Expression of a distinctive BCR-ABL oncogene in Ph1-positive acute lymphocytic leukemia (ALL)

Leukemic Stem Cells and Hematological Malignancies

The association between leukemic stem cells (LSCs) and leukemia development has been widely established in the context of genetic alterations, epigenetic pathways, and signaling pathway regulation. Hematopoietic stem cells are at the top of the bone marrow hierarchy and can self-renew and progressively generate blood and immune cells. The microenvironment, niche cells, and complex signaling pathways that regulate them acquire genetic mutations and epigenetic alterations due to aging, a chronic inflammatory environment, stress, and cancer, resulting in hematopoietic stem cell dysregulation and the production of abnormal blood and immune cells, leading to hematological malignancies and blood cancer. Cells that acquire these mutations grow at a faster rate than other cells and induce clone expansion. Excessive growth leads to the development of blood cancers. Standard therapy targets blast cells, which proliferate rapidly; however, LSCs that can induce disease recurrence remain after treatment, leading to recurrence and poor prognosis. To overcome these limitations, researchers have focused on the characteristics and signaling systems of LSCs and therapies that target them to block LSCs. This review aims to provide a comprehensive understanding of the types of hematopoietic malignancies, the characteristics of leukemic stem cells that cause them, the mechanisms by which these cells acquire chemotherapy resistance, and the therapies targeting these mechanisms.

Chronic myeloid leukemia with two rare fusion gene transcripts of atypical BCR::ABL: A case report and literature review

RATIONALE

Imatinib is a standard treatment for Philadelphia (Ph) chromosome-positive chronic myeloid leukemia (CML), but its efficacy in rare BCR::ABL variants is underexplored.

PATIENT CONCERNS

A 67-year-old woman was admitted to the Second Affiliated Hospital of Xi'an Jiaotong University in March 2022 due to elevated white blood cells.

DIAGNOSIS

Karyotype analysis revealed clonal abnormalities involving the variant t(9;22) and positive results for atypical BCR::ABL variants (e14a3 and e13a3). The clinical diagnosis was CML, chronic phase, Ph+, with rare BCR::ABL-e13a3- and BCR::ABL-e14a3-positive findings.

INTERVENTION

The patient was administered daily imatinib mesylate (400 mg).

OUTCOMES

After 4 weeks, a swift molecular response was observed: BCR::ABL-e13a3 transcript level at 2.82 × 10-1 (28.24%), and BCR::ABL-e14a3 transcript level at 4.68 × 10-1 (46.76%). Within 3 months, a complete cytogenetic response was achieved, with a Ph chromosome ratio of 0. Early molecular response was evident as BCR::ABL-e13a3 transcript level reached 5.11 × 10-3 (0.51%), and BCR::ABL-e14a3 transcript level at 6.26 × 10-3 (0.63%). The imatinib mesylate treatment continued without significant toxicity.

LESSONS

This case emphasizes the potential effectiveness of imatinib mesylate in managing rare BCR::ABL fusion gene variants of CML. Screening for these atypical variants is advised for suspected CML patients who test negative for common BCR::ABL fusion gene variants. The presented case underscores the positive outcomes achieved with imatinib treatment for a patient with rare BCR::ABL variants, contributing valuable insights for the management of similar cases. Screening for unusual fusion gene variants should be a consideration in CML diagnosis for comprehensive treatment strategies.

Jak2/STAT6/c-Myc pathway is vital to the pathogenicity of Philadelphia-positive acute lymphoblastic leukemia caused by P190BCR-ABL

BACKGROUND

The Philadelphia chromosome encodes the BCR-ABL fusion protein, which has two primary subtypes, P210 and P190. P210 and P190 cause Philadelphia-positive chronic myeloid leukemia (Ph+ CML) and Philadelphia-positive acute lymphoblastic leukemia (Ph+ ALL), respectively. The Ph+ ALL is more malignant than Ph+ CML in disease phenotype and progression. This implies the key pathogenic molecules and regulatory mechanisms caused by BCR-ABL in two types of leukemia are different. It is reported that STAT6 was significantly activated only in P190 transformed cells. However, the potential role and the mechanism of STAT6 activation in Ph+ ALL and its activation mechanism by P190 are still unknown.

METHODS

The protein and mRNA levels of STAT6, c-Myc, and other molecules were measured by western blot and quantitative real-time PCR. The STAT6 inhibitor AS1517499 was used to specifically inhibit p-STAT6. The effect of p-STAT6 inhibition on Ph+ CML and Ph+ ALL cells was identified by CCK-8 and FCM assay. Dual luciferase reporter and ChIP assay were performed to confirm the direct binding between STAT6 and c-Myc. The impact of STAT6 inhibition on tumor progression was detected in Ph+ CML and Ph+ ALL mouse models.

RESULTS

Our results demonstrated that P210 induced CML-like disease, and P190 caused the more malignant ALL-like disease in mouse models. STAT6 was activated in P190 cell lines but not in P210 cell lines. Inhibition of STAT6 suppressed the malignancy of Ph+ ALL in vitro and in vivo, whereas it had little effect on Ph+ CML. We confirmed that p-STAT6 regulated the transcription of c-Myc, and STAT6 was phosphorylated by p-Jak2 in P190 cell lines, which accounted for the discrepant expression of p-STAT6 in P190 and P210 cell lines. STAT6 inhibition synergized with imatinib in Ph+ ALL cells.

CONCLUSIONS

Our study suggests that STAT6 activation plays an essential role in the development of Ph+ ALL and may be a potential therapeutic target in Ph+ ALL. Video abstract.

Targeting eIF4F translation complex sensitizes B-ALL cells to tyrosine kinase inhibition

The mechanistic target of rapamycin (mTOR) is a kinase whose activation is associated with poor prognosis in pre-B cell acute lymphoblastic leukemia (B-ALL). These and other findings have prompted diverse strategies for targeting mTOR signaling in B-ALL and other B-cell malignancies. In cellular models of Philadelphia Chromosome-positive (Ph+) B-ALL, mTOR kinase inhibitors (TOR-KIs) that inhibit both mTOR-complex-1 (mTORC1) and mTOR-complex-2 (mTORC2) enhance the cytotoxicity of tyrosine kinase inhibitors (TKIs) such as dasatinib. However, TOR-KIs have not shown substantial efficacy at tolerated doses in blood cancer clinical trials. Selective inhibition of mTORC1 or downstream effectors provides alternative strategies that may improve selectivity towards leukemia cells. Of particular interest is the eukaryotic initiation factor 4F (eIF4F) complex that mediates cap-dependent translation. Here we use novel chemical and genetic approaches to show that selective targeting of either mTORC1 kinase activity or components of the eIF4F complex sensitizes murine BCR-ABL-dependent pre-B leukemia cells to dasatinib. SBI-756, a small molecule inhibitor of eIF4F assembly, sensitizes human Ph+ and Ph-like B-ALL cells to dasatinib cytotoxicity without affecting survival of T lymphocytes or natural killer cells. These findings support the further evaluation of eIF4F-targeted molecules in combination therapies with TKIs in B-ALL and other blood cancers.

