miRNA:mRNA interactions at the level of a single target gene

In nearly every cell of our body, DNA-encoded information is copied into messenger RNA (mRNA) that is subsequently used as a template to build protein molecules. Proteins perform the diverse biochemical functions reaching from cell proliferation to cellular suicide. Control mechanisms have evolved to prevent, allow, speed up or slow down protein production. Tiny molecules also encoded in our DNA, the microRNAs, help to control the amount of protein made by directly binding to mRNAs. Consequently, the microRNA-bound mRNAs are more rapidly degraded, and less protein is produced. The small size theoretically empowers any of the more than 1000 microRNAs encoded in our genome to bind to many different mRNAs. Amongst the sheer amount of possible interactions, however, only a minor fraction may indeed be relevant. Hence, it remains the holy grail in the scientific field to predict the amount of control imparted by each microRNA on individual mRNA targets. Identifying biologically true microRNA-mRNA interactions in the context of primary cells and whole organisms is critical as microRNA malfunction causes pathologies and impairs embryonic development. The proposed experiments are designed to characterize the specific relationship between defined microRNAs and one specific mRNA target, a tumor suppressor the allows for programmed cell death. With our experiments we aim to gain a better understanding on if and how individual microRNA:mRNA interactions slow down protein production and thus finally influence the behavior of a cell. More generally, we envision that our results will provide information on the logic of microRNA:mRNA binding. On a long run, it is the goal to identify microRNA:mRNA interactions that are suitable for therapeutic interventions in human diseases.

Within one day, more than 50 billion cells die in an adult human body. This "cellular suicide", also known as apoptosis, does not happen randomly but is controlled by the cells themselves. Usually, defective or old cells die due to apoptosis, keeping our body healthy. Incorrect cellular suicide, however, increases the risk of developing diseases. Apoptosis is a central process in preventing the transformation of cells and the development of cancer. In order to prevent premature apoptosis or to allow necessary apoptosis, the amount and activity of cell death-inducing proteins in the respective cells must be precisely regulated. One of the key proteins that can trigger a suicide cascade is BIM. In previous work, we and other researchers were able to show that apoptosis triggered by BIM plays an important role for maintaining a healthy immune system. We were also able to show that the amount of BIM in immune cells is regulated, among other mechanisms, by a specific group of small ribonucleic acid molecules, the miR17-92 microRNAs. Building on these insights, we have now investigated whether the regulation of the amount of BIM by the miR17-92 microRNAs promotes the transformation of immune cells into malignant lymphomas. For this purpose, we compared immune cells with normal regulation of BIM by the miR17-92 microRNAs with cells in which the amount of BIM can no longer be reduced by miR17-92. Notably, we performed these experiments with cells that divided uncontrollably, mimicking the initial stages of malignant transformation. In this project, we have found that blocking the binding of miR17-92 microRNAs to BIM permanently prevents the transformation of immune cells. We were also able to show that this effect is associated with increased cell death of the transforming cells. In addition, we experimentally established that malignant tumor cells are dependent on the binding of miR17-92 microRNAs to BIM for their survival. Blocking this interaction quickly leads to apoptosis of the cancer cells. Beyond the context of cancer, the reduction in BIM levels by the miR17-92 microRNAs also appears to play a positive role in the survival of antibody-producing cells after infections or vaccinations. Our results put the spotlight on miR-17~92:BIM interactions as a potential therapeutic target in lymphomas.