## 3 Formaldehyde-Induced Epigenetic Effects
### MicroRNAs  
MicroRNAs are small non-coding RNAs, of about 22 nucleotides, that regulate gene expression at the post-transcriptional level. The importance to investigate potential effects of FA on miRNAs relies on their ability to regulate gene expression through the binding of mRNA, causing rapid decay of the message, translational repression of the mRNA signals, and inducing cleavage of newly translated polypeptides [46]. Concerning in vivo responses induced by FA inhalation, the same group of researchers demonstrated a significant dysregulation in miRNA expression in the nose, olfactory bulb and white blood cells (WBCs) of rodents [35; 36], as well as in the nasal epithelium cells of non-human primates [34]. Interestingly, [PERSON] et al. [35] applied a multitiered approach to define possible genome-wide miRNA responses to FA, as well as time and tissue-related alterations in transcriptional profiles. MiRNA expression, in fact, was investigated within the nasal respiratory epithelium, circulating WBCs, and bone marrow (BM) and were affected in rats treated with FA via inhalation for 7 and 28 days [35]. The nose was the site with the greatest change in miRNA expression levels, followed by the WBCs, while no changes were evident in the BM samples. When alterations were analyzed according to the different 7 and 28 day exposures, FA-induced miRNA changes in nose showed high stability and no significant quantitative and qualitative differences over time. Such alterations failed to persist after a recovery period (7 days) following the 28-day treatment. Comparing the FA-responsive miRNAs across the different tissues revealed largely distinct miRNA responses, as only 10 alterations overlapped between nose and WBC samples and most with different directions of expression. Overall, these findings suggest that cells in direct contact with FA exposure are more responsive at the miRNA level than cells distant from exposure contact sites. Moreover, 7 of the 34 miRNAs with sustained decreased nose expression over time, i.e., let-7a, let-7c, let-7f, miR-10b, miR-126, miR-21, and miR-23a, have been previously shown to have decreased expression in cultured lung cells exposed to 1 ppm FA [33]. To integrate miRNA responses with transcriptional changes, mRNA profiles were assessed in the nose and WBCs. FA-responsive miRNAs were able to regulate a series of mRNAs, ranging from the 7% to 35% of those FA-related, determining an increase in immune system/inflammation signaling in both nose and WBCs. Considering the miRNA alterations reported in nasal epithelium of treated rats, and the potential regulating role of miRNAs in the neurogenesis of the olfactory bulb, [PERSON] et al. [36] investigated expression changes within olfactory bulb in mice after FA acute to subacute inhalation. Eighteen miRNAs were upregulated and seven were down-regulated after an acute (6-h) exposure to FA via inhalation. In the olfactory bulb of mice exposed to FA, the alterations in miRNA expression was more profound after 1 day of exposure for 6 h relative to 7 days of 6 h/day of exposure, as this last condition only altered the expression of nine miRNAs. The differentially expressed miRNA, i.e., mir-141-5p, mir-200a-5p, mir-199b-5p, are believed to be involved in axon guidance and MAPK signaling, which are associated with inter-neuronal maturation in the bulb, essential to assure a normal olfactory function [47; 48]. Additionally, also other pathways related to the immune system, such as toll-like receptor signaling and communication between innate and adaptive immune cells, molecular mechanisms of cancer, as well as cell cycle regulation have been also associated to the miRNA alterations detected after both 1- and 7-days of exposure. Potential changes in miRNA expression profiles have been also investigated in the nasal epithelial cells from the maxilla-turbinate region of _Cymomolgus macaques_, as the area with the maximum adsorption of FA, collected from animals treated via inhalation [34].