Abstract

Valproate (VPA) is a commonly used drug worldwide despite increased awareness of its adverse effects, especially when used during pregnancy. Fetal valproate syndrome, characterised by birth defects, learning difficulties, and autism, is observed in up to 50% of children exposed in utero. The teratogenic properties of VPA are thought to result from altered gene expression through its inhibition of the chromatin modifying enzyme family of histone deacetylases (Hdac). Brainstem serotonergic abnormalities are believed to be important in mediating the behavioural manifestations of fetal valproate syndrome. Abnormal differentiation of these neurons has been reported in rodent models of autism and this syndrome. However, the role of Hdac and the identity of the salient genetic targets in VPA–associated serotonergic dysgenesis has not been determined; this has hampered the development of therapies that can ameliorate fetal valproate syndrome. Here we show that VPA specifically blocks serotonergic differentiation in a zebrafish model by downregulating the expression of the proneural gene, Ascl1b, which encodes a critical determinant of serotonergic fate. Using a chemical–genetics approach we show that VPA exposes cryptic transcriptional repression by tonic Notch signalling, which prevents the activation of Ascl1b. Chemical blockade of Notch signalling results in Ascl1b activation and the rescue of serotonergic differentiation in the presence of VPA. We find that the ability of VPA to silence Ascl1b and block serotonergic neurogenesis is specifically dependent on inhibition of Hdac1. Consistent with this finding, Hdac1 mutants fail to activate Ascl1b and lack 5–HT neurons. Moroever, treatment of Hdac1 mutants with a Notch inhibitor drug rescues 5–HT neurons, which confirms that Hdac1 mediates the effects of VPA. The current model of Hdac function proposes that cyclical Hdac activity is needed to prime active genes, such as Ascl1b, for further rounds of transcription. Our discovery of the interplay between chromatin modifiers and Notch in gene regulation adds a new layer of complexity to the epigenetic molecular model. Fetal valproate syndrome thus highlights the crucial role of the epigenetic interface in genome–environment interactions in disease.