Transcriptional regulation of sinoatrial node development and function

A healthy condition of the sinoatrial node (SAN) is necessary for maintaining an adequate function of the heart. Therefore, knowledge regarding the genetics and physiology of the SAN is required. We used RNA-sequencing to investigate the expression pattern of the SAN in mouse and human and found numbers of genes to be more abundantly expressed in the SAN compared to the working myocardium. We found transcripts of transcription factor-encoding genes Tbx3, Shox2 and Isl1, and transcripts of Vsnl1 and Smoc2 to be SAN-enriched in both mice and humans. In addition, in both species SAN gene transcripts belonging to the NOTCH, BMP and HOX families of regulating proteins were identified.
In order to gain insight into the mechanisms underlying gene activation in the SAN, we performed ATAC-sequencing on human pacemaker cells derived from differentiation of pluripotent stem cells. We found non-coding regions in the SHOX2 and ISL1 loci to be functionally active in the SAN in mice and inflow tract in fish. Furthermore, a GWAS study indicated an association between prolonged heart rate recovery after exercise (a sinoatrial node phenotype) and genetic variants located at an intergenic region near MED13L, a million base pairs away from TBX3. We found this region contains regulatory sequences, active in pacemaker cells. Deletion of the mouse orthologue of this region resulted in lethality and absence of Tbx3 expression in the SAN. We detected slowed heart rates and increased heart rate variability in mice heterozygous for the deletion. Interestingly, in adult heterozygous mice we found prolonged heart rate recovery after excessive stimulation. With these studies we correlate multiple regions in the genome with expression regulation of genes in the sinoatrial node and provided a functional link between a region in the TBX3 locus harboring genetic variants and a SAN phenotype in humans.