Muscle autoantibodies in subgroups of myasthenia gravis patients

Myasthenia gravis (MG) is caused by autoantibodies to the acetylcholine receptor (AChR), but several other muscle autoantibodies have also been identified in patient sera. We studied muscle autoantibodies against AChR, striated muscle tissue sections (SH), titin, citric acid antigen (CA), and ryanodine receptor (RyR) in sera from 146 consecutive MG patients to evaluate whether a single test or several tests together can predict a thymoma. The MG patients were divided into five subgroups; ocular MG, early-onset MG (< 50 years), late-onset MG (≥ 50 years), MG with thymoma, and AChR antibody negative MG. AChR, SH, titin, CA, and RyR antibodies were detected in 85%, 34%, 34%, 25%, and 14% of the MG patients, respectively. For thymoma MG, AChR, SH, titin, CA, and RyR antibodies were detected in 100%, 75%, 95%, 70%, and 70% respectively. SH, titin, CA, RyR antibodies, and computed tomography of the anterior mediastinum have similar sensitivity for thymoma MG. The specificity of RyR, titin, CA, and SH antibodies for thymoma was 70%, 39%, 38%, and 31%, respectively, which is significantly higher for RyR antibodies than for the others. No single muscle antibody assay can predict a thymoma, and a combination of several antibody assays is preferred, although RyR antibody testing alone showed 70% sensitivity and specificity for thymoma MG. SH and CA antibodies provided only little additional information. Received: 23 September 1999, Received in revised form: 6 December 1999, Accepted: 19 January 2000     

Myasthenia gravis patients with ryanodine receptor antibodies have distinctive clinical features

Myasthenia gravis (MG) is an autoimmune disease caused in 85% of the patients by acetylcholine receptor (AChR) antibodies. Non-AChR muscle antibodies, against titin and ryanodine receptor (RyR) are mainly found in sera of patients with thymoma or late-onset MG. The occurrence of RyR antibodies increases the risk for severe MG and should lead to active immunomodulating treatment already at MG onset. The aim in this study was to describe the association between symptoms at MG onset and antibody profile in 152 patients. Patients with RyR antibodies had the highest rate of bulbar, respiratory and neck involvement at MG onset. They also had the highest frequency of non-limb MG symptoms. Neck weakness occurred in 40%. Respiratory difficulties at MG onset occurred in patients with titin antibodies, with and without RyR antibodies. Patients with RyR antibodies have a distinctive non-limb MG symptom profile, with bulbar, ocular, neck, and respiratory symptoms. These features, identified as early as at the first examination by a neurologist, characterize the RyR antibody positive subgroup at MG onset.     

Myasthenia gravis: disease severity and prognosis

Objectives –  To examine myesthenia gravis (MG) severity and long-term prognosis in seronegative, seropositive, and thymoma MG.
Materials and Methods –  Four series of patients were studied retrospectively. Severity and treatment were assessed each year, and muscle antibodies were assayed.
Results –  Seropositive MG patients had a more severe course than seronegative MG patients. MG severity was higher in non-thymectomized compared to thymectomized early-onset MG patients. MG severity did not differ between thymectomized and non-thymectomized late-onset patients. There was no significant difference in MG severity between thymoma and non-thymoma MG patients.
Conclusions –  MG is more severe in seropositive MG patients. With proper treatment, especially early thymectomy, the long-term prognosis is good in seropositive MG patients. The present studies indicate a benefit of thymectomy in early-onset MG, but no dramatic benefit in late-onset MG. Similar MG severity and outcome was seen in thymoma and non-thymoma MG.     

Thymectomy and anti-muscle autoantibodies in late-onset myasthenia gravis

Thymectomy is still widely carried out in myasthenia gravis (MG) patients, but its role, especially in late-onset MG patients, is not established. These patients are immunologically heterogeneous, some with thymoma-like and others with early onset-like features. We evaluated whether any therapeutic effects of thymectomy correlate with the presence of non-acetylcholine receptor (AChR) muscle antibodies. The severity of MG, and titin and ryanodine receptor (RyR) antibodies, were assessed yearly starting from MG onset in 21 thymectomized and 22 non-thymectomized AChR antibody positive late-onset MG patients, who were followed for 2, 3 and 5 years. Clinical or pharmacological remission were seen in six of 11 titin antibody negative but none of the 10 titin antibody positive thymectomized patients, however, the non-thymectomized cases showed an opposite trend. The three MG-related deaths were all in patients with titin antibodies. There was no significant difference in MG severity between thymectomized and non-thymectomized patients; 2 years after MG onset, both groups were significantly improved. This study showed no dramatic benefit from thymectomy in late-onset MG in general. Any limited improvement appeared less likely in cases with titin and/or RyR antibodies.     

