The contribution of genetic and epigenetic pathways in the molecular pathogenesis in neuroendocrine cancers of the gastro-entero-pancreatic system

Abstract

Introduction: Very little is known about the mechanisms contributing to the molecular pathogenesis of neuroendocrine tumors of the gastro-entero-pancreatic system. Apart from tumors belonging to the MEN1-syndrome knowledge of genetic events in sporadic neuroendocrine tumors is scarce. We aimed to investigate the role of genetic and epigenetic events leading to the formation of these rare tumors. Methods: Allelic losses, microsatellite instability (MSI) and tumor specific promoter methylation were assessed in 127 tumors. MSI testing was performed by PCR using 5 microsatellite markers. The promoter methylation status of hMLH1, p16, APC, O6-MGMT, HIC1, RASSF1a, TIMP3 and MEN1 were investigated by methylation specific PCR. The loss of heterozygosity (LOH) status of chromosomal loci on 5q and 11q was analysed using markers linked to the corresponding loci. Results: Of the tumors investigated, 80% were microsatellite stable. Only one tumor was highly unstable, 13% of the tumors were low-grade unstable. Of all tumors, 39% showed LOH in at least one of the markers investigated. In specific, 24% showed evidence of LOH close to the MEN1 locus, and 20% had allelic losses at the APC site. We further showed that promoter hypermethylation was the most prominent alteration in these tumors. While the prevalence of hMLH1, p16 and TIMP3 promoter methylation was close to zero, APC was methylated in 51.3%, O6-MGMT in 13.3%, HIC1 in 82.2%, RASSF1a in 65.7% and two putative regulatory areas of the MEN1 promoter in 5.2% and 11%, respectively. Discussion: This is the first study demonstrating the significance of genetic and epigenetic alterations in tumors of the gastro-entero-pancreatic system. While MSI does not seem to be involved in the pathogenesis of these tumors, LOH can be frequently detected. We were able to define a unique epigenetic profile characterizing these tumors. Interestingly, silencing of the MEN1 gene by promoter methylation does not seem to be strongly involved in the pathogenesis of neuroendocrine tumors. Further studies are in progress to investigate the biological significance of our findings and to correlate tumor differentiation to genes and mechanisms of the molecular pathogenesis of neuroendocrine tumors of the gastro-entero-pancreatic system.