MuSK-MG Patients Research Articles

Acetylcholinesterase inhibitors are ineffective in MuSK-antibody positive myasthenia gravis: Results of a study on 202 patients

BackgroundMyasthenia gravis (MG) with MuSK antibodies (MuSK-MG) represents a distinct subtype with different responses to treatments compared to patients with AChR antibodies, especially in terms of tolerance to acetylcholinesterase inhibitors (AChEI). However, AChEI are often used as first line symptomatic treatment in MuSK-MG, despite reports that they are poorly tolerated, seldom effective or even deleterious. MethodsWe analyzed demographic, clinical and therapeutic responses and side-effects in the large cohort of 202 MuSK-MG patients cared for at the MG Clinic of Azienda Ospedaliero-Universitaria Pisana. Results165 patients had received AChEI at first evaluation. Only 7/165 patients (4.2%) reported an initial clinical benefit. Conversely, 76.9% of patients reported at least one side effect, most commonly neuromuscular hyperexcitability (68.4%), gastrointestinal (53.9%) and neurovegetative (35.8%) disturbances. 56 (33.9%) patients reported a concomitant worsening of muscle weakness and twelve patients (7.3%) suffered a cholinergic crisis. According to these patients, the severity of cholinergic side effects was greater at higher doses of AChEI, but side effects occurred regardless of the dose administered and ceased once the drug was discontinued. ConclusionsThis is the largest population of MuSK-MG patients reported for perceived responsiveness and tolerance to AChEI treatment. Our obervations strongly suggest avoiding this treatment in MuSK-MG.

A retrospective multicenter study on clinical and serological parameters in patients with MuSK myasthenia gravis with and without general immunosuppression.

Muscle-specific kinase (MuSK)- myasthenia gravis (MG) is caused by pathogenic autoantibodies against MuSK that correlate with disease severity and are predominantly of the IgG4 subclass. The first-line treatment for MuSK-MG is general immunosuppression with corticosteroids, but the effect of treatment on IgG4 and MuSK IgG4 levels has not been studied. We analyzed the clinical data and sera from 52 MuSK-MG patients (45female, 7 male, median age 49 (range 17-79) years) from Italy, the Netherlands, Greece and Belgium, and 43 AChR-MG patients (22 female, 21 male, median age 63 (range 2-82) years) from Italy, receiving different types of immunosuppression, and sera from 46 age- and sex-matched non-disease controls (with no diagnosed diseases, 38 female, 8 male, median age 51.5 (range 20-68) years) from the Netherlands. We analyzed the disease severity (assessed by MGFA or QMG score), and measured concentrations of MuSK IgG4, MuSK IgG, total IgG4 and total IgG in the sera by ELISA, RIA and nephelometry. We observed that MuSK-MG patients showed a robust clinical improvement and reduction of MuSK IgG after therapy, and that MuSK IgG4 concentrations, but not total IgG4 concentrations, correlated with clinical severity. MuSK IgG and MuSK IgG4 concentrations were reduced after immunosuppression in 4/5 individuals with before-after data, but data from non-linked patient samples showed no difference. Total serum IgG4 levels were within the normal range, with IgG4 levels above threshold (1.35g/L) in 1/52 MuSK-MG, 2/43 AChR-MG patients and 1/45 non-disease controls. MuSK-MG patients improved within the first four years after disease onset, but no further clinical improvement or reduction of MuSK IgG4 were observed four years later, and only 14/52 (26.92%) patients in total, of which 13 (93.3%) received general immunosuppression, reached clinical remission. We conclude that MuSK-MG patients improve clinically with general immunosuppression but may require further treatment to reach remission. Longitudinal testing of individual patients may be clinically more useful than single measurements of MuSK IgG4. No significant differences in the serum IgG4 concentrations and IgG4/IgG ratio between AChR- and MuSK-MG patients were found during follow-up. Further studies with larger patient and control cohorts are necessary to validate the findings.

Change of voltage-gated sodium channel repertoire in skeletal muscle of a MuSK myasthenia gravis mouse model.

