Abstract 2544: The lncRNA Meg3 acts as an epigenetic determinant of oncogenic signaling in multiple endocrine neoplasia type 1

Abstract

Loss of tumor suppressors and gain of oncogenic properties is a hallmark of cancer. But the pathways leading to tumor formation are poorly understood. A good example is the multiple endocrine neoplasia type 1 (MEN1) syndrome, in which patients inherit germline mutations in the MEN1 gene, predisposing to tissue-specific loss of the encoded tumor suppressor protein menin. Consequently, tumors develop in multiple endocrine organs - the pituitary, the parathyroids, and the duodeno-pancreatic tissues, including pancreatic neuroendocrine tumors (PNETs). Insulinomas are the most common type of functioning PNETs that also occur in MEN1 patients. We investigated the molecular pathways associated with insulinomas. We previously showed that menin loss downregulated Maternally Expressed Gene 3 (MEG3), a long non-coding RNA (LncRNA), by eliciting promoter hypermethylation along with the loss of Histone H3 lysine 4 trimethylation (H3K4me3). Gene expression microarray analyses and RT-PCR, of mouse insulinoma MIN6 cells stably transfected with Meg3, showed a 5-fold decrease in the proto-oncogenic signaling receptor c-Met, suggesting a tumor suppressor function for Meg3. We also found MEG3 downregulation with concurrent upregulation of c-MET in PNETs. However, the molecular underpinnings of MEG3 governed c-MET repression remain elusive. To identify the mechanisms by which MEG3 inhibits oncogenic c-MET signaling to suppress tumorigenesis, we examined the effect of various Meg3 isoforms, explored Meg3 association with the epigenetic regulatory machinery, and RNA-DNA triplex formation. We provide direct evidence for the first time that ectopically expressed Meg3 isoforms could attenuate the highly abundant c-Met transcript in MIN6 insulinoma cells. Meg3 also interacted with components of the epigenetic machinery, such as the Polycomb Repressive Complex 2 (PRC2) protein conglomerate, to regulate c-Met RNA expression. Additionally, analyses of triplex forming oligos (TFOs) in MIN6 cells revealed that triple helix formation between Meg3 and dsDNA could potentially disrupt c-Met transcription. Combined, these data offer mechanistic insight into the dysregulation of c-MET repression in PNETs, and support the conclusion that MEG3 acts as an important determinant of oncogenic signaling in MEN1-associated endocrine tumor cells (insulinoma). These findings warrant further investigation into the tumorigenic pathways that may result from the loss of tumor suppressor MEG3 and gain of oncogenic c-MET signaling in other MEN1-associated endocrine tumors. Citation Format: Sucharitha Iyer, Sunita Agarwal. The lncRNA Meg3 acts as an epigenetic determinant of oncogenic signaling in multiple endocrine neoplasia type 1 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2544. doi:10.1158/1538-7445.AM2017-2544