Increased BCR-ABL Expression Research Articles

Targeting β-arrestin2 Enhances Survival in a Murine Model of Chronic Myeloid Leukemia

BackgroundChronic myeloid leukemia (CML) is a myeloproliferative neoplasm of hematopoietic stem cells characterized by presence of a dysregulated BCR-ABL fusion protein which leads to constitutive activation of tyrosine kinase activity. Classically, CML is treated with tyrosine kinase inhibitors (TKIs). However, TKI therapy is non-curative, and persistence of quiescent leukemia stem cells likely accounts for the inability of these agents to cure CML. β-arrestins are multifunctional adapter proteins which regulate G protein-coupled receptor (GPCR) signaling and trafficking and have recently been identified as mediators of distinct cellular signaling cascades independent of G proteins. β-arrestins also play a role in the Smoothened/Hedgehog pathway, as well as in the Wingless/Frizzled (Wnt/Fz) signaling axis, both of which have been associated with the development of CML. We, therefore, hypothesize that β-arrestin2 (βarr2) is necessary for the development and propagation of CML and may function as a therapeutic target. AimDemonstrate that loss of βarr2, using an inducible conditional knockout mouse model, slows progression of CML. MethodsWe used a standard murine retroviral transduction system to model chronic phase BCR-ABL positive CML. KLS cells (Lin-, Sca-1+, c-kit+) were harvested from bone marrow of donor C57BL6/J βarr2F/F-CreERT2+/- (Cre positive, CD45.2) and age matched C57BL6/J βarr2F/F-CreERT2-/- (Cre negative, CD54.2) male mice. KLS cells were retrovirally transduced with MSCV-BCR-ABL-IRES-GFP and were subsequently injected retro-orbitally into sublethally irradiated congenic wild type recipient male mice (CD45.1). Donor mice were engineered using global Cre-ER/loxP technology in order to induce site-specific recombination and loss of βarr2 when treated with tamoxifen. Mice who received Cre positive cells lost βarr2 only in hematopoietic cells. Recipient mice were treated with tamoxifen 75mg/kg daily via intraperitoneal injection for 5 days starting day 3 after transplant and were monitored for signs of leukemia development. Weekly blood analysis included WBC count, number of donor cells by flow cytometry, blood film, and qPCR for BCR-ABL expression. Survival was compared between animals receiving Cre positive (βarr2F/F-CreERT2+/-) and Cre negative (βarr2F/F-CreERT2-/-) donor cells as well as both tamoxifen treated and untreated conditions. ResultsTreatment of donor C57BL6/J βarr2F/F-CreERT2+/- mice with tamoxifen resulted in decreased βarr2 expression within 10 days in multiple tissues including spleen and bone marrow. By day 10, βarr2 expression was 9.5 ± 3.0% in Cre positive mice relative to pretreatment expression levels (Figure 1).In total, 8 mice received Cre positive cells and 11 mice received Cre negative cells. Ten of 11 (90.9%) Cre negative mice developed CML as evidenced by splenomegaly, leukocytosis, increased BCR-ABL expression measured by qPCR and increased number of donor cells detectable by flow cytometry. Median survival was 15 days. Six of 8 (75%) Cre positive mice developed disease with median survival of 27 days (HR 3.2, 95% CI; 1.525-12.2, p=0.013)(Figure 2). At day 11, flow cytometry for donor CD45.2 cells present in peripheral blood of recipient mice was 38 ± 2.05% in Cre negative versus 16.5± 3.9% in Cre positive mice (p<0.0001). Spleen size at death and WBC count at day 11 were not significantly different between groups. ConclusionsThese data demonstrate that targeting βarr2 prolongs the course of disease in chronic phase CML and raise the possibility that loss of βarr2 after disease onset may lead to disease regression. βarr2 may therefore represent an alternative therapeutic target for CML independent of tyrosine kinase inhibition. [Display omitted] [Display omitted] Disclosures:No relevant conflicts of interest to declare.

