Familial autoimmune myasthenia gravis: report of four families

Whole-exome sequencing and human leukocyte antigen analysis in familial myasthenia gravis with thymoma: Case report and literature review

Myasthenia gravis (MG) is an autoimmune disease characterized by impaired neurotransmission at the neuromuscular junction. MG is generally non-inherited but is rarely inherited. Here, we report two patients with MG in the same pedigree: a 62-year-old Japanese man and his 46-year-old daughter who were positive for anti-acetylcholine receptor antibodies and had thymoma. We performed whole-exome sequencing (WES) and human leukocyte antigen (HLA) analyses to investigate the genetic contribution to familial onset. WES analysis of both patients showed no known variations in candidate genes for familial MG, and HLA analysis failed to detect HLA haplotypes seen in early-onset and late-onset MG. These findings suggest the presence of an unknown genetic background. Previous genetic studies on familial MG have identified ENOX1 and IFNGR1 as candidate genes in patients without thymoma, whereas no studies have identified candidate genes in patients with thymoma. To explore causative genes, it may be necessary to consider whether the genetic background differs between patients with and without thymoma in familial autoimmune MG.

Case Series: A case of familial thymomatous myasthenia gravis in a family of three male brothers

Myasthenia gravis (MG) is an autoimmune disease that occurs as a consequence of anti-acetylcholine (Ach) antibodies specifically targeting postsynaptic Ach receptors (AchR). This leads to the evolution of the classic symptoms of the disease, which range from mild symptoms of diplopia, muscle fatigue with repetitive movement up to severe affection of the respiratory muscle. The disease can occur as an isolated finding or co-exist with a concomitant thymic tumor or hyperplasia. Careful diagnosis is crucial for the development of the management plan. Nearly 10-15% of MG cases coexist with a thymic pathology and in these cases, surgical resection leads to the resolution of symptoms. Although thymomatous MG occurrence is non-heritable, its polygenic nature accounts for its rare familial variant. In this case, we report a family of three brothers with familial thymomatous MG who underwent thymectomy and improved after thymic surgical resection. Myasthenia gravis can occur as an isolated finding or as an association of thymic pathology. Careful discrimination between the two should be made for the elaboration of a management plan. Familial variant thymomatous myasthenia gravis is exceedingly rare. A familial survey is crucial for its management.

A candidate gene for autoimmune myasthenia gravis

OBJECTIVE

We sought to identify a causative mutation in a previously reported kindred with parental consanguinity and 5 of 10 siblings with adult-onset autoimmune myasthenia gravis.

METHODS

We performed genome-wide homozygosity mapping, and sequenced all known genes in the one region of extended homozygosity. Quantitative and allele-specific reverse transcriptase PCR (RT-PCR) were performed on a candidate gene to determine the RNA expression level in affected siblings and controls and the relative abundance of the wild-type and mutant alleles in a heterozygote.

RESULTS

A region of shared homozygosity at chromosome 13q13.3-13q14.11 was found in 4 affected siblings and 1 unaffected sibling. A homozygous single nucleotide variant was found in the 3'-untranslated region of the ecto-NADH oxidase 1 gene (ENOX1). No other variants likely to be pathogenic were found in genes in this region or elsewhere. The ENOX1 sequence variant was not found in 764 controls. Quantitative RT-PCR showed that expression of ENOX1 decreased to about 20% of normal levels in lymphoblastoid cells from individuals homozygous for the variant and to about 50% in 2 unaffected heterozygotes. Allele-specific RT-PCR showed a 55%-60% reduction in the level of the variant transcript in heterozygous cells due to reduced mRNA stability.

CONCLUSION

These results indicate that this sequence variant in ENOX1 may contribute to the familial autoimmune myasthenia in these patients.

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.

Clinical evaluation and management of myasthenia gravis

Myasthenia gravis (MG) is a syndrome of fluctuating skeletal muscle weakness that worsens with use and improves with rest. Eye, facial, oropharyngeal, axial, and limb muscles may be involved in varying combinations and degrees of severity. Its etiology is heterogeneous, divided initially between those rare congenital myasthenic syndromes, which are genetic, and the bulk of MG, which is acquired and autoimmune. The autoimmune conditions are divided in turn between those that possess measurable serum acetylcholine receptor (AChR) antibodies and a smaller group that does not. The latter group includes those MG patients who have serum antibodies to muscle-specific tyrosine kinase (MuSK). Therapeutic considerations differ for early-onset MG, late-onset MG, and MG associated with the presence of a thymoma. Most MG patients can be treated effectively, but there is still a need for more specific immunological approaches.

Familial autoimmune myastenia gravis: four patients involving three generations

BACKGROUND

Familial autoimmune myasthenia gravis (MG) is rare, although a genetic role for the development of autoimmune MG is suggested by concordance in monozygotic twins and the increased frequency of other autoimmune diseases in family members of myasthenics.

METHODS

A patient with a family history of MG was evaluated in hospital. Relatives were interviewed and medical records examined for details regarding the diagnosis of MG in three other family members.

RESULTS

The index case first experienced symptoms of MG at age 75 years. She developed generalized MG and required corticosteroids and immunosuppressive therapy to control her disease. Her father developed predominantly bulbar symptoms of MG at age 75 years. He died of complications experienced following a gastrostomy placed for continued difficulty swallowing. His brother developed similar symptoms of MG in his early 60s and died shortly after thymectomy. A 46-year-old nephew of the index case is also beginning to exhibit signs of generalized MG. Acetylcholine receptor antibodies were strongly positive in the index case and her nephew. (The assay was not available for her father and uncle).

CONCLUSIONS

Four individuals in three successive generations had diagnoses of autoimmune MG. Study of familial cases such as these may clarify the contribution of genetic factors to the development of this disease.

Juvenile myasthenia gravis with prepubertal onset

Juvenile myasthenia gravis (JMG) with prepubertal onset is an uncommon disease. We studied 19 patients with age at onset ranging from 1.5 to 9.2 years and compared their clinical characteristics and response to therapy with 114 cases with MG onset after the prepubertal age, up to 20 years. Neither sex prevalence nor autoimmune diseases other than MG were found in younger patients. Although ocular myasthenia was more frequent than in later-onset JMG, children with generalized symptoms were often severely affected and respiratory involvement was present in 8/19 patients. Anti-acetylcholine receptor antibodies were detected at a lower rate and, in contrast with results in older patients, seronegativity was more frequent among children with generalized disease. Three out of six patients with onset before the age of five showed spontaneous remission. Nine prepubertal patients underwent thymectomy and, as most of them also received immunosuppressive therapy, the influence of surgery on disease outcome remains unclear; in no case was thymoma found. This is in contrast to the good results after thymectomy and the presence of thymoma in the later-onset group. Eleven patients in the prepubertal series were treated with immunosuppressive therapy. At the end of follow-up, most patients were in good condition. The frequency of immunosuppressive therapy and the rate of good therapeutic results did not differ from those observed in older patients.