Tetralogy of Fallot (TOF) is the most commonly observed conotruncal congenital heart defect. Treatment of these patients has evolved dramatically in the last few decades, yet a genetic explanation is lacking for the failure of cardiac development for the majority of children with TOF. Our goal was to perform genome wide analyses and characterize expression patterns in cardiovascular tissue (right ventricle, pulmonary valve and pulmonary artery) obtained at the time of reconstructive surgery from 19 children with tetralogy of Fallot.

The importance of microRNAs for maintaining stability in the developing vertebrate heart has recently become apparent. In addition, there is a growing appreciation for the significance of microRNAs in developmental pathology, including the formation of congenital heart defects. We examined the expression of microRNAs in right ventricular (RV) myocardium from infants with idiopathic tetralogy of Fallot (TOF, without a 22q11.2 deletion), and found 61 microRNAs to be significantly changed in expression in myocardium from children with TOF compared to normally developing comparison subjects (O’Brien et al. 2012). Predicted targets of microRNAs with altered expression were enriched for gene networks that regulate cardiac development. We previously derived a list of 229 genes known to be critical to heart development, and found 44 had significantly changed expression in TOF myocardium relative to normally developing myocardium. These 44 genes had significant negative correlations with 33 microRNAs, each of which also had significantly changed expression. Here, we focus on miR-421, as it is significantly upregulated in RV tissue from infants with TOF; is predicted to interact with multiple members of cardiovascular regulatory pathways; and has been shown to regulate cell proliferation. We knocked down, and over expressed miR-421 in primary cells derived from the RV of infants with TOF, and infants with normally developing hearts, respectively. We found a significant inverse correlation between the expression of miR-421 and SOX4, a key regulator of the Notch pathway, which has been shown to be important for the cardiac outflow track. These findings suggest that the dysregulation of miR-421 warrants further investigation as a potential contributor to tetralogy of Fallot.

Congenital heart disease (CHD) is a leading cause of death in children <1 year of age. Despite intense effort in the last 10 years, most CHDs (~70%) still have an unknown etiology. Conotruncal based defects, such as Tetralogy of Fallot (TOF), a common complex of devastating heart defects, typically requires surgical intervention in the first year of life. We reported that the noncoding transcriptome in myocardial tissue from children with TOF is characterized by significant variation in levels of expression of noncoding RNAs, and more specifically, a significant reduction in 12 small cajal body-associated RNAs (scaRNAs) in the right ventricle. scaRNAs are essential for the biochemical modification and maturation of small nuclear RNAs (spliceosomal RNAs), which in turn are critical components of the spliceosome. This is particularly important because we also documented that splicing of mRNAs that are critical for heart development was dysregulated in the heart tissue of infants with TOF. Furthermore, we went on to show, using the zebrafish model, that altering the expression of these same scaRNAs led to faulty mRNA processing and heart defects in the developing embryo. This review will examine how scaRNAs may influence spliceosome fidelity in exon retention during heart development and thus contribute to regulation of heart development.