Microsatellite unstable gastrointestinal neuroendocrine carcinomas: a new clinicopathologic entity.

Gastroenteropancreatic (GEP) neuroendocrine carcinomas (NECs) and mixed adenoneuroendocrine carcinomas (MANECs) are heterogeneous neoplasms characterized by poor outcome. Microsatellite instability (MSI) has recently been found in colorectal NECs showing a better prognosis than expected. However, the frequency of MSI in a large series of GEP-NEC/MANECs is still unknown. In this work we investigated the incidence of MSI in GEP-NEC/MANECs and we characterized their clinico-pathologic and molecular features. MSI analysis and immunohistochemistry for mismatch repair proteins (MLH1, MSH2, MSH6, and PMS2) were performed in 89 GEP-NEC/MANECs (6 esophageal, 77 gastrointestinal, 3 pancreatic and 3 of the gallbladder). Methylation of 34 genes was studied by MS-MLPA. BRAF and KRAS mutation analysis was assessed by PCR-pyrosequencing. MSI was observed in 11 NEC/MANECs (12.4%): 7 intestinal and 4 gastric. All but two MSI-cases showed MLH1 methylation and loss of MLH1 protein. The remaining two MSI-cancers showed lack of MSH2 or PMS2 immunohistochemical expression. MSI-NEC/MANECs showed higher methylation levels than microsatellite stable (MSS) NEC/MANECs (40.6% versus 20.2% methylated genes respectively, p<0.001). BRAF mutation was detected in six out of 88 cases (7%) and KRAS mutation was identified in 15 cases (17%). BRAF mutation was associated with MSI (p<0.0008) while KRAS status did not correlate with any clinico-pathologic or molecular feature. Vascular invasion (p=0.0003) and MSI (p=0.0084) were identified as the only independent prognostic factors in multivariate analysis. We conclude that MSI identifies a subset of gastric and intestinal NEC/MANECs with distinct biology and better prognosis. MSI-NEC/MANECs resemble MSI-gastrointestinal adenocarcinomas for frequency, molecular profile, and pathogenetic mechanisms.