Tyrosine kinase inhibitor resistance in chronic myeloid leukemia cell lines: investigating resistance pathways

There are three currently identified secondary resistance mechanisms observed in patients with chronic myeloid leukemia (CML) receiving tyrosine kinase inhibitors (TKIs). These are BCR–ABL kinase domain (KD) mutations, increased BCR–ABL expression, and overexpression of drug-efflux proteins (ABCB1 and ABCG2). To investigate the interplay between these three modes of resistance, three CML blast crisis cell lines (K562, its ABCB1-overexpressing variant K562 Dox, and KU812) were cultured in gradually increasing concentrations of imatinib to 2 μM, or dasatinib to 200 nM. Eight imatinib- and two dasatinib-resistant cell lines were established. Two imatinib-resistant K562 lines both had increased BCR–ABL expression as the apparent mode of resistance. However, when a dasatinib-resistant K562 culture was generated we observed gradually increasing BCR–ABL expression which peaked prior to identification of the T315I mutation. BCR–ABL overexpression followed by mutation development was observed in a further 4/10 cell lines, each with different KD mutations. In contrast, three imatinib-resistant K562 Dox lines exhibited only a further increase in ABCB1 expression. All TKI-resistant cell lines generated had increased IC50 (dose of drug required to reduce phosphorylation of the adaptor protein p-Crkl by 50%) to imatinib, dasatinib, and nilotinib, regardless of which TKI was used to induce resistance. This suggests that currently available TKIs share the same susceptibilities to drug resistance.

Modelling of TKI Resistance In CML Cell Lines: Kinase Domain Mutations Usually Arise In the Setting of BCR-ABL Overexpression.

AbstractAbstract 3383There are three currently identified secondary resistance mechanisms observed in chronic myeloid leukemia (CML) patients receiving tyrosine kinase inhibitors (TKIs). These include overexpression of drug-efflux proteins (ABCB1 and ABCG2), increased BCR-ABL expression, and mutations in the kinase domain (KD) of BCR-ABL. We investigated the interplay between these three modes of resistance in vitro, as well as looking for other mechanisms. Three CML blast crisis cell lines (K562, its ABCB1-overexpressing variant, K562 Dox, and KU812) were cultured in gradually increasing concentrations of imatinib (IM) to 2μ M, or dasatinib (DAS) to 200nM. Two IM-resistant K562 lines were established, both with increased IC50s for IM (from 13.7μ M in naïve cells, to ~50μ M), as well as increased IC50s for DAS and nilotinib (NIL). No cell-surface expression of ABCB1 or ABCG2 was observed, nor were KD mutations present. However, BCR-ABL expression was seen to steadily increase in both lines from 178% in naïve cells, to ~380% and 1200% in the resistant lines, suggesting this was the major mode of resistance. However, when a DAS-resistant K562 culture was generated the T315I mutation emerged. Studies of the intermediate stages of resistance revealed that BCR-ABL overexpression occurred in a step-wise fashion, peaking at 1915% in the 3.5nM intermediate, but then dropping significantly to ~1000% in the 5nM intermediate (P=0.0003). BCR-ABL expression then stabilised at this level, and the T315I mutation was subsequently detected. Thus, it appears that BCR-ABL overexpression was the first mechanism of resistance detectable, but was followed by the emergence of a KD mutation which had a clear selective advantage. This sequential selection was observed a further four times: in a DAS-resistant K562 Dox culture, and in three IM-resistant KU812 cultures. BCR-ABL expression in the DAS-resistant K562 Dox culture increased from 186% in naïve cells to 540% in the final culture. Studies of intermediate cultures revealed that BCR-ABL expression peaked at 850% in the 55nM intermediate, but then dropped significantly to ~500% in the 75nM intermediate (P=0.004). This drop in BCR-ABL expression coincided with the appearance of the V299L mutation. Interestingly, the K562 Dox DAS-resistant line also displayed resistance to NIL and IM, likely conferred by BCR-ABL overexpression as the 55nM intermediate (with the highest BCR-ABL expression levels) had the highest IC50s for NIL and IM, while the 75nM intermediate (with the V299L mutation) had increased IC50DAS but lower NIL and IM IC50s. Thus, BCR-ABL overexpression was the primary event, followed by the KD mutation. Likewise in three IM-resistant KU812 cultures, BCR-ABL expression levels rose from 443% in naïve cells, to peak levels of 6210%, 10,448% and 990% respectively, followed by drops in expression which coincided with the appearance of compound KD mutations, and the F359C mutation respectively (Table). In contrast, three IM-resistant K562 Dox cells were not found to have any KD mutations, nor BCR-ABL overexpression. Instead, the primary cause of resistance in these lines appears to be a further increase in ABCB1 expression. All three lines had increased IC50s for IM (from 12μ M in naïve cells, to ~27μ M), NIL (from 400nM to ~1000nM) and DAS (from 100nM to >625nM). The addition of PSC833 (an ABCB1 inhibitor) decreased the IM, NIL, and DAS IC50s for all three resistant lines to the level of the naïve control (~3μ M, ~250nM and ~10nM respectively), indicating that ABCB1 expression, facilitating active efflux of the drugs, is the primary mechanism of resistance in these lines. We have demonstrated that KD emergence is a stochastic event, as the same mutation did not always occur twice, however BCR-ABL and ABCB1 overexpression were more likely to arise recurrently in predisposed lines. Notably, different TKIs elicited different resistant mechanisms, but all were BCR-ABL dependent. Furthermore, all resistant lines showed cross-resistance to the three TKIs tested (IM, DAS and NIL), suggesting that currently available TKIs share the same susceptibilities to drug resistance.Table 1Summary of resistance mechanisms detected in three cell lines exposed to IM or DAS. ✓ = yes; × = no.Culture conditionK562K562 DoxKU812IMIMDASIMIMIMDASIMIMIMResistance to 3 TKIs✓✓✓✓✓✓✓✓✓✓KD MutationxxT315IxxxV299LE450QE459KE470KE459KE462KE466EF359CIncreased BCR-ABL✓✓✓xxx✓✓✓✓Increased ABCB1xxx✓✓✓xxxx Disclosures:White:Novartis Pharmaceuticals: Honoraria, Research Funding; Bristol-Myers Squibb: Honoraria, Research Funding. Hughes:Novartis Pharmaceuticals: Honoraria, Research Funding, Speakers Bureau; Bristol-Myers Squibb: Honoraria, Research Funding, Speakers Bureau.

