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The development and application of imatinib.

The hallmark characteristics of cancer include an unrestrained proliferation involving activation of growth signals, loss of negative regulation and dysfunctional apoptotic pathways. Targeting abnormal cell signalling pathways should provide a more selective approach to cancer treatment than conventional cytotoxic chemotherapy. Tyrosine kinases play an essential role in the signalling pathways involved in the control of cellular proliferation and growth. Imatinib is a small-molecule tyrosine kinase inhibitor of the ABL fusion gene, platelet derived growth factor receptors (PDGFR) and KIT. This agent has demonstrated considerable activity in chronic myeloid leukaemia (CML) by inhibiting the BCR-ABL fusion protein and gastrointestinal stromal tumours (GISTs), which are predominantly driven by activating mutations in KIT. A number of other rare conditions are also responsive, for example, dermatofibrosarcoma protuberans, which is driven by a chromosomal translocation involving PDGF-B and Col1A1, resulting in overexpression of PDGF-B, and hypereosinophillic syndrome, which can be caused by activating PDGFR mutations. The pivotal registration study for newly diagnosed CML was a large randomised trial comparing 400 mg/day of imatinib to a combination of IFN-alpha and cytarabine, which demonstrated a significantly higher complete haematological and cytogenetic response rate in the imatinib arm. In the case of GIST a randomised study in patients with inoperable or metastatic disease explored doses of 400 - 600mg and reported a response rate of > 50% in each arm plus disease stabilisation and an improvement in performance status. Large randomised trials have subsequently been performed, comparing 400 with 800mg/day. The first to report indicates that the larger dose is associated with improved progression-free survival, although it is not yet known whether or not this will translate into a difference in overall survival. The most common KIT mutation involves exon 11 and is associated with a statistically significant better response and prognosis compared with other mutations or no detectable mutations. Mutational analysis is likely to become increasingly important in the selection of patients for neoadjuvant and adjuvant treatment and in helping to understand the nature of acquired resistance.

Antineoplastic Agents↗

The fundamental prevalence of chronic myeloid leukemia-generating clonogenic cells in the light of the neutrality theory of evolution.

A variety of normal human tissues have been reported to harbor small cell populations carrying potentially oncogenic gene rearrangements. This backdrop of mutant cells may be present in the majority of healthy individuals and is apparently weakly selected against. This may provide empirical support for the concept of global neutrality, or near-neutrality (very weak selection), of many somatic mutations. Many healthy individuals, as well as patients with chronic myeloid leukemia, manifest the BCR-ABL fusion gene in blood cells. The presumed neutrality of the BCR-ABL rearrangement-carrying pluripotential hematopoietic stem cells and the relative uniformity of the incidence rate of CML worldwide were used to estimate the extent of the background of BCR-ABL-positive stem cells and the numerical size of the human pluripotential hematopoietic stem cell pool. Three different approaches (molecular-epidemiological, statistical, and population genetical) were employed. Each resulted in very similar estimates of the size of the stem cells carrying the BCR-ABL allele fusions (1.4 x 10(4) cells) and the size of the total human stem cell pool (1.6 x 10(9) cells per individual). The implication of these estimates in the context of the hierarchical nature of the stem cell pool is also considered. The presumptive smaller-sized population of CD34(-) stem cells could not be characterized by any of the approaches used as a "founding" population, representing an ultimate source of all hematopoietic progenitors, or as a subset of stem cells comprising a deeper "kinetic" segment of the total (10(9)-sized) stem cell compartment.

Cell Survival↗

Differential manipulation of normal and chronic myeloid leukemia stem cell proliferation in vitro.

