Immunoglobulin allotypes in caucasian and Chinese myasthenia gravis: differences from Japanese patients

Low conversion rate of ocular to generalized myasthenia gravis in Singapore

INTRODUCTION

Ocular myasthenia gravis (OMG) is a common condition of the neuromuscular junction that may convert to generalized myasthenia gravis (GMG). Our aim in this study was to determine the conversion rate and predictive factors for generalization in OMG, in an Asian population.

METHODS

The investigation consisted of a retrospective study of OMG patients with a minimum 2 years of follow-up.

RESULTS

Among 191 patients with OMG, 155 had the minimum 2-year follow-up. The conversion rate at median follow-up (40.8 months) was 10.6% (95% confidence interval 7.9%-13.3%), and at the 2-year follow-up it was 7.7% (95% confidence interval 5.6%-9.8%). At baseline, the predictive factors for generalization were positive acetylcholine receptor antibodies (hazard ratio 3.71, P = 0.024), positive repetitive nerve stimulation (RNS) studies (hazard ratio 4.42, P = 0.003), and presence of radiologically presumed or pathologically confirmed thymoma (hazard ratio 3.10, P = 0.013).

DISCUSSION

The conversion rate of OMG to GMG in Asian patients is low, as predicted by presence of acetylcholine receptor antibodies, presence of thymoma, and positive RNS studies. Muscle Nerve 57: 756-760, 2018.

Clinical and experimental features of MuSK antibody positive MG in Japan

We investigated the presence of antibodies (Abs) against muscle-specific tyrosine kinase (MuSK) in Japanese myasthenia gravis (MG) patients. MuSK Abs were found in 23 (27%) of 85 generalized seronegative MG (SNMG) patients but not in any of the ocular MG patients. MuSK Ab-positive patients were characterized as having female dominance (M:F, 5:18), age range at onset 18 to 72 (median 45) years old, and prominent oculobulbar symptoms (100%) with neck (57%) or respiratory (35%) muscle weakness. Limb muscle weakness was comparatively less severe (52%), thymoma absent. Most patients had good responses to simple plasma exchange and steroid therapy. MuSK IgG from all 18 patients was exclusively the IgG 4 subclass and bound mainly with the MuSK Ig 1-2 domain. Serial studies of 12 individuals showed a close correlation between the variation in MuSK Ab titers and MG clinical severity (P = 0.01 by Kruskal-Wallis). MuSK Ab titers were sharply decreased in patients who had a good response to early steroid therapy or simple plasma exchange, but there was no change, or a rapid increase on exacerbation after thymectomy. Measurement of MuSK Ab titers aids in the diagnosis of MG and the monitoring of clinical courses after treatment.

Genetics of autoimmune myasthenia gravis, a model for antibody-mediated autoimmunity in man

Autoimmune myasthenia gravis is an antibody-mediated autoimmune disease of the neuromuscular junction aimed at the muscle acetylcholine receptor. Recent genetic analyses focusing on a nonclass II HLA-linked locus, MYAS1, and on one of the genes encoding the AChR self-antigen, CHRNA1, are discussed.

Myasthenia gravis with thymoma is more common in the Maori and Pacific Island populations in New Zealand

BACKGROUND

The association of myasthenia gravis (MG) with thymoma is well recognized. Our clinical impression has been that MG associated with thymoma may be more common in patients of Polynesian descent than in other races.

AIM

To determine the influence of ethnicity on the association of MG with thymoma in our population.

METHOD

Review of all cases of thymectomy performed at Greenlane Hospital in Auckland for the 20-year period from June 1978 to June 1998.

RESULTS

There were 103 thymectomies performed in the study period. Fifty-five thymomas were identified, 15 in subjects of Maori or Pacific Island ethnicity and 40 in subjects of other races, predominantly Caucasian. Ten of 15 Maori or Pacific Island subjects with thymoma had MG (67%), compared with 15 of 40 subjects of other races (37.5%, P = 0.05). The mean age of Maori or Pacific Island subjects with thymoma and MG was 42.5 years, compared with 56.3 years in subjects from other races (P = 0.06). All five Maori and Pacific Island subjects with invasive thymoma had MG, whereas only four of 15 subjects (27%) from other races with invasive tumours had MG (P < 0.01). The overall incidence of thymoma and the proportion of thymomas that were invasive did not differ between the ethnic groups.

