Prenatal exposure to environmental chemicals, including heavy metals, essential metals, and nutrients, is associated with perinatal and childhood health outcomes. The Developmental Origins of Health and Disease (DOHaD) hypothesis postulates that environmental factors encountered in utero influence disease susceptibility and outcomes in adulthood. However, the biological mechanisms by which prenatal exposures promote postnatal health outcomes are not yet well understood. The majority of studies have examined single chemicals at a time, despite evidence that individuals are exposed to multiple chemicals simultaneously. Mixture analyses have the potential to elucidate combinations of chemicals that contribute the most to adverse health outcomes, as well as to identify susceptible sub-populations, for the prioritization and targeting of public health interventions. This dissertation aims to (1) evaluate individual and joint associations of prenatal metals with mitochondrial DNA copy number (mtDNAcn) and telomere length (TL) in maternal and umbilical cord blood, as biomarkers of inflammation and oxidative stress, (2) examine joint and individual associations between prenatal metals and childhood adiposity, and (3) determine whether maternal or umbilical cord blood mtDNAcn and TL, or umbilical cord blood hormones, mediate associations observed between prenatal metals and birthweight and childhood adiposity.
Chapter 1 provides background material.
Chapter 2 evaluates individual and joint associations of prenatal metals with mtDNAcn and TL in maternal and cord blood. Oxidative stress and inflammation may mediate adverse effects of heavy metals, while essential metals may act as antioxidants. Mitochondrial DNA is a prime target for metal induced oxidative damage. Telomere dysfunction is attributed to imbalances between reactive oxidant species and antioxidants. We measured six nonessential metals (arsenic, barium, cadmium, cesium, lead, mercury) and four essential metals (magnesium, manganese, selenium, zinc) in first trimester maternal red blood cells in Project Viva, a US pre-birth cohort. We measured relative mtDNAcn (N=898) and TL (N=893) in second trimester maternal blood and mtDNAcn (N=419) and TL (N=408) in cord blood. We used multivariable linear regression and quantile g-computation to estimate associations between prenatal metals and the biomarkers. We used generalized additive models and Bayesian kernel machine regression to examine nonlinearity and interactions. A two-fold increase in maternal magnesium was associated with lower maternal (β=-0.07, 95% CI: -0.10, -0.01) and cord blood (β=-0.08, 95% CI: -0.20, -0.01) mtDNAcn. Lead was associated with higher maternal mtDNAcn (β=0.04, 95% CI: 0.01, 0.06). Selenium was associated with longer cord blood TL (β=0.30, 95% CI: 0.01 0.50). An association was observed between the nonessential metal mixture and higher maternal mtDNAcn (β=0.04 per quartile, 95% CI: 0.01, 0.07). There was a nonlinear relationship between cord blood mtDNAcn and magnesium, maternal mtDNAcn and barium, lead, and mercury, and maternal TL and barium. Maternal exposure to metals such as lead, magnesium, and selenium, was associated with mtDNAcn and TL in maternal second trimester and cord blood. Future work will evaluate whether these biomarkers are associated with childhood health.
Chapter 3 examines associations between prenatal metals and adiposity at various timepoints during development. Prenatal nonessential metals may contribute to postnatal adiposity, while essential metals may have metabolic benefits. We measured six nonessential (arsenic, barium, cadmium, cesium, lead, mercury) and four essential (magnesium, manganese, selenium, zinc) metals in first trimester blood from Project Viva, a U.S. pre-birth cohort in eastern Massachusetts. We collected anthropometrics in early childhood, mid-childhood, and early adolescence including subscapular and tricep skinfold thickness sum (SS+TR; mm) (N=715-859), waist circumference (WC; cm) (N=717-882), and BMI (z-score) (N=716-875). We measured adiposity in mid-childhood and early adolescence, using DXA total-fat-mass-index (total FMI; kg/m2) and trunk FMI (kg/m2) (N=511-599). We used adjusted quantile g-computation and linear regressions to estimate associations. The nonessential mixture was associated with mid-childhood total FMI (β=0.07 kg/m2 per quartile, 95% CI: 0.01 kg/m2, 0.12 kg/m2), trunk FMI (β=0.12 kg/m2 per quartile, 95% CI: 0.02 kg/m2, 0.22 kg/m2), WC (β=0.01 cm per quartile, 95% CI: 0.00 cm, 0.01 cm), and BMI z-score (β=0.24 per quartile, 95% CI: 0.07, 0.41), and early adolescence total FMI (β=0.07 kg/m2 per quartile, 95% CI: 0.01 kg/m2, 0.14 kg/m2) and BMI z-score (β=0.19 per quartile, 95% CI: 0.02, 0.37). The essential metal mixture was associated with early adolescence total FMI (β=-0.07 kg/m2 per quartile, 95% CI: -0.12 kg/m2, -0.02 kg/m2), trunk FMI (β=-0.13 kg/m2 per quartile, 95% CI: -0.21 kg/m2, -0.05 kg/m2), SS+TR (β=-0.02 mm per quartile, 95% CI: -0.03 mm, -0.00 mm), WC (β=-0.004 cm per quartile, 95% CI: -0.01 cm, -0.00 cm), and BMI z-score (β=-0.16 per quartile, 95% CI: -0.28, -0.04). Cadmium and cesium were individually associated with childhood adiposity at different timepoints. Prenatal first trimester essential metals were associated with lower childhood adiposity, whereas nonessential metals were associated with higher adiposity into adolescence.
