A role for FOXO1 in BCR-ABL1-independent tyrosine kinase inhibitor resistance in chronic myeloid leukemia

An Overview of Myeloid Blast-Phase Chronic Myeloid Leukemia

Myeloid blast-phase chronic myeloid leukemia (MBP-CML) is a rare disease with a dismal prognosis. It is twice as common as lymphoid blast-phase CML, and its prognosis is poorer. Despite the success with tyrosine kinase inhibitors in the treatment of chronic-phase CML, the same does not hold true for MBP-CML. In addition to the Philadelphia chromosome, other chromosomal and molecular changes characterize rapid progression. Although some progress in elucidating the biology of MBP-CML has been made, there is need to discover more in order to develop more satisfactory treatment options. Currently, most common treatment options include tyrosine kinase inhibitors (TKIs) as monotherapy or in combination with acute myeloid leukemia-based intensive chemotherapy regimens. Some patients may develop resistance to TKIs via BCR-ABL1-dependent or BCR-ABL1-independent mechanisms. In this paper, we provide an overview of the biology of MBP-CML, the current treatment approaches, and mechanisms of resistance to TKIs. In order to improve treatment responses in these patients, more emphasis should be placed on understanding the biology of myeloid blastic transformation in CML and mechanisms of resistance to TKIs. Although patient numbers are small, randomized clinical trials should be considered.

SIRT1-mediated deacetylation of FOXO3 enhances mitophagy and drives hormone resistance in endometrial cancer

BACKGROUND

The complex interplay between Sirtuin 1 (SIRT1) and FOXO3 in endometrial cancer (EC) remains understudied. This research aims to unravel the interactions of deacetylase SIRT1 and transcription factor FOXO3 in EC, focusing on their impact on mitophagy and hormone resistance.

METHODS

High-throughput sequencing, cell experiments, and bioinformatics tools were employed to investigate the roles and interactions of SIRT1 and FOXO3 in EC. Co-immunoprecipitation (Co-IP) assay was used to assess the interaction between SIRT1 and FOXO3 in RL95-2 cells. Functional assays were used to assess cell viability, proliferation, migration, invasion, apoptosis, and the expression of related genes and proteins. A mouse model of EC was established to evaluate tumor growth and hormone resistance under different interventions. Immunohistochemistry and TUNEL assays were used to assess protein expression and apoptosis in tumor tissues.

RESULTS

High-throughput transcriptome sequencing revealed a close association between SIRT1, FOXO3, and EC development. Co-IP showed a protein-protein interaction between SIRT1 and FOXO3. Overexpression of SIRT1 enhanced FOXO3 deacetylation and activity, promoting BNIP3 transcription and PINK1/Parkin-mediated mitophagy, which in turn promoted cell proliferation, migration, invasion, and inhibited apoptosis in vitro, as well as increased tumor growth and hormone resistance in vivo. These findings highlighted SIRT1 as an upstream regulator and potential therapeutic target in EC.

CONCLUSION

This study reveals a novel molecular mechanism underlying the functional relevance of SIRT1 in regulating mitophagy and hormone resistance through the deacetylation of FOXO3 in EC, thereby providing valuable insights for new therapeutic strategies.

Exploiting the potential of the ubiquitin-proteasome system in overcoming tyrosine kinase inhibitor resistance in chronic myeloid leukemia

The advent of tyrosine kinase inhibitors (TKI) targeting BCR-ABL has drastically changed the treatment approach of chronic myeloid leukemia (CML), greatly prolonged the life of CML patients, and improved their prognosis. However, TKI resistance is still a major problem with CML patients, reducing the efficacy of treatment and their quality of life. TKI resistance is mainly divided into BCR-ABL-dependent and BCR-ABL-independent resistance. Now, the main clinical strategy addressing TKI resistance is to switch to newly developed TKIs. However, data have shown that these new drugs may cause serious adverse reactions and intolerance and cannot address all resistance mutations. Therefore, finding new therapeutic targets to overcome TKI resistance is crucial and the ubiquitin-proteasome system (UPS) has emerged as a focus. The UPS mediates the degradation of most proteins in organisms and controls a wide range of physiological processes. In recent years, the study of UPS in hematological malignant tumors has resulted in effective treatments, such as bortezomib in the treatment of multiple myeloma and mantle cell lymphoma. In CML, the components of UPS cooperate or antagonize the efficacy of TKI by directly or indirectly affecting the ubiquitination of BCR-ABL, interfering with CML-related signaling pathways, and negatively or positively affecting leukemia stem cells. Some of these molecules may help overcome TKI resistance and treat CML. In this review, the mechanism of TKI resistance is briefly described, the components of UPS are introduced, existing studies on UPS participating in TKI resistance are listed, and UPS as the therapeutic target and strategies are discussed.

Insights into existing and futuristic treatment approach for chronic myeloid leukaemia

Oncogenes play a crucial part in human cancer development, and when particular drugs obstruct the proteins produced by these oncogenes, the tumoural process can be ceased. For instance, in chronic myeloid leukaemia (CML), all pathological traits are associated with a single oncogene, BCR-ABL1. CML is a triphasic cancerous disorder of haematopoietic stem cells, marked by a balanced translocation between chromosomes 9 and 22, leading to the genesis of a Philadelphia chromosome encompassing the BCR-ABL1 fusion gene. This fusion oncogene further produces a constitutive active tyrosine kinase protein, enhancing the downstream signalling pathways and constitutes cancer. The treatment for CML has been entirely altered from chemotherapy and immunotherapy to targeted therapy with the emergence of tyrosine kinase inhibitors (TKIs) which inhibit BCR-ABL1 kinase activity. However, the inhibitory mechanism of TKIs is constrained by BCR-ABL1 dependent and independent resistance mechanisms, prompting the exploration of novel therapeutics through extensive clinical trials to develop next-generation drugs with enhanced potency. The persistent challenges posed by CML have motivated researchers to seek innovative strategies for its eradication, such as the application of the genome editing tool CRISPR/Cas9. This review provides insights into existing CML diagnoses, treatment modalities, resistance mechanisms, drugs under trial phases and new potential therapeutic drugs. Furthermore, the review looks ahead to a visionary perspective wherein the CRISPR/Cas9 approach holds the potential to evolve into a prospective curative measure for CML.

Compact CRISPR genetic screens enabled by improved guide RNA library cloning

CRISPR genome editing approaches theoretically enable researchers to define the function of each human gene in specific cell types, but challenges remain to efficiently perform genetic perturbations in relevant models. In this work, we develop a library cloning protocol that increases sgRNA uniformity and greatly reduces bias in existing genome-wide libraries. We demonstrate that our libraries can achieve equivalent or better statistical power compared to previously reported screens using an order of magnitude fewer cells. This improved cloning protocol enables genome-scale CRISPR screens in technically challenging cell models and screen formats.

Modulating retinoid-X-receptor alpha (RXRA) expression sensitizes chronic myeloid leukemia cells to imatinib in vitro and reduces disease burden in vivo

Introduction: The ligand-activated transcription factors, nuclear hormone receptors (NHRs), remain unexplored in hematological malignancies except for retinoic acid receptor alpha (RARA). Methods: Here we profiled the expression of various NHRs and their coregulators in Chronic myeloid leukemia (CML) cell lines and identified a significant differential expression pattern between inherently imatinib mesylate (IM)-sensitive and resistant cell lines. Results: Retinoid-X-receptor alpha (RXRA) was downregulated in CML cell lines inherently resistant to IM and in primary CML CD34⁺ cells. Pre-treatment with clinically relevant RXRA ligands improved sensitivity to IM in-vitro in both CML cell lines and primary CML cells. This combination effectively reduced the viability and colony-forming capacity of CML CD34⁺ cells in-vitro. In-vivo, this combination reduced leukemic burden and prolonged survival. Overexpression (OE) of RXRA inhibited proliferation and improved sensitivity to IM in-vitro. In-vivo, RXRA OE cells showed reduced engraftment of cells in the bone marrow, improved sensitivity to IM, and prolonged survival. Both RXRA OE and ligand treatment markedly reduced BCR::ABL1 downstream kinase activation, activating apoptotic cascades and improving sensitivity to IM. Importantly, RXRA OE also led to the disruption of the oxidative capacity of these cells. Conclusion: Combining IM with clinically available RXRA ligands could form an alternative treatment strategy in CML patients with suboptimal response to IM.

