The aim of the current study was to determine whether global hypomethylation of peripheral leukocyte DNA in male patients with schizophrenia was a secondary effect of their medication. As a model system for studying this question, we chose haloperidol-injected rats, since haloperidol, until recently, was a commonly used drug for the treatment of schizophrenia. In leukocyte DNA from control rats that were injected with buffer alone, there was a sex-dependent difference in mC content (females lower than males), similar to what was previously observed in humans [16, 19] The apparent paradox of a lower methylation state in females may be partly attributed to global hypomethylation of the inactive X chromosome . The global methylation level in rat liver and brain was also consistent with what has been observed in humans: liver < brain . Thus, the data obtained from rats and humans are comparable. In male rats injected with haloperidol, the amount of mC in leukocyte DNA was less than that seen in control male rats, but the difference was not statistically significant. Although further study is needed in order to fully understand the difference in mC content of peripheral leukocyte DNA between male patients with schizophrenia and healthy male subjects, the results of the current study present several interesting and novel findings, and implicate a role for haloperidol, and perhaps other antipsychotic medications, in the alteration of DNA methylation in schizophrenic male patients.
Haloperidol-treatment resulted in a decrease in the amount of mC in leukocyte DNA in male rats, although the difference did not reach a statistical significance. In contrast, we found a trend toward higher levels of mC in leukocyte DNA in female rats treated with haloperidol (P = 0.064). Although the reasons why haloperidol treatment would have opposite effects on global methylation in male and female rats are not clear from this study, it is possible that these differences are the result of disruptions in the balance of hormones in these animals. This is supported by observations that there are sex-dependent differences in the immune response to certain hormones, and in the frequency of occurrence of some autoimmune diseases [21–23]. It is also possible that the observed effect might be mediated by the alteration in the subset profile of white blood cells. The partial discrepancy between the results obtained from patients with schizophrenia and from rats injected with haloperidol will require additional study, in particular into the differences between humans and rodents in the regulation of epigenetic factors, or other systems that may effect the course of disease [24, 25].
In the liver, haloperidol increased global DNA methylation in both males and females, and the difference was statistically significant in males (P = 0.013). In the brain, haloperidol treatment resulted in a decrease in females, and the decrease was statistically significant (P = 0.026). The differential effect of haloperidol on DNA methylation in various tissues suggests that the regulation of global DNA methylation by this drug occurs through multiple, indirect pathways. In these pathways, sex-specific hormones may play a role in modifying the methylation state of DNA, since haloperidol also disturbs the normal regulation of sex-hormone secretion.
Several studies have established that there are gender differences among patients with psychiatric disorders, suggesting that sex-specific hormones play a role in the pathogenesis of these disorders, including schizophrenia [26–29]. It remains a topic of considerable interest whether and how these hormones are involved in disease progression. Clinical phenotypes are most frequently the manifestation of multiple alterations in genetic and environmental factors. In the current study, using a limited number of rats, we were unable to demonstrate a causative effect of haloperidol on the observed hypomethylation of leukocyte DNA in male patients with schizophrenia . However, we did uncover several sex- and tissue-specific effects of haloperidol on DNA methylation. Of note, valproate, a drug that is commonly used to treat bipolar disorder, has been shown to have a demethylating effect . Clarification of the cellular pathways that mediate haloperidol's effect on DNA methylation state will help elucidate the molecular etiologies of schizophrenia.