Contribution of anti-ryanodine receptor antibody to impairment of excitation–contraction coupling in myasthenia gravis

Pediatric myasthenia gravis with a combination of AChR and RyR is associated with an earlier onset and lower CSR rate: A cohort study in southwest China

BACKGROUND

The clinical manifestations and prognosis of myasthenia gravis are related to antibodies, and children are affected differently than adults. The presence of ryanodine receptor and titin antibodies in adults indicates late onset and severe disease related to thymoma, but their role in children is rarely reported.

METHODS

This study collected a cohort of children according to inclusion and exclusion criteria, consisting of antibody-negative, AChR-positive, and AChR with or without titin and RyR antibodies. The differences among groups in general conditions, clinical manifestation, treatment and prognosis were compared.

RESULTS

In total, 171 patients were included: 33 patients (19.30%) were antibody-negative, 84 patients (49.12%) were positve for AChR antibody, 22 patients (12.87%) were positve for AChR and RyR antibodies, 5 patients (2.92%) were positve for AChR and Titin antibodies, and 27 patients (15.79%) were positve for AChR, RyR and Titin antibodies. The median onset age of all the patients was 57.8 (9-177) months, and patients with AChR and RyR antibodies (p = 0.02) and AChR, RyR and Titin antibodies (p = 0.0006) had a younger onset age than patients with AChR antibodies. The rate of generalized MG and MG-ADL before treatment in the AChR-, RyR- and Titin antibody-positive groups was distinctly higher than that in the AChR antibody-positive group (p = 0.038, p = 0.0325). The rate of IVIG use in the AChR-, RyR- and Titin antibody-positive groups (p = 0.0388) was higher than that in the AChR antibody-positive group. The rate of immunosuppressant use in the AChR and RyR antibody-positive group (p = 0.0415) and in the AChR, RyR and Titin antibody-positive group (p = 0.0006) was higher than that in the AChR antibody-positive group. Plasmapheresis was performed in 1 case in the AChR-, RyR- and Titin antibody-positive groups. The CSR rate in the AChR and RyR antibody-positive group (p = 0.0423) and in the AChR, RyR and Titin antibody-positive group (p = 0.0152) was significantly lower than that in the AChR antibody-positive group. Gender, ptosis severity, and CSR time were not significantly different between groups.

CONCLUSIONS

We summarized one of the largest cohorts of pediatric MG patients and compared the clinical phenotype of patients with antibody-negative, AChR-positive, and AChR with or without titin and RyR antibodies. The results showed that patients with AChR and RyR antibodies had a younger onset age, a higher immunosuppressant use rate and a lower CSR rate.

Measurement of excitation-contraction coupling time in critical illness myopathy

OBJECTIVE

This study aimed to develop a simple and reliable technique to assess excitation-contraction (E-C) coupling for early diagnosis of critical illness myopathy (CIM).

METHODS

We prospectively performed clinical and electrophysiological examinations on patients admitted to intensive care unit (ICU). In addition to full neurological examinations and routine nerve conduction study, motor related potential (MRP) was recorded using an accelerometer attached to the base of hallux after tibial nerve stimulation, and E-C coupling time (ECCT) was measured from the latency difference between soleus compound muscle action potential (CMAP) and MRP.

RESULTS

Of 41 patients evaluated, 25 met the criteria for ICU-acquired weakness, 23 of whom had CIM. The time to the first electrophysiological examination (time to first test) correlated negatively with CMAP and with MRP. Conversely, a positive correlation was observed between the time to first test and ECCT. E-C coupling impairment occurred in most of our patients with CIM by the third day of ICU admission, and prolonged ECCT could be the earliest detectable abnormality.

CONCLUSIONS

The ECCT measurement is an easy and reliable technique to detect reduced muscle membrane excitability in the early stage of CIM.

SIGNIFICANCE

The ECCT measured by our method using an accelerometer may be a parameter that predicts the development of CIM.

Measurement of excitation-contraction coupling time in lower extremities

OBJECTIVE

The aim of this study was to apply a novel method to measure excitation-contraction coupling time (ECCT) in normal soleus muscles.

METHODS

We performed simultaneous recordings of soleus compound muscle action potential (CMAP) and foot movement-related potential (MRP), and measured ankle plantar flexion torque in 36 healthy subjects. We calculated ECCT and examined the relations between CMAP, MRP, ECCT and ankle plantar flexion torque.

RESULTS

Statistical analyses established reference ranges (mean ± SE) for CMAP (13.4 ± 0.9 mV), MRP (5.3 ± 0.4 m/s²), ECCT (5.2 ± 0.1 ms), torque (85.9 ± 6.4 Nm) and torque/body weight (1.4 ± 0.1 Nm/kg). The torque showed a positive linear correlation with CMAP (p = 0.041) and a negative linear correlation with ECCT (p = 0.045).

