CpG island methylation in carcinoid and pancreatic endocrine tumors

Carcinoid tumors and pancreatic endocrine tumors (PETs) are uncommon neuroendocrine neoplasms and their genetic alterations are not well characterized. CpG island methylation is a mechanism of gene silencing, and concordant methylation of multiple CpG islands as CpG island methylator phenotype (CIMP) has been described in tumors. The aim of this study was to evaluate CIMP in carcinoid tumors and PETs. We studied 16 carcinoid tumors, 11 PETs, and 22 associated normal mucosa or pancreas. Methylation status of the p14, p16, cyclooxygenase 2 (COX2), O⁶-methyl-guanine methyltransferase (MGMT), estrogen receptor (ER), thrombospondin 1 (THBS1), retinoic acid receptor beta 2 (RARβ), T-type calcium channel (CACNA1G), and multiple endocrine neoplasia type-1 (MEN1) genes, and of MINT1, MINT2, MINT25, MINT27 and MINT31 loci was evaluated by methylation-specific-PCR (MSP) or combined bisulfite restriction analysis (COBRA). Carcinoid tumors were frequently methylated at RARβ, MGMT, p16, COX2, p14, THBS1, and ER ranging from 25 to 63% of tumors. Other CpG islands were infrequently methylated or unmethylated. The adjoining normal mucosa was also methylated for ER, COX2, and RARβ, but methylation at p14, p16, THBS1, and MGMT was tumor-specific. By contrast, PETs and normal pancreas were frequently methylated only at ER. Methylation was more frequent in carcinoid tumors than PETs at MGMT (25 versus 0%, P=0.03), THBS1 (44 versus 9%, P=0.04), p14 (44 versus 9%, P=0.04) and RARβ (25 versus 0%, P=0.03). Loss of p16 protein expression correlated with methylation of p16 gene in carcinoid tumors (P=0.006). Our study indicates that methylation profile of carcinoid tumors differs from PETs, reflecting different molecular pathogenesis.