Serotonin and serotonin metabolism has for decades been understood as playing a critical role in mood disorders and has more recently also been implicated in brain tumour biology. However, in part due to the lack of direct investigation of genetic and epigenetic variation affecting serotonin pathways within human brain tissue this understanding has recently been challenged. We analysed genetic and epigenetic variation in the Monoamine oxidase A ( MAOA) and serotonin transporter ( 5HTT) genes using 232 biobanked glioma tissue samples from 216 adult patients. We further examined the association between use of antidepressants (targeting serotonergic pathways), serotonin levels and methylation. In male patients, genetic variation in the MAOA gene was significantly associated with tissue serotonin levels. Further analysis identified five single nucleotide variants (SNVs) that may contribute to this association. In contrast, 5HTT variants were not statistically associated with serotonin pathway metabolites, nor were MAOA variants in females. Increased methylation at several 5HTT CpG sites was positively correlated with serotonin levels and negatively correlated with 5-HIAA levels. In males, one CpG site in the MAOA gene was negatively associated with the 5-HIAA/serotonin ratio, suggesting reduced enzymatic degradation of serotonin due to lower MAOA activity. Patients using antidepressants had lower tissue serotonin levels. In males, genetic variation in the MAOA gene was significantly associated with tissue serotonin levels, although this association was not mediated by methylation. Our result supports the notion that the MAOA and 5HTT genes are related to serotonin metabolism and that such metabolism is related to antidepressant use.

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