Abstract 4413: Accelerated Inactivation of the L-type Calcium due to a Mutation in CACNB2b Underlies the Development of a Brugada ECG Phenotype
Background: Ion channelopathies are responsible for a number of genetic cardiac arrhythmia syndromes. Recent work demonstrated an association between mutations in CACNA1c and CACNB2b, the genes that encode the α and β subunits of the L-type calcium channel, and the Brugada syndrome (BrS). The mutations previously described all caused a loss of function secondary to a major reduction in peak calcium channel current (ICa). In the present study we describe a novel CACNB2b mutation associated with BrS in which loss of function was caused by accelerated inactivation of ICa.
Methods and Results: The proband, a 32 yo male, displayed a saddleback ST segment elevation in the right precordial leads that converted to a coved-type ECG following a procainamide challenge. His EP study was positive with double extrastimuli inducing polymorphic VT/VF. He was also diagnosed with vasovagal syncope. Genomic DNA was isolated from blood lymphocytes. All exons and intron borders of 12 ion channel genes were amplified and sequenced. The only mutation uncovered was a missense mutation (T11I) in CACNB2b. The effect of this mutation was studied by expression of WT or T11I CACNB2b in TSA201 cells co-transfected with WT CANCA1c and CACNA2d. Patch clamp analysis showed no difference in ICa density between WT and T11I (17.9±1.8 vs 22.5±4.3 pA/pF, respectively at +20mV). Similarly, steady-state inactivation and channel recovery was not different between WT and T11I mutant channels. However, both the fast and slow decay of ICa produced by T11I mutant were significantly faster compared to WT at potentials between −10 to +30 mV, suggesting a reduction in depolarizing current during the course of an action potential. Application of action potential voltage clamp pulses confirmed that T11I total charge was reduced by 42±2.3% compared to WT (p<0.05).
Conclusion: We report the first Brugada syndrome mutation in CaCNB2b resulting in accelerated inactivation of the L-type calcium channel. The T11I mutation caused a faster decay of cardiac L-type calcium current but did not significantly alter the magnitude of the peak current. Our results suggest that a reduced total charge carried by ICa during the plateau of the action potential predisposes to the Brugada phenotype.