Chronic myeloid leukemia stem cells and developing therapies

Usp5 links suppression of p53 and FAS levels in melanoma to the BRAF pathway

Usp5 is a deubiquitinase (DUB) previously shown to regulate unanchored polyubiquitin (Ub) chains, p53 transcriptional activity and double-strand DNA repair. In BRAF mutant melanoma cells, Usp5 activity was suppressed by BRAF inhibitor (vemurafenib) in sensitive but not in acquired or intrinsically resistant cells. Usp5 knockdown overcame acquired vemurafenib resistance and sensitized BRAF and NRAS mutant melanoma cells to apoptosis initiated by MEK inhibitor, cytokines or DNA-damaging agents. Knockdown and overexpression studies demonstrated that Usp5 regulates p53 (and p73) levels and alters cell growth and cell cycle distribution associated with p21 induction. Usp5 also regulates the intrinsic apoptotic pathway by modulating p53-dependent FAS expression. A small molecule DUB inhibitor (EOAI3402143) phenocopied the FAS induction and apoptotic sensitization of Usp5 knockdown and fully blocked melanoma tumor growth in mice. Overall, our results demonstrate that BRAF activates Usp5 to suppress cell cycle checkpoint control and apoptosis by blocking p53 and FAS induction; all of which can be restored by small molecule-mediated Usp5 inhibition. These results suggest that Usp5 inhibition can provide an alternate approach in recovery of diminished p53 (or p73) function in melanoma and can add to the targeted therapies already used in the treatment of melanoma.

p53 restoration kills primitive leukemia cells in vivo and increases survival of leukemic mice

Loss of p53 function is a common feature of human cancers and it is required for differentiated tumor cell maintenance; however, it is not known whether sustained inactivation of the p53 pathway is needed for cancer stem cell persistence. Chronic myeloid leukemia (CML) is caused by a chromosome translocation that generates the BCRABL oncogene encoding a constitutively active protein tyrosine kinase. The disease originates in a hematopoietic stem cell and is maintained by leukemic stem cells (LSCs). Treatment with specific tyrosine kinase inhibitors does not eliminate LSCs because they do not depend on the oncogene for survival. We have combined a switchable p53 knock-in mouse model, p53 (KI/KI) , with the well-characterized Sca1-BCRABLp210 CML transgenic model, to show that transient restoration of p53 slows disease progression and significantly extends the survival of leukemic animals, being the mechanism responsible for this effect, apoptotic death of primitive leukemia cells. In agreement with these in vivo findings, in vitro assays show that restoring p53 reduces hematopoietic colony formation by cells of leukemic animals. These results suggest that reestablishing p53 function may be a therapeutic strategy for the eradication of leukemic stem cells and to prevent disease progression.

Essential role for telomerase in chronic myeloid leukemia induced by BCR-ABL in mice

The telomerase protein is constitutively activated in malignant cells from many patients with cancer, including the chronic myeloid leukemia (CML), but whether telomerase is essential for the pathogenesis of this disease is not known. Here, we used telomerase deficient mice to determine the requirement for telomerase in CML induced by BCR-ABL in mouse models of CML. Loss of one telomerase allele or complete deletion of telomerase prevented the development of leukemia induced by BCR-ABL. However, BCR-ABL was expressed and active in telomerase heterozygous and null leukemic hematopoietic stem cells. These results demonstrate that telomerase is essential for oncogene-induced reprogramming of hematopoietic stem cells in CML development and validate telomerase and the genes it regulates as targets for therapy in CML.

Photocleavable peptide-oligonucleotide conjugates for protein kinase assays by MALDI-TOF MS