The long-term marrow culture (LTC) system allows the sustained production of primitive normal and neoplastic (chronic myeloid leukemia [CML]) hematopoietic cells in vitro for many weeks. This is achieved in the absence of exogenously added hematopoietic growth factors because of the presence in the cultures of supportive "stromal" cells of the fibroblast-endothelial-adipocyte lineages. These latter cells form a confluent adherent layer with which the most primitive hematopoietic cells become associated and which locally regulates their behavior. The LTC system has thus been considered as a model of the microenvironment of the bone marrow and used to delineate potentially physiologically relevant mechanisms that regulate the proliferation, self-renewal and differentiation of primitive normal hematopoietic cells. It has also been used to analyze the molecular basis of the altered proliferative behavior that characterizes primitive neoplastic cells from patients with CML. Most of the information obtained to date has emerged from experiments designed to shift the balance of stimulatory and inhibitory factors present in order to favor either the cycling or quiescence of primitive normal or CML cells in LTC. This has been achieved either by addition of soluble factors (or antagonists) to the LTC medium or by the use of genetically engineered factor-producing stromal cells. Such experiments have allowed the identification of a number of cytokines that promote one or the other of these responses (i.e., primitive progenitor cycling or quiescence), including some that are involved in control mechanisms endogenous to the LTC system. Recent studies suggest that the retention of primitive normal cells in a reversible G(o) state in this system is mediated by the cooperating action of limiting concentrations of at least two endogenously produced inhibitory factors (transforming growth factor-beta (TGF-beta) and macrophage inflammatory protein-1 alpha (MIP-1 alpha)), either of which, however, if added exogenously at a sufficient concentration, can exert this action on its own. Interestingly, the heightened turnover characteristic of primitive CML cells appears to be due to a selective unresponsiveness to only one of these two inhibitors (MIP-1 alpha). These findings are consistent with a complex model of the extrinsic regulation of primitive hematopoietic cells in which a multiplicity of intracellular signaling intermediates within the target cells converge at different points ultimately to control their entry into S phase. Our findings further suggest that only some of these pathways may be affected by intracellular expression of the BCR-ABL fusion gene.

Cell Differentiation↗

Fertilization promotes selective association of the Abl [correction of AbI] kinase with the egg cytoskeleton.

Fertilization of the sea urchin egg is known to involve activation of a variety of protein kinases including at least one protein tyrosine kinase. In the present study, fertilization was found to stimulate the association of the Abl [correction of AbI] protein kinase with the detergent-insoluble egg cytoskeleton. Increased levels of Abl protein and of protein tyrosine kinase activity were detected in cytoskeleton preparations made as early as 5 to 15 min after insemination. Immunofluorescence localization demonstrated that the Abl kinase becomes associated primarily with the cortical cytoskeleton of the fertilized egg. A separate, 57 kDa protein tyrosine kinase did not associate with the cytoskeleton indicating that fertilization results in the selective association of the Abl kinase with the cortical cytoskeleton.

Animals↗

Frequency and cell specificity of T-cell receptor interlocus recombination in human cells.

Immunoglobulin and T-cell receptor (TCR) genes are assembled by a site-specific rearrangement known as V(D)J [variable-(diversity)-joining] recombination. These rearrangements occur normally in pre-B- and pre-T-cells using signal sequences adjacent to coding exons for immunoglobulin and TCR genes, respectively. However, aberrant recombination may result in the generation of hybrid TCR genes by joining of TCR-beta with TCR-gamma specific sequences. Such hybrid TCR genes occur at a low frequency in peripheral blood lymphocytes (PBL) of healthy individuals, and can be detected by PCR amplification. We have determined the in vivo frequency of hybrid V gamma-J beta 1 TCR (hybrid TCR) genes in lymphocyte DNA from 12 healthy individuals. The average frequency was found to be 5.83 in 0.75 x 10(6) PBL, with a threefold difference between the highest and lowest individual value. The presence of similar TCR gene rearrangements in individual samples suggests that T-cells with a hybrid TCR gene are capable of clonal expansion in vivo. The individual hybrid TCR gene frequency remained relatively constant during 72 hours of in vitro cultivation. In long-term culture, the frequency gradually decreased, and after 28 days no hybrid TCR genes were detectable in lymphocyte DNA. These results show that T-cells with a hybrid TCR gene are able to respond to mitogen stimulation in vitro, and may have a proliferative disadvantage or are selected against during prolonged in vitro cultivation. No hybrid TCR genes were detected in ten proliferating T-cell clones, indicating that the rate of hybrid TCR gene formation is < 2.0 x 10(-8) per cell per cell division. No hybrid TCR genes were detected in DNA from B-lymphocytes, sperm, granulocytes, fibroblasts, keratinocytes, and three B-lymphoblastoid ataxia telangiectasia cell lines. In agreement with previous reports, the frequency of hybrid TCR genes in peripheral blood DNA from two ataxia telangiectasia patients was found to be more than 15-fold higher than in lymphocytes from normal individuals. These data show that formation of hybrid TCR genes is restricted to T-cells in vivo, and occurs at a very low frequency, if at all, in proliferating T-cells in vitro, and with an increased frequency in patients with ataxia telangiectasia.

