Chronic myeloid leukaemia as a model of disease evolution in human cancer

Silencing of membrane-associated sialidase Neu3 diminishes apoptosis resistance and triggers megakaryocytic differentiation of chronic myeloid leukemic cells K562 through the increase of ganglioside GM3

In chronic myeloid leukemia K562 cells, differentiation is also blocked because of low levels of ganglioside GM3, derived by the high expression of sialidase Neu3 active on GM3. In this article, we studied the effects of Neu3 silencing (40-70% and 63-93% decrease in protein content and activity, respectively) in these cells. The effects were as follows: (a) gangliosides GM3, GM1, and sialosylnorhexaosylceramide increased markedly; (b) cell growth and [(3)H]thymidine incorporation diminished relevantly; (c) as mRNA, cyclin D2, and Myc were much less expressed, whereas cyclin D1 was expressed more like its inhibitor p21; (d) as mRNA, pro-apoptotic proteins Bax and Bad increased with concurrent decrease and increase in the anti-apoptotic proteins Bcl-2 and Bcl-XL, respectively; (e) the apoptosis inducers etoposide and staurosporine were active on Neu3 silencing cells but not on mock cells; (f) as mRNA, the megakaryocytic markers CD10, CD44, CD41, and CD61 increased similar to the case of mock cells stimulated with PMA; (g) the signaling cascades mediated by PLC-beta2, PKC, RAF, ERK1/2, RSK90, and JNK were largely activated. The induction of a GM3-rich ganglioside pattern in K562 cells by treatment with brefeldin A elicited a phenotype similar to that of Neu3 silencing cells. In conclusion, upon Neu3 silencing, K562 cells show a decrease in proliferation, propensity to undergo apoptosis, and megakaryocytic differentiation.

A sensitive array-based assay for identifying multiple TMPRSS2:ERG fusion gene variants

Studies of gene fusions in solid tumors are not as extensive as in hematological malignancies due to several technical and analytical problems associated with tumor heterogeneity. Nevertheless, there is a growing interest in the role of fusion genes in common epithelial tumors after the discovery of recurrent TMPRSS2:ETS fusions in prostate cancer. Among all of the reported fusion partners in the ETS gene family, TMPRSS2:ERG is the most prevalent one. Here, we present a simple and sensitive microarray-based assay that is able to simultaneously determine multiple fusion variants with a single RT-PCR in impure RNA specimens. The assay detected TMPRSS2:ERG fusion transcripts with a detection sensitivity of <10 cells in the presence of more than 3000 times excess normal RNA, and in primary prostate tumors having no >1% of cancer cells. The ability to detect multiple transcript variants in a single assay is critically dependent on both the primer and probe designs. The assay should facilitate clinical and basic studies for fusion gene screening in clinical specimens, as it can be readily adapted to include multiple gene loci.

Resistance to chemotherapy: new treatments and novel insights into an old problem

Resistance to cancer chemotherapeutic treatment is a common phenomenon, especially in progressive disease. The generation of cellular models of drug resistance has been pivotal in unravelling the main effectors of resistance to traditional chemotherapy at the molecular level (i.e. intracellular drug inactivation, detoxifying systems, defects in DNA repair, apoptosis evasion, membrane transporters and cell adhesion). The development of targeted therapies has also been followed by resistance, reminiscent of an evolutionary arms race, as exemplified by imatinib and other BCR-ABL inhibitors for the treatment of chronic myelogenous leukaemia. Although traditionally associated with the last stages of the disease, recent findings with minimally transformed pretumorigenic primary human cells indicate that the ability to generate drug resistance arises early during the tumorigenic process, before the full transformation. Novel technologies, such as genome profiling, have in certain cases predicted the outcome of chemotherapy and undoubtedly have tremendous potential for the future. In addition, the novel cancer stem cell paradigm raises the prospect of cell-targeted therapies instead of treatment directed against the whole tumour.

