Integrating psychosocial stress into children’s molecular epidemiology research: An investigation of flame retardants, telomeres and neuroendocrine development

Abstract

Background & Objectives: This dissertation is comprised of two independent projects that seek to answer the research questions outlined in Aims 1 and 2. The first project is focused on measuring exposure to polybrominated diphenyl ethers (PBDE) throughout the early lifecourse, as well as investigating how exposure at different developmental periods relates to neuroendocrine endpoints. PBDEs are flame retardant chemicals that were used extensively in furniture and furnishings sold throughout the United States until their phase-out in 2004. Human exposure occurs primarily through incidental ingestion of PBDE-contaminated dust present in the indoor environment. The second project aimed to characterize telomere dynamics in maternal-child pairs and to evaluate associations between telomere dynamics and indicators of stress and stressful conditions. Telomeres are non-coding nucleotide repeats located at chromosome ends; they serve several functions, such as buffering against loss of important protein coding DNA regions during cell division. Both projects are focused on exposure-response relationships during early life and a central theme throughout this dissertation relates to the intersection of date, time and age in longitudinal cohort studies. Finally, the third aim seeks to integrate findings from Projects I and II and is focused on investigating whether telomere dynamics can be used as a biological indictor of stress in epidemiological research examining associations between low-level environmental chemical exposures and neurodevelopmental endpoints. Methods: Both projects were conducted using data and samples collected as part of the Columbia Center for Children’s Environmental Health (CCCEH) Mothers and Newborns study. In Project I, PBDEs were measured by the Centers for Disease Control and Prevention (CDC) using gas chromatography-mass spectrometry (GC-MS) in plasma samples collected repeatedly between birth and age 9 years. We examined determinants of 1) prenatal exposure to PBDEs (Chapter 2), and 2) trajectories of PBDE exposure over childhood, which we estimated using latent class growth analysis (LCGA) (Chapter 3). We also examined PBDE trajectories in relation to performance on tests of visual, verbal and working memory among early adolescents (Chapter 4) and investigated associations between prenatal exposure to PBDEs and thyroid hormone parameters, which were measured by radioimmunoassay in plasma samples collected at multiple ages (Chapter 5). In Project II, we used monochrome multiplex quantitative polymerase chain reaction (MMqPCR) to measure relative leukocyte telomere length (rLTL) in samples collected from mothers and newborns (umbilical cord blood) at the child’s delivery and from children at ages 2, 3, 5, 7 and 9-years (Chapter 6). We aimed to characterize rLTL dynamics over early life, examine the correlation between paired maternal-newborn rLTL, and examine associations between rLTL with measures of financial strain, perceived stress and maternal distress. Results: In Project I, we detected PBDEs in over 80% of cord blood samples and in multivariable models, sociodemographic and lifestyle factors explained 12% of cord blood PBDE variability. The largest determinant of exposure was ethnicity, with Dominican newborns having lower exposure compared to African American newborns, likely due to the reduced amount of time Dominican mothers had spent in the United States when they gave birth to the study child. Across postnatal life (2000 to 2013), PBDE concentrations in child blood decreased by approximately 12% per year, suggesting that exposure has continually declined since the PBDE phase-out in 2004. Trajectory analyses revealed several unique patterns of PBDE exposure over the early lifecourse, with the majority of children characterized by exposure that was persistently low or that peaked during toddler years. Smaller groups of children were characterized by exposure that was highest during the prenatal period and decreased after birth or by a pattern of high exposure during toddler years that remained elevated into middle childhood. We identified several important predictors of childhood PBDE exposure patterns, including modifiable factors, such as cleaning behaviors. In relation to neurodevelopmental outcomes, we found that children with sustained high exposure to PBDEs scored approximately 5-8 points lower on tests of visual memory. Associations between prenatal exposure and working memory significantly varied by sex, with inverse associations (approximately 8 points lower) observed only among girls. Children with PBDE plasma concentrations that peaked during toddler years performed better on verbal domains, however, these associations were significant only among children breastfed for more than 12 weeks. Finally, in relation to thyroid hormone levels, children with BDE-47 concentrations in the third and fourth quartiles of the exposure distribution (versus first quartile) had significantly lower TSH and free T4 levels, respectively. We did not detect associations between BDE-47 and total T4 levels; likewise, we did not detect associations between other pentaBDE congeners and any thyroid parameter. In Project II, we found that maternal-newborn rLTL in paired samples was moderately correlated and that maternal rLTL at delivery explained 8% of the variability (R2) in newborn rLTL. In relation to measures of hardship, perceived stress and demoralization, we found an inverse, albeit not statistically significant, association between maternal perceived stress and newborn rLTL. We did not detect an association with maternal rLTL, nor did we detect associations between material hardship or demoralization and maternal or newborn rLTL. When examining rLTL in child blood samples collected between birth and age 9 years, we observed a U-shaped pattern characterized by rapid shortening of rLTL between birth and 2 years, followed by gradual lengthening between ages 3 and 9 years. It remains unresolved whether this pattern reflects a true biological phenomenon or if it is an artifact of measurement error introduced by analytic or pre-analytic conditions. Conclusions: Despite the phase-out of PBDEs in 2004, exposure among children residing in New York City remained nearly ubiquitous through 2013, however, concentrations did decline over time. Our finding of several PBDE trajectories suggests that, despite the relatively long half-lives of PBDEs, a single measure may not accurately reflect exposure throughout childhood. Our findings of reduced scores on tests of working and visual memory during the prenatal and postnatal periods, respectively, support a growing body of literature linking early life PBDE exposure to disrupted neurodevelopment. The results of our analysis examining thyroid hormone disruption during childhood revealed a pattern consistent with hypothalamic or pituitary-level disruption during prenatal programming of the thyroid regulatory system. This is the first study to examine prenatal PBDE exposure in relation to childhood thyroid hormone levels, therefore, it is important that this finding is replicated by future research. Our finding of an inverse association between newborn rLTL and maternal perceived stress is consistent with results from previous research and suggests that the developing fetus may be sensitive to maternal stress perception during pregnancy, however, additional research is needed to more fully understand the mechanisms through which this transmission occurs. Our finding of increasing telomere length between toddler years and middle childhood is unexpected and raises questions about the suitability of the qPCR assay for analyzing telomere length in archived samples. Additional analyses are needed to determine whether the observed patterns reflect true biological changes or relate to measurement error introduced during sample processing, storage or analysis. Given these outstanding issues, we were ultimately unable to draw conclusions about the usefulness of telomere dynamics as a stress-sensitive biomarker