HDAC inhibitor chidamide overcomes drug resistance in chronic myeloid leukemia with the T315i mutation through the Akt-autophagy pathway

Currently, therapy for Chronic Myeloid Leukemia (CML) patients with the T315I mutation is a major challenge in clinical practice due to its high degree of resistance to first- and second-generation Tyrosine Kinase Inhibitors (TKIs). Chidamide, a Histone Deacetylase Inhibitor (HDACi) drug, is currently used to treat peripheral T-cell lymphoma. In this study, we investigated the anti-leukemia effects of chidamide on the CML cell lines Ba/F3 P210 and Ba/F3 T315I and primary tumor cells from CML patients with the T315I mutation. The underlying mechanism was investigated, and we found that chidamide could inhibit Ba/F3 T315I cells at G0/G1 phase. Signaling pathway analysis showed that chidamide induced H3 acetylation, downregulated pAKT expression and upregulated pSTAT5 expression in Ba/F3 T315I cells. Additionally, we found that the antitumor effect of chidamide could be exerted by regulating the crosstalk between apoptosis and autophagy. When chidamide was used in combination with imatinib or nilotinib, the antitumor effects were enhanced compared with chidamide alone in Ba/F3 T315I and Ba/F3 P210 cells. Therefore, we conclude that chidamide may overcome T315I mutation-related drug resistance in CML patients and works efficiently if used in combination with TKIs.

Comprehensive analysis of prognosis of cuproptosis-related oxidative stress genes in multiple myeloma

Introduction: Multiple myeloma (MM) is a highly heterogeneous hematologic malignancy. The patients’ survival outcomes vary widely. Establishing a more accurate prognostic model is necessary to improve prognostic precision and guide clinical therapy.

Methods: We developed an eight-gene model to assess the prognostic outcome of MM patients. Univariate Cox analysis, Least absolute shrinkage and selection operator (LASSO) regression, and multivariate Cox regression analyses were used to identify the significant genes and construct the model. Other independent databases were used to validate the model.

Results: The results showed that the overall survival of patients in the high-risk group was signifificantly shorter compared with that of those in the low-risk group. The eight-gene model demonstrated high accuracy and reliability in predicting the prognosis of MM patients.

Discussion: Our study provides a novel prognostic model for MM patients based on cuproptosis and oxidative stress. The eight-gene model can provide valid predictions for prognosis and guide personalized clinical treatment. Further studies are needed to validate the clinical utility of the model and explore potential therapeutic targets.

Activation of ERK1/2 by MOS and TPL2 leads to dasatinib resistance in chronic myeloid leukaemia cells

The development of BCR::ABL1 tyrosine kinase inhibitors (TKIs), such as dasatinib, has dramatically improved survival in cases of chronic myeloid leukaemia (CML). However, the development of resistance to BCR::ABL1 TKIs is a clinical problem. BCR::ABL1 TKI resistance is known to have BCR::ABL1-dependent or BCR::ABL1-independent mechanisms, but the mechanism of BCR::ABL1 independence is not well understood. In the present study, we investigated the mechanism of BCR::ABL1-independent dasatinib resistance. The expression and activation level of genes or proteins were evaluated using array CGH, real time PCR, or western blot analysis. Gene expression was modulated using siRNA-mediated knockdown. Cell survival was assessed by using trypan blue dye method. We found that dasatinib-resistant K562/DR and KU812/DR cells did not harbour a BCR::ABL1 mutation but had elevated expression and/or activation of MOS, TPL2 and ERK1/2. In addition, MOS siRNA, TPL2 siRNA and trametinib resensitized dasatinib-resistant cells to dasatinib. Moreover, expression levels of MOS in dasatinib non-responder patients with CML were higher than those in dasatinib responders, and the expression of TPL2 tended to increase in dasatinib non-responder patients compared with that in responder patients. Our results indicate that activation of ERK1/2 by elevated MOS and TPL2 expression is involved in dasatinib resistance, and inhibition of these proteins overcomes dasatinib resistance. Therefore, MOS, TPL2 and ERK1/2 inhibitors may be therapeutically useful for treating BCR::ABL1-independent dasatinib-resistant CML.

Prognostic impact of ASXL1 mutations in chronic phase chronic myeloid leukemia

While the clinical impact of mutations in the ABL1 gene on response to therapy in chronic phase chronic myeloid leukemia (CP-CML) is well established, less is known about how other mutations affect prognosis. In a retrospective analysis, we identified 115 patients with CML (71 chronic, 15 accelerated and 29 blast phase) where targeted next-generation sequencing of genes recurrently mutated in myeloid leukemias was performed. ASXL1 was the most frequently mutated gene in the chronic (14%) and accelerated phase (40%) CML patients, whereas RUNX1 (20%) was the most common mutation in blast phase. Compared with wild-type ASXL1, CP-CML with mutant ASXL1 was associated with worse event-free survival (EFS) (median of 32.8 vs 88.3 months; P = 0.002) and failure-free survival (median of 13.8 vs 57.8 months; P = 0.04). In a multivariate analysis, ASXL1 mutation was the only independent risk factor associated with worse EFS in chronic phase CML with a hazard ratio of 4.25 (95% CI 1.59–11.35, P = 0.004). In conclusion, mutations in ASXL1 are associated with worse outcomes when detected in chronic phase CML.

Novel Dicarboximide BK124.1 Breaks Multidrug Resistance and Shows Anticancer Efficacy in Chronic Myeloid Leukemia Preclinical Models and Patients' CD34+/CD38- Leukemia Stem Cells

Simple Summary

Chemotherapy is a first line treatment in many cancer types, but the constant exposition to chemotherapeutics often leads to therapy resistance. An example is chronic myeloid leukemia that, due to the use of tyrosine kinase inhibitors such as imatinib, remains manageable, however incurable. Overall, 20–25% of imatinib responders develop secondary resistance, and among them, 20–40% is due to mechanisms such as expression of P-glycoprotein (MDR1) or leukemia stem cells’ mechanisms of survival and cancer regrowth. This study provides the first evidence from animal and cellular models that this resistance can be overcome with the novel dicarboximide BK124.1. The compound causes no visible toxicity in mice, and has proper pharmacokinetics for therapeutic applications. It was efficient against both multidrug resistant CML blasts and CD34⁺/CD38⁻ leukemia stem cells coming from CML patients. Future development of BK124.1 could offer curative treatment of CML and of other cancers resistant or intolerant to current chemotherapy.

Abstract

The search is ongoing for new anticancer therapeutics that would overcome resistance to chemotherapy. This includes chronic myeloid leukemia, particularly suitable for the studies of novel anticancer compounds due to its homogenous and well-known genetic background. Here we show anticancer efficacy of novel dicarboximide denoted BK124.1 (C₃₁H₃₇ClN₂O₄) in a mouse CML xenograft model and in vitro in two types of chemoresistant CML cells: MDR1 blasts and in CD34⁺ patients’ stem cells (N = 8) using immunoblotting and flow cytometry. Intraperitoneal administration of BK124.1 showed anti-CML efficacy in the xenograft mouse model (N = 6) comparable to the commonly used imatinib and hydroxyurea. In K562 blasts, BK124.1 decreased the protein levels of BCR-ABL1 kinase and its downstream effectors, resulting in G2/M cell cycle arrest and apoptosis associated with FOXO3a/p21^waf1/cip1 upregulation in the nucleus. Additionally, BK124.1 evoked massive apoptosis in multidrug resistant K562-MDR1 cells (IC₅₀ = 2.16 μM), in CD34⁺ cells from CML patients (IC₅₀ = 1.5 µM), and in the CD34⁺/CD38⁻ subpopulation consisting of rare, drug-resistant cancer initiating stem cells. Given the advantages of BK124.1 as a potential chemotherapeutic and its unique ability to overcome BCR-ABL1 dependent and independent multidrug resistance mechanisms, future development of BK124.1 could offer a cure for CML and other cancers resistant to present drugs.

