UC Davis

The insecticide dichlorodiphenyltrichloroethane (DDT) and its metabolite, dichlorodiphenyldichloroethylene (DDE), together referred to as DDX, are persistent organic pollutants associated with increased adiposity and obesity in humans and rodents. Exposure to DDE begins in utero and continues through maternal transfer in breast milk, bioaccumulation in food, and direct exposure in countries that still manufacture and use DDT. Although DDT has been banned for decades in many countries due to its negative impact on the environment, DDE is still detected in human samples around the world. Rodent studies have found that exposure to DDX impaired thermogenesis in brown adipose tissue, which led to decreased energy expenditure and increased adiposity. The full molecular mechanism behind impaired thermogenesis in brown adipose tissue has not been revealed. It remains to be determined whether the impairment of thermogenesis is a culmination of physiological changes across tissues or primarily a cell-autonomous effect in brown adipocytes. Furthermore, studies of DDX exposure suggest that changes to the epigenome lead to increased adiposity across generations, although a stable epigenetic signature across studies has yet to be determined. To address these data gaps, we investigated the effect of perinatal DDX-exposure on the transcriptome and DNA methylome of brown adipose tissue from infant and adult mice. In this study we found that thermogenesis and related pathways, such as oxidative phosphorylation and mTOR signaling, were enriched with RNA transcription and DNA methylation in mice exposed to DDX, supporting the hypothesis that DDX exposure impairs thermogenesis. Human brown adipocytes cultured in DDT or DDE over the period of adipogenic differentiation exhibited limited changes in gene expression, however exposure to DDT and DDE decreased mitochondrial membrane potential, demonstrating that DDX has a cell-autonomous effect on brown adipocytes. To determine the effect of DDX on the blood methylome and whether changes in DNA methylation are translatable across species in a tissue more easily accessible than adipose, we compared the changes in DNA methylation in blood samples from infant mice perinatally exposed to DDT and humans exposed to DDE. The enrichment of pathways associated with insulin homeostasis, endocrine function, and cancer across human and mouse blood further support the hypothesis that DDX-associated diseases arise from alterations to the epigenome. In human blood, infant mouse blood, and infant mouse brown adipose tissue, we found hypomethylation of a novel gene, trinucleotide repeat containing 18 (TNRC18), in association with DDX. To investigate the function of TNRC18 with respect to adiposity, we used larval zebrafish with a tnrc18 knockout and found that a lack of tnrc18 led to an increase in lipid storage similarly to what is observed in DDX-exposed mice. The enrichment of the ubiquitin-mediated proteolysis pathway, known to be involved in thermogenesis and adaptive metabolism, in heterozygous tnrc18 knockout zebrafish larvae and DDX-exposed mammalian blood suggests a direct link between loss of TNRC18 function, DDX exposure, and energy balance. Finally, the associations between plasma DDE and lipoprotein levels were analyzed in human blood to further examine the link between DDE and lipid dysfunction. Plasma DDE was positively associated with lipoproteins, cholesterol, and apolipoproteins in a manner that resembled the atherogenic lipid profile associated with increased risk of cardiovascular disease. This body of work broadens our understanding of the molecular and epigenetic mechanisms driving the impairment of thermogenesis and increase in adiposity associated with DDX exposure.