MicroRNA Expression in Ileal Carcinoid Tumors: Down-regulation of MicroRNA-133a with Tumor Progression

MicroRNAs are involved in cell proliferation, differentiation, and apoptosis and can function as tumor suppressor genes or oncogenes. The role of microRNAs in neuroendocrine tumors such as ileal carcinoids is largely unknown. We examined the differential expression of 95 microRNAs by RT-PCR using the QuantiMir System in eight matching primary and metastatic carcinoid tumors from the ileum. All microRNAs chosen for the QuantiMir System Array were based on their potential functions related to cancer biology, cell development and apoptosis. The expression of microRNAs for the samples was normalized to microRNA-197, and the matching primary and metastatic tumors were compared. There was down-regulation of microRNA-133a, 145, 146, 222 and 10b in all samples between the primary and matching metastatic tumors and up-regulation of microRNA-183, 488 and 19a + b in six of eight metastatic carcinoids compared to the primary tumors. MicroRNA-133a was further analyzed by TaqMan Real Time RT-PCR and Northern hybridization using six additional matching primary and metastatic samples which supported the PCR Array findings. There were significant differences in microRNA-133a expression with down-regulation in the metastasis compared to the primary in the eight original cases (p<0.009) and in the six additional cases used for validation (p<0.014). Laser capture microdissection and Real Time RT-PCR analysis using normal ileum found microRNA-133a expression in normal enterochromaffin cells. In situ hybridization in normal ileum showed that some of the mucosal endocrine cells expressed microRNA-133a. Both primary and metastatic ileal carcinoid tumors expressed microRNA-133a by in situ hybridization. These results provide information about novel marker microRNAs that may be used as biomarkers and/or therapeutic targets in intestinal carcinoid tumors.