cAMP sensitivity of HCN pacemaker channels determines basal heart rate but is not critical for autonomic rate control.

BACKGROUND: HCN channels activate the pacemaker current I(f), which is thought to contribute significantly to generation and regulation of heart rhythm. HCN4 represents the dominant isotype in the sinoatrial node and binding of cAMP was suggested to be necessary for autonomic heart rate regulation. METHODS AND RESULTS: In a candidate gene approach, a heterozygous insertion of 13 nucleotides in exon 6 of the HCN4 gene leading to a truncated cyclic nucleotide-binding domain was identified in a 45-year-old woman with sinus bradycardia. Biophysical properties determined by whole-cell patch-clamp recording of HEK293 cells demonstrated that mutant subunits (HCN4-695X) were insensitive to cAMP. Heteromeric channels composed of wild-type and mutant subunits failed to respond to cAMP-like homomeric mutant channels, indicating a dominant-negative suppression of cAMP-induced channel activation by mutant subunits. Pedigree analysis identified 7 additional living carriers showing similar clinical phenotypes, that is, sinus node dysfunction with mean resting heart rate of 45.9±4.6 bpm (n=8) compared with 66.5±9.1 bpm of unaffected relatives (n=6; P<0.01). Clinical evaluation revealed no ischemic or structural heart disease in any family member. Importantly, mutant carriers exhibited normal heart rate variance and full ability to accelerate heart rate under physical activity or pharmacological stimulation. Moreover, mutant carriers displayed distinctive sinus arrhythmias and premature beats linked to adrenergic stress. CONCLUSIONS: In humans, cAMP responsiveness of I(f) determines basal heart rate but is not critical for maximum heart rate, heart rate variability, or chronotropic competence. Furthermore, cAMP-activated I(f) may stabilize heart rhythm during chronotropic response.

Biophysical characterization of KCNQ1 P320 mutations linked to long QT syndrome 1.

Hereditary long QT syndrome (LQTS) is a cardiovascular disorder characterized by prolongation of the QT interval on the surface ECG and a high risk for arrhythmia-related sudden death. Mutations in a cardiac voltage-gated potassium channel, KCNQ1, account for the most common form of LQTS, LQTS1. The objective of this study was the characterization of a novel KCNQ1 mutation linked to LQTS. Electrophysiological properties and clinical features were determined and compared to characteristics of a different mutation at the same position. Single-strand conformation polymorphism analysis followed by direct sequencing was performed to screen LQTS genes for mutations. A novel missense mutation in the KCNQ1 gene, KCNQ1 P320H, was identified in the index patient presenting with recurrent syncope and aborted sudden death triggered by physical stress and swimming. Electrophysiological analyses of KCNQ1 P320H and the previously reported KCNQ1 P320A mutation indicate that both channels are non-functional and suppress wild type I(Ks) in a dominant-negative fashion. Based on homology modeling of the KCNQ1 channel pore region, we speculate that the proline residue at position 320 limits flexibility of the outer pore and is required to maintain the functional architecture of the selectivity filter/pore helix arrangement. Our observations on the KCNQ1 P320H mutation are consistent with previous studies indicating that pore mutations in potassium channel alpha-subunits are associated with more severe electrophysiological and clinical phenotypes than mutations in other regions of these proteins. This study emphasizes the significance of mutation screening for diagnosis, risk-assessment, and mutation-site specific management in LQTS patients.

Adverse events in families with hypertrophic or dilated cardiomyopathy and mutations in the MYBPC3 gene.