Mechanisms of Resistance and Implications for Treatment Strategies in Chronic Myeloid Leukaemia

Simple Summary

Chronic myeloid leukaemia (CML) is a type of blood cancer that is currently well-managed with drugs that target cancer-causing proteins. However, a significant proportion of CML patients do not respond to those drug treatments or relapse when they stop those drugs because the cancer cells in those patients stop relying on that protein and instead develop a new way to survive. Therefore, new treatment strategies may be necessary for those patients. In this review, we discuss those additional survival pathways and outline combination treatment strategies to increase responses and clinical outcomes, improving the lives of CML patients.

Abstract

Tyrosine kinase inhibitors (TKIs) have revolutionised the management of chronic myeloid leukaemia (CML), with the disease now having a five-year survival rate over 80%. The primary focus in the treatment of CML has been on improving the specificity and potency of TKIs to inhibit the activation of the BCR::ABL1 kinase and/or overcoming resistance driven by mutations in the BCR::ABL1 oncogene. However, this approach may be limited in a significant proportion of patients who develop TKI resistance despite the effective inhibition of BCR::ABL1. These patients may require novel therapeutic strategies that target both BCR::ABL1-dependent and BCR::ABL1-independent mechanisms of resistance. The combination treatment strategies that target alternative survival signalling, which may contribute towards BCR::ABL1-independent resistance, could be a successful strategy for eradicating residual leukaemic cells and consequently increasing the response rate in CML patients.

BCR-ABL1 Tyrosine Kinase Complex Signaling Transduction: Challenges to Overcome Resistance in Chronic Myeloid Leukemia

The constitutively active BCR-ABL1 tyrosine kinase, found in t(9;22)(q34;q11) chromosomal translocation-derived leukemia, initiates an extremely complex signaling transduction cascade that induces a strong state of resistance to chemotherapy. Targeted therapies based on tyrosine kinase inhibitors (TKIs), such as imatinib, dasatinib, nilotinib, bosutinib, and ponatinib, have revolutionized the treatment of BCR-ABL1-driven leukemia, particularly chronic myeloid leukemia (CML). However, TKIs do not cure CML patients, as some develop TKI resistance and the majority relapse upon withdrawal from treatment. Importantly, although BCR-ABL1 tyrosine kinase is necessary to initiate and establish the malignant phenotype of Ph-related leukemia, in the later advanced phase of the disease, BCR-ABL1-independent mechanisms are also in place. Here, we present an overview of the signaling pathways initiated by BCR-ABL1 and discuss the major challenges regarding immunologic/pharmacologic combined therapies.

Overcoming Resistance to Kinase Inhibitors: The Paradigm of Chronic Myeloid Leukemia

Protein kinases (PKs) play crucial roles in cellular proliferation and survival, hence their deregulation is a common event in the pathogenesis of solid and hematologic malignancies. Targeting PKs has been a promising strategy in cancer treatment, and there are now a variety of approved anticancer drugs targeting PKs. However, the phenomenon of resistance remains an obstacle to be addressed and overcoming resistance is a goal to be achieved. Chronic myeloid leukemia (CML) is the first as well as one of the best examples of a cancer that can be targeted by molecular therapy; hence, it can be used as a model disease for other cancers. This review aims to summarize up-to-date knowledge on the main mechanisms implicated in resistance to PK inhibitory therapies and to outline the main strategies that are being explored to overcome resistance. The importance of molecular diagnostics and disease monitoring in counteracting resistance will also be discussed.

Description of PTPRG genetic variants identified in a cohort of Chronic Myeloid Leukemia patients and their ability to influence response to Tyrosine kinase Inhibitors

Tyrosine kinase inhibitors (TKIs) have remarkably transformed Ph+ chronic myeloid leukemia (CML) management; however, TKI resistance remains a major clinical challenge. Mutations in BCR-ABL1 are well studied but fail to explain 20-40% of resistant cases, suggesting the activation of alternative, BCR-ABL1-independent pathways. Protein Tyrosine Phosphatase Receptor Gamma (PTPRG), a tumor suppressor, was found to be well expressed in CML patients responsive to TKIs and down-regulated in resistant patients. In this study, we aimed to identify genetic variants in PTPRG that could potentially modulate TKIs response in CML patients. DNA was extracted from peripheral blood samples collected from two CML cohorts (Qatar and Italy) and targeted exome sequencing was performed. Among 31 CML patients, six were TKI-responders and 25 were TKI-resistant. Sequencing identified ten variants, seven were annotated and three were novel SNPs (c.1602_1603insC, c.85+86delC, and c.2289-129delA). Among them, five variants were identified in 15 resistant cases. Of these, one novel exon variant (c.1602_1603insC), c.841-29C>T (rs199917960) and c.1378-224A>G (rs2063204) were found to be significantly different between the resistant cases compared to responders. Our findings suggest that PTPRG variants may act as an indirect resistance mechanism of BCR-ABL1 to affect TKI treatment.

Emerging role of m6A methylation modification in ovarian cancer

m6A (N6-methyladenosine) methylation, a well-known modification in tumour epigenetics, dynamically and reversibly fine tunes the entire process of RNA metabolism. Aberrant levels of m6A and its regulators, which can predict the survival and outcomes of cancer patients, are involved in tumorigenesis, metastasis and resistance. Ovarian cancer (OC) ranks first among gynaecological tumours in the causes of death. At first diagnosis, patients with OC are usually at advanced stages owing to a lack of early biomarkers and effective targets. After treatment, patients with OC often develop drug resistance. This article reviews the recent experimental advances in understanding the role of m6A modification in OC, raising the possibility to treat m6A modification and its regulators as promising diagnostic markers and therapeutic targets for OC.

Leukemia Stem Cells as a Potential Target to Achieve Therapy-Free Remission in Chronic Myeloid Leukemia

Leukemia stem cells (LSCs, also known as leukemia-initiating cells) not only drive leukemia initiation and progression, but also contribute to drug resistance and/or disease relapse. Therefore, eradication of every last LSC is critical for a patient's long-term cure. Chronic myeloid leukemia (CML) is a myeloproliferative disorder that arises from multipotent hematopoietic stem and progenitor cells. Tyrosine kinase inhibitors (TKIs) have dramatically improved long-term outcomes and quality of life for patients with CML in the chronic phase. Point mutations of the kinase domain of BCR-ABL1 lead to TKI resistance through a reduction in drug binding, and as a result, several new generations of TKIs have been introduced to the clinic. Some patients develop TKI resistance without known mutations, however, and the presence of LSCs is believed to be at least partially associated with resistance development and CML relapse. We previously proposed targeting quiescent LSCs as a therapeutic approach to CML, and a number of potential strategies for targeting insensitive LSCs have been presented over the last decade. The identification of specific markers distinguishing CML-LSCs from healthy HSCs, and the potential contributions of the bone marrow microenvironment to CML pathogenesis, have also been explored. Nonetheless, 25% of CML patients are still expected to switch TKIs at least once, and various TKI discontinuation studies have shown a wide range in the incidence of molecular relapse (from 30% to 60%). In this review, we revisit the current knowledge regarding the role(s) of LSCs in CML leukemogenesis and response to pharmacological treatment and explore how durable treatment-free remission may be achieved and maintained after discontinuing TKI treatment.

