Fig. 1

Effects of X chromosome monosomy on gene transcription. X chromosome monosomy and epigenetic modifications. DNA is packed (compacted) into chromatin (DNA associated with histone and non-histone proteins) whose packaging unit is the nucleosome: 147 base pairs of DNA wound around an eight-histone protein (octamer) core. Gene expression rests on the chromatin state: (a) an open, accessible, and active chromatin (euchromatin) is associated with gene expression due to the combination of an histone code (H4Kac, H3K4, H3K36) and un(hypo)methylated CpG islands, which allow access of the transcriptional machinery (RNA polymerase and associated factors), and (b) a closed, inaccessible, and inactive chromatin (facultative heterochromatin) is associated with gene silencing due to the combination of an histone code (histone deacetylation, H3K9me3, H3K27me3) and (hyper)methylated CpG islands, which prevent access of the transcriptional machinery. H4Kac: lysine acetylation at histone 4; H3K4: methylation of lysine 4 residues at histone 3; H3K36: methylation of lysine 36 residues at histone 3; H3K9me3: trimethylation of lysine 9 residues at histone 3; H3K27me3: trimethylation of lysine 27 residues at histone 3; CpG: un-, hypo-, hyper-methylated: addition or removal of methyl groups to the 5′-carbon of cytosine, especially on CpG dinucleotides enriched in small regions of DNA (< 500 bp); MI or MII: meiosis I or meiosis II; ncRNA: noncoding RNA; miRNA: micro-noncoding RNA; IUGR: intrauterine growth retardation; HTA: arterial hypertension; T2DM type 2 diabetes mellitus