BackgroundSomatostatin receptor type 2 (SST2) expression is critical for the diagnosis and treatment of neuroendocrine tumors and is associated with improved patient survival. Recent data suggest that epigenetic changes such as DNA methylation and histone modifications play an important role in regulating SST2expression and tumorigenesis of NETs. However, there are limited data on the association between epigenetic marks and SST2expression in small intestinal neuroendocrine tumors (SI-NETs).MethodsTissue samples from 16 patients diagnosed with SI-NETs and undergoing surgical resection of the primary tumor at Erasmus MC Rotterdam were analysed for SST2expression levels and epigenetic marks surrounding the SST2promoter region, i.e. DNA methylation and histone modifications H3K27me3 and H3K9ac. As a control, 13 normal SI-tissue samples were included.ResultsThe SI-NET samples had high SST2protein and mRNA expression levels; a median (IQR) of 80% (70-95) SST2-positive cells and 8.2 times elevated SST2mRNA expression level compared to normal SI-tissue (p=0.0042). In comparison to normal SI-tissue, DNA methylation levels and H3K27me3 levels were significantly lower at five out of the eight targeted CpG positions and at two out of the three examined locations within the SST2gene promoter region of the SI-NET samples, respectively. No differences in the level of activating histone mark H3K9ac were observed between matched samples. While no correlation was found between histone modification marks and SST2expression, SST2mRNA expression levels correlated negatively with DNA methylation within the SST2promoter region in both normal SI-tissue and SI-NETs (p=0.006 and p=0.04, respectively).ConclusionSI-NETs have lower SST2promoter methylation levels and lower H3K27me3 methylation levels compared to normal SI-tissue. Moreover, in contrast to the absence of a correlation with SST2protein expression levels, significant negative correlations were found between SST2mRNA expression level and the mean level of DNA methylation within the SST2promoter region in both normal SI-tissue and SI-NET tissue. These results indicate that DNA methylation might be involved in regulating SST2expression. However, the role of histone modifications in SI-NETs remains elusive.