The molecular basis of cardiac arrhythmias in patients with cardiomyopathy

Escape capture bigeminy: phenotypic marker of cardiac sodium channel voltage sensor mutation R222Q

BACKGROUND

Electrocardiographic signature of escape capture bigeminy that spans generations and clusters in a family has not been linked to a sodium channel voltage sensor mutation.

OBJECTIVE

To characterize the clinical and biophysical consequences of the R222Q mutation in the voltage sensor of cardiac sodium channels.

METHODS

Comprehensive clinical assessment, invasive electrophysiologic study, genetic analysis, and patch-clamp studies were undertaken.

RESULTS

Uniquely, 5 members had the same electrocardiographic pattern of a junctional escape ventricular capture bigeminy. Genetic analysis of 3 family members revealed the same mutation (R222Q) in the cardiac sodium channel gene, SCN5A (nucleotide change was 665 G→A that led to missense amino acid substitution Arg 222 Gln, located in the S4 voltage sensor in domain I). Catheterization and mapping revealed that there was no consistent evidence of bundle branch reentry or fascicular potentials preceding ectopic beats. The bigeminy was suppressed by the intravenous administration of the sodium channel blocker, lidocaine. Patch-clamp studies revealed unique differential leftward voltage-dependent shifts in activation and inactivation properties of human voltage-gated Na(+) channels with the R222Q mutation, consistent with increasing channel excitability at precisely the voltages corresponding to the resting membrane potential of cardiomyocytes.

CONCLUSIONS

The R222Q mutation enhances cardiac sodium channel excitability, resulting in an unusual, highly penetrant phenotype of escape capture bigeminy and cardiomyopathy. These findings support the conclusion that a mutation in the voltage sensor of cardiac sodium channels can cause bigeminal arrhythmia associated with cardiomyopathy.

The ShcA phosphotyrosine docking protein uses distinct mechanisms to regulate myocyte and global heart function

RATIONALE

Although tyrosine kinases (TKs) are important for cardiac function, their relevant downstream targets in the adult heart are unknown. The ShcA docking protein binds specific phosphotyrosine (pTyr) sites on activated TKs through its N-terminal pTyr-binding (PTB) and C-terminal SH2 domains and stimulates downstream pathways through motifs such as pTyr sites in its central CH1 region. Therefore, ShcA could be a potential hub for downstream TK signaling in the myocardium.

OBJECTIVE

To define the role of ShcA, a TK scaffold, in the adult heart using a myocardial-specific knockout of murine ShcA (ShcA CKO) and domain knock-in models.

METHODS AND RESULTS

ShcA CKO mice developed a dilated cardiomyopathy phenotype involving impaired systolic function with enhanced cardiomyocyte contractility. This uncoupling of global heart and intrinsic myocyte functions was associated with altered collagen and extracellular matrix compliance properties, suggesting disruption of mechanical coupling. In vivo dissection of ShcA signaling properties revealed that selective inactivation of the PTB domain in the myocardium had effects resembling those seen in ShcA CKO mice, whereas disruption of the SH2 domain caused a less severe cardiac phenotype. Downstream signaling through the CH1 pTyr sites was dispensable for baseline cardiac function but necessary to prevent adverse remodeling after hemodynamic overload.

CONCLUSIONS

These data demonstrate a requirement for TK-ShcA PTB domain signaling to maintain cardiac function. In addition, analysis of the SH2 domain and CH1 pTyr sites reveals that ShcA mediates pTyr signaling in the adult heart through multiple distinct signaling elements that control myocardial functions and response to stresses.

Compartmentation of cyclic nucleotide signaling in the heart: the role of A-kinase anchoring proteins

The activation of the cyclic nucleotide protein kinase A (PKA) and PKG by their respective second messengers is responsible for the modulation of many cellular functions in the heart including cardiac hypertrophy, strength of contraction, and ion flux. However, several studies have revealed that a general increase in cyclic nucleotide concentration in the cell is not sufficient for the specific regulation of target proteins. These studies found that PKA and PKG must be colocalized with their targets to ensure spatial-temporal control of substrate phosphorylation. This compartmentation of cyclic nucleotide signaling is accomplished by tethering the protein kinases with their respective substrates through the association with scaffolding proteins. For cAMP signaling, A-kinase anchoring proteins (AKAPs) provide a molecular mechanism for cAMP compartmentation, allowing for the precise control of PKA-mediated phosphorylation events. (cAMP, PKA, AKAP, PKG).