Analysis of Molecular Pathways in Neuroendocrine Cancers of the Gastro-Entero-Pancreatic System

Introduction: Neuroendocrine gastro-entero-pancreatic (GEP) tumors are rare and present with variable clinical syndromes. The molecular pathogenesis is still not explained in detail. Tumors belonging to the MEN1-syndrome are caused by germ line mutations of the MEN1 gene. There do not exist detailed studies concerning the molecular pathogenesis of sporadic neuroendocrine GEP tumors. In our study, we investigated the role of microsatellite instability (MSI), loss of heterozygosity (LOH) and aberrant promoter methylation of several tumor suppressor (TSG) and mismatch repair genes in the molecular pathogenesis of non familiar neuroendocrine GEP tumors.

Methods: Neuroendocrine GEP tumors from 29 patients were assessed for their MSI status. MSI testing was performed using 5 MSI markers proposed for the detection of MSI at the NCI collaborative meeting on MSI. The promoter methylation status of hMLH1, p16 and APC were investigated by methylation specific PCR (MSP). The LOH status of APC and hMLH1 was investigated by MSI analysis using MSI markers linked to the hMLH1 locus on 3p23–21.3 and to the APC locus on 5q21.

Results: All tumors were MSS. One tumor showed LOH close to the APC locus, one tumor had LOH near the hMLH1 gene. The same tumor also showed LOH for the marker D17S250, which is not in vicinity to any known TSG. MSP for the hMLH1 promoter amplified in 23 of 29 tumors. One tumor showed hypermethylation of the hMLH1 promoter. Of all tumors, none was methylated at the p16 promoter. However,13 of 20 tumors (65%) were hypermethylated at the APC promoter. Nine tumors did not amplify. Of the hypermethylated tumors, none showed LOH of either the hMLH1 or the APC gene.

Discussion: The current study is the first report demonstrating that aberrant methylation of the APC promoter is strongly involved in the tumorigenesis of neuroendocrine GEP tumors. In contrast to other studies, p16 hypermethylation was not found in any of the cases investigated. MSI does not seem to be involved in the pathogenesis of these cancers, proven by the lack of instability of any of the markers tested and the low incidence of hMLH1 hypermethylation. Further studies are ongoing to validate these findings and to investigate the role of the APC pathway in neuroendocrine GEP tumors.