**2. Molecular background of circadian rhythm**

The primary clock genes show circadian expression in the SCN, and light is one of the key drivers that can reset the rhythm phases. There are several crucial proteins in SCN. Transcription activators are aryl hydrocarbon receptor nuclear translocatorlike (ARNTL or BMAL1) and circadian locomotor output cycle caps (CLOCK). Transcription inhibitors are period (PER) and cryptochrome (CRY) [30, 31]. Within 24 h, the entire process of activation and inhibition of gene expression takes place [32, 33]. The circadian system controls gene expression through various mechanisms as a basis of global gene regulation. The first is via E-boxes (promoter and enhancer regulatory elements) of oscillator proteins such as CLOCK, ARNTL, and NPAS2 (neuronal PAS domain protein 2). The second mechanism is using other oscillator proteins such as RORα (retinoic acid receptor-related orphan receptor) and REV-ERBα (or NR1D1, orphan nuclear receptor) via REV-ERB/ROR response element (RRE), which are present in the promoters of specific clock-controlled genes (CCGs) (**Figure 1**). The third mechanism is the daily chromatin remodeling [2, 19, 34, 35].

The ARNTL-CLOCK heterodimers enhance *CRY* and *PER* expression, as well as the expression of additional CCGs. Phosphorylated CRY-PER heterodimers repress the action of ARNTL-CLOCK heterodimer. As a result, *CRY* and *PER* gene transcription is decreased during the day, while ubiquitin degradation reduces the CRY and PER protein levels. The PER2 has histone deacetylase activity and modified chromatin structure, followed by transcription termination [36–39]. The new cycle begins with the termination of the ARNTL-CLOCK repression during the day. Casein kinase 1 (CK1) regulates the quantity of CRY-PER heterodimers' phosphorylation or degradation. CK1 controls protein activity via its phosphorylation [40]. An additional negative loop is REV-ERBα. It binds to the RRE of the *ARNTL* and *CLOCK* genes and inhibits their transcription. Overnight, REV-ERBα degrades, and RORα elevates the *ARNTL* gene transcription [2, 32, 41]. ARNTL-CLOCK heterodimers increase transcription of the nuclear receptors *RORα* and *REV-ERBα* and form an additional circadian rhythm loop [31, 42].

Nearly 10% of the transcripts show circadian rhythmicity [19]. Rhythmic expression of crucial metabolic genes is impaired due to clock gene mutations and lead to metabolic disorders [28]. Fasting glucose levels decrease, and insulin sensitivity

#### **Figure 1.**

*Circadian rhythm gene regulation in cardiovascular diseases. ARNTL and CLOCK activate transcription of* CRY *and* PER *and nuclear receptors (*REV-ERB*α and* ROR*α). CRY and PER heterodimerize and phosphorylate by casein kinases and translate into the nucleus where they prevent binding of the ARNTL-CLOCK to the regulatory regions of target genes. In the second feedback loop, REV-ERBα prevents the transcription of* ARNTL*, while overnight the RORα activates transcription of* ARNTL*. ARNTL—aryl hydrocarbon receptor nuclear translocator-like, CLOCK—circadian locomotor output cycle kaput, CRY cryptochrome, PER—period, P—phosphate, RORα—retinoic-related orphan receptor alpha, Ub—ubiquitin.*

**39**

*Epigenetics of Circadian Rhythm Disruption in Cardiovascular Diseases*

increases in overexpression of the *CRY1* [28]. *ARNTL* deletion and *CLOCK* mutation disturb lipid metabolism [28, 43]. REV-ERBα is involved in liver circadian lipid biosynthesis, and REV-ERBα and ARNTL manage adipocyte differentiation [28]. A primary regulator of bile acid synthesis is REV-ERBα, while the *PER1* and *PER2* deletion upregulates bile acid biosynthesis and causes hepatic cholestasis [28]. Based on circadian rhythms in SCN neurons and peripheral cells, epigenetic mechanisms participate in the formation of circadian rhythms of gene expression [2]. One of the primary circadian genes, CLOCK, has the function of histone acetyltransferase. Chromatin remodeling is an essential underlying mechanism of the clock rhythm and reveals an association between cellular physiology and histone acetylation [2]. ARNTL-CLOCK heterodimer or ARNTL-NPAS2 complex mobilizes HATs and HDACs [28, 44]. To maintain metabolic homeostasis and avoid metabolic disorders, the crosstalk between circadian rhythm and metabolism is necessary [28].

