# Vitamin D and Rehabilitation after Stroke: Status of Art
## 3 Genetic and Epigenetic of Vitamin D Pathway
### Epigenetic  
Another intriguing topic is the epigenetic regulation mechanism of the genes involved in the vitamin D metabolism due to a DNA methylation process. DNA methylation is an epigenetic mechanism characterized by the covalent attachment of a methyl group to the C5 position of cytosine residues in CpG dinucleotide (cytosine followed by guanine residue) [80; 81]. CpG methylation does not change the primary DNA structure, but rather alters the chromatin structure, regulating the gene expression patterns. These changes are heritable between generations with a specific pattern tissue. Indeed, DNA methylation is involved in several biological process, including cell cycle, differentiation and genomic imprinting. DNA regions with a relatively high CpG dinucleotide content are referred to as CpG islands [82] that are distributed in a non-random manner across the human genome and often span 5\({}^{\prime}\) untranslated region (UTR), 3\({}^{\prime}\) UTR, promoter region and the first exon of protein-coding genes. The methylation of CpG islands usually acts to turn off (silence) transcription by recruiting histone deacetylases, thereby inducing the formation of inactive chromatin [83]. Generally, hypermethylation is often associated with a decreased gene expression, while hypo-methylation is associated with an increased gene expression [83], so mapping the methylation patterns of CpG sites is an important tool for understanding both normal and pathogenic gene expression events [84; 85]. In the contest of vitamin D metabolism, _CYP2R1_, _CYP2R1_, _CYP24A1_ and _VDR_ have large CpG islands in their promoter regions and in the gene body [86; 87]. Consequently, the DNA methylation of CpG sites in these regions can regulate the transcriptional repression of these genes. _CYP2R1_ gene is located on chromosome11p15.2 (chr11:14899556-14913751) and the promoter region contains a CpG island that includes 77 CpG sites between +850 and -20 with respect to the start site of the gene, in agreement with the UCSC Genome Browser Homepage (GRCh37/hg19) [87]. Genome-wide association studies on leukocytes DNA from 22 African American male youths showed high methylation levels of the _CYP2R1_ promoter region in individuals with severe vitamin D deficiency versus the control group [30]. After one year of supplementation with vitamin D, the _CYP2R1_ promoter showed a significantly hypomethyllation [30]. In agreement with these data, [PERSON] et al. found that, in post-menopausal women, a methylation reduction in _CYP2R1_ after vitamin D supplementation [31], while [PERSON] et al. found, in 80 Australian subjects (principally women), that _CYP2R1_ methylation levels are negatively associated with plasma 25(OH)D [32]. Taken together, these data suggest that, when vitamin D serum levels are low, the _CYP2R1_ promoter is hypermethylated and that a vitamin D supplementation can contribute to modulating the methylation levels of this gene. The _CYP2 TB1_ gene is located on 12q14.1 chromosome (chr12:58156117-58160976) and, in the promoter region, a CpG island (91 CpG sites) is present between +2120 and +976 positions with respect to the start site of the gene [88]. The promoter region contains a negative VDRE (nVDRE) that, through recruitment of both histone deacetylases (HDACs) and DNA methyltransferases (DNMTs), by VDR/retiinoid X receptor (RXR) is responsible for 1,25-D3-dependent _trans_ repression [89]. In different types of tumor, such as primary lymphoma, leukemia, chorioocarcinoma and breast cancer cells, methylation levels of the _CYP27B1_ promoter region are increased when compared with normal cells [39; 40; 41; 42]. Moreover, hypomethyllation of the _CYP27B1_ gene positively influences the rate of body mass index (BMI) observed in obese children (82 obese children, age: 3-18 years) [43], while no methylation differences in this gene were found by previous studies [30; 31; 32].