FISH mapping of Philadelphia negative BCR/ABL1 positive CML

Background

Chronic myeloid leukaemia (CML) is a haematopoietic stem cell disorder, almost always characterized by the presence of the Philadelphia chromosome (Ph), usually due to t(9;22)(q34;q11) or its variants. The Ph results in the formation of the BCR/ABL1 fusion gene, which is a constitutively activated tyrosine kinase. Around 1% of CML patients appear to have a Ph negative karyotype but carry a cryptic BCR/ABL1 fusion that can be located by fluorescence in situ hybridisation (FISH) at chromosome 22q11, 9q34 or a third chromosome. Here we present FISH mapping data of BCR and ABL1 flanking regions and associated chromosomal rearrangements in 9 Ph negative BCR/ABL1 positive CML patients plus the cell line CML-T1.

Results

BCR/ABL1 was located at 9q34 in 3 patients, 22q11 in 5 patients and CML-T1 and 22p11 in 1 patient. In 3 of 6 cases with the fusion at 22q11 a distal breakpoint cluster was found within a 280 Kb region containing the RAPGEF1 gene, while in another patient and the CML-T1 the distal breakpoint fell within a single BAC clone containing the 3' RXRA gene. Two cases had a duplication of the masked Ph while genomic deletions of the flanking regions were identified in 3 cases. Even more complex rearrangements were found in 3 further cases.

Conclusion

BCR/ABL1 formation resulted from a direct insertion (one step mechanism) in 6 patients and CML-T1, while in 3 patients the fusion gene originated from a sequence of rearrangements (multiple steps). The presence of different rearrangements of both 9q34 and 22q11 regions highlights the genetic heterogeneity of this subgroup of CML. Future studies should be performed to confirm the presence of true breakpoint hot spots and assess their implications in Ph negative BCR/ABL1 positive CML.

BCR-ABL in chronic myelogenous leukemia--how does it work?

The discovery of the BCR-ABL fusion gene on the Philadelphia (Ph) chromosome in 1985 was the start of a new era in understanding the molecular basis of hematologic malignancies. It provided the rationale for producing first imatinib and then a series of small molecules designed to inhibit the tyrosine kinase activity of the Bcr-Abl oncoprotein, all of which can induce complete cytogenetic remissions in the majority of patients with chronic myelogenous leukemia (CML) in the chronic phase. However, we still do not know for sure whether the BCR-ABL fusion gene is really the initiating lesion for the chronic phase of CML and we have an incomplete understanding of the so-called genomic instability that underlies the production of the fusion gene and predisposes the Ph-positive clone to acquire further genetic events that lead to advanced-phase disease. Moreover, it is clear that though some of the mutant Ph-positive subclones that develop in patients taking tyrosine kinase inhibitors (TKIs) are the direct cause of the resistance observed, in other cases, its cause is unclear. It is likely that in the next few years we will see (1) improved methods for predicting responses to TKIs, (2) the use of TKIs in combination with other effective molecules such as farnesyl transferase inhibitors, and (3) a gradual reduction in the proportion of chronic-phase patients resistant to therapy.

Emerging stem cell concepts for imatinib-resistant chronic myeloid leukaemia: implications for the biology, management, and therapy of the disease

Chronic myeloid leukaemia (CML) is a myeloid neoplasm defined by the BCR/ABL oncoprotein that is considered essential for leukaemogenesis and accumulation of neoplastic cells. The BCR/ABL kinase inhibitor imatinib is an effective agent in most patients and can now be regarded as front-line therapy. Hence, intrinsic and acquired resistance to imatinib has been described and is an emerging challenge in clinical practice. While CML stem cells display primary resistance, stem cell subclones may, in addition, acquire imatinib-resistant mutants of BCR/ABL. Other factors that are considered to contribute to stem cell resistance include the genetic background, clonal evolution, additional biological features of subclones, gene amplifications, silencing of tumour suppressor genes and specific pharmacological aspects. In this article, mechanisms of resistance of CML (stem) cells against imatinib and other BCR/ABL inhibitors are discussed, together with strategies to overcome and/or to prevent resistance with available drugs or novel anti-leukaemic approaches.

