The electrocardiogram in complete transposition of the great vessels

An Anterior Second Heart Field Enhancer Regulates the Gene Regulatory Network of the Cardiac Outflow Tract

BACKGROUND

Conotruncal defects due to developmental abnormalities of the outflow tract (OFT) are an important cause of cyanotic congenital heart disease. Dysregulation of transcriptional programs tuned by NKX2-5 (NK2 homeobox 5), GATA6 (GATA binding protein 6), and TBX1 (T-box transcription factor 1) have been implicated in abnormal OFT morphogenesis. However, there remains no consensus on how these transcriptional programs function in a unified gene regulatory network within the OFT.

METHODS

We generated mice harboring a 226-nucleotide deletion of a highly conserved cardiac enhancer containing 2 GATA-binding sites located ≈9.4 kb upstream of the transcription start site of Nkx2-5 (Nkx2-5∆enh) using CRISPR-Cas9 gene editing and assessed phenotypes. Cardiac defects in Nkx2-5∆enh/∆enh mice were structurally characterized using histology and scanning electron microscopy, and physiologically assessed using electrocardiography, echocardiography, and optical mapping. Transcriptome analyses were performed using RNA sequencing and single-cell RNA sequencing data sets. Endogenous GATA6 interaction with and activity on the NKX2-5 enhancer was studied using chromatin immunoprecipitation sequencing and transposase-accessible chromatin sequencing in human induced pluripotent stem cell-derived cardiomyocytes.

RESULTS

Nkx2-5∆enh/∆enh mice recapitulated cyanotic conotruncal defects seen in patients with NKX2-5, GATA6, and TBX1 mutations. Nkx2-5∆enh/∆enh mice also exhibited defects in right Purkinje fiber network formation, resulting in right bundle-branch block. Enhancer deletion reduced embryonic Nkx2-5 expression selectively in the right ventricle and OFT of mutant hearts, indicating that enhancer activity is localized to the anterior second heart field. Transcriptional profiling of the mutant OFT revealed downregulation of important genes involved in OFT rotation and septation, such as Tbx1, Pitx2, and Sema3c. Endogenous GATA6 interacted with the highly conserved enhancer in human induced pluripotent stem cell-derived cardiomyocytes and in wild-type mouse hearts. We found critical dose dependency of cardiac enhancer accessibility on GATA6 gene dosage in human induced pluripotent stem cell-derived cardiomyocytes.

CONCLUSIONS

Our results using human and mouse models reveal an essential gene regulatory network of the OFT that requires an anterior second heart field enhancer to link GATA6 with NKX2-5-dependent rotation and septation gene programs.

"Who" can be found in and beyond of an electrocardiographic strip

Over the years, an electrocardiogram had become the basic tool to study the heart physiology and pathophysiology. Many authors gave a substantial contribution in understanding the electrophysiological basis for numerous electrocardiographic changes. Some of them were named after authors themselves, or others used the names of scientists who first discovered or explained the nature of a particular electrocardiographic finding. In this article, electrocardiographic phenomena and eponyms used in today's electrocardiography are described.

Electrophysiologic residua and sequelae of surgery for congenital heart defects

Postoperative arrhythmias may occur in any patient who undergoes intracardiac surgery for a congenital heart defect. The correction of certain intracardiac heart defects predisposes to a large incidence of cardiac arrhythmias. Ventricular arrhythmias and conduction disturbances are seen after correction of tetralogy of Fallot, ventricular septal defect and atrioventricular canal defect. Supraventricular arrhythmias and sinus nodal dysfunction may be seen after surgery for transposition of the great arteries or atrial septal defect. The identification, evaluation and treatment of these patients are discussed.

Transposition of the great vessels with hypoplasia of the right ventricle

Eight cases are reported in which complete transposition of the great vessels was associated with varying degrees of hypoplasia of the right ventricle. In three, the correct diagnosis was suspected on the basis of certain electrovectorcardiographic findings and confirmed by selective angiography. Quantitative analysis of ventricular volumes and valve circumferences demonstrated that these cases represent a distinct pathological entity.

The diagnosis of complete transposition with hypoplastic right ventricle may be suggested by electrocardiographic evidence of left ventricular hypertrophy, relative absence of right ventricular QRS vectors, and leftward axis for age. The vectorcardiogram reveals a counterclockwise QRS loop in the horizontal plane, situated to the left and posteriorly. These findings occur infrequently in the young infant with complete transposition.

The presence of a hypoplastic right ventricle is confirmed by angiography, which demonstrates a small right ventricular chamber located centrally in the cardiac silhouette, not extending beyond the left border of the thoracic spine.