Heterogeneity in myasthenia gravis: considerations for disease management

Introduction: Myasthenia gravis is a rare disease of the neuromuscular junction and a prototype of B cell-driven immunopathology. Pathogenic antibodies target post-synaptic transmembrane proteins, most commonly the nicotinic acetylcholine receptor and the muscle-specific tyrosine kinase, inducing end-plate alterations and neuromuscular transmission impairment. Several clinical subtypes are distinct on the basis of associated antibodies, age at symptom onset, thymus pathology, genetic factors, and weakness distribution. These subtypes have distinct pathogenesis that can account for different responses to treatment. Conventional therapy is based on the use of symptomatic agents, steroids, immunosuppressants and thymectomy. Of late, biologics have emerged as effective therapeutic options.Areas covered: In this review, we will discuss the management of myasthenia gravis in relation to its phenotypic and biological heterogeneity, in the light of recent advances in the disease immunopathology, new diagnostic tools, and results of clinical trialsExpert opinion: Clinical management is shaped on serological subtype, and patient age at onset, lifestyle and comorbidities, balancing therapeutic needs and safety. Although reliable biomarkers predictive of clinical and biologic outcome are still lacking, recent developments promise a more effective and safe treatment. Disease subtyping according to serological testing and immunopathology is crucial to the appropriateness of clinical management.

Late-onset myasthenia gravis - CTLA4(low) genotype association and low-for-age thymic output of naïve T cells

Late-onset myasthenia gravis (LOMG) has become the largest MG subgroup, but the underlying pathogenetic mechanisms remain mysterious. Among the few etiological clues are the almost unique serologic parallels between LOMG and thymoma-associated MG (TAMG), notably autoantibodies against acetylcholine receptors, titin, ryanodine receptor, type I interferons or IL-12. This is why we checked LOMG patients for two further peculiar features of TAMG - its associations with the CTLA4(high/gain-of-function) +49A/A genotype and with increased thymic export of naïve T cells into the blood, possibly after defective negative selection in AIRE-deficient thymomas. We analyzed genomic DNA from 116 Caucasian LOMG patients for CTLA4 alleles by PCR/restriction fragment length polymorphism, and blood mononuclear cells for recent thymic emigrants by quantitative PCR for T cell receptor excision circles. In sharp contrast with TAMG, we now find that: i) CTLA4(low) +49G(+) genotypes were more frequent (p = 0.0029) among the 69 LOMG patients with age at onset ≥60 years compared with 172 healthy controls; ii) thymic export of naïve T cells from the non-neoplastic thymuses of 36 LOMG patients was lower (p = 0.0058) at diagnosis than in 77 age-matched controls. These new findings are important because they suggest distinct initiating mechanisms in TAMG and LOMG and hint at aberrant immuno-regulation in the periphery in LOMG. We therefore propose alternate defects in central thymic or peripheral tolerance induction in TAMG and LOMG converging on similar final outcomes. In addition, our data support a 60-year-threshold for onset of 'true LOMG' and an LOMG/early-onset MG overlapping group of patients between 40 and 60.

The auto-antigen repertoire in myasthenia gravis

Myasthenia Gravis (MG) is an antibody-mediated autoimmune disorder affecting the postsynaptic membrane of the neuromuscular junction (NMJ). MG is characterized by an impaired signal transmission between the motor neuron and the skeletal muscle cell, caused by auto-antibodies directed against NMJ proteins. The auto-antibodies target the nicotinic acetylcholine receptor (nAChR) in about 90% of MG patients. In approximately 5% of MG patients, the muscle specific kinase (MuSK) is the auto-antigen. In the remaining 5% of MG patients, however, antibodies against the nAChR or MuSK are not detectable (idiopathic MG, iMG). Although only the anti-nAChR and anti-MuSK auto-antibodies have been demonstrated to be pathogenic, several other antibodies recognizing self-antigens can also be found in MG patients. Various auto-antibodies associated with thymic abnormalities have been reported, as well as many non-MG-specific auto-antibodies. However, their contribution to the cause, pathology and severity of the disease is still poorly understood. Here, we comprehensively review the reported auto-antibodies in MG patients and discuss their role in the pathology of this autoimmune disease.