Deletions adjacent to BCR and ABL1 breakpoints occur in a substantial minority of chronic myeloid leukemia patients with masked Philadelphia rearrangements

Deletions at the t(9;22) breakpoint regions, found in 15% of chronic myeloid leukemia patients (CML) with an overt Philadelphia (Ph) translocation, are associated with an adverse disease prognosis in patients receiving interferon-alpha therapy. The incidence of deletions has been shown to vary for different cytogenetic subgroups of CML, with a significantly higher incidence of deletion in patients with a variant Ph translocation. To date, however, the frequency of such deletions in the subgroup of CML patients in whom the BCR/ABL1 fusion arises via submicroscopic chromosomal insertion (masked Ph) has not been investigated. We report the evaluation of 14 patients with masked Ph-positive CML for the presence of deletions extending 3' from BCR and 5' from ABL1 using two triple-color BCR/ABL probes. Deletions were identified in 3 patients (21%), encompassing sequences 5' to ABL1 in two of these and sequences 3' to BCR in the remaining patient, thus demonstrating that the phenomenon is a significant feature of the masked Ph CML subgroup. Furthermore, our findings are consistent with the notion that loss of genomic material is a potential side effect of any DNA breakage event at the 9q34.1 and 22q11.2 chromosomal regions, regardless of the subsequent mechanism of chromosomal rearrangement.

Abi1 gene silencing by short hairpin RNA impairs Bcr-Abl-induced cell adhesion and migration in vitro and leukemogenesis in vivo

Abl interactor (Abi) 1 was first identified as the downstream target of Abl tyrosine kinases and was found to be dysregulated in leukemic cells expressing oncogenic Bcr-Abl and v-Abl. Although the accumulating evidence supports a role of Abi1 in actin cytoskeleton remodeling and growth factor/receptor signaling, it is not clear how it contributes to Bcr-Abl-induced leukemogenesis. We show here that Abi1 gene silencing by short hairpin RNA attenuated the Bcr-Abl-induced abnormal actin remodeling, membrane-type 1 metalloproteinase clustering and inhibited cell adhesion and migration on fibronectin-coated surfaces. Although the knock down of Abi1 expression did not affect growth factor-independent growth of Bcr-Abl-transformed Ba/F3 cells in vitro, it impeded competitive expansion of these cells in non obese diabetic (NOD)/ severe combined immuno-deficiency (SCID) mice. Remarkably, the knock down of Abi1 expression in Bcr-Abl-transformed Ba/F3 cells impaired the leukemogenic potential of these cells in NOD/SCID mice. Abi1 contributes to Bcr-Abl-induced leukemogenesis in part through Src family kinases, as the knock down of Abi1 expression attenuates Bcr-Abl-stimulated activation of Lyn. Together, these data provide for the first time the direct evidence that supports a critical role of Abi1 pathway in the pathogenesis of Bcr-Abl-induced leukemia.

Experimental non-ATP-competitive therapies for chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) is a hematopoietic stem cell malignancy driven by the BCR-ABL fusion tyrosine kinase. The central role played by BCR-ABL1 in the pathogenesis of CML facilitated the development of the tyrosine kinase inhibitor (TKI) imatinib mesylate, the first actual targeted therapy in cancer history. Imatinib competes with ATP at the active site of BCR-ABL1 kinase. Despite outstanding clinical results, imatinib as well as other BCR-ABL1 TKIs have been associated with limited rates of complete molecular response and the development of mutations within the kinase domain of BCR-ABL1 that impairs TKI binding. To override such drawbacks, an array of novel non-ATP-competitive therapies with distinct mechanisms of action is undergoing preclinical, and in some cases, early clinical stages of development. This review focuses on the most promising among such therapeutics.

Getting to the stem of chronic myeloid leukaemia

Tyrosine kinase inhibitor (TKI) therapy for chronic myeloid leukaemia (CML) is the consummate success story for targeted therapy, yet relapse is a nearly inevitable consequence of cessation or interruption of therapy. Primitive TKI-refractory CML stem cells are the likely source of these relapses, as they provide sanctuary for the Philadelphia chromosome. In advanced disease, their progressively anaplastic progeny ultimately maintain CML independently of the CML haematopoietic stem cell (HSC). Interestingly, there are at least two distinct cell types capable of self-renewal in different phases of CML: first, a primitive HSC with BCR-ABL mutation, which maintains the more indolent chronic-phase disease and, second, a coexisting mutated progenitor cell which acquires stem cell characteristics responsible for rapid cell expansion in advanced disease.

