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PubMed1994

A temperature sensitive p210 BCR-ABL mutant defines the primary consequences of BCR-ABL tyrosine kinase expression in growth factor dependent cells.

The Philadelphia translocation commonly observed in chronic myeloid leukaemia (CML) and a proportion of cases of acute leukaemia results in the creation of a chimeric fusion protein, BCR-ABL. The fusion protein exhibits an elevated tyrosine kinase activity as compared to normal ABL. Using a temperature sensitive mutant of p210 BCR-ABL (ts-p210) we find that the primary effect of BCR-ABL expression in an IL-3 dependent cell line is to prolong survival following growth factor withdrawal; only a small proportion of cells remain viable and rapidly evolve to complete growth factor independence. During passage in the presence of IL-3 at the temperature permissive for kinase activity, ts-p210 expressing cultures become dominated by completely growth factor independent cells within 10-30 days. There is also a significant difference between BCR-ABL and IL-3 mediated signalling with respect to the MAP kinase pathway; in contrast to IL-3 stimulation or v-ABL expression, BCR-ABL does not signal ERK 2 (MAP 2 kinase) activation, underlining the apparent inability of BCR-ABL to deliver an immediate proliferative signal in Ba/F3 cells. Our data suggest that growth factor independence does not simply reflect the convergence of BCR-ABL and IL-3 mediated signalling pathways and its development, at least in Ba/F3 cells, requires prolonged exposure to BCR-ABL kinase activity. We suggest that the myeloid expansion characteristic of CML may result from the prolongation of survival of myeloid progenitor cells under conditions of limiting growth factor rather than their uncontrolled proliferation.

Animals↗

PubMed2005

Engineering 3-alkyltriazenes to block bcr-abl kinase: a novel strategy for the therapy of advanced bcr-abl expressing leukemias.

Recently, within the framework of a new strategy termed "combi-targeting," we designed ZRCM5 to contain a 2-phenylaminopyrimidopyridine moiety targeted to bcr-abl kinase and a triazene tail capable of generating a methyldiazonium species upon hydrolysis. The ability of ZRCM5 to block tyrosine kinase activity was tested in a short 10 min exposure ELISA involving isolated bcr-abl kinase and Western blotting assays. The results showed that: (a) ZRCM5 was hydrolyzed with a half-life of 27 min in cell culture media, (b) it blocked bcr-abl autophosphorylation in promyeloblastic leukemia K562 cells in a dose-dependent manner (IC(50)=14.01 microM) and (c) it induced dose-dependent levels of DNA strand breaks. In contrast, temozolomide (TEM), a clinical DNA damaging triazene capable of generating, like ZRCM5, a methyldiazonium species, could neither block bcr-abl tyrosine kinase activity in isolated enzyme nor in whole cell autophosphorylation assays. In cells expressing varied levels of bcr-abl, ZRCM5 was consistently more potent than TEM. The significant potency of ZRCM5 against the leukemia cells was attributed to its ability to simultaneously to block bcr-abl and related DNA repair activity while inducing significant DNA lesions in bcr-abl expressing leukemia cells. Further studies are ongoing to increase the affinity of ZRCM5 with the purpose of further enhancing its potency in bcr-abl expressing cells.

Apoptosis↗

PubMed2001

Bcr in vascular smooth muscle cells involvement of Ras and Raf-1 activation by Bcr.

Bcr gene was originally identified by its presence in the chimeric Bcr/Abl oncogene. In vascular smooth muscle cells, platelet-derived growth factor-BB (PDGF) stimulated Bcr kinase activity. The mutant PDGF receptor for PI3-K, but not for PLC-gamma binding sites, showed significantly reduced Bcr kinase activity. Bcr wild-type enhanced, whereas Bcr kinase negative form inhibited PDGF-stimulated ERK1/2 activity. A dominant negative Ras did not inhibit Bcr kinase activation, and overexpression of Bcr increased Ras/Raf-1 activity and DNA synthesis. These results demonstrated the importance of Bcr in PDGF-mediated events such as activation of Ras, Raf-1, and ERK1/2 and stimulation of DNA synthesis.