Proteomics analysis reveals the correlation of programmed ROS-autophagy loop and dysregulated G1/S checkpoint with imatinib resistance in chronic myeloid leukemia cells

Although tyrosine kinase inhibitors (TKIs), including imatinib, have greatly improved clinical treatment of patients with chronic myeloid leukemia (CML), drug resistance remains a major obstacle. Studies on the mechanisms underlying imatinib resistance and other alternative drugs are urgently needed. Liquid chromatography tandem mass spectrometry was applied to investigate the differences in proteomics and phosphoproteomics between K562 and K562/G (imatinib resistant K562). Multiple bioinformatics analyses were performed to unveil the differential signal pathways. CCK-8 was used to detect cell proliferation. Flow cytometry was performed to analyze reactive oxygen species (ROS), cell cycle, and cell apoptosis. Western blotting and quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR) were used to observe the changes of ROS and autophagy associated with imatinib resistance in CML. Our results indicated that ROS-autophagy formed one negative feedback loop and was associated with imatinib resistance. Additionally, the limited-rate enzymes of serine synthesis pathway were escalated in K562/G, which could contribute to the increased cyclin-dependent kinases and cell proliferation index. According to phosphoproteomics data, K562/G cells exhibited abnormal phosphorylation of splicing signals. These results revealed that it could be one useful strategy to correct metabolism shift and oxidative stress, or moderately regulate autophagy. Future research should focus on the discovery of potential targets in ROS-autophagy loop.

Resistance to Tyrosine Kinase Inhibitors in Chronic Myeloid Leukemia-From Molecular Mechanisms to Clinical Relevance

Simple Summary

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasia associated with a molecular alteration, the fusion gene BCR-ABL1, that encodes the tyrosine kinase oncoprotein BCR-ABL1. This led to the development of tyrosine kinase inhibitors (TKI), with Imatinib being the first TKI approved. Although the vast majority of CML patients respond to Imatinib, resistance to this targeted therapy contributes to therapeutic failure and relapse. Here we review the molecular mechanisms and other factors (e.g., patient adherence) involved in TKI resistance, the methodologies to access these mechanisms, and the possible therapeutic approaches to circumvent TKI resistance in CML.

Abstract

Resistance to targeted therapies is a complex and multifactorial process that culminates in the selection of a cancer clone with the ability to evade treatment. Chronic myeloid leukemia (CML) was the first malignancy recognized to be associated with a genetic alteration, the t(9;22)(q34;q11). This translocation originates the BCR-ABL1 fusion gene, encoding the cytoplasmic chimeric BCR-ABL1 protein that displays an abnormally high tyrosine kinase activity. Although the vast majority of patients with CML respond to Imatinib, a tyrosine kinase inhibitor (TKI), resistance might occur either de novo or during treatment. In CML, the TKI resistance mechanisms are usually subdivided into BCR-ABL1-dependent and independent mechanisms. Furthermore, patients’ compliance/adherence to therapy is critical to CML management. Techniques with enhanced sensitivity like NGS and dPCR, the use of artificial intelligence (AI) techniques, and the development of mathematical modeling and computational prediction methods could reveal the underlying mechanisms of drug resistance and facilitate the design of more effective treatment strategies for improving drug efficacy in CML patients. Here we review the molecular mechanisms and other factors involved in resistance to TKIs in CML and the new methodologies to access these mechanisms, and the therapeutic approaches to circumvent TKI resistance.

The Levels of FoxO3a Predict the Failure of Imatinib Mesylate Therapy among Chronic Myeloid Leukemia Patients

INTRODUCTION: Forkhead Transcription Factor 3a (FoxO3a) has been proposed to have a high efficacy to predict the failure of imatinib mesylate (IM) therapy among Chronic Myeloid Leukemia (CML) patients. However, the limited evidence had made this marker remained controversy. OBJECTIVES: We aimed to investigate the correlation between the levels of FoxO3a and the risk of treatment failure of IM therapy in CML patients. METHODS: A prospective cohort study was carried out between February 2019 and February 2020 in Saiful Anwar Hospital, Malang, Indonesia. All CML patients treated with IM on our hospital during the study period were included. The levels of FoxO3a was determined using the Enzyme-linked immunosorbent assay (ELISA) using Cusabio Biotech Kit (Cusabio Biotech Co., New York, USA). The treatment response was assessed using the European Leukemia criteria. The correlation and effect estimate between the levels of FoxO3a and treatment response of CML patients was assessed using multiple logistic regression. RESULTS: 53 CML patients receiving IM in our hospital were included, consisting of 29 patients with good response and 24 patients with non-response. Our study found that CML patients with lower levels of FoxO3a was associated with increased risk to develop treatment failure when treated with IM. Moreover, we also found that higher risk of treatment failure of IM therapy was also found in patients with increased levels of thrombocytes, basophils, and leukocytes, and lower levels of hemoglobin. CONCLUSION: We reveal that FoxO3a is the prominent marker to predict the treatment response of CML patients treated with IM.

Development of BCR-ABL1 Transgenic Zebrafish Model Reproducing Chronic Myeloid Leukemia (CML) Like-Disease and Providing a New Insight into CML Mechanisms

Zebrafish has proven to be a versatile and reliable experimental in vivo tool to study human hematopoiesis and model hematological malignancies. Transgenic technologies enable the generation of specific leukemia types by the expression of human oncogenes under specific promoters. Using this technology, a variety of myeloid and lymphoid malignancies zebrafish models have been described. Chronic myeloid leukemia (CML) is a clonal myeloproliferative neoplasia characterized by the BCR-ABL1 fusion gene, derived from the t (9;22) translocation causing the Philadelphia Chromosome (Ph). The BCR-ABL1 protein is a constitutively activated tyrosine kinas inducing the leukemogenesis and resulting in an accumulation of immature leukemic cells into bone marrow and peripheral blood. To model Ph+ CML, a transgenic zebrafish line expressing the human BCR-ABL1 was generated by the Gal4/UAS system, and then crossed with the hsp70-Gal4 transgenic line. The new line named (BCR-ABL1pUAS:CFP/hsp70-Gal4), presented altered expression of hematopoietic markers during embryonic development compared to controls and transgenic larvae showed proliferating hematopoietic cells in the caudal hematopoietic tissue (CHT). The present transgenic zebrafish would be a robust CML model and a high-throughput drug screening tool.

Targeting USP47 overcomes tyrosine kinase inhibitor resistance and eradicates leukemia stem/progenitor cells in chronic myelogenous leukemia

Identifying novel drug targets to overcome resistance to tyrosine kinase inhibitors (TKIs) and eradicating leukemia stem/progenitor cells are required for the treatment of chronic myelogenous leukemia (CML). Here, we show that ubiquitin-specific peptidase 47 (USP47) is a potential target to overcome TKI resistance. Functional analysis shows that USP47 knockdown represses proliferation of CML cells sensitive or resistant to imatinib in vitro and in vivo. The knockout of Usp47 significantly inhibits BCR-ABL and BCR-ABLT315I-induced CML in mice with the reduction of Lin-Sca1+c-Kit+ CML stem/progenitor cells. Mechanistic studies show that stabilizing Y-box binding protein 1 contributes to USP47-mediated DNA damage repair in CML cells. Inhibiting USP47 by P22077 exerts cytotoxicity to CML cells with or without TKI resistance in vitro and in vivo. Moreover, P22077 eliminates leukemia stem/progenitor cells in CML mice. Together, targeting USP47 is a promising strategy to overcome TKI resistance and eradicate leukemia stem/progenitor cells in CML.

Involvement of Oxidative Stress in Resistance to Tyrosine-Kinase Inhibitors Therapy in Chronic Myeloid Leukemia

Oxidative stress involves disruption of the cellular redox status through excessive production of reactive oxygen species or through deficiency in the cellular antioxidant capacity. It is involved in the pathogeny of multiple entities (hematological diseases, metabolic disorders, cardiovascular and renal pathology etc.), as well as in the pharmacokinetics of specific treatments for these pathologies. Chronic myeloid leukemia is a chronic myeloproliferative disease for which current standard treatment is BCR-ABL tyrosine kinase inhibitors. The innovation of this therapy has significantly improved life expectancy for patients with chronic myeloid leukemia, but in some cases, this treatment becomes ineffective, installing the resistance to tyrosine kinase inhibitors therapy. There were described two types of tyrosin kinase inhibitors resistance: primary and secondary resistance. In the present paper we proposed to evaluate the involvement of oxidative in the resistance to tyrosine kinase inhibitors therapy, in the clonal instability in chronic myeloid leukemia and in the progression of the disease to an advanced stage. We concluded that oxidative stress can play a dual role in the evolution of chronic myeloid leukemia: on the one hand it can promote genomic instability and accelerate the progression of the disease to advanced stages associated with tyrosin kinase inhibitors resistance and, on the other hand, it can contribute to leukemic cell apoptosis. It seems to be outlined a fragile balance between the pro- and anti-apoptotic effects of the reactive oxygen species, closely related to their level in the leukemic cells.