Rac GTPases as key regulators of p210-BCR-ABL-dependent leukemogenesis

Chronic myelogenous leukemia (CML) is a malignant disease characterized by expression of p210-BCR-ABL, the product of the Philadelphia chromosome. Survival of CML patients has been significantly improved with the introduction of tyrosine kinase inhibitors that induce long-term hematologic remissions. However, mounting evidence indicates that the use of a single tyrosine kinase inhibitor does not cure this disease due to the persistence of p210-BCR-ABL at the molecular level or the acquired resistance in the stem cell compartment to individual inhibitors. We have recently shown in a murine model that deficiency of the Rho GTPases Rac1 and Rac2 significantly reduces p210-BCR-ABL-mediated proliferation in vitro and myeloproliferative disease in vivo, suggesting Rac as a potential therapeutic target in p210-BCR-ABL-induced disease. This target has been further validated using a first-generation Rac-specific small molecule inhibitor. In this review we describe the role of Rac GTPases in p210-BCR-ABL-induced leukemogenesis and explore the possibility of combinatorial therapies that include tyrosine kinase inhibitor(s) and Rac GTPase inhibitors in the treatment of CML.

Phenotypic and gene expression diversity of malignant cells in human blast crisis chronic myeloid leukemia

Chronic myeloid leukemia (CML) is considered as a paradigm of neoplasias developing through multistep track. It is believed that in the blast crisis (BC) terminal phase of the disease, blood-circulating blasts represent an expansion of a single CML clone. However, although these blasts grow mostly in suspension under standard culture conditions, a relatively small cell-fraction adheres to the plastic dish. Yet, it is unknown whether these two cell-fractions are distinct sub-populations that originated from a common CML clone and whether they have different biological and malignant properties. To address these questions, we have characterized the plastic-adherent and non-adherent sub-populations of various cell lines and primary cells derived from patients with CML in BC. This study indicated that the adherent-subsets retain repopulating ability with indications of increased malignant properties as greater anchorage-independent clonogenicity, impairment of cell-cell contact inhibition, loss of serum-dependent attenuation of plastic-adhesion, and a significant up-regulation of the oncogenes BCR-ABL, c-JUN, and c-FOS along with the adhesion-related genes KiSS-1, THBS3, and ITGB5. The adherent blasts stably retain their unique properties even after elimination of the adherence selection pressure. Sub-cloning analyses indicated that the adherent cells could be continuously evolved from any parental non-adherent clone in a unidirectional manner. This study provides new insights into the biology and the malignant evolution of CML, indicating that at the BC phase, circulating blasts are heterogeneous and consisting of at least two distinct populations of a common clonal origin. The existence of a minor "pool" of blasts of greater clonogenic capacity along with significantly higher expression level of BCR-ABL, individually or in conjunction with other cancer and adhesion-related genes, might also signify clonal evolution toward subsequent increased malignancy and lower therapeutic sensitivity.

Solution conformations and dynamics of ABL kinase-inhibitor complexes determined by NMR substantiate the different binding modes of imatinib/nilotinib and dasatinib

Current structural understanding of kinases is largely based on x-ray crystallographic studies, whereas very little data exist on the conformations and dynamics that kinases adopt in the solution state. ABL kinase is an important drug target in the treatment of chronic myelogenous leukemia. Here, we present the first characterization of ABL kinase in complex with three clinical inhibitors (imatinib, nilotinib, and dasatinib) by modern solution NMR techniques. Structural and dynamical results were derived from complete backbone resonance assignments, experimental residual dipolar couplings, and (15)N relaxation data. Residual dipolar coupling data on the imatinib and nilotinib complexes show that the activation loop adopts the inactive conformation, whereas the dasatinib complex preserves the active conformation, which does not support contrary predictions based upon molecular modeling. Nanosecond as well as microsecond dynamics can be detected for certain residues in the activation loop in the inactive and active conformation complexes.