Weighted gene correlation network analysis identifies microenvironment-related genes signature as prognostic candidate for Grade II/III glioma

Glioma is the most common malignant tumor in the central nervous system. Evidence shows that clinical efficacy of immunotherapy is closely related to the tumor microenvironment. This study aims to establish a microenvironment-related genes (MRGs) model to predict the prognosis of patients with Grade II/III gliomas. Gene expression profile and clinical data of 459 patients with Grade II/III gliomas were extracted from The Cancer Genome Atlas. Then according to the immune/stromal scores generated by the ESTIMATE algorithm, the patients were scored one by one. Weighted gene co-expression network analysis (WGCNA) was used to construct a gene co-expression network to identify potential biomarkers for predicting the prognosis of patients. When adjusting clinical features including age, histology, grading, IDH status, we found that these features were independently associated with survival. The predicted value of the prognostic model was then verified in 440 samples in CGGA part B dataset and 182 samples in CGGA part C dataset by univariate and multivariate cox analysis. The clinical samples of 10 patients further confirmed our signature. Our findings suggested the eight-MRGs signature identified in this study are valuable prognostic predictors for patients with Grade II/III glioma.

The Perplexity of Synergistic Duality: Inter-molecular Mechanisms of Communication in BCR-ABL1

BACKGROUND

Aberrant and proliferative expression of the oncogene BCR-ABL in bone marrow cells is one of the prime causes of Chronic Myeloid Leukemia (CML). It has been established that the tyrosine kinase domain of the BCR-ABL protein is a potential therapeutic target for the treatment of CML. Although the first and second line inhibitors against the enzyme are available, recent studies have indicated that monotherapeutic resistance has become a great challenge.

OBJECTIVE

In recent studies, the dual inhibition of BCR-ABL by Nilotinib and Asciminib has been shown to overcome drug resistance. This prompted us to investigate the dynamics behind this novel drug combination.

METHODS

By the utilization of a wide range of computational tools, we defined and compared BCR-ABL's structural and dynamic characteristics when bound as a dual inhibitor system.

RESULTS

Conformational ensemble analysis presented a sustained inactive protein, as the activation loop, inclusive of the characteristic Tyr257, remained in an open position due to the unassailable binding of Asciminib at the allosteric site. Nilotinib also indicated stronger binding at the catalytic site in the presence of Asciminib, thus exposing new avenues in treating Nilotinib-resistance. This was in accordance with intermolecular hydrogen bond interactions with key binding site residues GLU399, Asn259 and Thr252.

CONCLUSION

The investigations carried out in this study gave rise to new possibilities in the treatment of resistance in CML, as well as assisting in the design of novel and selective inhibitors as dual anti-cancer drugs.

Synthesis, Structural Characterization, and Biological Activity of New Pyrazolo[4,3-e][1,2,4]triazine Acyclonucleosides

A series of new pyrazolo[4,3-e][1,2,4]triazine acyclonucleosides 2–5 and 8 were prepared and evaluated for their anticancer activity against human cancer cell lines (MCF-7, K-562) and CDK2/E, as well as Abl protein kinases inhibitors. Lipophilicity of the compounds was determined using C-18 and immobilized artificial membrane (IAM) chromatography. In order to confirm the molecular structures and synthesis pathway of new acyclonucleosides, X-ray analysis was performed for model compound 3. Theoretical calculations at the DFT/B3LYP/6-311++G(d,p) level were used for the characterization of electronic structures of 1–8. The potential antiviral activity of acyclonucleosides 2–8 was tested in silico using molecular docking method.

Molecular dynamics investigation on the Asciminib resistance mechanism of I502L and V468F mutations in BCR-ABL

Asciminib, a highly selective non-ATP competitive inhibitor of BCR-ABL, has demonstrated to be a promising drug for patients with chronic myeloid leukemia. It is a pity that two resistant mutations (I502L and V468F) have been found during the clinical trial, which is a challenge for the curative effect of Asciminib. In this study, molecular dynamics simulations and molecular mechanics generalized Born surface area (MM-GB/SA) calculations were performed to investigate the molecular mechanism of Asciminib resistance induced by the two mutants. The obtained results indicate that the mutations have adversely influence on the binding of Asciminib to BCR-ABL, as the nonpolar contributions decline in the two mutants. In addition, I502L mutation causes α-helix I' (αI') to shift away from the helical bundle composed of αE, αF, and αH, making the distance between αI' and Asciminib increased. For V468F mutant, the side chain of Phe468 occupies the bottom of the myristoyl pocket (MP), which drives Asciminib to shift toward the outside of MP. Our results provide the molecular insights of Asciminib resistance mechanism in BCR-ABL mutants, which may help the design of novel inhibitors.

Methods of Exploring Protein-Ligand Interactions to Guide Medicinal Chemistry Efforts

We present a number of techniques to analyze protein-ligand interactions in the context of medicinal chemistry: crystal Contract Preferences, Electrostatic Maps and pharmacophore screening using Hückel Theory. Contact Preferences is a statistical technique to predict hydrophobic and hydrophilic geometry in receptor active sites. Electrostatic Maps use the Poisson-Boltzmann Equation to model solvation effects and are particularly useful for predicting hydrophobic regions. Pharmacophore annotation with Hückel Theory provides finer detail of hydrogen bonding interactions, including CH..O interactions. Applications to AblK:Gleevec and CDK2 virtual screening are presented.

Pleckstrin homology domain of p210 BCR-ABL interacts with cardiolipin to regulate its mitochondrial translocation and subsequent mitophagy

Chronic myeloid leukemia (CML) is caused by the chimeric protein p210 BCR-ABL encoded by a gene on the Philadelphia chromosome. Although the kinase domain of p210 BCR-ABL is an active driver of CML, the pathological role of its pleckstrin homology (PH) domain remains unclear. Here, we carried out phospholipid vesicle-binding assays to show that cardiolipin (CL), a characteristic mitochondrial phospholipid, is a unique ligand of the PH domain. Arg726, a basic amino acid in the ligand-binding region, was crucial for ligand recognition. A subset of wild-type p210 BCR-ABL that was transiently expressed in HEK293 cells was dramatically translocated from the cytosol to mitochondria in response to carbonyl cyanide m-chlorophenylhydrazone (CCCP) treatment, which induces mitochondrial depolarization and subsequent externalization of CL to the organelle's outer membrane, whereas an R726A mutant of the protein was not translocated. Furthermore, only wild-type p210 BCR-ABL, but not the R726A mutant, suppressed CCCP-induced mitophagy and subsequently enhanced reactive oxygen species production. Thus, p210 BCR-ABL can change its intracellular localization via interactions between the PH domain and CL to cope with mitochondrial damage. This suggests that p210 BCR-ABL could have beneficial effects for cancer proliferation, providing new insight into the PH domain's contribution to CML pathogenesis.

Molecular techniques for the personalised management of patients with chronic myeloid leukaemia

Chronic myeloid leukemia (CML) is the paradigm for targeted cancer therapy. RT-qPCR is the gold standard for monitoring response to tyrosine kinase-inhibitor (TKI) therapy based on the reduction of blood or bone marrow BCR-ABL1. Some patients with CML and very low or undetectable levels of BCR-ABL1 transcripts can stop TKI-therapy without CML recurrence. However, about 60 percent of patients discontinuing TKI-therapy have rapid leukaemia recurrence. This has increased the need for more sensitive and specific techniques to measure residual CML cells. The clinical challenge is to determine when it is safe to stop TKI-therapy. In this review we describe and critically evaluate the current state of CML clinical management, different technologies used to monitor measurable residual disease (MRD) focus on comparingRT-qPCR and new methods entering clinical practice. We discuss advantages and disadvantages of new methods.

