Aryl hydrocarbon receptor-interacting protein (AIP) n-terminus gene mutations identified in pituitary adenoma patients alter protein stability and function

Abstract

Mutations spanning the entire aryl hydrocarbon receptor–interacting protein (AIP) gene have been found in isolated familial cases of pituitary adenomas (PA). Missense mutations located in the N-terminus of the gene have been identified in several patients. However, the functional significance of these mutations remains a matter of controversy. In most studies, the N-terminus of AIP has been shown to regulate protein stability and subcellular localization of the AIP-AHR-HSP90 complex but not to be involved in protein–protein interactions. Other studies found that the N-terminal domain interacts directly with other proteins. The aim of this study was to analyze whether specific N-terminus AIP mutations identified in PA patients would be functionally different from wild-type (WT) AIP. In vitro analyses were used to assess the role of known N-terminus variants, a locally identified mutant, R9Q, and three other commonly genotyped N-terminus mutations R16H, V49M and K103R are found in PA patients. Given the functional effect of WT AIP on cAMP signalling alterations caused by N-terminus mutants on this pathway were also analyzed in GH3 cells. Results indicate that N-terminus mutations lead to de-regulation of the effect of WT AIP on cAMP signalling and increased cAMP thresholds in GH3 cells resulting in increased growth hormone (GH) secretion. Cycloheximide chase analysis identified a variation in protein degradation patterns between WT and N-terminus variants. Therefore, both functional and structural studies reveal that N-terminus mutations in the AIP gene alter protein behaviour significantly and hence can truly be pathogenic in nature.