Traditional toxicological studies have not incorporated genetic variability, but rather have focused on homogenous models, such as inbred mouse strains. The lack of incorporation of genetic heterogeneity provides a challenge in defining safe-exposure limits that encompass all individuals within the population. The goal of this dissertation was to use a population-based approach to characterize the impact of genetic heterogeneity within TCDD-induced toxicity. TCDD is a pervasive and persistent environmental contaminant that is associated with a plethora of adverse health effects in humans. TCDD-elicited toxicity is mediated through activation of a ligand-activated transcription factor called the aryl hydrocarbon receptor (AHR). While the Ahr gene sequence inherited is known to impact TCDD-induced toxicity, we hypothesize that other genomic factors will impact susceptibility to TCDD-elicited toxicity. To test this hypothesis, a mixture of in vitro and in vivo-based methods were employed to quantify the variability in response across heterogeneous individuals and to identify genetic modulators of TCDD-induced immunosuppression and alterations in liver homeostasis. First, an in vitro-based approach was used to identify the inherent variability in the human population to TCDD-elicited suppression of B cells. The results showed that there was a wide range of response (>70-fold) at high doses of TCDD. B cells were isolated from a genetically-diverse mouse panel and exposed to TCDD to scan for genetic modulators that may explain the wide-degree of variability across human individuals. Our study implicated Serpinb2, which encodes for serine peptidase inhibitor, clade B, member 2, as a modulator of TCDD-elicited suppression of the B cell. Further downstream functional analysis identified that Serpinb2 plays a protective role against TCDD-elicited suppression of the B cell in mice. Secondly, an in vivo mouse population-based approach was used to scan for genetic modulators of TCDD-elicited alterations in liver homeostasis. Hepatic sequestration of TCDD was found to be dependent on AHR-mediated transcription. Inter-strain differences in expression of AHR-responsive genes implicated Tgfbr2, which encodes for transforming growth factor receptor II (TGFBR2), as a potential modulator of TCDD-elicited liver toxicity. Functional analyses suggested that TGFBR2-activity protects against TCDD-elicited inflammation, but increases hepatic lipid accumulation in the livers of male, but not female, mice. Finally, TCDD-elicited change in body weight across our mouse panel implicated Hmgcr, which encodes the rate-limiting enzyme of cholesterol biosynthesis called HMG-CoA reductase (HMGCR). While reports indicate that TCDD-impacts cholesterol homeostasis in rodents, the phenotype has not been demonstrated in the human population. Multiple linear regression analysis using data derived from the National Health and Nutrition Examination Survey (NHANES) suggests that, like in previous rodent studies, serum TCDD levels are also associated with cholesterol levels in humans in a sex-specific manner. Further functional mouse analyses suggest that HMGCR is a modulator of TCDD-elicited liver phenotypes. More specifically, inhibition of HMGCR was found to protect against AHR-mediated steatosis in both sexes, but increase TCDD-elicited liver injury in males and alters glycogen metabolism in females. The results outlined in this dissertation indicate the power in using population-based models in characterizing the degree of variability and identifying modulating genes within adverse responses to chemical exposures, such as TCDD. We hope that our data will impact real-world risk assessment in ensuring that safe-exposure guidelines for TCDD reflect population-wide variability. While many of the findings outlined still need confirmation in the human population, our results may be used to identify individuals within the human population that may be more susceptible to TCDD-induced toxicity which, ultimately, has potential to impact public health.
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