Of meis and men: lessons from a microarray study of teratogen action

Key developments in endocrine disrupter research and human health

Environmental etiologies involving exposures to chemicals that mimic endogenous hormones are proposed for a number of adverse human health effects, including infertility, abnormal prenatal and childhood development, and reproductive cancers (National Research Council, 1999; World Health Organization, 2002). Endocrine disrupters represent a significant area of environmental research with important implications for human health. This article provides an overview of some of the key developments in this field that may enhance our ability to assess the human health risks posed by exposure to endocrine disrupters. Advances in methodologies of hazard identification (toxicogenomics, transcriptomics, proteomics, metabolomics, bioinformatics) are discussed, as well as epigenetics and emerging biological endpoints.

Molecular approaches to developmental malformations using analogous forms of valproic acid

The teratogenic potential of valproic acid has been well established both in experimental models and in human clinical studies. Evidence from many previous studies has shown that VPA is an appropriate drug model for studying chemical structure-teratogenicity relationships. Using molecular techniques of DNA microarray (GeneChip system) or quantitative real-time polymerase chain reaction with low teratogenic VPA analogs as comparative control drugs, we attempted to identify the genes involved with the molecular mechanisms of VPA teratogenicity in the neural tube and the axial skeleton of the mouse embryo. The recent development of DNA microarray enables a genome-wide approach to the identification of genes correlated with the teratogenicity of chemicals (teratogenomics). The VPA-induced changes in gene expression seen during mouse embryogenesis provides information for understanding how VPA disrupts normal embryonic development, and also provides leads for the development of safer medicines.

Animal carcinogenicity studies: 3. Alternatives to the bioassay

Conventional animal carcinogenicity tests take around three years to design, conduct and interpret. Consequently, only a tiny fraction of the thousands of industrial chemicals currently in use have been tested for carcinogenicity. Despite the costs of hundreds of millions of dollars and millions of skilled personnel hours, as well as millions of animal lives, several investigations have revealed that animal carcinogenicity data lack human specificity (i.e. the ability to identify human non-carcinogens), which severely limits the human predictivity of the bioassay. This is due to the scientific inadequacies of many carcinogenicity bioassays, and numerous serious biological obstacles, which render profoundly difficult any attempts to accurately extrapolate animal data in order to predict carcinogenic hazards to humans. Proposed modifications to the conventional bioassays have included the elimination of mice as a second species, and the use of genetically-altered or neonatal mice, decreased study durations, initiation-promotion models, the greater incorporation of toxicokinetic and toxicodynamic assessments, structure-activity relationship (computerised) systems, in vitro assays, cDNA microarrays for detecting changes in gene expression, limited human clinical trials, and epidemiological research. The potential advantages of non-animal assays when compared to bioassays include the superior human specificity of the results, substantially reduced time-frames, and greatly reduced demands on financial, personnel and animal resources. Inexplicably, however, the regulatory agencies have been frustratingly slow to adopt alternative protocols. In order to decrease the enormous cost of cancer to society, a substantial redirection of resources away from excessively slow and resource-intensive rodent bioassays, into the further development and implementation of non-animal assays, is both strongly justified and urgently required.

Identification of early-responsive genes correlated to valproic acid-induced neural tube defects in mice

BACKGROUND

Valproic acid (VPA) causes the failure of neural tube closure in newborn mice. However, the molecular mechanism of its teratogenesis is unknown. This study was conducted to investigate the genomewide effects of VPA disruption of normal neural tube development in mice.

METHODS

Microarray analysis was performed on the head part of NMRI mouse embryos treated for 1 hr with VPA on gestational day (GD) 8. Subsequently, we attempted to isolate genes that changed in correlation with the teratogenic action of VPA by employing reduced teratogenic VPA analogs, valpromide (VPD) and valnoctamide (VCD), in a real-time PCR study.

RESULTS

Microarray results demonstrated that during neurulation, many genes, some of whose functions are known and some unknown, were either increased or decreased after VPA injection. Some genes were affected by VPD or VCD in the same way as VPA, but others were not changed by the analogs. In this way, our system identified 11 increased and 20 decreased genes. Annotation analysis revealed that the increased genes included gadd45b, ier5, per1, phfl3, pou3f1, and sox4, and the decreased genes included ccne2, ccnl, gas5, egr2, sirt1, and zfp105.

CONCLUSIONS

These findings demonstrate that expression changes in genes having roles in the cell cycle and apoptosis pathways of neural tube cells were strongly expected to relate to the teratogenic, but not antiepileptic, activity of VPA. Our approach has allowed the expansion of the catalog of molecules immediately affected by VPA in the developing neural tube.

Valproic acid teratogenicity: a toxicogenomics approach

Embryonic development is a highly coordinated set of processes that depend on hierarchies of signaling and gene regulatory networks, and the disruption of such networks may underlie many cases of chemically induced birth defects. The antiepileptic drug valproic acid (VPA) is a potent inducer of neural tube defects (NTDs) in human and mouse embryos. As with many other developmental toxicants however, the mechanism of VPA teratogenicity is unknown. Using microarray analysis, we compared the global gene expression responses to VPA in mouse embryos during the critical stages of teratogen action in vivo with those in cultured P19 embryocarcinoma cells in vitro. Among the identified VPA-responsive genes, some have been associated previously with NTDs or VPA effects [vinculin, metallothioneins 1 and 2 (Mt1, Mt2), keratin 1-18 (Krt1-18)], whereas others provide novel putative VPA targets, some of which are associated with processes relevant to neural tube formation and closure [transgelin 2 (Tagln2), thyroid hormone receptor interacting protein 6, galectin-1 (Lgals1), inhibitor of DNA binding 1 (Idb1), fatty acid synthase (Fasn), annexins A5 and A11 (Anxa5, Anxa11)], or with VPA effects or known molecular actions of VPA (Lgals1, Mt1, Mt2, Id1, Fasn, Anxa5, Anxa11, Krt1-18). A subset of genes with a transcriptional response to VPA that is similar in embryos and the cell model can be evaluated as potential biomarkers for VPA-induced teratogenicity that could be exploited directly in P19 cell-based in vitro assays. As several of the identified genes may be activated or repressed through a pathway of histone deacetylase (HDAC) inhibition and specificity protein 1 activation, our data support a role of HDAC as an important molecular target of VPA action in vivo.

Gene-Teratogen Interactions in Chemically Induced Congenital Malformations

Exposure of the embryo to environmental chemicals can result in congenital malformations or abortion. Although experimental teratology data are considered sufficient for risk assessment, only knowledge of their mechanisms of action permits a justifiable extrapolation of animal data to humans. Mechanistic studies of some teratogenic agents such as retinoic acids, valproic acid, diethylstilbestrol, and cyclopamine provided evidence of interference with regulation of genes controlling the embryonic development. The new genomic technologies are important tools in this field and may represent a real improvement in understanding the mechanisms of action of chemical teratogens.