Abstract 15270: Circadian Variation in SCN5A Driven by the Molecular Clock
Circadian rhythms, endogenously generated rhythms with a 24 hour-period, are driven by an intrinsic molecular clock which is described as a transcription/translation feedback system. In mammals Bmal1 and Clock encode transcription factors that are core components of the molecular clock. BMAL1:CLOCK heterodimers also transcriptionally regulate a group of genes referred to as clock-controlled genes which are believed to be necessary for maintenance of normal cell physiology. We developed a tamoxifen-induced cardiac-specific Bmal1 knockout mouse strain that lacked Bmal1 only in cardiomyocytes. We treated mice with either vehicle or tamoxifen and found the recombination efficiency with tamoxifen treatment to be 95.6±2.4% (n=4). In vivo electrocardiography using telemetry showed that cardiac specific loss of BMAL1 prolonged the corrected QT (QTc) interval during both day(vehicle QTc = 39.02±1.34, n=3; tamoxifen QTc = 42.98±1.19, n=3, p<0.05) and night(vehicle QTc = 37.7±0.87, n=3; tamoxifen QTc = 45.18±1.92, n=3, p<0.05). Based on these results, we tested whether different cardiac ion channel genes or their regulatory proteins had a circadian expression pattern. We analyzed gene expression over 24 hrs and found that expression of Scn5a exhibited a circadian pattern with greater than 2 fold change from peak to trough in the ventricle from C57BL/6J mice. In contrast, the expression of other channels and regulators of Scn5a, including Kcnj2, Kchip2, Kcnd2,Gpd1l, Scn4a,Cav3 and Cacna1c, were not circadian. Scn5a showed a similar pattern of expression in ventricular tissue isolated from WKY rats demonstrating that this was not a species-specific phenomenon. To test whether human Scn5a promoter activity was regulated by the molecular clock, NIH/3T3 cells were co-transfected with the Scn5a promoter-reporter construct, and expression vectors for molecular clock components, Clock and Bmal1. Compared to control cells, normalized luciferase activity increased by 4.0±0.41 in cells co-expressing BMAL1 and CLOCK (n=8 wells/ condition, p<0.05). Together the data suggest that the normal electrical activity of the heart depends on an intact cardiac molecular clock and Scn5a is a candidate clock-controlled gene.
- © 2011 by American Heart Association, Inc.