Naturally occurring mutations in the cardiac ryanodine receptor (RyR2) have been associated with both cardiac arrhythmias and cardiomyopathies. It is clear that delayed afterdepolarization resulting from abnormal activation of sarcoplasmic reticulum Ca²⁺ release is the primary cause of RyR2-associated cardiac arrhythmias. However, the mechanism underlying RyR2-associated cardiomyopathies is completely unknown.

In the present study, we investigate the role of the NH₂-terminal region of RyR2 in and the impact of a number of cardiomyopathy-associated RyR2 mutations on the termination of Ca²⁺ release.

The 35-residue exon-3 region of RyR2 is associated with dilated cardiomyopathy. Single-cell luminal Ca²⁺ imaging revealed that the deletion of the first 305 NH₂-terminal residues encompassing exon-3 or the deletion of exon-3 itself markedly reduced the luminal Ca²⁺ threshold at which Ca²⁺ release terminates and increased the fractional Ca²⁺ release. Single-cell cytosolic Ca²⁺ imaging also showed that both RyR2 deletions enhanced the amplitude of store overload-induced Ca²⁺ transients in HEK293 cells or HL-1 cardiac cells. Furthermore, the RyR2 NH₂-terminal mutations, A77V, R176Q/T2504M, R420W, and L433P, which are associated with arrhythmogenic right ventricular displasia type 2, also reduced the threshold for Ca²⁺ release termination and increased fractional release. The RyR2 A1107M mutation associated with hypertrophic cardiomyopathy had the opposite action (ie, increased the threshold for Ca²⁺ release termination and reduced fractional release).

These results provide the first evidence that the NH₂-terminal region of RyR2 is an important determinant of Ca²⁺ release termination, and that abnormal fractional Ca²⁺ release attributable to aberrant termination of Ca²⁺ release is a common defect in RyR2-associated cardiomyopathies.