## 3 Formaldehyde-Induced Epigenetic Effects
### DNA Methylation  
DNA methylation is a major epigenetic modification in mammals [29; 37]. It involves the addition of a methyl group to the cytosine nucleotide at CpG sites via the donation from S-adenosylmethionine, catalyzed by DNA methyltransferases (DNMTs) [37]. DNA methylation can affect the chromatin structure, and therefore, control the selective transcription or silencing of tissue-specific genes potentially involved in disease pathogenesis [29]. The possible effects of occupational exposure to FA on global DNA methylation were investigated in a population of Brazilian beauty salon workers [28]. These were exposed during hair treatments employing creams illegally added with FA as a straightening agent. Environmental monitoring allowed to rank workers into three increasing groups (A, B, C) of exposure according to the FA environmental workplace levels: \(<\)0.01 ppm; 0.03 ppm \(<\) FA \(<\) 0.06 ppm; and 0.08 ppm \(<\) FA \(<\) 0.24 ppm. Interestingly, significantly higher levels of global DNA methylation were detected in the whole blood samples from the two groups with the highest environmental exposures in comparison to the group with the lowest one, but not between each other. A weak, but significant correlation was also found between global DNA methylation and FA in personal exposure sampling. Furthermore, methylation changes were positively associated with the number of straightening procedures carried out per month. In this latter regard, to better define the possible dose-response relationships, the hairdressers from the two groups with the highest levels of exposure were further divided depending on whether they were directly involved in hair straightening procedures or not. A significant increase in FA personal exposure levels (median values: 0.19 ppm vs. 0.041 ppm), and percentages of global DNA methylation (4.63% vs. 4.29%) was determined in the directly exposed group compared to the other one. This may suggest that the proximity these workers experienced to the source of contamination may play a significant role in affecting exposure levels and biological responses [12; 38]. Interestingly, these epigenetic changes were detected at exposure levels lower than occupational limits proposed by different agencies in Brazil [39], by the USA Occupational Safety and Health Administration [40], and by the European Chemical Agency [41] supporting their role as early biological effect indicators in response to chemical insults. Unfortunately, no other data are currently available on occupational or general living FA exposure and DNA methylation changes for comparison and therefore, caution is necessary for a correct interpretation of the results and the extrapolation of biological monitoring conclusions. An in vitro study reported a time-dependent decrease in global DNA methylation following chronic (24 weeks), low-dose FA exposure of 16 human bronchial epithelial (16 HE) cells compared to controls [29]. The expression of DNA methyltransferases (DNMTs), as the enzymes responsible for maintaining the methylation status in the genome, was tested both at mRNA and protein level to define the initial mechanisms of global genomic hypomethylation induced by FA. Also, the expression of the methyl-CpG-binding protein DNA-binding domain protein 2 (MBD2), responsible for binding methylated DNA to suppress transcription from a methylated gene was investigated. A significant reduction in the expression of DNMT3a and DNMT3b was detected with the increasing exposure time to FA, while that of DNMT1 and MBD2 time-dependently increased. The loss of global DNA methylation, an epigenetic alteration associated with genomic instability, after long term exposure to a low-dose, may be one of the possible underlying carcinogenic mechanisms of FA. Moreover, it cannot be excluded that the different DNMT expression responses may be related to their diverse role in DNA methylation.