MicroRNA gene regulatory networks in osteosarcoma

Osteosarcoma (OS) is the common histological form of primary bone cancer and one of the leading aggressive cancers in children under age fifteen. Although several genetic predisposing conditions have been associated with OS the understanding of its molecular etiology is limited. Here, we show that microRNAs (miRNAs) at the chr.14q32 locus are significantly downregulated in osteosarcoma compared to normal bone tissues. Bioinformatic predictions identified that a subset of 14q32 miRNAs (miR-382, miR-369-3p, miR-544 and miR-134) could potentially target cMYC transcript. The physical interaction between these 14q32 miRNAs and cMYC was validated using reporter assays. Further, restoring expression of these four 14q32 miRNAs decreased cMYC levels and induced apoptosis in Saos2 cells. We also show that exogenous expression of 14q32 miRNAs in Saos2 cells significantly downregulated miR-17-92, a transcriptional target of cMYC. The pro-apoptotic effect of 14q32 miRNAs in Saos2 cells was rescued either by overexpression of cMYC cDNA without the 3'UTR or with miR-17-92 cluster. Further, array comparative genomic hybridization studies showed no DNA copy number changes at 14q32 locus in OS patient samples suggesting that downregulation of 14q32 miRNAs are not due to deletion at this locus. Together, our data support a model where the deregulation of a network involving 14q32 miRNAs, cMYC and miR-17-92 miRNAs could contribute to osteosarcoma pathogenesis.

Role of microRNAs in malignant transformation

Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive soft tissue tumors that occur either sporadically or in patients with neurofibromatosis. Malignant transformation of the benign neurofibroma to MPNST is incompletely understood at the molecular level. Our preliminary genome-wide gene expression and microRNA profiling studies in a series of peripheral nerve sheath tumors identified unique expression signature for the benign and malignant tumors. We found p53 inactivation and significant downregulation of several miRNAs including miR-34a (a direct transcriptional target of p53) in majority of MPNSTs compared to neurofibromas. Recently we showed that microRNAs regulated by p53 may play a role in the malignant transformation of peripheral nerve sheath tumors. Our objectives are to define miRNA profiles of MPNST susceptibility and to functionally characterize the role of miRNAs in the malignant transformation process.

ceRNA: Competing endogenous RNAs

A given mRNA can be regulated by interactions with miRNAs and in turn the availability of these miRNAs can be regulated by their interactions with alternate mRNAs.  The concept of regulation of a given mRNA by alternate mRNA (competing endogenous mRNA) by virtue of interactions with miRNAs through shared miRNA response elements, is poised to become one of the fundamental gene regulatory mechanisms.  The molecular basis of the mRNA-mRNA cross talks is via miRNA response elements, which can be predicted based on both molecular interaction and evolutionary conservation.  By examining the co-occurrence of miRNA response elements in the mRNAs on a genome-wide basis we predict competing endogenous RNA for specific mRNAs targeted by miRNAs.