Pulmonary Carcinoids and Low-Grade Gastrointestinal Neuroendocrine Tumors Show Common MicroRNA Expression Profiles, Different from Adenocarcinomas and Small Cell Carcinomas

Background: It is still uncertain whether carcinoids of the lung and gastrointestinal (GI) tract have a common origin or whether they are closer in origin to carcinomas of the same organs. MicroRNA (miRNA) expression may clarify their nature and origin. Methods: First, to verify whether formalin-fixed paraffin-embedded (FFPE) samples retain the expression signature of the tissue, miRNA expression was compared between FFPE and frozen samples. Second, we selected surgically resected FFPE samples of pulmonary and GI carcinoids, as well as other types of tumors and normal tissues from each organ, and we compared the comprehensive expression patterns of miRNAs by microarray. These data were analyzed by hierarchical clustering and consensus clustering with non-negative matrix factorization (NMF). Results: The miRNA expression profiles of FFPE and frozen samples correlated quite well. In the first hierarchical clustering, most of the carcinoids formed one major cluster with loose subpartitioning into each organ type, while the second major cluster mainly comprised adenocarcinomas and normal tissues. The NMF approach largely supported hierarchical clustering. In the additional cluster analysis comparing carcinoids to small-cell lung carcinomas (SCLCs), carcinoids formed a distinct cluster, while SCLCs grouped together with pulmonary adenocarcinomas and normal lung tissues in another major cluster. Furthermore, we found some miRNAs that exhibited significant expression in carcinoids. Conclusion: Carcinoids had a characteristic pattern of miRNA expression, suggesting a common origin for pulmonary and GI carcinoids. The expression profiles were different in carcinoids and SCLCs, indicating distinct histogenesis of these neuroendocrine tumors. A total of 50 carcinoids, 9 SCLCs, 9 adenocarcinomas, 7 olfactory neuroblastomas and 2 schwannomas were enrolled in this study. The 50 carcinoids included 22 pulmonary carcinoids, and 28 NETs G1/G2 (7 gastric, 9 duodenal and 12 rectal tumors.) The adenocarcinomas included 3 pulmonary, 3 gastric and 3 rectal adenocarcinomas. Additionally, 15 normal (non-tumorous) tissues were taken for analysis from 6 pulmonary, 3 gastric, 3 duodenal and 3 rectal carcinoid cases.