Epigenetic mechanisms in the regulation of the reelin and glutamic acid decarboxylase 67 genes

Reduced reelin and glutamic acid decarboxylase 67 (GAD67) mRNA and protein levels have been among the most consistent findings reported in post-mortem brains of schizophrenia patients. Increasing evidence suggests that the observed down-regulation of gene expression may be caused by dysfunction of the epigenetic regulatory mechanisms operative in cortical GABAergic neurons. To explore mechanisms involved in the reelin and GAD67 gene regulation, we studied the effects of two classes of epigenetic drugs, DNA methyltransferase (DNMT) inhibitors and histone deacetylase (HDAC) inhibitors, in NT-2 neuronal progenitor cells. We have first demonstrated that DNMT inhibitors coordinately increase reelin and GAD67 mRNAs. We have further shown that HDAC inhibitors activate these two genes with a comparable dose- and time-dependence. In parallel, both groups of drugs decrease DNMT1, DNMT3A and DNMT3B protein levels, and reduce DNMT enzyme activity. Furthermore, induction of the reelin and GAD67 mRNAs is accompanied by the dissociation of repressor complexes, containing all three DNMTs, MeCP2 and HDAC1, from the corresponding promoters and increased local histone acetylation. Our data imply that drug-induced promoter demethylation is relevant for maximal activation of reelin and GAD67 transcription. The results suggest that HDAC and DNMT inhibitors activate reelin and GAD67 expression through similar mechanisms. Both classes of drugs attenuate, directly or indirectly, the enzymatic and transcriptional repressor activities of DNMTs and HDACs. These data provide a mechanistic rationale for the use of epigenetic drugs, individually or in combination, as a potential novel therapeutic strategy for correcting the reelin and GAD67 mRNA insufficiencies, and therefore GABAergic deficits associated with schizophrenia.