Paraneoplastic myasthenia gravis: immunological and clinical aspects

Paraneoplastic myasthenia gravis (MG) is accompanied by a neoplasm, usually thymoma. In patients with thymoma and a specific genetic make‐up, the paraneoplastic immune response develops further in thymic remnant or peripheral lymphatic tissue. Paraneoplastic MG and late‐onset MG (age ≥ 50 years) share a similar immunological profile with high titin and ryanodine receptor (RyR) antibody prevalence. This profile is the most important predictor of clinical outcome in paraneoplastic MG. The presence of a thymoma per se does not cause more severe MG. MG severity is linked to the patient’s immunological profile. Paraneoplastic MG causes a distinctive non‐limb symptom profile at MG onset, characterized by bulbar, ocular, neck, and respiratory symptoms. When the diagnosis of paraneoplastic MG is established, the neoplasm should be removed surgically. Pre‐thymectomy plasmapheresis or iv‐IgG should be considered in these patients to minimize post‐thymectomy MG exacerbation risk. Paraneoplastic MG usually continues after thymectomy. The pharmacological treatment of paraneoplastic MG does not differ from non‐paraneoplastic MG, except for tacrolimus that should be considered in difficult cases. Tacrolimus is an immunosuppressant acting specifically in RyR antibody positive patients through enhancing RyR‐related sarcoplasmic calcium release that in theory might be blocked by RyR antibodies, causing symptomatic relief in paraneoplastic MG.     

重症肌无力患者血清中Ryanodine受体抗体检测及其临床意义

目的　探讨Ryanodine受体 (RyR)抗体在重症肌无力 (MG)诊断中的临床意义。方法　以差速离心法提取富含RyR的肌质网 (SR)提取物 ,建立ELISA RyR抗体检测系统 ,分析 6 6例伴胸腺瘤的MG(MGT)、98例非胸腺瘤MG(NTMG)和 5 0例非重症肌无力 (NMG)及 12 3名健康人 (NC)血清中RyR抗体水平。 结果　MGT组RyR抗体阳性检出率显著性高于NTMG组和NMG组 (P <0 0 1) ,敏感性和特异性分别达 81 8%和 94 5 %。不同胸腺组织学类型MGT之间RyR抗体阳性检出率差异无显著意义 (P >0 0 5 )。RyR抗体呈阳性MG组的年龄、临床评分和乙酰胆碱受体抗体水平均显著高于RyR抗体阴性MG组 (P <0 0 1)。RyR抗体水平同MG患者临床症状的严重程度呈正相关 ,尤其是MGT组 (r =0 6 2 6 ,P <0 0 1)。不同胸腺组织学类型MGT中以上皮细胞型相关性最高 (r =0 5 92 ,P <0 0 1)。结论　RyR抗体检测对诊断MGT具有较高的敏感性和特异性 ,并且与MG患者临床症状的严重程度呈正相关。     

重症肌无力患者血清Ryanodine受体抗体检测及意义

目的探讨Ryanodine受体（RyR）抗体在重症肌无力（myasthenia gravis，MG）诊断中的临床意义。方法采用ELISA法检测89例MG患者、66例其他神经系统疾病患者和66名正常对照者血清RyR抗体水平。结果MG组血清RyR抗体阳性率显著高于其他神经系统疾病组和正常对照组（P〈0．05），其敏感性和特异性分别为55．0％和91．7％。晚发型MG患者血清RyR抗体阳性率（74．4％）明显高于早发型MG（37．0％，P〈0．05）。MG合并胸腺瘤（MGT）和未合并胸腺瘤（nMGT）患者血清RyR抗体阳性率差异无统计学意义（P〉0．05）。将MG患者根据Osserman评分进行分型，各型血清RyR抗体阳性率及其吸光度值大小与病情严重程度无相关性（P〉0．05）。结论RyR抗体多见于晚发型MG，对诊断MG具有较高的敏感性和特异性。     

