Molecular insights from a novel cardiac troponin I mouse model of familial hypertrophic cardiomyopathy

Abstract

Gene mutations in cardiac troponin I (cTnI) account for up to 5% of genotyped families with familial hypertrophic cardiomyopathy (FHC). Little is known about how cTnI mutations cause disease. Five lines of transgenic mice were generated which overexpress the human disease-causing cTnI gene mutation, Gly203Ser (designated cTnI-G203S), in a cardiac-specific manner. Mice were compared to transgenic mice that overexpress normal cTnI (cTnI-wt) and non-transgenic littermates (NTG). cTnI-G203S mice developed all the characteristic features of FHC by age 21 weeks. Left ventricular hypertrophy was observed on echocardiography (1.25 ± 0.05 mm vs. 0.86 ± 0.02 mm in cTnI-wt, P < 0.01), associated with a significant 4-fold increase in RNA markers of hypertrophy, ANF and BNP. Myocyte hypertrophy, myofiber disarray and interstitial fibrosis were observed in cTnI-G203S mice. Expression of the cTnI-G203S mutation in neonatal cardiomyocytes resulted in a significant increase in myocyte volume, and reduced interactions with both troponins T and C. Ca²⁺ cycling was abnormal in adult cardiomyocytes extracted from cTnI-G203S mice, with a prolonged decay constant in Ca²⁺ transients and a reduced decay constant in response to caffeine treatment. Mice with the cTnI-G203S gene mutation develop all the phenotypic features of human FHC. The cTnI-G203S mutation disrupts interactions with partner proteins, and results in intracellular Ca²⁺ dysregulation early in life, suggesting a pathogenic role in development of FHC.

Introduction

Familial hypertrophic cardiomyopathy (FHC) is a primary disorder of the myocardium characterized by cardiac hypertrophy in the absence of other loading conditions such as hypertension [1]. The clinical course of FHC is variable, with inter- and intra-familial variations ranging from benign asymptomatic disease to a malignant phenotype with a high risk of cardiac failure or sudden cardiac death [2], [3], [4]. Genetic studies over the last decade have shown that FHC is an autosomal dominant condition caused by defects in at least 12 genes, the majority of which encode sarcomeric proteins [5], [6], [7].

Cardiac troponin I (cTnI) is an important component of the troponin complex, the main function of which is to regulate cardiac muscle contraction and relaxation in response to changes in intracellular Ca²⁺ [8]. Mutations in the cTnI gene have been identified in up to 5% of families with FHC [9], [10], [11], and more recently, in families with restrictive cardiomyopathy and recessively inherited idiopathic dilated cardiomyopathy [12], [13]. The mutation in the cTnI gene at codon 203, in which glycine is replaced by a serine residue (cTnI-G203S), has been previously reported to cause human FHC, with affected members showing clinical features of FHC, and in some cases, associated with both supraventricular and ventricular arrhythmias [14]. The cTnI-G203S mutation is located near the COOH-terminal end of the cTnI protein, a region considered important in modulating actomyosin kinetics during unloaded sliding, potentially leading to enhanced cross-bridge cycling [8]. Previous cellular studies of the cTnI-G203S mutation also indicate the mutation may enhance Ca²⁺ sensitivity of force generation. Such a change in mechanical performance may contribute to the hypertrophic response [15], [16], [17].

This study sought to develop a murine model of FHC caused by the cTnI-G203S human disease-causing mutation, and to investigate how defects in cTnI may disrupt normal cellular processes related to Ca²⁺ cycling and interactions with partner proteins.