Molecular regulation of the oncogenic miR-17-92 cluster

MicroRNAs (miRNAs) are a type of short, non-protein-coding RNAs that have been shown to have a tremendous influence on almost all biological processes in mammals. While it has become clear that these miRNAs exert their function by regulating a defined set of genes and thus influence the cellular behavior, it still remains to be determined how the miRNAs itself are regulated on the molecular level, i.e. which events determine whether a miRNA becomes active. In principle, the regulation of miRNAs can be controlled on several levels, including their production, their processing from precursors to mature forms and their stability once active and fully functional, but little is known about how these steps are specifically regulated. In this proposal, I plan to provide a comprehensive understanding of the molecular regulation of a prominent oncogenic group of miRNAs, the miR-17-92 cluster, which is strongly expressed in several human cancers such as lymphoma. Thus, understanding how these miRNAs are regulated is not only of basal scientific interest, but also of clinical importance and may help to design novel therapies against cancer. To unravel the molecular mechanisms that regulate the activity of the miR-17-92 cluster, we first plan to conduct a several genome-wide loss-of-function screens. In short, we will simply simultaneously disrupt each of the about 20000 genes in the human body in a defined cellular system and will then measure whether this disruption has any positive or negative effect on the function of the individual miRNAs from the clusters. The genes that indeed influence the miRNA levels upon their loss will then be further analyzed and characterized, i.e. we want to find out how exactly they are able to regulate the miRNAs. In a last step, we will then test the most promising candidates in a tumor model for lymphoma to determine whether their manipulation may counteract cancer cells in an experimental setting that is relatively close to the patient. If so, this may be a promising first step towards a miRNA-targeted anti-cancer therapy. Together, we expect that these studies will form the basis for a comprehensive understanding of the molecular events upstream of miR-17-92 both under physiological as well as under pathophysiological conditions. In the long term, this may lay the groundwork for therapeutic approaches that target miRNAs to manipulate disease-related transcriptional programs.

MicroRNAs (miRNAs) are a type of short, non-protein-coding RNAs that have been shown to have a tremendous influence on almost all biological processes in mammals. While it has become clear that these miRNAs exert their function by regulating a defined set of genes and thus influence the cellular behaviour, it still remains to be determined how the miRNAs itself are regulated on the molecular level. In principle, the activity of miRNAs can be controlled on several levels, including their production, their processing from precursors to mature forms and their stability once active and fully functional, but little is known about how these steps are regulated or whether they require specific co-factors. Of note, this is not only of basal scientific interest, but also of potential clinical and therapeutic relevance, as aberrant expression of miRNAs has been linked to human diseases such as cancer. In this project, we have simultaneously disrupted each of the about 20000 genes in the human body in a defined cellular system and have then measured whether this disruption has any positive or negative effect on the function of miRNAs. On the one hand, this led to the identification of a protein called SPF30 as a general facilitator of miRNA biogenesis. Disruption of SPF30, which previously has only been described in the context of protein-coding RNA processing, reduces the levels of all mature miRNAs. At the same time, cells lacking this factor accumulate specific miRNA precursors, indicating that SPF30 is normally involved in facilitating this particular processing step. Using different experimental tools, we have been able to show that SPF30 associates with one of the key enzymatic complexes of early miRNA biogenesis, the so-called Microprocessor, which cleaves long RNA molecules and releases a shorter RNA strand containing the miRNA molecule. Here, our data indicate that SPF30 facilitates those processes that directly follow this initial cleavage, thereby enhancing general miRNA biogenesis. On the other hand, we have been able to describe a completely new mode of miRNA biogenesis in which one miRNA precursor helps and assists in the processing of a neighbouring precursor on the same RNA molecule. This process, which we have termed "cluster assistance", depends on the two factors SAFB2 and ERH, both of which bind to and appear to make the Microprocessor less selective when it chooses which RNA molecules to cleave. Of note, cluster assistance plays a general role in miRNA biogenesis, i.e. we have identified a number of clustered miRNAs that require SAFB2 and ERH for their processing. At the moment, we can only speculate about the role this peculiar process for cellular function, but one possible explanation is that it enables the emergence of new miRNAs in the course of evolution.