Ataxia Telangiectasia↗

Molecular cloning of a novel rat salt-tolerant protein by functional complementation in yeast.

To elucidate the genetic basis of salt-sensitivity in mammalian hypertension, we isolated six rat complementary DNAs by functional complementation in yeast. These genes were able to substitute for the salt-tolerant activity of HALI which confers salt tolerance by modulating the cation transport system in yeast. We identified these genes as beta-globin, lambda-crystallin, androgen-regulated protein, mitochondrial cytochrome b, a homologue of infant brain cDNA, and a novel gene, called salt-tolerant protein (STP). STP contains 1964 bp nucleotides and an open reading frame which encodes 496 amino acid residues. Northern blot analysis showed that STP mRNA is expressed in various rat tissues.

ATPases Associated with Diverse Cellular Activitie↗

An intron promotes the anti-bcr-abl activities of a retrovirally expressed ribozyme in chronic myeloid leukemia cells.

Ribozymes have been developed to cleave the bcr-abl transcripts and thereby suppress transforming activities of chronic myelogenous leukemia (CML) cells. However, the intracellular efficacy of vector-dependent ribozymes usually depends in part on their expression cassettes, which may affect their intracellular trafficking and distribution. In order to test effects of an intron in pre-fusion ribozyme on the anti-bcr-abl activities in CML cells, retroviral vectors harboring ribozyme expression cassettes with (RzI) or without (Rz) an intron-encoding sequence were used to transduce K562 cells. In terms of both reduction of the target bcr-abl mRNA and suppression of colony formation in soft agar and xenograft growth on SCID mice, the anti-bcr-abl efficacy of the RzI fusion ribozyme was significantly superior to that of Rz. These results also correlate with more cytoplasmic accumulation of the RzI fusion ribozymes than that of the Rz. This study suggests activities of a RNA polymerase II-driven fusion ribozyme against its targeted spliced mRNA are improved by incorporating an intron in its pre-splicing transcript. Noticeably, the improvement is contributed in part by subcellular co-localization.

Animals↗

Betulinic acid induces apoptosis in human chronic myelogenous leukemia (CML) cell line K-562 without altering the levels of Bcr-Abl.

Betulinic acid (BA), a plant derived triterpenoid, isolated from various sources shows cytotoxicity in cell lines of melanoma, neuroectodermal and malignant brain tumors. Chronic myelogenous leukemia (CML) is characterized by Philadelphia chromosome (Bcr-Abl), a molecular abnormality leading to the intrinsic tyrosine kinase activity that provides growth and survival advantage to the cells. Present study describes the cytotoxicity of BA on human CML cell line K-562, positive for Bcr-Abl. The decrease in the viability of K-562 cells treated with BA at different concentrations and time intervals was assessed using MTT assay. Cell death induced by BA was determined to be apoptotic as measured by FACS analysis of PI stained nuclei, PS externalization by Annexin-V fluorescence and characteristic DNA fragmentation. DiOC6(3) fluorescent probe determined alterations in the mitochondrial membrane potential (MMP). RT-PCR confirmed the expression levels of Bcr-Abl in controls and K-562 cells treated with BA. The rapid loss of MMP of K-562 cells upon treatment with BA shows the direct activation of apoptosis at the level of mitochondria, overcoming the resistance of the high levels of expression of Bcr-Abl.

Annexin A5↗

Specificity of novel allosterically trans- and cis-activated connected maxizymes that are designed to suppress BCR-ABL expression.

