Dual variation in SCN5A and CACNB2b underlies the development of cardiac conduction disease without Brugada syndrome.
BACKGROUND
Inherited loss of function mutations in SCN5A have been linked to overlapping syndromes including cardiac conduction disease and Brugada syndrome (BrS). The mechanisms responsible for the development of one without the other are poorly understood.


METHODS
Direct sequencing was performed in a family with cardiac conduction disease. Wild-type (WT) and mutant channels were expressed in TSA201 cells for electrophysiological study. Green fluorescent protein (GFP)-fused WT or mutant genes were used to assess channel trafficking.


RESULTS
A novel SCN5A mutation, P1008S, was identified in all family members displaying first-degree atrioventricular block, but not in unaffected family members nor in 430 reference alleles. Peak P1008S current was 11.77% of WT (P < 0.001). Confocal microscopy showed that WT channels tagged with GFP were localized on the cell surface, whereas GFP-tagged P1008S channels remained trapped in intracellular organelles. Trafficking could be rescued by incubation at room temperature, but not by incubation with mexiletine (300 muM) at 37 degrees C. We also identified a novel polymorphism (D601E) in CACNB2b that slowed inactivation of L-type calcium current (I(Ca,L)), significantly increased total charge. Using the Luo-Rudy action potential (AP) model, we show that the reduction in sodium current (I(Na)) can cause loss of the right ventricular epicardial AP dome in the absence but not in the presence of the slowed inactivation of I(Ca,L). Slowed conduction was present in both cases.


CONCLUSIONS
Our results suggest genetic variations leading to a loss-of-function in I(Na) coupled with a gain of function in I(Ca,L) may underlie the development of cardiac conduction disease without BrS.
Hu
Dan
D
Masonic Medical Research Laboratory, Utica, New York, NY 13501-1787, USA.
Barajas-Martinez
Hector
H
Nesterenko
Vladislav V
VV
Pfeiffer
Ryan
R
Guerchicoff
Alejandra
A
Cordeiro
Jonathan M
JM
Curtis
Anne B
AB
Pollevick
Guido D
GD
Wu
Yuesheng
Y
Burashnikov
Elena
E
Antzelevitch
Charles
C
D000293:Adolescent; D000483:Alleles; D000704:Analysis of Variance; D001919:Bradycardia / Q000235:genetics* / Q000503:physiopathology; D053840:Brugada Syndrome / Q000235:genetics / Q000503:physiopathology; D020746:Calcium Channels, L-Type / Q000235:genetics*; D022062:Electrophysiologic Techniques, Cardiac; D005260:Female; D006327:Heart Block / Q000235:genetics* / Q000503:physiopathology; D006329:Heart Conduction System / Q000503:physiopathology*; D006801:Humans; D008297:Male; D018613:Microscopy, Confocal; D008875:Middle Aged; D009124:Muscle Proteins / Q000235:genetics*; D009154:Mutation; D062554:NAV1.5 Voltage-Gated Sodium Channel; D010375:Pedigree; D010641:Phenotype; D016133:Polymerase Chain Reaction; D020641:Polymorphism, Single Nucleotide; D015222:Sodium Channels / Q000235:genetics*
C107597:CACNB2 protein, human; D020746:Calcium Channels, L-Type; D009124:Muscle Proteins; D062554:NAV1.5 Voltage-Gated Sodium Channel; C568320:SCN5A protein, human; D015222:Sodium Channels