Vesicular acetylcholine transporter defect underlies devastating congenital myasthenia syndrome

Aran A, Segel R, Kaneshige K, Gulsuner S, Renbaum P, Oliphant S, Meirson T, Weinberg-Shukron A, Hershkovitz Y, Zeligson S, Lee MK, Samson AO, Parsons SM, King MC, Levy-Lahad E, Walsh T. Vesicular acetylcholine transporter defect underlies devastating congenital myasthenia syndrome. Neurology. 2017 Mar 14;88(11):1021-1028.

Abstract

OBJECTIVE:

To identify the genetic basis of a recessive congenital neurologic syndrome characterized by severe hypotonia, arthrogryposis, and respiratory failure.

METHODS:

Identification of the responsible gene by exome sequencing and assessment of the effect of the mutation on protein stability in transfected rat neuronal-like PC12A123.7 cells.

RESULTS:

Two brothers from a nonconsanguineous Yemeni Jewish family manifested at birth with severe hypotonia and arthrogryposis. The older brother died of respiratory failure at 5 days of age. The proband, now 4.5 years old, has been mechanically ventilated since birth with virtually no milestones achievement. Whole exome sequencing revealed homozygosity of SLC18A3 c.1078G>C, p.Gly360Arg in the affected brothers but not in other family members. SLC18A3 p.Gly360Arg is not reported in world populations but is present at a carrier frequency of 1:30 in healthy Yemeni Jews. SLC18A3 encodes the vesicular acetylcholine transporter (VAChT), which loads newly synthesized acetylcholine from the neuronal cytoplasm into synaptic vesicles. Mice that are VAChT-null have been shown to die at birth of respiratory failure. In human VAChT, residue 360 is located in a conserved region and substitution of arginine for glycine is predicted to disrupt proper protein folding and membrane embedding. Stable transfection of wild-type and mutant human VAChT into neuronal-like PC12A123.7 cells revealed similar mRNA levels, but undetectable levels of the mutant protein, suggesting post-translational degradation of mutant VAChT.

CONCLUSION:

Loss of function of VAChT underlies severe arthrogryposis and respiratory failure. While most congenital myasthenic syndromes are caused by defects in postsynaptic proteins, VAChT deficiency is a presynaptic myasthenic syndrome.