Action Potential Clamp as a Tool for Risk Stratification of Sinus Bradycardia Due to Loss-of-Function Mutations in HCN4

We performed computer simulations using the Fabbri-Severi model of a human sinus node cell to assess whether action potential (AP) clamp experiments on transfected cells could serve as a useful tool for risk stratification of sinus bradycardia due to loss-of-function mutations in the HCN4 gene, which encodes the ion channels carrying the hyperpolarization-activated 'funny' current (I_{f}). For a total of 12 well-documented HCN4 mutations from the literature, we simulated AP clamp experiments on transfected cells and computed the charge carried by the wild-type or mutant I_{f} channels during the diastolic depolarization (Q_{f}). For each of the mutations tested, we then incorporated the mutation-induced changes in fully-activated conductance and kinetics of I_{f} into the Fabbri-Severi model and determined the cycle length in the presence of the specific mutation at different levels of autonomic tone. At each level, the beating rate of the model cell showed a close correlation with the charge carried by the HCN4 channels in the simulated AP clamp experiments (R^{2} of 0.90-0.99). More importantly, the clinically observed minimum or resting heart rates showed a strong correlation with Q_{f} (R^{2}=0.73 and R^{2}=0.71, respectively). We conclude that AP clamp on transfected cells is a promising tool for risk stratification of sinus bradycardia due to loss-of-function mutations in HCN4.