Aims For the majority of congenital heart diseases (CHDs), the full complexity of the causative molecular network, which is driven by genetic, epigenetic and environmental factors, is yet to be elucidated. Epigenetic alterations are suggested to play a pivotal role in modulating the phenotypic expression of CHDs and their clinical course during life. Candidate approaches implied that DNA methylation might have a developmental role in CHD and contributes to the long-term progress of non-structural cardiac diseases. The aim of the present study is to define the postnatal epigenome of two common cardiac malformations, representing epigenetic memory and adaption to hemodynamic alterations, which are jointly relevant for the disease course.

Methods and Results We present the first analysis of genome-wide DNA methylation data obtained from myocardial biopsies of Tetralogy of Fallot (TOF) and ventricular septal defect (VSD) patients. We defined stringent sets of differentially methylated regions between patients and controls, which are significantly enriched for genomic features like promoters, exons and cardiac enhancers. For TOF, we linked DNA methylation with genome-wide expression data and found a significant overlap for hypermethylated promoters and down-regulated genes, and vice versa. We validated and replicated the methylation of selected CpGs and performed functional assays. We identified a hypermethylated novel developmental CpG island in the promoter of SCO2 and demonstrate its functional impact. Moreover, we discovered methylation changes co-localized with novel, differential splicing events among sarcomeric genes as well as transcription factor binding sites. Finally, we demonstrated the interaction of differentially methylated and expressed genes in TOF with mutated CHD genes in a molecular network.

Conclusions By interrogating DNA methylation and gene expression data, we identify two novel mechanism contributing to the phenotypic expression of CHDs: aberrant methylation of promoter CpG islands and methylation alterations leading to differential splicing.