title: A mutation in a rare type of intron in a sodium-channel gene results in aberrant splicing and causes myotonia.
abstract: Many mutations in the skeletal-muscle sodium-channel gene SCN4A have been associated with myotonia and/or periodic paralysis, but so far all of these mutations are located in exons. We found a patient with myotonia caused by a deletion/insertion located in intron 21 of SCN4A, which is an AT-AC type II intron. This is a rare class of introns that, despite having AT-AC boundaries, are spliced by the major or U2-type spliceosome. The patient's skeletal muscle expressed aberrantly spliced SCN4A mRNA isoforms generated by activation of cryptic splice sites. In addition, genetic suppression experiments using an SCN4A minigene showed that the mutant 5' splice site has impaired binding to the U1 and U6 snRNPs, which are the cognate factors for recognition of U2-type 5' splice sites. One of the aberrantly spliced isoforms encodes a channel with a 35-amino acid insertion in the cytoplasmic loop between domains III and IV of Nav1.4. The mutant channel exhibited a marked disruption of fast inactivation, and a simulation in silico showed that the channel defect is consistent with the patient's myotonic symptoms. This is the first report of a disease-associated mutation in an AT-AC type II intron, and also the first intronic mutation in a voltage-gated ion channel gene showing a gain-of-function defect.
authors.affiliation: Department of Neurology, Osaka University Graduate School of Medicine, 2-2 Yamadaoaka, Suita, Osaka, Japan.
authors.forename: Tomoya
authors.initials: T
authors.lastname: Kubota
authors.forename: Xavier
authors.initials: X
authors.lastname: Roca
authors.forename: Takashi
authors.initials: T
authors.lastname: Kimura
authors.forename: Yosuke
authors.initials: Y
authors.lastname: Kokunai
authors.forename: Ichizo
authors.initials: I
authors.lastname: Nishino
authors.forename: Saburo
authors.initials: S
authors.lastname: Sakoda
authors.forename: Adrian R
authors.initials: AR
authors.lastname: Krainer
authors.forename: Masanori P
authors.initials: MP
authors.lastname: Takahashi
mesh_terms: D000328:Adult; D017398:Alternative Splicing / Q000235:genetics*; D004594:Electrophysiology; D006801:Humans; D054643:INDEL Mutation / Q000235:genetics; D007438:Introns / Q000235:genetics; D007564:Japan; D008297:Male; D009154:Mutation / Q000235:genetics; D009222:Myotonia / Q000235:genetics* / Q000473:pathology; D062553:NAV1.4 Voltage-Gated Sodium Channel; D015222:Sodium Channels / Q000235:genetics*
chemical_list: D062553:NAV1.4 Voltage-Gated Sodium Channel; C106251:SCN4A protein, human; D015222:Sodium Channels