Ravi Bhatia

miR-328 Functions as an RNA Decoy to Modulate hnRNP E2 Regulation of mRNA Translation in Leukemic Blasts

MicroRNAs and heterogeneous ribonucleoproteins (hnRNPs) are posttranscriptional gene regulators that bind mRNA in a sequence-specific manner. Here, we report that loss of miR-328 occurs in blast crisis chronic myelogenous leukemia (CML-BC) in a BCR/ABL dose- and kinase-dependent manner through the MAPK-hnRNP E2 pathway. Restoration of miR-328 expression rescues differentiation and impairs survival of leukemic blasts by simultaneously interacting with the translational regulator poly(rC)-binding protein hnRNP E2 and with the mRNA encoding the survival factor PIM1, respectively. The interaction with hnRNP E2 is independent of the microRNAs seed sequence and it leads to release of CEBPA mRNA from hnRNP E2-mediated translational inhibition. Altogether, these data reveal the dual ability of a microRNA to control cell fate both through base pairing with mRNA targets and through a decoy activity that interferes with the function of regulatory proteins.

Activation of p53 by SIRT1 inhibition enhances elimination of CML leukemia stem cells in combination with imatinib.

BCR-ABL tyrosine kinase inhibitors (TKI) fail to eliminate quiescent leukemia stem cells (LSC) in chronic myelogenous leukemia (CML). Thus, strategies targeting LSC are required to achieve cure. We show that the NAD(+)-dependent deacetylase SIRT1 is overexpressed in human CML LSC. Pharmacological inhibition of SIRT1 or SIRT1 knockdown increased apoptosis in LSC of chronic phase and blast crisis CML and reduced their growth in vitro and in vivo. SIRT1 effects were enhanced in combination with the BCR-ABL TKI imatinib. SIRT1 inhibition increased p53 acetylation and transcriptional activity in CML progenitors, and the inhibitory effects of SIRT1 targeting on CML cells depended on p53 expression and acetylation. Activation of p53 via SIRT1 inhibition represents a potential approach to target CML LSC.

Solid Cancers After Bone Marrow Transplantation

PURPOSE To evaluate the incidence and associated risk factors of solid cancers after bone marrow transplantation (BMT). PATIENTS AND METHODS We analyzed 2,129 patients who had undergone BMT for hematologic malignancies at the City of Hope National Medical Center between 1976 and 1998. A retrospective cohort and nested case-control study design were used to evaluate the role of pretransplantation therapeutic exposures and transplant conditioning regimens. RESULTS Twenty-nine patients developed solid cancers after BMT, which represents a two-fold increase in risk compared with a comparable normal population. The estimated cumulative probability (+/- SE) for development of a solid cancer was 6.1% +/- 1.6% at 10 years. The risk was significantly elevated for liver cancer (standardized incidence ratio [SIR], 27.7; 95% confidence interval [CI], 1.9 to 57.3), cancer of the oral cavity (SIR, 17.4; 95% CI, 6.3 to 34.1), and cervical cancer (SIR, 13.3; 95% CI, 3.5 to 29.6). Each of the two patients with liver cancer had a history of chronic hepatitis C infection. All six patients with squamous cell carcinoma of the skin had chronic graft-versus-host disease. The risk was significantly higher for survivors who were younger than 34 years of age at time of BMT (SIR, 5.3; 95% CI, 2.7 to 8.6). Cancers of the thyroid gland, liver, and oral cavity occurred primarily among patients who received total-body irradiation. CONCLUSION The risk of radiation-associated solid tumor development after BMT is likely to increase with longer follow-up. This underscores the importance of close monitoring of patients who undergo BMT.

