These substances are used in the composition of a wide range of products, including, but not limited to, plastic derivatives, which are now known to enter living organisms via several routes: the water they absorb, foods they eat or even the air they breathe.
Admittedly, in everyday life, the levels of exposure to individual chemical substances are often lower than the legal limit values, determined experimentally. But the problem is that these individual exposures do not always reflect the risks these products pose to human health.
Indeed, chemical substances which, taken separately, have only a limited effect can have much more deleterious effects. when present in complex mixtures.
This “cocktail effect” was again highlighted by a study published this year in the journal Science.
By combining data from epidemiological studies with experiments carried out on cellular and aquatic animal models, an international research group, of which we are a part, has shown that the hormonal disruption induced by exposure to a mixture of eight chemicals in the daily has an impact on brain development and language acquisition in children. Back on these results, and on the perspectives they open up.
A cohort to study the impact of chemicals on unborn children
Unlike the majority of previous studies, which had focused on the effects of single compounds, the work in which we participated aimed to analyze the consequences of exposure to a mixture of chemical compounds with endocrine disrupting properties, actual exposure levels.
For the record, endocrine disruptors are substances capable of interfering with the functioning of hormones (chemical messengers), and this, at extremely low concentrations. The consequences of these interactions are potentially very deleterious, because hormones are involved in a large number of fundamental processes: cell proliferation and migration during fetal development, metabolism, reproduction, stress, nutrition, sleep, etc.
Our results were obtained using data from the SELMA cohort study, conducted at Karlstad University, Sweden. This study follows approximately 2 mother-child pairs from early pregnancy, through childbirth, and until the child reaches school age.
The general objective of SELMA is to study the impact of exposure to chemical substances suspected or proven to disrupt the endocrine system in early pregnancy on the health and development of the child later in life. As a reminder, the SELMA study has already made it possible to establish a link between exposure to different chemicals and the child sex development, respiratory problems, cognitive development and growth during childhood.
The originality of the approach, which gave rise to a publication in the scientific journal Science, is to have integrated not only epidemiological data such as that of the SELMA study, but also experimental toxicology data and finally propose a new approach for assessing the risk associated with exposure to mixtures. To do this, this work was carried out in three stages.
Identification of the chemical mixture
First, the overrepresentation of a mixture of eight chemicals in the blood and urine of pregnant women in the SELMA cohort was correlated with language delay in children at 30 months of age (less than fifty spoken words). Several of the constituents of this blend were known to have endocrine disrupting effects.
This was, for example, the case of bisphenol A (a compound used to make plastics and some epoxy resins) or some chemical compounds perfluorinated (used in a large number of consumer and industrial products, from cosmetics to fire-fighting foam to waterproof clothing). Other compounds in this mixture, such as certain phthalates (diethyl phthalate, dibutyl phthalate and benzyl and butyl phthalate), had been associated with language delay in children at 30 months by previous research.
These effects had previously been identified through association research conducted on each product individually. This time, the goal was to determine their effects as a mixture. Once the mixture was identified, it was therefore recreated by chemists in order to study it in more detail.
After this first identification step, the scientists then conducted experiments to study the mode of action of the mixture of chemicals. For this, they used various experimental models, in order to identify the molecular targets via which this mixture could act in the body.
The aim was to assess its ability to disrupt regulation mediated by hormones, but also genes involved in brain development or associated with cognitive and intellectual impairment at levels of exposure relevant to humans.
This step was carried out in particular on human brain organoids (cultures of cells capable of organizing themselves to reproduce certain functions of the tissues of the organ they represent. They are in some sorts of "mini-organs", editor's note). Thanks to these tools, it has been possible to reproduce the main aspects of the development of our brain. The researchers were thus able, for the first time, to directly study the molecular effects of this mixture of chemicals on human fetal brain tissue.
Computer modeling has also made it possible to analyze the effects of the mixture on networks of genes involved in the differentiation of neurons and regulated by numerous hormones, in particular hormones thyroid. In this same study, aspects of thyroid disturbance and disturbance of swimming behavior could be demonstrated in an amphibian and in zebrafish.
The data obtained on all of these experimental models were then analyzed in order to identify the hormonal pathways that are mainly disturbed. The results revealed a conservation of the disruptive properties of the chemical mixture in vertebrates.
Thirdly, the results of these experimental studies were used to develop new tools for assessing the risks associated with exposure to mixtures of chemicals.
All of this work has made it possible to demonstrate that at realistic concentrations, the mixture of products studied disrupts regulatory networks under hormonal influence in the organoids of the human brain as in animal models. Xenopus leavis et Danio laughed.
By analyzing the epidemiological data, we were able to show that up to 54% of the children had experienced prenatal exposures above the levels considered to be of concern in our study (which were determined experimentally).
Children in the top decile of exposure had a 3,3 times higher risk of language delay than those in the bottom decile (language delay was chosen as the endpoint because it is a marker early intellectual disability).
One of the main hormonal pathways affected is that of thyroid hormones. Knowing that optimal levels of maternal thyroid hormones are necessary in early pregnancy for brain growth and development, it is not surprising that there is an association between prenatal exposure to these products and delayed speech.
These results demonstrate that it is imperative to change the approach to take into account mixtures of chemicals when evaluating them. The use of new tools for analyzing the risk linked to exposure to mixtures (rather than to individual compounds) could have prevented 54% of children from being exposed in utero to levels judged retrospectively, at the light of the results of this worrying study.
Adapt legislation to better assess risks
This work demonstrates that the risk identified by this research can only be detected by considering the products not one by one, but as a "cocktail", since the effects of chemical substances in complex mixtures may differ from their individual properties.
This is an important point because, at present, risk assessment exclusively addresses the effects of individual chemicals.
In addition, the biological effects of endocrine disruptors can manifest themselves at doses well below the limit values set by conventional toxicology tests. However, current legislation does not sufficiently take into account the “endocrine disrupting” nature of certain substances. By way of illustration, it is only since 2018 that the texts allow the identification of endocrine disruptors. And again, only the biocidal phytosanitary products are concerned.
Researchers have been trying to clarify the modes of action of endocrine disruptors and measure their effects on health (human and animal) for over 30 years. Our work demonstrates that combining epidemiological data and experimental results is a promising way to improve our understanding of these very specific pollutants. It will then remain to adapt the legislation accordingly.