Familial isolated pituitary adenoma (FIPA) is an autosomal dominant condition 
with variable genetic background and incomplete penetrance. Germline mutations 
of the aryl hydrocarbon receptor interacting protein (AIP) gene have been 
reported in 15-40% of FIPA patients. Limited data are available on the 
functional consequences of the mutations or regarding the regulation of the AIP 
gene. We describe a large cohort of FIPA families and characterize missense and 
silent mutations using minigene constructs, luciferase and beta-galactosidase 
assays, as well as in silico predictions. Patients with AIP mutations had a 
lower mean age at diagnosis (23.6+/-11.2 years) than AIP mutation-negative 
patients (40.4+/-14.5 years). A promoter mutation showed reduced in vitro 
activity corresponding to lower mRNA expression in patient samples. Stimulation 
of the protein kinase A-pathway positively regulates the AIP promoter. Silent 
mutations led to abnormal splicing resulting in truncated protein or reduced AIP 
expression. A two-hybrid assay of protein-protein interaction of all missense 
variants showed variable disruption of AIP-phosphodiesterase-4A5 binding. In 
summary, exonic, promoter, splice-site, and large deletion mutations in AIP are 
implicated in 31% of families in our FIPA cohort. Functional characterization of 
AIP changes is important to identify the functional impact of gene sequence 
variants.