Obesity is a highly complex heterogeneous disease caused by multiple environmental, lifestyle, and genetic factors. Results from family and twin studies have suggested that genetic factors explain 40–70% of the inter-individual variation in obesity susceptibility (15). Individuals with pediatric obesity frequently remain obese in adulthood. It is important to elucidate the interaction between genetic factors and pediatric obesity, and to manage obesity in childhood. Recently, BAT has been identified as a novel target of obesity, as it increases energy expenditure by non-shivering thermogenesis. BAT-related gene polymorphisms are associated with obesity (8–11). However, these relationships have not yet been fully elucidated, especially in pediatric obesity. Therefore, we carried out a case-control study to investigate the association of pediatric obesity with three SNPs (DIO2 Thr92Ala, UCP1-3826 A/G, β3AR Trp64Arg) related to BAT. We demonstrated a significant correlation between the DIO2 Thr92Ala 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 (15.1% vs. 6.3%, P = 0.004) (Table 2). These frequencies resulted in an OR of 3.393 (95% CI 1.498–7.687) 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 (Fig. 2). 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 (8). Individuals with DIO2 Ala/Ala tend to have insulin resistance with a lower glucose disposal rate in adult obese Caucasians (11). There are three isoforms of DIO; 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 (16, 17). DIO2 exhibits a higher catalytic capacity than DIO1; thus, DIO2 plays a critical role in the production of T3 (18, 19). TH (T4 and T3) regulates metabolic processes, such as body weight, lipolysis, and thermogenesis (20). DIO2 Ala mutation impairs DIO2-mediated conversion of T4 to T3 in thyroid-deficient patients (7, 21–24), and DIO2 Ala mutation reduces intracellular conversion of T4 to T3 compared to DIO2 WT (23). Therefore, DIO2 Ala mutation could reduce the TH-mediated thermogenesis of BAT, leading to obesity. However, not all studies examining this genotype have verified the association with obesity, despite subjects having insulin resistance (25, 26). This discrepancy with the present study might be due to differences in age. In the present study, we enrolled young people aged ≤ 18 years. In contrast, all previous studies enrolled older people aged more than 40 years. BAT mass is highly present in infancy and declines in adulthood (27). Therefore, BAT might impact childhood more than adulthood. Furthermore, the activity of BAT increases during childhood, reaching its peak at approximately 12 years, and then declines during adulthood (28). The average age of children enrolled in the present study was 12 years. This concordance in age might have affected the results. We believe that adolescents approximately 12 years of age are suitable for assessing the association of obesity and gene polymorphisms related to BAT, because other factors, such as environmental, lifestyle, and genetic factors, are likely to be more involved in the pathogenesis of obesity in adulthood. In contrast, in adulthood, insulin resistance might be the main phenotype caused by DIO2 polymorphism. DIO2 is also expressed in skeletal muscles, 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 in adulthood is more abundant than in children. More importantly, there were no significant differences in insulin levels between DIO2 genotypes after adjusting for body weight. Higher insulin levels in the DIO2 Ala/Ala genotype might be secondary to obesity.
Previous studies have demonstrated that DIO2, UCP1, and β3AR have a mutual impact on obesity in adulthood (10, 11). Therefore, in the present study, we examined the gene-gene interaction of the DIO2 Thr92Ala genotype with UCP1-3826 A/G and β3AR Trp64Arg in pediatric obesity. We found no significant association between DIO2 Thr92Ala and UCP1-3826 A/G and β3AR Trp64Ar in pediatric obesity. This discrepancy from studies investigating adult obesity might also be due to age. The combination of UCP1 and β3AR polymorphisms is related to reduced BAT mass with age (29), suggesting that aging uncovers the effects of the 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.
There are a number of limitations in the present study. 1) Due to its exploratory nature, the sample size was not determined on a statistical basis, and multiplicity of tests was not considered in statistical analysis. 2) There is difference in mean age between the NOB and OB groups. The mean height is expected to increase with age; the taller children are, the higher are their BMIs. 3) There is a lack of data on pubertal stage. Data on the pubertal stage are important for assessing insulin levels, as children physiologically develop insulin resistance and insulin levels increase with age, during puberty. The OB group had a higher insulin concentration, even though the median age in the OB group (11.1 years (9.1–13.8)) was younger than that in the NOB group (13.5 years (12.9–14.1) ). Therefore, we believe that higher insulin levels in the DIO2 Ala/Ala genotype represent insulin resistance (30). 4) Puberty alters body composition differently according to sex. During puberty, boys gain only lean mass, but girls gain both lean mass and fat mass. We cannot assess the effect of altered body composition by puberty due to a lack of data on the pubertal stage. 5) Some longitudinal studies have shown that girls have a significant decline in physical activity compared to boys during puberty (31, 32). This decline in physical activity due to sex difference during puberty might affect obesity. 6) We used random glucose and insulin levels in the present study. However, fasting glucose and insulin levels should be used to assess insulin resistance. 7) We included population-based non-obese children as controls in the present study. However, we should have enrolled hospital-based non-obese children as controls to align the background of the participants. Due to the various limitations of this study, further studies are needed to validate the results.
In conclusion, our results indicate that the homozygous Ala/Ala allele of the DIO2 gene is associated with an increased risk of obesity and insulin resistance in children. The present study also suggests that pediatric obesity might be suitable for assessing the association with gene polymorphisms related to BAT. These findings advance our understanding of the genetic contribution to the pathogenesis of obesity and might develop new strategies against obesity. However, this is an exploratory study; thus, further studies, including a genome wide association study, are needed to validate these findings.