**3. Epigenetic changes in circadian rhythm in cardiovascular diseases**

risk of a disease or in monitoring the response to a particular treatment [14]. In the process of DNA methylation, homocysteine, an amino acid that does not enter into protein composition, is essential [46]. The lack of folate in the diet leads to an increase in plasma homocysteine, which contributes to the rise of S-adenosyl homocysteine. It represses transmethylation reactions and decreases methylation all over the epigenome [1, 46]. In atherogenesis are included homocysteine-induced changes in DNA methylation in smooth muscle vascular cells [1, 47, 48]. Endothelial dysfunction and different aspects of CVD are epigenetically associated with folic acid deficiency [16]. Genomic DNA is hypomethylated in human atherosclerotic lesions [1, 2, 12]. Inflammatory processes involved in the development of atherosclerotic plaques are associated with hypermethylation [1, 49]. There are rhythmic changes in global DNA methylation in human blood, and there is an increased level at night [35]. Changes in circadian rhythm genes methylation were observed in aging mice, but are tissue-dependent [35, 50]. For example, in the stomach of older mice, the methylation of the *PER1* promoter decreased, while the methylation of the *ARNTL*, *CRY1*, and *NPAS2* promoters in the spleen was increased [35]. Sleep disorders affect circadian rhythm gene methylation, especially *ARNTL*, *CRY1*, and *PER1* [35, 51]. Temporary epigenetic changes linked with rhythmic gene expression lead to circadian epiphenotypes [2]. Based on this, it can be concluded that DNA methylation may be reversed by conventional drugs, independent of DNA replication [2].

The histone code is involved in many aspects of cardiovascular physiology, from endothelial cell responses to hypoxia to recovery from MI [16]. CLOCK has enzymatic properties of histone acetyltransferase (HAT). It performs acetylation at Lys537 of H3 histone and ARNTL, which is necessary for circadian rhythm [1, 9]. CLOCK works in collaboration with other HATs to maintain circadian rhythm in the acetylation state of histones at CCG promoters [6]. HDAC activity has an essential function in defining

Rapid adaptation of cells to environmental changes is facilitated by epigenetic mechanisms that also offer a link between genes and the environment [1]. The phenotypic variations observed in humans are more significant than genotype variations alone, and changes in epigenetic gene modification explain them [1, 45]. CVDs, such as atherosclerosis, cardiac hypertrophy, myocardial infarction, and heart failure, are associated with epigenetic mechanisms ranging from DNA methylation, histone modification, to ncRNAs [13]. An essential way of developing CVD early in life involves epigenetic changes [12]. The underlying mechanism providing the link between the early life environment and the subsequent CVD risk is epigenetic modifications [12]. The association of methylation with specific genes may be useful in assessing the

*DOI: http://dx.doi.org/10.5772/intechopen.92057*

#### *Epigenetics of Circadian Rhythm Disruption in Cardiovascular Diseases DOI: http://dx.doi.org/10.5772/intechopen.92057*