CONCLUSION

Soleus ECCT can be recorded easily, and is useful to assess the impairment of E-C coupling in muscles of the lower extremities.

Autoantibodies in acquired myasthenia gravis: Clinical phenotype and immunological correlation

BACKGROUND

Data on antibody profile in myasthenia gravis (MG) from India are limited.

OBJECTIVES

To investigate antibody profile in patients with MG and their clinical correlates.

PATIENTS AND METHODS

Patients of MG (n = 85, M:F::1.1:1, mean age: 39.29 ± 17.3 years, mean symptom duration: 72.94 ± 91.8 months) were evaluated for clinical features, MG foundation of America (MGFA) score, response to treatment, and outcome at last follow-up. Antibodies to acetylcholine receptor (AChR), muscle-specific kinase (MUSK), titin and ryanodine receptor (RYR) were analysed using ELISA.

RESULTS

Based on the regional distribution of weakness, the cohort could be categorized as: generalized: 60, ocular: 16 and oculo-bulbar: 9. Sixty patients were followed up for a mean duration of 26.74 ± 13.8 months. Outcome at last follow-up was as follows: remission-22, no remission-33 and dead-5. AChR and MUSK antibodies were detected in 58 and 8 patients, respectively. Frequency of generalized MG, worse MGFA score during the disease course and thymomatous histology significantly correlated with presence of AChR-antibodies, though outcome at last follow-up was comparable between AChR-antibody positive and negative groups. Patients with MUSK antibodies had oculo-bulbar or generalized MG and frequent respiratory crisis, but majority improved or remitted with treatment. Titin antibodies were detected in 31.8% and RYR antibodies in 32.9%. Their presence did not correlate with age at onset of MG, severity or presence of thymoma.

CONCLUSION

This report highlights the spectrum of antibodies in MG in an Indian cohort. AChR-antibody positivity correlated with clinical severity. Outcome was good in majority of MUSK antibody-positive MG. The role of other antibodies, complementary vs epiphenomenon, remains open.

Cytometric cell-based assays for anti-striational antibodies in myasthenia gravis with myositis and/or myocarditis

The purposes of the present study were to identify anti-striational antibodies in myasthenia gravis (MG) patients with myositis and/or myocarditis using a combination of cell-based assays and flow cytometry (cytometric cell-based assays) and to describe the main clinical implications. Among 2,609 stored samples collected from all over Japan between 2003 and 2016, we had serum samples from 30 MG patients with myositis and/or myocarditis. Cytometric cell-based assays with titin, ryanodine receptor, and voltage-gated Kv1.4 were performed. Autoantibodies were determined by differences in phycoerythin fluorescence between the 293F cells and titin-transfected cells. MG patients with myositis and/or myocarditis as well as late-onset and thymoma-associated MG had anti-titin, anti-ryanodine receptor, and anti-Kv1.4 antibodies. In contrast, patients with early-onset MG, those with other myopathies and healthy controls did not have anti-titin or anti-Kv1.4 antibodies with some exceptions, but they possessed anti-ryanodine receptor antibodies. Thirty MG patients with myositis and/or myocarditis showed a severe generalized form, and 21 of them had thymoma. Anti-titin and anti-Kv1.4 antibodies were found in 28 (93%) and 15 (50%) patients, respectively, and all patients had at least one of these antibodies. Cytometric cell-based assays thus demonstrated that anti-striational antibodies are biomarkers of MG with myositis and/or myocarditis.

Ryanodine receptor dysfunction in human disorders

Regulation of intracellular calcium (Ca²⁺) is critical in all cell types. The ryanodine receptor (RyR), an intracellular Ca²⁺ release channel located on the sarco/endoplasmic reticulum (SR/ER), releases Ca²⁺ from intracellular stores to activate critical functions including muscle contraction and neurotransmitter release. Dysfunctional RyR-mediated Ca²⁺ handling has been implicated in the pathogenesis of inherited and non-inherited conditions including heart failure, cardiac arrhythmias, skeletal myopathies, diabetes, and neurodegenerative diseases. Here we have reviewed the evidence linking human disorders to RyR dysfunction and describe novel approaches to RyR-targeted therapeutics.