Muscle-specific kinase myasthenia gravis (MuSK MG) is caused by autoantibodies against MuSK in the neuromuscular junction (NMJ). MuSK MG patients have fluctuating, fatigable skeletal muscle weakness, in particular of bulbar muscles. Severity differs greatly between patients, in spite of comparable autoantibody levels. One explanation for inter-patient and inter-muscle variability in sensitivity might be variations in compensatory muscle responses. Previously, we developed a passive transfer mouse model for MuSK MG. In preliminary ex vivo experiments, we observed that muscle contraction of some mice, in particular those with milder myasthenia, had become partially insensitive to inhibition by μ-Conotoxin-GIIIB, a blocker of skeletal muscle NaV1.4 voltage-gated sodium channels. We hypothesised that changes in NaV channel expression profile, possibly co-expression of (μ-Conotoxin-GIIIB insensitive) NaV1.5 type channels, might lower the muscle fibre's firing threshold and facilitate neuromuscular synaptic transmission. To test this hypothesis, we here performed passive transfer in immuno-compromised mice, using 'high', 'intermediate' and 'low' dosing regimens of purified MuSK MG patient IgG4. We compared myasthenia levels, μ-Conotoxin-GIIIB resistance and muscle fibre action potential characteristics and firing thresholds. High- and intermediate-dosed mice showed clear, progressive myasthenia, not seen in low-dosed animals. However, diaphragm NMJ electrophysiology demonstrated almost equal myasthenic severities amongst all regimens. Nonetheless, low-dosed mouse diaphragms showed a much higher degree of μ-Conotoxin-GIIIB resistance. This was not explained by upregulation of Scn5a (the NaV1.5 gene), lowered muscle fibre firing thresholds or histologically detectable upregulated NaV1.5 channels. It remains to be established which factors are responsible for the observed μ-Conotoxin-GIIIB insensitivity and whether the NaV repertoire change is compensatory beneficial or a bystander effect.

The mechanisms of immunopathology underlying B cell depletion therapy-mediated remission and relapse in patients with MuSK MG

The application of reverse translational medicine allows for the understanding of immune pathogenesis via therapeutic intervention. We applied this approach to the MuSK subtype of myasthenia gravis. Treatment with the CD20-specific B cell depletion therapy (BCDT) demonstrated that MuSK MG patients respond remarkably well; the majority invariably reach remission accompanied by a remarkable drop in autoantibody levels. Circulating antibodies are primarily produced by bone marrow resident plasma cells, which do not express CD20. So, how does BCDT diminish MuSK autoantibodies and induce rapid remission? We developed a mechanistic model, which hypothesized that plasmablasts, which are short-lived antibody secreting B cell populations, produce MuSK-specific autoantibodies. Anti-CD20-mediated BCDT is expected to deplete CD20-expressing plasmablasts or CD20 expressing memory cells that supply the plasmablast population. To test this hypothesis, we performed a series of investigations, which were reported over the last seven years and are summarized in this review. First, we isolated plasmablasts from patients and generated human recombinant monoclonal autoantibodies (mAb) which bound MuSK and had pathogenic capacity, demonstrating that MuSK autoantibodies can be produced by this specific cell population. The characterization of the mAbs showed that MuSK autoantibodies can include unique properties including unusually high antigen binding affinity, and an elevated frequency of N-linked glycosylation in their binding domains. Further characterization suggested that MuSK autoantibody-producing cells may form in the early stages of B cell development due to defective tolerance mechanisms. Finally, we sought to determine how these pathogenic B cell clones behave over time. High throughput B cell receptor sequencing was applied to investigate longitudinally collected samples from patients treated with anti-CD20-mediated BCDT. MuSK-specific clonal variants were detected at multiple timepoints spanning more than five years and reemerged after BCDT-induced remission, predating disease relapse by several months. These collective investigations provide a more detailed mechanistic understanding that MuSK MG, the key features of which include production of autoantibodies by circulating plasmablasts that can be targeted by CD20-specific BCDT, and that pathogenic clones can survive BCDT and reemerge prior to manifestation of clinical relapse.

IgG1-3 MuSK Antibodies Inhibit AChR Cluster Formation, Restored by SHP2 Inhibitor, Despite Normal MuSK, DOK7, or AChR Subunit Phosphorylation.

