**4. Interplay between DNA sequence variation and DNA methylation**

The role of DNA methylation in gene regulation and the interplay between genomic sequence variations and various environmental features as well as their implications on disease pheno‐ types remains to be investigated. However, studies support the role of DNA methylation in regulatory interactions influencing gene expression [51]. It is also established that there is an interplay between sequence variation, DNA methylation, and gene expression [52]. A recent study has highlighted the fact that mutated CpG sites (CpG‐SNPs) could play a critical role in the cause and treatment of the disease [34].

Our lab recently identified several genes that differ between the affected and unaffected members of two monozygotic twin pairs discordant for schizophrenia. The differences were assessed using a genome‐wide methylation promoter array and complete genome sequen‐ ces. The results shed light on a number of facts. First, monozygotic twins differ in both DNA methylation as well as *de novo* sequence variations. Antipsychotic drugs could have caused the observed DNA methylation changes as was evidenced in our rat model study discussed above. Also, there is a possibility that DNA methylation changes could be caused by *de novo* events. It has been reported that differences in genetic components underlie the differences in DNA methylation profiles observed between individuals [53]. Second, some genes that were affected by differential promoter methylation between discordant monozygotic twins also harbor a number of different types of sequence variations [2]. Some of the variations may represent *de novo* sequence variations. Third, a number of the observed changes are located in known candidate genes for schizophrenia. Thus, the genes harboring promoter DNA methylation changes could contribute to neuropsychiatric disorders, including schizo‐ phrenia. Moreover, when the DNA sequence differences were analyzed independently, ad‐ ditional previously reported candidate genes of schizophrenia were identified (unpublished data) suggesting that these findings may reflect that many of the previously identified can‐ didate genes of schizophrenia can be revealed by differences in DNA sequence. These find‐ ings also highlight the patient‐specific nature of these differences. Any sequence variation or promoter DNA methylation between a patient and their healthy cotwin was considered as a potential predisposing factor for the disease. Overall, the findings to date argue that it is not only sequence variation but also their interactions with chromatin structure and other epige‐ netic signatures which regulate disease outcomes. Therefore, future studies need to focus on investigating the interactions of sequence variation, including nuclear and mitochondrial DNA sequence variations, with epigenetic signatures, in subjects with psychosis and healthy controls.

Studies in the past support the notion that DNA methylation may play a critical role in the therapeutic efficacy of olanzapine. For example, findings suggest that DNA methylation changes in the promoter regions of several genes including genes located in the 22q11.2 microdeletion region and the cadherin/protocadherin genes impact the response of olanzapine treatment [47]. These impacts have been revealed through the identified pathways that have been previously implicated in psychosis.

In conclusion, the results from our lab and elsewhere corroborated the fact that various types of *de novo* sequence, including copy number variants and their interactions with epigenetic signatures, may underlie the etiology of schizophrenia and also may hold the key to discovery of drug targets in developing personalized medicine for psychosis. Epigenetic changes, DNA methylation in particular, may play a critical role in the therapeutic efficacy of antipsychotic drugs. Overall, the known functions of genes affected by olanzapine‐induced DNA methyla‐ tion changes suggest that DNA methylation differences may underlie the amelioration of psychosis symptoms as well as account for certain adverse effects of drugs used to treat the disorder.