Chapter 4 examines (1) associations between prenatal metals and nutrients and umbilical cord blood hormones, and (2) whether associations between prenatal metals and childhood adiposity from Chapter 3 are mediated by maternal or umbilical cord blood mtDNAcn and TL, or umbilical cord blood hormones. Prenatal heavy metals may influence umbilical cord blood hormones through endocrine disruption, whereas essential metals and nutrients at therapeutic levels may mitigate the endocrine disruption promoted by heavy metals through their roles as antioxidants. We measured six nonessential metals (arsenic, barium, cadmium, cesium, lead, mercury), four essential metals (magnesium, manganese, selenium, zinc), and two nutrients (Vitamin B12 and folate) in first trimester blood from participants in Project Viva, a U.S. pre-birth cohort in eastern Massachusetts. In plasma, we measured concentrations of adiponectin (μg/mL), C-peptide (ng/mL), IGF-1 (ng/mL), IGF-2 (ng/mL), IGFBP-3 (ng/mL), leptin (ng/mL), and insulin (μU/mL). We used adjusted quantile g-computation and linear regressions to estimate associations, as well as generalized additive models and Bayesian kernel machine regression to examine nonlinearity. For individual metal mediation, we used causal mediation analysis to estimate natural direct and indirect effects, and for metal mixture mediation, we used BKMR causal mediation analysis (BKMR-CMA). Higher levels of the prenatal essential metal mixture were associated with higher IGF-1 (β=0.09 ng/ml per quartile, 95% CI: 0.01 ng/ml, 0.16 ng/ml), higher IGF-2 (β=0.04 ng/ml per quartile, 95% CI: 0.00 ng/ml, 0.08 ng/ml), higher IGFBP-3 (β=0.05 ng/ml per quartile, 95% CI: 0.00 ng/ml, 0.10 ng/ml), and higher leptin (β=0.14 ng/ml per quartile, 95% CI: 0.00 ng/ml, 0.28 ng/ml). Higher levels of the prenatal essential metal mixture were associated with birthweight-for-gestational age and sex z-score (β=0.07 per quartile, 95% CI: 0.00, 0.14). A doubling in barium was associated with higher IGF-1 (β=1.63 ng/ml, 95% CI: 0.24 ng/ml, 3.01 ng/ml). A doubling in cadmium was associated with lower C-peptide (β=-0.04 ng/ml, 95% CI: -0.09 ng/ml, 0.00 ng/ml) and IGF-2 (β=-8.41 ng/ml, 95% CI: -15.26 ng/ml, -1.55 ng/ml). A doubling in prenatal cesium was associated with higher C-peptide (β=0.11 ng/ml, 95% CI: 0.02 ng/ml, 0.21 ng/ml). A doubling in prenatal lead was associated with lower C-peptide (β=-0.11 ng/ml, 95% CI: -0.18 ng/ml, -0.03 ng/ml). Finally, a doubling in magnesium was associated with higher leptin (β=2.90 ng/ml, 95% CI: 0.89 ng/ml, 4.91 ng/ml). We observed evidence of nonlinear relationships. In individual metal causal mediation analyses, umbilical cord blood C-peptide mediated 25% (95% CI: 5%, 101%) of the inverse association observed between lead and birthweight-for-gestational age and sex z-score. Future studies should validate our findings and examine associations between prenatal metal and nutrient mixtures and cord blood hormones during different trimesters of pregnancy.
Chapter 5 highlights overall conclusions and future research directions.