Up to 50% of patients with myasthenia gravis (MG) without acetylcholine receptor antibodies (AChR-Abs) have antibodies to muscle-specific kinase (MuSK). Most MuSK antibodies (MuSK-Abs) are IgG4 and inhibit agrin-induced MuSK phosphorylation, leading to impaired clustering of AChRs at the developing or mature neuromuscular junction. However, IgG1-3 MuSK-Abs also exist in MuSK-MG patients, and their potential mechanisms have not been explored fully. C2C12 myotubes were exposed to MuSK-MG plasma IgG1-3 or IgG4, with or without purified agrin. MuSK, Downstream of Kinase 7 (DOK7), and βAChR were immunoprecipitated and their phosphorylation levels identified by immunoblotting. Agrin and agrin-independent AChR clusters were measured by immunofluorescence and AChR numbers by binding of 125I-α-bungarotoxin. Transcriptomic analysis was performed on treated myotubes. IgG1-3 MuSK-Abs impaired AChR clustering without inhibiting agrin-induced MuSK phosphorylation. Moreover, the well-established pathway initiated by MuSK through DOK7, resulting in βAChR phosphorylation, was not impaired by MuSK-IgG1-3 and was agrin-independent. Nevertheless, the AChR clusters did not form, and both the number of AChR microclusters that precede full cluster formation and the myotube surface AChRs were reduced. Transcriptomic analysis did not throw light on the pathways involved. However, the SHP2 inhibitor, NSC-87877, increased the number of microclusters and led to fully formed AChR clusters. MuSK-IgG1-3 is pathogenic but seems to act through a noncanonical pathway. Further studies should throw light on the mechanisms involved at the neuromuscular junction.

Complement Activation Profile in Myasthenia Gravis Patients: Perspectives for Tailoring Anti-Complement Therapy.

The complement system plays a key role in myasthenia gravis (MG). Anti-complement drugs are emerging as effective therapies to treat anti-acetylcholine receptor (AChR) antibody-positive MG patients, though their usage is still limited by the high costs. Here, we searched for plasma complement proteins as indicators of complement activation status in AChR-MG patients, and potential biomarkers for tailoring anti-complement therapy in MG. Plasma was collected from AChR-MG and MuSK-MG patients, and healthy controls. Multiplex immunoassays and ELISA were used to quantify a panel of complement components (C1Q, C2, C3, C4, C5, Factor B, Factor H, MBL, and properdin) and activation products (C4b, C3b, C5a, and C5b-9), of classical, alternative and lectin pathways. C2 and C5 levels were significantly reduced, and C3, C3b, and C5a increased, in plasma of AChR-MG, but not MuSK-MG, patients compared to controls. This protein profile was indicative of complement activation. We obtained sensitivity and specificity performance results suggesting plasma C2, C3, C3b, and C5 as biomarkers for AChR-MG. Our findings reveal a plasma complement “C2, C3, C5, C3b, and C5a” profile associated with AChR-MG to be further investigated as a biomarker of complement activation status in AChR-MG patients, opening new perspectives for tailoring of anti-complement therapies to improve the disease treatment.

Spotlight on MuSK positive myasthenia gravis: clinical characteristics, treatment and outcomes

BackgroundTo investigate the clinical characteristics, treatments and outcomes of patients with myasthenia gravis with antibodies to muscle-specific tyrosine kinase (MuSK-MG).MethodsWe retrospectively reviewed the cases of 21 patients with confirmed MuSK-MG between January 2012 and January 2020 in our centre. Detailed clinical data and long-term follow-up information were summarized.ResultsFemales (17/21, 81%) predominated among these MuSK-MG patients, and the mean age of onset in this group was 51.86 ± 16.16 years. MuSK-MG patients were divided into three subgroups according to the symptoms of muscle weakness at onset: ocular myasthenia gravis (OMG, 47.6%), bulbar myasthenia gravis (BMG, 42.9%), and generalized myasthenia gravis (GMG, 9.5%). The mean progression time from symptom onset to other muscle group involvement in OMG patients was 4.38 ± 2.54 months. Pyridostigmine bromide was adopted in 81.0% of patients, and 90.5% of patients received corticosteroids. Compared to usage in hospitals, the median daily dose of corticosteroids decreased significantly at the last follow-up. A total of 85.7% of patients received a long-term follow-up, with an average time of 1202.17 ± 976.73 days. At the end of the follow-up period, 4.8% of patients had achieved complete stable remission, 42.9% of patients had minimal manifestations, 19.0% had improved, the condition of 4.8% of patients remained unchanged, and 9.5% of patients died.ConclusionFemale patients were more prevalent in this study, and MuSK-MG patients rapidly progressed to a generalized state. Although approximately 50% of MuSK-MG patients can achieve a favourable outcome with conventional immunosuppressants, complete stable remission is rare, and approximately 15% respond poorly. More effective medications should be explored in these patients.

