BACKGROUND: A human genetic variant (Ser96Ala) in the sarcoplasmic reticulum 
(SR) histidine-rich Ca(2+)-binding (HRC) protein has been linked to ventricular 
arrhythmia and sudden death in dilated cardiomyopathy. However, the precise 
mechanisms affecting SR function and leading to arrhythmias remain elusive.
METHODS AND RESULTS: We generated transgenic mice with cardiac-specific 
expression of human Ala96 HRC or Ser96 HRC in the null background to assess 
function in absence of endogenous protein. Ala96 HRC decreased (25% to 30%) 
cardiomyocyte contractility and Ca2+ kinetics compared with Ser96 HRC in the 
absence of any structural or histological abnormalities. Furthermore, the 
frequency of Ca2+ waves was significantly higher (10-fold), although SR Ca2+ 
load was reduced (by 27%) in Ala96 HRC cells. The underlying mechanisms involved 
diminished interaction of Ala96 HRC with triadin, affecting ryanodine receptor 
(RyR) stability. Indeed, the open probability of RyR, assessed by use of 
ryanodine binding, was significantly increased. Accordingly, stress conditions 
(5 Hz plus isoproterenol) induced aftercontractions (65% in Ala96 versus 12% in 
Ser96) and delayed afterdepolarizations (70% in Ala96 versus 20% in Ser96). The 
increased SR Ca2+ leak was accompanied by hyperphosphorylation (1.6-fold) of RyR 
at Ser2814 by calmodulin-dependent protein kinase II. Accordingly, inclusion of 
the calmodulin-dependent protein kinase II inhibitor KN93 prevented Ser2814 
phosphorylation and partially reversed the increases in Ca2+ spark frequency and 
wave production. Parallel in vivo studies revealed ventricular ectopy on 
short-term isoproterenol challenge and increased (4-fold) propensity to 
arrhythmias, including nonsustained ventricular tachycardia, after myocardial 
infarction in Ala96 HRC mice.
CONCLUSIONS: These findings suggest that aberrant SR Ca2+ release and increased 
susceptibility to delayed afterdepolarizations underlie triggered arrhythmic 
activity in human Ala96 HRC carriers.