasthenia gravis (MG) subtypes. METHODS: Fifty MG patients underwent clinical examination, MuSK-Ab and AChR-Ab analysis. The majority underwent quantitative and single-fiber electromyography (QEMG, SFEMG), repetitive nerve stimulation and deltoid muscle biopsy. From muscle specimens with histological mitochondrial dysfunction, we amplified mitochondrial DNA (mtDNA). In specimens with mtDNA deletions, the nuclear gene POLG1 was sequenced. RESULTS: Five AChR-Ab seropositive [AChR(+)] and 5 seronegative [AChR(-)] patients were MuSK-Ab seropositive [MuSK(+)]. Five of 7 neurophysiologically examined MuSK(+) patients (71%) had proximal myopathic pattern, compared to 7 of 31 MuSK(-)/AChR(+) patients (23%) (P=0.012). SFEMG was abnormal in all examined MuSK(+) patients. All 7 biopsied MuSK(+) and 32 MuSK(-) patients (89%) had cytochrome c oxidase (COX) negative fibers. Three of five MuSK(+) and 13 of 20 MuSK(-) patients analyzed had multiple mtDNA deletions but no POLG1 mutations. CONCLUSIONS: Similar degree of SFEMG abnormalities was present in proximal muscles among MuSK(+) and AChR(+) patients. Proximal myopathy was over-represented in MuSK(+) patients; however, both MuSK(+) and MuSK(-) patients had mild myopathy with frequent mitochondrial abnormalities. SIGNIFICANCE: The weakness in MuSK(+) patients is most likely due to disturbed neuromuscular transmission. The frequently encountered mitochondrial dysfunction in MG warrants further study.     

Antibodies against Wnt receptor of muscle-specific tyrosine kinase in myasthenia gravis

Muscle-specific tyrosine kinase (MuSK) antibodies are detected in a proportion of myasthenia gravis (MG) patients who are negative for acetylcholine receptor (AChR) antibodies and have prominent bulbar weakness and crises. In the MuSK ectodomains, the immunoglobulin-like 1 and 2 domains (Ig1/2) mediate the agrin–Lrp4–MuSK signaling and the cysteine-rich domain (CRD) mediates the Wnt–MuSK–Dishevelled signaling; both contribute to AChR clustering. Immunoblotting against recombinant proteins showed MuSK Ig1/2 antibodies in 33 anti-AChR-negative MG patients; 10 patients of them (30%) were additionally positive for MuSK CRD antibodies. The result suggests that MuSK antibodies have heterogeneity in their binding to functional domains of MuSK.     

Anti-MuSK antibody myasthenia gravis: clinical findings and response to treatment in two large cohorts

Introduction: Myasthenia gravis (MG) patients with autoantibodies to muscle‐specific tyrosine kinase (MuSK) represent a distinct subset of those with this disease. Treatment and outcomes data in these patients are limited and conflicting. Methods: We reviewed 110 MuSK‐MG patients from two large clinics in Italy and the USA. Results: Thirty‐nine to 49% of patients with generalized, acetylcholine receptor antibody (AChR‐Ab)–negative MG had MuSK‐MG. Eighty‐five percent were female, with disease onset typically in the fourth decade. Ocular and/or bulbar symptoms were present at onset in 79% of those studied. Eighty‐five percent were MGFA class III or greater, and crisis occurred in 28%. Plasma exchange (PLEX) produced improvement in 93%, whereas only 61% improved after intravenous immunoglobulin. Long‐term outcomes were comparable to those of patients with AChR‐Ab–positive MG. Conclusions: MuSK‐MG has a marked female predominance with frequent oculobulbar weakness and crises. Many patients deteriorate rapidly early in the disease, and PLEX is usually the preferred treatment. Long‐term outcomes are similar to those of patients with AChR‐Ab⁺ MG. Muscle Nerve 44: 36–40, 2011     

Clinical comparison of muscle-specific tyrosine kinase (MuSK) antibody-positive and -negative myasthenic patients