Correlation between clinical manifestation, prognosis and antibody in anti-muscle-specific tyrosine kinase antibody positive myasthenia gravis

Objective: To determine the correlation between the titer of anti-muscle-specific tyrosine kinase (anti-MuSK) antibodies (Abs) and the severity and prognosis of Musk-associated myasthenia gravis (Musk-MG). Methods: A total of 33 MuSK-MG patients diagnosed at Department of Neurology, Peking Union Medical College Hospital from May 2018 to June 2020 were prospectively included. Patients were divided into different groups according to immune state, and the immune naive patients were further divided by the Myasthenia Gravis Foundation of America (MGFA) classification. There were 28 Musk-MG patients who completed the follow-up and subdivided into different groups according to post-intervention status (PIS). Twenty-five patients who received immunotherapy were divided into corticosteroid monotherapy group (n=17) and corticosteroid combined with immunosuppressant group (n=8). The comparison of Ab titers between different MGFA groups and PIS groups was determined by Kruskal-Wallis method, and the comparison of Ab titers between different time points was analyzed by Mann-Whitney U method. Results: There were 11 males and 22 females included in the study, with an onset age of 48 (18, 73) years, of which 16 cases were immune naive and 17 cases were treated with corticosteroids or immunosuppressant at least once. In immune naive population, a significant difference of Ab titers among different MGFA phenotypes was detected (P=0.04). Ab titers were reduced by immunosuppression therapy (the median value decreased from 1.20 to 0.87, P=0.01). Twenty-four (85.7%) MuSK-MG patients achieved a good prognosis (PIS-PR/MM), 1 (3.6%) case achieved improvement (PIS-I), and 3 (10.7%) patients' condition worsened (PIS-W), there was no significant difference of Ab titers among the three groups (P=0.21). Meanwhile, there was no significant difference of Ab titers between different treatment groups (P=0.95). Conclusions: In the immune naive state, the concentration of MuSK-Ab is consistent with the severity of the disease, and the Ab titers decrease after immunotherapy. Change of Ab titers is related to the daily dosage of corticosteroid and is not consistent with PIS grades.

Lower BAFF Levels in Myasthenic Patients Treated with Glucocorticoids

B-cell activating factor (BAFF), a member of tumor necrosis factor family, activates B cells, promotes their survival and proliferation. BAFF is considered to have an influence on development of autoimmune diseases including myasthenia gravis (MG). We aimed to evaluate BAFF serum levels in MG patients, their potential connection with therapy and course of MG. Cross-sectional study. Two hundred eighteen adult patients with MG (67% women, age: 18–89 years, 82.6% AChR antibody seropositive (AChRAb(+)). Serum BAFF levels, their relationship with severity of clinical symptoms, therapy conducted, clinical and demographic features and other factors were analyzed. Patients with AChRAb(+) MG demonstrated significantly higher BAFF levels than MuSK-MG patients (831.2 ± 285.4 pg/ml vs. 745.6 ± 633.4 pg/ml, respectively; p = 0.030). Serum BAFF levels in women were significantly higher than in men (855.9 ± 302.5 vs. 756.6 ± 289.4, respectively; p = 0.017). Mean serum BAFF level was significantly decreased in patients who were ever treated with corticosteroids (CS) (770.4 ± 327.8 pg/ml vs. 891.3 ± 246.1 pg/ml, respectively; p = 0.001). Thymoma-MG patients demonstrated significantly lower BAFF levels (671.2 ± 244.9 vs. 833.5 ± 302.4, respectively; p = 0.044). Thymectomized patients did not differ in BAFF levels from the MG patients who had not undergone thymectomy. In multiple linear regression model, recent CS therapy and male sex were found to be independent predictors of lower BAFF levels. Serum BAFF level is decreased in patients treated with CS, which may suggest inhibiting influence of CS on BAFF—a potential mechanism contributing to the effectiveness of such therapy.

Symposium 4: Neuromuscular junction and related disorders SY4.1. Myasthenia gravis and related disorders