Animals↗

PubMed2003

Bcr and Abl interaction: oncogenic activation of c-Abl by sequestering Bcr.

c-Abl tyrosine kinase is under rigorous control because of an unknown cellular inhibitor that maintains c-Abl in a relatively inactive state. Because SH2 domains are positive regulators of the nonreceptor tyrosine kinases, we tested whether this putative inhibitor would bind to an Abl SH2 protein construct and thus activate the c-Abl tyrosine kinase. Expression of a Mr 10,000 Abl SH2 protein in COS-1 and Rat-1 cells activated the tyrosine kinase activity of p145 ABL and induced both morphological transformation and foci formation in Rat-1 cells. Importantly, the R to L mutant of the FLVRES sequence of the Abl SH2 protein also activated the c-Abl tyrosine kinase and induced oncogenic transformation. Addition of the Abl kinase inhibitor STI-571 to ABL SH2-transformed Rat-1 cells inhibited tyrosine phosphorylation of p145 ABL. Overexpression of Bcr has been shown to inhibit the Bcr-Abl oncoprotein, and the endogenous Bcr protein forms a complex with c-Abl in hematopoietic cells and insect cells. Therefore, we determined whether Bcr is the putative c-Abl inhibitor that interacts with the Mr 10,000 Abl SH2 protein. Bcr expression in Rat-1 cells transformed by the Mr 10,000 Abl SH2 protein reduced the activated c-Abl tyrosine kinase activity to near normal levels and reversed the oncogenic effects (morphology changes and foci formation) seen in the Abl SH2-treated cells. We additionally demonstrated that Bcr and the Mr 10,000 Abl SH2 protein are present in a complex. We conclude from these studies that Bcr is a major tyrosine kinase inhibitor of cytoplasmic c-Abl and that procedures that sequester Bcr will release the c-Abl protein from the Bcr/c-Abl complex, which leads to c-Abl oncogenic activation.

Animals↗

PubMed2002

Dual transcription of b2a2 and b3a2 BCR-ABL transcripts in chronic myeloid leukaemia is confined to patients with a linked polymorphism within the BCR gene.

We propose a mechanism for dual expression of b2a2 and b3a2 BCR-ABL in chronic myeloid leukaemia (CML). We have identified a BCR allele, present in approximately 29% of the population, which includes an adenine to guanine polymorphism in the putative branchpoint of BCR intron 13. CML patients who expressed both b2a2 and b3a2 transcripts had the allele, which was also associated with alternative transcription of the normal BCR allele. We conclude that the BCR intronic polymorphism is associated with activation of a cryptic branchpoint resulting in reduced efficiency of RNA splicing and exon 14 (b3) skipping in BCR and BCR-ABL.

Alternative Splicing↗

PubMed2005

B cell receptor (BCR) cross-talk: CD40 engagement creates an alternate pathway for BCR signaling that activates I kappa B kinase/I kappa B alpha/NF-kappa B without the need for PI3K and phospholipase C gamma.

BCR signaling is propagated by a series of intermediaries and eventuates in NF-kappaB activation, among other outcomes. Interruption of several mediators that constitute the signalosome, such as PI3K and phospholipase Cgamma2, completely blocks BCR signaling for NF-kappaB. We show here that this accepted, conventional paradigm is, in fact, limited to naive B cells. CD40L treatment reprograms normal B cells such that a novel, alternate pathway for BCR signaling is created. Through this alternate pathway BCR triggering induces nuclear NF-kappaB without the need for PI3K or for phospholipase Cgamma2. Induction of NF-kappaB via the alternate pathway is accompanied by IkappaB kinase beta (IKKbeta) phosphorylation, IkappaBalpha phosphorylation, and IkappaBalpha degradation, and inhibition of IKKbeta blocked IkappaBalpha degradation. Several key events in the conventional pathway, including early protein tyrosine phosphorylation, were unimpeded by generation of the alternate pathway which appears to operate in parallel, rather than in competition, with classical BCR signaling. These results demonstrate cross-talk between CD40 and BCR, such that the requirements for BCR signaling are altered by prior B cell exposure to CD40L. The alternate BCR signaling pathway bypasses multiple signalosome elements and terminates in IKKbeta activation.

Adaptor Proteins, Signal Transducing↗

PubMed2005

B cell receptor (BCR) cross-talk: CD40 engagement enhances BCR-induced ERK activation.

