Faculty Bibliography

AIMS: Current mechanisms driving cardiac pacemaker function have focused on ion channel and gap junction channel function, which are essential for action potential generation and propagation between pacemaker cells. However, pacemaker cells also harbor desmosomes that structurally anchor pacemaker cells to each other in tissue, but their role in pacemaker function remains unknown. METHODS AND RESULTS: To determine the role of desmosomes in pacemaker function, we generated a novel mouse model harboring cardiac conduction-specific ablation (csKO) of the central desmosomal protein, desmoplakin (DSP) using the Hcn4-Cre-ERT2 mouse line. Hcn4-Cre targets cells of the adult mouse sinoatrial node (SAN) and can ablate DSP expression in the adult DSP csKO SAN resulting in specific loss of desmosomal proteins and structures. Dysregulation of DSP via loss-of-function (adult DSP csKO mice) and mutation (clinical case of a patient harboring a pathogenic DSP variant) in mice and man, respectively, revealed that desmosomal dysregulation is associated with a primary phenotype of increased sinus pauses/dysfunction in the absence of cardiomyopathy. Underlying defects in beat-to-beat regulation were also observed in DSP csKO mice in vivo and intact atria ex vivo. DSP csKO SAN exhibited migrating lead pacemaker sites associated with connexin 45 loss. In vitro studies exploiting ventricular cardiomyocytes that harbor DSP loss and concurrent early connexin loss phenocopied the loss of beat-to-beat regulation observed in DSP csKO mice and atria, extending the importance of DSP-associated mechanisms in driving beat-to-beat regulation of working cardiomyocytes. CONCLUSIONS: We provide evidence of a mechanism that implicates an essential role for desmosomes in cardiac pacemaker function, which has broad implications in better understanding mechanisms underlying beat-to-beat regulation as well as sinus node disease and dysfunction.

Sudden unexpected death in epilepsy (SUDEP) is the most common cause of epilepsy-related mortality. We hypothesized that electrocardiography (ECG) features may distinguish SUDEP cases from living subjects with epilepsy. Using a matched case-control design, we compared ECG studies of 12 consecutive cases of SUDEP over 10 years and 22 epilepsy controls matched for age, sex, epilepsy type (focal, generalized, or unknown/mixed type), concomitant antiepileptic, and psychotropic drug classes. Conduction intervals and prevalence of abnormal ventricular conduction diagnosis (QRS >/=110 msec), abnormal ventricular conduction pattern (QRS <110 msec, morphology of incomplete right or left bundle branch block or intraventricular conduction delay), early repolarization, and features of inherited cardiac channelopathies were assessed. Abnormal ventricular conduction diagnosis and pattern distinguished SUDEP cases from matched controls. Abnormal ventricular conduction diagnosis was present in two cases and no controls. Abnormal ventricular conduction pattern was more common in cases than controls (58% vs. 18%, p = 0.04). Early repolarization was similarly prevalent in cases and controls, but the overall prevalence exceeded that of published community-based cohorts.

Studies have demonstrated non-myocytes, including fibroblasts, can electrically couple to myocytes in culture. However, evidence demonstrating current can passively spread across scar tissue in the intact heart remains elusive. We hypothesize electrotonic conduction occurs across non-myocyte gaps in the heart and is partly mediated by Connexin43 (Cx43). We investigated whether non-myocytes in ventricular scar tissue are electrically connected to surrounding myocardial tissue in wild type and fibroblast-specific protein-1 driven conditional Cx43 knock-out mice (Cx43fsp1KO). Electrical coupling between the scar and uninjured myocardium was demonstrated by injecting current into the myocardium and recording depolarization in the scar through optical mapping. Coupling was significantly reduced in Cx43fsp1KO hearts. Voltage signals were recorded using microelectrodes from control scars but no signals were obtained from Cx43fsp1KO hearts. Recordings showed significantly decreased amplitude, depolarized resting membrane potential, increased duration and reduced upstroke velocity compared to surrounding myocytes, suggesting that the non-excitable cells in the scar closely follow myocyte action potentials. These results were further validated by mathematical simulations. Optical mapping demonstrated that current delivered within the scar could induce activation of the surrounding myocardium. These data demonstrate non-myocytes in the scar are electrically coupled to myocytes, and coupling depends on Cx43 expression.

