BACKGROUND: Familial hypertrophic cardiomyopathy (FHC) is characterized by 
genetic and clinical heterogeneity. Five percent of FHC families have 2 
FHC-causing mutations, which results in earlier disease onset, increased cardiac 
dysfunction, and a higher incidence of sudden death events. These observations 
suggest a relationship between the number of gene mutations and phenotype 
severity in FHC.
METHODS AND RESULTS: We sought to develop, characterize, and investigate the 
pathogenic mechanisms in a double-mutant murine model of FHC. This model 
(designated TnI-203/MHC-403) was generated by crossbreeding mice with the 
Gly203Ser cardiac troponin I (TnI-203) and Arg403Gln alpha-myosin heavy chain 
(MHC-403) FHC-causing mutations. The mortality rate in TnI-203/MHC-403 mice was 
100% by age 21 days. At age 14 days, TnI-203/MHC-403 mice developed a 
significantly increased ratio of heart weight to body weight, marked 
interstitial myocardial fibrosis, and increased expression of atrial natriuretic 
factor and brain natriuretic peptide compared with nontransgenic, TnI-203, and 
MHC-403 littermates. By age 16 to 18 days, TnI-203/MHC-403 mice rapidly 
developed a severe dilated cardiomyopathy and heart failure, with inducibility 
of ventricular arrhythmias, which led to death by 21 days. Downregulation of 
mRNA levels of key regulators of Ca(2+) homeostasis in TnI-203/MHC-403 mice was 
observed. Increased levels of phosphorylated STAT3 were observed in 
TnI-203/MHC-403 mice and corresponded with the onset of disease, which suggests 
a possible cardioprotective response.
CONCLUSIONS: TnI-203/MHC-403 double-mutant mice develop a severe cardiac 
phenotype characterized by heart failure and early death. The presence of 2 
disease-causing mutations may predispose individuals to a greater risk of 
developing severe heart failure than human FHC caused by a single gene mutation.