Current studies have consistently shown that the prefrontal cortex (PFC) GABAergic interneurons of psychotic patients [schizophrenia (SZ), and bipolar disorder with psychosis (BDP)] express a downregulation of glutamic acid decarboxylase (GAD)67 (one of the two decarboxylases that synthesize GABA) and reelin (an extracellular matrix protein that is preferentially synthesized and secreted by GABAergic interneurons) (Benes et al., 1992, Akbarian et al., 1995, Impagnatiello et al., 1998, Guidotti et al., 2000, Fatemi et al., 2000, Eastwood et al., 2003, Woo et al., 2004, Lewis et al., 2005). Upon secretion into the extracellular matrix, reelin adheres to the dendritic shafts and surrounds dendritic spines of cortical pyramidal neurons. This protein perhaps by impinging on synaptically located integrin receptors, modulates event-related protein synthesis and may influence the dendritic spine expression density (Costa et al., 2001, Liu et al., 2001, Dong et al., 2003), markedly changing LTP and cognitive function expression (Larson et al., 2003, Carboni et al., 2004, Bufferet et al., 2005, Qiu et al., 2006).
Reelin and GAD67 promoters are embedded in large CpG islands and express methylation consensuses (Grayson et al., 2005). In the PFC of SZ patients, a decrease of reelin expression was found to be associated with cytosine hypermethylation in the promoter region of the gene encoding for this protein (Grayson et al., 2005, Abdolmaleky et al., 2005). The regulatory role played by promoter CpG island methylation in the expression level of reelin can be inferred by the increase by up to 80 fold occuring the human reelin promoter following hypomethylation (Chen et al., 2002).
In the PFC of SZ and BDP patients, we have also quantified the expression of DNA methyltrasferase 1 (DNMT1), which catalyzes the methylation of the carbon atom in position 5 of cytosines in CpG dinucleotides of various gene promoter regions of GABAergic neurons. We found that in these PFC neurons DNMT1 is highly expressed whereas this enzyme cannot be detected in pyramidal neurons (Veldic et al., 2004, Ruzicka et al., 2006). Moreover, DNMT1 expression is increased in a subset of cortical GABAergic interneurons in SZ and BDP patients. For example, it is increased in cortical layers I, II, and IV GABAergic neurons but not in GABAergic neurons of layers III, V and VI (Veldic et al., 2005, Ruzicka et al., 2006). In PFC GABAergic neurons of SZ and BDP, the extent of the DNMT1 increase was accompanied by related decrease of reelin and GAD67 expression (Veldic et al., 2004, 2005). Hence, these studies suggest that the downregulation of GAD67 and reelin, or that of other genes expressed in cortical GABAergic neurons of psychotic patients, may be mediated by a 5-cytosine hypermethylation of the promoter CpG dinucleotides elicited by the increased expression of DNMT1 (Grayson et al., 2006).
Postmortem studies of human brain suggest that SZ may be associated with a GABAergic neuron downregulation detected not only in the cortex but also in the striatum (Impagnatiello et al., 1998). High affinity binding studies with [3H]muscimol show an increase in the number of GABAA recognition sites in the caudate nucleus (CN) of SZ patients (Hanada et al., 1987). In addition the expression levels of reelin mRNA are decreased by more than 70% in the CN of SZ patients compared to matched nonpsychiatric subjects (NPS) (Impagnatiello et al., 1998). In previous studies, we have shown that CN GABAergic neurons of SZ patients overexpress DNMT1 whereas reelin expression is downregulated (Veldic et al., 2004).
The goal of the present study is to replicate in the McLean 66 Cohort (Harvard Brain Tissue Resource Center, McLean Hospital, Belmont, MA) earlier findings of a DNMT1 mRNA increase in GABAergic neurons in Brodmann’s area (BA) 9 and BA10 (Veldic et al. 2004, 2005, Ruzicka et al., 2006) and verify whether there is an overexpression of DNMT1 and a parallel downregulation of reelin and GAD67 in CN and putamen (PT) medium spiny GABAergic neurons, in SZ and BDP patients.