Characterization of functional polymorphisms and glucocorticoid-responsive elements in the promoter of TDO2, a candidate gene for ethanol-induced behavioural disorders.

In response to acute ethanol consumption, tryptophan 2,3-dioxygenase (TDO) induces the kynurenine pathway (KP) through a glucocorticoid-mediated mechanism, which could lead to a dramatic accumulation of neurotoxic metabolites in association with serotonin depletion. As a result, interindividual variability in ethanol-induced behavioural disorders, such as black-outs and violent impulsive behaviours (BOVIBs) following binge drinking, could be partly explained by genetic polymorphisms affecting the KP. The aim of this study was to identify polymorphisms on the promoter of the TDO2 gene that could affect expression and/or activity of TDO through glucocorticoid induction. Polymorphisms were screened using a PCR-sequencing strategy applied to 31 alcohol-dependent patients and 49 unrelated healthy volunteers, and functionally analysed with bioinformatic prediction tools and gene reporter assays in HepG2 and A549 cell lines. We identified 12 polymorphisms in the human TDO2 promoter region, 2 of them corresponding to previously unknown single-nucleotide polymorphisms (SNPs) and 3 of them located in putative glucocorticoid-responsive elements (GREs). Gene reporter assays using HepG2 and A549 cell lines confirmed the presence of several functional GREs in the promoter region of TDO2 and revealed that some of the identified polymorphisms affect the promoter activity under glucocorticoid receptor over-expression and dexamethasone exposure conditions. Correlational studies in larger samples could help to determine whether these polymorphisms are responsible for variations of expression and/or activity of TDO, in particular under conditions where release of glucocorticoids is increased, such as acute ethanol intake. If confirmed, such results would be of major interest in explaining part of the interindividual variability observed in behavioural responses to acute ethanol consumption.