Hematopoietic cell kinase (HCK) as a therapeutic target in immune and cancer cells

The hematopoietic cell kinase (HCK) is a member of the SRC family of cytoplasmic tyrosine kinases (SFKs), and is expressed in cells of the myeloid and B-lymphocyte cell lineages. Excessive HCK activation is associated with several types of leukemia and enhances cell proliferation and survival by physical association with oncogenic fusion proteins, and with functional interactions with receptor tyrosine kinases. Elevated HCK activity is also observed in many solid malignancies, including breast and colon cancer, and correlates with decreased patient survival rates. HCK enhances the secretion of growth factors and pro-inflammatory cytokines from myeloid cells, and promotes macrophage polarization towards a wound healing and tumor-promoting alternatively activated phenotype. Within tumor associated macrophages, HCK stimulates the formation of podosomes that facilitate extracellular matrix degradation, which enhance immune and epithelial cell invasion. By virtue of functional cooperation between HCK and bona fide oncogenic tyrosine kinases, excessive HCK activation can also reduce drug efficacy and contribute to chemo-resistance, while genetic ablation of HCK results in minimal physiological consequences in healthy mice. Given its known crystal structure, HCK therefore provides an attractive therapeutic target to both, directly inhibit the growth of cancer cells, and indirectly curb the source of tumor-promoting changes in the tumor microenvironment.

Galectin-3 mediates bone marrow microenvironment-induced drug resistance in acute leukemia cells via Wnt/β-catenin signaling pathway

BACKGROUND

Acute leukemia is currently the major cause of death in hematological malignancies. Despite the rapid development of new therapies, minimal residual disease (MRD) continues to occur and leads to poor outcomes. The leukemia niche in the bone marrow microenvironment (BMM) is thought to be responsible for such MRD development, which can lead to leukemia drug resistance and disease relapse. Consequently further investigation into the way in which the leukemia niche interacts with acute leukemia cells (ALCs) and development of strategies to block the underlying process are expected to improve disease prognosis. Recent studies indicated that galectin-3 (gal-3) might play a pivotal role in this process. Thus we aimed to elucidate the exact role played by gal-3 in this process and clarify its mechanism of action.

METHODS

We used human bone marrow-derived mesenchymal stromal cells (hBM-MSCs) to mimic the leukemia BMM in vitro, and investigated their effects on drug resistance of ALCs and the possible mechanisms involved, with particular emphasis on the role of gal-3.

RESULTS

In our study, we demonstrated that hBM-MSCs induced gal-3 up-regulation, promoting β-catenin stabilization and thus activating the Wnt/β-catenin signaling pathway in ALCs, which is critical in cytotoxic drug resistance of leukemia. This effect could be reversed by addition of gal-3 short hairpin RNA (shRNA). We also found that up-regulation of gal-3 promoted Akt and glycogen synthase kinase (GSK)-3β phosphorylation, thought to constitute a cross-bridge between gal-3 and Wnt signaling.

CONCLUSIONS

Our results suggest that gal-3, a key factor mediating BMM-induced drug resistance, could be a novel therapeutic target in acute leukemia.

High frequency of BTG1 deletions in patients with BCR-ABL1-positive acute leukemia

Deletions affecting the B-cell translocation gene 1 (BTG1) have recently been reported in 9% of patients with B-cell precursor acute lymphoblastic leukemia (BCP-ALL), and occur even more frequently in ETV6-RUNX1-positive and BCR-ABL1-positive subgroups. To investigate whether the BTG1 deletions occur in other BCR-ABL1-positive acute leukemias besides BCP-ALL, we analyzed 44 leukemia cases harboring the BCR-ABL1 transcript [32 BCP-ALL, six mixed-phenotype acute leukemia (MPAL), and six chronic myeloid leukemia in B-lineage blast crisis (CML-BC)] by array-based comparative genomic hybridization and reverse transcription-PCR. BTG1 deletions were present in 31.8% of BCR-ABL1-positive acute leukemia patients, including 31.3% of BCP-ALL (10/32), 33.3% of MPAL (2/6), and 33.3% of CML-BC (B-lineage) (2/6) patients. Of note, the intragenic deletion breakpoints, mapping to 5 different positions at the proximal end of the breakpoint, clustered tightly within exon 2 of BTG1, which were located within a stretch of 20 bp from nucleotide 284 to nucleotide 304 and led to truncated BTG1 transcripts. There were no significant differences in the median white blood cell count, hemoglobin concentration, platelet count, bone marrow blast count, sex, age, or overall complete remission rate between patients with and without BTG1 deletions. Taken together, our data suggest that BTG1 deletions might play a role in leukemogenesis of BCP-ALL as well as of BCR-ABL1-positive MPAL and CML-BC (B-lineage).

Clinical targeting of mutated and wild-type protein tyrosine kinases in cancer

Clinical therapies for cancer have evolved from toxic, nontargeted agents to manageable, highly targeted therapies. Protein tyrosine kinases are a family of signaling molecules implicated in nearly every cancer type and are the foundation for the development of modern targeted agents. Recent genomic analyses have identified activating mutations, translocations, and amplifications of tyrosine kinases. Selective targeting of these genetically altered tyrosine kinases has resulted in significant clinical advances, including increased patient survival. This indicates that altered protein tyrosine kinases are the main drivers of many different cancers. However, lost during analyses of genetic lesions are the contributions of activated, wild-type kinases on tumor-dependent pathways. New approaches in phosphoproteomic technologies have identified several wild-type tyrosine kinase activation states, suggesting that non-genetically altered kinases can be essential "nodes" for signal transduction. Here, we summarize the evidence supporting the common mechanisms of protein tyrosine kinase activation in cancer and provide a personal perspective on the kinases BCR-ABL and BTK, as well as nonmutated kinase targets in prostate cancer, through our work. We outline the mechanisms of tyrosine kinase activation in the absence of direct mutation and discuss whether non-genetically altered tyrosine kinases or their associated downstream signaling pathways can be effectively targeted.

Combating the epigenome: epigenetic drugs against non-Hodgkin's lymphoma

Non-Hodgkin's lymphomas (NHLs) comprise a large and diverse group of neoplasms of lymphocyte origin with heterogeneous molecular features and clinical manifestations. Current therapies are based on standard chemotherapy, immunotherapy, radiation or stem cell transplantation. The discovery of recurrent mutations in epigenetic enzymes, such as chromatin modifiers and DNA methyltransferases, has provided researchers with a rationale to develop novel inhibitors targeting these enzymes. Several clinical and preclinical studies have demonstrated the efficacy of epigenetic drugs in NHL therapy and a few specific inhibitors have already been approved for clinical use. Here, we provide an overview of current NHL classification and a review of the present literature describing epigenetic alterations in NHL, including a summary of different epigenetic drugs, and their use in preclinical and clinical studies.

