Novel delta subunit mutation in slow-channel syndrome causes severe weakness by novel mechanisms
Article first published online: 28 DEC 2001
Copyright © 2001 Wiley-Liss, Inc.
Annals of Neurology
Volume 51, Issue 1, pages 102–112, January 2002
How to Cite
Gomez, C. M., Maselli, R. A., Vohra, B. P. S., Navedo, M., Stiles, J. R., Charnet, P., Schott, K., Rojas, L., Keesey, J., Verity, A., Wollmann, R. W. and Lasalde-Dominicci, J. (2002), Novel delta subunit mutation in slow-channel syndrome causes severe weakness by novel mechanisms. Ann Neurol., 51: 102–112. doi: 10.1002/ana.10077
- Issue published online: 31 DEC 2001
- Article first published online: 28 DEC 2001
- Manuscript Accepted: 25 SEP 2001
- Manuscript Revised: 24 SEP 2001
- Manuscript Received: 26 APR 2001
- National Institutes of Health. Grant Numbers: RO1 NS33202, NIH RR 06009
We investigated the basis for a novel form of the slow-channel congenital myasthenic syndrome presenting in infancy in a single individual as progressive weakness and impaired neuromuscular transmission without overt degeneration of the motor endplate. Prolonged low-amplitude synaptic currents in biopsied anconeus muscle at 9 years of age suggested a kinetic disorder of the muscle acetylcholine receptor. Ultrastructural studies at 16 months, at 9 years, and at 15 years of age showed none of the typical degenerative changes of the endplate associated with the slow-channel congenital myasthenic syndrome, and acetylcholine receptor numbers were not significantly reduced. We identified a novel C-to-T substitution in exon 8 of the δ-subunit that results in a serine to phenylalanine mutation in the region encoding the second transmembrane domain that lines the ion channel. Using Xenopus oocyte in vitro expression studies we confirmed that the δS268F mutation, as with other slow-channel congenital myasthenic syndrome mutations, causes delayed closure of acetylcholine receptor ion channels. In addition, unlike other mutations in slow-channel congenital myasthenic syndrome, this mutation also causes delayed opening of the channel, a finding that readily explains the marked congenital weakness in the absence of endplate degeneration. Finally, we used serial morphometric analysis of electron micrographs to explore the basis for the progressive weakness and decline of amplitude of endplate currents over a period of 14 years. We demonstrated a progressive widening and accumulation of debris in the synaptic cleft, resulting in loss of efficacy of released neurotransmitter and reduced safety factor. These studies demonstrate the role of previously unrecognized mechanisms of impairment of synaptic transmission caused by a novel mutation and show the importance of serial in vitro studies to elucidate novel disease mechanisms.