Autoimmune myasthenia gravis (MG) results from autoantibodies that bind to the neuromuscular junction (NMJ). Incidence and prevalence vary worldwide; the estimated US prevalence is 20/100,000, or ∼60,000 patients. Women develop MG more often before age 40, men after 50. Drooping eyelids or double vision is the first symptom in 60%; weakness remains limited to periocular muscles in 10–15% – ocular myasthenia. The voice may be nasal or hoarse, especially after prolonged talking, and swallowed liquids may escape through the nose. Weakness typically worsens throughout the day. Spontaneous improvement may occur early on but is rarely permanent. The diagnosis depends on recognizing distinctive patterns of weakness and confirmation by serologic and electrodiagnostic tests. 80–85% of patients have circulating anti-AChR antibodies; repetitive nerve stimulation (RNS) demonstrates abnormal neuromuscular transmission (NMT) in up to 90% of generalized MG and 60% of ocular myasthenia. Increased jitter is found in almost all patients if appropriate muscles are examined. Approximately 10% of patients have antibodies to muscle specific tyrosine kinase (MuSK). Clinical findings resemble non-MuSK MG in some patients; many have predominant weakness in cranio-bulbar muscles with marked atrophy. Others have predominant neck, shoulder and respiratory weakness. With selected treatment most non-MuSK and MuSK-MG patients have only minimal long-term morbidity. Congenital myasthenic syndromes result from genetic defects of muscle endplate molecules involved in NMT; most have autosomal recessive inheritance. The diagnosis is suggested by clinical features and response to cholinesterase inhibitors, or electrodiagnostic findings. Identification of specific genetic or physiologic defects requires genetic studies or specialized morphological and electrophysiological studies. Lambert-Eaton myasthenia results from antibodies to voltage-gated calcium channels (VGCC) on presynaptic nerve terminals, which produces the classic triad of weakness, reduced muscle stretch reflexes and autonomic dysfunction. Approximately 50% have an underlying malignancy, usually small cell lung cancer. The diagnosis is confirmed by a decrementing pattern during low frequency RNS and low muscle responses that increase at least 60% after muscle activation. Antibodies to the VGCC are found in most cases. Treatment is initially directed against any underlying malignancy. Amifampridine produces symptomatic improvement in most patients; immunomodulatory therapy may provide modest benefit in some.

Cholinergic hyperactivity in patients with myasthenia gravis with MuSK antibodies: A neurophysiological study

ObjectivePatients with myasthenia gravis associated with muscle-specific tyrosine kinase antibodies (MuSK-MG) often manifest signs of cholinergic hyperactivity with standard doses of acetylcholinesterase inhibitors (AChE-Is). Aim of the study was to investigate whether repetitive compound muscle action potential (R-CMAP), the neurophysiological correlate of cholinergic hyperactivity, was present in MuSK-MG irrespective of AChE-I treatment. MethodsPatients with confirmed diagnosis of MuSK-MG were consecutively enrolled during follow-up visits, from January 2019 to April 2020. All these subjects underwent the same neurophysiological protocol, including motor nerve conduction studies and repetitive nerve stimulation. In patients taking pyridostigmine, neurophysiological testing was performed at least 12 hours after the last dose. For comparison, the presence of R-CMAP was investigated in 20 consecutive acetylcholine receptor antibody positive myasthenia gravis (AChR-MG) patients. ResultsWe enrolled 25 MuSK-MG patients (20 females), aged 16–79 years at the study time, with disease duration ranging 0.6–48.8 years (median: 17.7 years). R-CMAP was detected in 12/25 (48%) MuSK-MG cases and in none of the AChR-MG controls (p = 0.0003). In the MuSK-MG population, a history of muscle cramps and fasciculations, during low-dose pyridostigmine therapy, was significantly more frequent in R-CMAP positive than in R-CMAP negative patients (100% vs 31%, p = 0.001). At the time of the study, the proportion of patients still symptomatic for MG was higher among R-CMAP positive cases (92% vs 23%, p = 0.0005). ConclusionsCholinergic hyperactivity is a relatively common finding in MuSK-MG patients, independent of AChE-I treatment, and may constitute an intrinsic feature of the disease. SignificanceR-CMAP detection can represent a useful diagnostic clue for MuSK-MG and predicts poor tolerance to AChE-Is.

Short-term effect of low-dose rituximab on myasthenia gravis with muscle-specific tyrosine kinase antibody.

The study aims to investigate the short-term efficacy of low-dose rituximab and its effect on immunological biomarker levels in myasthenia gravis (MG) patients with antibodies against muscle-specific tyrosine kinase (MuSK-MG). Twelve MuSK-MG patients were enrolled in this prospective, open-label, self-controlled pilot study. Clinical severity was evaluated at baseline and 6mo after a single rituximab treatment (600 mg). B lymphocyte subtypes, MuSK antibody titers, together with levels of immunoglobulins, serum B-cell activating factor (BAFF), a proliferation-inducing ligand (APRIL), soluble CD40L, and four exosomal microRNAs were evaluated. A correlation matrix to reveal pairwise relationships among above variables was also generated. The single rituximab treatment significantly lowered the clinical severity scores and reduced daily dosage of prednisone (P=.032) at 6mo. MuSK antibody titers decreased (P=.035) without significant changes in immunoglobulin levels. Serum BAFF level increased (P=.010), which negatively correlated with the percentages of B cells in lymphocytes as well as clinical severity. Additionally, serum exosomal miR-151a-3p showed a reduction of 28.1% (P=.031). We confirmed the clinical efficacy of low-dose rituximab in MuSK-MG, accompanied by a decrease in MuSK antibody titers and an increase in serum BAFF. Serum BAFF levels negatively correlated with B-cell counts as well as clinical severity.