Complement activation by titin and ryanodine receptor autoantibodies in myasthenia gravis. A study of IgG subclasses and clinical correlations

To elucidate the mechanism of immune damage caused by titin and ryanodine receptor (RyR) autoantibodies in myasthenia gravis (MG), we studied the complement-activating capacity and the IgG subclass distribution of these autoantibodies in sera from 49 MG patients. Complement activation occurred in 38 out of 49 titin antibody positive sera, and in 14 out of 21 RyR antibody positive sera. The titin antibodies occurred only in the IgG 1 and IgG 4 subclasses, whereas the RyR antibodies occurred in all four IgG subclasses but with IgG 1 predominance. Complement-activating RyR antibodies occurred with higher frequency in sera of thymoma MG than of late-onset MG. RyR IgG 1 antibodies occurred more often in severe MG than in mild and moderate disease groups. Mean total IgG and IgG 1 titin and RyR antibody titers fell during long-time patient observation together with an improvement of the MG symptoms.     

Titin and ryanodine receptor epitopes are expressed in cortical thymoma along with costimulatory molecules

Cortical-type thymomas are associated with myasthenia gravis (MG) in 50% of the cases. MG is caused by antibodies against the acetylcholine receptors (AChR), but additional non-AChR muscle autoantibodies such as those against titin and ryanodine receptor (RyR) are found in up to 95% of MG patients with thymoma. To elucidate the induction of non-AChR autoantibodies in thymoma-associated MG, we studied cortical-type thymomas from seven thymoma MG patients, and sera from six of them. All six had titin antibodies, and four had RyR antibodies. Titin and RyR epitopes were co-expressed along with LFA3 and B7 (BB1) costimulatory molecules on thymoma antigen-presenting cells (APC) in all thymomas. In normal thymus, the staining by anti-titin, anti-RyR, anti-LFA3, and anti-BB1 antibodies was weak and occurred exclusively in the medulla and perivascularly. Our results indicate a primary autosensitization against titin and RyR antigens inside the thymoma. In MG-associated thymoma, the mechanisms involved in the initial autosensitization against titin and RyR are probably similar to those implicated in the autosensitization against AChR. In all cases, there is an overexpression of muscle-like epitopes and costimulatory molecules indicating that the T-cell autoimmunization is actively promoted by the pathogenic microenvironment inside the thymoma.     

Thymectomy in nonthymoma early-onset myasthenia gravis in correlation with disease severity and muscle autoantibodies

OBJECTIVE: To study the clinical effect of thymectomy in a well-defined early-onset MG subgroup and to correlate it to MG severity, the presence of circulating muscle autoantibodies, and the need for pharmacological treatment in a long-term setting. METHODS: Fifty-two consecutive AChR antibody-positive early-onset MG patients (34 thymectomized and 18 nonthymectomized) were included. Severity was assessed and the pharmacological treatment monitored on a yearly basis, starting from the year of MG onset, for 5, 10, 15, and 20 consecutive years; AChR, titin, and RyR antibodies were assayed. RESULTS: In the four follow-up groups, MG severity was significantly higher in nonthymectomized compared to thymectomized MG patients. The postthymectomy MG improvement was significant and persistent. There were 21/34 remissions in thymectomized patients and only 4/18 in the nonthymectomized group. Patients with initially high or low AChR antibody concentration had a similar thymectomy outcome. Only 6 patients had titin antibodies, and none had RyR antibodies. CONCLUSION: The present study indicates a benefit of thymectomy in early-onset MG. The muscle autoantibody concentration does not influence the outcome of thymectomy in early-onset MG.     

