Translational Diagnostics and Therapeutics in Pancreatic Neuroendocrine Tumors

Abstract

Pancreatic neuroendocrine tumors (PNET) are rare tumors, but have been increasing in incidence. Although typically thought of as indolent, more than half of patients present with metastatic disease. For many years, the only mutations commonly known in these tumors were those in the MEN1 gene. Recently, the genetics underlying PNETs have been further defined through exome sequencing. The most frequent alterations found in sporadic PNETs are in MEN1, DAXX/ATRX, and a variety of genes in the mTOR pathway. Confirmation of these mutations has prompted trials with a number of drugs active in these pathways, and two drugs were eventually approved in 2011-sunitinib and everolimus. New data additionally identify the MET and CD47 receptors as potential novel drug targets. Yet despite improvements in progression-free survival with sunitinib and everolimus, further studies defining when to use these agents and factors associated with limitations in their utility are needed. As more discoveries are made in the laboratory that elucidate additional molecular mechanisms important in the initiation and metastasis of PNETs, continued efforts to translate these discoveries into distinct new therapies will be needed to improve patient survival. Clin Cancer Res; 22(20); 5022-9. ©2016 AACR SEE ALL ARTICLES IN THIS CCR FOCUS SECTION, "ENDOCRINE CANCERS REVISING PARADIGMS".

Figure 1. Mechanisms of genetic alteration and tumor formation

Mutations in both MEN1 and DAXX/ATRX are proposed to follow a two-hit model.(15) In multiple endocrine neoplasia type 1, an inherited heterozygous MEN1 mutation is then accompanied by a somatic second mutation in MEN1 or loss of heterozygosity (LOH), leading to tumor formation. In sporadic PNETs, alterations in MEN1 and DAXX/ATRX are frequently found to involve both copies, and tumor formation likely results from a heterozygous somatic mutation in the setting of LOH/deletion, or from a somatic heterozygous mutation with epigenetic silencing of the remaining allele.(15)

Figure 2. Role of MEN1 and DAXX/ATRX mutations in PNETS

The functions of Menin and DAXX/ATRX are incompletely understood. Menin acts as a tumor suppressor for endocrine tumors, but is required for development of some leukemias through its interaction with mixed-lineage leukemia (MLL) fusion proteins.(70) Menin appears to interact with numerous proteins and transcription factors, such as SMADs (TGFβ family), cell cycle regulators (71)(such as p27, shown to be important in SBNET tumorigenesis (72, 73)) as well as proteins involved in DNA damage-dependent cell cycle arrest or DNA repair.(24, 25) It may also act as a protein scaffold to up- and down-regulate transcription and inhibit signaling through interactions with the Map-kinase (MAPK), phosphatidylinositol-3-kinase/Akt (PI3K/Akt), and transforming growth factor beta (TGFβ) pathways, among others. Menin localizes to telomeres during cell division and has a role in regulation of histone methylation.(70) Mutations in DAXX and ATRX are found in sporadic PNETs and have been reported to correlate with both better and worse clinical outcomes compared to tumors without these mutations.(15, 19) DAXX and ATRX are directly-interacting proteins that function in histone recruitment and incorporation and maintenance of telomeres.(27) As with Menin, they likely play a role in chromatin remodeling and genomic stability, but their exact functions remain to be defined.(19)

Figure 3. Targetable pathways in pancreatic neuroendocrine tumors (PNETs)

The mitogen-activated protein kinase (MAPK) and phosphatidyl-inositol-3-kinase/mammalian target of rapamycin (PI3K/mTOR) pathways are dysregulated in sporadic and familial neuroendocrine tumors and may be targeted for treatment. Patients with multiple endocrine neoplasia type 1 (mutation in menin protein encoded by MEN1), von Hippel-Lindau syndrome (VHL), Neurofibromatosis-1 (NF1) and Tuberous Sclerosis (TSC1/2) develop neuroendocrine tumors. In non-familial PNETs, somatic mutations occur in MEN1 (44%), DAXX/ATRX (43%), and the mammalian target of rapamycin (mTOR) pathway (15%), including mutations in the phosphatidyl inositol-3-kinase (PI3K) p110 subunit PIK3CA, phosphatase and tensin homologue (PTEN), and tuberous sclerosis-2 (TSC2).(15) In both pathways, binding of ligands by receptor tyrosine kinases (RTKs) including epidermal growth factor receptors (EGFR), vascular endothelial growth factor receptors (VEGFR), platelet derived growth factor receptors (PDGFR), insulin-like growth factor 1 receptor (IGF-1R), fibroblast growth factor receptors (FGFR), transforming growth factor receptors alpha (TGFαR) and beta (TGFβR), and Met, leads to pathway activation and ultimately to transcription of genes promoting angiogenesis, growth, proliferation, cell-survival, and metastasis.(, –76) VEGFR blockade by the antibody bevacizumab blocks angiogenesis and slows tumor growth in a mouse model.(77) The multi-kinase inhibitor sunitinib blocks VEGFR1-3, PDGFR-alpha and –beta, and c-kit (not shown), and was associated in a phase-III trial with significant improvement in progression-free survival (PFS, 11.4 months vs. 5.5 months with placebo, p<0.001).(54) Everolimus acts to inhibit the mTOR complex-1 (mTORC1) and is approved for treatment of metastatic PNETS.(34) VHL protein acts to stabilize hypoxia-inducible factors (HIF) to promote angiogenesis. Finally, new data suggests that PNETS overexpress MET receptors, which receive high levels of paracrine stimulation by production of their ligand, hepatocyte growth factor (HGF), by the tumor stroma, activating the MAPK and PI3K pathways. Additionally, they may evade immune surveillance by expression of CD47, blockade of which decreased tumor growth in a mouse model.(63)