Endocrine Abstracts

The tumour suppressor menin, encoded by the multiple endocrine neoplasia type 1 (MEN1) gene, has been reported to be a component of the Wnt/β-catenin signalling pathway. To investigate the effects of menin loss on this pathway, we have determined the cDNA expression profiles of pituitary tumours from 5 Men1^+/− mice and in normal pituitaries from 5 Men1^+/+ littermates by extracting total RNA and by hybridizing it to Affymetrix Mouse Genome arrays. The pituitary tumours, which were mostly somatolactotrophinomas, when compared to normal pituitaries were found to have significant alterations of several Wnt/β-catenin pathway components including up-regulation of the inhibitors carboxypeptidase Z (Cpz) and dickkopf 3 (Dkk3) (+15.5- and +3.8-fold respectively, FDR<0.01), and down-regulation of the target genes axin2 and calcium/calmodulin-dependent protein kinase II γ (Camk2g) (−1.9- and −3.0-fold, respectively, FDR<0.05). These alterations in gene expression were confirmed by quantitative RT-PCR which showed the pituitary tumours to have the following fold differences: Cpz +22.2; Dkk3 +4.5; Axin2 −1.7; and Camk2g −2.4, P<0.01. To further characterize the functional effects, expression of total and active β-catenin protein was assessed by western blot and densitometry analysis, using extracts from pituitary tumours and normal pituitaries. Total β-catenin expression was similar in normal pituitaries and tumours but active β-catenin was reduced by >50% in the tumours, although immunohistochemistry revealed the tumours to have increased nuclear localisation of the active β-catenin, which was predominantly cytoplasmic in normal pituitaries. Thus, our results show that the absence of menin in pituitary tumours inhibits Wnt target gene expression, and thus supports a role for menin in increasing histone H3K4 trimethylation of the Axin2 promoter and thereby a transcriptional activation of Wnt/β-catenin target genes. These studies provide further insights into the role of menin in the Wnt/β-catenin signalling pathway and may allow the development of novel therapeutic targets for endocrine tumours.