Philadelphia Chromosome-Positive Leukemia in the Lymphoid Lineage-Similarities and Differences with the Myeloid Lineage and Specific Vulnerabilities

Philadelphia chromosome (Ph) results from a translocation between the breakpoint cluster region (BCR) gene on chromosome 9 and ABL proto-oncogene 1 (ABL1) gene on chromosome 22. The fusion gene, BCR-ABL1, is a constitutively active tyrosine kinase which promotes development of leukemia. Depending on the breakpoint site within the BCR gene, different isoforms of BCR-ABL1 exist, with p210 and p190 being the most prevalent. P210 isoform is the hallmark of chronic myeloid leukemia (CML), while p190 isoform is expressed in majority of Ph-positive B cell acute lymphoblastic leukemia (Ph+ B-ALL) cases. The crucial component of treatment protocols of CML and Ph+ B-ALL patients are tyrosine kinase inhibitors (TKIs), drugs which target both BCR-ABL1 isoforms. While TKIs therapy is successful in great majority of CML patients, Ph+ B-ALL often relapses as a drug-resistant disease. Recently, the high-throughput genomic and proteomic analyses revealed significant differences between CML and Ph+ B-ALL. In this review we summarize recent discoveries related to differential signaling pathways mediated by different BCR-ABL1 isoforms, lineage-specific genetic lesions, and metabolic reprogramming. In particular, we emphasize the features distinguishing Ph+ B-ALL from CML and focus on potential therapeutic approaches exploiting those characteristics, which could improve the treatment of Ph+ B-ALL.

The Lyn-SIRT1 signaling pathway is involved in imatinib resistance in chronic myeloid leukaemia

BACKGROUND

Imatinib resistance is commonly associated with the activation of BCR-ABL signaling in chronic myeloid leukaemia (CML). The activation of Lyn can result in imatinib resistance by regulating the formation of BCR-ABL protein complexes. SIRT1 is a novel survival pathway activated by BCR-ABL expression in haematopoietic progenitor cells. This study aimed to investigate whether the signaling pathway of Lyn/BCR-ABL/SIRT1 could mediate imatinib resistance in CML.

METHODS

The MTT assay was used to detect cell viability. Apoptosis was measured by a flow cytometry assay. Protein expression was detected by Western blotting. Knockdown CML cells were constructed by shRNA interference. The CML mouse model was used to investigate the role of SIRT1 in CML in vivo.

RESULTS

Lyn was overexpressed in K562R cells. BCR-ABL phosphorylation and activation were promoted by Lyn. Imatinib suppressed BCR-ABL phosphorylation in both K562 and K562R cells. BCR-ABL positively regulated SIRT1 and Foxo1 but negatively regulated acetylated Foxo1 (Ac-Foxo1) and p53 expression. Pharmacological inhibition of SIRT1 or knockdown of SIRT1 increased apoptosis and reduced growth in vitro and in vivo. Foxo1 was downregulated by SIRT1 inhibition or knockdown, while Ac-Foxo1 and p53 were upregulated. In vivo experiments showed that imatinib and/or SIRT1 inhibition both prolonged the survival of the CML mouse model and that the effects of imatinib were enhanced in combination with SIRT1 inhibition.

CONCLUSION

We proposed a novel molecular mechanism of imatinib resistance in CML in which the high expression of Lyn in imatinib-resistant cells inhibited Ac-Foxo1 and p53 expression through the BCR-ABL/SIRT1/Foxo1 signaling pathway, thus reducing apoptosis and mediating imatinib resistance.

CML - Not only BCR-ABL1 matters

BCR-ABL1 is in the center of chronic myeloid leukemia (CML) pathology, diagnosis and treatment, as confirmed by the success of tyrosine kinase inhibitor (TKI) therapy. However, additional mechanisms and events, many of which function independently of BCR-ABL1, play important roles, particularly in terms of leukemic stem cell (LSC) persistence, primary and secondary resistance, and disease progression. Promising therapeutic approaches aim to disrupt pathways which mediate LSC survival during successful TKI treatment, in the hope of improving long-term treatment-free-remission and perhaps provide a functional cure for some patients. Over the years through advances in sequencing technology frequent molecular aberrations in addition to BCR-ABL1 have been identified not only in advanced disease but also in chronic phase CML, often affecting epigenetic regulators such as ASXL1, DNMT3A and TET2. Analyses of serial samples have revealed various patterns of clonal evolution with some mutations preceding the BCR-ABL1 acquisition. Such mutations can be considered to be important co-factors in the pathogenesis of CML and could potentially influence therapeutic strategies in the future.

Imatinib mesylate affects extracellular ATP catabolism and expression of NTPDases in a chronic myeloid leukemia cell line

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm, characterized by the occurrence of the t(9;22)(q34;q11) translocation. First-line therapy for CML consists of treatment with imatinib mesylate, which selectively inhibits the BCR-ABL protein by competing for its ATP-binding site. Adenine nucleotide signaling is modulated by the ectonucleotidases and this pathway is related to tumorigenic processes. Considering the relationship between ATP and cancer, we aimed to evaluate the influence of imatinib mesylate on the expressions and functions of the NTPDase and ecto-5'-nucleotidase (CD73) enzymes in imatinib-sensitive and -resistant K-562 cell lines. mRNA analysis showed that K-562 cells express all ENTPDs and NT5E. However, when treated with imatinib mesylate for 24 h, the expression of ENTPD1, -2, -3 and -5 increased, leading to a higher nucleotides hydrolysis rate. HPLC analysis identified increased ATP degradation in cells after 24 h of treatment, with consequent ADP and AMP formation, corroborating the increase in gene and protein expression of ectonucleotidases as observed in previous results. On the other hand, we observed that imatinib-resistant K-562 cells presented a decrease in nucleotide hydrolysis and expressions of ENTPD1 and -5. These results suggest an involvement of imatinib in modulating ectonucleotidases in CML that will need further investigation. Since these ectonucleotidases have important catalytic activities in the tumor microenvironment, their modulation in CML cells may represent an important therapeutic approach to regulate levels of extracellular adenine nucleotides.

Novel therapeutic approaches in chronic myeloid leukemia

The tyrosine kinase inhibitors (TKIs) have revolutionized the management of chronic myeloid leukemia (CML) and BCR-ABL1 inhibitors form the mainstay of CML treatment. Although patients with CML generally do well under TKI therapy, there is a subgroup of patients who are resistant and/or intolerant to TKIs. In these group of patients, there is the need of additional treatment strategies. In this review, we provide an update on the current knowledge of these novel treatment approaches that can be used alone and/or in combination with TKIs.