Bcr-Abl tyrosine kinase inhibitors- current status

Bcr-Abl plays a central role in the development of chromosome positive leukaemia. Chronic Myeloid leukaemia occurs due to increase proliferation and resistance to apoptosis by Bcr-Abl positive cells. Imatinib (STI571) is the first drug in the family of Bcr-Abl tyrosine kinase inhibitors while Nilotinib (AMN107) and Dasatinib (BMS-345825) are second generation drugs that are intended to have less resistance and intolerance than imatinib. Ponatinib (AP24534) an orally active Bcr-Abl Tyrosine Kinase Inhibitor and Bafetinib (INNO-406) have efficacy against various point mutations in the Bcr-Abl kinase. 1, 3, 4 thiadiazole derivatives has also displayed moderate inhibitory action on both Abl and Src kinase family. However there are varieties of Bcr-Abl inhibitors but Nilotinib is still the frontline tyrosine kinase inhibitors.

p210bcr-abl induces amoeboid motility by recruiting ADF/destrin through RhoA/ROCK1

We previously demonstrated that the Bcr-Abl oncogene, p210(bcr-abl), through its unique GEF domain, specifically activates RhoA and induces spontaneous amoeboid motility. We intend to study the pathways downstream RhoA controlling amoeboid motility. Mouse prolymphoblastic cells (Ba/F3 cell line) expressing different forms of Bcr-Abl were embedded in 3-dimensional (3D) Matrigel to study motility and explore the effects of inhibiting Rho pathway (inhibitors and siRNAs). The phosphorylation levels of cofilin-1 and destrin were analyzed by 2-dimensional electrophoresis. Composition of Bcr-Abl signalplex in different conditions was determined by coimmunoprecipitation. Ba/F3p190 and Ba/F3 expressing a mutant form of p210(bcr-abl) (unable to activate RhoA) cells presented a spontaneous motility, but not an amoeboid type. p210(bcr-abl)-induced amoeboid motility in a 3D matrix requires isoform-specific RhoA/ROCK-1/destrin signaling. Next to the conventional Rho/ROCK/MLC/myosin pathway, this pathway is a crucial determinant for amoeboid motility, specific for the destrin isoform (and not its coexpressed homologue cofilin-1). Also, the presence of destrin (and not cofilin-1) in the p210(bcr-abl) complex is dependent on ROCK1, and this signalplex is required for amoeboid motility. This underscores isoform-specific function within the ADF/cofilin family and provides new insight into Bcr-Abl signaling to amoeboid motility and possible impact on understanding chronic myeloid leukemia progression.

The PI3K/PKB signaling module as key regulator of hematopoiesis: implications for therapeutic strategies in leukemia

An important mediator of cytokine signaling implicated in regulation of hematopoiesis is the PI3K/protein kinase B (PKB/c-Akt) signaling module. Constitutive activation of this signaling module has been observed in a large group of leukemias. Because activation of this signaling pathway has been demonstrated to be sufficient to induce hematologic malignancies and is thought to correlate with poor prognosis and enhanced drug resistance, it is considered to be a promising target for therapy. A high number of pharmacologic inhibitors directed against either individual or multiple components of this pathway have already been developed to improve therapy. In this review, the safety and efficacy of both single and dual-specificity inhibitors will be discussed as well as the potential of combination therapy with either inhibitors directed against other signal transduction molecules or classic chemotherapy.

Regulation of hTERT by BCR-ABL at multiple levels in K562 cells

BACKGROUND

The cytogenetic characteristic of Chronic Myeloid Leukemia (CML) is the formation of the Philadelphia chromosome gene product, BCR-ABL. Given that BCR-ABL is the specific target of Gleevec in CML treatment, we investigated the regulation of the catalytic component of telomerase, hTERT, by BCR-ABL at multiple levels in K562 cells.

METHODS

Molecular techniques such as over expression, knockdown, real-time PCR, immunoprecipitation, western blotting, reporter assay, confocal microscopy, telomerase assays and microarray were used to suggest that hTERT expression and activity is modulated by BCR-ABL at multiple levels.

RESULTS

Our results suggest that BCR-ABL plays an important role in regulating hTERT in K562 (BCR-ABL positive human leukemia) cells. When Gleevec inhibited the tyrosine kinase activity of BCR-ABL, phosphorylation of hTERT was downregulated, therefore suggesting a positive correlation between BCR-ABL and hTERT. Gleevec treatment inhibited hTERT at mRNA level and significantly reduced telomerase activity (TA) in K562 cells, but not in HL60 or Jurkat cells (BCR-ABL negative cells). We also demonstrated that the transcription factor STAT5a plays a critical role in hTERT gene regulation in K562 cells. Knockdown of STAT5a, but not STAT5b, resulted in a marked downregulation of hTERT mRNA level, TA and hTERT protein level in K562 cells. Furthermore, translocation of hTERT from nucleoli to nucleoplasm was observed in K562 cells induced by Gleevec.

CONCLUSIONS

Our data reveal that BCR-ABL can regulate TA at multiple levels, including transcription, post-translational level, and proper localization. Thus, suppression of cell growth and induction of apoptosis by Gleevec treatment may be partially due to TA inhibition. Additionally, we have identified STAT5a as critical mediator of the hTERT gene expression in BCR-ABL positive CML cells, suggesting that targeting STAT5a may be a promising therapeutic strategy for BCR-ABL positive CML patients.

c-Myb and its target Bmi1 are required for p190BCR/ABL leukemogenesis in mouse and human cells

Expression of c-Myb is required for normal hematopoiesis and for proliferation of myeloid leukemia blasts and a subset of T cell leukemia but its role in B-cell leukemogenesis is unknown.

We tested the role of c-Myb in p190^BCR/ABL-dependent B-cell leukemia in mice transplanted with p190^BCR/ABL-transduced marrow cells with a c-Myb allele (Myb^f/d) and in double transgenic p190^BCR/ABL/Myb^w/d mice. In both models, loss of a c-Myb allele caused a less aggressive B-cell leukemia.

In p190^BCR/ABL expressing human B-cell leukemia lines, knockdown of c-Myb expression suppressed proliferation and colony formation.

Compared to c-Myb^w/f cells, expression of Bmi1, a regulator of stem cell proliferation and maintenance, was decreased in pre-B cells from Myb^w/d p190^BCR/ABL transgenic mice. Ectopic expression of a mutant c-Myb or Bmi1 enhanced the proliferation and colony formation of Myb^w/d p190^BCR/ABL B-cells; by contrast, Bmi1 downregulation inhibited colony formation of p190^BCR/ABL-expressing murine B cells and human B-cell leukemia lines. Moreover, c-Myb interacted with a segment of the human Bmi1 promoter and enhanced its activity.

In blasts from nineteen Ph¹ adult ALL patients, levels of c-Myb and Bmi1 showed a positive correlation. Together, these findings support the existence of a c-Myb-Bmi1 transcription regulatory pathway required for p190^BCR/ABL leukemogenesis.

Tyrosine phosphorylation enhances RAD52-mediated annealing by modulating its DNA binding

The DNA recombination mediator and annealing factor RAD52 is a target of c-ABL activated in response to DNA damage. Engineering of recombinant tyrosine-phosphomimetic RAD52 facilitated studying the consequences of this phosphorylation.