OBJECTIVES: Cardiovascular disease (CVD) is a leading cause of death in systemic lupus erythematosus (SLE) and in rheumatoid arthritis (RA). Although only explored in one study, ECG non-specific ST-T abnormalities, in addition to corrected QT-interval (QTc) prolongation, were recently reported in an SLE inception cohort. Importantly, these ECG abnormalities are known predictors of CVD mortality in the general population, yet their prevalence in patients with established SLE has not been evaluated. METHODS: We cross-sectionally investigated the presence of non-specific ST-T and QTc abnormalities in 50 patients with SLE, predominantly Hispanic and black, without CVD or SLE-related cardiac involvement and compared them with 139 patients with RA without CVD. Demographics, disease-specific characteristics and CVD risk factors were ascertained and adjusted for. RESULTS: Patients with SLE (mean age 36+/-13 years, 92% women, 6 years median disease duration, 96% Hispanics and blacks) had a 3.3-fold higher adjusted prevalence of non-specific ST-T abnormalities (56% vs 17%; p <0.0001) compared with RA, despite the older age and higher percentage of men in the RA group. The QTc was 26 ms longer in SLE compared with RA (p=0.002) in the setting of a higher percentage of women, blacks, Hispanics and higher C reactive protein levels in the SLE group. CONCLUSIONS: This study demonstrates a high prevalence of ECG abnormalities in predominantly Hispanic and black patients with SLE. Longitudinal evaluation of the progression to potentially life-threatening arrhythmias and/or cardiovascular events is warranted.

SCN5A encodes the alpha subunit of the major cardiac sodium channel NaV1.5. Mutations in SCN5A are associated with conduction disease and ventricular fibrillation (VF); however, the mechanisms that link loss of sodium channel function to arrhythmic instability remain unresolved. Here, we generated a large-animal model of a human cardiac sodium channelopathy in pigs, which have cardiac structure and function similar to humans, to better define the arrhythmic substrate. We introduced a nonsense mutation originally identified in a child with Brugada syndrome into the orthologous position (E558X) in the pig SCN5A gene. SCN5AE558X/+ pigs exhibited conduction abnormalities in the absence of cardiac structural defects. Sudden cardiac death was not observed in young pigs; however, Langendorff-perfused SCN5AE558X/+ hearts had an increased propensity for pacing-induced or spontaneous VF initiated by short-coupled ventricular premature beats. Optical mapping during VF showed that activity often began as an organized focal source or broad wavefront on the right ventricular (RV) free wall. Together, the results from this study demonstrate that the SCN5AE558X/+ pig model accurately phenocopies many aspects of human cardiac sodium channelopathy, including conduction slowing and increased susceptibility to ventricular arrhythmias.

Mutations in proteins of the desmosome are associated with arrhythmogenic cardiomyopathy (AC; also referred to as "ARVC" or "ARVD"). Life-threatening ventricular arrhythmias often occur in the concealed phase of the disease before the onset of structural changes. Among the various potential mechanisms for arrhythmogenesis in AC, in this article, we concentrate on the relation between desmosomes and sodium channel function. We review evidence indicating that (1) loss of desmosomal integrity (including mutations or loss of expression of plakophilin-2; PKP2) leads to reduced sodium current (INa), (2) the PKP2-INa relation could be partly consequent to the fact that PKP2 facilitates proper trafficking of proteins to the intercalated disc, and (3) PKP2 mutations can be present in patients diagnosed with Brugada syndrome (BrS), thus supporting the previously proposed notion that AC and BrS are not two completely separate entities, but "bookends" in a continuum of variable sodium current deficiency and structural disease.

This review summarizes data in support of the notion that the cardiac intercalated disc is the host of a protein interacting network, called "the connexome", where molecules classically defined as belonging to one particular structure (e.g., desmosomes, gap junctions, sodium channel complex) actually interact with others, and together, control excitability, electrical coupling and intercellular adhesion in the heart. The concept of the connexome is then translated into the understanding of the mechanisms leading to two inherited arrhythmia diseases: arrhythmogenic cardiomyopathy, and Brugada syndrome. The cross-over points in these two diseases are addressed to then suggest that, though separate identifiable clinical entities, they represent "bookends" of a spectrum of manifestations that vary depending on the effect that a particular mutation has on the connexome as a whole.