**6. Steatosis and genetic of steatosis**

[144]. Epigenetics can be defined as heritable changes that are mitotically stable (and poten‐ tially meiotically) and affect gene function but do not involve changes in the DNA se‐

Currently, there is a growing interest in the study of the relationship between genetic variation, epigenetic variation and disease simultaneously. The two main mechanisms that lead to epigenetic changes are DNA methylation, and the alterations to histone proteins that alter the

Interindividual variations in epigenetic changes like CpG methylation can potentially alter gene function and predispose to obesity. The variation in the degree of methylation, in fact, is able to modulate the expression of genes involved in controlling hypothalamic appetite [148]. Using a genome‐wide approach, obesity has been related to changes in DNA methylation status in peripheral blood leukocytes of lean and obese adolescents for two genes: in the *UBASH3A* (*ubiquitin‐associated and SH3 domain‐containing protein A*, OMIM \*605736, 21q22.3) gene, a CpG site showed higher methylation levels in obese cases, and one CpG site in the promoter region *TRIM3* (*tripartite motif‐containing protein 3*, OMIM \*605493, 11p15.4) gene, showed lower methylation levels in the obese cases [149]. Also the obesity risk allele of *FTO* has been associated with higher methylation of sites within the first intron of the *FTO* gene, suggesting an interaction between genetic and epigenetic factors [150]. In addition, the obesity risk allele of *FTO* affects the methylation status of sites related to other genes (*KARS* [16q23.1; OMIM \*601421], *TERF2IP* [16q23.1; OMIM \*605061], *MSI1* [12q24.31; OMIM \*603328], *STON1* [2p16.3; OMIM \*605357] and *BCAS3* [OMIM \*607470]), showing that the *FTO* gene may influence the methylation level of other genes [151]. Finally, a recent work has demonstrated that hypermethylation of the *POMC* gene plays an important role in preparing to obesity by

Epigenetic changes usually occur during prenatal development or the early post‐natal period. Already *in utero*, in fact, there may be a switch of energy balance resulting from exposure to specific environmental factors, resulting in epigenetic changes that can affect the potential of the fat mass of offspring. For example in a recent work, the methylation status of CpG from five candidate genes in umbilical cord tissue DNA from healthy neonates was measured, and it was found that higher methylation levels within promoter region of *RXRA* (*retinoid X receptor, alpha*, OMIM \*180245, 9q34.2) gene, measured at birth, were strongly correlated with greater adiposity in later childhood [152]. Maternal nutrition is a major factor leading to epigenetic changes. Thus, the levels of vitamins consumed in pregnancy such as folate, methionine and vitamin B12, which affect methylation, become very important [147]. One study showed that prenatal exposure to malnutrition can determine abnormal DNA methylation resulting in epigenetic modifications that remain for the whole existence and that predispose to obesity and metabolic and cardiovascular risk in later life [153]. On the other hand also glycemic status during pregnancy is an important factor; in fact, hyperglycemia, as well as having a strong impact on the child's weight, can increase the risk of developing insulin

likelihood that specific genes are transcribed [146, 147].

reducing the expression of the gene itself [148].

resistance and obesity [147].

quence [145].

234 Adiposity - Omics and Molecular Understanding

Non‐alcoholic fatty liver disease (NAFLD) actually represents the most frequent cause of chronic liver disease in industrialized countries in children and adolescents, as a direct consequence of the rise in childhood obesity [154]. Italian epidemiological data indicate that NAFLD affects approximately 3–10% of general pediatric population. This percentage increases up to >70%, with a male‐to‐female ratio of 2:1, in obese children [155]. NAFLD is defined by hepatic fat infiltration >5% hepatocytes, in the absence of other causes of liver pathology (such as daily alcohol utilization and either viral, autoimmune or drug‐induced liver disease). It includes a spectrum of disease ranging from intrahepatic fat accumulation (steatosis) to various degrees of necrotic inflammation and fibrosis (non‐alcoholic steatohepa‐ tatis [NASH]); simple steatosis has generally a benign course, but, rarely in children, NASH may progress to advanced and severe liver damage like cirrhosis and its complications (hepatocellular carcinoma and portal hypertension) [154, 156].

