Duodenal Gastrinomas Exhibit a Unique Transcriptome and Mutational Profile to Pancreatic Neuroendocrine Tumors

Abstract

Gastrinomas are malignant gastroenteropancreatic neuroendocrine tumors (GEP-NETs) that occur more commonly in the duodenum than in the pancreas and are associated with mutations in MEN1. Neuroendocrine cells, which give rise to GEP-NETs, are thought to arise from endodermally-derived cells. We previously showed that duodenal gastrinomas express glial cell markers, suggesting that duodenal neuroendocrine tumors (DNETs) and pancreatic neuroendocrine tumors (PNETs) may have a different origin. AIM: To determine if DNETs and PNETs exhibit a transcriptome that reflects parallel mechanisms for the development of GEP-NETs and whether MEN1 influences the tissue-specific differences. METHODS: Blood and tumor whole exome sequencing (WES) was performed in three PNETs, three DNETs, and two ileal carcinoids. RNA-Seq was performed on two of the DNET gastrinomas and two non-functional PNETs. We conducted a western blot and immunohistochemistry of menin to determine if mutations led to a loss of the protein or change in location. RESULTS: All four tumor samples showed elevated but equivalent levels of RNA transcripts for neuroendocrine markers such as chromogranin A, neuron-specific enolase and synaptophysin. Gastrin mRNA was 700-fold greater in both DNETs compared to the PNETs, which is consistent with their clinical diagnoses of gastrinoma. The non-functional PNETs expressed >25-fold higher mRNA levels of vasoactive intestinal peptide relative to the DNETs. DNETs expressed elevated mRNA levels of genes related to gastrin-cell specification (e.g., NKX6-3). In PNETs, elevated transcripts were related to genes expressed during the early stages of enteroendocrine cell differentiation (ASCL-1, IRX2). WES analysis of all samples showed a germline mutation of MEN1Ala541Thr. The minor allele frequency of the MEN1Ala541Thr mutation is 16.2%. In addition, a germline frameshift at MEN1 Arg 521 was identified in a DNET. A germline non-frameshift deletion at Glu363 and two somatic insertions consisting of 14 and 32 nucleotides at Gly2 and Met1 respectively, were identified in a PNET. All DNETs exhibited germline and somatic MEN1 mutations in the C-terminus of the protein near the nuclear export signal within exon 10. By contrast, in the non-functional PNETs, the mutations were located near the N-terminus of the protein. Surprisingly, a MEN1 mutation was present in the ileal carcinoids at MEN1Ala541Thr. Menin protein expression was observed in GEP-NETs and was cytoplasmic. CONCLUSION: The transcriptome and mutation profiles of DNET gastrinomas differed from PNETs. Elevated levels of transcripts in DNETs were related to gastrin cell specification, while elevated PNET transcripts were related to neuronal and enteroendocrine cell differentiation. Regardless of the MEN1 mutation profile, menin protein was expressed in all samples. MEN1 mutations may not be sufficient to initiate tumorigenesis, suggesting that other parallel mechanisms might inactivate the function of menin.