RAD52 protein has an important role in homology-directed DNA repair by mediating RAD51 nucleoprotein filament formation on single-stranded DNA (ssDNA) protected by replication protein-A (RPA) and annealing of RPA-coated ssDNA. In human, cellular response to DNA damage includes phosphorylation of RAD52 by c-ABL kinase at tyrosine 104. To address how this phosphorylation modulates RAD52 function, we used an amber suppressor technology to substitute tyrosine 104 with chemically stable phosphotyrosine analogue (p-Carboxymethyl-L-phenylalanine, pCMF). The RAD52^Y104pCMF retained ssDNA-binding activity characteristic of unmodified RAD52 but showed lower affinity for double-stranded DNA (dsDNA) binding. Single-molecule analyses revealed that RAD52^Y104pCMF specifically targets and wraps ssDNA. While RAD52^Y104pCMF is confined to ssDNA region, unmodified RAD52 readily diffuses into dsDNA region. The Y104pCMF substitution also increased the ssDNA annealing rate and allowed overcoming the inhibitory effect of dsDNA. We propose that phosphorylation at Y104 enhances ssDNA annealing activity of RAD52 by attenuating dsDNA binding. Implications of phosphorylation-mediated activation of RAD52 annealing activity are discussed.

Sphingosine kinase-1 and sphingosine 1-phosphate receptor 2 mediate Bcr-Abl1 stability and drug resistance by modulation of protein phosphatase 2A

The mechanisms by which sphingosine kinase-1 (SK-1)/sphingosine 1-phosphate (S1P) activation contributes to imatinib resistance in chronic myeloid leukemia (CML) are unknown. We show herein that increased SK-1/S1P enhances Bcr-Abl1 protein stability, through inhibition of its proteasomal degradation in imatinib-resistant K562/IMA-3 and LAMA-4/IMA human CML cells. In fact, Bcr-Abl1 stability was enhanced by ectopic SK-1 expression. Conversely, siRNA-mediated SK-1 knockdown in K562/IMA-3 cells, or its genetic loss in SK-1(-/-) MEFs, significantly reduced Bcr-Abl1 stability. Regulation of Bcr-Abl1 by SK-1/S1P was dependent on S1P receptor 2 (S1P2) signaling, which prevented Bcr-Abl1 dephosphorylation, and degradation via inhibition of PP2A. Molecular or pharmacologic interference with SK-1/S1P2 restored PP2A-dependent Bcr-Abl1 dephosphorylation, and enhanced imatinib- or nilotinib-induced growth inhibition in primary CD34(+) mononuclear cells obtained from chronic phase and blast crisis CML patients, K562/IMA-3 or LAMA4/IMA cells, and 32Dcl3 murine progenitor cells, expressing the wild-type or mutant (Y253H or T315I) Bcr-Abl1 in situ. Accordingly, impaired SK-1/S1P2 signaling enhanced the growth-inhibitory effects of nilotinib against 32D/T315I-Bcr-Abl1-derived mouse allografts. Since SK-1/S1P/S1P2 signaling regulates Bcr-Abl1 stability via modulation of PP2A, inhibition of SK-1/S1P2 axis represents a novel approach to target wild-type- or mutant-Bcr-Abl1 thereby overcoming drug resistance.

ABL1 fusion genes in hematological malignancies: a review

Chromosomal rearrangements involving the ABL1 gene, leading to a BCR-ABL1 fusion gene, have been mainly associated with chronic myeloid leukemia and B-cell acute lymphoblastic leukemia (ALL). At present, six other genes have been shown to fuse to ABL1. The kinase domain of ABL1 is retained in all chimeric proteins that are also composed of the N-terminal part of the partner protein that often includes a coiled-coil or a helix-loop-helix domain. These latter domains allow oligomerization of the protein that is required for tyrosine kinase activation, cytoskeletal localization, and neoplastic transformation. Fusion genes that have a break in intron 1 or 2 (BCR-ABL1, ETV6-ABL1, ZMIZ1-ABL1, EML1-ABL1, and NUP214-ABL1) have transforming activity, although NUP214-ABL1 requires amplification to be efficient. The NUP214-ABL1 gene is the second most prevalent fusion gene involving ABL1 in malignant hemopathies, with a frequency of 5% in T-cell ALL. Both fusion genes (SFPQ-ABL1 and RCSD1-ABL1) characterized by a break in intron 4 of ABL1 are associated with B-cell ALL, as the chimeric proteins lacked the SH2 domain of ABL1. Screening for ABL1 chimeric genes could be performed in patients with ALL, more particularly in those with T-cell ALL because ABL1 modulates T-cell development and plays a role in cytoskeletal remodeling processes in T cells.

Treatment of Philadelphia-chromosome-positive acute lymphoblastic leukemia with imatinib in combination with chemotherapy

The presence of the Philadelphia chromosome (Ph) is associated with a very poor prognosis in acute lymphoblastic leukemia (ALL). Although hematologic complete remission (CR) is achieved in 50% to 80% of adult patients by intensive chemotherapy in multicenter studies, long-term outcome is dismal, with overall survival of approximately 10%. Currently, allogeneic hematopoietic stem cell transplantation (allo-SCT) is thought to be the only curative therapeutic modality for this leukemia in adults, but the long-term survival rates are about 40% or less, far from satisfactory. Imatinib mesylate, a recently introduced specific tyrosine kinase inhibitor of BCR-ABL, in combination with chemotherapy, resulted in more than 90% hematologic CR in adult Ph-positive ALL, including molecular CR in more than 50% of patients. The higher CR rate and less frequent relapse gave more patients a chance to receive SCT. Patients who did not qualify for allo-SCT because of the lack of a suitable donor, advanced age, or underlying medical conditions apparently showed better survival than historical control patients treated with chemotherapy alone. Although longer follow-up is required to determine the effect on survival, imatinib in combination with chemotherapy clearly has a major potential to improve the treatment of Ph-positive ALL and may cure a substantial proportion of patients without SCT.

Novel molecular and cellular therapeutic targets in acute lymphoblastic leukemia and lymphoproliferative disease

While the outcome for pediatric patients with lymphoproliferative disorders (LPD) or lymphoid malignancies, such as acute lymphoblastic leukemia (ALL), has improved dramatically, patients often suffer from therapeutic sequelae. Additionally, despite intensified treatment, the prognosis remains dismal for patients with refractory or relapsed disease. Thus, novel biologically targeted treatment approaches are needed. These targets can be identified by understanding how a loss of lymphocyte homeostasis can result in LPD or ALL. Herein, we review potential molecular and cellular therapeutic strategies that (i) target key signaling networks (e.g., PI3K/AKT/mTOR, JAK/STAT, Notch1, and SRC kinase family-containing pathways) which regulate lymphocyte growth, survival, and function; (ii) block the interaction of ALL cells with stromal cells or lymphoid growth factors secreted by the bone marrow microenvironment; or (iii) stimulate innate and adaptive immune responses.

Recent advances in the treatment of Philadelphia chromosome-positive acute lymphoblastic leukemia

The advent of imatinib, a selective inhibitor of the ABL tyrosine kinase, has revolutionized the treatment of Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL). Combined with chemotherapy, imatinib exerts remarkable efficacy in patients with newly diagnosed disease with a complete remission (CR) rate of 95% and a survival rate of 55% at 3 years. Profound eradication of leukemia cells not only provides patients with a better chance for receiving allogeneic hematopoietic stem cell transplantation during first CR but also contributes to durable CR even without transplantation. Despite such improvement, however, relapse does occur, mainly owing to acquisition of resistance. Growing comprehension of the molecular mechanisms of resistance to imatinib has led to the development of novel BCR-ABL inhibitors that yield higher affinity for BCR-ABL and/or potent inhibitory activity against other target molecules such as SRC family kinases. The second-generation ABL kinase inhibitors, namely dasatinib and nilotinib, are already showing clinical activity in patients with imatinib-resistant Ph+ ALL, and other novel agents are undergoing preclinical and early clinical evaluation. Further improvement in treatment results will be achieved by identifying each patient's disease profile based on information obtained before and during treatment and by optimizing subsequent treatment accordingly.