The role of complement in myasthenia gravis: serological evidence of complement consumption in vivo

BACKGROUND: Antibodies to the acetylcholine receptor (AChR) titin and the ryanodine receptor (RyR) occur in myasthenia gravis (MG). These antibodies are capable of complement activation in vitro. The involvement of the complement system should cause consumption of complement components such as C3 and C4 in vivo. MATERIALS AND METHODS: Complement components C3 and C4 were assayed in sera from 78 AChR antibody-positive MG patients and 52 healthy controls. Forty-eight of the patient sera contained titin antibodies as well, and 20 were also RyR antibody-positive. RESULTS: MG patients with AChR antibody concentrations above the median (11.2 nmol/l) had significantly lower mean C3 and C4 concentrations in serum compared to those with AChR antibody concentrations below the median. Titin antibody-positive MG patients, titin antibody-negative early-onset MG patients, titin antibody-negative late-onset MG patients, and controls had similar C3 and C4 concentrations. Nor did mean C3 and C4 concentrations differ in MG patients with RyR antibodies. Patients with severe MG (grades 4 and 5) had similar C3 and similar C4 levels compared to those with mild MG (grades 1 and 2). CONCLUSION: An increased in vivo complement consumption was detected in MG patients with high AChR antibody concentrations, unrelated to MG severity and non-AChR muscle antibodies.     

The severity of myasthenia gravis correlates with the serum concentration of titin and ryanodine receptor antibodies

BACKGROUND: Myasthenia gravis (MG) is caused by autoantibodies to the acetylcholine receptor (AChR). Non-AChR muscle autoantibodies are present in many MG serum samples, mainly from patients with thymoma or late-onset MG. The exact relationship between MG severity and several non-AChR muscle antibodies is unknown. OBJECTIVE: To study the correlation between the severity of MG and the concentration of antibodies against striated muscle tissue sections, titin, citric acid antigen, ryanodine receptor, and AChR. SETTING: The severity of MG was graded in 146 consecutive patients with MG, and their serum samples were tested for the presence of autoantibodies. Ten patients who were titin antibody positive were observed in longitudinal follow-up. RESULTS: No significant difference was found in MG severity between late-onset and thymoma MG. Titin, citric acid antigen, and ryanodine receptor antibodies occurred significantly more often among patients with severe MG than among patients with less severe disease. Changes in MG severity correlated with changes in titin antibody titer in the individual patient. Titin antibodies showed a better longitudinal correlation with disease severity than the AChR antibodies. CONCLUSIONS: Non-AChR muscle autoantibodies occurred more frequently in severe MG regardless of MG subgroup. Thymoma per se does not generate a more severe MG. It may well be the presence of a humoral immune response to non-AChR muscle antigens such as titin, citric acid antigen, and ryanodine receptor that leads to a severe disease, not the presence of thymoma or a late age of onset. These antibodies can serve as important prognostic markers in MG regardless of the presence of thymoma.     

Myasthenia gravis sera containing antiryanodine receptor antibodies inhibit binding of [3H]-ryanodine to sacroplasmic reticulum

Myasthenia gravis (MG) patients with thymoma often have antibodies against the calcium-release channel of the sarcoplasmic reticulum (SR) in striated muscle, the ryanodine receptor (RyR). RyR function can be tested in vitro by measuring the degree of [³H]-ryanodine binding to SR. In this study, sera from 9 out of 14 MG patients containing RyR antibodies inhibited [³H]-ryanodine binding to SR membranes from rat skeletal muscle. The 9 patients with antibodies inhibiting ryanodine binding had more severe MG than those with noninhibiting antibodies (P = 0.006). Sera from MG patients with acetylcholine receptor and titin muscle antibodies but no antibodies against RyR and blood-donor sera did not have an inhibiting effect in the [³H]-ryanodine binding assay. The results show that RyR antibodies in MG patients have high affinity for the RyR, and that the binding of antibodies probably affects calcium release from SR by locking the RyR ion channel in a closed position. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:329–335.     

Ryanodine receptor antibodies are associated with severe myasthenia gravis

Sera from 32 thymoma patients, 29 of them with myasthenia gravis (MG), were tested for the presence of circulating antibodies to the ryanodine receptor (RyR) in Western blot RyR is a channel protein essential for the excitation-contraction coupling in skeletal muscle. MG severity was scored according to the Osserman classification during 1–17 years of follow-up (mean 7 years). Fifteen patients (14 MG and 1 non-MG) were RyR-antibody positive. RyR-positive patients had a significantly higher frequency of invasive thymomas (p = 0.01), and also a more severe MG than RyR-antibody negative patients (p = 0.04). The use of immunosuppressive drugs at latest follow-up was more frequent in RyR-antibody positive than in RyR-antibody negative patients (p = 0.02). Thus the presence of RyR-antibodies in thymoma patients is associated with a more severe disease and can be used as a prognostic marker.     

