Biophysical Characterization of a Novel KCNJ8 Rare Variant Linked With Inherited and Acquired J Wave Syndrome.

BACKGROUND

Adenosine triphosphate (ATP)-sensitive potassium cardiac channels (KATP) are composed of inward rectifying potassium channel (Kir) subunit Kir6.1 or Kir6.2, encoded by KCNJ8 or KCNJ11, and the sulfonylurea receptor SUR2 or SUR1, encoded by ABCC9 or ABCC8.

OBJECTIVES

The aim of this study is to functionally characterize a potential novel high-frequent mutation in KCNJ8 and its possible association with congenital and acquired J-wave syndrome (JWS).

METHODS

A next-generation sequencing-based large gene panel of susceptibility genes was performed on 413 JWS and 25 consecutive probands who developed ventricular fibrillation (VF) during acute myocardial infarction. Whole-cell and inside-out patch-clamp techniques were used to characterize mutant channels expressed in HEK293 cells. Homology models of Kir6.1 and molecular dynamics simulations were developed to explore the molecular mechanisms affected by the Kir6.1 mutation.

RESULTS

A novel highly conserved missense mutation in KCNJ8, A88G, was uncovered in 3 probands diagnosed with Brugada syndrome and/or early repolarization syndrome, one of which presented with VF after myocardial infarction. Whole-cell patch-clamp experiments revealed a 2-fold gain of function in KATP current when KCNJ8-A88G was coexpressed with ABCC9-wide type (WT) compared with KCNJ8-WT. Current-voltage relationships assessed using the inside-out patch configuration demonstrated a conductance of approximately 73.4 ± 1.5 pS for KCNJ8-A88G, in contrast to 46.2 ± 1.2 pS for KCNJ8-WT (P < 0.01). The evaluation of open probability showed a significant increase in open state duration, with τ values of 3.5 ± 1.2 ms for KCNJ8-WT and 27.5 ± 2.2 ms for KCNJ8-A88G. Moreover, mutant channels exhibited a markedly higher half-maximal inhibitory ATP concentration (IC50), measuring 44.3 ± 3.9 μmol/L for WT and 1,702.4 ± 218.9 μmol/L for the mutant (P < 0.01). The molecular dynamics results showed that the A88G mutation could release the restriction of residue F131, causing a greater separation of residues in the selectivity filter, resulting in increased channel conductance.

CONCLUSIONS

Our findings demonstrate that the A88G mutant in KCNJ8 induces a gain-of-function in KATP current secondary to a major increase in open probability of the KATP channel responsible for increased channel conductance as well as a reduced sensitivity to inhibition of the channel by intracellular ATP. These findings provide further evidence pointing to KCNJ8 as a JWS and VF susceptibility gene.