CONCLUSIONS

Myasthenia gravis with thymoma occurs more frequently among Maori or Pacific Island people than in other racial groups in our population. This is due to an increase in the proportion of cases with thymoma who have MG in this group, while the overall frequency of cases of thymoma is similar between groups. MG with thymoma in the Maori or Pacific Island populations also presents at a younger age and is more often associated with tumour invasion.

Autoimmunity to ryanodine receptor and titin in myasthenia gravis is associated with GM allotypes

Myasthenia gravis (MG) is mediated by autoantibodies against the acetylcholine receptor at the muscle endplate. Some MG patients have in addition antibodies (Ab) to the skeletal muscle proteins ryanodine receptor (RyR) and titin. We have examined GM and KM allotypes, RyR and titin Ab in 44 MG patients (37 thymoma patients and 7 non-thymoma, late-onset patients) and 292 non-MG controls to see if GM/KM allotypes associate with differences in autoantibody production. All patients had titin Ab, and 15 thymoma patients had also RyR Ab. The phenotype GM 1, 2, 3 23 5, 21 was significantly increased in the patients with titin Ab compared with the non-MG controls (chi2 = 4.93, p < 0.05). Thymoma patients with RyR Ab had a higher frequency of the GM 3 23 5 phenotype compared with RyR Ab negative patients and controls (chi2 = 7.1, p < 0.05). KM allotypes did not differ between RyR Ab positive or titin Ab positive patients and controls. GM phenotypes may thus be associated with an autoimmune response against the muscle proteins titin and RyR in MG patients.

Ocular myasthenia: a protean disorder

Ocular myasthenia is a localized form of myasthenia clinically involving only the extraocular, levator palpebrae superioris, and/or orbicularis oculi muscles. Ocular manifestations can masquerade as a variety of ocular motility disorders, including cranial nerve and gaze palsies. A history of variable and fatiguable muscle weakness suggests this diagnosis, which may be confirmed by the edrophonium (Tensilon) test and acetylcholine receptor antibody titer. Anticholinesterases, corticosteroids and other immunosuppressive agents, and other therapeutic modalities, including thymectomy and plasmapheresis, are used in treatment. As the pathophysiology of myasthenia has been elucidated in recent years, newer treatment strategies have evolved, resulting in a much more favorable prognosis than several decades ago. This review provides historical background, pathophysiology, immuno-genetics, diagnostic testing, and treatment options for ocular myasthenia, as well as a discussion of drug-induced myasthenic syndromes.

Association of Km3 allotype with increased serum levels of autoantibodies against muscle acetylcholine receptor in myasthenia gravis

Association of immunoglobulin allotypes with myasthenia gravis was examined in a set of 84 patients presenting autoantibodies against muscle acetylcholine receptor (AChR). Km and G1m(f)/G1m(z) allotypes were determined by PCR-amplification followed by hybridization with allele-specific oligonucleotides. The Km3 kappa light chain allotype was found to be associated with significantly increased serum levels of anti-AChR autoantibodies in myasthenic patients (P = 0.014). We hypothesize that an allelic polymorphism of a regulatory (enhancer) sequence closely linked to the C kappa gene segment could account for our finding.

Identification by genomic typing of non-DR3 HLA class II genes associated with myasthenia gravis

