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

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 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

the intensity of myocardial ischemia, especially after MI [16]. Inhibition of HDAC can promote angiogenesis and reduce myocardial damage after MI [16], such as valproic acid (VPA), which is an HDAC inhibitor [2]. Histone deacetylases, SIRT1 (sirtuin 1), and SIRT6 participate in the histone modification, thus controlling gene expression [35] and providing a molecular connection among metabolism and circadian rhythm [6]. SIRT1 deacetylates regulatory proteins and acts as a rhythm-promoting agent in circadian oscillators [35]. SIRT1 has a unique role in central and peripheral circadian rhythms [35]. The purpose of histone phosphorylation in CVDs is minimal [14], while SUMO proteins influence the activity of several essential factors that are important for cardiac development [14]. There are connections between circadian rhythm regulators, chromatin modifications, and cellular metabolism [1, 52].

Numerous lncRNAs have essential regulatory functions in various CVDs [14]. The miRNAs regulate cholesterol metabolism, oxidative stress, and endothelial dysfunction, diverse cellular processes involved in atherosclerosis [14]. MiRNAs may be relevant regulators of circadian rhythm [1]. Circulating miRNA-145 and miRNA-126 are decreased in patients with coronary artery disease, while miRNA-1, miRNA-499, and miRNA-133b are increased during acute myocardial infarction [13]. All those miRNAs can be biomarkers of CVD.

Circadian rhythms combine metabolic and environmental signals and alter gene expression when adapting the organism to particular circumstances [6]. Many epigenetic regulators in some tissues are controlled in a circadian fashion [19, 53]. The challenge is to determine whether epigenetic variations happen in a rhythmic pattern in tissues included in the CVD development [12, 19]. Epigenetics can contribute to enhancing CVD therapies and finding new markers for CVD screening [16, 54].