MuSK-Associated Myasthenia Gravis: Clinical Features and Management.

Muscle-specific tyrosine kinase (MuSK) myasthenia gravis (MG) is a rare, frequently more severe, subtype of MG with different pathogenesis, and peculiar clinical features. The prevalence varies among countries and ethnic groups, affecting 5–8% of all MG patients. MuSK-MG usually has an acute onset affecting mainly the facial-bulbar muscles. The symptoms usually progress rapidly, within a few weeks. Early respiratory crises are frequent. The disease may lead to generalized muscle weakness up to muscle atrophy. The main bulbar involvement, the absence of significant thymus alterations, and the association with HLA class II DR14, DR16, and DQ5 alleles have been confirmed. Atypical onset, such as ocular involvement, lack of symptom fluctuations, acetylcholinesterase inhibitors failure, and negative results of electrophysiologic testing, if not specifically performed in the mainly involved muscle groups, makes MuSK-MG diagnosis challenging. In most cases, steroids are effective. Conventional immunosuppressants are not commonly able to replace steroids in maintaining a satisfactory long-term control of symptoms. However, the majority of MuSK-MG patients are refractory to treatment. In these cases, the use of rituximab showed promising results, resulting in sustained symptom control.

Long-Lasting Rituximab-Induced Reduction of Specific-But Not Total-IgG4 in MuSK-Positive Myasthenia Gravis.

The use of rituximab (RTX), an anti-CD20 monoclonal antibody (Ab), in refractory myasthenia gravis (MG) is associated with a better response in patients with Abs to the muscle-specific tyrosine kinase (MuSK) than in other MG subgroups. Anti-MuSK Abs are mostly IgG4 with proven pathogenicity and positive correlation with clinical severity. The rapid and sustained response to RTX may be related to MuSK Ab production by short-lived Ab-secreting cells derived from specific CD20+ B cells. Here, we investigated the long-term effects of RTX in nine refractory MuSK-MG patients with a follow-up ranging from 17 months to 13 years. In patients’ sera, we titrated MuSK-specific IgG (MuSK-IgG) and MuSK-IgG4, along with total IgG and IgG4 levels. Optimal response to RTX was defined as the achievement and maintenance of the status of minimal manifestations (MM)-or-better together with a ≥ 50% steroid reduction, withdrawal of immunosuppressants, and no need for plasma-exchange or intravenous immunoglobulin. After a course of RTX, eight patients improved, with optimal response in six, while only one patient did not respond. At baseline, MuSK-IgG and MuSK-IgG4 serum titers were positive in all patients, ranging from 2.15 to 49.5 nmol/L and from 0.33 to 46.2 nmol/L, respectively. MuSK Abs mostly consisted of IgG4 (range 63.80–98.86%). RTX administration was followed by a marked reduction of MuSK Abs at 2–7 months and at 12–30 months (p < 0.02 for MuSK-IgG and p < 0.01 for MuSK-IgG4). In patients with a longer follow-up, MuSK Ab titers remained suppressed, paralleling clinical response. In the patient who achieved long-term complete remission, MuSK-IgG4 was no longer detectable within 2 years, while MuSK-IgG remained positive at very low titers up to 10 years after RTX. In the patient who did not respond, MuSK-IgG and MuSK-IgG4 remained unchanged. In this patient series, total IgG and IgG4 transiently decreased (p < 0.05) at 2–7 months after RTX. The different trends of reduction between MuSK-IgG4 and total IgG4 after RTX support the view that short-lived Ab-secreting cells are the main producers of MuSK Abs. The ratio between short-lived Ab-secreting cells and long-lived plasma cells may influence the response to RTX, and B-cell severe depletion may reduce self-maintaining autoimmune reactivity.

