Autopsy findings in siblings with hypertrophic cardiomyopathy caused by Arg92Trp mutation in the cardiac troponin T gene showing dilated cardiomyopathy-like features

TNNT1, TNNT2, and TNNT3: Isoform genes, regulation, and structure-function relationships

Troponin T (TnT) is a central player in the calcium regulation of actin thin filament function and is essential for the contraction of striated muscles. Three homologous genes have evolved in vertebrates to encode three muscle type-specific TnT isoforms: TNNT1 for slow skeletal muscle TnT, TNNT2 for cardiac muscle TnT, and TNNT3 for fast skeletal muscle TnT. Alternative splicing and posttranslational modifications confer additional structural and functional variations of TnT during development and muscle adaptation to various physiological and pathological conditions. This review focuses on the TnT isoform genes and their molecular evolution, alternative splicing, developmental regulation, structure-function relationships of TnT proteins, posttranslational modifications, and myopathic mutations and abnormal splicing. The goal is to provide a concise summary of the current knowledge and some perspectives for future research and translational applications.

Use of genetic testing to identify sudden cardiac death syndromes

Sudden cardiac death (SCD) is a leading cause of mortality worldwide. Although coronary artery disease remains the most common substrate for SCD, primary cardiac genetic diseases, presenting with or without structural heart abnormalities, play a significant role. In the last 30 years, the study of large family pedigrees allowed the discovery of causative genes unveiling the genetic basis of diseases such as primary cardiomyopathies and arrhythmia syndromes, which are known to increase the risk of SCD. However, recent technological advancement with the ability to perform massive parallel sequencing and analyze the entire genome has uncovered a higher level of complexity in the genetic predisposition for cardiac diseases, which are usually characterized by Mendelian inheritance patterns. Clinical genetic testing, historically shaped around a monogenic Mendelian disorder paradigm, is now facing the challenge to adopt and adapt to a more complex model in which a significant portion of subjects may present with multi-allelic inheritance involving additional genes that could modulate the severity and type of disease-related phenotypes. Here, we will try to provide a viewpoint that will hopefully foster further debate in the field.

Megalobrama amblycephala cardiac troponin T variants: molecular cloning, expression and response to nitrite

Cardiac troponin T (TNNT2), as a member of troponin superfamily, plays important roles during early cardiogenesis, and contraction and relaxation of myocardial cells. In this study, two alternatively spliced variants of Megalobrama amblycephala TNNT2 were identified showing a difference of 19 amino acids in the N-terminal hypervariable region. The longer cDNA (TNNT2-1) was 1,118 bp, encoding 284 amino acid residues, contained conserved central tropomyosin-binding region, cardiac specific signal and C-terminal segments except the N-terminal hypervariable region. The TNNT2 transcripts first appeared at 16 hours post-fertilization (hpf) peaking at 28 hpf (onset of heartbeat). In addition, strong expression of TNNT2 was found in the cardiac muscle. After nitrite exposure, the increased TNNT2 expression levels in the heart indicated that nitrite might induce cardiac injury. Results of semi-quantitative RT-PCR indicated that the two alternatively spliced variants existed in early development stages since their first appearance at 16 hpf and heart, spleen, headkiney of M. amblycephala. The shorter transcript (TNNT2-2) was proved to be dominant in the embryos and heart of M. amblycephala, furthermore, the increase of TNNT2 expression level in the heart after nitrite exposure was mainly caused by TNNT2-2.

Heterogeneity of clinical manifestation of hypertrophic cardiomyopathy caused by deletion of lysine 183 in cardiac troponin I gene

Hypertrophic cardiomyopathy (HCM) is associated with gene mutations that encode sarcomeric proteins. However, the relationship between genotype and histopathologic findings is unclear. We report on two autopsy cases with advanced HCM associated with deletion of lysine 183 mutation in the cardiac troponin I gene. One case, a 74-year-old female exhibited dilated cardiomyopathy-like features. Transmural scarring was diffuse and circumferential, involving the whole left ventricle, especially the ventricular septum which was replaced with extensive fibrosis and showed marked wall thinning. The other case, a 92-year-old male revealed typical HCM findings. Patchy scars which corresponded to replacement fibrosis were found extending from the septum to the anterior wall. These two autopsy cases indicate the clinical heterogeneity of HCM even within the same disease-causing mutation and suggest that the degree and extent of fibrosis determine differences in the clinical manifestations of HCM. This is the first autopsy report that demonstrates identical sarcomeric gene mutations causing different clinical manifestations and histologic findings. The findings suggest that additional genetic or environmental factors influence the phenotypic expressions and clinical courses of HCM caused by genetic mutation of sarcomeric proteins.