Bystander B cells may be initially stimulated through CD40, which enhances susceptibility to Fas-mediated apoptosis, before encountering Ag, which produces Fas resistance. A key issue in this process is to what extent CD40 cross-talk might affect subsequent BCR signaling. It has previously been shown that CD40 engagement bypasses or mitigates the need for Bruton's tyrosine kinase in subsequent BCR signaling for NF-kappaB activation. However, the full extent of the effects of CD40 on BCR signaling has not been delineated. In the present study we evaluated the possibility that CD40-mediated cross-talk also affects another principal outcome of BCR signaling: MAPK activation. We found that prior stimulation of primary murine B cells with CD40L markedly enhanced the level of ERK and JNK (but not p38 MAPK) phosphorylation produced by subsequently added anti-Ig Ab, and much, but not all, of this enhancement was independent of PI3K and phospholipase C. CD40L treatment similarly enhanced BCR-induced MAPK kinase (MEK) phosphorylation, and MEK was required for enhancement of ERK. Although BCR-induced c-Raf phosphorylation was also enhanced by prior CD40L treatment, c-Raf was not required for MEK/ERK phosphorylation. These results identify a novel system of receptor cross-talk between CD40 and BCR and indicate that the effects of CD40 engagement on subsequent BCR stimulation spread beyond NF-kappaB to involve the MAPK pathway.

Animals↗

PubMed1996

Signaling through CD38 augments B cell antigen receptor (BCR) responses and is dependent on BCR expression.

mAbs directed against the ectoenzyme CD38 will induce B cell proliferation in normal resting B lymphocytes, but cannot induce proliferation in B cells that are unresponsive to B cell Ag receptor (BCR) cross-linking. Using the CD38- murine B cell line A20 we have examined the relationship between CD38- and BCR-mediated signaling after transfection of wild-type or mutant CD38 molecules. Although association between CD38 and the BCR was not detectable, co-cross-linking of CD38 and the BCR gave rise to a synergistic response, and expression of CD38 lowered the threshold for BCR-induced responses. Generation of Ig loss variant clones established that coexpression of the BCR was required for CD38-mediated signal transduction. The cytoplasmic tail of Ig alpha or Ig beta rescued CD 38 responsiveness in the CD38+Ig- cells provided that the chimeric molecules were coligated with CD38. Separate experiments indicated that the cytoplasmic tail of CD38 is not required for CD38 signaling. The anti-CD38-induced response was dependent on the influx of extracellular calcium but was not accompanied by detectable tyrosine phosphorylation of any cellular proteins. Together, these data demonstrate that the CD38 molecule can influence BCR-induced responses and that CD38 signaling is dependent on the BCR complex, perhaps to utilize a functional cytoplasmic tail(s) for intracellular signaling.

ADP-ribosyl Cyclase↗

PubMed2002

BCR-ABL point mutants isolated from patients with imatinib mesylate-resistant chronic myeloid leukemia remain sensitive to inhibitors of the BCR-ABL chaperone heat shock protein 90.

Clinical resistance to imatinib mesylate is commonly observed in patients with advanced Philadelphia chromosome- positive (Ph(+)) leukemias. Acquired resistance is typically associated with reactivation of BCR-ABL due to kinase domain mutations or gene amplification, indicating that BCR-ABL remains a viable target for inhibition in these patients. Strategies for overcoming resistance can be envisioned through exploitation of other molecular features of the BCR-ABL protein, such as its dependence on the molecular chaperone heat shock protein 90 (Hsp90). To determine whether inhibition of Hsp90 could induce degradation of imatinib mesylate-resistant, mutant BCR-ABL proteins, hematopoietic cells expressing 2 mutant BCR-ABL proteins found in imatinib mesylate-resistant patients (T315I and E255K) were examined for sensitivity to geldanamycin and 17-allylaminogeldanamycin (17-AAG). Both compounds induced the degradation of wild-type and mutant BCR-ABL and inhibited cell growth, with a trend indicating more potent activity against mutant BCR-ABL proteins. These data support clinical investigations of 17-AAG in imatinib mesylate-resistant Ph(+) leukemias.

Antineoplastic Agents↗

PubMed1989

Fine mapping of chromosome 22 breakpoints within the breakpoint cluster region (bcr) implies a role for bcr exon 3 in determining disease duration in chronic myeloid leukemia.