The pathogenesis of NAFLD appears to be multifactorial. The principal risk factor for fatty liver in childhood is obesity, but several other factors contribute to NAFLD development, including race/ethnicity, genetic factors, environmental exposures and alterations in the gut microbiome [157]. The dramatic rise in the prevalence of pediatric NAFLD is closely associated with the epidemic of obesity and metabolic syndrome; as in adulthood, pediatric NAFLD is associated with severe metabolic impairments such as insulin resistance, hypertension and abdominal obesity, determining an increased risk of developing type 2 diabetes mellitus, the metabolic syndrome and cardiovascular diseases [157, 158]. In addition, unhealthy food choices and the excessive fructose consumption in particular the fructose contained in the most common soda can promote the development of fatty liver [159].

The prevalence of hepatic steatosis varies among different ethnic groups. The ethnic group with the highest prevalence is the American Hispanic one (45%) followed by the Caucasian (33%) and the African‐American (24%). The fatty liver prevalence in Europe, Australia and Middle East encompasses from 20 to 30%. In India, the fatty liver prevalence in urban popu‐ lations encompasses from 16 to 32%; but in rural India, where there are traditional diets and lifestyles, the prevalence is lower (about 9%); this evidence suggests that a sedentary lifestyle and globalization of Western diet could be associated with an increase in the fatty liver prevalence in developing nations. In all the ethnicity, NAFLD is more prevalent in boys than in girls with a male to female ratio of 2:1 [160, 161].

Regarding to genetic factors, one of the most important gene involved in determining hepatic steatosis is the *patatin‐like phospholipase‐containing domain 3* gene (*PNPLA3*). Genome‐wide association studies and other pediatric studies have revealed that the *rs738409* (I148M) variant for *PNPLA3* confers susceptibility to NAFLD‐promoting hepatic accumulation of triglycerides and cholesterol by inhibition of triglyceride hydrolysis [162]. In addition, a recent case‐control study has demonstrated that the *rs9939609A* allele of the fat mass and obesity‐associated gene (*FTO*) increases the risk of NAFLD [157].

Another gene that acts together with *PNPLA3* in determining hepatic steatosis is the glucoki‐ nase regulatory protein (*GCKR*) gene which encodes for the glucokinase regulatory protein (GCKRP) that inhibits the glucokinase (GCK) activity competing with the glucose, substrate of GCK. It has been demonstrated that the GCKRP L466 variant encodes for a protein that indirectly increased GCK activity. This increase in GCK hepatic activity promotes hepatic glucose metabolism, raises the concentrations of malonyl coenzyme A, a substrate for de novo lipogenesis, and contributes in liver fat accumulation [160, 163]. In addition, a study conducted in Chinese children has shown that that the polymorphism *rs11235972* of the *uncoupling protein 3* (*UCP3*) gene is associated with the occurrence of NAFLD. *UCP3* is a mitochondrial protein with a highly selective expression in skeletal muscle, a major site of thermogenesis in humans. Genetic variants of *UCP3* have been associated with NIDDM and obesity [164].

*Apolipoprotein C3* gene (*APOC3*) *rs2854117* and *rs2854116* variants and *farnesyl‐diphosphate farnesyltransferase 1* (*FDFT1*) gene *rs2645424* variant have been also associated with NAFLD in adult [160]. Also in the recent years, genetic studies have demonstrated that single‐nucleotide polymorphisms (SNPs) in genes involved in lipid metabolism (*Lipin 1, LPIN1*), oxidative stress (*superoxide dismutase 2, —SOD2*), insulin signaling (*insulin receptor substrate‐1, IRS‐1*) and fibrogenesis (*Kruppel‐like factor 6, KLF6*) have been associated with a high risk for NAFLD development and progression [154]. Finally, a recent study evaluated the combined effect of four‐polymorphisms genetic risk score in predicting NASH in NAFLD obese children with increased liver enzymes to help NASH diagnosis with the other non‐invasive diagnostic tests [165].

In conclusion, obesity and fatty liver disease often go hand in hand even in the pediatric population, and both are pathologies related to genetic and environmental factors.
