Hypomethylation of BDNF gene promoter in bipolar disorder patients

INTRODUCTION: Bipolar disorder-1 (BD-1) is a mood disorder characterized with recurrent manic or mixed episodes, and the lifetime (and 12-month) prevalence estimate is 1%. The biological basis of the disease is still not very clear. It is known that genetic and environmental factors are involved in the pathogenesis of the disease. Family, twin and adoption studies provide strong evidence for the importance of genetic etiology of bipolar disorder. There could not be determined a major disease-associated locus because of the occurrence of bipolar disorder in interaction with multiple genes and environmental factors. All these data suggest that the genetics of bipolar disorder is very complex. Epigenetics has an important role in gene and environment interactions. This means environmental factors such as nutrition, maternal care and behavior, hormones and drugs, the early life experiences, and environmental agents in early development stages influence the gene expressions through epigenetic mechanisms1 . Brain Derived Neurotrophic Factor (BDNF) is a neurotrophin that regulates synaptic transmission and plasticity, and it has a role in proliferation, differentiation, survival and death of neuronal and non-neuronal cells2 . BDNF may also play a role in the pathophysiology of bipolar disorder. We hypothesized that there is a defect in DNA methylation mechanisms, and the BDNF gene is affected by these processes in bipolar disorder patients. Therefore, in order to test our hypothesis we aimed to investigate the DNA methylation status of two regions in the BDNF gene in patients with bipolar disorder.

MATERIALS AND METHODS: The study included 100 BD patients (Bipolar depression: 18, Mania: 33, Euthymic: 49; aged 30.8±9.3 years, 56 male and 44 female) with bipolar disorder and 59 healthy controls (aged 30.0±6.3 years, 29 male and 30 female). DNA was extracted from blood samples by using the salt-chloroform method. Determination of the methylation pattern of CpG islands was based on the principle that bisulfite treatment of DNA would result in conversion of unmethylated cytocine residues into uracil, whereas methylated cytocine residues would remain unmodified. Methylation-specific PCR was performed with primers specific for either methylated or unmethylated DNA.

RESULTS: In this study we found that there was a hypomethylation in BDNF gene promoter 1 in bipolar disorder patients compared to healthy controls (p<0.001). The comparisons of the methylated or un-methylated status for each area according to the study groups are presented in Table 1. When patients were analyzed according to their attack type, there was a hypomethylation in BDNF gene promoter 1 in patients in depressive or manic episodes compared to healthy controls and euthymic episode (p=0.011). However, BDNF gene promoter 2 was hypermethylated in patients in euthymic episode compared to healthy controls (p=0.010).

CONCLUSION: DNA methylation plays a role in neuronal cell survival and maturation; balance of the methylation level is important for neuronal survival, and hypomethylation causes abnormalities in neuronal function. Although it is not clear, it has been mentioned that DNA hypomethylation may cause cell death by apoptosis. A normal level of DNA methylation is required for controlling genomic expressions. An animal study found that DNA hypomethylation is associated with genomic instability which may lead to cancer development. Cancer studies show that DNA hypomethylation caused chromosomal instability, abnormal gene expression, and loss of imprinting. Considering these data, it can be thought that DNA hypomethylation may cause diseases by affecting gene functions such as genomic instability and cell survival and maturation, and even through apoptosis. It is found that the methylation status is correlated with antidepressant treatment, and it is suggested that the methylation status is not associated with only the disease itself but can also be associated with pharmacological treatment3 . Another study showed that antidepressants are associated with increased methylation, and mood stabilizers are associated with decreased methylation4 . The methylation status was found lower in patients who received valproate or lithium. But the methylation status is higher in patients with antidepressant treatment compared to antidepressant and mood stabilizer combined (4). In addition to antidepressants and mood stabilizers, it is shown that antipsychotics have effects on DNA methylation. Except haloperidol and olanzapine, sulpirid ans clozapin have demethylation effects on rats5 . As seen from these data, antidepressants, mood stabilizers, and antipsychotics influence DNA methylation levels. However, it is not exactly known how these drugs affect the methylation status. The effects of gene methylation on treatment and clinical symptoms are unclear, too. It is thought that DNA methylation levels of related genes in bipolar disorder are involved in the etiopathogenesis of the disease. The level of methylation status of other candidate genes in relation with clinical disease should be investigated. Regulating DNA methylation may be a new therapeutic target for treatment of the disease.