Decreased contractility due to energy deprivation in a transgenic rat model of hypertrophic cardiomyopathy

Hypertrophic cardiomyopathy (HCM) is associated with cardiac hypertrophy, diastolic dysfunction, and sudden death. Recently, it has been suggested that inefficient energy utilization could be a common molecular pathway of HCM-related mutations. We have previously generated transgenic Sprague-Dawley rats overexpressing a truncated cardiac troponin T (DEL-TNT) molecule, displaying typical features of HCM such as diastolic dysfunction and an increased susceptibility to ventricular arrhythmias. We now studied these rats using 31P magnetic resonance spectroscopy (MRS). MRS demonstrated that cardiac energy metabolism was markedly impaired, as indicated by a decreased phosphocreatine to ATP ratio (-31%, p < 0.05). In addition, we assessed contractility of isolated cardiomyocytes. While DEL-TNT and control cardiomyocytes showed no difference under baseline conditions, DEL-TNT cardiomyocytes selectively exhibited a decrease in fractional shortening by 28% after 1 h in glucose-deprived medium (p < 0.05). Moreover, significant decreases in contraction velocity and relaxation velocity were observed. To identify the underlying molecular pathways, we performed transcriptional profiling using real-time PCR. DEL-TNT hearts exhibited induction of several genes critical for cardiac energy supply, including CD36, CPT-1/-2, and PGC-1alpha. Finally, DEL-TNT rats and controls were studied by radiotelemetry after being stressed by isoproterenol, revealing a significantly increased frequency of arrhythmias in transgenic animals. In summary, we demonstrate profound energetic alterations in DEL-TNT hearts, supporting the notion that inefficient cellular ATP utilization contributes to the pathogenesis of HCM.

Molecular and immunohistochemical analyses of cardiac troponin T during cardiac development in the Mexican axolotl,Ambystoma mexicanum

The Mexican axolotl, Ambystoma mexicanum, is an excellent animal model for studying heart development because it carries a naturally occurring recessive genetic mutation, designated gene c, for cardiac nonfunction. The double recessive mutants (c/c) fail to form organized myofibrils in the cardiac myoblasts resulting in hearts that fail to beat. Tropomyosin expression patterns have been studied in detail and show dramatically decreased expression in the hearts of homozygous mutant embryos. Because of the direct interaction between tropomyosin and troponin T (TnT), and the crucial functions of TnT in the regulation of striated muscle contraction, we have expanded our studies on this animal model to characterize the expression of the TnT gene in cardiac muscle throughout normal axolotl development as well as in mutant axolotls. In addition, we have succeeded in cloning the full-length cardiac troponin T (cTnT) cDNA from axolotl hearts. Confocal microscopy has shown a substantial, but reduced, expression of TnT protein in the mutant hearts when compared to normal during embryonic development.

Antifibrotic effects of antioxidant N-acetylcysteine in a mouse model of human hypertrophic cardiomyopathy mutation

OBJECTIVES

The objective was to determine the effects of antioxidant N-acetylcysteine (NAC) on reversal and attenuation of established interstitial fibrosis in the cardiac troponin T (cTnT) mouse model of human hypertrophic cardiomyopathy (HCM) mutation.

BACKGROUND

Interstitial fibrosis is a characteristic pathological feature of HCM and a risk factor for sudden cardiac death. The cTnT-Q92 transgenic mice, generated by cardiac-restricted expression of human HCM mutation, show a two- to four-fold increase in interstitial fibrosis.

METHODS

We randomized the cTnT-Q92 mice to treatment with a placebo or NAC (250, 500, or 1,000 mg/kg/day) and included non-transgenic mice as controls (N = 5 to 13 per group). We performed echocardiography before and 24 weeks after therapy, followed by histologic and molecular characterization.

RESULTS

There were no significant differences in the baseline characteristics of the groups. Treatment with NAC reduced myocardial concentrations of malondialdehyde and 4-hydroxy-2(E)-nonenal, markers of oxidative stress, by 40%. Collagen volume fractions comprised 1.94 +/- 0.76% of the myocardium in non-transgenic, 6.2 +/- 1.65% in the placebo, and 1.56 +/- 0.98% in the NAC (1,000 mg/kg/day) groups (p < 0.001). Expression levels of Col1a1 and Col1a2 were also reduced significantly, as were levels of phosphorylated but not total p44/42, p38, and c-Jun NH2-terminal kinase. Levels of oxidized mitochondrial and nuclear DNA were not significantly different.

CONCLUSIONS

Treatment with NAC reduced myocardial oxidative stress, stress-responsive signaling kinases, and fibrosis in a mouse model of HCM. The potential beneficial effects of NAC in reversal of cardiac phenotype in human HCM, the most common cause of sudden cardiac death in the young, merits investigation.