We assayed cryopreserved sera from 38 acetylcholine receptor (AChR) antibody-negative patients with myasthenia gravis (MG) who were followed clinically for muscle-specific tyrosine kinase (MuSK) antibodies and analyzed and compared their clinical characteristics. None of 13 sera from patients with purely ocular MG were positive. Sera from 10 of 25 patients (40%) with generalized MG were positive for MuSK antibodies. The age at onset of myasthenic symptoms was significantly earlier in MuSK antibody-positive patients (P = 0.02). MuSK antibodies were present in AChR antibody-negative patients of either gender, with virtually identical prevalence in women (41.2%) and men (37.5%). The distribution of weakness more commonly involved neck muscles in MuSK antibody-positive patients, and limb muscles in MuSK antibody-negative patients. Patients responded to immunosuppressive treatment regardless of whether MuSK antibody was present. We conclude that MuSK antibodies are present and diagnostically useful in a subset of myasthenic patients without AChR antibodies. Although the distribution of weakness differs somewhat depending on whether MuSK antibodies are present, responses to anticholinesterase and immunosuppressive treatments are similar.     

MuSK-antibody positive myasthenia gravis: clinical and electrodiagnostic patterns.

OBJECTIVE: To examine the clinical manifestations and patterns of electromyographic abnormalities in MuSK-Ab positive myasthenia gravis. METHODS: The clinical evaluations and electrodiagnostic testing results of 20 MuSK-Ab positive myasthenia gravis patients were retrospectively reviewed and compared with matched AChR-Ab positive (N = 72) and MuSK-Ab negative/AChR-Ab negative (N = 24) patients. RESULTS: MuSK-Ab positive patients were younger and more frequently female and African-American, and compared to the AChR-Ab positive patients, were more likely to present with respiratory symptoms or neck extension weakness. MuSK-Ab positive patients were less likely to have abnormal jitter in a limb muscle: only 59% had abnormal jitter in the extensor digitorum communis muscle, compared to 80% of the AChR-Ab negative/MuSK-negative patients and 91% of the AChR-Ab positive patients. CONCLUSIONS: Our MuSK-Ab positive patients not only differ demographically from our MuSK-Ab negative cohort, but they also appear to have a more limited distribution of SFEMG abnormalities. SIGNIFICANCE: The possibility that electrophysiologic abnormalities may not be widely distributed should be considered during electrodiagnostic evaluation of suspected MuSK-Ab positive MG and in selecting muscles for molecular, morphologic, or microphysiologic studies in this condition.     

Effect of sera from AChR-antibody negative myasthenia gravis patients on AChR and MuSK in cell cultures

A proportion of patients with myasthenia gravis (MG) do not have antibodies to the acetylcholine receptor (AChR). Some of these patients have antibodies to muscle specific kinase (MuSK), whereas others have neither antibody (seronegative MG, SNMG). Both MuSK antibody positive MG (MuSK-MG) and SNMG are antibody-mediated diseases but how they cause neuromuscular junction failure is not clear. One possibility is that they reduce the clustering and expression of AChRs. We looked at the effects of MuSK-MG and SNMG sera/IgG on surface AChR distribution and expression, and AChR subunit and MuSK mRNA by quantitative RT-PCR, in TE671 and C2C12 myotubes. In TE671 cells MuSK-MG sera reduced AChR expression by about 20%, but had no effect on AChR subunit or MuSK mRNA expression. In C2C12 myotubes, MuSK-MG sera caused a reduction in the number of agrin-induced clusters, but the clusters became larger and there was no significant effect on total surface AChR numbers or AChR subunit or MuSK mRNA. By contrast, SNMG sera not only reduced AChR numbers by about 20% in TE671 cells, but modestly upregulated AChR gamma subunit expression in TE671 cells and both AChR gamma subunit and MuSK expression in C2C12 myotubes. Thus, although these results have, disappointingly, demonstrated little effect of MuSK antibodies on AChR expression, they do imply that SNMG antibodies act on AChR-associated pathways.     

