Redefining tumor suppressor microRNAs: functional complexities & nanoparticle mediated delivery

MicroRNAs (miRNAs) are 22-24 nucleotides non-coding RNAs that can simultaneously influence the levels of multiple target genes and have the potential to strongly silence multiple gene networks. MYCN and ALK are well established oncogenes implicated in the pathogenesis and progression of neuroblastoma. We have identified the miRNAs that are directly repressed by the oncogene MYCN; however, overexpression of a MYCN-repressed miRNA resulted in an unexpected oncogenesis. Then using web-based algorithms I have identified miR-1323 as potential tumor suppressor that can control the pathogenesis and/or progression of neuroblastoma by targeting both MYCN and ALK. Over-expression of miR-1323 led to a decrease in MYCN and ALK expression; however, it revealed increased proliferation, colony formation, and tumor growth of xenograft tumors. SiRNA against MYCN-ALK strongly limits proliferation indicating miR-1323 targets an alternative tumor suppressor which by-passes the growth suppressive effects of MYCN-ALK inhibition and drives proliferation. Bio-informatics analysis of putative miR-1323 targets revealed PAG1 as a possible tumor suppressor target of miR-1323. Analyses of PAG1 in clinical cohorts of neuroblastoma revealed that low expression of PAG1 strongly correlates with poor overall survival compared to patients with high PAG1 expression. We therefore propose that miR-1323 mediated repression of PAG1 activates the Src-dependent oncogenic pathways, overriding its effect on MYCN or ALK. These studies further demonstrate the complexity of microRNA functions in normal and cancer cells and mandate careful evaluation of putative ‘tumor suppressive’ microRNAs as therapeutic tools.

Lastly, the large number of miRNAs and genes that are typically dysregulated in the majority of diseases makes it impractical to systematically analyze the impact of each gene and miRNA candidate on disease phenotypes. To overcome these challenges, we developed a miR-AuNP delivery system that can overcome the toxicity and low miRNA entrapment capacity of liposomal delivery and release functional miRNAs into living cells quite efficiently. In vitro studies indicated that the AuNP platform was able to release functional miRNAs, those efficiently down regulate target genes and modulate the rate of proliferation.