Epigenetic and post-transcriptional regulation of somatostatin receptor subtype 5 (SST5 ) in pituitary and pancreatic neuroendocrine tumors.

Sergio Pedraza‐Arevalo,Alfonso Soto‐Moreno,Aura D Herrera‐Martínez,Justo P Castaño,María A Gálvez‐Moreno,Eva Venegas‐Moreno,Marika Charalambous,Mari C Vázquez‐Borrego,Manuel D Gahete,Miguel R Branco,Raquel Serrano‐Blanch,Álvaro Arjona‐Sánchez,Marta Korbonits,Ricardo Blázquez‐Encinas,Raúl M Luque,Alejandro Ibáñez‐Costa

doi:10.1002/1878-0261.13107

Abstract

Somatostatin receptor subtype 5 (SST5) is an emerging biomarker and actionable target in pituitary (PitNETs) and pancreatic (PanNETs) neuroendocrine tumors. Transcriptional and epigenetic regulation of SSTR5 gene expression and mRNA biogenesis is poorly understood. Recently, an overlapping natural antisense transcript, SSTR5‐AS1, potentially regulating SSTR5 expression, was identified. We aimed to elucidate whether epigenetic processes contribute to the regulation of SSTR5 expression in PitNETs (somatotropinomas) and PanNETs. We analyzed the SSTR5/SSTR5‐AS1 human locus in silico to identify CpG islands. SSTR5 and SSTR5‐AS1 expression was assessed by quantitative real‐time PCR (qPCR) in 27 somatotropinomas, 11 normal pituitaries (NPs), and 15 PanNETs/paired adjacent (control) samples. We evaluated methylation grade in four CpG islands in the SSTR5/SSTR5‐AS1 genes. Results revealed that SSTR5 and SSTR5‐AS1 were directly correlated in NP, somatotropinoma, and PanNET samples. Interestingly, selected CpG islands were differentially methylated in somatotropinomas compared with NPs. In PanNETs cell lines, SSTR5‐AS1 silencing downregulated SSTR5 expression, altered aggressiveness features, and influenced pasireotide response. These results provide evidence that SSTR5 expression in PitNETs and PanNETs can be epigenetically regulated by the SSTR5‐AS1 antisense transcript and, indirectly, by DNA methylation, which may thereby impact tumor behavior and treatment response.

Highlights

Neuroendocrine tumors (NETs) comprise a heterogeneous group of neoplasms, with rising incidence over the last decades [1,2,3]
There are four cytosine preceding a guanine (CpG) islands, named hereafter as CpG1-4, which are susceptible zones of methylation, along both genes, which could regulate their expression. Some of those CpG islands are in sites of interest, for they could be important in the control of the expression of these genes
CpG4 was the largest region identified and was subdivided into three subzones for the purpose of the study: CpG4.1 overlaps with the start of the natural antisense transcripts (NATs), possibly with its promoter, and the intron of SSTR5; CpG4.2 falls in the exon of SSTR5 and coincides with the coding sequence of the canonical Somatostatin receptor subtype 5 (SST5); CpG4.3 overlaps with the center of the large exon of SSTR5 gene, including its zone of alternative splicing, and the zone immediately previous to the SSTR5-AS1 gene

Summary

Introduction

Neuroendocrine tumors (NETs) comprise a heterogeneous group of neoplasms, with rising incidence over the last decades [1,2,3]. These tumors arise from cells of (neuro)endocrine origin, which share common features like the synthesis, storage and secretion of hormones and neurotransmitters. PanNETs have been classically thought to be derived from hormoneproducing cells of the pancreatic Langerhans’ islets [5], recent evidence has arisen challenging this concept, and it is presently under debate whether NETs can be originated from a common cell progenitor from the pancreas [6]. Genetic alterations contributing to PanNETs tumorigenesis include frequent mutations in MEN1, ATRX or DAXX genes [7]

Objectives

Methods

Results

Discussion

Conclusion