SRSF1 mediates cytokine-induced impaired imatinib sensitivity in chronic myeloid leukemia

Patients with chronic myeloid leukemia (CML) who are treated with tyrosine kinase inhibitors (TKIs) experience significant heterogeneity regarding depth and speed of responses. Factors intrinsic and extrinsic to CML cells contribute to response heterogeneity and TKI resistance. Among extrinsic factors, cytokine-mediated TKI resistance has been demonstrated in CML progenitors, but the underlying mechanisms remain obscure. Using RNA-sequencing, we identified differentially expressed splicing factors in primary CD34⁺ chronic phase (CP) CML progenitors and controls. We found SRSF1 expression to be increased as a result of both BCR-ABL1- and cytokine-mediated signaling. SRSF1 overexpression conferred cytokine independence to untransformed hematopoietic cells and impaired imatinib sensitivity in CML cells, while SRSF1 depletion in CD34⁺ CP CML cells prevented the ability of extrinsic cytokines to decrease imatinib sensitivity. Mechanistically, PRKCH and PLCH1 were upregulated by elevated SRSF1 levels, and contributed to impaired imatinib sensitivity. Importantly, very high SRSF1 levels in the bone marrow of CML patients at presentation correlated with poorer clinical TKI responses. In summary, we find SRSF1 levels to be maintained in CD34⁺ CP CML progenitors by cytokines despite effective BCR-ABL1 inhibition, and that elevated levels promote impaired imatinib responses. Together, our data support an SRSF1/PRKCH/PLCH1 axis in contributing to cytokine-induced impaired imatinib sensitivity in CML.

Betulinic acid restores imatinib sensitivity in BCR-ABL1 kinase-independent, imatinib-resistant chronic myeloid leukemia by increasing HDAC3 ubiquitination and degradation

Although imatinib (IM) has been demonstrated to be an efficient treatment in chronic myeloid leukemia (CML), some patients still experience IM resistance and disease relapse. Through in vitro studies, we observed that HDAC3 levels were elevated in BCR-ABL1 kinase-independent, IM-resistant primary cells from CML patients and in IM-resistant K562 (K562R) cells and that downregulation of HDAC3 could enhance IM efficacy in K562R cells. Furthermore, betulinic acid (BA), a lupane-type pentacyclic triterpenoid saponin isolated from birch trees, restored IM sensitivity in the BCR-ABL1 kinase-independent, IM-resistant primary cells and in K562R cells, as well as in primary CD34⁺ bone marrow cells from CML patients. We found that BA restored IM sensitivity through inhibition of HDAC3 accumulation in cells, and that this was mediated by BA-dependent ubiquitination and degradation of HDAC3. BA at low dosage significantly increased IM antitumor effects on murine xenografts bearing K562R cells and inhibited HDAC3 expression in tumor tissue. Our findings demonstrated that HDAC3 is an essential factor in BCR-ABL1 kinase-independent IM resistance, and that BA in combination with IM may be a novel treatment strategy for overcoming IM resistance in CML.

Mechanisms of Disease Progression and Resistance to Tyrosine Kinase Inhibitor Therapy in Chronic Myeloid Leukemia: An Update

Chronic myeloid leukemia (CML) is characterized by the presence of the BCR-ABL1 fusion gene, which encodes a constitutive active tyrosine kinase considered to be the pathogenic driver capable of initiating and maintaining the disease. Despite the remarkable efficacy of tyrosine kinase inhibitors (TKIs) targeting BCR-ABL1, some patients may not respond (primary resistance) or may relapse after an initial response (secondary resistance). In a small proportion of cases, development of resistance is accompanied or shortly followed by progression from chronic to blastic phase (BP), characterized by a dismal prognosis. Evolution from CP into BP is a multifactorial and probably multistep phenomenon. Increase in BCR-ABL1 transcript levels is thought to promote the onset of secondary chromosomal or genetic defects, induce differentiation arrest, perturb RNA transcription, editing and translation that together with epigenetic and metabolic changes may ultimately lead to the expansion of highly proliferating, differentiation-arrested malignant cells. A multitude of studies over the past two decades have investigated the mechanisms underlying the closely intertwined phenomena of drug resistance and disease progression. Here, we provide an update on what is currently known on the mechanisms underlying progression and present the latest acquisitions on BCR-ABL1-independent resistance and leukemia stem cell persistence.

Management of Chronic Myeloid Leukemia in Advanced Phase

Management of chronic myeloid leukemia (CML) in advanced phases remains a challenge also in the era of tyrosine kinase inhibitors (TKIs) treatment. Cytogenetic clonal evolution and development of resistant mutations represent crucial events that limit the benefit of subsequent therapies in these patients. CML is diagnosed in accelerated (AP) or blast phase (BP) in <5% of patients, and the availability of effective treatments for chronic phase (CP) has dramatically reduced progressions on therapy. Due to smaller number of patients, few randomized studies are available in this setting and evidences are limited. Nevertheless, three main scenarios may be drawn: (a) patients diagnosed in AP are at higher risk of failure as compared to CP patients, but if they achieve optimal responses with frontline TKI treatment their outcome may be similarly favorable; (b) patients diagnosed in BP may be treated with TKI alone or with TKI together with conventional chemotherapy regimens, and subsequent transplant decisions should rely on kinetics of response and individual transplant risk; (c) patients in CP progressing under TKI treatment represent the most challenging population and they should be treated with alternative TKI according to the mutational profile, optional chemotherapy in BP patients, and transplant should be considered in suitable cases after return to second CP. Due to lack of validated and reliable markers to predict blast crisis and the still unsatisfactory results of treatments in this setting, prevention of progression by careful selection of frontline treatment in CP and early treatment intensification in non-optimal responders remains the main goal. Personalized evaluation of response kinetics could help in identifying patients at risk for progression.

Overcoming Tyrosine Kinase Inhibitor Resistance in Transformed Cell Harboring SEPT9-ABL1 Chimeric Fusion Protein

Hematological malignancies harboring various ABL1 fusions are expected to be sensitive to tyrosine kinase inhibitors (TKIs), similar to those with BCR-ABL1. However, SEPT9-ABL1 exhibits TKI resistance both in vitro and in vivo. SEPT9-ABL1 has the same ABL1 region as seen in BCR-ABL1 but no point mutation in its kinase domain, which is one of the main mechanisms underlying TKI resistance in the leukemic cells harboring BCR-ABL1. The purpose of this study was to reveal the mechanism underlying TKI resistance induced by SEPT9-ABL1. We focused on the TP53 status because TKI-induced apoptosis in BCR-ABL1–positive cells is achieved through TP53. Mouse TP53 homologue TRP53 was downregulated and less phosphorylated in the cells expressing SEPT9-ABL1 than in those with BCR-ABL1, resulting in the prevention of apoptosis induced by TKIs. The CRM1 inhibitor KPT-330 accumulated nuclear TRP53 and NFKB1A (also known as IκBα), which is thought to capture TRP53 in the cytoplasm, and induced apoptosis in the hematopoietic cells expressing SEPT9-ABL1. In addition, the combination treatment of KPT-330 and imatinib, which induced the marked nuclear accumulation of PP2A and SET, reactivated PP2A through its dephosphorylation and inhibited SET expression, resulting in the effective induction of the apoptosis in the cells expressing SEPT9-ABL1. The combination treatment with KPT-330 and imatinib successfully reduced the subcutaneous masses expressing SEPT9-ABL1 and extended the survival of the mice intraperitoneally transplanted with SEPT9-ABL1–expressing cells. These results show that therapy with CRM1 inhibitors may be effective for overcoming TKI resistance induced by SEPT9-ABL1.

Current outlook on drug resistance in chronic myeloid leukemia (CML) and potential therapeutic options

Chronic myeloid leukemia cells are armed with several resistance mechanisms that can make current drugs ineffective. A better understanding of resistance mechanisms is yielding new approaches to management of the disease. Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm the hallmark of which, the breakpoint cluster region-Abelson (BCR-ABL) oncogene, has been the target of tyrosine kinase inhibitors (TKIs), which have significantly improved the survival of patients with CML. However, because of an increase in TKI resistance, it is becoming imperative to identify resistance mechanisms so that drug therapies can be better prescribed and new agents developed. In this review, we discuss the various BCR-ABL-dependent and -independent mechanisms of resistance observed in CML, and the range of therapeutic solutions available to overcome such resistance and to ultimately improve the survival of patients with CML.