BCR-ABL1 lymphoblastic leukaemia is characterized by the deletion of Ikaros

The Philadelphia chromosome, a chromosomal abnormality that encodes BCR-ABL1, is the defining lesion of chronic myelogenous leukaemia (CML) and a subset of acute lymphoblastic leukaemia (ALL). To define oncogenic lesions that cooperate with BCR-ABL1 to induce ALL, we performed a genome-wide analysis of diagnostic leukaemia samples from 304 individuals with ALL, including 43 BCR-ABL1 B-progenitor ALLs and 23 CML cases. IKZF1 (encoding the transcription factor Ikaros) was deleted in 83.7% of BCR-ABL1 ALL, but not in chronic-phase CML. Deletion of IKZF1 was also identified as an acquired lesion at the time of transformation of CML to ALL (lymphoid blast crisis). The IKZF1 deletions resulted in haploinsufficiency, expression of a dominant-negative Ikaros isoform, or the complete loss of Ikaros expression. Sequencing of IKZF1 deletion breakpoints suggested that aberrant RAG-mediated recombination is responsible for the deletions. These findings suggest that genetic lesions resulting in the loss of Ikaros function are an important event in the development of BCR-ABL1 ALL.

Sphingosine kinase-1 is a downstream regulator of imatinib-induced apoptosis in chronic myeloid leukemia cells

We examined the involvement of sphingosine kinase-1 (SphK1), which governs the ceramide/sphingosine-1-phosphate balance, in susceptibility to imatinib of either sensitive or resistant chronic myeloid leukemia cells. Imatinib-sensitive LAMA84-s displayed marked SphK1 inhibition coupled with increased content of ceramide and decreased pro-survival sphingosine-1-phosphate. Conversely, no changes in the sphingolipid metabolism were observed in LAMA84-r treated with imatinib. Overcoming imatinib resistance in LAMA84-r with farnesyltransferase or MEK/ERK inhibitors as well as with cytosine arabinoside led to SphK1 inhibition. Overexpression of SphK1 in LAMA84-s cells impaired apoptosis and inhibited the effects of imatinib on caspase-3 activation, cytochrome c and Smac release from mitochondria through modulation of Bim, Bcl-xL and Mcl-1 expression. Pharmacological inhibition of SphK1 with F-12509a or its silencing by siRNA induced apoptosis of both imatinib-sensitive and -resistant cells, suggesting that SphK1 inhibition was critical for apoptosis signaling. We also show that imatinib-sensitive and -resistant primary cells from chronic myeloid leukemia patients can be successfully killed in vitro by the F-12509a inhibitor. These results uncover the involvement of SphK1 in regulating imatinib-induced apoptosis and establish that SphK1 is a downstream effector of the Bcr-Abl/Ras/ERK pathway inhibited by imatinib but upstream regulator of Bcl-2 family members.

Effective killing of Gleevec-resistant CML cells with T315I mutation by a natural compound PEITC through redox-mediated mechanism

Mutation of Bcr-Abl is an important mechanism by which chronic myelogenous leukemia (CML) cells become resistant to Gleevec. The T315I mutation is clinically significant since CML cells harboring this mutation are insensitive to Gleevec and other Bcr-Abl-targeted drugs. Identification of new agents capable of effectively killing CML cells with T315I mutation would have important therapeutic implications in Gleevec-resistant CML. Here, we showed that beta-phenylethyl isothiocyanate (PEITC), a natural compound found in vegetables, is effective in killing CML cells expressing T315I BCR-ABL. Treatment of leukemia cell lines harboring wild-type or mutant Bcr-Abl with 10 microM PEITC resulted in an elevated ROS stress and a redox-mediated degradation of the BCR-ABL protein, leading to massive death of the leukemia cells. Antioxidant NAC attenuated the PEITC-induced oxidative stress in CML cells and prevented the degradation of BCR-ABL, caspase-3 activation and cell death. We further showed that the ROS-induced degradation of BCR-ABL was mediated partially by caspase-3 and the proteasome pathway. The ability of PEITC to effectively kill T315I-positive CML cells was further confirmed using primary leukemia cells isolated from CML patients. Our results suggest that PEITC is a promising compound capable of killing Gleevec-resistant CML cells through a ROS-mediated mechanism and warrants further investigations.

