Highlight report: Stem cell-based developmental toxicity tests
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Developmental toxicity testing is a particularly challenging field of toxicology (Colaianna et al. 2013, 2017; Leist et al. 2017; Zimmer et al. 2014). Up to 50% of the animals used for testing in the context of REACH have been estimated to be required for evaluation of developmental and reproductive toxicity (Seiler et al. 2011). Therefore, there is a demand for accurate and fast in vitro tests of this form of toxicity (Shinde et al. 2015; Meganathan et al. 2015; Reif 2015; Weng et al. 2014). Recently, several test systems have been established based on human embryonic stem cells (hESC) that differentiate to neuronal precursor cells (Krug et al. 2013; Kuegler et al. 2012; Weng et al. 2012; Balmer et al. 2012, 2014; Pallocca et al. 2016). Alternatively, multilineage differentiation into ecto-, meso- and endoderm can be recapitulated in vitro (Jagtap et al. 2011; Krug et al. 2013; Meganathan et al. 2012). During the differentiation period large numbers of genes (named ‘developmental genes’) are either up- or downregulated. The principle of many assays is to study whether test compounds influence expression of these genes compared to solvent controls. Concentration-dependent experiments with the developmental toxicant valproic acid have shown that three concentration ranges can be differentiated: a concentration range of tolerance, where the test compound does not influence expression of any gene; a concentration range of deregulation, where expression of genes is altered but no cytotoxicity occurs; and finally a concentration range of cytotoxicity, where expression of additional cell death associated genes is observed (Waldmann et al. 2014, 2017). Patterns of deregulated genes have been shown to be compound specific. It has, for example, been shown that histone deacetylase inhibitors can be identified by a classifier based on eight genes and can be differentiated from other classes of compounds, e.g., mercurials (Rempel et al. 2015). For quantification of the effect of test compounds, a concept of two indices, ‘developmental potency’ and ‘developmental index’, has been established (Shinde et al. 2017). The developmental potency is the fraction of all developmental genes (genes up- or downregulated in spontaneously differentiating cells) that are up- or downregulated by a test compound. The developmental index is the ratio of overrepresentation of developmental genes among all genes deregulated by a test compound (Shinde et al. 2017). Using the developmental index allows a particularly sensitive hazard identification, because some developmental toxicants deregulate expression of only a small number of genes but a high fraction of them are developmental genes (Shinde et al. 2017). Although many developmental toxicity test systems used hESC, recent work has shown that also induced pluripotent stem cells (hiPSC), which are not facing ethical issues such as hESC, allow the identification of developmental toxicants in a similar way (Shinde et al. 2016). However, until now the sensitivity and specificity are major limitations and the future challenges of human stem cell-based developmental toxicity tests. To determine both, analysis of large numbers of positive and negative control compounds (which do versus do not induce developmental toxicity in humans) should be tested at concentrations relevant for the human in vivo situation.
Much progress has already been achieved but there is still a long way to go until in vitro assays will replace animal tests of developmental toxicity.
Compliance with ethical standards
Conflict of interest
The author declares that he has no conflict of interest.
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