Autoantibodies in thymoma-associated myasthenia gravis with myositis or neuromyotonia

BACKGROUND: About 50% of patients with thymoma have paraneoplastic myasthenia gravis (MG). Myositis and myocarditis or neuromyotonia (NMT) will also develop in some. Patients with thymoma-associated MG produce autoantibodies to a variety of neuromuscular antigens, particularly acetylcholine receptor (AChR), titin, skeletal muscle calcium release channel (ryanodine receptor [RyR]), and voltage-gated potassium channels (VGKC). OBJECTIVE: To examine whether neuromuscular autoantibodies in patients with thymoma correlate with specific clinical syndromes. METHODS: Serum and plasma samples from 19 patients with thymoma-associated MG, of whom 5 had myositis and 6 had NMT, underwent testing for antibodies to AChR, titin, RyR, and VGKC. RESULTS: Antibodies to AChR and titin were found in 19 and 17 patients, respectively. Antibodies to RyR correlated with the presence of myositis (P = .03); they were found in all 5 patients with myositis and in only 1 patient with NMT, but also in 4 of 8 patients with neither disease. Antibodies to VGKC were found in 4 patients with NMT, 1 of 3 patients undergoing testing for myositis, and 2 of 7 patients undergoing testing with neither comorbidity. Presence of RyR antibodies correlated with high levels of titin antibodies. CONCLUSIONS: The results appear to distinguish partially between 3 groups of patients with thymoma-associated MG: the first with RyR antibodies and myositis or myocarditis, the second with NMT without RyR antibodies, and the third without RyR antibodies, myositis, or NMT. Differences in the thymoma may underlie these pathologic associations.     

TNFA and TNFB polymorphisms in myasthenia gravis

BACKGROUND: Tumor necrosis factor (TNF) alpha and TNF-beta are proinflammatory cytokines thought to be involved in the pathogenesis of myasthenia gravis (MG). OBJECTIVE: To examine whether TNF polymorphisms are associated with MG, MG subgroups, and the presence of titin and ryanodine-receptor antibodies. PATIENTS AND METHODS: We did genotyping on 30 patients with MG and 92 healthy blood donors for 2 biallelic TNFA polymorphisms (G to A at positions -238 and -308) and 1 TNFB polymorphism (NcoI digestive site) using methods based on the polymerase chain reaction. RESULTS: Patients with thymoma were typically homozygous for both the TNFA*T1 and the TNFB*2 alleles, but patients having an early onset of MG without thymoma were carriers of the TNFA*T2 and TNFB*1 alleles. Patients without thymoma who had the titin antibody had the same high frequency of TNFA*T1 and TNFB*2 as patients with thymoma, whereas patients without the titin antibody carried the same allele, TNFA*T2 and TNFB*1, regardless of age and thymic disease. No association was found with acetylcholine-receptor levels or disease severity for any of the TNFA or TNFB polymorphisms. CONCLUSION: Patients having MG, including those with thymoma, who have the titin antibody are most often homozygous for the TNFA*T1 and TNFB*2 alleles, whereas the presence of the TNFA*T2 and TNFB*1 alleles correlates with early-onset MG and the absence of titin antibodies.     

The significance of titin antibodies in myasthenia gravis

Myasthenia gravis (MG) is caused by autoantibodies to acetylcholine receptors (AChR). Non-AChR muscle autoantibodies, such as titin antibodies, are present in sera of many MG patients. To study the correlation between titin antibodies and the features of MG, the cDNA segment encoding MGT-30 was amplified and sequenced. The cloned MGT-30 cDNA was expressed in vector pET-30a, and then transfected into E.coli. BL21. We examined titin antibodies in sera of 265 normal subjects, 154 MG patients with different thymic pathology and 48 patients with other neurological diseases. Titin antibodies occurred more frequently in MGT, especially in MG with epithelial predominant-thymoma, and were correlated with the severity of disease. The levels of titin antibodies were reduced 6 months after thymectomy. The specificity of titin antibodies for the detection of thymoma was higher than that of CT examination in MG with thymoma. These results suggest that titin antibodies could be useful in both diagnosis and follow-up of MG patients with thymoma.     

