Shen, Xin-Ming

AbstractObjectiveTo dissect the kinetic defects of acetylcholine receptor (AChR) γ subunit variant in an incomplete form of the Escobar syndrome without pterygium and compare it with those of a variant of corresponding residue in the AChR ε subunit in a congenital myasthenic syndrome (CMS).MethodsWhole exome sequencing, α‐bungarotoxin binding assay, single channel patch‐clamp recordings, and maximum likelihood analysis of channel kinetics.ResultsWe identified compound heterozygous variants in AChR γ and ε subunits in three Escobar syndrome (1–3) and three CMS patients (4–6), respectively. Each Escobar syndrome patient carries γP121R along with γV221Afs*44 in patients 1 and 2, and γY63* in patient 3. Three CMS patients share εP121T along with εR20W, εG‐8R, and εY15H in patients 4, 5, and 6, respectively. Surface expressions of γP121R‐ and εP121T‐AChR were 80% and 138% of the corresponding wild‐type AChR, whereas εR20W, εG‐8R, and εY15H reduced receptor expression to 27%, 35%, and 30% of wild‐type εAChR, respectively. γV221Afs*44 and γY63* are null variants. Thus, γP121R and εP121T determine the phenotype. γP121R and εP121T shorten channel opening burst duration to 28% and 18% of corresponding wild‐type AChR by reducing the channel gating equilibrium constant 44‐ and 63‐fold, respectively.InterpretationSimilar impairment of channel gating efficiency of a corresponding P121 residue in the acetylcholine‐binding site of the AChR γ and ε subunits causes Escobar syndrome without pterygium and fast‐channel CMS, respectively, suggesting that therapy for the fast‐channel CMS will benefit Escobar syndrome.

AbstractObjectiveUse of tacrolimus in mild to moderate myasthenia gravis (MG) is generally limited to glucocorticoid‐refractory cases; the advantage of mono‐tacrolimus over mono‐glucocorticoids is unknown.MethodsWe included mild to moderate MG patients treated with mono‐tacrolimus (mono‐TAC) or mono‐glucocorticoids (mono‐GC). The correlation between the immunotherapy options and the treatment efficacy and side effects were examined in 1:1 propensity‐score matching. The main outcome was time to minimal manifestations status or better (MMS or better). Secondary outcomes include time to relapse, the mean changes in Myasthenia Gravis‐specific Activities of Daily Living (MG‐ADL) scores and the rate of adverse events.ResultsBaseline characteristics showed no difference between matched groups (49 matched pairs). There were no differences in median time to MMS or better between the mono‐TAC group and mono‐GC group (5.1 vs. 2.8 months: unadjusted hazard ratio [HR], 0.73; 95% CI, 0.46–1.16; p = 0.180), as well as in median time to relapse (data unavailable for the mono‐TAC group since 44 of 49 [89.8%] participants remained in MMS or better; 39.7 months in mono‐GC group: unadjusted HR, 0.67; 95% CI, 0.23–1.97; p = 0.464). Changes in MG‐ADL scores between the two groups were similar (mean differences, 0.3; 95% CI, −0.4 to 1.0; p = 0.462). The rate of adverse events was lower in the mono‐TAC group compared to the mono‐GC group (24.5% vs. 55.1%, p = 0.002).InterpretationMono‐tacrolimus performs superior tolerability with non‐inferior efficacy compared to mono‐glucocorticoids in mild to moderate myasthenia gravis patients who refuse or have a contraindication to glucocorticoids.

Congenital myasthenic syndromes (CMS) are a heterogeneous group of disorders characterized by impaired neuromuscular signal transmission due to germline pathogenic variants in genes expressed at the neuromuscular junction (NMJ). A total of 35 genes have been reported in CMS (AGRN, ALG14, ALG2, CHAT, CHD8, CHRNA1, CHRNB1, CHRND, CHRNE, CHRNG, COL13A1, COLQ, DOK7, DPAGT1, GFPT1, GMPPB, LAMA5, LAMB2, LRP4, MUSK, MYO9A, PLEC, PREPL, PURA, RAPSN, RPH3A, SCN4A, SLC18A3, SLC25A1, SLC5A7, SNAP25, SYT2, TOR1AIP1, UNC13A, VAMP1). The 35 genes can be classified into 14 groups according to the pathomechanical, clinical, and therapeutic features of CMS patients. Measurement of compound muscle action potentials elicited by repetitive nerve stimulation is required to diagnose CMS. Clinical and electrophysiological features are not sufficient to identify a defective molecule, and genetic studies are always required for accurate diagnosis. From a pharmacological point of view, cholinesterase inhibitors are effective in most groups of CMS, but are contraindicated in some groups of CMS. Similarly, ephedrine, salbutamol (albuterol), amifampridine are effective in most but not all groups of CMS. This review extensively covers pathomechanical and clinical features of CMS by citing 442 relevant articles.

AbstractObjectiveTo characterize the molecular and phenotypic basis of a severe slow‐channel congenital myasthenic syndrome (SCCMS).MethodsIntracellular and single‐channel recordings from patient endplates; alpha‐bungarotoxin binding studies; direct sequencing of AChR genes; microsatellite analysis; kinetic analysis of AChR activation; homology modeling of adult human AChR structure.ResultsAmong 24 variants reported to cause SCCMS only two appear in the AChR δ‐subunit. We here report a 16‐year‐old patient harboring a novel δL273F mutation (δL294F in HGVS nomenclature) in the second transmembrane domain (M2) of the AChR δ subunit. Kinetic analyses with ACh and the weak agonist choline indicate that δL273F prolongs the channel opening bursts 9.4‐fold due to a 75‐fold increase in channel gating efficiency, whereas a previously identified εL269F mutation (εL289F in HGVS nomenclature) at an equivalent location in the AChR ε‐subunit prolongs channel opening bursts 4.4‐fold due to a 30‐fold increase in gating efficiency. Structural modeling of AChR predicts that inter‐helical hydrophobic interactions between the mutant residue in the δ and ε subunit and nearby M2 domain residues in neighboring α subunits contribute to structural stability of the open relative to the closed channel states.InterpretationThe greater increase in gating efficiency by δL273F than by εL269F explains why δL273F has more severe clinical effects. Both δL273F and εL269F impair channel gating by disrupting hydrophobic interactions with neighboring α‐subunits. Differences in the extent of impairment of channel gating in δ and ε mutant receptors suggest unequal contributions of ε/α and δ/α subunit pairs to gating efficiency.