Elucidation of AIP related pituitary tumorigenesis

Germline mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene were recently shown to predispose to pituitary adenomas, which are neoplasms of the anterior pituitary gland. AIP mutations were first discovered in a cluster of familial pituitary adenomas in Northern Finland and afterwards in several populations worldwide. AIP associated pituitary adenomas are predominantly growth hormone secreting, thus several patients present with acromegaly or gigantism. The aim of this study was to elucidate AIP related pituitary tumorigenesis. AIP is suggested to be involved in several cellular pathways such as the xenobiotic response. In the cytosol, AIP interacts directly with aryl hydrocarbon receptor (AHR). After interaction with xenobiotic compounds AHR translocates to the nucleus and binds aryl hydrocarbon receptor nuclear translocator (ARNT). The AHR/ARNT heterodimer regulates the expression of several xenobiotic metabolizing enzymes. The AHR pathway is linked to e.g. the hypoxia response and estrogen signaling through ARNT and possibly through ARNT2, an ARNT homolog. A previously generated heterozygous Aip (Aip+/-) mouse model was used to study the role of these pathways in AIP-mediated tumorigenesis. Immunohistochemical studies revealed that expression of either ARNT or ARNT2 protein was lost in the mouse tumors. Aip+/- mice were also crossed and pregnant mice dissected to acquire Aip knockout (Aip-/-) and Aip wildtype (Aip+/+) embryos. Mouse embryonic fibroblast cell lines were prepared from these embryos and protein extracts were used for western blot analysis. This analysis revealed that expression of ARNT and ARNT2 was uniform between Aip-/- and Aip+/+ cell lines, which suggests that aberrant ARNT/ARNT2 expression is pituitary specific. In addition, Ki-67 analysis indicated that Aip deficient tumors have higher proliferation rates as compared with Aip proficient tumors. These results suggest that mechanisms of AIP related pituitary tumorigenesis involve aberrant ARNT/ARNT2 function, possibly via the AHR, hypoxia or estrogen pathways. Furthermore, the comparatively high proliferation rates of Aip deficient tumors may correlate with a more aggressive disease. Concentration is next focused on acquiring gene expression data of early Aip-/- embryos, which will be compared with expression data of Aip+/+ embryos. Differential expression patterns in these embryos could give new insights on different pathways involving AIP. Revealing the molecular basis of AIP related pituitary tumorigenesis is important in order to develop new diagnostic and therapeutic strategies for several patients suffering with pituitary adenomas

No evidence of RET germline mutations in familial pituitary adenoma

Pituitary adenomas are common in the general population. Although most of them are sporadic, some occur in a familial setting. In familial pituitary adenoma patients it is common that no germline defects are found after screening of aryl hydrocarbon receptor interacting protein (AIP) and other genes known to underlie the condition, suggesting the existence of yet unknown predisposition genes. Recently, the RET proto-oncogene was found to be a novel in vivo interaction partner of AIP in the pituitary gland. Here, we have screened patients from 16 AIP mutation negative (AIPmut-) pituitary adenoma families for RET germline mutations to assess whether RET could play a role in pituitary adenoma predisposition, similar to AIP. We found five novel germline RET changes: one in RET Exon 4 and the rest in noncoding regions of RET. Two changes, c.1560*G&gt;A and -1285G&gt;A, were segregated in affected family members. We also analyzed the RET region with enhancer element locator (EEL) to identify RET regulatory elements, and to see whether the changes resided in these. None of the variants mapped to the regions predicted by EEL. Expression of RET was examined in ten AIPmut- and seven AIP mutation positive (AIPmut+) somatotropinomas by immunohistochemistry, with a trend showing reduced expression in the latter (P=0.05). We conclude that the RET variants are presumably not related to pituitary adenoma predisposition, although reduced RET expression may play a role in AIP-related genesis of somatotropinomas.</p

Mice with Inactivation of Aryl Hydrocarbon Receptor-Interacting Protein (Aip) Display Complete Penetrance of Pituitary Adenomas with Aberrant ARNT Expression

Mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene have been shown to predispose to pituitary adenoma predisposition, a condition characterized by growth hormone (GH)-secreting pituitary tumors. To study AIP-mediated tumorigenesis, we generated an Aip mouse model. Heterozygous mice developed normally but were prone to pituitary adenomas, in particular to those secreting GH. A complete loss of AIP was detected in these lesions, and full penetrance was reached at the age of 15 months. No excess of any other tumor type was found. Ki-67 analysis indicated that Aip-deficient tumors have higher proliferation rates compared with Aip-proficient tumors, suggesting a more aggressive disease. Similar to human AIP-deficient pituitary adenomas, immunohistochemical studies showed that expression of aryl hydrocarbon receptor nuclear translocator 1 or 2 (ARNT or ARNT2) protein was lost in the mouse tumors, suggesting that mechanisms of AIP-related tumorigenesis involve aberrant ARNT function. The Aip+/− mouse appears to be an excellent model for the respective human disease phenotype. This model constitutes a tool to further study AIP-associated pituitary tumorigenesis and may be potentially valuable in efforts to develop therapeutic strategies to treat pituitary adenomas