Functional heterogeneity of ryanodine receptor mutations associated with sudden cardiac death

Objectives: Point mutations in the cardiac ryanodine receptor (RyR2) mediate abnormal intracellular Ca2+ release and are associated with stress-induced ventricular tachycardia (VT), leading to sudden cardiac death (SCD). Although the precise molecular basis of RyR2 dysfunction in SCD remains controversial, there is consensus that the mutations characterised to date all exhibit gain-of-function Ca2+ release properties following cell stimulation. We investigated the functional impact of a distinct set of SCD-linked RyR2 mutations (L433P, N2386I, R176Q/T2504M) on intracellular Ca2+ handling. Methods: We expressed full-length recombinant human wild-type (WT) and SCD-linked RyR2 mutations in human embryonic kidney (HEK) cells, and profiled the spatial and amplitude characteristics of caffeine-evoked Ca2+ release through homo-tetrameric channels in living cells using rapid confocal laser scanning microscopy. Results: Analysis of the precise mode of Ca2+ release in HEK cells expressing RyR2 mutants demonstrated profound differences when compared with WT channels. The SCD-linked RyR2 mutations characterised in this study exhibited heterogeneous Ca2+ release profiles, including the novel observation that one of the mutants, (L433P), exhibited a marked reduction in sensitivity to channel activation. However, all SCD-linked RyR2 mutations characterised in this study resulted in an increased duration of elevated cytoplasmic Ca2+ levels following channel activation. Conclusions: Our live cell-based data demonstrates functional heterogeneity of Ca2+ release through SCD-linked RyR2 mutants, suggesting that the mechanistic basis of RyR2 dysfunction in SCD may be more complex than previously anticipated. These findings may have profound consequences for the therapeutic modulation of RyR2 in stress-induced VT and SCD