Obesity is a highly complex heterogeneous disease caused by multiple factors such as the influence of the environment, lifestyle, and genes. The incidence of obesity in children is increasing worldwide. Pediatric obesity is occurring in younger generations and is becoming more severe compared with previous decades [
18]. Obesity is strongly influenced by genetic and environmental factors. Our genes have not changed significantly over the last few decades. Even with melanocortin-4 receptor deficiency, which causes obesity, the rate of obesity differs among generations [
19]. In recent years, childhood obesity has increased during the COVID-19 pandemic [
20]. The review of this information suggests that modern society is an “obesogenic” environment. However, not everyone will become obese because they are in the same environment. Results from family and twin studies have suggested that genetic factors explain 40% to 70% of the inter-individual variation in obesity susceptibility [
2]. Individuals with pediatric obesity frequently remain obese through adulthood. It is important to elucidate the interaction between genetic factors and pediatric obesity and manage obesity in childhood. Recently, BAT has been identified as a novel target for obesity, as it increases energy expenditure by non-shivering thermogenesis. BAT-related gene polymorphisms are associated with obesity [
4,
5,
6,
7]. However, these relationships have not yet been fully elucidated, especially in pediatric obesity. Therefore, a case-control study was conducted to investigate the association of pediatric obesity with three SNPs (DIO2 Thr92Ala (rs225014), UCP1-3826 A/G (rs1800592), β3AR Trp64Arg (rs4994)) related to BAT. We demonstrated a significant correlation between DIO2 Thr92Ala (rs225014) polymorphism and childhood obesity.
In the present case-control study, the frequency of the homozygous Ala allele of DIO2 was significantly higher in the OB than in the NOB group (17.1% vs. 5.8%,
p = 0.004) (
Table 2). These frequencies resulted in an OR of 4.036 (95% CI 1.785–9.124) for the DIO2 Ala/Ala genotype in the OB group (
Table 3). More than 40% of subjects with DIO2 Ala/Ala were obese, which was the highest prevalence of obesity among all genotypes. This result is similar to that of a previous study showing that the DIO2 Ala/Ala genotype is strongly associated with obesity, concomitant with insulin resistance in a large cohort of patients with type 2 diabetes mellitus [
5]. Individuals with DIO2 Ala/Ala tend to have insulin resistance, with a lower glucose disposal rate among adult obese Caucasians [
7]. DIO has three isoforms: DIO1, DIO2, and DIO3. DIO1 and DIO2 convert T4 to T3 by catalyzing 5′-deiodination, and DIO1 and DIO3 convert T4 to an inactive metabolite, rT3, by catalyzing 5-deiodination [
21]. DIO2 exhibits a higher catalytic capacity than DIO1 and therefore plays a critical role in the production of T3 [
22,
23]. TH (T4 and T3) regulates metabolic processes, such as body weight, lipolysis, and thermogenesis [
24]. DIO2 Ala mutation impairs DIO2-mediated conversion of T4 to T3 in thyroid-deficient patients [
11,
25,
26,
27,
28], and DIO2 Ala mutation reduces intracellular conversion of T4 to T3 compared to the DIO2 wild type [
27]. Therefore, DIO2 Ala mutation could reduce TH-mediated thermogenesis of BAT, leading to obesity. However, not all studies examining this genotype have verified its association with obesity, despite subjects having insulin resistance [
23,
29]. A similar discrepancy was observed in this study, which might be due to differences in age. Young individuals aged ≤18 years were enrolled in the present study. In contrast, all previous studies enrolled people aged ≥40 years. BAT mass is high during infancy and declines through adulthood [
30]. Therefore, BAT might impact childhood more than adulthood. Furthermore, the activity of BAT increases during childhood, reaches its peak at approximately 12 years, and then declines through adulthood [
31]. The average age of children enrolled in the present study was 12 years. Thus, this concordance in age may have affected the results. We hypothesized that adolescents approximately 12 years of age are suitable for assessing the association between obesity and gene polymorphisms related to BAT because other factors, such as environment, lifestyle, and genetic factors, were likely to be more involved in the pathogenesis of obesity during adulthood. Further, insulin resistance may be the main phenotype caused by DIO2 polymorphism in adulthood. DIO2 is also expressed in skeletal muscle, and TH upregulates the expression of glucose transporter 4, which is responsible for glucose uptake. Therefore, skeletal muscle is the main glucose-consuming tissue. DIO2 polymorphism might lead to insulin resistance in adulthood, as skeletal muscle mass is more abundant in adulthood than in children. More importantly, there were no significant differences in the insulin levels between the DIO2 genotypes after adjusting for body weight. The higher insulin levels in the DIO2 Ala/Ala genotype may be secondary to obesity.
Previous studies have demonstrated that DIO2, UCP1, and β3AR have a mutual impact on obesity in adulthood [
6,
7]. Therefore, the present study examined the gene–gene interaction of the DIO2 Thr92Ala (rs225014) genotype with UCP1-3826 A/G (rs1800592) and β3AR Trp64Arg (rs4994) in pediatric obesity [
32]. There was no significant association between DIO2 Thr92Ala (rs225014) and UCP1-3826 A/G (rs1800592) or β3AR Trp64Ar (rs4994) in pediatric obesity. This discrepancy between studies that investigated adult obesity and the present study may also be due to age. The combination of UCP1 and β3AR polymorphisms is related to reduced BAT mass with age [
33], suggesting that aging uncovers the effects of this gene combination on obesity. Further studies are needed to investigate the effect of UCP1 and/or the combination of DIO2 and UCP1 on pediatric obesity. Another possibility may be sex-related BAT activity. Females have greater BAT mass than males, and sex differences may be greater in younger generations. Additionally, females can regulate their thermogenesis more than males, as sympathetic innervation in BAT is more abundant in females. However, the effects of these SNP differences remain unknown. Sex-related differences in BAT are of great concern. In fact, the effect of the DIO2 polymorphism differed by sex in the present study. DIO2 Ala/Ala was associated with obesity in boys but not in girls. Further studies are needed to investigate the association between gene polymorphisms related to BAT with aging and sex differences. Some studies have reported that polymorphisms in UCP1 and β3AR have an effect on obesity even in children. This difference from our findings may be related to differences in the ethnicities of the study populations. However, Endo et al. reported an association between β3AR Trp64Arg and obesity in Japanese school children. The difference in the sex ratio in each study may also explain the inconsistency between the results. As this was an exploratory study, further verification of our results is warranted.
This is the first study to report an association between the DIO2 Thr92Ala (rs225014) polymorphism and pediatric obesity. It also suggests that BAT-related SNPs should be assessed in patients with pediatric obesity. However, this study had several limitations. (1) Owing to its exploratory nature, the sample size was not determined statistically, and the multiplicity of tests was not considered during analysis. (2) There was a difference in the mean age between the NOB and OB groups. The mean height is expected to increase with age: the taller the children, the higher their BMIs. (3) There is a lack of data on the pubertal stage. Puberty drastically alters body composition according to sex. During puberty, boys gain only lean mass, whereas girls gain both lean mass and fat mass. Therefore, the effect of altered body composition during puberty could not be assessed due to a lack of data on the pubertal stage. (4) Population-based non-obese children were included as controls in the present study. However, perhaps hospital-based non-obese children should have been enrolled as controls to match the backgrounds of all the participants. (5) We did not have data on thyroid function. DIO2 Thr92Ala (rs225014) might be associated with obesity via hypothyroidism [
34]. Due to these limitations, further studies are needed to validate the results.