Chronic myelogenous leukemia (CML) is associated with the presence of the Philadelphia chromosome, which is generated by the reciprocal translocation of chromosomes 9 and 22. In the case of L6 (b2a2) mRNA, it is difficult to cleave the abnormal mRNA specifically because the mRNA includes no sequences that can be cleaved efficiently by conventional hammerhead ribozymes near the BCR-ABL junction. We recently succeeded in designing a novel maxizyme, which specifically cleaves BCR-ABL fusion mRNA, as a result of the formation of a dimeric structure. As an extension of our molecular engineering of maxizymes, as well as to improve their potential utility, we examined whether an analogous conformational change could be induced within a single molecule when two maxizymes were connected via a linker sequence. An active conformation was achieved by binding of the construct to the BCR-ABL junction in trans, with part of the linker sequence then acting as an antisense modulator in cis (within the complex) to adjust the overall structure. Results of studies in vitro in the presence of cetyltrimethylammonium bromide (CTAB) (but not in its absence) suggested that a certain kind of connected maxizyme (cMzB) might be able to undergo a desired conformational change and, indeed, studies in vivo confirmed this prediction. Therefore, we successfully created a fully functional, connected maxizyme and, moreover, we found that the activity and specificity of catalytic RNAs in vivo might be better estimated if their reactions are monitored in vitro in the presence of CTAB.

Allosteric Regulation↗

Increased circulating normal and BCR-ABL+Ve progenitor numbers in Philadelphia chromosome-positive acute myeloid leukaemia.

We recorded elevated numbers of circulating myeloid and erythroid colony-forming cells in 15 adult patients with acute myeloid leukaemia (AML) who presented with high blood white cell counts. Since leukaemic blasts from three of these patients were Philadelphia chromosome-positive (Ph+), we were able to determine if blood progenitors from these particular patients arose from the leukaemic clone or from residual normal progenitors. Blasts and colonies were intensively investigated using a combination of cell surface marker analysis by flow cytometry, RT-PCR and interphase fluorescence in situ hybridization (FISH). FISH detected rearrangements within the major breakpoint BCR (M-BCR) region in blasts and in some myeloid and erythroid colonies from patients 1 and 2. The minor breakpoint (m-BCR) region was detected in blasts and in some myeloid and erythroid colonies from patient 3. RT-PCR detected long b2a2 BCR-ABL transcripts in blasts from patients 1 and 2, although misspliced short e1a2 transcripts were also seen in patient 1. Only e1a2 transcripts were found in blasts from patient 3. Flow sorting demonstrated the B-cell marker CD19 on blasts and on a proportion of myeloid and erythroid progenitors from patients 1 and 3. RT-PCR also detected IgH rearrangements, further evidence of B-cell differentiation, in blasts from these two patients. We conclude that both normal and clonal circulating progenitor numbers can be raised in both M-BCR and m-BCR Ph+ AML. The underlying cause, perhaps efflux from a congested marrow, may be common to AML patients with a high blood white cell count.

Acute Disease↗

BCR-ABL rearrangement frequencies in chronic myeloid leukemia and acute lymphoblastic leukemia in Ecuador, South America.

Different BCR-ABL transcript variants occur more or less frequently, according to the leukemia type. We report the frequencies of BCR-ABL transcript variants studied in chronic myeloid leukemia (CML) and acute lymphoblastic leukemia (ALL) patients in the Ecuadorian population. The frequencies found for CML patients in this study were 94.6% for the b2/a2 rearrangement and 5.4% for the b3/a2 rearrangement; whereas in ALL, all cases (100%) that presented the BCR-ABL rearrangement had the e1/a2 junction. Since our results differ from the frequencies previously reported, we suggest that this may be due to a different genetic background in the population involved in this study when compared to the populations analyzed in prior studies. Furthermore, we recommend a survey of the BCR-ABL transcript variants and their frequencies in different ethnic groups.

Adolescent↗

Two different Philadelphia chromosomes in a cell line from an AML-M0 patient.

A second Philadelphia (Ph) chromosome is one of the most common nonrandom secondary chromosome changes in leukemias with 9;22 translocations. It has been suggested, and observed in two studies of masked t(9;22), that the second Ph chromosome is an exact duplication of the entire derivative chromosome 22. In a cytogenetic study of bone marrow cells from an acute myelogenous leukemia patient, a cell line carrying two different Ph chromosomes evidenced by a chromosome 22 centromeric heteromorphism was found. From this observation arose the question whether the second der(22) was a true Ph chromosome or whether it was a deleted chromosome derived from the normal chromosome 22 that did not contain the bcr-abl rearrangement. A fluorescent in situ hybridization (FISH) study with the t(9;22) probe revealed two bcr-abl positive signals on 60 of 100 interphase nuclei. The second Ph could have resulted from a mitotic crossing over; or, analogously to late-appearing Philadelphia chromosomes, it may be derived from a new chromatid translocation between the chromosomes 9 and 22 not involved in the initial t(9;22).

