A novel gain-of-function HCN4 variant responsible for familial inappropriate sinus tachycardia

Inappropriate Sinus Tachycardia (IST) is an arrhythmia characterized by rapid sinus rates of over 100 BPM at rest. The differential diagnosis for this persistent observation is broad, including medication side effects or serendipitous use of chronotropic stimulating drugs. Genetic causes of IST are seldom considered. Only two mutations have been linked to this condition, both of which affect the gene encoding the hyperpolarization-activated cyclic nucleotide-gated potassium channel (HCN4), which is essential for generating pacemaker activity of the SA node.

The objective of this study was to investigate the genetic basis of IST in a mother and daughter, and to functionally characterize a novel HCN4 variant identified in the family.

Standard clinical genetic testing was performed on a child with IST, her affected mother, and two healthy siblings. A novel HCN4 channel variant identified in the family was studied by whole-cell patch clamp analysis. 3D protein structures of mutant and wild-type (WT) HCN4 channels were generated and subjected to 200 ns of unrestricted molecular dynamics (MD) simulation.

A novel heterozygous, missense variant was identified in the HCN4 gene (p. Asn299Ser) in the affected child and mother, while absent in 2 healthy siblings of the child. Patch-clamp analysis revealed significantly increased HCN4 current density and a rightward-shifted activation curve in cells expressing p.Asn299Ser-HCN4 versus WT channels, suggesting constitutive activity of the mutant HCN4 channel. In MD simulations, the voltage sensor of p.Asn299Ser-HCN4 channels adopted a resting conformation mimicking that of cAMP-bound WT HCN4, providing a structural basis for the functional observations. Ivabradine application returned the gain-of-function properties of mutant channels to baseline levels.

We identified a novel gain-of-function HCN4 variant in a family with IST that mimics the effects of cAMP activation. This study furthers our understanding of the mechanisms underlying IST and provides data supporting the efficacious effect of ivabradine in genetic-based IST.