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. The results suggest that clozapine, or sulpiride associated with valproate, by increasing DNA demethylation with an unknown mechanism, causes a chromatin remodeling that brings about a beneficial change in the epigenetic GABAergic dysfunction typical of schizophrenia and bipolar disorder patients.

In this review, we discuss changes in the regulation of gene expression in the central nervous system (CNS) associated with DNA (cytosine-5) methylation, chromatin remodeling and post-translational covalent modifications of histones. During brain development, abnormal intrinsic or extrinsic cues may compromise epigenetic processes regulating neural stem cell proliferation and differentiation and thus directly or indirectly could contribute to altered epiphenotypes leading to psychiatric disorders. These mechanisms, that include chromatin remodeling and reversible changes in promoter methylation patterns, are largely expressed by terminally differentiated cortical GABAergic neurons. These neurons are unique among various brain cell subtypes because they express high levels of DNA-methyltransferase-1 (DNMT1). Moreover, DNMT1 expression is further increased in schizophrenia (SZ) and bipolar (BP) disorder brains. To unravel how this pathological DNMT1 overexpression induces GABAergic neuronal dysfunction in SZ and in other psychoses, we report on how alterations in methylation modify the expression of susceptible vulnerability genes such as reelin or GAD67 in these neurons. The results encourage the view that promoter hypermethylation in GABAergic neurons that occurs in SZ represents a testable target for novel therapeutic strategies to treat this disorder.

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. The present study expands upon these findings using nested competitive reverse transcription-polymerase chain reaction combined with laser-assisted microdissection to quantitate the extent of DNMT1 mRNA overexpression in distinct populations of GABAergic neurons obtained from either layer I or layer V of the PFC of SZP. In a cohort of eight SZP and eight non-psychiatric subject (NPS) post-mortem BA9 tissue samples, DNMT1 mRNA was found to be selectively expressed in GABAergic interneurons and virtually absent in pyramidal neurons. DNMT1 mRNA expression was approximately threefold higher in GABAergic interneurons microdissected from layer I of SZP relative to the same neurons microdissected from NPS. GABAergic interneurons obtained from layer V of the same samples displayed no difference in DNMT1 mRNA expression between groups. In the same samples, the GABAergic neuron-specific glutamic acid-decarboxylase(67)(GAD(67)) and reelin mRNAs were underexpressed twofold in GABAergic interneurons isolated from layer I of SZP relative to GABAergic interneurons microdissected from layer I of NPS, and unaltered in GABAergic interneurons of layer V. These findings implicate an epigenetically mediated layer I GABAergic dysfunction in the pathogenesis of schizophrenia, and suggest novel strategies for treatment of the disease.Molecular Psychiatry advance online publication, 30 January 2007; doi:10.1038/sj.mp.4001954.

Prefrontal cortex (Brodmann's area 9) levels of the methyl donor S-adenosyl methionine were increased by about two-fold in schizophrenia and bipolar disorder patients, but not in unipolar depressed patients compared with nonpsychiatric subjects from the Stanley Foundation Neuropathology Consortium (Bethesda, Maryland, USA). Neither age, brain weight and pH, hemisphere, post-mortem interval, disease onset/duration, nor cumulative dose of fluphenazine affected S-adenosyl methionine content. In schizophrenia and bipolar disorder patients, the increase of S-adenosyl methionine is associated with an overexpression of DNA methyltransferase-1 mRNA in Brodmann's area 9 GABAergic neurons. Hence, the increased expression of S-adenosyl methionine and DNA methyltransferase-1 may contribute to promoter cytosine 5-methylation and to downregulation of the expression of mRNAs encoding for reelin and GAD67 in cortical GABAergic neurons of schizhophrenia and bipolar disorder patients.

Reelin and glutamic acid decarboxylase (GAD)67 expressed by cortical gamma-aminobutyric acid-ergic interneurons are down-regulated in schizophrenia. Because epidemiological studies of schizophrenia fail to support candidate gene haploinsufficiency of Mendelian origin, we hypothesize that epigenetic mechanisms (i.e., cytosine hypermethylation of CpG islands present in the promoter of these genes) may be responsible for this down-regulation. Protracted l-methionine (6.6 mmolkg for 15 days, twice a day) treatment in mice elicited in brain an increase of S-adenosyl-homocysteine, the processing product of the methyl donor S-adenosyl-methionine, and a marked decrease of reelin and GAD67 mRNAs in both WT and heterozygous reeler mice. This effect of l-methionine was associated with an increase in the number of methylated cytosines in the CpG island of the reelin promoter region. This effect was not observed for GAD65 or neuronal-specific enolase and was not replicated by glycine doses 2-fold greater than those of l-methionine. Prepulse inhibition of startle declined at a faster rate as the prepulsestartle interval increased in mice receiving l-methionine. Valproic acid (2 mmolkg for 15 days, twice a day) reverted l-methionine-induced down-regulation of reelin and GAD67 in both WT and heterozygous reeler mice, suggesting an epigenetic action through the inhibition of histone deacetylases. The same dose of valproate increased acetylation of histone H3 in mouse brain nearly 4-fold. This epigenetic mouse model may be useful in evaluating drug efficacy on schizophrenia vulnerability. Hence the inhibition of histone deacetylases could represent a pharmacological intervention mitigating epigenetically induced vulnerability to schizophrenia in individuals at risk.