Increased Ca²⁺ Transient Underlies RyR2-Related Left Ventricular Noncompaction

A loss-of-function cardiac ryanodine receptor (RyR2) mutation, I4855M^+/-, has recently been linked to a new cardiac disorder termed RyR2 Ca²⁺ release deficiency syndrome (CRDS) as well as left ventricular noncompaction (LVNC). The mechanism by which RyR2 loss-of-function causes CRDS has been extensively studied, but the mechanism underlying RyR2 loss-of-function-associated LVNC is unknown. Here, we determined the impact of a CRDS-LVNC-associated RyR2-I4855M^+/- loss-of-function mutation on cardiac structure and function.

We generated a mouse model expressing the CRDS-LVNC-associated RyR2-I4855M^+/- mutation. Histological analysis, echocardiography, ECG recording, and intact heart Ca²⁺ imaging were performed to characterize the structural and functional consequences of the RyR2-I4855M^+/- mutation.

As in humans, RyR2-I4855M^+/- mice displayed LVNC characterized by cardiac hypertrabeculation and noncompaction. RyR2-I4855M^+/- mice were highly susceptible to electrical stimulation-induced ventricular arrhythmias but protected from stress-induced ventricular arrhythmias. Unexpectedly, the RyR2-I4855M^+/- mutation increased the peak Ca²⁺ transient but did not alter the L-type Ca²⁺ current, suggesting an increase in Ca²⁺-induced Ca²⁺ release gain. The RyR2-I4855M^+/- mutation abolished sarcoplasmic reticulum store overload-induced Ca²⁺ release or Ca²⁺ leak, elevated sarcoplasmic reticulum Ca²⁺ load, prolonged Ca²⁺ transient decay, and elevated end-diastolic Ca²⁺ level upon rapid pacing. Immunoblotting revealed increased level of phosphorylated CaMKII (Ca²⁺-calmodulin dependent protein kinases II) but unchanged levels of CaMKII, calcineurin, and other Ca²⁺ handling proteins in the RyR2-I4855M^+/- mutant compared with wild type.

The RyR2-I4855M^+/- mutant mice represent the first RyR2-associated LVNC animal model that recapitulates the CRDS-LVNC overlapping phenotype in humans. The RyR2-I4855M^+/- mutation increases the peak Ca²⁺ transient by increasing the Ca²⁺-induced Ca²⁺ release gain and the end-diastolic Ca²⁺ level by prolonging Ca²⁺ transient decay. Our data suggest that the increased peak-systolic and end-diastolic Ca²⁺ levels may underlie RyR2-associated LVNC.