A novel fast-channel myasthenia caused by mutation in β subunit of AChR reveals subunit-specific contribution of the intracellular M1-M2 linker to channel gating

Impaired gating of γ- and ε-AChR respectively causes Escobar syndrome and fast-channel myasthenia

OBJECTIVE

To 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).

METHODS

Whole exome sequencing, α-bungarotoxin binding assay, single channel patch-clamp recordings, and maximum likelihood analysis of channel kinetics.

RESULTS

We 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.

INTERPRETATION

Similar 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.

Clinical and Pathologic Features of Congenital Myasthenic Syndromes Caused by 35 Genes-A Comprehensive Review

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.

Unmasking coupling between channel gating and ion permeation in the muscle nicotinic receptor

Whether ion channel gating is independent of ion permeation has been an enduring, unresolved question. Here, applying single channel recording to the archetypal muscle nicotinic receptor, we unmask coupling between channel gating and ion permeation by structural perturbation of a conserved intramembrane salt bridge. A charge-neutralizing mutation suppresses channel gating, reduces unitary current amplitude, and increases fluctuations of the open channel current. Power spectra of the current fluctuations exhibit low- and high-frequency Lorentzian components, which increase in charge-neutralized mutant receptors. After aligning channel openings and closings at the time of transition, the average unitary current exhibits asymmetric relaxations just after channel opening and before channel closing. A theory in which structural motions contribute jointly to channel gating and ion conduction describes both the power spectrum and the current relaxations. Coupling manifests as a transient increase in the open channel current upon channel opening and a decrease upon channel closing.

CHRNB1-associated congenital myasthenia syndrome: Expanding the clinical spectrum

CHRNB1 encodes the β subunit of the acetylcholine receptor (AChR) at the neuromuscular junction. Inherited defects in the neuromuscular junction can lead to congenital myasthenia syndrome (CMS), a clinically and genetically heterogeneous group of disorders which includes fetal akinesia deformation sequence (FADS) on the severe end of the spectrum. Here, we report two unrelated families with biallelic CHRNB1 variants, and in each family, one child presented with lethal FADS. We contrast the diagnostic odysseys in the two families, one of which lasted 16 years while the other, utilizing rapid exome sequencing, led to specific treatment in the first 2 weeks of life. Furthermore, we note that CHRNB1 variants may be under-recognized because in both families, one of the variants is a single exon deletion that has been previously described but may not easily be detected in clinically available genetic testing.