Increased eIF4E Expression and Phosphorylation in Late Phase Chronic Myelogenous Leukemia Occurs in a Bcr-Abl-Dependent Manner, and Can Be Targeted by a Novel Mnk Kinase Inhibitor, CGP57380, To Overcome Imatinib-Resistance.

The molecular mechanisms which mediate progression of chronic phase (CP) CML to accelerated and blast phase (BP) disease remain unclear, although one feature that correlates with progression is increased expression of the Bcr-Abl protein itself (Barnes et al., Can. Res. 2005). Increased Bcr-Abl expression is likely to contribute to the more aggressive behavior of BP disease, but the downstream factors that are dysregulated by the increased amounts of Bcr-Abl protein remain to be determined. In these studies we turned our attention to eIF4E since forced expression of eIF4E is transforming, and because increased levels of eIF4E have been found in BP but not CP CML (Topisirovic et al., Mol. Cell. Bio. 2003). eIF4E plays a critical role in cap-dependent translation and allows recruitment of the translation machinery to mRNA. eIF4E is phosphorylated at Ser209, and phosphorylation correlates with exposure to growth factors and increased cap-dependent translation. Using a panel of primary CML cells representing patients at various stages of disease, we confirmed that both Bcr-Abl and eIF4E protein levels were elevated in BP samples compared to those in CP, and furthermore that phosphorylation at Ser209 was dependent on Bcr-Abl kinase activity in BP but not CP samples. We next went on to explore the role of eIF4E phosphorylation in BP CML. Because eIF4E is exclusively phosphorylated at Ser209 by the MAPK signal-integrating kinases (Mnk1/2), we used a small molecule inhibitor of Mnk1/2, CGP57380, to inhibit eIF4E phosphorylation (kind gift of Dr. H. Gram, Novartis). Using MTS assays, we found that CGP57380 exhibited synergistic activity with imatinib mesyalte (IM) against Ba/F3-Bcr-Abl and K562 cells, and that this was associated with increased caspase-3 activation. Consistent with a role for eIF4E phosphorylation in cap-dependent translation, we found that CGP57380 augmented the IM-mediated inhibition of cap-binding complex (eIF4F) formation, as well as loading of mRNA onto polysomes. Interestingly, we also uncovered the existence of a novel negative-feedback loop regulating Mnk kinase. Here, treatment with CGP57380 resulted in increased phosphorylation of Mnk1 as well as its upstream activator, ERK, in a time- and dose-dependent manner. Because activation of the MEK/ERK pathway is essential to Bcr-Abl-mediated transformation, this finding suggested that the full activity of CGP57380 might be obscured by this feedback loop. In support of this, the addition of the MEK inhibitor, U0126, to the IM/CGP57380combination resulted in increased activity against CML cells. The triple combination was also effective against Ba/F3-Bcr-Abl cells harboring the E255K and T315I mutations, but not parental Ba/F3 cells (reduced by 50, 23, and 15% respectively of DMSO-treated controls by MTS assay). Colony forming assays also demonstrated the activity of the IM/CGP57380 combination against CML progenitor cells. In conclusion, our data demonstrate that:eIF4E protein expression and phosphorylation are upregulated in a Bcr-Abl-dependent manner in BP CML;Inhibition of eIF4E phosphorylation by the novel Mnk kinase inhibitor, CGP57380, synergizes with IM in killing CML cells, as well as overcomes certain forms of IM-resistance;The addition of CGP57380 to IM results in inhibition of key steps in cap-dependent mRNA translation, and may provide a mechanistic explanation for the activity of this agent in CML.