BACKGROUND: Mutations in MYBPC3 encoding myosin binding protein C belong to the most frequent causes of hypertrophic cardiomyopathy (HCM) and may also lead to dilated cardiomyopathy (DCM). MYBPC3 mutations initially were considered to cause a benign form of HCM. The aim of this study was to examine the clinical outcome of patients and their relatives with 18 different MYBPC3 mutations. METHODS: 87 patients with HCM and 71 patients with DCM were screened for MYBPC3 mutations by denaturing gradient gel electrophoresis and sequencing. Close relatives of mutation carriers were genotyped for the respective mutation. Relatives with mutation were then evaluated by echocardiography and magnetic resonance imaging. A detailed family history regarding adverse clinical events was recorded. RESULTS: In 16 HCM (18.4%) and two DCM (2.8%) index patients a mutation was detected. Seven mutations were novel. Mutation carriers exhibited no additional mutations in genes MYH7, TNNT2, TNNI3, ACTC and TPM1. Including relatives of twelve families, a total number of 42 mutation carriers was identified of which eleven (26.2%) had at least one adverse event. Considering the twelve families and six single patients with mutations, 45 individuals with cardiomyopathy and nine with borderline phenotype were identified. Among the 45 patients, 23 (51.1%) suffered from an adverse event. In eleven patients of seven families an unexplained sudden death was reported at the age between 13 and 67 years. Stroke or a transient ischemic attack occurred in six patients of five families. At least one adverse event occurred in eleven of twelve families. CONCLUSION: MYBPC3 mutations can be associated with cardiac events such as progressive heart failure, stroke and sudden death even at younger age. Therefore, patients with MYBPC3 mutations require thorough clinical risk assessment.

Absence of auto-antibodies against cardiac troponin I predicts improvement of left ventricular function after acute myocardial infarction.

AIMS: Application of antibodies against cardiac troponin I (cTnI-Ab) can induce dilation and dysfunction of the heart in mice. Recently, we demonstrated that immunization with cTnI induces inflammation and fibrosis in myocardium of mice. Others have shown that auto-antibodies to cTnI are present in patients with acute coronary syndrome, but little is known about the clinical relevance of detected cTnI-Ab. METHODS AND RESULTS: First, anti-cTnI and anti-cTnT antibody titres were measured in sera from 272 patients with dilated- (DCM) and 185 with ischaemic- (ICM) cardiomyopathy. Secondly, 108 patients with acute myocardial infarction (AMI) were included for a follow-up study. Heart characteristics were determined by magnetic resonance imaging 4 days and 6-9 months after AMI. Altogether in 7.0% of patients with DCM and in 9.2% with ICM, an anti-cTnI IgG antibody titre >/=1:160 was measured. In contrast, only in 1.7% of patients with DCM and in 0.5% with ICM, an anti-cTnT IgG antibody titre >/=1:160 was detected. Ten out of 108 patients included in the follow-up study were tested positive for cTnI-Ab with IgG Ab titres >/=1:160. TnI-Ab negative patients showed a significant increase in left ventricular ejection fraction (LVEF) and stroke volume 6-9 months after AMI. In contrast, there was no significant increase in LVEF and stroke volume in TnI-Ab positive patients. CONCLUSION: We demonstrate for the first time that the prevalence of cTnI-Abs in patients with AMI has an impact on the improvement of the LVEF over a study period of 6-9 months.

Computer based modeling of the congenital long-QT 2 syndrome in the Visible Man torso: from genes to ECG.

The congenital long-QT syndrome is commonly associated with a high risk for polymorphic ventricular tachy-cardia and sudden cardiac death. This is probably due to an intensification of the intrinsic heterogeneities present in ventricular myocardium. Increasing the electrophysiological heterogeneities amplifies the dispersion of repolarization which directly affects the morphology of the T wave in the ECG. The aim of this work is to investigate the effects of LQT2, a specific subtype of the long-QT syndrome (LQTS), on the Body Surface Potential Maps (BSPM) and the ECG. In this context a three-dimensional, heterogeneous model of the human ventricles is used to simulate both physiological and pathological excitation propagation. The results are used as input for the forward calculation of the BSPM and ECG. Characteristic QT prolongation is simulated correctly. The main goal of this study is to prepare and evaluate a simulation environment that can be used prospectivley to find features in the ECG or the BSPM that are characteristic for the LQTS. Such features might be used to facilitate the identification of LQTS patients.