Robust methods for highly parallel, quantitative analysis of cellular protein tyrosine kinase activities may provide tools critically needed to decipher oncogenic signaling, discover new targeted drugs, diagnose cancer and monitor patients. Here, we describe proof-of-principle for a novel protein kinase assay with the potential to help overcome these challenges. MALDI-TOF mass spectrometry provides an ideal tool for label-free multiplexed analysis of peptide phosphorylation, but is poorly matched to homogeneous assays and complex samples. Thus, we conjugated a common oligonucleotide tag to multiple peptide substrates, offering efficient capture from solution-phase kinase reactions by annealing to the complementary sequence tethered to PEG-passivated superparamagnetic microparticles. To enable reversible conjugation, we developed a novel bifunctional cross-linker allowing simple and efficient preparation of photocleavable peptide-oligonucleotide conjugates. After washing away contaminants and following photorelease, MALDI-TOF analysis yielded relative phosphorylation of each peptide with high sensitivity and specificity. Validating the hybridization-mediated multiplexed kinase assay, when three peptide substrate-oligonucleotide conjugates were mixed with the tyrosine kinase c-Abl and ATP, we readily observed their differential phosphorylation yet measured a common IC(50) for the Abl kinase inhibitor imatinib. This new assay enables analysis of protein kinase activities in a multiplexed format amenable to screening inhibitors against multiple kinases in parallel, an important capability for drug discovery and predictive diagnostics.

Chronic myeloid leukemia stem cells in the era of targeted therapies: resistance, persistence and long-term dormancy

Targeted therapies of chronic myeloid leukemia (CML) using tyrosine kinase inhibitors (TKI) have profoundly changed the natural history of the disease with a major impact on survival. Molecular monitoring with BCR-ABL quantification shows that a status of undetectable molecular residual disease (UMRD) is obtained in a significant minority of patients. However, it remains unclear whether these patients are definitively cured of their leukemia. Imatinib mesylate withdrawal trials have demonstrated the rapid appearance of the malignant clone in the majority of the patients whereas some patients remain in a state of UMRD. It has clearly been demonstrated that the most primitive stem cells are refractory to all TKIs used in clinical practice. In addition, long-term dormancy is one of the most fundamental characteristics of hematopoietic stem cells. In this context, we have recently undertaken a systematic analysis of the bone marrow stem cell compartment in several patients in durable UMRD. We have demonstrated the long-term persistence of a considerable amount of BCR-ABL-expressing stem cells, even in the absence of relapse. The phenomenon of long-term leukemic stem cell dormancy is of major importance in CML and one of the key questions in cancer biology in general. We discuss, here, the potential mechanisms, including intrinsic and microenvironmental factors, that control the response of leukemic stem cells (LSCs) to targeted therapies and potential novel strategies currently in progress with a curative intent. Moreover, we propose a molecular evaluation of the residual LSC compartment in selected patients in order to develop rational TKI-cessation strategies in CML.

Bortezomib has little ex vivo activity in chronic myeloid leukemia: individual tumor response testing comparative study in acute and chronic myeloid leukemia

Aim of the study

Resistance to imatinib is one of the most important issues in treatment of chronic myeloid leukemia (CML) patients. The objective of the study was to analyze the ex vivo drug resistance profile to bortezomib and 22 other antileukemic drugs, including three tyrosine kinase inhibitors (TKIs), in CML in comparison to acute myeloid leukemia (AML).

Material and methods

A total of 82 patients entered the study, including 36 CML and 46 AML adults. Among CML patients, 19 had advanced disease, 16 were resistant to imatinib, and 6 had ABL-kinase domain mutations. The ex vivo drug resistance profile was studied by the MTT assay.

Results

CML cells were more resistant than AML blasts to the following drugs: prednisolone, vincristine, doxorubicin, etoposide, melphalan, cytarabine, fludarabine, thiotepa, 4-HOO-cyclophosphamide, thioguanine, bortezomib, topotecan, and clofarabine. CML cells were 2-fold more sensitive to busulfan than AML cells. CML patients with clinical imatinib resistance had higher ex vivo resistance to vincristine, daunorubicin, etoposide, and busulfan. No significant differences to all tested drugs, including TKIs, were observed between CML patients with non-advanced and advanced disease. CML patients with mutation had higher ex vivo resistance to vincristine, idarubicin, thiotepa, and busulfan.

Conclusions

CML cells are ex vivo more resistant to most drugs than acute myeloid leukemia blasts. Busulfan is more active in CML than AML cells. In comparison to AML cells, bortezomib has little ex vivo activity in CML cells. No differences between CML subgroups in sensitivity to 3 tested TKIs were detected.