2. NATURALEZA JURÍDICA DE LAS SOLUCIONES ALTERNATIVAS AL
2.16. EL IUS PUNIENDI VERSUS SOLUCIONES ALTERNATIVAS
As further intensification of the current multi-drug chemotherapy regimens is not feasible due to toxicity in healthy tissues, identification of agents that are more selective and better targeted in their action are necessary to improve survival rates in AML. The goal of targeted therapy is to induce a potent and specific anti-leukemic effect while producing minimal toxicity to non-leukemic cells. Targeted therapy is usually directed at leukemia driving genes or their downstream effectors. Because most healthy tissues are spared, these treatments tend to cause fewer and less severe side effects than other standard treatments. Genetic and epigenetic aberrations in leukemic cells may serve as potential therapeutic targets. In CML, leukemogenesis is driven by the BCR-ABL fusion gene. The fusion between the ABL (Abelson) tyrosine kinase gene at chromosome 9 and the BCR (break point cluster) gene at chromosome 22, results in the formation of the BCR-ABL fusion gene which encodes for a constitutively active tyrosine kinase. In CML, the availability of bcr-abl targeting tyrosine kinase inhibitors (TKI) such as imatinib significantly improved the 5-year overall survival rates from 30% to 90%.115 Although survival rates of CML patients are
Summary, General Discussion and Future Perspectives
193 TKI-treatment has turned CML into a chronic disease for most patients, requiring long-term or even life-long treatment. For a definitive cure, eradication of residual CML initiating cells is necessary. With the development of more potent TKIs, investigators hope that complete disease eradication will become possible. In APL, oncogenic transformation originates from a chromosomal translocation t(15;17), fusing the retinoic acid receptor-alpha (RARA) gene on chromosome 17 with the promyelocytic leukemia (PML) gene on chromosome 15 in the majority of patients. The PML-RARα oncoprotein serves as a therapeutic target which is degraded by ATRA and/or ATO, resulting in dramatic improvement of the long-term survival of APL patients from 35% to 80%.44 Treatment with ATRA can induce transient complete
remission, but for the induction of long-term remission, combination of ATRA with ATO or chemotherapy is necessary. The combination of ATRA and ATO is highly effective in PML-RARα degradation as both agents target the fusion protein in two distinct ways.51;53;116
These example have proven that an impressive clinical benefit may be achieved when the disease-driving mutation is targeted efficiently. As the genetic background of other malignancies, including AML, is more complex, similar spectacular effects may be hard to achieve. In AML, a diverse spectrum of underlying genetic and epigenetic aberrations has been detected, including cytogenetic aberrations, somatic mutations, and altered expression of specific genes. The complex network of genetic and epigenetic aberrations makes it more difficult to develop targeted therapy. Activating mutations in the receptor tyrosine kinases FLT3 have emerged as a promising molecular target in therapy of AML using specific TKIs.117 In addition, epigenetic
deregulation in AML can be targeted by DNA methyltransferase (DNMT) inhibitors and histone deacetylase (HDAC) inhibitor.118 Monotherapy will probably only be
effective in leukemias that are mostly driven by a single genetic aberrations, such as CML. The combination of different (targeted) treatment modalities will probably be more effective in AML than monotherapy, as illustrated by the beneficial effect of combination therapy with ATRA and ATO in APL. The more targeted the therapeutic agents become, the less side-effects may be expected. A potential draw-back of drugs that are extremely specific, is the development of resistance by activating alternative biological routes. Moreover, additional mutations can induce drug resistance. Mutations in the kinase domain of BCR-ABL that impair drug binding are the best characterized mechanism of TKI-resistance in patients with CML.115 In
addition, the more specific the drugs are, the less capable they will be to deal with the genetic and epigenetic variability that may occur in tumors to a varying degree. Challenges in the development of targeted therapy in AML include distinguishing the driver mutations from passenger mutations and finding combination of drugs that effectively eradicate all tumor cells and prevent recurrence of disease. It may be
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anticipated that the more complex the genetic disturbances in the cells are, the more we have to rely on combinations of drugs that are able to deal with the complete, highly heterogeneous spectrum of malignant cells that need to be eradicated. Various investigators have suggested a role for microRNA-based therapeutic approaches in leukemia.119 MicroRNAs can function both as tumor suppressors by
targeting of oncogenes, or display an oncogenic role through targeting of tumor suppressor genes.120;121 In general, oncogenic microRNAs may be used as therapeutic
targets, whereas tumor-suppressive microRNAs may be used as therapeutic agents.119
Targeting of oncogenic microRNAs with anti-microRNA molecules (such as antagomiRs or locked nucleic acid-anti-microRNA oligonucleotides) may induce anti-leukemic effects by reactivating the expression of tumor suppressor genes. In leukemic cells, silencing of the oncogenic miR-19 by anti-microRNA molecules was shown to reactivate the expression of the tumor suppressor PTEN and thereby antagonize the miR-19 driven leukemogenesis.122 Therapeutic agents based on tumor suppressive
microRNAs may induce anti-leukemic effects by downregulating the expression of oncogenes. In this respect, miR-132 might be an interesting therapeutic tool against
WT1 expressing leukemic cells. In vitro studies have demonstrated that targeting of WT1 with siRNAs can induce an anti-leukemic effect, by inhibition of proliferation and
induction of apoptosis in leukemic cells.65;123 In addition to WT1 targeting, miR-132
may also target GATA2 expressing leukemic cells. Simultaneous targeting of these two hematopoietic transcription factors by miR-132 may induce a synergistic anti-leukemic effect. Background expression of WT1 and GATA2 in normal peripheral blood and bone marrow cells may limit the specificity of this approach in leukemia treatment. An advantage of microRNA-based therapeutics is that one microRNA is able to target multiple genes and thereby might have a broad effect on gene expression. This advantage may however also be a complicating factor, as potentially, many off-target effects may be expected. In addition, in order to be effective essentially all tumor cells should be targeted by the micro-RNA modulating therapeutic agent, which may be difficult in vivo .108
Conclusion
AML is a clinically and molecularly heterogeneous disease characterized by the presence of a recurrent cytogenetic and molecular aberrations. One of the hallmarks of AML is deregulated gene expression. The goal of this thesis was to extend our knowledge on the (post-) transcriptional regulation of gene expression in myelopoiesis to improve prognostic classification and identify novel targets for therapy in AML. We describe the (de)regulation of two hematopoietic transcription factors, WT1 and
Summary, General Discussion and Future Perspectives
195 GATA2, which are both implicated in normal and malignant myelopoiesis, and
determine a role for miR-132 in the regulation of both transcription factors. We show that GATA2 is a novel independent poor prognostic factor in pediatric AML. Recent advances in genomic technologies have identified a diverse spectrum of genetic aberrations in AML. The prognostic relevance of various recently identified molecular markers merits further investigation as it is mostly evaluated in retrospective studies and the interaction with other molecular markers is not always taken into account. To improve survival rates in AML, other more effective therapeutic options are needed. In APL, oncoprotein targeted therapy with ATRA and/or ATO has resulted in a dramatic improvement of the long-term survival of APL patients. Development of novel targeted therapeutic approaches directed at the underlying genetic aberrations in AML have the potential to contribute to a significant improvement of outcome while simultaneously decreasing therapy-related toxicity. Identification of the driver mutations in AML is an important step towards the development of targeted therapy.
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