Novel therapeutic strategies in aggressive pediatric brain tumors: navigating the epigenetic landscape

Chapter 1 introduces the types of childhood brain tumors that are described in subsequent chapters of this PhD. It includes an explanation of the epidemiology, pathological features, and classifications of high-grade childhood brain tumors. Furthermore, the current treatment options, long-term effects and molecular characteristics of these tumors are briefly discussed. In chapter 2 we describe the development of a new laboratory protocol for effective viral transduction in pHGG cell culture models. Conventional viral transduction protocols are often not suitable for primary cell cultures that are recently isolated from patients. With this new method, we show how viral transduction can be applied significantly more efficient to primary cells from brain tumors, resulting in researchers being able to more effectively create genetic modifications in these cells. In chapter 3 we identify FANCD2 as a promising therapeutic target in pHGG. In this chapter we show that a natural plant extract called 'celastrol' can effectively inhibit the growth of childhood DMG and glioblastoma cells. We show that this acts via inhibition of the DNA-damage regulator FAND2, which happens to be overexpressed in these aggressive childhood brain tumors. Next, we show that the loss of FANCD2, by celastrol treatment, causes the cancer cells to become extremely sensitive to the DNA-crosslinker drug carboplatin. In summary, this chapter describes how combination therapy with celastrol and carboplatin can synergistically inhibit the growth and viability of childhood HGG tumors and significantly prolong survival of tumor bearing mice. Chapter 4 provides a comprehensive overview of the different radiosensitization strategies described in pHGG to date. Furthermore, the preclinical studies on this topic are critically discussed. Radiotherapy is one of the pillars of today's cancer therapy and the only effective tool against the most aggressive childhood brain tumors. In this thesis, we often use radiotherapy as part of the presented novel treatment strategy, because it can be applied very locally and does not impinge on the BBB. In this chapter, we conclude that radiotherapy is often forgotten as a means of making brain tumor cells more sensitive to other treatment modalities and that more research into radiosensitization could have a major impact on the prognosis of children suffering from brain tumors. In chapter 5 we present AURKA and PLK1 as two potential therapeutic targets in DMG. Initially, we discover that the AURKA inhibitor 'phthalazinone pyrazole' is an effective radiosensitizer in our DMG culture models. Using extensive CRISPR/Cas9 screening, we next identified that PLK1 is an essential protein for DMG cells to survive phthalazinone pyrazole treatment. When we jointly inhibit AURKA and PLK1 in DMG cells, the tumor cells die. To bring this treatment strategy to the clinic, we then use other AURKA and PLK1 inhibitors that are already used in patients (for other indications) and show the mechanisms through which their synergism works. In chapter 6 we show that a well-known and widely used chemotherapeutic agent, called gemcitabine, is highly effective against ATRT. We show that gemcitabine utilizes the protein SIRT1, which is overexpressed in ATRT. By inhibiting SIRT1, gemcitabine activates the tumor suppressor gene p53, which is rarely mutated in ATRT. Subsequently, we describe that sonic hedgehog (SHH) subtype ATRT are extra sensitive to gemcitabine because loss of SIRT1 inhibits the SHH machinery. Finally, we show that different ATRT xenograft mice live significantly longer after gemcitabine treatment, without signs of toxicity. Due to these results and the translational potential of gemcitabine, we expect this treatment to be rapidly incorporated in the clinic for patients suffering from (SHH)-ATRT. Finally, the relevance and future perspectives for children with brain tumors and a personal view on the pediatric neuro-oncology research field are discussed.