Background: Conditional gene inactivation in mice using the bacteriophage P1 Cre-loxP recombination system requires transgenic expression of Cre-recombinase driven by a tissue-specific or inducible promoter.
Methods and results: Using the cardiac alpha-myosin-heavy-chain promoter, the most commonly used myocardial-specific transgenic promoter, we created transgenic mice expressing Cre-recombinase in the heart. Seven transgenic lines developed dilated cardiomyopathy and premature death from congestive heart failure. One founder line that survived long enough to propagate had extremely high-level Cre recombinase expression. Transgenic lines that expressed low levels remained healthy. The high-expressing strain developed heart failure over a very predictable and reproducible time course. Detailed examination of the high-expressing strain revealed important molecular, cellular, and pharmacologic hallmarks of cardiomyopathy. First, "fetal genes" such as atrial natriuretic factor and brain natriuretic protein were expressed, a marker of pathologic cardiac hypertrophy and heart failure. Second, an increased incidence of cardiac myocyte apoptosis was present. Third, treatment of mice with captopril or metoprolol, drugs that delay the progression of heart failure, improved survival.
Conclusion: Cre-recombinase when expressed at high levels may cause organ dysfunction, which could be mistaken for an effect of conditional gene inactivation. In addition, the stereotypic cardiomyopathy and disease progression in the characterized, high-expressing transgenic strain suggests its utility as a model to study the effects of pharmacologic or genetic manipulations in heart failure.