Novel modulators of DOT1L in cartilage homeostasis and osteoarthritis.

Osteoarthritis (OA) is the most common chronic joint disease, and an increasing problem in our society. Current therapy options are limited to symptom relief, and in severe cases joint-replacement surgery. Thus, there is a high unmet need for OA disease-modifying therapy. Better in-depth knowledge of molecular mechanisms of cartilage biology and OA pathogenesis is necessary to achieve this. In OA articular cartilage, H3K79me is reduced. Previously, we provided evidence that maintaining DOT1L and its associated H3K79me is crucial to protect the joints against OA. However, despite DOT1L’s and H3K79me’s important functions throughout the human body, mechanisms that control DOT1L or H3K79me remained largely elusive. In this project, we characterized the functional role of DOT1L in vivo in ageing and posttraumatic OA using two new cartilage specific Dot1l deficient genetic mouse models. We demonstrated that these novel Dot1l-deficient mice display more severe cartilage damage and osteophyte formation than the corresponding controls upon ageing or DMM joint trauma. Then, we studied the regulation of DOT1L and H3K79me in the articular chondrocyte to identify targets for OA therapy. First, we identified potential transcription factors that regulate the expression of the human DOT1L gene, using a novel bioinformatics pipeline. With this analysis, we demonstrated that hypoxia positively regulates DOT1L expression via HIF1A. Moreover, we provide evidence that targeting hypoxia and DOT1L could be an attractive strategy for OA therapy since intraarticular injections with a hypoxia mimetic could successfully halt OA progression in mice. Finally, we identified 230 positive regulators of H3K79me using a high-throughput siRNA screening. Taken together, this thesis unraveled novel signaling networks in OA biology and identified innovative and promising targets for OA modifying strategies.