Functional analysis of DROSHA and DGCR8 microprocessor mutations as a cause of Wilms tumors

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Altered miRNA patterns are well known features of cancer, but only the recent identification of recurrent mutations in miRNA processing genes in Wilms tumors has now established deregulation of miRNA biogenesis as a driver of tumor formation. Wilms tumors are embryonal neoplasms that are derived from renal precursor cells. Especially in blastemal-type tumors, embryonic precursor cells apparently fail to terminally differentiate and remain in a highly proliferative state. By exome sequencing, we have identified recurrent mutations in the microprocessor genes DROSHA and DGCR8 in the blastemal subgroup with poor outcome. Most mutations are predicted either to inactivate the catalytic center of DROSHA or to change the RNA binding domain of DGCR8, both of which should lead to altered RNA metabolism. Together with evidence for a role of DICER1, DIS3L2 and LIN28B in rare cases of (inherited) Wilms tumors, this clearly suggests that miRNAs and their processing machinery must be critical for regulating cell fate and malignant transformation in kidney precursor cells, even if the underlying mechanism is completely unknown at this point.We plan to characterize the consequences of such mutations on clinical tumor characteristics. The focus of our work will be on in vitro analyses of the effects of mutant DROSHA/DGCR8 on cellular RNA representations and functions in cultured tumor cells as well as in mouse ES cells as genetically more tractable models. These will also be used to test the functional relevance of critical target RNAs identified in this screen to evaluate if just single genes or global effects on miRNA or even on mRNA/lncRNA may be important. To generate a model of microprocessor dependent Wilms tumor formation, we will establish mouse lines with inducible mutant DROSHA and DGCR8 cDNAs that will allow us to extend our functional studies to the in vivo situation. These experiments should provide us with unique insights into this novel pathway of miRNA driven tumor formation that should be relevant in a much broader context.