Therapeutic target identification and drug-target validation for acute myeloid leukemia

Acute myeloid leukemia (AML) is a hematological malignancy characterized by accumulation of immature myeloid cells with aberrant proliferation, differentiation and survival capacity in the patient’s bone marrow (BM) and peripheral blood. Most AML patients are treated with standard combination chemotherapy consisting of cytarabine and an anthracycline, aiming at achieving complete remission (CR, <5% leukemic blasts). Although most AML patients achieve CR after initial treatment, the overall survival is less than 40%. The major cause of treatment failure in AML is intrinsic heterogeneity in treatment response. After therapy, a small population of leukemic cells survives (minimal or measurable residual disease, MRD) and has the capacity to develop into a relapse. Therapy-resistant AML cells might exist prior to drug treatment or might become resistant upon treatment exposure. Relapse-initiating AML cells residing within MRD have stem cell features and are therefore called leukemic stem cells (LSCs). For development of successful treatment strategies that potentially delay or prevent relapse initiation, unraveling the mechanisms that drive (chemo)therapy resistance and leukemia-initiating potential during the disease course and unraveling the mechanisms that drive persistence of AML MRD and LSCs after therapy are crucial. The overall aim of the studies described in this thesis is to identify novel therapeutic targets and to validate these targets as potential effective and safe treatment options for AML patients. In chapter 2 we reviewed the different faces of (chemo)therapy-resistant AML (stem) cell populations, how these mechanisms affect treatment outcome of AML. and we discussed potential strategies to target dynamic treatment resistance and LSCs. In chapter 3 we identified insulin-like growth factor binding protein 7 (IGFBP7) as an efficient therapeutic approach to specifically eliminate AML (stem) cells. Low IGFBP7 is a feature of LSCs and is correlated with reduced chemotherapy sensitivity. In chapter 4 we identified a subpopulation of reversible anthracycline-tolerant leukemia cells (ATCs) and elucidated their transcriptional and epigenetic features. The ATCs lacked upregulation of H3K27 methylation after doxorubicin, and could be eliminated by targeting the H3K27 demethylase KDM6B using the inhibitor GSK-J4. GSK-J4 could also reduce MRD load and eradicated LSCs at diagnosis and after chemotherapy, suggesting that there is a potential for epigenetic-based therapy to eliminate chemotherapy-resistant AML cells. Acute promyelocytic leukemia (APL) is the only curable AML subtype, due to effective application of all-trans retinoic acid (ATRA). In chapter 5 we discussed the current knowledge on molecular, transcription and epigenetic mechanisms and potential biomarkers associated with ATRA susceptibility in APL and non-APL AML. In chapter 6 we identified IGFBP7 as part of ATRA-induced responses in APL cells, and we showed that IGFBP7 has the potential to induce susceptibility for ATRA-driven responses in non-APL AML patients by lowering the transcription factor GFI1. We found that high RARA expression is a biomarker predicting sensitivity for rhIGFBP7-induced activation of ATRA-driven responses in non-APL AML patients. In chapter 7 we studied whether AML (stem) cells could be efficiently targeted using epigenetic drugs directed to proteins involved in the regulation of transcription by chromatin modulation. In AML cells, we characterized the biological activity of two inhibitors targeting both bromodomain and extraterminal domain (BET) proteins and cyclic AMP response binding protein-binding protein (CBP) and its paralogue E1A interacting protein of 300 kDa (p300), NEO1132 and NEO2734. Additionally, we examined whether NEO2734 could serve as novel therapeutic strategy to increase effectiveness of combination chemotherapy in AML. The studies performed in this thesis contribute to improved knowledge on the biological properties of therapy-resistant AML cells. In chapter 8 we further evaluated the potential of the identified novel therapeutic strategies to target AML (stem) cells and to enhance (chemo)therapy efficacy in relation to the improvement of AML prognosis.