Covalent modifications of DNA and its surrounding chromatin constitute an essential and powerful regulatory mechanism for gene transcription. Epigenetics is the study of this regulatory system. There is now strong albeit indirect evidence that epigenetic mechanisms contribute to the pathophysiology of schizophrenia. Furthermore, the discovery that valproic acid, a widely used psychotropic, has powerful epigenetic effects in clinically relevant concentrations suggests new therapeutic possibilities, i.e., drugs that act on chromatin structure.
Proceedings of the National Academy of Sciences of the United States of America
The polygenic nature of complex psychiatric disorders suggests a common pathway that may be involved in the down-regulation of multiple genes through an epigenetic mechanism. To investigate the role of methylation in down-regulating the expression of mRNAs that may be associated with the schizophrenia phenotype, we have adopted a cell-culture model amenable to this line of investigation.
A recent report suggests that the down-regulation of reelin and glutamic acid decarboxylase (GAD(67)) mRNAs represents 2 of the more consistent findings thus far described in post-mortem material from schizophrenia (SZ) patients [reviewed in. Neurochemical markers for schizophrenia, bipolar disorder amd major depression in postmortem brains. Biol Psychiatry 57, 252-260]. To study mechanisms responsible for this down-regulation, we have analyzed the promoter of the human reelin gene.
Reelin and glutamic acid decarboxylase 67 (GAD67) mRNAs and protein levels are substantially reduced in postmortem brains of patients with schizophrenia. Increasing evidence suggests that the observed down-regulation of reelin and GAD67 gene expression may be caused by dysfunction of the epigenetic regulatory mechanisms operative in cortical GABAergic interneurons.
Among the most consistent results of studies of post-mortem brain tissue from schizophrenia patients (SZP) is the finding that in this disease, several genes expressed by GABAergic neurons are downregulated. This downregulation may be caused by hypermethylation of the relevant promoters in affected neurons. Indeed, increased numbers of GABAergic interneurons expressing DNA methyltransferase 1 (DNMT1) mRNA have been demonstrated in the prefrontal cortex (PFC) of SZP using in situ hybridization.
Histone deactylase enzymes are responsible for the deacetylation of histone tails, and consequently influence gene regulation through their ability to modify chromatin structure surrounding promoter regions. We analyzed the microarray collection of the National Brain Databank to investigate differential expression of these enzymes in the prefrontal cortices of control, schizophrenia and bipolar subjects. HDAC1 expression levels were significantly higher in schizophrenia versus normal subjects.
The epigenetic down-regulation of genes is emerging as a possible underlying mechanism of the GABAergic neuron dysfunction in schizophrenia. For example, evidence has been presented to show that the promoters associated with reelin and GAD67 are down-regulated as a consequence of DNA methyltransferase (DNMT)-mediated hypermethylation. Using neuronal progenitor cells to study this regulation, we have previously demonstrated that DNMT inhibitors coordinately increase reelin and GAD67 mRNAs.
The neuronal GABAergic mechanisms that mediate the symptomatic beneficial effects elicited by a combination of antipsychotics with valproate (a histone deacetylase inhibitor) in the treatment of psychosis (expressed by schizophrenia or bipolar disorder patients) are unknown. This prompted us to investigate whether the beneficial action of this combination results from a modification of histone tail covalent esterification or is secondary to specific chromatin remodeling.
OBJECTIVE: The emerging field of psychiatric epigenetics is constrained by the dearth of research methods feasible in living patients. With this focus, we report on two separate approaches, one in vitro and one in vivo, developed in our laboratory. METHOD: In the first approach, we isolated lymphocytes from 12 subjects and cultured their cells with either 0.7 mM valproic acid (VPA), 100 nM Trichostatin A (TSA), or DMSO (control) for 24h based upon previous dose response experiments.
Several lines of schizophrenia (SZ) research suggest that a functional downregulation of the prefrontal cortex GABAergic neuronal system is mediated by a promoter hypermethylation, presumably catalyzed by an increase in DNA-methyltransferase-1 (DNMT-1) expression. This promoter hypermethylation may be mediated not only by DNMT-1 but also by an entire family of de novo DNA-methyltransferases, such as DNA-methyltransferase-3a (DNMT-3a) and -3b (DNMT-3b).