**Acknowledgements**

*Pheochromocytoma, Paraganglioma and Neuroblastoma*

caused the inhibition of the growth of tumors.

the maintenance of cellular homeostasis.

NC-siRNA (**Figure 9A** and **B**).

**4. Discussion**

neuroblastoma.

combination of siRNA-HDAC8 + siRNA-MYC compared to tumors treated with

The results indicate that a decrease in the tumor targets HDAC 8 and MYC

miRNAs are both oncogenic and tumor suppressors. In normal cells homeostasis is maintained by keeping equilibrium between oncogenic and suppressor miRNA. If this equilibrium is disrupted that can cause dysfunction with increases in oncogenic miRNA and decreases in suppressor miRNA. A decrease in a specific suppressor miRNA can cause overexpression of HDCAs, c MYC and MYCN which can alter gene expression and cause cancer. However, when suppressor miRNAs are added exogenously to these cells then these cells restore normal properties and show growth arrest and apoptosis Therefore, suppressor miRNAs seem to be critical in

A decrease in suppressor miRNA can over express genes like c MYC, MYCN and HDACs and cause cancers. Over expression of c MYC and MYCN cause the down regulation of suppressor miRNAs. HDACs indirectly effect gene expression by the deacetylation of histones, therefore, this process can also effect the expression of miRNA. Transfection of miR-665 into murine NB cells caused growth inhibition, cell cycle arrest, decreased total HDAC activity, decreased HDAC8 and MYC protein expression, activated caspase 3 and increased the acetylation of histones. miR-665 targets HDAC8, c MYC and MYCN oncogene and decreases their expression. These targets are validated by the co- transfection of luciferase reporter with target 3' UTR and miRNA-665. Therefore, miRNA 665 directly targets HDAC 8, MYCN and c MYC in the inhibition of mouse neuroblastoma cells. This is the first report to show that miR-665 is a suppressor miRNA of mouse

In targeted therapy of cancer, critical genes and proteins involved in the tumorigenesis are identified and therapeutic agents' miRNA and siRNA are used to inhibit the expression of target genes to inhibit the growth of cells in vitro and in vivo. SiRNA-mediated gene knockdown is much more potent and specific with only one mRNA target, whereas miRNA has multiple mRNA targets. siRNA therapeutic approach was used in gene targeting overexpressed cancer proteins in inhibiting cancer cell growth in vitro and inhibited tumor growth in vivo in the following mouse models: breast cancer mouse model, Glioma cells tumor and colon cancer tumor. MYCN, c-MYC, and HDAC8 may each contribute to neuroblastoma tumorigenesis. We reported that transfection of mimic suppressor miR-665 inhibited the expression of c-MYC and HDAC 8 and increased caspase 3 involved in apoptosis

Our data also indicate that both c-MYC and HDAC 8 are critical targets and targeting these two targets with siRNA inhibited cell growth by 86% in vitro. The combination of siRNAs inhibited tumor growth in vivo by 80%, therefore, inhibiting more than one target is critical for the successful treatment of tumors in vivo.

Neuroblastoma is the most frequently diagnosed extracranial solid tumor in children. These tumors account for 15% of childhood deaths from cancer. Survival

and inhibited the growth of neuroblastoma cells in vitro [10].

in one- year-old children is <30% despite aggressive therapies.

**114**

**5. Conclusion**

The author would like to thank Texas Children's Hospital Flow Cytometry Core Facilities, Houston, Texas, for their help in performing cell cycle analysis and Dr. George Calin and associates at MD Anderson Cancer Center, Houston, Texas, for access to the luminometer plate reader.
