Fishing for cures : the zebrafish as a powerful tool to identify novel therapies against glioblastoma by targeting MTH1 and beyond

Abstract

Glioblastoma (GBM) is the most aggressive form of brain cancer. Despite today’s combinatory therapy consisting of surgery, radio- and chemotherapy, the prognosis remains dismal. Fostered by extensive tumor heterogeneity, cancer cell plasticity and the presence of cancer stem cells, GBM evades almost any therapeutic strategy, leading to high mortality. Thus, the development of novel therapies is of urgent need. With the identification of the Hallmarks of Cancer several cancer specific characteristics have been described that could serve as promising anti-cancer targets, including the combination of an elevated proliferation rate, crucial changes in cancer metabolism and consequently, an altered redox environment. Cancer cells and GBM in particular depend on effective anti-oxidant defense systems and non-oncogenic addiction enzymes such as MTH1, an enzyme that detoxifies oxidized bases to prevent DNA damage and subsequent cell death.

While potential anti-cancer targets are constantly being identified, the development of novel therapies against GBM is, amongst other reasons, hampered by the lack of orthotopic animal models that support large drug discovery screens. During the last decade, the zebrafish has been introduced as a clinically relevant model for human malignancies including cancer. Owing its biological and technical advantages, the zebrafish is the only vertebrate animal suitable for automated drug discovery screens to facilitate the identification and validation of novel cancer therapies. In this thesis, we primarily focused on complementing established biochemical and cellular assays with a broad application of the zebrafish model to: 1. Describe factors that render cancer cells sensitive to MTH1 inhibitors; 2. Validate MTH1 as a target in GBM and GBM stem cells; 3. Develop a new orthotopic in vivo model for GBM.

In Paper I we have demonstrated that the cellular redox environment and activation of the hypoxia signaling axis determine sensitivity to MTH1 inhibition in vitro and in vivo, thus suggesting that MTH1 inhibition may present a promising approach to treat cancers characterized by deregulated hypoxia signaling and redox imbalance. In Paper II we have tested this hypothesis and showed that depletion or inhibition of MTH1 efficiently reduces viability of patient-derived GBM cultures independent of aggressiveness i in vitro and in vivo, thus providing supporting data that MTH1 represents a promising target for GBM therapy in particular. In Paper III we addressed the lack of an orthotopic animal model for GBM which is suitable for large drug discovery screens. We found that GBM cultures transplanted into the blastoderm of zebrafish embryos form a congregated tumor in the central nervous system, fully recapitulating the human disease. As no intracranial transplantation is required, we have developed an orthotopic animal model for GBM that could readily be implemented in fully automatable drug discovery screens in order to accelerate the identification and development of novel therapies against GBM.