In psychosis, cortical interneurons overexpress DNA-methyltransferase 1

Abstract

Cortical DNA-methyltransferase 1 (DNMT1) is preferentially expressed in interneurons secreting GABA where it very likely contributes to promoter CpG island hypermethylation, thus causing a down-regulation of promoter functions. To consolidate and expand on previous findings that, in the cortex of schizophrenia (SZ) brains, glutamic acid decarboxylase 67 (GAD67) expression is down-regulated whereas that of DNMT1 is up-regulated, we studied both parameters in Brodmann's area (BA) 9 from the McLean 66 Cohort Collection (Harvard Brain Tissue Resource Center, Belmont, MA). In BA9 of SZ and bipolar disorder patients with psychosis, DNMT1 mRNA and protein expression preferentially increases in layer I, II, and IV interneurons, and this increase is paralleled by a decreased number of GAD67 mRNA-positive neurons. The increase in DNMT1 and the decrease in GAD67-expressing neurons were unrelated to postmortem interval, pH, RNA quality, or to the presence, dose, or duration of antipsychotic (APS) medication, with the exception of a subgroup of SZ patients treated with a combination of valproate and APS in which the expression of DNMT1 failed to change. The DNMT1 increase and the GAD67 decrease in BA9 interneurons are significant features of SZ and bipolar disorder with psychosis. Interestingly, the DNMT1 increase failed to occur when patients with psychosis received a combination of valproate and APS treatment but not APS monotherapy.

Fig. 1.

DNMT1 mRNA-positive neuronal counts in various layers of BA9 from nonpsychiatric subjects and psychiatric patients. For NPS, n = 26; for SZP, n = 19; for BP+ patients, n = 14; for BP- patients, n = 5. The difference between SZP and BP patients with or without psychosis versus NPS was calculated by ANOVA, and P values were evaluated by Dunnett's two-sided test comparison. ***, P < 0.001; **, P < 0.01; *, P < 0.05. Error bars represent SEM.

Fig. 2.

Photomicrographs showing examples of DNMT1 mRNA and DNMT1 protein neuronal expression in layers I and II of BA9 from one NPS and one SZP. (A) DNMT1 mRNA in situ hybridization signal. (B) DNMT1 protein immunolabeling. Note an increase of neuronal DNMT1 mRNA and protein labeling in SZP. (Scale bars, 20 μm.)

Fig. 3.

Negative correlation between GAD₆₇ and DNMT1 mRNAs-positive neurons in layer I of BA9 from NPS (n = 26) and psychotic (SZ plus BP+) patients (PP) (n = 33). (A) The difference between NPS and PP for GAD₆₇ and DNMT1 mRNAs-positive neurons was calculated by ANOVA followed by Bonferroni comparison. *, P < 0.001. Error bars represent SEM. (B) Pearson correlation between number of DNMT1 mRNA-positive neurons and number of GAD₆₇ mRNA-positive neurons.

Fig. 4.

DNMT1 mRNA-positive interneurons in layer II of BA9 from NPS (n = 26), SZP (n = 15), and BP patients (n = 12). APS, patients treated with antipsychotics; VPA, patients treated with valproate. *, P < 0.001 measured by ANOVA followed by Bonferroni multiple comparison.