The chromosomal translocation that fuses the phl gene with the c-abl proto-oncogene appears to be a pivotal step in the pathogenesis of some leukemias. In chronic myeloid leukemia (CML) the breakage within the phl gene is largely confined to a 5.8-kb segment referred to as the breakpoint cluster region (bcr). To determine whether the presence of specific bcr exons on the Philadelphia chromosome has any clinical significance, we have analyzed the bcr breakpoints in 134 patients with CML. As many as five probes were used in this analysis, including a synthetic oligonucleotide probe homologous to the bcr exon 3 (phl exon 14) region. The distribution of breakpoints indicates that, in fact, breakage is largely confined to a 3.1-kb segment lying between bcr exon 2 and exon 4 (phl exons 13-15). In 61 CML patients analyzed within 1 year of diagnosis, the distribution of breakpoints appeared to be random within the 3.1-kb region. However, a significant excess of 5' breakpoints was observed in the total population studied, consistent with previous data showing that patients with 3' breakpoints have shorter disease durations. Analysis using the bcr exon 3 sequence probe indicated it was probably the presence or absence of bcr exon 3 on the Philadelphia chromosome that accounts for some of the variability in disease duration seen in CML. The data suggest that the phl/abl protein product may influence the timing of the onset of blast crisis and imply a continuing role for this protein during the evolution of the disease.

Blotting, Southern↗

PubMed1998

Selective inhibition of cell proliferation and BCR-ABL phosphorylation in acute lymphoblastic leukemia cells expressing Mr 190,000 BCR-ABL protein by a tyrosine kinase inhibitor (CGP-57148).

The excessive proliferation of the myeloid marrow compartment in Philadelphia chromosome (Ph)-positive acute and chronic leukemias has been largely attributed to a hyperactive and autonomously acting hybrid tyrosine kinase BCR-ABL, a product of the fusion between the second exon of the c-ABL proto-oncogene and 5' portions of the BCR gene on chromosome 22. This specific molecular event, amenable to attack with specifically designed inhibitors, has recently been successfully influenced by the drug CGP-57148 in mammalian cells transfected with full-length BCR-ABL gene and expressing full-length p210Bcr-Abl protein, as well as in primary human leukemic cells expressing p210Bcr-Abl fusion protein. In view of the heterogeneity of BCR-ABL transcripts associated with various phenotypes, we investigated the effect of CGP-57148 on p190Bcr-Abl- and p210Bcr-Abl-expressing, patient-derived cell lines and primary intact blast cells. In particular, we were interested in whether the variations in molecular events and/or the phenotype of Ph-positive cells would affect their susceptibility to the specific tyrosine kinase inhibitor CGP-57148. We have demonstrated that the sensitivity of human cells with lymphoblastic immunophenotype expressing p190Bcr-Abl protein is comparable to that for leukemic myeloid cells expressing p210Bcr-Abl protein. After documenting profound and phenotype-independent suppression of both autophosphorylation and cell growth, we explored the importance of time and dose of exposure on the manifestation and stability of the induced events. Although there were variations between target cells, in vitro exposure for 24-48 h induced extensive and apparently irreversible apoptosis in BCR-ABL-expressing but not other normal or BCR-ABL-negative leukemic cells. These findings support the potential use of CGP-57148 to purge Ph-positive cells from autologous bone marrow in vitro. Another important finding was the comparable suppressive effect of temporary CGP-57148 exposure on both clonogenic KBM-5 cells and the whole cell population. Exposure time and dose appeared to be important variables among various cell types. Moreover, effective doses appeared uniformly harmless to cells lacking BCR-ABL protein functioning as tyrosine kinase. Thus, the continuous exposure of target cells, at least during the initial period of 24-48 h, may prove to be an important variable in the design of in vitro and in vivo therapy using tyrosine kinase inhibitors.

Apoptosis↗

PubMed2006

Src family kinases phosphorylate the Bcr-Abl SH3-SH2 region and modulate Bcr-Abl transforming activity.

Bcr-Abl is the oncogenic protein-tyrosine kinase responsible for chronic myelogenous leukemia. Recently, we observed that inhibition of myeloid Src family kinase activity (e.g. Hck, Lyn, and Fyn) induces growth arrest and apoptosis in Bcr-Abl-transformed cells, suggesting that cell transformation by Bcr-Abl involves Src family kinases (Wilson, M. B., Schreiner, S. J., Choi, H. J., Kamens, J., and Smithgall, T. E. (2002) Oncogene 21, 8075-8088). Here, we report the unexpected observation that Hck, Lyn, and Fyn strongly phosphorylate the SH3-SH2 region of Bcr-Abl. Seven phosphorylation sites were identified by matrix-assisted laser desorption ionization time-of-flight mass spectrometry: Tyr89 and Tyr134 in the Abl-derived SH3 domain; Tyr147 in the SH3-SH2 connector; and Tyr158, Tyr191, Tyr204, and Tyr234 in the SH2 domain. SH3 domain Tyr89, the most prominent phosphorylation site in vitro, was strongly phosphorylated in chronic myelogenous leukemia cells in a Src family kinase-dependent manner. Substitution of the SH3-SH2 tyrosine phosphorylation sites with phenylalanine substantially reduced Bcr-Abl-mediated transformation of TF-1 myeloid cells to cytokine independence. The positions of these tyrosines in the crystal structure of the c-Abl core and the transformation defect of the corresponding Bcr-Abl mutants together suggest that phosphorylation of the SH3-SH2 region by Src family kinases impacts Bcr-Abl protein conformation and signaling.