Chronic myeloid leukemia stem cells are not dependent on Bcr-Abl kinase activity for their survival

Recent evidence suggests chronic myeloid leukemia (CML) stem cells are insensitive to kinase inhibitors and responsible for minimal residual disease in treated patients. We investigated whether CML stem cells, in a transgenic mouse model of CML-like disease or derived from patients, are dependent on Bcr-Abl. In the transgenic model, after retransplantation, donor-derived CML stem cells in which Bcr-Abl expression had been induced and subsequently shut off were able to persist in vivo and reinitiate leukemia in secondary recipients on Bcr-Abl reexpression. Bcr-Abl knockdown in human CD34(+) CML cells cultured for 12 days in physiologic growth factors achieved partial inhibition of Bcr-Abl and downstream targets p-CrkL and p-STAT5, inhibition of proliferation and colony forming cells, but no reduction of input cells. The addition of dasatinib further inhibited p-CrkL and p-STAT5, yet only reduced input cells by 50%. Complete growth factor withdrawal plus dasatinib further reduced input cells to 10%; however, the surviving fraction was enriched for primitive leukemic cells capable of growth in a long-term culture-initiating cell assay and expansion on removal of dasatinib and addition of growth factors. Together, these data suggest that CML stem cell survival is Bcr-Abl kinase independent and suggest curative approaches in CML must focus on kinase-independent mechanisms of resistance.

Effective Targeting of Quiescent Chronic Myelogenous Leukemia Stem Cells by Histone Deacetylase Inhibitors in Combination with Imatinib Mesylate

Imatinib mesylate (IM) induces remission in chronic myelogenous leukemia (CML) patients but does not eliminate leukemia stem cells (LSCs), which remain a potential source of relapse. Here we investigated the ability of HDAC inhibitors (HDACis) to target CML stem cells. Treatment with HDACis combined with IM effectively induced apoptosis in quiescent CML progenitors resistant to elimination by IM alone, and eliminated CML stem cells capable of engrafting immunodeficient mice. In vivo administration of HDACis with IM markedly diminished LSCs in a transgenic mouse model of CML. The interaction of IM and HDACis inhibited genes regulating hematopoietic stem cell maintenance and survival. HDACi treatment represents an effective strategy to target LSCs in CML patients receiving tyrosine kinase inhibitors.

Altered Microenvironmental Regulation of Leukemic and Normal Stem Cells in Chronic Myelogenous Leukemia

We characterized leukemia stem cells (LSC) in chronic phase chronic myelogenous leukemia (CML) using a transgenic mouse model. LSC were restricted to cells with long-term hematopoietic stem cell (LTHSC) phenotype. CML LTHSC demonstrated reduced homing and retention in the bone marrow (BM), related to decreased CXCL12 expression in CML BM, resulting from increased G-CSF production by leukemia cells. Altered cytokine expression in CML BM was associated with selective impairment of normal LTHSC growth and a growth advantage to CML LTHSC. Imatinib (IM) treatment partially corrected abnormalities in cytokine levels and LTHSC growth. These results were validated using human CML samples and provide improved understanding of microenvironmental regulation of normal and leukemic LTHSC and their response to IM in CML.

T cells expressing CD123-specific chimeric antigen receptors exhibit specific cytolytic effector functions and antitumor effects against human acute myeloid leukemia.

Induction treatments for acute myeloid leukemia (AML) have remained largely unchanged for nearly 50 years, and AML remains a disease of poor prognosis. Allogeneic hematopoietic cell transplantation can achieve cures in select patients and highlights the susceptibility of AML to donor-derived immunotherapy. The interleukin-3 receptor α chain (CD123) has been identified as a potential immunotherapeutic target because it is overexpressed in AML compared with normal hematopoietic stem cells. Therefore, we developed 2 chimeric antigen receptors (CARs) containing a CD123-specific single-chain variable fragment, in combination with a CD28 costimulatory domain and CD3-ζ signaling domain, targeting different epitopes on CD123. CD123-CAR-redirected T cells mediated potent effector activity against CD123+ cell lines as well as primary AML patient samples. CD123 CAR T cells did not eliminate granulocyte/macrophage and erythroid colony formation in vitro. Additionally, T cells obtained from patients with active AML can be modified to express CD123 CARs and are able to lyse autologous AML blasts in vitro. Finally, CD123 CAR T cells exhibited antileukemic activity in vivo against a xenogeneic model of disseminated AML. These results suggest that CD123 CAR T cells are a promising immunotherapy for the treatment of high-risk AML.

