Jacob, J., Ribes, V., Moore, S., Constable, S.C., Sasai, N., Gerety, S.S., Martin, D.J., Sergeant, C.P., Wilkinson, D.G., and Briscoe, J. (2014) Valproic Acid silencing of ascl1b/ascl1 results in the failure of serotonergic differentiation in a zebrafish model of Fetal Valproate Syndrome. Disease models & mechanisms. 7(1):107-17.
Fetal valproate syndrome (FVS) is caused by in utero exposure to the drug sodium valproate. Valproate is used worldwide for
the treatment of epilepsy, as a mood stabiliser and for its pain relieving properties. In addition to birth defects, FVS
is associated with an increased risk of autism spectrum disorder (ASD), which is characterised by abnormal behaviours. Valproate
perturbs multiple biochemical pathways and alters gene expression through its inhibition of histone deacetylases. Which, if
any, of these mechanisms is relevant to the genesis of its behavioural side-effects is unclear. Neuroanatomical changes associated
with FVS have been reported and amongst these, altered serotonergic neuronal differentiation is a consistent finding. Altered
serotonin homeostasis is also associated with autism. Here we have used a chemical-genetics approach to investigate the underlying
molecular defect in a zebrafish FVS model. Valproate causes the selective failure of zebrafish central serotonin expression.
It does so by downregulating the proneural gene ascl1b, an ortholog of Ascl1 and a known determinant of serotonergic identity in the mammalian brainstem. Ascl1b is sufficient to rescue serotonin expression in valproate treated embryos. Chemical and genetic blockade of the histone deacetylase
Hdac1 downregulates ascl1b, consistent with the Hdac1 mediated silencing of ascl1b expression by valproate. Moreover, tonic Notch signalling is critical for ascl1b repression by valproate. Concomitant blockade of Notch signalling restores ascl1b expression and serotonin expression in both valproate-exposed and hdac1 mutant embryos. Together these data provide a molecular explanation for serotonergic defects in FVS and highlight an epigenetic
mechanism for genome-environment interaction in disease.