MuSK EAMG: Immunological Characterization and Suppression by Induction of Oral Tolerance

Myasthenia gravis (MG) with antibodies to the muscle-specific receptor tyrosine kinase (MuSK) is a distinct sub-group of MG, affecting 5–8% of all MG patients. MuSK, a receptor tyrosine kinase, is expressed at the neuromuscular junctions (NMJs) from the earliest stages of synaptogenesis and plays a crucial role in the development and maintenance of the NMJ. MuSK-MG patients are more severely affected and more refractory to treatments currently used for MG. Most patients require long-term immunosuppression, stressing the need for improved treatments. Ideally, preferred treatments should specifically delete the antigen-specific autoimmune response, without affecting the entire immune system. Mucosal tolerance, induced by oral or nasal administration of an auto-antigen through the mucosal system, resulting in an antigen-specific immunological systemic hyporesponsiveness, might be considered as a treatment of choice for MuSK-MG. In the present study we have characterized several immunological parameters of murine MuSK-EAMG and have employed induction of oral tolerance in mouse MuSK-EAMG, by feeding with a recombinant MuSK protein one week before disease induction. Such a treatment has been shown to attenuate MuSK-EAMG. Both induction and progression of disease were ameliorated following oral treatment with the recombinant MuSK fragment, as indicated by lower clinical scores and lower anti-MuSK antibody titers.

Anti-musk antibody myasthenia gravis: Clinical findings and long term follow up

Myasthenia gravis (MG) is adisorder neuro-muscular junction where 50-85% patients have acetylcholine receptor antibody (AChR-Ab). About 30-40% sero-negative patients have antibodies to muscle-specific tyrosine kinase (MuSK). This disorder represents a distinct subset of MG where clinical features and treatment options are different. We retrospectively analysed MuSK-MG patients seen at our center in last 4 years. From 2015 to 2019, there were 6 MuSK-MG patients. Eighty-three percent were female, with disease onset in the fourth to fifth decade. Oculobulbar symptoms were present at onset in 83%. Eventually ocular involvement was seen in 83%, bulbar in 83% and limbs weakness in 83%. Fifty percent patients had myasthenic crisis. No patient had antibodies to AchR and of those tested, CT scan of thorax did not show any thymic tumour. No patients responded to pyridostigmine. Sixty six percent patients had moderate to good response to steroids. Sixty six percent patients were treated with plasma exchange (PLEX) which produced improvement while none was treated with intravenous immunoglobulin. Immunosupression with azathioprine (50%), mycophenolate (16%) and rituximab (50%) was used during the course of illness. All patients achieved good remission over a median follow up of 2 years. MuSK-MG has a marked female predominance with frequent oculobulbar weakness and crises. Pyridostigmine does not help to relieve symptoms and PLEx shows better results compared to steroids. Most patients need immunosuppression and remission can be achieved with it.

Thymectomy may not be associated with clinical improvement in MuSK myasthenia gravis.

A randomized trial demonstrated benefit from thymectomy in nonthymomatous acetylcholine receptor (AChR)-antibody positive myasthenia gravis (MG). Uncontrolled observational and histologic studies suggest thymectomy may not be efficacious in anti-muscle-specific kinase (MuSK)-MG. The therapeutic impact of thymectomy was evaluated from data collected for a multicenter, retrospective blinded review of rituximab in MuSK-MG. Baseline characteristics were similar between thymectomy (n = 26) and nonthymectomy (n = 29) groups, including treatment with rituximab (42% vs. 45%). At last visit, 35% of thymectomy subjects reached the primary endpoint, a Myasthenia Gravis Foundation of America (MGFA) post-intervention status (PIS) score of minimal manifestations (MM) or better, compared with 55% of controls (P = 0.17). After controlling for age at onset of MG, rituximab, prednisone, and intravenous immunoglobulin/plasma exchange treatment, thymectomy was not associated with greater likelihood of favorable clinical outcome (odds ratio = 0.43, 95% confidence interval 0.12-1.53, P = 0.19). Thymectomy was not associated with additional clinical improvement in this multicenter cohort of MuSK-MG patients. Muscle Nerve 59:404-410, 2019.

HLA and MuSK-positive myasthenia gravis: A systemic review and meta-analysis.