Binding Sites↗

PubMed2004

Protein kinase D2 mediates activation of nuclear factor kappaB by Bcr-Abl in Bcr-Abl+ human myeloid leukemia cells.

The Bcr-Abl tyrosine kinase activates various signaling pathways including nuclear factor kappaB that mediate proliferation, transformation, and apoptosis resistance in Bcr-Abl(+) myeloid leukemia cells. Here we report that protein kinase (PK) D2, a serine threonine kinase of the PKD family, is a novel substrate of Bcr-Abl. PKD2 was found to be the major isoform of the PKD family expressed in chronic myeloid leukemia cells and is tyrosine phosphorylated by Bcr-Abl in its pleckstrin homology domain. A mutant that mimicks tyrosine phosphorylation of PKD2 in the pleckstrin homology domain activates nuclear factor kappaB independently of its catalytic activity. Furthermore, our data show that Bcr-Abl-induced activation of the nuclear factor kappaB cascade in LAMA84 cells is largely mediated by tyrosine-phosphorylated PKD2. These data present a novel mechanism of Bcr-Abl-induced nuclear factor kappaB activation in myeloid leukemia. Targeting PKD2 tyrosine phosphorylation, not its kinase activity, could be a novel therapeutic approach for the treatment of Bcr-Abl(+) myeloid leukemia.

Cell Line, Tumor↗

PubMed1992

Absence in Ph-negative, M-BCR rearrangement-positive chronic myelogenous leukemia of linkage between 5' ABL and 3' M-BCR sequences in Philadelphia translocation.

The Philadelphia (Ph) chromosome can be detected in the vast majority of patients with chronic myelogenous leukemia (CML). We performed a long-range analysis of chromosomal translocation junction by pulsed-field gel electrophoresis (PFGE) techniques, to examine whether molecular evidence of a reciprocal Ph translocation exists in Ph-positive CML as well as Ph-negative, M-BCR rearrangement-positive CML. The rearrangement within M-BCR and ABL was detected in all patients including nine Ph-positive CML, and three Ph-negative CML. The rearranged 3'-abl fragments showed comigration with rearranged 5'-bcr fragment in rare-cutting restriction enzyme digests in all patients with Ph-positive CML. Thus, the physical linkage of the 3' part of ABL to the 5' side of M-BCR on 22q-chromosome was shown. The same linkage was also demonstrated in all three patients with Ph-negative CML. Meanwhile, the rearranged 3'-bcr fragments showed comigration with rearranged pHabl5' (or T39-1-2) fragments in all patients with Ph-positive CML, indicating the linkage of the 5' end of ABL to the 3' part of M-BCR on 9q+ chromosome. However, this linkage was absent in two Ph-negative CML patients who could be studied. The results suggest that a genomic insertion of 3' ABL into M-BCR in Ph-negative CML occurs by a single cytogenetic event rather than a two-translocation mechanism.

Adult↗

PubMed1999

3' BCR recombines with IGL locus in BCR-ABL-positive philadelphia-negative chronic myeloid leukemia.

We have isolated the 3' BCR breakpoint junction of a complex BCR-ABL1 rearrangement found in leukemic cells with a cytogenetically normal karyotype, and the corresponding germline fragment that spanned the 3' BCR recombination site. Fluorescence in situ hybridization localized the 3' BCR recombination site to 22q11, about 350-600 kb proximal to BCR. Restriction map and DNA sequence comparisons indicated that 3' M-Bcr had recombined at a site within the variable region (Itv Region IV) of the immunoglobulin lambda (IGL) locus. Somatic rearrangement of DNA sequences (variable, joining, and constant regions) within the IGL locus, as in other Ig and TCR loci, represents the basis for human antibody diversity. Misrecombination of these somatically rearranging sites has been associated with chromosomal rearrangements in lymphoid leukemia and lymphoma, but there are no previous descriptions of IGL involvement in genomic aberrations associated with myeloid leukemia. Genes Chromosomes Cancer 26:366-371, 1999.