Titin and ryanodine receptor antibodies and neuromuscular involvement in myasthenia gravis

Of sera of patients with myasthenia gravis (MG), 30% contains titin and ryanodine receptor (RyR) antibodies. In early-onset MG (i.e., before 50 years of age), the presence of titin antibodies strongly suggests a thymoma. Late-onset MG comprises one MG group, characterized by a broad antimuscle immune response, including both titin and the acetylcholine receptor (AChR). Another group is preferentially associated with the HLA-A3, B7 and DRw2 antigens, representing a delayed early-onset of the disease that has a selective AChR immune response. The presence of titin and RyR antibodies is associated with more severe disease. Titin antibodies may, in some patients with rippling muscle disease, affect the contractile machinery of myofibers, thereby affecting their mechanical sensitivity. It is not known whether this occurs in MG. RyR antibodies may impair excitation–contraction coupling and contribute to muscle weakness in MG patients. Titin antibodies may serve as tumor markers in early-onset MG patients.

Seronegative myasthenia gravis: disease severity and prognosis

Around 10-20% of myasthenia gravis (MG) patients do not have acetylcholine receptor (AChR) antibodies (seronegative), of whom some have antibodies to a membrane-linked muscle specific kinase (MuSK). To examine MG severity and long-term prognosis in seronegative MG compared with seropositive MG, and to look specifically at anti-AChR antibody negative and anti-MuSK antibody negative patients. Seventeen consecutive seronegative non-thymomatous MG patients and 34 age and sex matched contemporary seropositive non-thymomatous MG controls were included in a retrospective follow-up study for a total period of 40 years. Clinical criteria were assessed each year, and muscle antibodies were assayed. There was no difference in MG severity between seronegative and seropositive MG. However, when thymectomized patients were excluded from the study at the year of thymectomy, seropositive MG patients had more severe course than seronegative (P < 0.001). One seropositive patient died from MG related respiratory insufficiency. The need for thymectomy in seronegative MG was lower than in seropositive MG. None of the seronegative patients had MuSK antibodies. This study shows that the presence of AChR antibodies in MG patients correlates with a more severe MG. With proper treatment, especially early thymectomy for seropositive MG, the outcome and long-term prognosis is good in patients with and without AChR antibodies.

Myasthenia gravis: clinical, immunological, and therapeutic advances

We give an update on clinical, immunological, and therapeutic advances in the field of myasthenia gravis, including a summary of suggested therapeutic recommendations.

Disease severity and outcome in thymoma myasthenia gravis: a long-term observation study

Thymomas occurring in myasthenia gravis (MG) are usually of the cortical subtype and are usually treated by thymectomy. However, the factors that influence MG outcome in thymoma MG patients are not known. In a long-term study, MG severity and treatment was observed in 24 thymoma and 24 non-thymoma MG patients for up to 30 years, and the occurrence of muscle autoantibodies was assayed. The rate of complete stable remission was low and did not differ between the two groups. There was no significant difference in MG severity between thymoma and non-thymoma MG patients at any time during the study. Titin and ryanodine receptor (RyR) antibody occurrence was significantly higher in thymoma MG patients. Four thymoma (all titin and RyR antibody positive) and two non-thymoma (both titin and one RyR antibody positive) MG patients died from MG-related respiratory insufficiency. Seventy percent of thymoma and 75% of non-thymoma MG patients were treated with immunosuppressive drugs. The number of patients who received plasmapheresis did not differ in those who were treated because of acute MG deterioration, irrespective of planned surgery.This study shows equal MG severity and outcome in thymoma and non-thymoma MG, but the presence of RyR antibodies in thymoma MG and titin/RyR in non-thymoma MG indicates a less favorable prognosis.

Thymectomy and antimuscle antibodies in nonthymomatous myasthenia gravis

The clinical effect of thymectomy in early- and late-onset myasthenia gravis (MG) and the correlation to MG severity, pharmacological treatment, and antimuscle antibodies were examined in two series of consecutive acetylcholine receptor (AChR) antibody-positive nonthymoma MG patients. The results indicate a benefit of thymectomy in early-onset MG, but no obvious clinical benefit in late-onset MG. The presence of muscle autoantibodies did not influence the outcome of thymectomy in early-onset MG. In late-onset MG, improvement is least likely in patients with titin and/or RyR antibodies. Thymectomy should always be considered shortly after MG onset in early-onset MG patients and might only be considered in late-onset patients who have early-onset-like immunological characteristics.

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.

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.

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.