Compound mutations: a common cause of severe long-QT syndrome.
BACKGROUND
Long QT syndrome (LQTS) predisposes affected individuals to sudden death from cardiac arrhythmias. Although most LQTS individuals do not have cardiac events, significant phenotypic variability exists within families. Probands can be very symptomatic. The mechanism of this phenotypic variability is not understood.


RESULTS
Genetic analyses of KVLQT1, HERG, KCNE1, KCNE2, and SCN5A detected compound mutations in 20 of 252 LQTS probands (7.9%). Carriers of 2 mutations had longer QTc intervals (527+/-54 versus 489+/-44 ms; P<0.001); all had experienced cardiac events (20 of 20 [100%] versus 128 of 178 [72%]; P<0.01) and were 3.5-fold more likely to have cardiac arrest (9 of 16 [56%] versus 45 of 167 [27%]; P<0.01; OR, 3.5; 95% CI, 1.2 to 9.9) compared with probands with 1 or no identified mutation. Two-microelectrode voltage clamp of Xenopus oocytes was used to characterize the properties of variant slow delayed rectifier potassium (I(Ks)) channels identified in 7 of the probands. When wild-type and variant subunits were coexpressed in appropriate ratios to mimic the genotype of the proband, the reduction in I(Ks) density was equivalent to the additive effects of the single mutations.


CONCLUSIONS
LQTS-associated compound mutations cause a severe phenotype and are more common than expected. Individuals with compound mutations need to be identified, and their management should be tailored to their increased risk for arrhythmias.
Westenskow
Peter
P
Department of Physiology, University of Utah, 95 South 2000 East, Salt Lake City, UT 84112-5000, USA.
Splawski
Igor
I
Timothy
Katherine W
KW
Keating
Mark T
MT
Sanguinetti
Michael C
MC
D000818:Animals; D002478:Cells, Cultured; D000072237:ERG1 Potassium Channel; D004553:Electric Conductivity; D051638:Ether-A-Go-Go Potassium Channels; D005260:Female; D006801:Humans; D007473:Ion Channels / Q000235:genetics*; D051656:KCNQ Potassium Channels; D051657:KCNQ1 Potassium Channel; D008133:Long QT Syndrome / Q000175:diagnosis / Q000235:genetics*; D008297:Male; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D062554:NAV1.5 Voltage-Gated Sodium Channel; D018408:Patch-Clamp Techniques; D010375:Pedigree; D015221:Potassium Channels / Q000235:genetics / Q000502:physiology; D024642:Potassium Channels, Voltage-Gated; D015222:Sodium Channels / Q000235:genetics; D014982:Xenopus laevis
D000072237:ERG1 Potassium Channel; D051638:Ether-A-Go-Go Potassium Channels; D007473:Ion Channels; C000606913:KCNH2 protein, human; D051656:KCNQ Potassium Channels; D051657:KCNQ1 Potassium Channel; C496909:KCNQ1 protein, human; D062554:NAV1.5 Voltage-Gated Sodium Channel; D015221:Potassium Channels; D024642:Potassium Channels, Voltage-Gated; C568320:SCN5A protein, human; D015222:Sodium Channels; C059983:potassium channel protein I(sk)