Next-Generation Sequencing Identifies Extended HLA Class I and II Haplotypes Associated With Early-Onset and Late-Onset Myasthenia Gravis in Italian, Norwegian, and Swedish Populations

Genetic predisposition to autoimmune encephalitis and paraneoplastic neurological syndromes

PURPOSE OF REVIEW

We summarize the recent discoveries on genetic predisposition to autoimmune encephalitis and paraneoplastic neurological syndromes (PNS), emphasizing clinical and pathophysiological implications.

RECENT FINDINGS

The human leukocyte antigen (HLA) is the most studied genetic factor in autoimmune encephalitis and PNS. The HLA haplotype 8.1, which is widely known to be related to systemic autoimmunity, has been only weakly associated with a few types of autoimmune encephalitis and PNS. However, the strongest and most specific associations have been reported in a subgroup of autoimmune encephalitis that comprises antileucine-rich glioma-inactivated 1 (LGI1) limbic encephalitis, associated with DRB1∗07 : 01 , anticontactin-associated protein-like 2 (CASPR2) limbic encephalitis, associated with DRB1∗11 : 01 , and anti-IgLON5 disease, associated with DRB1∗10 : 01∼DQA1∗01∼DQB1∗05 . Non-HLA genes have been poorly investigated so far in autoimmune encephalitis, mainly in those lacking HLA associations such as anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis, with only a few genome-wide association studies (GWAS) reporting equivocal results principally limited by small sample size.

SUMMARY

Genetic predisposition seems to be driven mostly by HLA in a group of autoimmune encephalitis characterized by being nonparaneoplastic and having predominantly IgG4 autoantibodies. The contribution of non-HLA genes, especially in those diseases lacking known or strong HLA associations, will require large cohorts enabling GWAS to be powerful enough to render meaningful results.

Myasthenia gravis and congenital myasthenic syndromes

Myasthenia gravis is an autoimmune disorder caused by antibodies against elements in the postsynaptic membrane at the neuromuscular junction, which leads to muscle weakness. Congenital myasthenic syndromes are rare and caused by mutations affecting pre- or postsynaptic function at the neuromuscular synapse and resulting in muscle weakness. MG has a prevalence of 150-250 and an annual incidence of 8-10 individuals per million. The majority has disease onset after age 50 years. Juvenile MG with onset in early childhood is more common in East Asia. MG is subgrouped according to type of pathogenic autoantibodies, age of onset, thymus pathology, and generalization of muscle weakness. More than 80% have antibodies against the acetylcholine receptor. The remaining have antibodies against MuSK, LRP4, or postsynaptic membrane antigens not yet identified. A thymoma is present in 10% of MG patients, and more than one-third of thymoma patients develop MG as a paraneoplastic condition. Immunosuppressive drug therapy, thymectomy, and symptomatic drug therapy with acetylcholine esterase inhibitors represent cornerstones in the treatment. The prognosis is good, with the majority of patients having mild or moderate symptoms only. Most congenital myasthenic syndromes are due to dysfunction in the postsynaptic membrane. Symptom debut is in early life. Symptomatic drug treatment has sometimes a positive effect.

Identification of genetic risk loci and prioritization of genes and pathways for myasthenia gravis: a genome-wide association study

Myasthenia gravis is a chronic autoimmune disease characterized by autoantibody-mediated interference of signal transmission across the neuromuscular junction. We performed a genome-wide association study (GWAS) involving 1,873 patients diagnosed with acetylcholine receptor antibody-positive myasthenia gravis and 36,370 healthy individuals to identify disease-associated genetic risk loci. Replication of the discovered loci was attempted in an independent cohort from the UK Biobank. We also performed a transcriptome-wide association study (TWAS) using expression data from skeletal muscle, whole blood, and tibial nerve to test the effects of disease-associated polymorphisms on gene expression. We discovered two signals in the genes encoding acetylcholine receptor subunits that are the most common antigenic target of the autoantibodies: a GWAS signal within the cholinergic receptor nicotinic alpha 1 subunit (CHRNA1) gene and a TWAS association with the cholinergic receptor nicotinic beta 1 subunit (CHRNB1) gene in normal skeletal muscle. Two other loci were discovered on 10p14 and 11q21, and the previous association signals at PTPN22, HLA-DQA1/HLA-B, and TNFRSF11A were confirmed. Subgroup analyses demonstrate that early- and late-onset cases have different genetic risk factors. Genetic correlation analysis confirmed a genetic link between myasthenia gravis and other autoimmune diseases, such as hypothyroidism, rheumatoid arthritis, multiple sclerosis, and type 1 diabetes. Finally, we applied Priority Index analysis to identify potentially druggable genes/proteins and pathways. This study provides insight into the genetic architecture underlying myasthenia gravis and demonstrates that genetic factors within the loci encoding acetylcholine receptor subunits contribute to its pathogenesis.