Epigenetics refers to all the modifications that alter gene activity without nucleotide sequence modification, but including the chromatin structure alteration as a direct consequence. Indeed, the most widely studied epigenetic mechanism is DNA methylation, which involves the addition of a methyl group onto cytosine nucleotide. DNA methylation may be modified by environmental stimuli including dietary patterns and nutrients. The DNA methylation pattern alteration has been associated with the development of obesity, inflammation and metabolic disturbances (type 2 diabetes, hypercholesterolemia, hypertension, cardiovascular disease). In this context, obesity is considered a contributing factor to the onset and aggravation of the conditions that lead to metabolic syndrome. In the last years it has been reported that low-grade inflammation underlies the pathological processes that are tied to obesity and metabolic syndrome, meanwhile the disruption of the circadian system has also been associated with higher risk to develop obesity-related comorbidities. Furthermore, in the era of personalized nutrition , the DNA methylation pattern of each individual has emerged as a promising tool for the prediction, screening, diagnosis and prognosis of obesity and related pathologies. Likewise, the modulation of DNA methylation marks by different dietary compounds may be a target for newer therapeutic strategies concerning the prevention and treatment of these diseases.
In this context, this research work has taken advantage of omics and high-throughput screening technologies in order to address the following aims: 1) to analyse the association between DNA methylation in white blood cells and the development of obesity in a pediatric population; 2) to investigate the influence of a weight loss intervention in the DNA methylation levels of genes involved in the circadian system, and the association between DNA methylation and changes in the lipid profile; 3) to identify potential epigenetic biomarkers for weight loss within a weight-loss program by integrating transcriptome and methylome microarray data; 4) to evaluate whether a low intake of folic acid is related to adverse metabolic features in obese subjects through changes in gene-specific DNA methylation pattern, and 5) to study whether folic acid and other dietary methyl donors can prevent the inflammatory response in an in vitro model through epigenetic mechanisms.
In relation to the first objective, the results of the first chapter of this thesis suggest a role for DNA methylation, particularly in PTPRS and PER3 genes, in childhood obesity development. Concerning the second objective, we observe that DNA methylation in circadian genes, particularly in BMAL1, is dependent on dietary factors such as energy and carbohydrate intake, and could be used as a biomarker of the lipid profile response to the diet. The third chapter demonstrates that CD44 may have a role in body weight regulation, and its methylation levels can be used as a predictor of the success to a weight-loss intervention. The fourth chapter evidences that subjects with lower folate intake showed more adiposity and higher circulating levels of insulin, glucose, PAI-1, and cortisol, but lower CAMKK2 methylation levels. Moreover, CAMKK2 methylation was negatively associated with HOMA-IR index whereas CAMKK2 expression positively correlated with insulin resistance, suggesting that the methylation of this gene could be an epigenetic mechanism underlying low folic acid intake-mediated insulin resistance. Finally, in relation to the fifth objective, an in vitro study conducted in THP-1 monocytes and macrophages confirms that methyl donors, particularly folic acid, are able to decrease the expression and secretion of several pro-inflammatory mediators like IL-1β and TNF-α, which was accompanied by epigenetic modifications such as increased methylation of IL1B, SERPINE1 and IL18
