Abstract LB-249: Reduced expression of the long non-coding RNA MEG3 in sporadic and MEN1-associated tumors.
Authors: Modali Sita D. , Desai Shruti S. , Parekh Vaishali I. , Lonser Russel R. , Kebebew Electron , Emmert-Buck Michael , Agarwal Sunita K. 
Published: August 2014

Rapidly growing evidence shows the emerging tumor suppressor role of large non-coding RNAs (lncRNAs) in cancer pathogenesis. Maternally expressed gene 3 (MEG3) is a lncRNA that belongs to the imprinted DLK1-MEG3 locus located on human chromosome 14q32.3. Recent studies have shown reduced MEG3 expression and promoter hyper-methylation in common pituitary adenomas, classifying MEG3 as an epigenetically-regulated tumor suppressor gene. Pituitary tumor is one of the tumor types observed in the multiple endocrine neoplasia type 1 (MEN1) syndrome. MEN1 is caused by heterozygous germline mutations (1st hit) in the MEN1 tumor suppressor gene encoding menin. Subsequent tissue-specific inactivation of the normal copy of the MEN1 gene (2nd hit) leads to tumors in the parathyroids, anterior pituitary, and endocrine pancreas. Somatic MEN1 mutations are also observed in sporadic parathyroid and pancreatic endocrine tumors. We have recently demonstrated by ChIP-seq and gene expression analysis of mouse embryonic stem cells that, loss of menin results in significant loss of H3K4me3 at the Meg3 locus, and reduced expression of Meg3. These observations suggest menin-regulated epigenetic modification and expression of MEG3 as a probable mechanism for tumorigenesis. Characterization of the expression levels of MEG3 in the different tumor types associated with the MEN1 syndrome can provide valuable insights into the role of MEG3 in the pathogenesis of these tumors as well as sporadic tumors. In the current study, we used quantitative real-time PCR to evaluate the expression of MEG3 in sporadic parathyroid tumors (4 carcinoma, 8 hyperplasia, and 8 adenoma), pituitary tumors (3 sporadic ACTH-secreting, and 4 MEN1-associated: 1 ACTH secreting, 2 PRL-secreting, 1 non-functional), and sporadic pancreatic islet tumors (30 insulinomas). For pituitary tumor samples, the total RNAs were reverse transcribed to cDNA and used as templates for the RT-PCR. However, for parathyroid tumor RT-PCR templates, the total RNAs were subjected to one round of amplification prior to the cDNA synthesis. For insulinoma RT-PCR templates, we performed laser capture microdissection using PixCell IIe system on insulinoma tissue sections to obtain tumor and normal islets, isolated total RNA and performed cDNA synthesis. Our RT-PCR results showed decreased MEG3 expression in 13 out of 20 parathyroid samples (3 cancer, 6 hyperplasia, 4 adenoma) (p = 0.001-0.05). In pituitary tumors, PRL-secreting tumors from MEN1 patients showed less MEG3 compared to all 4 ACTH-secreting tumors. Analysis of sporadic insulinomas is in progress. Our results strongly indicate that MEG3 is silenced in sporadic or MEN1-associated endocrine tumor types supporting the tumor suppressor role of MEG3 in endocrine cells, and warrant further study to explore the possibility of restoring MEG3 expression as a potential therapeutic option or marker for the management of endocrine tumors.

Citation Format: Sita D. Modali, Shruti S. Desai, Vaishali I. Parekh, Russel R. Lonser, Electron Kebebew, Michael Emmert-Buck, Sunita K. Agarwal. Reduced expression of the long non-coding RNA MEG3 in sporadic and MEN1-associated tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr LB-249. doi:10.1158/1538-7445.AM2013-LB-249