Congenital myasthenic syndromes in Turkey: Clinical clues and prognosis with long term follow-up.

Congenital myasthenic syndromes (CMS) are a group of hereditary disorders affecting the neuromuscular junction. Here, we present clinical, electrophysiological and genetic findings of 69 patients from 51 unrelated kinships from Turkey. Genetic tests of 60 patients were performed at Mayo Clinic.

Mutations causing congenital myasthenia reveal principal coupling pathway in the acetylcholine receptor ε-subunit.

We identify 2 homozygous mutations in the ε-subunit of the muscle acetylcholine receptor (AChR) in 3 patients with severe congenital myasthenia: εR218W in the pre-M1 region in 2 patients and εE184K in the β8-β9 linker in 1 patient. Arg218 is conserved in all eukaryotic members of the Cys-loop receptor superfamily, while Glu184 is conserved in the α-, δ-, and ε-subunits of AChRs from all species. εR218W reduces channel gating efficiency 338-fold and AChR expression on the cell surface 5-fold, whereas εE184K reduces channel gating efficiency 11-fold but does not alter AChR cell surface expression.

Novel synaptobrevin-1 mutation causes fatal congenital myasthenic syndrome.

Objective: To identify the molecular basis and elucidate the pathogenesis of a fatal congenital myasthenic syndrome.

Methods: We performed clinical electrophysiology studies, exome and Sanger sequencing, and analyzed functional consequences of the identified mutation.

Results: Clinical electrophysiology studies of the patient revealed several-fold potentiation of the evoked muscle action potential by high frequency nerve stimulation pointing to a presynaptic defect.

Investigation of Congenital Myasthenia Reveals Functional Asymmetry of Invariant Acetylcholine Receptor (AChR) Cys-loop Aspartates.

We identify two heteroallelic mutations in the acetylcholine receptor δ-subunit from a patient with severe myasthenic symptoms since birth: a novel δD140N mutation in the signature Cys-loop and a mutation in intron 7 of the δ-subunit gene that disrupts splicing of exon 8. The mutated Asp residue, which determines the disease phenotype, is conserved in all eukaryotic members of the Cys-loop receptor superfamily. Studies of the mutant acetylcholine receptor expressed in HEK 293 cells reveal that δD140N attenuates cell surface expression and apparent channel gating, predicting a reduced magnitude and an accelerated decay of the synaptic response, thus reducing the safety margin for neuromuscular transmission.

Mutant SNAP25B causes myasthenia, cortical hyperexcitability, ataxia, and intellectual disability.

Objective: To identify and characterize the molecular basis of a syndrome associated with myasthenia, cortical hyperexcitability, cerebellar ataxia, and intellectual disability.

Methods: We performed in vitro microelectrode studies of neuromuscular transmission, performed exome and Sanger sequencing, and analyzed functional consequences of the identified mutation in expression studies.

Results: Neuromuscular transmission at patient endplates was compromised by reduced evoked quantal release.

DPAGT1 myasthenia and myopathy: genetic, phenotypic, and expression studies.

Objective: To investigate patients with DPAGT1 (UDP-N-acetylglucosamine-dolichyl-phosphate N-acetylglucosaminephosphotransferase 1)-associated myasthenic syndrome.

Methods: We performed exome and Sanger sequencing, determined glycoprotein expression in patient muscles, assessed pathogenicity of the mutant proteins by examining their expression and enzymatic activity in transfected cells, evaluated structural changes in muscle and the neuromuscular junction, and examined electrophysiologic aspects of neuromuscular transmission in vitro.

Results: Patients 1 and 2, 16 and 14 years of age, had progressive fatigable weakness since infancy and are intellectually disabled.

GFPT1-myasthenia: clinical, structural, and electrophysiologic heterogeneity.

Objective: To identify patients with GFPT1-related limb-girdle myasthenia and analyze phenotypic consequences of the mutations.

Methods: We performed genetic analysis, histochemical, immunoblot, and ultrastructural studies and in vitro electrophysiologic analysis of neuromuscular transmission.

Results: We identified 16 recessive mutations in GFPT1 in 11 patients, of which 12 are novel.

Myasthenic syndrome AChRα C-loop mutant disrupts initiation of channel gating.

Congenital myasthenic syndromes (CMSs) are neuromuscular disorders that can be caused by defects in ace-tylcholine receptor (AChR) function. Disease-associated point mutants can reveal the unsuspected functional significance of mutated residues. We identified two pathogenic mutations in the extracellular domain of the AChR α subunit (AChRα) in a patient with myasthenic symptoms since birth: a V188M mutation in the C-loop and a heteroallelic G74C mutation in the main immunogenic region.

Highly fatal fast-channel syndrome caused by AChR ε subunit mutation at the agonist binding site.

Objective: To characterize the molecular basis of a novel fast-channel congenital myasthenic syndrome.

Methods: We used the candidate gene approach to identify the pathogenic mutation in the acetylcholine receptor (AChR) ε subunit, genetically engineered the mutant AChR into HEK cells, and evaluated the level of expression and kinetic properties of the mutant receptor.

Results: An 8-year-old boy born to consanguineous parents had severe myasthenic symptoms since birth.

Functional consequences and structural interpretation of mutations of human choline acetyltransferase.

Choline acetyltransferase (ChAT; EC 2.3.1.

E-box mutations in the RAPSN promoter region in eight cases with congenital myasthenic syndrome.

Myogenic determination factors are basic helix-loop-helix proteins that govern specification and differentiation of muscle cells, and bind to the E-box consensus sequence CANNTG in promoter regions of muscle-specific genes. No E-box mutation has been reported to date. RAPSN encodes rapsyn, a 43 kDa postsynaptic peripheral membrane protein that clusters the nicotinic acetylcholine receptor at the motor endplate.

Mutation causing severe myasthenia reveals functional asymmetry of AChR signature cystine loops in agonist binding and gating.

We describe a highly disabling congenital myasthenic syndrome (CMS) associated with rapidly decaying, low-amplitude synaptic currents, and trace its cause to a valine to leucine mutation in the signature cystine loop (cys-loop) of the AChR alpha subunit. The recently solved crystal structure of an ACh-binding protein places the cys-loop at the junction between the extracellular ligand-binding and transmembrane domains where it may couple agonist binding to channel gating. We therefore analyzed the kinetics of ACh-induced single-channel currents to identify elementary steps in the receptor activation mechanism altered by the alphaV132L mutation.

Rapsyn mutations in humans cause endplate acetylcholine-receptor deficiency and myasthenic syndrome.

Congenital myasthenic syndromes (CMSs) stem from genetic defects in endplate (EP)-specific presynaptic, synaptic, and postsynaptic proteins. The postsynaptic CMSs identified to date stem from a deficiency or kinetic abnormality of the acetylcholine receptor (AChR). All CMSs with a kinetic abnormality of AChR, as well as many CMSs with a deficiency of AChR, have been traced to mutations in AChR-subunit genes.