Leukemic Stem Cells of Chronic Phase CML Patients Possess Uniquely Elevated BCR-ABL Kinase Activity and Acquire Spontaneous BCR-ABL Kinase Domain Mutations at a High Frequency.

Growing evidence indicates that the therapeutic potential of imatinib mesylate (IM) for the treatment of CML may be limited initially by a relative innate resistance of the leukemic stem cells and eventually by an accumulation of cells with BCR-ABL tyrosine kinase domain mutations. We now show that the amount and tyrosine kinase activity of p210-BCR-ABL in the most primitive and relatively IM-unresponsive lin−CD34+CD38− CML cells is 3 to 10-fold higher than in the majority of the lin−CD34+CD38+ CML progenitors (n=3). These results confirm previous BCR-ABL transcript data and identify elevated p210-BCR-ABL expression to be a likely important factor in the characteristic IM-insensitivity of very primitive CML cells. To determine whether in vivo, CML stem cells also accumulate gene mutations affecting the BCR-ABL kinase domain, cDNAs were prepared from RNA extracts of purified lin−CD34+CD38− cells isolated from 3 chronic phase patients that had not received IM therapy. Bidirectional sequencing of individually cloned cDNAs from these samples revealed BCR-ABL kinase domain mutations in 2 of the 3 patients at frequencies of 10% (1/10), 20% (2*/10,*identical mutations). Incubation of these lin−CD34+CD38− cells in vitro for 2–3 wk ± a high concentration of IM (up to 10 μM, which was sufficient to reduce the tyrosine kinase activity in the input cells by 70±12% and in their 2 wk progeny by 10±5%) selected a subpopulation of more differentiated and completely IM-resistant cells. This was shown in Western blots by the inability of 10 μM IM to reduce either their p210-BCR-ABL tyrosine kinase activity or CrkL phosphorylation and in methylcellulose assays ±5 μM IM. As predicted, IM-selected cells showed a higher frequency of kinase domain mutations (13–20% vs 0–20% of cDNA clones analyzed from 3 wk cells cultured ±IM). Analysis of individual colonies produced from CFCs in the cultured cells showed all (21/21) colonies from IM-selected cells had mutations vs 50% (5/10) in those cultured without IM. The total frequency of mutant cDNAs detected was also increased in the IM-resistant cells (35–55% vs 10–25% mutant cDNAs in selected vs control cells). Interestingly, in most cases, both wild-type and mutant cDNAs were identified in the same colony, indicating de novo generation of mutations in vitro. Overall, >50 different mutations were identified. These included 10 point mutations previously associated with clinical IM resistance (including G250 and T315), another 13 point mutations previously identified in a comprehensive mutational screen, and >20 previously undescribed mutations. Several of the latter affect the critical region of the P loop, the c-helix and the activation loop and would be predicted to confer significant IM resistance. To investigate the possibility that the observed genomic instability of very primitive CML cells might be related to their elevated innate p210-BCR-ABL activity, BCR-ABL transcript levels in individual IM-selected, fully resistant and control (similarly treated but no IM exposure) colonies were compared. This showed that BCR-ABL transcripts were ~20-fold higher (P<0.05) in the resistant colonies (30 assessed from 3 patients). These findings suggest that the increased BCR-ABL expression and activity that uniquely characterizes the most primitive CML cells may contribute not only to their innate insensitivity to IM but also to a deregulation of genomic stability leading to the emergence of IM-resistant mutants and other subclones associated with disease progression.

Various Mechanisms Underlie Cytogenetic Refractoriness to Imatinib.

