Human SCN5A gene mutations alter cardiac sodium channel kinetics and are associated with the Brugada syndrome.
BACKGROUND
Primary dysrhythmias other than those associated with the long QT syndrome, are increasingly recognized. One of these are represented by patients with a history of resuscitation from cardiac arrest but without any structural heart disease. These patients exhibit a distinct electrocardiographic (ECG) pattern consisting of a persistent ST-segment elevation in the right precordial leads often but not always accompanied by a right bundle branch block (Brugada syndrome). This syndrome is associated with a high mortality rate and has been shown to display familial occurrence.


RESULTS
Pharmacological sodium channel blockade elicits or worsens the electrocardiographic features associated with this syndrome. Hence, a candidate gene approach directed towards SCN5A, the gene encoding the alpha-subunit of the cardiac sodium channel, was followed in six affected individuals. In two patients missense mutations were identified in the coding region of the gene: R1512W in the DIII-DIV cytoplasmic linker and A1924T in the C-terminal cytoplasmic domain. In two other patients mutations were detected near intron/exon junctions. To assess the functional consequences of the R1512W and A1924T mutations, wild-type and mutant sodium channel proteins were expressed in Xenopus oocytes. Both missense mutations affected channel function, most notably a 4-5 mV negative voltage shift of the steady-state activation and inactivation curves in R1512W and a 9 mV negative voltage shift of the steady-state activation curve in A1924T, measured at 22 degrees C. Recovery from inactivation was slightly prolonged for R1512W channels. The time dependent kinetics of activation and inactivation at -20 mV were not significantly affected by either mutation.


CONCLUSIONS
Two SCN5A mutations associated with the Brugada syndrome, significantly affect cardiac sodium channel characteristics. The alterations seem to be associated with an increase in inward sodium current during the action potential upstroke.
Rook
M B
MB
Department of Medical Physiology, Utrecht University, The Netherlands.
Bezzina Alshinawi
C
C
Groenewegen
W A
WA
van Gelder
I C
IC
van Ginneken
A C
AC
Jongsma
H J
HJ
Mannens
M M
MM
Wilde
A A
AA
D000200:Action Potentials / Q000235:genetics; D000818:Animals; D002037:Bundle-Branch Block / Q000235:genetics* / Q000378:metabolism / Q000503:physiopathology; D004562:Electrocardiography; D015870:Gene Expression; D006323:Heart Arrest / Q000235:genetics* / Q000378:metabolism / Q000503:physiopathology; D006801:Humans; D015640:Ion Channel Gating / Q000235:genetics; D020125:Mutation, Missense; D009206:Myocardium / Q000378:metabolism*; D062554:NAV1.5 Voltage-Gated Sodium Channel; D009865:Oocytes; D018807:Polymorphism, Single-Stranded Conformational; D017422:Sequence Analysis, DNA; D015222:Sodium Channels / Q000235:genetics* / Q000378:metabolism; D013577:Syndrome; D014981:Xenopus
D062554:NAV1.5 Voltage-Gated Sodium Channel; C568320:SCN5A protein, human; D015222:Sodium Channels