Reelin and glutamic acid decarboxylase (GAD)67 expressed by cortical ␥-aminobutyric acid-ergic interneurons are downregulated 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 downregulation. Protracted L-methionine (6.6 mmol͞kg 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 GAD 67 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 prepulse͞startle interval increased in mice receiving L-methionine. Valproic acid (2 mmol͞kg 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. S tudies of heterozygous reeler mice (HRM) have provided preliminary evidence of a relationship between reelin haploinsufficiency, the decrease of dendritic spine expression density in frontal cortex (FC) pyramidal neurons and associated neuropil hypoplasticity, the down-regulation of glutamic acid decarboxylase (GAD) 67 expression, and the decrease in ␥-aminobutyric acid (GABA) turnover (1-3). Similar neurochemical and structural abnormalities were detected in the FC of schizophrenia postmortem brains (4-9). Hence, HRM may be a model to evaluate the efficacy of novel treatments for schizophrenia by monitoring drug actions on (i) reelin and GAD 67 mRNA expression, (ii) GABA turnover, and (iii) cortical neuropil plasticity including dendritic spine expression.The HRM model several aspects of the molecular neuropathology expressed in schizophrenia, although the mechanisms operative in these pathologies may be different. In fact, demographic studies of schizophrenia inheritance in identical twins show a concordance of Ϸ50%, which supports an epigenetic model but not gene haploinsufficiency of Mendelian origin. Investigation of a putative epigenetic mechanism attending schizophrenia vulnerability may therefore be in order (10). Along this line of thinking we have hypothesiz...

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.

IMPORTANCE Dysfunction related to γ-aminobutyric acid (GABA)-ergic neurotransmission in the pathophysiology of major psychosis has been well established by the work of multiple groups across several decades, including the widely replicated downregulation of GAD1. Prior gene expression and network analyses within the human hippocampus implicate a broader network of genes, termed the GAD1 regulatory network, in regulation of GAD1 expression. Several genes within this GAD1 regulatory network show diagnosis-and sector-specific expression changes within the circuitry of the hippocampus, influencing abnormal GAD1 expression in schizophrenia and bipolar disorder.OBJECTIVE To investigate the hypothesis that aberrant DNA methylation contributes to circuit-and diagnosis-specific abnormal expression of GAD1 regulatory network genes in psychotic illness. DESIGN, SETTING, AND PARTICIPANTSThis epigenetic association study targeting GAD1 regulatory network genes was conducted between July 1, 2012, and June 30, 2014. Postmortem human hippocampus tissue samples were obtained from 8 patients with schizophrenia, 8 patients with bipolar disorder, and 8 healthy control participants matched for age, sex, postmortem interval, and other potential confounds from the Harvard Brain Tissue Resource Center, McLean Hospital, Belmont, Massachusetts. We extracted DNA from laser-microdissected stratum oriens tissue of cornu ammonis 2/3 (CA2/3) and CA1 postmortem human hippocampus, bisulfite modified it, and assessed it with the Infinium HumanMethylation450 BeadChip (Illumina, Inc). The subset of CpG loci associated with GAD1 regulatory network genes was analyzed in R version 3.1.0 software (R Foundation) using the minfi package. Findings were validated using bisulfite pyrosequencing. MAIN OUTCOMES AND MEASURESMethylation levels at 1308 GAD1 regulatory network-associated CpG loci were assessed both as individual sites to identify differentially methylated positions and by sharing information among colocalized probes to identify differentially methylated regions.RESULTS A total of 146 differentially methylated positions with a false detection rate lower than 0.05 were identified across all 6 groups (2 circuit locations in each of 3 diagnostic categories), and 54 differentially methylated regions with P < .01 were identified in single-group comparisons. Methylation changes were enriched in MSX1, CCND2, and DAXX at specific loci within the hippocampus of patients with schizophrenia and bipolar disorder.CONCLUSIONS AND RELEVANCE This work demonstrates diagnosis-and circuit-specific DNA methylation changes at a subset of GAD1 regulatory network genes in the human hippocampus in schizophrenia and bipolar disorder. These genes participate in chromatin regulation and cell cycle control, supporting the concept that the established GABAergic dysfunction in these disorders is related to disruption of GABAergic interneuron physiology at specific circuit locations within the human hippocampus.

Schizophrenia is a devastating mental disorder with a high societal burden, complex pathophysiology, and diverse genetic and environmental risk factors. Its complexity, polygenicity, and small-effect-size and cell-type-specific contributors have hindered mechanistic elucidation and the search for new therapeutics. Here, we present the first single-cell dissection of schizophrenia, across 500,000+ cells from 48 postmortem human prefrontal cortex samples, including 24 schizophrenia cases and 24 controls. We annotate 20 cell types/states, providing a high-resolution atlas of schizophrenia-altered genes and pathways in each. We find neurons are the most affected cell type, with deep-layer cortico-cortical projection neurons and parvalbumin-expressing inhibitory neurons showing significant transcriptional changes converging on genetically-implicated regions. We discover a novel excitatory-neuron cell-state indicative of transcriptional resilience and enriched in schizophrenia subjects with less-perturbed transcriptional signatures. We identify key trans-acting factors as candidate drivers of observed transcriptional perturbations, including MEF2C, TCF4, SOX5, and SATB2, and map their binding patterns in postmortem human neurons. These factors regulate distinct gene sets underlying fetal neurodevelopment and adult synaptic function, bridging two leading models of schizophrenia pathogenesis. Our results provide the most detailed map to date for mechanistic understanding and therapeutic development in neuropsychiatric disorders.