Adolescent↗

The in vitro activity of the tyrosine kinase inhibitor STI571 in BCR-ABL positive chronic myeloid leukaemia cells: synergistic interactions with anti-leukaemic agents.

Chronic myeloid leukaemia is typically characterised by the presence of dysregulated BCR-ABL tyrosine kinase activity, which is central to the oncogenic feature of being resistant to a wide range of cytotoxic agents. We have investigated whether the inhibition of this tyrosine kinase by the novel compound STI571 (formerly CGP57148B) would render K562, KU812 cell lines and chronic myeloid leukaemia-progenitor cells sensitive to induction of cell kill. Proliferation assays showed STI571 to be an effective cytotoxic agent in chronic myeloid leukaemia-derived cell lines (IC(50) on day 5 of 4.6 microg ml(-1) and 3.4 microg ml(-1) for K562 and KU812 respectively) and in leukaemic blast cells (per cent viability on day 3 at 4 microg ml(-1): 55.5+/-8.7 vs 96.4+/-3.7%). STI571 also appeared to specifically target bcr-abl expressing cells, as results from colony forming assays using the surviving cell fraction from STI571-treated peripheral CD34(+) chronic myeloid leukaemia blast cells, indicated a reduction in the expansion of colonies of myeloid lineage, but no effect on normal colony formation. Our data also showed synergy between STI571 and other anti-leukaemic agents; as an example, there were significant increases in per cent cell kill in cell lines cultured with both STI571 and etoposide compared to the two alone (per cent cell kill on day 3: 73.7+/-11.3 vs 44.5+/-8.7 and 17.8+/-7.0% in cultures with STI571 and etoposide alone respectively; P<0.001). This study confirms the central oncogenic role of BCR-ABL in the pathogenesis of chronic myeloid leukaemia, and highlights the role of targeting this tyrosine kinase as a useful tool in the clinical management of the disease.

Antineoplastic Agents↗

Transcription-mediated amplification and hybridisation protection assay to determine BCR-ABL transcript levels in patients with chronic myeloid leukaemia.

Detection of BCR-ABL transcripts in chronic myeloid leukaemia (CML) is used to confirm the diagnosis and to monitor residual disease. Quantitative techniques are required to predict response to therapy or early relapse. We have evaluated an assay in which transcription-mediated amplification (TMA) of BCR-ABL and ABL transcripts is achieved using reverse transcriptase and RNA polymerase. The products are quantified in the hybridisation protection assay (HPA) using acridinium ester-labelled DNA probes and chemiluminescence. The method is a single tube procedure which uses small amounts of RNA (<500 ng/triplicate analysis), is technically simple (requiring just two waterbaths and a luminometer), rapid (total assay time <4 h) and sensitive (capable of detecting one BCR-ABL-positive K562 cell in the presence of 10(4)-10(5) BCR-ABL-negative cells). BCR-ABL signals from patient RNA samples were quantified relative to known amounts of K562 RNA and normalised to levels of ABL. BCR-ABL/ABL ratios ranged from 0.15 to 1.59 (median 0.65) in RNA from diagnostic blood or bone marrow of 18 CML patients and were < or =0.0001 in 20 normal controls. Sequential samples analysed from six CML patients post-allogeneic bone marrow transplantation who relapsed and received donor lymphocyte infusions showed BCR-ABL/ABL ratios which reflected patient status or treatment. A BCR-ABL/ABL ratio of 0.01 served as a useful arbitrary indicator value, with results above and below this value generally correlating with relapse or remission, respectively.

Acridines↗

Jak2 is involved in c-Myc induction by Bcr-Abl.

