Understanding Circadian Mechanisms of Sudden Cardiac Death: A Report From the National Heart, Lung, and Blood Institute Workshop, Part 1: Basic and Translational Aspects

Abstract

Sudden cardiac death (SCD), the unexpected death due to acquired or genetic cardiovascular disease, follows distinct 24-hour patterns in occurrence. These 24-hour patterns likely reflect daily changes in arrhythmogenic triggers and the myocardial substrate caused by day/night rhythms in behavior, the environment, and endogenous circadian mechanisms. To better address fundamental questions regarding the circadian mechanisms, the National Heart, Lung, and Blood Institute convened a workshop, Understanding Circadian Mechanisms of Sudden Cardiac Death. We present a 2-part report of findings from this workshop. Part 1 summarizes the workshop and serves to identify research gaps and opportunities in the areas of basic and translational research. Among the gaps was the lack of standardization in animal studies for reporting environmental conditions (eg, timing of experiments relative to the light dark cycle or animal housing temperatures) that can impair rigor and reproducibility. Workshop participants also pointed to uncertainty regarding the importance of maintaining normal circadian rhythmic synchrony and the potential pathological impact of desynchrony on SCD risk. One related question raised was whether circadian mechanisms can be targeted to reduce SCD risk. Finally, the experts underscored the need for studies aimed at determining the physiological importance of circadian clocks in the many different cell types important to normal heart function and SCD. Addressing these gaps could lead to new therapeutic approaches/molecular targets that can mitigate the risk of SCD not only at certain times but over the entire 24-hour period.

Keywords: National Heart, Lung, and Blood Institute; cardiovascular disease; circadian clocks; circadian rhythm; sudden cardiac death.

Figure 1.. SCD and life-threatening triggers for arrhythmias can exhibit a 24-hour pattern of occurrence.

This 24-hour pattern suggests that the aspects of usual day/night rhythms, including 24-hour environmental changes (daily cycles in light, temperature and ambient air composition), the 24-hour behaviors (e.g., postural change and increased activity upon awakening, daily sleep/wake cycle, daily fasting/feeding cycle), or 24-hour internal body clock changes (endogenous circadian rhythms) could, alone or in combination, lead to a day/night rhythm in the risk for SCD. Circadian rhythms are generated by cell autonomous circadian clocks, present in nearly every cell/tissue in the body and resonate with a periodicity of ~24 hours.

Figure 2.. Theoretical framework for generating day/night patterns in arrhythmias.

There are day/night rhythms in both arrhythmogenic triggers (red line, A) and anti-arrhythmic or protective mechanisms (blue line, P). A disruption in these rhythms could impact arrhythmia risk (AR, blue = low risk and red = higher risk) during the sleep (S) wake (W) cycle. A. In a healthy individual these patterns may synchronize to produce an overall low arrhythmia risk throughout the sleep-wake cycle. Increase in arrhythmia susceptibility at certain times during the sleep-wake cycle may be caused by B, disease-states that alter the myocardial substrate to cause a loss in protection; C, stressors that increase the relative amplitude of the arrhythmogenic triggers; or D, disruptions that result in a misalignment of arrhythmogenic and protection rhythms.

Figure 3.. Schematic of the circadian clock, a transcription/translation feedback loop that drives cellular circadian rhythms.

Heterodimerized BMAL1 and CLOCK bind E-box (or E-box related) elements to activate transcription of core clock genes including Period (PER), Cryptochrome (CRY), Rev-erb, and ROR. Top loop: PER and CRY proteins dimerize and negatively feedback on BMAL1 and CLOCK activity. Bottom loop: ROR and Rev-erb proteins directly increase and decrease, respectively, the transcription of BMAL1. Box: The circadian clock mechanism also regulates the cell and tissue-specific expression of “clock controlled genes” that, although outside the timekeeping network, are important in the regulation of physiology and behavior.

Figure 4.. Quantifying circadian rhythms in vivo and in vitro.

The standard representation of circadian timing involves plotting variables across numerous consecutive days. A, Mouse rhythms (eg, behavioral or physiological) are often shown as each day double-plotted on the abscissa and stacked along the ordinate to produce a temporal raster plot or “actogram.” Mice maintained in the standard laboratory 12- hour light (open bar):12-hour dark cycle (solid bar) display outputs that align to the light dark (LD) cycle. Shown is an example of nocturnal wheel running activity. Switching to constant darkness (DD, shaded region) causes mice to display a consistently advancing phase in outputs because the endogenous cycle length of the suprachiasmatic nucleus (SCN) is shorter than 24-hours. B, Measuring circadian clock/molecular rhythms in tissues/cells requires genetic reporters that typically use luciferase (due to the short half-life of the protein). Cultured cells/tissues can be entrained by the use of several tissue culture techniques that synchronize/align circadian clocks (yellow cells). Bioluminescence activity can then be quantified in living cells/tissue over the course of several days to clearly delineate reporters that generate robust molecular rhythms (green) from those that do not (red).