# The Double Face of Metals: The Intriguing Case of Chromium
## 7 Chromium Epigenetic Effects on DNA, Histones and MicroRNAs  
Epigenetics refers to hereditary phenotype changes in gene expression without implying changes in the sequence of DNA. Epigenetic regulation of gene expression may be mediated by several mechanisms including DNA hypo- or hypermethylation, the post-translational modifications of histone tail and small noncoding RNA molecules (microRNA, miRNA), that may interfere with the packaging of DNA around nucleosomes and with gene transcription and translation. DNA methyltransferases, histone methyltransferases, HATs and HDACs are the enzymes implicated in epigenetic mechanisms [26; 120]. The covalent methylation of DNA in the presence of DNA methyltransferase and SAM (S-adenosyl methionine) takes place through the formation of a bond between a methyl group and cytosine forming 5-methylcytosine (Figure 3A); moreover, 5-methylcytosine can be actively or passively demethylated. The H3 and H4 histone tails undergo post-translational reactions that include methylation, acetylation, phosphorylation, ubiquitination and sumoylation. Acetylation (Figure 3B) of the N-terminal lysine residue removes the positive chargeson the histones abolishing the ionic bonds between the -NH3+ terminal groups on the histones with the negative charges of the phosphate groups of DNA. The negative charges, destabilizing the compact DNA-histone structure, lead to a relaxed chromatin structure. Histone acetylation is regulated by enzymatic systems, as histone acetyltransferase (HATs) in the presence of acetyl coenzyme A and the histone deacetylases (HDACs). Histones are also methylated on lysine (Figure 3B), arginine and histidine residues. Unlike acetylation, methylation does not alter the charge of histone. Finally, small noncoding miRNA, formed by 25-30 nucleotides, are involved in the post-transcriptional regulation of protein expression binding to a complementary region on target mRNA and silencing their translation, also reducing expression through induced decapping and deadenylation [121; 122]. Small noncoding RNAs are subjected to a covalent methylation modification (6-methyladenosine; 6 MeA) (Figure 3C) coordinated by methyltransferases and demethylases. Several authors suggested that Cr(VI) is able to alter gene expression and induce cancer development through different epigenetic mechanisms. The results of the study of [PERSON] et al. (2013) suggest that G1 phase cell cycle arrest is induced by Cr(VI) [123]. The exposure to potassium dichromate (K2 Cr2O7) or lead chromate (PbCrO4) for 2-24 h can be related to hypomethylulation and DNA methylation of p16 gene, which maintained for 20 h. [PERSON] et al. (2016) used in vitro human bronchial epithelial (16 HBE) cells to analyze the epigenetic role of DNA damage following Cr(VI) exposure [124]. CpG1, CpG31 and CpG32 sites of p16 gene were significatively hypermethylated in Cr(VI) treated groups than controls. Cellular toxicity and p16 expression were demonstrated to be Cr(VI)-concentration dependent. In a following study by the same research group, [PERSON] et al. (2018) examined a cross-sectional study in workers exposed to Cr(VI) as well as 16 HBE cells treated in vitro with Cr(VI) [125]. Hypermethylation of CpG sites were observed in both occupationally exposed workers and in 16 HBE cells. In addition, in workers a positive relationship existed between blood chromium concentration and methylation of CpG sites in DNA repair genes: MGMT, HOGG1, and RAD51. [PERSON] and co-authors (2009) showed that exposing human lung A549 cells to hexavalent chromium increased di-and tri-methylated histone H3 lysine 9 (H3K9) and H3 lysine 4 (H3K4) levels, while reducing tri-methylated histone H3 lysine 27 (H3K27) and dimethylated histone H3 histidine 2 (H3R2) levels [44]. Furthermore, supplementation of reductant ascorbate, a cofactor necessary for histone demethylase activity, inverted the H3K9 demethylation.