Seronegative generalised myasthenia gravis: clinical features,antibodies, and their targets

Myasthenia gravis (MG) is a well-recognised disorder of neuromuscular transmission that can be diagnosed by the presence of antibodies to the acetylcholine receptor (AChR). However, some patients (about 15%) with generalised MG do not have detectable AChR antibodies. There is some evidence, however, that this "seronegative" MG is an antibody-mediated disorder. Plasma from patients with the disorder seems to contain various distinct humoral factors: IgG antibodies that reversibly inhibit AChR function; a non-IgG (possibly IgM) factor that indirectly inhibits AChR function; and an IgG antibody against the muscle-specific kinase (MuSK). The presence of antibodies against MuSK appears to define a subgroup of patients with seronegative MG who have predominantly localised, in many cases bulbar, muscle weaknesses (face, tongue, pharynx, etc) and reduced response to conventional immunosuppressive treatments. Moreover, muscle wasting may be present, which prevents complete response to these therapies.     

Seronegative myasthenia gravis

Some myasthenia gravis (MG) patients do not have detectable acetylcholine receptor (AChR) antibodies and have been termed "seronegative" (SNMG) in many previous studies. A high proportion of patients with purely ocular symptoms, ocular MG, are seronegative; this may be because the sensitivity of the assay is insufficient to detect low levels of circulating AChR antibodies and because of intrinsic differences in the ocular muscles that make them more susceptible to circulating factors. Seronegative generalized myasthenia is proving to be heterogeneous both clinically and immunologically. Plasma from SNMG patients often contains a factor, probably an immunoglobulin M antibody, that alters AChR function in in vitro assays, but its target is not yet clear. A variable proportion of SNMG patients have antibodies to the muscle-specific tyrosine kinase (MuSK). These antibodies are directed against the extracellular domain of MuSK and inhibit agrin-induced AChR clustering in muscle myotubes. Although the role of these antibodies in causing myasthenic symptoms in vivo has not been elucidated, MuSK antibodies appear to define a group of patients who are often female with bulbar weakness, contrasting with MuSK antibody-negative SNMG patients who are more likely to have generalized weakness. MuSK antibody-positive patients may also differ in the distribution of their electrophysiological abnormalities and their responses to treatments.     

Detection and characterization of MuSK antibodies in seronegative myasthenia gravis

Antibodies to rat muscle specific kinase, MuSK, have recently been identified in some generalized "seronegative" myasthenia gravis (SNMG) patients, who are often females with marked bulbar symptoms. Using immunoprecipitation of (125)I-labelled-human MuSK, 27 of 66 (41%) seronegative patients were positive, but 18 ocular SNMG patients, 105 AChR antibody positive MG patients, and 108 controls were negative. The antibodies are of high affinity (Kds around 100 pM) with titers between 1 and 200 nM. They bind to the extracellular Ig-like domains of soluble or native MuSK. Surprisingly they are predominantly in the IgG4 subclass. MuSK-antibody associated MG may be different in etiological and pathological mechanisms.     

AChR phosphorylation and indirect inhibition of AChR function in seronegative MG

BACKGROUND: Approximately 10% to 20% of patients with autoimmune MG do not have antibodies to the acetylcholine receptor (AChR), so-called seronegative MG (SNMG). IgG antibodies from up to 70% of SNMG patients bind to the muscle-specific receptor tyrosine kinase, MuSK. The plasmas and non-IgG fractions from SNMG patients (and some with AChR antibodies) also contain a factor, perhaps an IgM antibody, that inhibits AChR function, but it is not clear how this factor acts and whether it is related to the MuSK IgG antibodies. METHODS: The authors studied 12 unselected SNMG plasmas and their non-IgG fractions; seven were positive for MuSK IgG antibodies. Ion flux assays, electrophysiology, phosphorylation, and kinase assays were used to look at mechanisms of action. RESULTS: Eight of the 12 plasmas and their non-IgG fractions inhibited AChR function, but the inhibitory activity was transient and did not correlate with the presence of MuSK IgG antibodies. Two of three plasmas added outside of a cell-attached patch pipette inhibited AChR function within the patch, and these two plasmas also increased AChR phosphorylation. CONCLUSIONS: The authors propose that a plasma factor(s) in SNMG patients, distinct from MuSK IgG antibodies, binds to a muscle membrane receptor and activates a second messenger pathway leading to AChR phosphorylation and reduced AChR function. Identifying the target for this factor should lead to improved diagnosis of MG in MuSK antibody-negative patients and may provide new insights into the function of the neuromuscular junction and pathophysiological mechanisms in MG.