Clinical Implications of Discordant Early Molecular Responses in CML Patients Treated with Imatinib

A reduction in BCR-ABL1/ABL1^IS transcript levels to <10% after 3 months or <1% after 6 months of tyrosine kinase inhibitor therapy are associated with superior clinical outcomes in chronic myeloid leukemia (CML) patients. In this study, we investigated the reliability of multiple BCR-ABL1 thresholds in predicting treatment outcomes for 184 subjects diagnosed with CML and treated with standard-dose imatinib mesylate (IM). With a median follow-up of 61 months, patients with concordant BCR-ABL1/ABL1^IS transcripts below the defined thresholds (10% at 3 months and 1% at 6 months) displayed significantly superior rates of event-free survival (86.1% vs. 26.6%) and deep molecular response (≥ MR⁴; 71.5% vs. 16.1%) compared to individuals with BCR-ABL1/ABL1^IS levels above these defined thresholds. We then analyzed the outcomes of subjects displaying discordant molecular transcripts at 3- and 6-month time points. Among these patients, those with BCR-ABL1/ABL1^IS values >10% at 3 months but <1% at 6 months fared significantly better than individuals with BCR-ABL1/ABL1^IS <10% at 3 months but >1% at 6 months (event-free survival 68.2% vs. 32.7%; p < 0.001). Likewise, subjects with BCR-ABL1/ABL1^IS at 3 months >10% but <1% at 6 months showed a higher cumulative incidence of MR⁴ compared to patients with BCR-ABL1/ABL1^IS <10% at 3 months but >1% at 6 months (75% vs. 18.2%; p < 0.001). Finally, lower BCR-ABL1/GUS^IS transcripts at diagnosis were associated with BCR-ABL1/ABL1^IS values <1% at 6 months (p < 0.001). Our data suggest that when assessing early molecular responses to therapy, the 6-month BCR-ABL1/ABL1^IS level displays a superior prognostic value compared to the 3-month measurement in patients with discordant oncogenic transcripts at these two pivotal time points.

Resistance in chronic myeloid leukemia: definitions and novel therapeutic agents

PURPOSE OF REVIEW

Chronic myeloid leukemia (CML) is hallmarked by the presence of fusion protein kinase derived from a reciprocal translocation between chromosome 9 and 22, breakpoint cluster region (BCR)-Abelson leukemia virus (ABL) 1, causing aberrant regulation of the downstream pathways leading to unchecked CML leukemia stem cells (LSCs) proliferation. Since the discovery of tyrosine kinase inhibitors (TKI), CML, once a fatal disease, has become a chronic illness if managed appropriately. Changing treatment landscape has unsurfaced the challenge of TKI resistance that is clinically difficult to overcome.

RECENT FINDINGS

In this review, we discuss the concept of TKI resistance and pathways leading to the resistance which allows for a survival advantage to CML LSCs. Aside from BCR-ABL-dependent mechanisms of resistance which involves aberrant expression in the regulatory pumps involving efflux and influx of the TKI affecting drug bioavailability, activation of alternate survival pathways may be accountable for primary or secondary resistance. Activation of these pathways, intrinsically and extrinsically to LSCs, may be mediated through various upstream and downstream signaling as well as conditions affecting the microenvironment. Several therapeutic approaches that combine TKI with an additional agent that inhibits the activation of an alternate pathway have been studied as part of clinical trials which we will discuss here.

SUMMARY

We categorize the resistance into BCR-ABL-dependent and BCR-ABL-independent subgroups to further describe the complex molecular pathways which can potentially serve as a therapeutic target. We further discuss novel combination strategies currently in early or advanced phase clinical trials aimed to overcome the TKI resistance. We further highlight the need for further research despite the tremendous strides already made in the management of CML.

The Dominant Role of Forkhead Box Proteins in Cancer

Forkhead box (FOX) proteins are multifaceted transcription factors that are significantly implicated in cancer, with various critical roles in biological processes. Herein, we provide an overview of several key members of the FOXA, FOXC, FOXM1, FOXO and FOXP subfamilies. Important pathophysiological processes of FOX transcription factors at multiple levels in a context-dependent manner are discussed. We also specifically summarize some major aspects of FOX transcription factors in association with cancer research such as drug resistance, tumor growth, genomic alterations or drivers of initiation. Finally, we suggest that targeting FOX proteins may be a potential therapeutic strategy to combat cancer.

miR-217 sensitizes chronic myelogenous leukemia cells to tyrosine kinase inhibitors by targeting pro-oncogenic anterior gradient 2

BCR-ABL1-independent mechanisms had been thought to mediate drug resistance to tyrosine kinase inhibitors (TKIs) in patients with chronic myelogenous leukemia (CML). The pro-oncogenic anterior gradient 2 (AGR2) mediates drug resistance of cancer cells. In this study, we observed an increased level of AGR2 in TKI-resistant CML cells. Silence of AGR2 in dasatinib-resistant K562 (K562DR) cells led to restored sensitivity to dasatinib both in vitro and in vivo. Exposure to dasatinib induced upregulation of AGR2 in K562 cells, which indicated a probable treatment-related drug resistance. We further investigated the potential interaction between microRNA (miRNA) and AGR2 in K562DR cells and found that downregulation of miR-217 was associated with overexpression of AGR2 in K562DR cells. Luciferase reporter assay identified that miR-217 negatively regulated expression of AGR2 through binding the 3'-untranslated region of AGR2. Hypermethylation of the CpG island on the promoter region of the MIR217 gene is a probable reason for the downregulation of miR-217 in dasatinib-treated K562 cells. Forced expression of miR-217 led to decreased expression of AGR2 as well as compromised TKI-resistant potential of K562DR cells. Similarly, overexpression of miR-217 resensitized K562DR cells to dasatinib treatment in a murine xenograft transplantation model. TKI treatment-induced drug resistance is correlated with a decrease of miR-217 and upregulation of AGR2. The miR-217/AGR2 interaction might be a potential therapeutic target in treating CML patients with TKI resistance.

Crosstalk between BCR-ABL and protease-activated receptor 1 (PAR1) suggests a novel target in chronic myeloid leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by the presence of the Philadelphia chromosome, which generates the oncogene BCR-ABL1. Protease-activated receptor 1 (PAR1) is involved in tumor progression and angiogenesis. We have previously reported that PAR1 expression is elevated in human leukemias that display a more aggressive clinical behavior, including the blast crisis of CML. In this study, we analyzed the crosstalk between the oncoprotein BCR-ABL and PAR1 in CML. Leukemic cell lines transfected with the BCR-ABL1 oncogene showed significantly higher expression levels of PAR1 compared with that of wild-type counterparts. This phenomenon was reversed by treatment with tyrosine kinase inhibitors (TKIs). Conversely, treatment with the PAR1 antagonist SCH79797 inhibited BCR-ABL expression. The PAR1 antagonist induced apoptosis in a dose- and time-dependent manner. Higher vascular endothelial growth factor (VEGF) levels were observed in cells transfected with BCR-ABL1 than in their wild-type counterparts. VEGF expression was strongly inhibited after treatment with either TKIs or the PAR1 antagonist. Finally, we evaluated PAR1 expression in CML patients who were either in the blast or chronic phases and had either received TKI treatment or no treatment. A significant decrease in PAR1 expression was observed in treatment-responsive patients, as opposed to a significant increase in PAR1 expression levels in treatment-resistant patients. Patients classified as high risk according to the Sokal index showed higher PAR1 expression levels. Our results demonstrate the crosstalk between BCR-ABL and PAR1. These data may offer important insight into the development of new therapeutic strategies for CML.

