Selective DNA methylation of BDNF promoter in bipolar disorder: differences among patients with BDI and BDII

Abstract

The etiology of bipolar disorder (BD) is still poorly understood, involving genetic and epigenetic mechanisms as well as environmental contributions. This study aimed to investigate the degree of DNA methylation at the promoter region of the brain-derived neurotrophic factor (BDNF) gene, as one of the candidate genes associated with major psychoses, in peripheral blood mononuclear cells isolated from 94 patients with BD (BD I=49, BD II=45) and 52 healthy controls. A significant BDNF gene expression downregulation was observed in BD II 0.53±0.11%; P<0.05), but not in BD I (1.13±0.19%) patients compared with controls (CONT: 1±0.2%). Consistently, an hypermethylation of the BDNF promoter region was specifically found in BD II patients (CONT: 24.0±2.1%; BDI: 20.4±1.7%; BDII: 33.3±3.5%, P<0.05). Of note, higher levels of DNA methylation were observed in BD subjects on pharmacological treatment with mood stabilizers plus antidepressants (34.6±4.2%, predominantly BD II) compared with those exclusively on mood-stabilizing agents (21.7±1.8%; P<0.01, predominantly BD I). Moreover, among the different pharmacological therapies, lithium (20.1±3.8%, P<0.05) and valproate (23.6±2.9%, P<0.05) were associated with a significant reduction of DNA methylation compared with other drugs (35.6±4.6%). Present findings suggest selective changes in DNA methylation of BDNF promoter in subjects with BD type II and highlight the importance of epigenetic factors in mediating the onset and/or susceptibility to BD, providing new insight into the mechanisms of gene expression. Moreover, they shed light on possible mechanisms of action of mood-stabilizing compounds vs antidepressants in the treatment of BD, pointing out that BDNF regulation might be a key target for their effects.

Figure 1

Sequence of the human brain-derived neurotrophic factor (BDNF) exon I promoter (chr11: 27 743 605–27 744 379). Amplicon is highlighted with the 17 cytosine guanine dinucleotide (CpG) sites present (also bolded) and primer sequences (methylated) underlined. The transcriptional start site (+1) is indicated and the predicted CpG island is shown (criteria used: island size >100, % GC >50.0, obs/exp >0.60).

Figure 2

Levels of brain-derived neurotrophic factor (BDNF) mRNA in peripheral blood mononuclear cells from patients diagnosed with bipolar disorders type 1 (BD I; n=16) and BD type 2 (BD II; n=16). Box plots with whiskers from minimum to maximum represent 2^−DDCt values calculated by the Delta-Delta Ct (DDCt) method. Means of mRNA levels are expressed relative to control subjects (n=14).

Figure 3

Amount of methylated DNA in the promoter region of brain-derived neurotrophic factor (BDNF) in controls, patients diagnosed with bipolar disorders type 1 (BD I; n=16) and BD type 2 (BD II; n=16). Scatter dot plots with mean values are shown.

Figure 4

Correlation between brain-derived neurotrophic factor (BDNF) gene expression and percentage change in DNA methylation at BDNF promoter in bipolar disorders type 2 (BD II) subjects. Data are compared by Spearman's rank correlation coefficient (P<0.01, r=−0.7343).

Figure 5

Amount of methylated DNA in the promoter region of brain-derived neurotrophic factor (BDNF) in peripheral blood mononuclear cells from patients diagnosed with bipolar disorders type 1 (BD I) + BDs type 2 (BD II) in therapy with (+) or without antidepressant drug (−). Scatter dot plots with mean values are shown.

Figure 6

Amount of methylated DNA in the promoter region of brain-derived neurotrophic factor (BDNF) in peripheral blood mononuclear cells from patients diagnosed with bipolar disorders type 1 (BD I) or BD type 2 (BD II) in therapy with (+) or without antidepressant drug (−). Scatter dot plots with mean values are shown.

Figure 7

Amount of methylated DNA in the promoter region of brain-derived neurotrophic factor (BDNF) in patients diagnosed with BD (bipolar disorders type 1 (BD I; n=49) + BD type 2 (BD II; n=45)) treated with lithium, valproate or other drugs. Scatter dot plots with mean values are shown.

Figure 8

Amount of methylated DNA in the promoter region of brain-derived neurotrophic factor (BDNF) in patients diagnosed with (a) bipolar disorders type 1 (BD I; n=49); (b) BD type 2 (BD II; n=45) or (c) BD I and BD II according to their mood state.