**Abstract**

HDAC8, c MYC and MYCN are involved in the tumorigenesis of neuroblastoma. A mouse Neuroblastoma (NB) tumor model was used to understand the role of miRNA, miR-665 in NB tumorigenesis and cellular differentiation. During cellular differentiation of NB cells there is an up regulated miRNA-665. We found that HDAC 8, c MYC and MYCN are the direct targets of mimic miR-665 which was validated by luciferase reporter plasmid with 3' UTR and ELISA. Mimic miR-665 inhibited cell proliferation, arrested cells in G1 stage and decreased S Phase in cell cycle. miR-665 increased the acetylation of histones and activated Caspase 3. This is the first report to recognize miRNA 665 as a suppressor miRNA of NB. The effects of miR-665 were confirmed with the transfection of siRNA for HDAC8 and siRNA for MYC. Individual siRNA- HDAC8 or siRNA-MYC inhibited 40–50% of cell proliferation in vitro, however, the treatment with the combination of both siRNA-MYC + siRNA- HDAC8 inhibited 86% of cell proliferation. Indicating that both the targets c MYC and HDAC 8 should be reduced to obtain a significant inhibition of cell proliferation. Intratumoral treatment of xenograft tumors in mice with the combination of siRNA-MYC + siRNA- HDAC8 reduced the levels of target c-MYC protein by 64% and target HDAC 8 protein by 85% and the average tumor growth reduced by 80% compared to control tumors treated with NC-siRNA. Our results suggest the potential therapeutic effect of suppressor miR-665 and the combination of siRNA-MYC + siRNA-HDAC8 for neuroblastoma treatment.

**Keywords:** neuroblastoma, miRNA, siRNA, MYC, HDAC8