The histomorphology of the thymus in late onset, non-thymoma myasthenia gravis

Twenty-two late-onset, non-thymoma myasthenia gravis (MG) patients were selected for study based on the combined presence of MG and involution of the thymus. Thymectomy specimens from all the patients were examined histologically and immunohistologically and compared with an age-matched control group in which tissue was removed from the thymic gland during operations for various cardiac conditions. We were not able to find any morphological differences between thymi from MG patients and patients in the control group. Germinal centres were present in 7/22 in the patient group and 6/20 in the control group. Seven of our 22 MG patients had auto-antibodies to titin. Germinal centres in the thymus were found in only one. In contrast germinal centres were present in the thymus of 6/15 MG patients with no detectable antibody response to titin ( p = 0.01). Our study suggests that late-onset MG is pathogenetically heterogeneic with different modes of auto-sensitisation.     

Autoantibodies in thymoma-associated myasthenia gravis and their clinical significance

Myasthenia gravis (MG) is characterized by the development of antibodies that act against the acetylcholine receptor (AChR) present at the postsynaptic site of neuromuscular junctions. Some MG patients have antibodies that bind in a cross-striational pattern to skeletal and heart muscle tissue sections (striational antibodies). These antibodies react with the epitopes on the muscle protein titin, ryanodine receptor (RyR), and voltage-gated K channel α subunit, Kv1.4. Since these 3 molecules are expressed in the thymoma tissue of MG patients, striational antibodies are frequently detected in thymoma-associated MG. More severe MG symptoms in thymoma-associated MG may be due to the presence of striational autoantibodies. The anti-titin antibody, usually detected by enzyme-linked immunosorbent assay (ELISA), is found in 20-40% of all MG patients, and is more common in late-onset MG patients. The anti-RyR antibody, detected by Western blotting or ELISA, is found in 13-38% of all MG patients, and is known to inhibit Ca2+ release from sarcoplasmic reticulum and excitation-contraction coupling of the muscle. The anti-Kv1.4 antibody, detected by the immunoprecipitation assay with 35S-labeled extract from rhabdomyosarcoma cells, is found in 12-15% of all MG patients. Autoimmune myocarditis may develop in MG patients who have the anti-Kv1.4 antibody. In addition, the anti-Kv1.4 antibody is a useful marker to check the response to calcineurin inhibitors. In summary, the detection of striational antibodies provides more specific clinical information in MG patients.     

Do titin and cytokine antibodies in MG patients predict thymoma or thymoma recurrence?

BACKGROUND: Patients with MG often have other autoantibodies in addition to those against the acetylcholine receptor (AChR). It has been suggested that antibodies to the muscle protein titin may be diagnostic of a thymoma, but they have also been found in patients with late-onset MG. Antibodies to certain cytokines have also been detected in patients with MG and thymoma, and it is not clear whether these antibodies could be more useful clinically. The authors measured antibodies against titin and the cytokines interferon alpha (IFNalpha) and interleukin 12 (IL12) in patients with MG and thymoma or thymoma recurrence, and in patients with MG but without thymoma presenting before (early-onset MG) or after (late-onset MG) 40 years of age. METHOD: Levels of titin, IFNalpha, and IL12 antibodies were determined by radioimmunoassay in 191 patients with MG and 82 controls. RESULTS: As previously reported, titin antibodies were uncommon in patients with early-onset MG. However, in patients with late-onset MG, titin antibodies had similar prevalence and levels to those in patients with MG and thymoma, although the antibodies were uncommon in patients between 40 and 60 years of age presenting without a tumor. By contrast, cytokine antibodies were more common in patients with thymoma than in patients without thymoma, and cytokine antibodies typically increased substantially if the thymoma recurred. CONCLUSIONS: Measurement of titin antibodies has limited use in predicting the presence of a tumor, unless the patient is less than 60 years of age, but measurement of IFNalpha and IL12 antibodies may be helpful in identifying patients with a thymoma recurrence, particularly when mediastinal imaging is equivocal.