Molecular oncology: current trends in diagnostics

Applications of molecular diagnostics to oncology have been slow to make their way to the clinical laboratory. While numerous genes and mutation spectra have been found to be involved in tumorigenesis, it is only recently that these findings begin to become useful in a clinical setting. Building on the technical knowledge obtained from molecular infectious disease testing, new instruments and assays have been developed to answer similar questions regarding qualitative, quantitative and genotyping issues. In this manuscript we describe two current examples of clinical molecular diagnostic applications, the assessment of BCR–ABL in chronic myelogenous leukemia patients and the detection of tumor cells in the sentinel lymph nodes of breast cancer patients, to demonstrate the role of molecular techniques in a routine clinical setting.

Expression of BCR-ABL1 oncogene relative to ABL1 gene changes overtime in chronic myeloid leukemia

Using a quantitative single nucleotide polymorphism (SNP) assay we have investigated the changes in the expression of the BCR-ABL1 oncogene relative to the wild-type ABL1 and BCR alleles in cells from chronic myeloid leukemia (CML) patients not responding to therapy. The results show a progressive increase in the BCR-ABL1 oncogene expression at the expense of decreased expression of the ABL1 allele, not involved in the fusion. No relative changes in the expression of the two BCR alleles were found. These results demonstrate that allele-specific changes in gene expression, with selective, progressive silencing of the wild-type ABL1 allele in favor of the oncogenic BCR-ABL1 allele occur in CML patients with therapy-resistant disease.

Cancer stem cells with genetic instability: the best vehicle with the best engine for cancer

Our understanding of the role of stem cells in cancer development is evolving quickly. In the course of tumor expansion, a subpopulation of tumor cells with stem cell-like features has been noted. These cancer stem cells give rise to transit amplifying tumor cells, which comprise the majority of the tumor mass prior to terminal differentiation. Combining this finding with genetic instability, a well-known engine for cancer development and metastases, a new model emerges for cancer where normal stem cells and their cellular pathway acquire stochastic malignant abilities. In this model, when cancer stem cells self-renew, many genetic variants are produced. Just as microbes 'learn' to defeat antibiotics, genetically heterogeneous cancer stem cells may possibly acquire resistance to various chemotherapeutic approaches. Drug-resistant microorganisms selected by spontaneous mutation of bacterial DNA may not be so different than the drug-resistant and genetically instable cancer stem cells recurring after chemotherapeutic treatment. In this gloomy view of cancer, cancer stem cells with genetic instability can be considered as 'the best vehicle with the best engine', a formidable challenge for the future development of new anticancer therapies.

The impact of a negligent G2/M checkpoint on genomic instability and cancer induction

DNA damage responses (DDR) encompass DNA repair and signal transduction pathways that effect cell cycle checkpoint arrest and/or apoptosis. How DDR pathways respond to low levels of DNA damage, including low doses of ionizing radiation, is crucial for assessing environmental cancer risk. It has been assumed that damage-induced cell cycle checkpoints respond to a single double strand break (DSB) but the G2/M checkpoint, which prevents entry into mitosis, has recently been shown to have a defined threshold of 10-20 DSBs. Here, we consider the impact of a negligent G2/M checkpoint on genomic stability and cancer risk.

The Biology of CML Blast Crisis

The natural history of chronic myeloid leukemia (CML) progresses from a relatively benign chronic phase into a fatal blast crisis, which resembles acute leukemia, but is incurable by chemotherapy. Fortunately, the progression can usually be blocked by tyrosine kinase therapy or allogeneic transplantation. The seemingly stereotypical march of progression involves changes in genetic instability and DNA repair, proliferation, differentiation, and apoptosis, and thus may serve as a unique model of cancer evolution and progression. Given that all treatments work much better in chronic-phase than advanced-phase disease, the clinical dilemma is predicting and detecting patients bound to evolve into advanced disease. This is especially important in the age of tyrosine kinase inhibition (TKI) therapy. The purpose of this review is to address the biology of blast crisis in the age of tyrosine kinase therapy, with an emphasis on what genes or pathways may be future targets of predictive assays or treatments of progression.