Allogeneic hematopoietic cell transplantation for adult Philadelphia-positive acute lymphoblastic leukemia in the era of tyrosine kinase inhibitors

Allogeneic hematopoietic cell transplantation in first complete remission (CR1) is considered the standard of care, and the only established therapy that offers a possibility of cure for patients with Philadelphia-positive acute lymphoblastic leukemia (Ph+ ALL). Unfortunately, a number of patients, with suitable HLA-matched donors, are unable to receive an allograft because they fail to respond, or relapse shortly after induction chemotherapy. Incorporating imatinib during the induction/consolidation phase is facilitating a higher number of potentially curative allografts by improving both remission rates and/or the durability of responses in patients with Ph+ ALL. Imatinib and other tyrosine kinase inhibitors are also improving outcomes in elderly patients with Ph+ ALL, ineligible for allografting, when combined with glucocorticoids, and/or conventional chemotherapy. The addition of imatinib or other tyrosine kinase inhibitors to the therapeutic armamentarium of Ph+ ALL is reshaping the treatment algorithm and improving prognosis of this dreadful disease.

Differential motility of p190bcr-abl- and p210bcr-abl-expressing cells: respective roles of Vav and Bcr-Abl GEFs

The chimeric oncogene Bcr-Abl is known to induce autonomous motility of leukemic cells. We show here that p210(bcr-abl) responsible for chronic myelogenous leukemia induces an amoeboid type of motility while p190(bcr-abl), associated with acute lymphoid leukemia, induces a rolling type of motility. We previously reported that p210(bcr-abl) activates RhoA and Rac1, while p190(bcr-abl) although devoid of a Dbl-homology (DH) domain activates Rac1, but not RhoA. We investigated the regulation of GDP/GTP exchange factor (GEF) activities in the Bcr-Abl complex. For that purpose, different GEF activity mutants of Vav and of Bcr-Abl were constructed and stably transfected in Ba/F3 cells. Using these mutants, we demonstrate that RhoA is exclusively activated by the DH domain of p210(bcr-abl), while Rac1 activation is mostly due to Vav. Inhibition of Rac1 by Vav GEF mutant leads to immobilization of cells. Vav depletion using shRNA also induces immobilization of cells and suppression of GTP-bound Rac1. RhoA inactivation induces the specific loss of amoeboid movements. These results suggest that Rac1 activation by Vav triggers the motility of Bcr-Abl-expressing Ba/F3 cells, while the specific amoeboid mode of motility induced by p210(bcr-abl) is a consequence of RhoA activation.

Imatinib combined chemotherapy for Philadelphia chromosome-positive acute lymphoblastic leukemia: major challenges in current practice

The prognosis of Philadelphia chromosome-positive (Ph+) and/or BCR-ABL-positive acute lymphoblastic leukemia (ALL) is extremely poor, and for decades allogeneic hematopoietic stem cell transplantation (HSCT) has been considered the only option for a cure. However, the treatment for Ph+ ALL has been rapidly changing since imatinib, a selective inhibitor of the ABL tyrosine kinase, was introduced. Earlier clinical trials in which a moderate anti-leukemic effect of imatinib monotherapy was demonstrated have prompted investigators to explore the combination of imatinib and chemotherapy. The results of multiple studies indicate that chemotherapy combined with imatinib is well tolerated, induces complete hematological remission in almost every patient with newly diagnosed Ph+ ALL, and molecular remission in more than half of the cases. Future clinical studies need to focus on how imatinib can be incorporated into chemotherapy more effectively by determining the optimal dosage of imatinib, the optimal combinational schedule, and the role of allogeneic HSCT.

Fusion tyrosine kinases: a result and cause of genomic instability

Reciprocal chromosomal translocations may arise as a result of unfaithful repair of spontaneous DNA double-strand breaks, most probably induced by oxidative stress, radiation, genotoxic chemicals and/or replication stress. Genes encoding tyrosine kinases are targeted by these mechanisms resulting in the generation of chimera genes encoding fusion tyrosine kinases (FTKs). FTKs display transforming activity owing to their constitutive kinase activity causing deregulated proliferation, apoptosis, differentiation and adhesion. Moreover, FTKs are able to facilitate DNA repair, prolong activation of G(2)/M and S cell cycle checkpoints, and elevate expression of antiapoptotic protein Bcl-X(L), making malignant cells less responsive to antitumor treatment. FTKs may also stimulate the generation of reactive oxygen species and enhance spontaneous DNA damage in tumor cells. Unfortunately, FTKs compromise the fidelity of DNA repair mechanisms, which contribute to the accumulation of additional genetic abnormalities leading to the resistance to inhibitors such as imatinib mesylate and malignant progression of the disease.

High complete remission rate and promising outcome by combination of imatinib and chemotherapy for newly diagnosed BCR-ABL-positive acute lymphoblastic leukemia: a phase II study by the Japan Adult Leukemia Study Group

PURPOSE

A novel therapeutic approach is urgently needed for BCR-ABL-positive acute lymphoblastic leukemia (ALL). In this study, we assessed the efficacy and feasibility of chemotherapy combined with imatinib.

PATIENTS AND METHODS

A phase II study of imatinib-combined chemotherapy was conducted for newly diagnosed BCR-ABL-positive ALL in adults. Eighty patients were entered into the trial between September 2002 and January 2005.

RESULTS

Remission induction therapy resulted in complete remission (CR) in 77 patients (96.2%), resistant disease in one patient, and early death in two patients, as well as polymerase chain reaction negativity of bone marrow in 71.3%. The profile and incidence of severe toxicity were not different from those associated with our historic chemotherapy-alone regimen. Relapse occurred in 20 patients after median CR duration of 5.2 months. Allogeneic hematopoietic stem-cell transplantation (HSCT) was performed for 49 patients, 39 of whom underwent transplantation during their first CR. The 1-year event-free and overall survival (OS) rates were estimated to be 60.0%, and 76.1%, respectively, which were significantly better than those for our historic controls treated with chemotherapy alone (P < .0001 for both). Among the current trial patients, the probability for OS at 1 year was 73.3% for those who underwent allogeneic HSCT, and 84.8% for those who did not.

CONCLUSION

Our results demonstrated that imatinib-combined regimen is effective and feasible for newly diagnosed BCR-ABL-positive ALL. Despite a relatively short period of observation, a major potential of this treatment is recognized. Longer follow-up is required to determine its overall effect on survival.

NS-187, a potent and selective dual Bcr-Abl/Lyn tyrosine kinase inhibitor, is a novel agent for imatinib-resistant leukemia

Although the Abelson (Abl) tyrosine kinase inhibitor imatinib mesylate has improved the treatment of breakpoint cluster region–Abl (Bcr-Abl)–positive leukemia, resistance is often reported in patients with advanced-stage disease. Although several Src inhibitors are more effective than imatinib and simultaneously inhibit Lyn, whose overexpression is associated with imatinib resistance, these inhibitors are less specific than imatinib. We have identified a specific dual Abl-Lyn inhibitor, NS-187 (elsewhere described as CNS-9), which is 25 to 55 times more potent than imatinib in vitro. NS-187 is also at least 10 times as effective as imatinib in suppressing the growth of Bcr-Abl–bearing tumors and markedly extends the survival of mice bearing such tumors. The inhibitory effect of NS-187 extends to 12 of 13 Bcr-Abl proteins with mutations in their kinase domain but not to T315I. NS-187 also inhibits Lyn without affecting the phosphorylation of Src, Blk, or Yes. These results suggest that NS-187 may be a potentially valuable novel agent to combat imatinib-resistant Philadelphia-positive (Ph+) leukemia.

