• anti-musk antibodies
• myasthenia gravis
• refractory myasthenia gravis
• thymoma

Background: although there are a large number of treatment options, approximately 10% of patients with generalized MG do not respond to conventional treatments. This subgroup of subjects falls under the definition of “Refractory Myasthenia Gravis patients”. In literature Authors agree that, compared to those with non-refractory myasthenia, these patients have an earlier onset, are predominantly female, have a thymoma and a form of generalized myasthenia gravis associated with anti-MuSK antibodies (anti-MuSK Abs). In order to improve the picture of patients with refractory MG it is of utmost importance defining clinically and serologically the different subtypes of MG.
Aim: to analyze the clinical, serological and histopathological characteristics of patients with Refractory Myasthenia associated with anti-MuSK antibodies and with thymoma afferent at Myasthenia Clinic of Azienda Ospedaliera Universitaria Pisana (AOUP). Further aims were: to determine if inhibitor of the tyrosine phosphatase Shp2 (NSC-87877) could represent a new treatment strategy for MuSK-MG, to identify clinical and serological features capable of predicting the risk of relapse in thymoma patients and to define the percentage of both MuSK and thymoma patients who achieve MG remission.
Methods and Materials: A total number of 268 patients with thymomatous MG and 71 patients with MuSK-MG were studied retrospectively. Of both cohorts we assessed: age of MG onset, MG clinical status according to MGFA (Myasthenia Gravis Foundation of America), treatment and post-intervention status. For patients with thymoma we also evaluated: epoch of thymectomy, oncological features and surgical approach. Sera of MuSK-MG patients were sent in Oxford and anti-MuSK Abs were detected both with radioimmunoassay (RIA) and cell-based assay (CBA). Moreover, we tested NSC-87877 for its ability to increase MuSK phosphorylation and to reverse or prevent the effects of MuSK-Abs in vitro models.
In thymoma patients, AChR-Ab dosages were measured both before and after thymectomy and those with symptoms of spontaneous muscle overactivity were selected for autoantibody testing using immunohistology for neuronal cell-surface proteins and cell-based assays for contactin-associated protein 2 (CASPR2), leucine-rich glioma inactivated 1 (LGI1), glycine receptor and Netrin-1 receptor antibodies.
Results: pharmacological/complete stable remission (PR/CSR) was reached in 26/69 (37.7%) of Musk patients. There were no differences in rate of remission between different MuSK-MG phenotypes (p>0.05). 80% of our MuSK-MG cohort positive by RIA responded poorly to cholinesterase inhibitors or with cholinergic side effects. Our in-vitro studies demonstrated the beneficial effect of SHP2 inhibition in reversing the effects of MuSK-Abs on AChR clusters.
In our cohort of patients with thymoma, PR/CSR was attained in 120 (56.6%) patients and CSR was achieved in a higher percentage of patients with a mild MG status before thymectomy than others. There were no differences in rate of remission between thymoma recurrences and monophasic thymoma (p=0.08). AChR-Ab titres decreased after first thymectomy (p < 0.001) and this reduction was more pronounced in female patients (p = 0.05), in patients diagnosed with MG at an older age (p = 0.003), and in those with a milder MG stage before surgery (p = 0.02) or higher Masaoka-Koga stage (p=0.005) than others. The risk of relapse was closely linked with the age of the patient, the Masaoka-Koga stage, the surgical approach and the presence of Neuromyotonia (p < 0.001).
Conclusions: The reduction of AChR-Ab titres after thymectomy confirms an immunological role of thymoma in the pathogenesis of MG. Our observations on thymoma recurrences provide a pragmatic risk stratification for tumour vigilance in patients with thymomatous MG. A relevant percentage of thymoma and anti-MuSK patients can achieve a pharmacological/complete stable remission. Our study shows that the knowledge of the molecular mechanisms that occur in the development and progression of MG is essential to develop a more precise targeting treatment for patients with Refractory Myasthenia Gravis.