BCR-ABL1 mediated miR-150 downregulation through MYC contributed to myeloid differentiation block and drug resistance in chronic myeloid leukemia

The fusion oncoprotein BCR-ABL1 exhibits aberrant tyrosine kinase activity and it has been proposed that it deregulates signaling networks involving both transcription factors and non-coding microRNAs that result in chronic myeloid leukemia (CML). Previously, microRNA expression profiling showed deregulated expression of miR-150 and miR-155 in CML. In this study, we placed these findings into the broader context of the MYC/miR-150/MYB/miR-155/PU.1 oncogenic network. We propose that up-regulated MYC and miR-155 in CD34⁺ leukemic stem and progenitor cells, in concert with BCR-ABL1, impair the molecular mechanisms of myeloid differentiation associated with low miR-150 and PU.1 levels. We revealed that MYC directly occupied the -11.7 kb and -0.35 kb regulatory regions in the MIR150 gene. MYC occupancy was markedly increased through BCR-ABL1 activity, causing inhibition of MIR150 gene expression in CML CD34⁺ and CD34^- cells. Furthermore, we found an association between reduced miR-150 levels in CML blast cells and their resistance to tyrosine kinase inhibitors (TKIs). Although TKIs successfully disrupted BCR-ABL1 kinase activity in proliferating CML cells, this treatment did not efficiently target quiescent leukemic stem cells. The study presents new evidence regarding the MYC/miR-150/MYB/miR-155/PU.1 leukemic network established by aberrant BCR-ABL1 activity. The key connecting nodes of this network may serve as potential druggable targets to overcome resistance of CML stem and progenitor cells.

A transcriptional MAPK Pathway Activity Score (MPAS) is a clinically relevant biomarker in multiple cancer types

KRAS- and BRAF-mutant tumors are often dependent on MAPK signaling for proliferation and survival and thus sensitive to MAPK pathway inhibitors. However, clinical studies have shown that MEK inhibitors are not uniformly effective in these cancers indicating that mutational status of these oncogenes does not accurately capture MAPK pathway activity. A number of transcripts are regulated by this pathway and are recurrently identified in genome-based MAPK transcriptional signatures. To test whether the transcriptional output of only 10 of these targets could quantify MAPK pathway activity with potential predictive or prognostic clinical utility, we created a MAPK Pathway Activity Score (MPAS) derived from aggregated gene expression. In vitro, MPAS predicted sensitivity to MAPK inhibitors in multiple cell lines, comparable to or better than larger genome-based statistical models. Bridging in vitro studies and clinical samples, median MPAS from a given tumor type correlated with cobimetinib (MEK inhibitor) sensitivity of cancer cell lines originating from the same tissue type. Retrospective analyses of clinical datasets showed that MPAS was associated with the sensitivity of melanomas to vemurafenib (HR: 0.596) and negatively prognostic of overall or progression-free survival in both adjuvant and metastatic CRC (HR: 1.5 and 1.4), adrenal cancer (HR: 1.7), and HER2+ breast cancer (HR: 1.6). MPAS thus demonstrates potential clinical utility that warrants further exploration.

Non ABL-directed inhibitors as alternative treatment strategies for chronic myeloid leukemia

The introduction of ABL Tyrosine Kinase Inhibitors (TKIs) has significantly improved the outcome of Chronic Myeloid Leukemia (CML) patients that, in large part, achieve satisfactory hematological, cytogenetic and molecular remissions. However, approximately 15-20% fail to obtain optimal responses according to the current European Leukemia Network recommendation because of drug intolerance or resistance.Moreover, a plethora of evidence suggests that Leukemic Stem Cells (LSCs) show BCR-ABL1-independent survival. Hence, they are unresponsive to TKIs, leading to disease relapse if pharmacological treatment is discontinued.All together, these biological events generate a subpopulation of CML patients in need of alternative therapeutic strategies to overcome TKI resistance or to eradicate LSCs in order to allow cure of the disease.In this review we update the role of "non ABL-directed inhibitors" targeting signaling pathways downstream of the BCR-ABL1 oncoprotein and describe immunological approaches activating specific T cell responses against CML cells.

Chronic myeloid leukemia: the paradigm of targeting oncogenic tyrosine kinase signaling and counteracting resistance for successful cancer therapy

Deregulated activity of BCR-ABL1, a nonreceptor tyrosine kinase encoded by the fusion gene resulting from the t(9;22)(q34;q11) chromosomal translocation, is thought to be the driver event responsible for initiation and maintenance of chronic myeloid leukemia (CML). BCR-ABL1 was one of the first tyrosine kinases to be implicated in a human malignancy and the first to be successfully targeted. Imatinib mesylate, the first tyrosine kinase inhibitor (TKI) to be approved for therapeutic use, was hailed as a magic bullet against cancer and remains one of the safest and most effective anticancer agents ever developed. Second- and third-generation TKIs were later introduced to prevent or counteract the problem of drug resistance, that may arise in a small proportion of patients. They are more potent molecules, but have been associated to more serious side effects and complications. Patients achieving stable optimal responses to TKI therapy are predicted to have the same life expectancy of the general population. However, TKIs do not ‘cure’ CML. Only a small proportion of cases may attempt therapy discontinuation without experiencing subsequent relapse. The great majority of patients will have to assume TKIs indefinitely – which raises serious pharmacoeconomic concerns and is now shifting the focus from efficacy to compliance and quality of life issues. Here we retrace the steps that have led from the biological acquisitions regarding BCR-ABL1 structure and function to the development of inhibitory strategies and we discuss drug resistance mechanism and how they can be addressed.

The impact of Mir-9 regulation in normal and malignant hematopoiesis

MicroRNA-9 (MiR-9) dysregulation has been observed in various cancers. Recently, MiR-9 is considered to have a part in hematopoiesis and hematologic malignancies. However, its importance in blood neoplasms is not yet well defined. Thus, this study was conducted in order to assess the significance of MiR-9 role in the development of hematologic neoplasia, prognosis, and treatment approaches. We have shown that a large number of MiR-9 targets (such as FOXOs, SIRT1, CCND1, ID2, CCNG1, Ets, and NFkB) play essential roles in leukemogenesis and that it is overexpressed in different leukemias. Our findings indicated MiR-9 downregulation in a majority of leukemias. However, its overexpression was reported in patients with dysregulated MiR-9 controlling factors (such as MLLr). Additionally, prognostic value of MiR-9 has been reported in some types of leukemia. This study generally emphasizes on the critical role of MiR-9 in hematologic malignancies as a prognostic factor and a therapeutic target.

The regulation of FOXO1 and its role in disease progression

Cell proliferation, apoptosis, autophagy, oxidative stress and metabolic dysregulation are the basis of many diseases. Forkhead box transcription factor O1 (FOXO1) changes in response to cellular stimulation and maintains tissue homeostasis during the above-mentioned physiological and pathological processes. Substantial evidences indicate that FOXO1's function depends on the modulation of downstream targets such as apoptosis- and autophagy-associated genes, anti-oxidative stress enzymes, cell cycle arrest genes, and metabolic and immune regulators. In addition, oxidative stress, high glucose and other stimulations induce the regulation of FOXO1 activity via PI3k-Akt, JNK, CBP, Sirtuins, ubiquitin E3 ligases, etc., which mediate multiple signalling pathways. Subsequent post-transcriptional modifications, including phosphorylation, ubiquitination, acetylation, deacetylation, arginine methylation and O-GlcNAcylation, activate or inhibit FOXO1. The regulation of FOXO1 and its role might provide a significant avenue for the prevention and treatment of diseases. However, the subtle mechanisms of the post-transcriptional modifications and the effect of FOXO1 remain elusive and even conflicting in the development of many diseases. The determination of these questions potentially has implications for further research regarding FOXO1 signalling and the identification of targeted drugs.

Mechanisms of Resistance to ABL Kinase Inhibition in Chronic Myeloid Leukemia and the Development of Next Generation ABL Kinase Inhibitors

Chronic myeloid leukemia is increasingly viewed as a chronic illness; most patients have a life expectancy close to that of the general population. Despite progress made using BCR-ABL1 tyrosine kinase inhibitors (TKIs), drug resistance via BCR-ABL1-dependent and BCR-ABL1-independent mechanisms continues to be an issue. BCR-ABL1-dependent resistance is primarily mediated through oncoprotein kinase domain mutations and usually results in overt resistance to TKIs. However, BCR-ABL1-independent resistance in the setting of effective BCR-ABL1 inhibition is recognized as a major contributor to minimal residual disease. Efforts to eradicate persistent leukemic stem cells have focused on combination therapy.