BCR/ABL modifies the kinetics and fidelity of DNA double-strand breaks repair in hematopoietic cells

The oncogenic BCR/ABL tyrosine kinase facilitates the repair of DNA double-strand breaks (DSBs). We find that after gamma-irradiation BCR/ABL-positive leukemia cells accumulate more DSBs in comparison to normal cells. These lesions are efficiently repaired in a time-dependent fashion by BCR/ABL-stimulated non-homologous end-joining (NHEJ) followed by homologous recombination repair (HRR) mechanisms. However, mutations and large deletions were detected in HRR and NHEJ products, respectively, in BCR/ABL-positive leukemia cells. We propose that unfaithful repair of DSBs may contribute to genomic instability in the Philadelphia chromosome-positive leukemias.

Rho GTPases in hematopoietic cells

The ubiquitous Rho GTPases are instrumental in the organization of the actin cytoskeleton, but also for the control of gene expression. Here we review the role of the major members of this family, i.e., RhoA, Rac1, Rac2, and Cdc42, and their intracellular signaling in hematopoietic cells. Although these proteins have been classically implicated in chemotaxis, there are now clear indications on how differential signaling toward other, more specific functions, such as phagocytosis or the production of reactive oxygen species, is regulated by relatively small differences in primary sequence. The identification of mutations in these GTPases or their regulators has provided novel insights in their function as well as their relevance for the development of hematological diseases.

Biological impediments to monoclonal antibody–based cancer immunotherapy

The ability of antibodies to exploit antigenic differences between normal and malignant tissues and to exact a variety of antitumor responses offers significant advantages to conventional forms of therapy. Several monoclonal antibodies (mAb) have already proved to be relatively well tolerated and effective for the treatment of many different malignant diseases. However, mAbs must overcome substantial obstacles to reach antigens presented on target cells to be of therapeutic value. Intravenously administered antibodies must avoid host immune response and contend with low or heterogeneous expression of antigen on tumor cells. Antibodies must also overcome significant physical barriers en route to a solid tumor mass, including the vascular endothelium, stromal barriers, high interstitial pressure, and epithelial barriers. Here we review the application and evolution of mAbs as effective forms of treatment, with particular attention to the barriers and impediments to successful treatment and discuss strategies to overcome these barriers and improve the efficacy of mAb-based therapy.

Bcr (breakpoint cluster region) protein binds to PDZ-domains of scaffold protein PDZK1 and vesicle coat protein Mint3

The breakpoint cluster region protein (Bcr) is a large soluble oligomeric multidomain protein best known because of its involvement in chronic myelogenous leukemia (CML). A chromosomal translocation between its gene and that of the c-abl kinase (`Philadelphia chromosome') plays a major causative role in that malignancy. Thus most attention has been paid to the role of the protein in hemopoietic cells. However, Bcr is also expressed in other cell types including epithelia. Bcr is generally considered to be a cytoplasmic protein but in addition to its kinase and GTPase exchange and activating domains it contains potentially membrane-interacting pleckstrin homology and C2 domains as well as a PDZ-binding C terminus mediating an interaction with a PDZ-domain protein at intercellular junctions of epithelial cells. We have examined the ability of Bcr to interact with other epithelial PDZ proteins and found specific binding to both the apical PDZK1 protein and the Golgi-localized Mint3. The former is important in the organization of several apical functions and the latter in vesicular trafficking in the secretory pathway. Hence these findings extend the interactions and likely signaling impact of Bcr in epithelia from the cytosol to at least these two membrane compartments.

Effects of a conditionally active v-ErbB and an EGF-R inhibitor on transformation of NIH-3T3 cells and abrogation of cytokine dependency of hematopoietic cells

Epidermal growth factor (EGF) and its cognate receptor (EGF-R) are often dysregulated in human neoplasia. Moreover, EGF-R-transformed cell lines have constitutive EGF-R activity, which makes elucidation of its effects difficult to determine. In the following studies, the effects of a novel conditionally activated form of EGF-R, v-ErbB:ER, on the morphological transformation of NIH-3T3 cells and the abrogation of hematopoietic cell cytokine dependence were investigated. The v-ErbB ES-4 oncogene was fused to the hormone binding domain of the estrogen receptor (ER). This construct, v-ErbB:ER, requires beta-estradiol or 4-OH tamoxifen for activation. v-ErbB:ER conditionally transformed NIH-3T3 cells and abrogated cytokine dependence of hematopoietic cells. Stimulation of v-ErbB:ER activity resulted in the activation of the phosphatidylinositol 3-kinase (PI3K)/Akt and Raf/MEK/ERK kinase cascades. To determine the importance of these signal transduction pathways, the conditionally transformed hematopoietic cells were treated with EGF-R, PI3K and MEK inhibitors. The EGF-R inhibitor AG1478 effectively inhibited MEK, ERK and Akt activation, and induced apoptosis when the cells were grown in response to v-ErbB:ER. Apoptosis was observed at 100- to 1000-fold lower concentrations of AG1478 when the cells were grown in response to v-ErbB:ER as opposed to IL-3. Furthermore, the parental, BCR-ABL- and Raf-transformed cells were only susceptible to the apoptosis-inducing effects of AG1478 at the highest concentrations demonstrating the specificity of these inhibitors. MEK or PI3K inhibitors suppressed ERK or Akt activation, respectively, and induced apoptosis in the v-ErbB:ER-responsive cells. However, MEK and PI3K inhibitors only induced apoptosis at 1000-fold higher concentrations than the EGFR inhibitor. This novel v-ErbB:ER construct and these conditionally transformed cell lines will be useful to further elucidate ErbB-mediated signal transduction and to determine the effectiveness of various inhibitors in targeting different aspects of EGF-R-mediated signal transduction and malignant transformation.

BCR/ABL oncogenic kinase promotes unfaithful repair of the reactive oxygen species-dependent DNA double-strand breaks

The oncogenic BCR/ABL tyrosine kinase induces constitutive DNA damage in Philadelphia chromosome (Ph)-positive leukemia cells. We find that BCR/ABL-induced reactive oxygen species (ROSs) cause chronic oxidative DNA damage resulting in double-strand breaks (DSBs) in S and G(2)/M cell cycle phases. These lesions are repaired by BCR/ABL-stimulated homologous recombination repair (HRR) and nonhomologous end-joining (NHEJ) mechanisms. A high mutation rate is detected in HRR products in BCR/ABL-positive cells, but not in the normal counterparts. In addition, large deletions are found in NHEJ products exclusively in BCR/ABL cells. We propose that the following series of events may contribute to genomic instability of Ph-positive leukemias: BCR/ABL --> ROSs --> oxidative DNA damage --> DSBs in proliferating cells --> unfaithful HRR and NHEJ repair.

