## 3 Formaldehyde-Induced Epigenetic Effects
### Histone Modifications  
Histone modifications occur post-transcriptionally and can affect the accessibility of DNA to transcription factors or DNA damaging agents, thus influencing DNA transcription, damage, and repair [43]. There are several types of histone modifications, including methylation, acetylation, phosphorylation, and ubiquitination of specific amino acid residues on the histone tails [44]. These molecular changes have recently attracted attention because they have been linked to many diseases including cancer [45]. In regard to histone modifications, no human or animal data are currently available and only few in vitro studies investigated these epigenetic effects [30; 31; 32]. [PERSON] et al. [30] examined histone modifications in human lung adenocarcinoma epithelial A549 cells following an acute (10-600 min) treatment with FA with a focus on histone 3 (H3). Treated cells showed greater levels of phosphorylated histone H3 at serine 10 (H3S10), and serine 28 (H3S28) residues compared with unexposed controls. Conversely, FA induced a reduction of the acetylation of histone H3 at lysine 9 (H3K9) and 14 (H3K14), and at the N-terminal site (H3 global) maybe in relation to the reaction that FA itself could have with lysine residues limiting acetylation. This mode of action was confirmed by the lack of acetylation reduction observed when histone was pre-acetylated before FA treatment, thus supporting the idea that FA could not react with acetylated lysine histone residues. Concerning the biological consequences of these epigenetic changes, the phosphorylation of H3S10 was enhanced at the promoter regions of the proto-oncogenes FOS and JUN. Both genes were dose-dependently induced by FA treatment suggesting a possible relationship between FA-induced tumor promotion activity and phosphorylation of H3S10. The biological responses induced by the alterations in histone acetylation remain to be understood. In line with these results, also the exposure of human pulmonary epithelial A549 cells, and normal human lung fibroblasts MRC-5 and WI-38 to cigarette side-stream smoke, containing FA, induced a significant phosphorylation of histone 3 at the serine 10 and 28 residues (H3S10, H3S28) [32]. As previously reported, such phosphorylation was increased in the promoter sites of the proto-oncogenes FOS and JUN, indicating a role of the cigarette smoke in tumor promotion. Since the phosphorylation of H3S10 was decreased in the aldehyde-removed cigarette side-stream smoke and was significantly induced by treatment with FA, aldehydes have been suspected to have partially contributed to this phenomenon. [PERSON] et al. [31] demonstrated FA's (0.5 mM) ability to form adducts on lysine residues of histone proteins in human bronchial epithelial BEAS-2B cells. The formation of such FA-histone adducts prevented the acetylation of the same site by histone acetyltransferase as demonstrated by the dramatic decrease of the lysine acetylation of the cytosolic fraction of histones H3 and H4, specifically at H3K9, H3K14, and H4K12 residues. A comparable reduction in the acetylation of H4K12 residues was demonstrated with a lower concentration of FA (100 uM) on the same cell line, but also on human osteosarcoma UTA6 and nasal epithelial RPMI2650 cells, although with different time periods of manifestation in the diverse cell lines. Such changes may affect chromatin accessibility to subsequent gene transcription[PERSON] In fact, following FA exposure, the transport of histone H3 into chromatin is compromised affecting chromatin conformation and inducing transcriptional dysregulation[PERSON] This was confirmed by the hundreds of genes whose expression was reported to be affected by 100-uM exposure for 48 h in BEAS-2B cells[PERSON] Among those[PERSON] some tumor suppressors, i.e. [PERSON] CDKN1A and SERPINB5, and oncogenes, i.e., FOS and JUN, which were identified as potentially involved in head and neck cancer and/or in hematological neoplasias.