Thyroid hormone disruption after mixture exposure

29 October, 2017

We are exposed to multiple chemicals that make their way into the environment through several routes. Hormonal disruptive effects of many of these chemicals have been reported in experimental and/or epidemiological studies. Most of these disruptive effects have, however, been reported as an individual chemical rather than a mixture.

Research group led by Prof. Barbara Demeneix (France) in the EDC-MixRisk project has been tasked to identify and characterize the thyroid disrupting effect of endocrine disruptive chemical (EDC) mixtures using Xenopus laevis (an amphibian model for studying thyroid hormones, THs). These mixtures were shown within another work package to affect brain development (mix N0) and metabolism (mix G0) in humans.

The Xenopus Embryonic Thyroid Assay (XETA) was used to identify mixtures effect on thyroid signalling disruption. XETA was developed ten years ago by Prof. Demeneix and is currently being validated within the test guideline process at the OECD. The two mixtures suspected to disrupt either neurodevelopment or metabolism in human were found to disrupt TH signalling (at concentrations found in human concentration). Gene expression analysis of key genes in thyroid hormone signaling further confirmed TH disruption after mixture exposure. Finally, a phenotypical effect on mobility was reported as it was observed that all concentrations tested affected the light-induced movement of tadpoles.

A new research approach was applied, confirming experimentally the epidemiologically based hypothesis about these worrisome chemicals. It was shown that the two mixtures, N0 and G0, disrupted thyroid hormone signalling and adversely affected gene expression and mobility in amphibians. This amphibian model is relevant for thyroid disruption studies as metamorphosis is a highly thyroid hormone dependent process.

A matter of concern

Thyroid hormones are essential for normal brain development. A complete lack of thyroid hormone induces cretinism but a slight change in maternal thyroid hormone during pregnancy has also been shown to affect IQ and brain structure in children (Korevaar et al., 2016). Given this crucial role of thyroid hormone during early brain development, a correlation with N0 mixture effect is not surprising. Moreover, thyroid hormones play a critical role on metabolism, both peripherally and centrally. Therefore, it wasn’t unexpected that the mix G0 disrupted thyroid hormone signaling.

Currently, there is a sharp increase in neurodevelopmental and metabolic diseases in modern western countries. For example type II diabetes incidence doubled in the last 10 years while in US, 1 out of 42 boys were diagnosed autistic in 2016 (1 out of 5000 in 70’s). This increase is regardless of changes in diagnostic methods or protocols and access to better healthcare. Furthermore, in France, a 3% increase in congenital hypothyroidism has been observed for the last 20 years with exactly same diagnostic threshold (Barry et al. 2016). This gives importance to the environmental hypothesis on the increase in neurodevelopmental disorder incidence rate.

Best experimental practices and new methods

EDC-Mix Risk project offers a new strategy in risk assessment. Conventionally, compounds of concern are identified and a battery of in vitro or in vivo assays are conducted in order to extrapolate the human risk assessment. For endocrine disruptor risk assessment specifically, this strategy is extremely difficult. First, one should provide a burden of evidence, e.g. proof of exposure, adverse effects and a proof of mechanism of action. Second, the fact that we are exposed to a mixture of compounds makes the legislation process slow and difficult to handle.

In EDC-MixRisk, proof of exposure and correlation with adverse outcome are done on the basis of real-life exposure, derived from epidemiological data. Mechanism of action is provided by in vitro and in vivo assays such as the before mentioned the Xenopus Embryonic Thyroid Assay. New regulations through new data, protocols and approaches developed within EDC Mix-Risk could provide for safer chemicals for our next generation.

References:

Barry Y, Bonaldi C, Goulet V, Coutant R, Léger J, Paty AC, Delmas D, Cheillan D, Roussey M. Increased incidence of congenital hypothyroidism in France from 1982 to 2012: a nationwide multicenter analysis. Ann Epidemiol. 2016 Feb;26(2):100-105.e4. doi: 10.1016/j.annepidem.2015.11.005. Epub 2015 Dec 12. PubMed PMID: 26775052.

Korevaar, T. I. M. et al. (2016) ‘Association of maternal thyroid function during early pregnancy with off spring IQ and brain morphology in childhood: a population-based prospective cohort study’, Articles Lancet Diabetes Endocrinol. Elsevier Ltd, 4(4), pp. 35–43. doi: 10.1016/S2213-8587(15)00327-7.