Differential RNA Expression Profile of Skeletal Muscle Induced by Experimental Autoimmune Myasthenia Gravis in Rats

The differential susceptibility of skeletal muscle by myasthenia gravis (MG) is not well understood. We utilized RNA expression profiling of extraocular muscle (EOM), diaphragm (DIA), and extensor digitorum (EDL) of rats with experimental autoimmune MG (EAMG) to evaluate the hypothesis that muscles respond differentially to injury produced by EAMG. EAMG was induced in female Lewis rats by immunization with acetylcholine receptor purified from the electric organ of the Torpedo. Six weeks later after rats had developed weakness and serum antibodies directed against the AChR, animals underwent euthanasia and RNA profiling performed on DIA, EDL, and EOM. Profiling results were validated by qPCR. Across the three muscles between the experiment and control groups, 359 probes (1.16%) with greater than 2-fold changes in expression in 7 of 9 series pairwise comparisons from 31,090 probes were identified with approximately two-thirds being increased. The three muscles shared 16 genes with increased expression and 6 reduced expression. Functional annotation demonstrated that these common expression changes fell predominantly into categories of metabolism, stress response, and signaling. Evaluation of specific gene function indicated that EAMG led to a change to oxidative metabolism. Genes related to muscle regeneration and suppression of immune response were activated. Evidence of a differential immune response among muscles was not evident. Each muscle had a distinct RNA profile but with commonality in gene categories expressed that are focused on muscle repair, moderation of inflammation, and oxidative metabolism.

Understanding autoimmunity: The ion channel perspective

Ion channels are integral membrane proteins that orchestrate the passage of ions across the cell membrane and thus regulate various key physiological processes of the living system. The stringently regulated expression and function of these channels hold a pivotal role in the development and execution of various cellular functions. Malfunction of these channels results in debilitating diseases collectively termed channelopathies. In this review, we highlight the role of these proteins in the immune system with special emphasis on the development of autoimmunity. The role of ion channels in various autoimmune diseases is also listed out. This comprehensive review summarizes the ion channels that could be used as molecular targets in the development of new therapeutics against autoimmune disorders.

Impaired post-tetanic potentiation of muscle twitch in myasthenia gravis

OBJECTIVE

The aim of this study was to evaluate post-tetanic potentiation of muscle twitch in myasthenia gravis (MG).

METHODS

Post-tetanic potentiation was evaluated by recording the compound muscle action potential (CMAP) of abductor pollicis brevis and movement-related potential (MRP) of the thumb using an accelerometer after tetanic stimulation of the median nerve at the wrist. After baseline recording, tetanic stimulation was delivered to the median nerve at a frequency of 10 Hz for 10s. The CMAP and MRP were successively recorded at baseline and at 5, 10, 30, 60, 90 and 120 s after tetanic stimulation. The chronological changes of CMAPs and MRPs were recorded bilaterally in 11 patients with MG, 9 patients with myopathies (disease controls), and 25 healthy control subjects.

RESULTS

Maximal acceleration of MRP was significantly elevated during 10s after tetanic stimulation without any CMAP changes in all groups. However, statistical analysis detected a significant decrease in post-tetanic potentiation of maximal acceleration of MRP in MG patients only compared to healthy controls, but not in myopathy patients, which may imply impairment of excitation-contraction coupling in MG.

CONCLUSIONS

Post-tetanic potentiation of muscle twitch is significantly diminished in MG, suggesting impaired excitation-contraction coupling.

SIGNIFICANCE

Measurement of post-tetanic potentiation using an accelerometer is a simple and sensitive method to detect impairment of excitation-contraction coupling in MG.

[Thymoma-associated myasthenia gravis: Clinical features and surgical results]

OBJECTIVE

The aim of this study was to compare the characteristics of myasthenic patients with and without thymoma, and the results of thymectomy in both types of patients.

MATERIAL AND METHODS

A retrospective study was conducted among 66 patients who underwent thymectomy for myasthenia gravis in our department over a 10-year period (2000-2010). The surgical approach was sternotomy or anterolateral thoracotomy. Patients were divided into two groups according to the presence of thymoma: with (T-MG) and without (NT-MG) thymoma. Complete stable remission (CSR) was the primary endpoint.

RESULTS

Median age was 35.09±9.89 years. The NT-MG group had 38 patients (57.57%) and the T-MG group 28 patients (42.43%). There was no difference between the two groups regarding the surgical approach (P=0.52). T-MG patients were older (40.54±15.16 vs. 31.37±9.46) (P=0.008) and predominantly male. There were more generalized forms (P=0.01) and more bulbar involvement (P=0.02) in the T-MG group. The rate of CSR at 5 years was 7% and 17% in the T-MG and NT-MG patients respectively (P=0.70). At 10 years, it was 36% and 94.73% respectively (P=0.03).

CONCLUSION

Thymomatous myasthenia gravis is characterized by the severity of its clinical features. Remission rate at 10 years was significantly lower in the myasthenia with thymoma group.