We have previously shown that the Jak2 tyrosine kinase is activated in Bcr-Abl positive cell lines and blood cells from CML blast crisis patients by tyrosine phosphorylation. We are searching for downstream targets of Jak2 in Bcr-Abl positive cells. It is known that c-Myc expression is required for the oncogenic effects of Bcr-Abl, and that over-expression of c-Myc complements the transformation defect of the Bcr-Abl SH2 deletion mutant. Moreover, the Bcr-Abl SH2 deletion mutant and an Abl C-terminal deletion mutant are deficient in activating c-Myc expression. Since the Jak2 binds to the C-terminal domain of Bcr-Abl and optimal Jak2 activation requires the SH2 domain, we tested whether Jak2 was involved in c-Myc protein induction by Bcr-Abl. We treated the 32Dp210 Bcr-Abl cells with the Jak2 specific tyrosine kinase inhibitor, AG490, and found that this drug, like the Abl tyrosine kinase inhibitor STI-571, inhibited c-Myc protein induction by Bcr-Abl. Treatment of 32Dp210 Bcr-Abl cells with AG490 also inhibited c-MYC RNA expression. It is also known that c-Myc protein is a labile protein that is increased in amounts in response to various growth factors by a mechanism not involving new Myc protein formation. Treatment of 32Dp210 Bcr-Abl cells with both the proteasome inhibitor MG132 and AG490 blocked the reduction of the c-Myc protein observed by AG490 alone. An adaptor protein SH2-Bbeta is involved in the enhancement of the tyrosine kinase activity of Jak2 following ligand/receptor interaction. In this regard we showed that the Jak2/Bcr-Abl complex contains SH2-Bbeta. Expression of the SH2-Bbeta R555E mutant in 32Dp210 Bcr-Abl cells reduced c-Myc expression about 40% compared to a vector control. Interestingly, we found the reduction of the c-Myc protein in several clones of dominant-negative (DN) Jak2 expressing K562 cells correlated very well with the reduction of tumor growth of these cells in nude mice as compared to vector transfected K562 cells. Both STI-571 and AG490 also induced apoptosis in 32Dp210 cells. Of interest, IL-3 containing medium reversed the STI-571 induced apoptosis of 32Dp210 cells but did not reverse the induction of apoptosis by AG490, which strongly supports the specificity of the inhibitory effects of AG490 on the Jak2 tyrosine kinase. In summary, our findings indicate that Jak2 mediates the increase in c-Myc expression that is induced by Bcr-Abl. Our results indicate that activated Jak2 not only mediates an increase of c-MYC RNA expression but also interferes with proteasome-dependent degradation of c-Myc protein.

Adaptor Proteins, Signal Transducing↗

Changes in the proteome associated with the action of Bcr-Abl tyrosine kinase are not related to transcriptional regulation.

Chronic myeloid leukemia (CML) is a hematopoietic stem cell disease, the hallmark of which is the Bcr-Abl protein tyrosine kinase (PTK). Without intervention the disease progresses from a benign chronic phase to a rapidly fatal blast crisis. To identify the molecular mechanisms underlying disease progression we used two-dimensional gel electrophoresis on a model we have previously described using the expression of a conditional mutant of Bcr-Abl PTK in a multipotent stem cell line, FDCP-Mix. Long term exposure of FDCP-Mix cells to Bcr-Abl mimics disease progression in CML. Four major differences were observed as a consequence of long term exposure to the Bcr-Abl PTK compared with cells exposed short term. The proteins were identified using matrix-assisted laser desorption ionization-time of flight mass spectrometry-generated peptide mass fingerprint data and liquid chromatography-tandem mass spectrometry-generated sequence information. Leukotriene A4 hydrolase, an enzyme known to be deregulated in CML, was found to be up-regulated. Annexin VI, vacuolar ATP synthase catalytic subunit A, and mortalin were found to be down-regulated. Poly(A) PCR cDNA analysis showed there was no correlation between the protein expression changes and mRNA levels. Western blot analysis also indicated no change in the levels of mortalin or leukotriene A4 hydrolase, indicating that post-translational events may modify protein content of the specific spots. Leukotriene B4 levels (product of leukotriene A4 hydrolase) were, however, reduced in cells exposed long term to Bcr-Abl activity. This study demonstrates the potential of proteomic analysis to define novel effects of oncogenes.

Animals↗