Heart rate reduction after heart transplantation with beta-blocker versus the selective If channel antagonist ivabradine.

BACKGROUND: Graft denervation in heart transplant recipients causes sinus tachycardia, occasionally requiring pharmacologic heart rate reduction. The If channel antagonist ivabradine has not been compared to beta-blocker after heart transplantation. Heart rate control, tolerability, short-term safety, and effects on exercise capacity were studied consecutively with an established heart rate-reducing drug (metoprolol succinate) compared to a novel agent (ivabradine) in heart transplant recipients. METHODS: In 25 heart transplant recipients, heart rate, exercise capacity, and patient preference were assessed under no medication (baseline) and after consecutive 8-week treatment periods under metoprolol and ivabradine. RESULTS: Drug discontinuation following side effects occurred in 5 patients (metoprolol: 4, ivabradine: 1); per-protocol analysis was performed on 20 patients completing both consecutive treatment periods. Mean heart rate was reduced from baseline (96.5+/-7.0 bpm) to 84.4+/-8.8 bpm on beta-blocker (P=0.0004 vs. baseline) and to 76.2+/-8.9 bpm with ivabradine (P=0.0001 vs. baseline and P=0.003 vs. beta-blocker). Exercise capacity by spiroergometry was not altered by either drug. Relevant pharmacokinetic interaction with immunosuppressants was not seen under ivabradine; safety laboratory values were unchanged. Mild adverse effects were noted in 45% of patients during beta-blocker and 20% during ivabradine treatment. Questionnaire analysis demonstrated patient preference for heart rate reduction with ivabradine. CONCLUSIONS: Heart rate reduction with ivabradine is effective and potentially better tolerated than beta-blocker therapy in heart transplant recipients. Although the prognostic role of heart rate after HTX is unknown, ivabradine may offer relevant symptomatic benefit, especially in cases of beta-blocker intolerance.

Identification and genotype related classification of children with long QT-syndrome using 24 h Holter recordings.

The long QT syndrome (LQTS) belongs to the family of hereditary diseases and can cause life-threatening arrhythmias and leads to sudden cardiac death. Mutations on six genes are responsible for changes in the electrophysiological properties of myocardial cells that are involved in the repolarization phase. In the surface ecg this is expressed by a prolonged QT interval and genotypespecific shapes for the T-Wave. The aim of the study was to find parameters that quantify properties of the repolarization phase which can be used in addition to the established Schwartz score in the process of diagnosing LQTS. Furthermore, ecg features were evaluated for the separation of the LQT subtypes LQT1, LQT2 and LQT3. The combination of the features PtA50 and QTc yielded with 93% sensitivity and 100% specificity the best results in the field of patient identification. Despite the small dataset consisting of 14 patients that was available for the second aim, the achieved results for the morphology indices motivate further research in this field.

Cardiac-specific activation of Cre expression at late fetal development.

In a first step towards dissecting molecular mechanisms that contribute to the development of cardiac diseases, we have generated transgenic mice that express a Cre-GFP fusion protein under the transcriptional control of a 4.3kb murine cardiac Troponin I gene (cTnI) promoter. Cre-GFP expression, similar in three transgenic lines, is described in one line. In mouse embryos, transgenic for the Cre-GFP and ROSA lacZ reporter allele, first Cre-mediated recombination appeared at 16.5 dpc selectively at the heart. Like the endogenous cTnI gene, transgenic Cre expression showed a slow rise through fetal development that increased neonatally. Bitransgenic hearts, stained at 30 days of age, showed intense signals in ventricular and atrial myocytes while no recombination occurred in other tissues. The delayed onset of Cre activity in cTnI-Cre mice could provide a useful genetic tool to evaluate the function of loxP targeted cardiac genes without interference of recombination during early heart development.