The ANKK1 Protein Associated with Addictions has Nuclear and Cytoplasmic Localization and Shows a Differential Response of Ala239Thr to Apomorphine

The TaqIA single-nucleotide polymorphism (SNP), which is the most widely studied genetic polymorphism in addictions, is located at the gene that encodes the RIP kinase ANKK1 near the gene for dopamine receptor D2. The TaqIA SNP is in strong linkage disequilibrium with the SNP rs7118900, which changes the alanine at position 239 to threonine in the ANKK1 protein (Ala239/A2; Thr239/A1). In silico analysis has predicted that this polymorphic substitution creates an additional phosphorylation site in the kinase domain of ANKK1. To investigate the contribution of ANKK1 to the pathophysiology of TaqIA-associated phenotypes, we analyzed transfected HEK293T cells with the human ANKK1-kinase^Ala239 and ANKK1-kinase^Thr239 variants tagged with GFP. We observed that the ANKK1-kinase is located in both the nucleus and the cytoplasm, suggesting that there is nucleocytoplasmic shuttling of this putative signal transducer. In addition, we found that the Ala239Thr ANKK1-kinase polymorphism exhibited strong expression differences in both the nucleus and the cytoplasm at basal level and when stimulated with the dopamine agonist apomorphine. Specifically, the ANKK1-kinase^Thr239 variant showed the highest level of basal protein expression, while ANKK1-kinase^Ala239 was 0.64-fold lower. After treatment with apomorphine, ANKK1-kinase^Ala239 showed a 2.4-fold increment in protein levels, whereas a 0.67-fold reduction was observed in ANKK1-kinase^Thr239. Thus, here we provide the first evidence of functional ANKK1 differences that are marked by TaqIA and could be associated with vulnerability to addiction.