*Cardiac Diseases - Novel Aspects of Cardiac Risk, Cardiorenal Pathology and Cardiac Interventions*

in SCN. Transcription activators are aryl hydrocarbon receptor nuclear translocatorlike (ARNTL or BMAL1) and circadian locomotor output cycle caps (CLOCK). Transcription inhibitors are period (PER) and cryptochrome (CRY) [30, 31]. Within 24 h, the entire process of activation and inhibition of gene expression takes place [32, 33]. The circadian system controls gene expression through various mechanisms as a basis of global gene regulation. The first is via E-boxes (promoter and enhancer regulatory elements) of oscillator proteins such as CLOCK, ARNTL, and NPAS2 (neuronal PAS domain protein 2). The second mechanism is using other oscillator proteins such as RORα (retinoic acid receptor-related orphan receptor) and REV-ERBα (or NR1D1, orphan nuclear receptor) via REV-ERB/ROR response element (RRE), which are present in the promoters of specific clock-controlled genes (CCGs) (**Figure 1**). The third mechanism is the daily chromatin remodeling [2, 19, 34, 35]. The ARNTL-CLOCK heterodimers enhance *CRY* and *PER* expression, as well as the expression of additional CCGs. Phosphorylated CRY-PER heterodimers repress the action of ARNTL-CLOCK heterodimer. As a result, *CRY* and *PER* gene transcription is decreased during the day, while ubiquitin degradation reduces the CRY and PER protein levels. The PER2 has histone deacetylase activity and modified chromatin structure, followed by transcription termination [36–39]. The new cycle begins with the termination of the ARNTL-CLOCK repression during the day. Casein kinase 1 (CK1) regulates the quantity of CRY-PER heterodimers' phosphorylation or degradation. CK1 controls protein activity via its phosphorylation [40]. An additional negative loop is REV-ERBα. It binds to the RRE of the *ARNTL* and *CLOCK* genes and inhibits their transcription. Overnight, REV-ERBα degrades, and RORα elevates the *ARNTL* gene transcription [2, 32, 41]. ARNTL-CLOCK heterodimers increase transcription of the nuclear receptors *RORα* and *REV-ERBα* and form an additional circadian rhythm loop [31, 42]. Nearly 10% of the transcripts show circadian rhythmicity [19]. Rhythmic expres-

sion of crucial metabolic genes is impaired due to clock gene mutations and lead to metabolic disorders [28]. Fasting glucose levels decrease, and insulin sensitivity

*Circadian rhythm gene regulation in cardiovascular diseases. ARNTL and CLOCK activate transcription of* CRY *and* PER *and nuclear receptors (*REV-ERB*α and* ROR*α). CRY and PER heterodimerize and phosphorylate by casein kinases and translate into the nucleus where they prevent binding of the ARNTL-CLOCK to the regulatory regions of target genes. In the second feedback loop, REV-ERBα prevents the transcription of* ARNTL*, while overnight the RORα activates transcription of* ARNTL*. ARNTL—aryl hydrocarbon receptor nuclear translocator-like, CLOCK—circadian locomotor output cycle kaput, CRY cryptochrome, PER—period, P—phosphate, RORα—retinoic-related orphan receptor alpha, Ub—ubiquitin.*

**38**

**Figure 1.**

increases in overexpression of the *CRY1* [28]. *ARNTL* deletion and *CLOCK* mutation disturb lipid metabolism [28, 43]. REV-ERBα is involved in liver circadian lipid biosynthesis, and REV-ERBα and ARNTL manage adipocyte differentiation [28]. A primary regulator of bile acid synthesis is REV-ERBα, while the *PER1* and *PER2* deletion upregulates bile acid biosynthesis and causes hepatic cholestasis [28].

Based on circadian rhythms in SCN neurons and peripheral cells, epigenetic mechanisms participate in the formation of circadian rhythms of gene expression [2]. One of the primary circadian genes, CLOCK, has the function of histone acetyltransferase. Chromatin remodeling is an essential underlying mechanism of the clock rhythm and reveals an association between cellular physiology and histone acetylation [2]. ARNTL-CLOCK heterodimer or ARNTL-NPAS2 complex mobilizes HATs and HDACs [28, 44]. To maintain metabolic homeostasis and avoid metabolic disorders, the crosstalk between circadian rhythm and metabolism is necessary [28].
