Abstract 2294: Let-7a inhibits neuroblastoma cell proliferation by directly targeting the MYCN oncogene
Authors: Fernandez Raquel D. , Bryan Kenneth , Buckley Patrick G. G. , Bray Isabella , Alcock Leah , Stallings Raymond L. L. 
Published: August 2012

Abstract
Neuroblastoma is a heterogeneous pediatric cancer consisting of clinically favorable and unfavorable tumor subtypes, with tumors having MYCN amplification or loss of chromosome 11q being associated with particularly poor patient survival. We have previously profiled a set of 145 primary neuroblastoma tumors for the expression 450 miRNA loci and have identified miRNAs that have been dysregulated by MYCN or by large scale genomic imbalances (Bray et al Plos One 2009), as well as miRNAs that are significantly correlated with clinically aggressive disease (Chen et al., Cancer Res, 2007). Here, we have carried out a refined analysis of the let-7 family of miRNAs, which map to 9 different human chromosomes. These miRNAs display a remarkable degree of coordinated expression in the tumors, in spite of their differing genomic sites. Although the expression of let-7 family members was not predictive of either overall (OS) or event free (EFS) survival in the entire tumor data set, let-7d and let-7f family members were independently predictive (p= 0.039, HR= 3.71 and p=0.024, HR=3.37 respectively) of poor clinical outcome when down-regulated in a stratified data set consisting of only 11q- tumors. We demonstrate how let-7a, let-7b, let-7c, let-7d, let-f, let-7g and let-7i miRNAs represent a signature for predicting patient survival for the 11q- tumor subtype (p&lt;0.05). The coordinated expression of let-7 family members across tumors is likely a consequence of LIN28B, which is known to inhibit the processing of primary and/or pre-let-7 sequences into mature miRNAs. Thus, a primary function of LIN28B would be to down-regulate the tumor suppressive let-7 miRNAs. Consistent with this hypothesis is the fact that LIN28B is up-regulated in tumors from patients with poor survival, as determined using 38 tumors from the R2 data analysis tool. As a partial mechanism for up-regulation of LIN28B, we further demonstrate that miR-125b, which maps to the chromosome 11q deletion region, is significantly down-regulated in 11q- tumors (p&lt; 0.05), and that this miRNA directly targets LIN28B. Thus, down-regulation of miR-125b through genomic deletion of one allele leads to up-regulation of LIN28B, which in turn reduces levels of let-7 members. At a functional level, we demonstrate that let-7a significantly diminishes neuroblastoma cell viability when ectopically over-expressed in Kelly cells, through direct targeting and inhibition of the MYCN oncogenic transcription factor. We conclude that let-7 family members act as tumor suppressors in MYCN amplified and 11q- neuroblastoma tumor subtypes, although each of the tumor subtypes appear to differ in their endogenous levels of these miRNAs.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2294. doi:1538-7445.AM2012-2294