Muscle striation antibodies in myasthenia gravis. Diagnostic and functional significance

Improving laboratory diagnostics in myasthenia gravis

Introduction: Myasthenia gravis (MG) is a prototypical autoimmune disease, characterized by pathogenic autoantibodies targeting structures of the neuromuscular junction. Radioimmunoprecipitation assays (RIPAs) represent the gold standard for their detection. However, new methods are emerging to complement, or overcome RIPAs, also with the perspective of eliminating the use of radioactive reagents.Areas covered: We discuss advances in laboratory methods, prompted especially by cell-based assays (CBAs), for the detection of the autoantibodies of MG diagnostics, above all those to the nicotinic acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low molecular-weight receptor-related low-density lipoprotein-4 (LRP4).Expert opinion: CBA technology makes AChRs aggregate on cell membranes, thus allowing to detect autoantibodies to clustered AChRs, with reduction of seronegative MG cases. The diagnostic relevance of RIPA/CBA-measurable LRP4 antibodies is still unclear, in Caucasian patients at least. Live CBAs for the detection of AChR, MuSK, and LRP4 antibodies might represent an alternative to RIPAs, but first require full validation. CBAs could be used as screening tests, limiting RIPAs for antibody quantification. To this end, ELISAs might be an alternative.Fixation procedures preserving enough degree of antigen conformationality could yield AChR and MuSK CBAs suitable for a wide use in clinical-chemistry laboratories.

Myasthenia Gravis: Pathogenic Effects of Autoantibodies on Neuromuscular Architecture

Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction (NMJ). Autoantibodies target key molecules at the NMJ, such as the nicotinic acetylcholine receptor (AChR), muscle-specific kinase (MuSK), and low-density lipoprotein receptor-related protein 4 (Lrp4), that lead by a range of different pathogenic mechanisms to altered tissue architecture and reduced densities or functionality of AChRs, reduced neuromuscular transmission, and therefore a severe fatigable skeletal muscle weakness. In this review, we give an overview of the history and clinical aspects of MG, with a focus on the structure and function of myasthenic autoantigens at the NMJ and how they are affected by the autoantibodies' pathogenic mechanisms. Furthermore, we give a short overview of the cells that are implicated in the production of the autoantibodies and briefly discuss diagnostic challenges and treatment strategies.

Is excitation–contraction coupling impaired in myasthenia gravis?

OBJECTIVE

To investigate whether excitation-contraction (E-C) coupling of muscle is impaired in patients with myasthenia gravis (MG).

METHODS

In 51 patients with generalized MG and 35 normal subjects, compound muscle action potentials (CMAPs) of the abductor pollicis brevis, and movement-related potentials using an accelerometer placed at the thumb tip were simultaneously recorded after median nerve stimulation at the wrist. The E-C coupling time (ECCT) was estimated by a latency difference between CMAP and movement-related potential. Antibodies against acetylcholine receptor (AChR), ryanodine receptor (RyR), and muscle specific receptor tyrosine kinase (MuSK) were measured by immunoassays.

RESULTS

The mean ECCT was significantly longer in patients with MG (mean+/-SEM; 2.79+/-0.1 ms; p=0.002) than in normal controls (2.52+/-0.1 ms). Among MG patients, the mean ECCT was longer for patients with thymoma than for those without it (P=0.04), and was shorter for patients treated with FK506 (an immunosuppressant and also an enhancer of RyR related Ca(2+) release) than for those not receiving this treatment (p=0.04). ECCT had no significant correlation with anti-AChR, anti-RyR, or anti-MuSK antibodies.

CONCLUSIONS

In MG, E-C coupling appears to be impaired, particularly in patients with thymoma, and FK506 possibly facilitates E-C coupling.

SIGNIFICANCE

The functional implication of impaired E-C coupling is not established, but it may contribute to muscle weakness in patients with MG.

The significance of titin antibodies in myasthenia gravis--correlation with thymoma and severity of 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 Role of Thymomas in the Development of Myasthenia Gravis

Thymic pathology occurs in 80-90% of myasthenia gravis patients. Significant associations between different thymic alterations and clinical findings are discussed. To highlight peculiarities in thymoma-associated myasthenia gravis, we briefly review myasthenia gravis associated with thymic lymphofollicular hyperplasia (TFH) and thymic atrophy.

Disorders of the respiratory muscles

The act of breathing depends on coordinated activity of the respiratory muscles to generate subatmospheric pressure. This action is compromised by disease states affecting anatomical sites ranging from the cerebral cortex to the alveolar sac. Weakness of the respiratory muscles can dominate the clinical manifestations in the later stages of several primary neurologic and neuromuscular disorders in a manner unique to each disease state. Structural abnormalities of the thoracic cage, such as scoliosis or flail chest, interfere with the action of the respiratory muscles-again in a manner unique to each disease state. The hyperinflation that accompanies diseases of the airways interferes with the ability of the respiratory muscles to generate subatmospheric pressure and it increases the load on the respiratory muscles. Impaired respiratory muscle function is the most severe consequence of several newly described syndromes affecting critically ill patients. Research on the respiratory muscles embraces techniques of molecular biology, integrative physiology, and controlled clinical trials. A detailed understanding of disease states affecting the respiratory muscles is necessary for every physician who practices pulmonary medicine or critical care medicine.

Cardiac disease in myasthenia gravis: a literature review

Patients with myasthenia gravis might develop various cardiac disorders, yet a causal relationship remains unestablished. Because causes of sudden death in this population have not been ascertained, further attention to possible cardiac disease in this population is warranted. We summarize the current literature and describe the possible etiologies and implications of cardiac disease in myasthenics.