Background: Imatinib is the first line drug therapy for patients with chronic myeloid leukemia (CML). Drug resistance, after an initial response, is frequently due to reactivation of the BCR-ABL kinase as a result of point mutations within the kinase domain (KD) or overexpression of the protein. In contrast, mechanisms underlying disease persistence (refractoriness) at the cytogenetic level are unknown.Patients and Methods: We investigated potential causes for cytogenetic refractoriness in 28 patients with CML who achieved a complete hematological response (CHR) but remained at least 65% Philadelphia chromosome positive (Ph+) during the first 10 months of imatinib therapy. At the time of starting imatinib, 5 patients were in accelerated phase (AP), of which 3 had only cytogenetic clonal evolution as the defining criteria for AP; the remaining 23 were in first chronic phase (CP). All patients initiated therapy with at least 400mg of imatinib daily, with 3 also receiving imatinib in combination with low dose cytarabine. Median time from diagnosis to starting imatinib was 43.3 months (range, 9 to 103.4) and median time to CHR was 41.5 days (range, 19 to 220). Patients were analyzed for evidence of clonal evolution (conventional cytogenetics), mutations in the KD (D-HPLC and direct sequencing of PCR products generated with BCR-ABL-specific primers), expression of BCR-ABL, and MDR1 mRNA (quantitative RT-PCR). Samples prior to starting imatinib were available in 16 patients. Median follow up was 28.3 months (range, 10 to 58.3).Results: Between 9 and 15 months from starting imatinib, 4 of 28 patients had a single KD mutation detected (M351T, G250E, Y253H, Y253F). Three out of 16 patients demonstrated a >2 fold increase in BCR-ABL expression (range, 2.68 to 15.14), and 4 out of 16 patients showed a >2 fold increased expression of MDR1 (range, 2.72 to 47.57), compared to pre-imatinib levels. One patient demonstrated an increase in both BCR-ABL and MDR1 expression. (None of the patients with increased BCR-ABL or MDR1 expression had a KD mutation detected.) Overall, 14 patients progressed, including all patients with a KD mutation. The majority of patients with increased BCR-ABL (2/3) or MDR1 (3/4) expression, including the patient with dual increased expression, did not progress during follow up but remained at least 95% Ph+.Conclusions: Twenty eight patients with cytogenetic refractoriness to imatinib were examined and KD mutations were detected in 4. Of the patients with samples available prior to starting imatinib, a >2 fold increase in expression of BCR-ABL and the drug resistance gene MDR1 were seen in 3/16 and 4/16 patients respectively. Overall, these data suggest that various mechanisms, including increased BCR-ABL expression and induction of drug resistance gene expression, underlie cytogenetic refractoriness in our patient population. Studies are ongoing to further examine the role of drug resistance genes in patients with cytogenetic refractoriness, as compared to patients who do achieve a major cytogenetic response within the first year of imatinib therapy.

Imatinib mesylate resistance through BCR-ABL independence in chronic myelogenous leukemia.

Imatinib mesylate (IM) binds to the BCR-ABL protein, inhibiting its kinase activity and effectively controlling diseases driven by this kinase. IM resistance has been associated with kinase mutations or increased BCR-ABL expression. However, disease progression may be mediated by other mechanisms that render tumor cells independent of BCR-ABL. To demonstrate this potential, IM-resistant cells were found in chronic myelogenous leukemia patients with continuous BCR-ABL gene expression but undetectable BCR-ABL protein expression. These cells were unresponsive to IM and acquired BCR-ABL-independent signaling characteristics. IM resistance in some patients may be mediated through loss of kinase target dependence.

Elevated Bcr-Abl expression levels are sufficient for a haematopoietic cell line to acquire a drug-resistant phenotype.

A characteristic feature of chronic myeloid leukaemia (CML) is the inevitable advancement from a treatable chronic phase to a fatal, drug-resistant stage referred to as blast crisis. The molecular mechanisms responsible for this disease transition remain unknown. As increased expression of Bcr-Abl has been associated with blast crisis CML, we have established transfectants in 32D cells that express low and high levels of Bcr-Abl, and assessed their drug sensitivity. Cells with high Bcr-Abl expression levels are resistant to conventional cytotoxic drugs, and also require higher levels of STI571 (an inhibitor of Bcr-Abl), to induce cell death. Co-treatment with cytotoxic drugs and STI571 increased the sensitivity of the drug-resistant cells. Despite the drug-resistant phenotype, high Bcr-Abl levels concomitantly increased the expression of p53, p21, Bax and down-regulated Bcl-2. These cells maintain a survival advantage irrespective of a reduced proportion of cycling cells and the pro-apoptotic shift in gene expression. In addition, the level of Bcr-Abl expression (high or low) does not alter the growth factor independence and elevated Bcl-xL expression observed. Our study indicates that drug resistance can be primarily attained by increased Bcr-Abl expression, and highlights the potential of therapy which combines STI571 with conventional cytotoxic drugs.