Persistence of leukemia stem cells in chronic myelogenous leukemia patients in prolonged remission with imatinib treatment

Imatinib mesylate treatment markedly reduces the burden of leukemia cells in chronic myelogenous leukemia (CML) patients. However, patients remain at risk for relapse on discontinuing treatment. We have previously shown that residual BCR-ABL(+) progenitors can be detected in CML patients within the first 2 years of imatinib treatment. However, reduced rates of relapse and continued decline of BCR-ABL levels with prolonged treatment, together with the ability of selected patients to maintain remission after discontinuing treatment, led us to investigate whether prolonged imatinib exposure resulted in reduction or elimination of BCR-ABL(+) stem cells. We evaluated BCR-ABL expression in CD34(+)CD38(+) (38(+)) committed progenitors and CD34(+)CD38(-) (38(-)) stem/primitive progenitor cells in samples from CML patients on imatinib treatment for at least 4 years with cytogenetic and molecular response. High levels of BCR-ABL expression were maintained over time in the 38(-) stem cell fraction. The absolute frequency of BCR-ABL(+) cells as determined by limiting dilution analysis was consistently higher in 38(-) compared with 38(+) cells. Transplantation into NOD/SCID-IL2Rγ-chain knockout mice demonstrated that BCR-ABL(+) cells had long-term in vivo repopulating capacity. These results directly demonstrate that BCR-ABL(+) stem cells persist in CML patients despite prolonged treatment with imatinib, and support ongoing efforts to target this population.

Interferon-alpha restores normal adhesion of chronic myelogenous leukemia hematopoietic progenitors to bone marrow stroma by correcting impaired beta 1 integrin receptor function

Treatment of chronic myelogenous leukemia (CML) with interferon-alpha frequently results in normalization of peripheral blood counts and, in up to 20% of patients, reestablishment of normal hematopoiesis. We hypothesize that interferon-alpha may restore normal adhesive interactions between CML progenitors and the bone marrow microenvironment and restore normal growth regulatory effects resulting from these progenitor-stroma interactions. We demonstrate that treatment with interferon-alpha induces a significant, dose-dependent increase in the adhesion of primitive long-term culture initiating cells and committed colony-forming cells (CFC) from CML bone marrow to normal stroma. Adhesion of CFC seen after interferon-alpha treatment could be inhibited by blocking antibodies directed at the alpha 4, alpha 5, and beta 1 integrins and vascular cell adhesion molecule, but not CD44 or intracellular adhesion molecule, suggesting that interferon-alpha induces normalization of progenitor-stroma interactions in CML. Because FACS analysis showed that the level of alpha 4, alpha 5, and beta 1 integrin expression after interferon-alpha treatment is unchanged, this suggests that interferon-alpha may restore normal beta 1 integrin function. Normalization of interactions between CML progenitors and the bone marrow microenvironment may then result in the restoration of normal regulation of CML progenitor proliferation, and explain, at least in part, the therapeutic efficacy of interferon-alpha in CML.

Microenvironmental protection of CML stem and progenitor cells from tyrosine kinase inhibitors through N-cadherin and Wnt-β-catenin signaling.

Tyrosine kinase inhibitors (TKIs) are highly effective in treatment of chronic myeloid leukemia (CML) but do not eliminate leukemia stem cells (LSCs), which remain a potential source of relapse. TKI treatment effectively inhibits BCR-ABL kinase activity in CML LSCs, suggesting that additional kinase-independent mechanisms contribute to LSC preservation. We investigated whether signals from the bone marrow (BM) microenvironment protect CML LSCs from TKI treatment. Coculture with human BM mesenchymal stromal cells (MSCs) significantly inhibited apoptosis and preserved CML stem/progenitor cells following TKI exposure, maintaining colony-forming ability and engraftment potential in immunodeficient mice. We found that the N-cadherin receptor plays an important role in MSC-mediated protection of CML progenitors from TKI. N-cadherin-mediated adhesion to MSCs was associated with increased cytoplasmic N-cadherin-β-catenin complex formation as well as enhanced β-catenin nuclear translocation and transcriptional activity. Increased exogenous Wnt-mediated β-catenin signaling played an important role in MSC-mediated protection of CML progenitors from TKI treatment. Our results reveal a close interplay between N-cadherin and the Wnt-β-catenin pathway in protecting CML LSCs during TKI treatment. Importantly, these results reveal novel mechanisms of resistance of CML LSCs to TKI treatment and suggest new targets for treatment designed to eradicate residual LSCs in CML patients.