The BCR-ABL story: bench to bedside and back

The twenty-first century is beginning with a sharp turn in the field of cancer therapy. Molecular targeted therapies against specific oncogenic events are now possible. The BCR-ABL story represents a notable example of how research from the fields of cytogenetics, retroviral oncology, protein phosphorylation, and small molecule chemical inhibitors can lead to the development of a successful molecular targeted therapy. Imatinib mesylate (Gleevec, STI571, or CP57148B) is a direct inhibitor of ABL (ABL1), ARG (ABL2), KIT, and PDGFR tyrosine kinases. This drug has had a major impact on the treatment of chronic myelogenous leukemia (CML) as well as other blood neoplasias and solid tumors with etiologies based on activation of these tyrosine kinases. Analysis of CML patients resistant to BCR-ABL suppression by Imatinib mesylate coupled with the crystallographic structure of ABL complexed to this inhibitor have shown how structural mutations in ABL can circumvent an otherwise potent anticancer drug. The successes and limitations of Imatinib mesylate hold general lessons for the development of alternative molecular targeted therapies in oncology.

Chlorogenic acid inhibits Bcr-Abl tyrosine kinase and triggers p38 mitogen-activated protein kinase-dependent apoptosis in chronic myelogenous leukemic cells

We report that chlorogenic acid (Chl) induces apoptosis of several Bcr-Abl-positive chronic myelogenous leukemia (CML) cell lines and primary cells from CML patients in vitro and destroys Bcr-Abl-positive K562 cells in vivo. In contrast, this compound has no effect on the growth and viability of Bcr-Abl-negative lymphocytic and myeloid cell lines and primary CML cells. Sodium chlorogenate (NaChl) exhibits 2-fold higher efficiency in killing K562 cells compared with Chl. NaChl also induces growth inhibition of squamous cell carcinoma (HSC-2) and salivary gland tumor cells (HSG), although at 50-fold higher concentration. NaChl inhibits autophosphorylation of p210(Bcr-Abl) fusion protein rapidly. We demonstrate that p38 phosphorylation is increased in Bcr-Abl-positive cells after treatment with NaChl and closely paralleled the inhibition of Bcr-Abl phosphorylation. NaChl did not increase phosphorylation of p38 in Bcr-Abl-negative cells including HSC-2 and HSG that are responsive to this compound, indicating that p38 activation by NaChl is dependent on Bcr-Abl kinase inhibition. Inhibition of p38 activity by SB203580 significantly reduced NaChl-induced apoptosis of K562 cells, whereas activation of p38 by anisomycin augmented the apoptosis. These findings indicate that inhibition of Bcr-Abl kinase leading to activation of p38 mitogen-activated protein (MAP) kinase may play an important role in the anti-CML activity of Chl.

Split-signal FISH for detection of chromosome aberrations in acute lymphoblastic leukemia

Chromosome aberrations are frequently observed in precursor-B-acute lymphoblastic leukemias (ALL) and T-cell acute lymphoblastic leukemias (T-ALL). These translocations can form leukemia-specific chimeric fusion proteins or they can deregulate expression of an (onco)gene, resulting in aberrant expression or overexpression. Detection of chromosome aberrations is an important tool for risk classification. We developed rapid and sensitive split-signal fluorescent in situ hybridization (FISH) assays for six of the most frequent chromosome aberrations in precursor-B-ALL and T-ALL. The split-signal FISH approach uses two differentially labeled probes, located in one gene at opposite sites of the breakpoint region. Probe sets were developed for the genes TCF3 (E2A) at 19p13, MLL at 11q23, ETV6 at 12p13, BCR at 22q11, SIL-TAL1 at 1q32 and TLX3 (HOX11L2) at 5q35. In normal karyotypes, two colocalized green/red signals are visible, but a translocation results in a split of one of the colocalized signals. Split-signal FISH has three main advantages over the classical fusion-signal FISH approach, which uses two labeled probes located in two genes. First, the detection of a chromosome aberration is independent of the involved partner gene. Second, split-signal FISH allows the identification of the partner gene or chromosome region if metaphase spreads are present, and finally it reduces false-positivity.

Mechanisms involved in the induced differentiation of leukemia cells

Despite the remarkable progress achieved in the treatment of leukemias over the last several years, many problems (multidrug resistance [MDR], cellular heterogeneity, heterogeneous molecular abnormalities, karyotypic instability, and lack of selective action of antineoplastic agents) still remain. The recent progress in tumor molecular biology has revealed that leukemias are likely to arise from disruption of differentiation of early hematopoietic progenitors that fail to give birth to cell lineage restricted phenotypes. Evidence supporting such mechanisms has been derived from studying bone marrow leukemiogenesis and analyzing differentiation of leukemic cell lines in culture that serve as models of erythroleukemic (murine erythroleukemia [MEL] and human leukemia [K562] cells) and myeloid (human promyelocytic leukemia [HL-60] cells) cell maturation. This paper reviews the current concepts of differentiation, the chemical/pharmacological inducing agents developed thus far, and the mechanisms involved in initiation of leukemic cell differentiation. Emphasis was given on commitment and the cell lineage transcriptional factors as key regulators of terminal differentiation as well as on membrane-mediated events and signaling pathways involved in hematopoietic cell differentiation. The developmental program of MEL cells was presented in considerable depth. It is quite remarkable that the erythrocytic maturation of these cells is orchestrated into specific subprograms and gene expression patterns, suggesting that leukemic cell differentiation represents a highly coordinated set of events that lead to irreversible growth arrest and expression of cell lineage restricted phenotypes. In MEL and other leukemic cells, differentiation appears to be accompanied by differentiation-dependent apoptosis (DDA), an event that can be exploited chemotherapeutically. The mechanisms by which the chemical inducers promote differentiation of leukemic cells have been discussed.

Tyrosine kinase inhibitor as a therapeutic drug for chronic myelogenous leukemia and gastrointestinal stromal tumor

The Philadelphia chromosome found in leukemia cells of chronic myelogenous leukemia (CML) patients is produced by translocation between chromosomes 9 and 22, resulting in expression of a chimera protein of Bcr and Abl kinase in the cytoplasm. Bcr-Abl kinase attracted oncology researchers as a molecular target for CML therapy, and a variety of small Abl kinase inhibitors were synthesized. STI571 (imatinib mesylate) was produced by modification of 2-phenylaminopyrimidine, a core structure of protein kinase C inhibitor, to improve selectivity, stability, solubility, and bioavailability. STI571 competitively binds to the ATP binding site of Bcr-Abl kinase and inhibits Abl tyrosine kinase activity. STI571 showed significant efficacy in the clinical study with CML patients at all stages: chronic phase, accelerated phase, and blast crisis. More than 90% of the patients showed good hematologic response to STI571. STI571 is also a potent inhibitor of a receptor-type c-Kit tyrosine kinase. Therefore, STI571 was examined for therapeutic efficacy against malignant Gastro-Intestinal Stromal Tumors (GIST), which are mainly caused by aberrant expression of a mutated c-Kit that is constitutively active without binding of a ligand, stem cell factor (SCF). More than a half of the metastatic GIST patients enrolled in the clinical study responded to STI571. Thus, STI571 is now used as a therapeutic drug for both CML and GIST in more than 80 countries worldwide. Certain point mutations in the ATP binding site were found to be a cause of resistance to STI571 in both Bcr-Abl and c-Kit kinases. Therefore, it would be better to make a precise therapeutic strategy with STI571 based on the gene analysis data. It is also expected that it will be possible to design an inhibitor to overcome such resistance by using the structural information on the mutants.