3' Untranslated Regions↗

PubMed1988

Undetectable bcr-abl rearrangements in some CML patients are due to a deletion mutation in the bcr gene.

Most patients with chronic myelogenous leukemia (CML) have Philadelphia (Ph) chromosome. Breakpoints on chromosome 22 in CML occur in a small region designated as the breakpoint cluster region (bcr). More than 90 percent of CML patients have breakpoints in the bcr; the remaining patients had no detectable rearrangement. In our study, a commercially available 1.2 kb HindIII-BglII (1.2 HBg) bcr probe was used to locate breakpoints in the bcr, which were found in 22 of 24 patients. Furthermore, using a probe upstream from the 1.2 HBg probe, rearranged bands were clearly detected in the two patients in whom no extra bands had been found with the 1.2 HBg probe. These results strongly suggest that these two patients carry a deletion at the acr-abl recombination point encompassing the area of the 1.2 HBg probe. Therefore, in our series, all CML patients eventually had breakpoints in the bcr, and the involvement of rearrangement was demonstrated to be highly specific for CML. Our data indicate that hybridization of CML cellular DNA with several bcr probes is important in examining accurately the frequency of bcr-abl rearrangements in CML, as some cases contain a deletion within the region.

Chromosome Aberrations↗

PubMed1993

Consistent involvement of the 3' half part of the first BCR intron in adult Philadelphia-positive leukaemia without M-bcr rearrangement.

About one half of Philadelphia (Ph)-positive acute leukaemia (AL) patients and rare cases of Ph-positive chronic myelogenous leukaemia (CML) show rearrangements within the first intron of the BCR gene on chromosome 22. We studied breakpoints within the first BCR intron in 22 adult patients with Ph-positive leukaemia; 21 with AL and one with CML, which lacked rearrangements within the major bcr (M-bcr). With a series of genomic probes from this intronic region, we detected chromosomal breaks in all 22 patients within the 35 kb area, corresponding almost to the 3' half portion of the intron. The breakpoints were distributed throughout this region but we could not identify any special cluster of breakpoints in this area. Our data support consistent involvement of the 3' half part of the first BCR intron in Ph-positive leukaemias without M-bcr rearrangement, and indicate relatively wide scattering of breakpoints in this portion of the intron.

Adult↗

PubMed1993

Subcellular localization of Bcr, Abl, and Bcr-Abl proteins in normal and leukemic cells and correlation of expression with myeloid differentiation.

We used specific antisera and immunohistochemical methods to investigate the subcellular localization and expression of Bcr, Abl, and Bcr-Abl proteins in leukemic cell lines and in fresh human leukemic and normal samples at various stages of myeloid differentiation. Earlier studies of the subcellular localization of transfected murine type IV c-Abl protein in fibroblasts have shown that this molecule resides largely in the nucleus, whereas transforming deletion variants are localized exclusively in the cytoplasm. Here, we demonstrate that the murine type IV c-Abl protein is also found in the nucleus when overexpressed in a mouse hematopoietic cell line. However, in both normal and leukemic human hematopoietic cells, c-Abl is discerned predominantly in the cytoplasm, with nuclear staining present, albeit at a lower level. In contrast, normal endogenous Bcr protein, as well as the aberrant p210BCR-ABL and p190BCR-ABL proteins consistently localize to the cytoplasm in both cell lines and fresh cells. The results with p210BCR-ABL were confirmed in a unique Ph1-positive chronic myelogenous leukemia (CML) cell line, KBM5, which lacks the normal chromosome 9 and hence the normal c-Abl product. Because the p210BCR-ABL protein appears cytoplasmic in both chronic phase and blast crisis CML cells, as does the p190BCR-ABL in Ph1-positive acute leukemia, a change in subcellular location of Bcr-Abl proteins between cytoplasm and nucleus cannot explain the different spectrum of leukemias associated with p210 and p190, nor the transition from the chronic to the acute leukemia phenotype seen in CML. Further analysis of fresh CML and normal hematopoietic bone marrow cells reveals that p210BCR-ABL, as well as the normal Bcr and Abl proteins, are expressed primarily in the early stages of myeloid maturation, and that levels of expression are reduced significantly as the cells mature to polymorphonuclear leukocytes. Similarly, a decrease in Bcr and Abl levels occurs in HL-60 cells induced by DMSO to undergo granulocytic differentiation. The action of p210BCR-ABL and its normal counterparts may, therefore, take place during the earlier stages of myeloid development.

Animals↗