From cohorts to molecules: Adverse impacts of endocrine disrupting mixtures

Abstract

Convergent evidence associates exposure to endocrine disrupting chemicals (EDCs) with major human diseases, even at regulation-compliant concentrations. This might be because humans are exposed to EDC mixtures, whereas chemical regulation is based on a risk assessment of individual compounds. Here, we developed a mixture-centered risk assessment strategy that integrates epidemiological and experimental evidence. We identified that exposure to an EDC mixture in early pregnancy is associated with language delay in offspring. At human-relevant concentrations, this mixture disrupted hormone-regulated and disease-relevant regulatory networks in human brain organoids and in the model organisms Xenopus leavis and Danio rerio , as well as behavioral responses. Reinterrogating epidemiological data, we found that up to 54% of the children had prenatal exposures above experimentally derived levels of concern, reaching, for the upper decile compared with the lowest decile of exposure, a 3.3 times higher risk of language delay.

Mixed chemicals’ potential risks

Exposure to endocrine-disrupting chemicals in the environment can cause a variety of potential health problems in human patients. Such exposures are not isolated, however, and environmental safety regulations that only consider doses of individual chemicals do not take multisubstance exposures into account. Caporale et al . examined chemical exposure data in a large cohort of human mother-child pairs and identified a set of common endocrine-disrupting chemicals (see the Perspective by Liew and Guo). The authors then showed that a mixture of these substances could cause abnormalities in two model organisms and may also be associated with language delay in humans. These findings suggest a need for further research into the effects of chemical mixtures and regulations that take such combinatorial effects into account. —YN

Abstract

Exposure to mixed endocrine-disrupting chemicals is common in pregnant women and may be associated with language delay in their children.

Abstract

INTRODUCTION

Endocrine disrupting chemicals (EDCs) are compounds that interfere with physiological hormonal regulation. Humans are pervasively exposed to many different EDCs, and a growing body of evidence indicates that early life exposure to such EDC mixtures can induce changes in the human organism that underlie increased susceptibility to diseases throughout the life span, including neurodevelopmental disorders. Chemical regulation is, however, entirely based on the risk assessment of individual compounds, leaving the real-life impact of chemical mixtures unexamined and unregulated. This is relevant insofar as cumulative exposure to multiple compounds may be associated with adverse health outcomes even when the concentrations of individual chemicals fall below the regulatory dose.

RATIONALE

We set out to make the epidemiological associations between exposure to mixtures and health outcomes experimentally tractable, defining molecular pathways and dose responses that could be translated back to actual human exposures and thereby refine current risk assessment practices. As opposed to previous studies that focused on single compounds, we identified and tested an EDC mixture associated with adverse neurodevelopmental outcomes in the Swedish Environmental Longitudinal, Mother and child, Asthma and allergy (SELMA) pregnancy cohort (MIX N) by integrating epidemiological data with experimental toxicology and characterized real life–relevant exposure.

RESULTS

We used weighted quantile sum (WQS) regression to identify chemicals associated with language delay in children and included those chemicals in MIX N. MIX N was synthesized following the relative proportions and total concentrations found in the SELMA cohort. It was then tested in both in vitro and in vivo models. In human fetal primary neural stem cells and three-dimensional cortical brain organoids differentiated from human pluripotent stem cells, transcriptomic analysis showed that MIX N interferes with hormonal pathways and dysregulates expression of genes and biological pathways that are causally linked to autism spectrum disorders. Data from experiments in Xenopus leavis and Danio rerio , in vivo models validated by the Organisation for Economic Co-operation and Development (OECD), confirmed thyroid function as one of the key and unifying points of vulnerability to MIX N and linked thyroid disruption to neurodevelopmental effects measured as alterations in locomotor activity. The resulting dose-response relationships were then used to estimate a point of departure (POD), which is the toxicological measure to estimate no-effect concentration. This enabled us to apply a similar mixture approach (SMACH) where we (i) identified individuals in the SELMA study who were sufficiently similarly exposed compared with the experimental mixtures and (ii) determined the proportion of the SELMA children with exposure ranges of concern using the POD as reference.

CONCLUSION

Integrating experimental and epidemiological evidence, we established mechanistic and correlative evidence for neurodevelopmental adversities of an EDC mixture associated with language delay. Using the generated experimental data in a risk assessment concept, we found increased odds of language delay in offspring of up to 54% of pregnant women. These results emphasize the need to take mixtures into account during chemical testing and risk assessment and provide an integrative framework to guide risk assessment strategies.