Measurement of antiacetylcholine receptor antibody in patients with thymoma without myasthenia gravis complications

OBJECTIVE

Some patients with thymoma reported to show higher antiacetylcholine receptor antibody titers without the preoperative occurrence of myasthenia gravis and some have suffered postoperative complications of myasthenia gravis despite being negative for antiacetylcholine receptor antibody preoperatively. We evaluated changes in antiacetylcholine receptor antibody titers and the occurrence of myasthenia gravis in thymoma patients.

METHODS

Subjects were 31 of 44 patients with thymoma undergoing thymothymectomy at Tokyo Women's Medical University Hospital between 1987 to 1999 in whom antiacetylcholine receptor antibody titers were measured preoperatively. We studied postoperative changes in antiacetylcholine receptor antibody titers and the presence or absence of myasthenia gravis.

RESULTS

Eight patients were positive for antiacetylcholine receptor antibody preoperatively, suggesting the presence of subclinical myasthenia gravis. Neither postoperative changes in antiacetylcholine receptor antibody titers nor the occurrence of myasthenia gravis was observed in these 8 patients. Recurrent thymoma and rapid elevation of antiacetylcholine receptor antibody titers were observed postoperatively in 1 patient negative for antiacetylcholine receptor antibody preoperatively, resulting in manifestation of myasthenia gravis symptoms.

CONCLUSION

We found no correlation between preoperative titers and myasthenia gravis symptoms. Rapid titer elevation indicates the occurrence of myasthenia gravis symptoms or the recurrence of thymoma.

Auto-antibodies to the receptor tyrosine kinase MuSK in patients with myasthenia gravis without acetylcholine receptor antibodies

Myasthenia gravis (MG) is an antibody-mediated autoimmune disease of the neuromuscular junction. In approximately 80% of patients, auto-antibodies to the muscle nicotinic acetylcholine receptor (AChR) are present. These antibodies cause loss of AChR numbers and function, and lead to failure of neuromuscular transmission with muscle weakness. The pathogenic mechanisms acting in the 20% of patients with generalized MG who are seronegative for AChR-antibodies (AChR-Ab) have not been elucidated, but there is evidence that they also have an antibody-mediated disorder, with the antibodies directed towards another, previously unidentified muscle-surface-membrane target. Here we show that 70% of AChR-Ab-seronegative MG patients, but not AChR-Ab-seropositive MG patients, have serum auto-antibodies against the muscle-specific receptor tyrosine kinase, MuSK. MuSK mediates the agrin-induced clustering of AChRs during synapse formation, and is also expressed at the mature neuromuscular junction. The MuSK antibodies were specific for the extracellular domains of MuSK expressed in transfected COS7 cells and strongly inhibited MuSK function in cultured myotubes. Our results indicate the involvement of MuSK antibodies in the pathogenesis of AChR-Ab-seronegative MG, thus defining two immunologically distinct forms of the disease. Measurement of MuSK antibodies will substantially aid diagnosis and clinical management.

Autoimmunity against the ryanodine receptor in myasthenia gravis

Some myasthenia gravis (MG) patients have antibodies against skeletal muscle antigens in addition to the acetylcholine receptor (AChR). A major antigen for these antibodies is the Ca2+ release channel of the sarcoplasmic reticulum the ryanodine receptor (RyR). These antibodies are found mainly in MG patients with a thymoma MG and correlate with severe MG symptoms. The antibodies recognize a region near the N-terminus on the RyR, which seems to be of importance for RyR regulation. The antibodies cause allosteric inhibition of RyR function in vitro, inhibiting Ca2+ release from sarcoplasmic reticulum.

From basic immunobiology to the upcoming WHO-classification of tumors of the thymus. The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors and related recent developments

The Second Conference on Biological and Clinical Aspects of Thymic Epithelial Tumors in Leiden, The Netherlands, 1998, set the stage for an interdisciplinary meeting of immunologists, pathologists and members of various clinical disciplines to exchange their recent findings in the field of thymus-related biology, pathology, and medicine. The contributions covered such diverse subjects as the role of transcription factors and cytokines in the development of the thymic microenvironment, thymic T, B and NK cell development, the pathogenesis of myasthenia gravis and other thymoma-associated autoimmunities, the pathology of thymic epithelial tumors and germ cell neoplasms, and new approaches to their diagnosis and treatment. This editorial will briefly sum up the data presented at the Conference and will comment on related novel findings that have been reported since then. Because it was also at the Leiden Conference, that the proposal of the WHO committee for the classification of thymic tumors was discussed for the first time, a description of the upcoming WHO Classification of Tumors of the Thymus is given with emphasis on the diagnostic criteria of thymic epithelial tumors, that should now be termed as type A, AB, B1-3 and type C thymomas, to make pathological and clinical studies comparable in the future.

Myasthenia Gravis

The treatment of patients with myasthenia gravis should be individualized according to the extent (ocular versus generalized) and severity (mild to severe) of disease, the presence or absence of concomitant disease (including but not limited to other autoimmune diseases and thymoma), and, to a lesser degree, the age of the patient. Thymectomy should be performed in patients with generalized disease, especially those who have detectable levels of circulating antibodies to acetylcholine receptor (anti-AChR), as it should be in all patients thought to have an operable thymoma (observed on imaging studies of the chest). Symptomatic therapy consists of anticholinesterase drugs (usually pyridostigmine); occasionally, other drugs are required to reduce the muscarinic side effects. At times, patients need immunosuppressive or immunomodulatory therapy with glucocorticoids, azathioprine, cyclospor-ine or cyclophosphamide, plasma exchange, and intravenous immunoglobulin. Remission, whether spontaneous or pharmacologically induced, or significant improvement can be achieved in most patients, but some treatments entail significant side effects and considerable cost.