Myasthenia gravis (MG) represents a spectrum of clinical subtypes with differences in disease mechanisms and treatment response. MG with muscle-specific tyrosine kinase (MuSK) antibodies accounts for 1%-10% of all MG patients. We conducted a meta-analysis to evaluate the association between HLA genes and MuSK-MG susceptibility. Studies were searched in Pubmed, EMBASE database and other sources between 2001 and 2018. Genotype, allele and haplotype frequencies of HLA loci in MuSK-MG patients and healthy controls were extracted from each included study. The meta-analysis showed that HLA DQB1*05, DRB1*14 and DRB1*16 were strongly associated with an increased risk of MuSK-MG (P<.0001), whereas HLA DQB*03 was less frequent in MuSK patients compared with healthy controls (P<.05). Haplotype analysis showed that these DQB1 and DRB1 alleles were closely linked, forming both risk (DQ5-DR14, DQ5-DR16, P<.0001) and protective (DQ3-DR4, DQ3-DR11, P<.05) haplotypes. The distinct genetic patterns of MuSK-MG indicate that variation in HLA class II genes plays an important role in the pathogenesis of MuSK-MG patients.

F43. Electrophysiological tests in patients with anti-MuSK myasthenia gravis: A retrospective study

Introduction Myasthenia gravis (MG) with autoantibodies to muscle-specific tyrosine kinase (MuSK) represents a distinct type of disease, as compared MG with antibodies against acetylcholine receptor (AchR-Abs). MuSK-MG patients have peculiar clinical features characterized by predominant oculo-bulbar symptoms and a lack of response to conventional MG treatments. Since the role of electrophysiological tests in MuSK patients is still argued, our aim was to investigate their electrophysiological profile in order to arise a light over this darkness. Methods We retrospectively evaluated repetitive nerve stimulation (RNS) and single-fibre electromyography (SF-EMG) reports of 63 MuSK patients, at the time of MG diagnosis, and we compared them with 35 non-timomatous AChR-Abs positive patients matched with MuSK group according to gender, age at onset of MG, age at MG diagnosis and severity of the disease. Disease severity was graded according to Myasthenia Gravis Foundation of America (MGFA) classification. Seronegative patients and thymoma-associated MG patients were excluded from the study. Results Sixty-three MuSK patients (11 males; 52 females; mean age at MG onset: 37 ± 2.0; mean age at diagnosis: 38.7 ± 2.0) and 35 AChR-Abs positive patients (9 males; 26 females; mean age at MG onset: 31.2 ± 2.7: mean age at diagnosis: 33.1 ± 2.5) were enrolled in the study. Fifty-nine MuSK patients (93.6%) and 31 AChR-Abs positive patients (88.5%) underwent RNS, whereas SF-EMG was tested in 17 MuSK patients and in 7 AChR-Abs patients. RNS showed a significant decrement in a higher percentage of AChR-Abs positive patients than the MuSK group (p Conclusion While SR-EMG positivity was significantly higher in AChR-Abs group, there was no difference regarding SF-EMG. In MusK group, the decrement of RNS was more evident in proximal muscles while RNS in distal muscles can be negative. Although neurophysiological evaluation is still important for MG diagnosis, it plays a secondary role after anti-MuSK titration.

Serological Immunoglobulin-Free Light Chain Profile in Myasthenia Gravis Patients.

Background Serological levels of free immunoglobulin light chains (FLCs), produced in excess of heavy chains during synthesis of immunoglobulins by plasma cells, can be considered a direct marker of B cell activity in different systemic inflammatory-autoimmune conditions and may represent a useful predictor of rituximab (RTX) therapeutic efficacy, as reported for rheumatoid arthritis and systemic lupus erythematosus. Myasthenia gravis (MG) is an autoimmune disease of the neuromuscular junction with antibodies (abs) targeting the acetylcholine receptor (AChR) or the muscle-specific tyrosine kinase (MuSK), inducing muscle weakness and excessive fatigability. As MG course may be remarkably variable, we evaluated the possible use of FLCs as biomarkers of disease activity. Subjects and Methods We assessed FLC levels in 34 sera from 17 AChR-MG and from 13 MuSK-MG patients, in comparison with 20 sera from patients with systemic autoimmune rheumatic diseases and 18 from healthy blood donors, along with titers of specific auto-abs and IgG subclass distribution. Results We found a statistically significant increase in free κ chains in both AChR- and MuSK-MG patients, while free λ chain levels were increased only in AChR-MG. We also observed a significant reduction of both free κ and λ chains in 1/4 MuSK-MG patients along with specific abs titer, two months after RTX treatment. Conclusions From our data, FLCs appear to be a sensitive marker of B cell activation in MG. Further investigations are necessary to exploit their potential as reliable biomarkers of disease activity.