The RNA binding protein LIN28B is an essential regulator of stem cell self-renewal and has been identified as a bona fide oncogene in neuroblastoma. LIN28B is known to enhance MYCN expression through downregulation of let-7 microRNAs (miRNAs). As part of a broader study of dynamic miRNA/mRNA regulation during neuroblastoma development, we observed unexpected dynamic upregulation of LIN28B in MYCN-driven hyperplasia and tumors in mice. Hence, we hypothesized that MYCN and LIN28B are involved in a positive feedback loop through one or more miRNAs that are acting as regulatory switches between the hubs in this regulatory network. In order to fully explore this putative network, we experimentally studied all possible LIN28B-miRNA interactions through an unbiased LIN28B 3’UTR-miRNA library screen for a total of 470 miRNAs.
This LIN28B 3’UTR-miRNA library screen identified 30 miRNAs potentially targeting LIN28B. Subsequently, integrated analyses with miRNA expression profiles of a large cohort of primary neuroblastoma tumors, yielded miR-26a-5p as top candidate miRNA for LIN28B targeting in neuroblastoma. Importantly, miR-26a-5p is directly downregulated by MYCN in the MYCN-inducible MYCN3 cell line, thus providing strong evidence for MYCN-driven LIN28B upregulation. In vivo assessment using xenograft experiments with miR-26a-5p modulated neuroblastoma cell lines is currently ongoing. The discovery of this MYCN/ miR-26a-5p/ LIN28B regulatory axis marks LIN28B as an important effector of the MYCN oncogenic phenotype and underlines once more the importance of MYCN-regulated miRNAs in establishing the MYCN-driven oncogenic process. Consequently, LIN28B can be regarded as a prominent therapeutic target for MYCN-driven neuroblastoma tumors. Finally, given the role of both MYCN and LIN28B as bona fide stem cell markers, these novel findings are of broader significance for normal development and cancer biology.