HLA association with myasthenia gravis (MG) has been studied in a series of 114 patients using class I and class II genotyping after PCR amplification. Positive association was found with DR3, particularly in women (RR = 2.6) and in early MG onset (RR = 3.4). DRB1, DRB3, DQB1, DQA1 and B (B8 and B18) genotyping revealed that the association was predominantly with the B8 DRB1*03 DRB3*0101 DQB1*0201 DQA1*0501 ancestral haplotype. This haplotype frequency was also increased in patients with thymic hyperplasia (RR = 3.5) and was greatly reduced in patients with thymoma (RR = 0.35). Sixteen out of 48 patients carrying this 8.1 ancestral haplotype showed absence of B8 (n = 4) or of DR3 (n = 12). HLA class II genotyping further revealed the existence of two other significant associations. MG was positively associated with the DQB1*0604 allele (RR = 3.4), particularly in patients with thymoma (RR = 5.7). Furthermore, the disease was negatively associated with DR1 in females (RR = 0.32). These data suggest that MG is placed under the control of at least three distinct genes: (1) a class II predisposing gene in the 8.1 ancestral haplotype; (2) a thymoma-associated class II allele on the DQB1*0604 haplotype; and (3) a protective allele DR1.

Association of HLA-Bw46DR9 combination with juvenile myasthenia gravis in Chinese

One hundred and fifty two Chinese patients with myasthenia gravis in Taiwan were investigated for HLA-A, B, C and DR/DQ typing. HLA-Bw46 and DR9 frequencies were significantly increased in patients compared with the control group, and there was a decrease in DR3. Further analysis between different subgroups of patients showed Bw46 and DR9 were more significantly increased in the juvenile group than in the adult group. No single HLA allele was associated with either clinical type or thymic pathology, but there was an excess of BW46DR9 combination in both juvenile and ocular type patients. The Chinese population with myasthenia gravis is characterised by earlier age at onset, more ocular forms and less clinically severe illness than in whites, and these characteristics indicate a special subgroup that correlates with the strong Bw46DR9 association.

The contribution of non-MHC genes to susceptibility to autoimmune diseases

Genetic studies of experimental models of autoimmune diseases, including systemic lupus-like syndromes and organ-specific autoimmunity, provide major information on genetic control of autoimmune diseases. In addition to genes known to be linked to the major histocompatibility complex (MHC), these studies point to multiple genes located outside the MHC that influence the onset and the progression of autoimmune diseases. Identification of these genes and of their interrelationships is now a major task that will be facilitated by recent progress in molecular biology and gene mapping. Among candidate genes, antigen-receptor genes (i.e., immunoglobulin- and T-cell receptor genes) most likely contribute an important part of the autoimmune susceptibility in several of these animal models. Available linkage data suggest a similar involvement of these antigen-receptor genes in several human autoimmune diseases. In addition to a better understanding of pathogenic mechanisms associated with autoimmunity, the knowledge of these disease-predisposing genes is expected to permit a better classification of often complex syndromes as well as the design of new treatments.

Immunoglobulin allotypes in myasthenia gravis patients with a thymoma

Gm and Km allotypes were examined in 29 myasthenia gravis patients with a thymoma and non-receptor skeletal muscle antibodies. The frequency of the phenotype Gm1,2,3;23;5,21 was significantly higher in the patients than in 292 healthy controls (P less than 0.01). Km allotype frequencies did not differ in patients and controls.

Linkage disequilibrium study of RFLPs detected at the human muscle nicotinic acetylcholine receptor subunit genes

We screened DNA from unrelated individuals for RFLPs in the muscle nicotinic acetylcholine receptor (AcChoR) genes. These RFLP markers can be used for genetic linkage and association studies to test the hypothesis that receptor structure or regulation is involved in the development of myasthenia gravis (MG). The cDNAs from four subunits (alpha, beta, gamma, and delta) of the murine muscle AcChoR were used as probes to identify RFLPs in the homologous human genes. Digestion of DNA from 15 unrelated individuals with a set of 10 restriction enzymes revealed 11 RFLPs. At least one RFLP was found for each subunit gene. Eight RFLPs were found at the linked gamma and delta gene loci, six with minor allele frequencies greater than 15%, making that linkage group a very informative marker locus (PIC = .72). PIC values were calculated for the RFLPs from allele and haplotype frequency estimates obtained from a population sample of 53 individuals. The delta gene was assigned by in situ hybridization to region q31----q34 of chromosome 2. All pairs of RFLPs were analyzed for linkage disequilibrium. Of the 16 pairs of RFLPs from the same gene or from the linked gamma and delta genes, 13 pairs showed evidence of disequilibrium that was significant, with P less than .05. The implications of these results are discussed.