Targeted signal transduction therapies in myeloid malignancies

A Genome-Wide CRISPR Screen Identifies Genes Critical for Resistance to FLT3 Inhibitor AC220

Acute myeloid leukemia (AML) is a malignant hematopoietic disease and the most common type of acute leukemia in adults. The mechanisms underlying drug resistance in AML are poorly understood. Activating mutations in FMS-like tyrosine kinase 3 (FLT3) are the most common molecular abnormality in AML. Quizartinib (AC220) is a potent and selective second-generation inhibitor of FLT3. It is in clinical trials for the treatment of relapsed or refractory FLT3-ITD-positive and -negative AML patients and as maintenance therapy. To understand the mechanisms of drug resistance to AC220, we undertook an unbiased approach with a novel CRISPR-pooled library to screen new genes whose loss of function confers resistance to AC220. We identified SPRY3, an intracellular inhibitor of FGF signaling, and GSK3, a canonical Wnt signaling antagonist, and demonstrated reactivation of downstream FGF/Ras/ERK and Wnt signaling as major mechanisms of resistance to AC220. We confirmed these findings in primary AML patient samples. Expression of SPRY3 and GSK3A was dramatically reduced in AC220-resistant AML samples, and SPRY3-deleted primary AML cells were resistant to AC220. Intriguingly, expression of SPRY3 was greatly reduced in GSK3 knockout AML cells, which positioned SPRY3 downstream of GSK3 in the resistance pathway. Taken together, our study identified novel genes whose loss of function conferred resistance to a selective FLT3 inhibitor, providing new insight into signaling pathways that contribute to acquired resistance in AML. Cancer Res; 77(16); 4402-13. ©2017 AACR.

The protein phosphatase 2A regulatory subunit B55α is a modulator of signaling and microRNA expression in acute myeloid leukemia cells

We recently discovered that the protein phosphatase 2A (PP2A) B55α subunit (PPP2R2A) is under-expressed in primary blast cells and is unfavorable for remission duration in AML patients. In this study, reverse phase protein analysis (RPPA) of 230 proteins in 511 AML patient samples revealed a strong correlation of B55α with a number of proteins including MYC, PKC α, and SRC. B55α suppression in OCI-AML3 cells by shRNA demonstrated that the B subunit is a PKCα phosphatase. B55α does not target SRC, but rather the kinase suppresses protein expression of the B subunit. Finally, the correlation between B55α and MYC levels reflected a complex stoichiometric competition between B subunits. Loss of B55α in OCI-AML3 cells did not change global PP2A activity and the only isoform that is induced is the one containing B56α. In cells containing B55α shRNA, MYC was suppressed with concomitant induction of the competing B subunit B56α (PPP2R5A). A recent study determined that FTY-720, a drug whose action involves the activation of PP2A, resulted in the induction of B55α In AML cells, and a reduction of the B subunit rendered these cells resistant to FTY-720. Finally, reduction of the B subunit resulted in an increase in the expression of miR-191-5p and a suppression of miR-142-3p. B55α regulation of these miRs was intriguing as high levels of miR-191 portend poor survival in AML, and miR-142-3p is mutated in 2% of AML patient samples. In summary, the suppression of B55α activates signaling pathways that could support leukemia cell survival.

STAT5 acetylation: Mechanisms and consequences for immunological control and leukemogenesis

The cytokine-inducible transcription factors signal transducer and activator of transcription 5A and 5B (STAT5A and STAT5B) are important for the proper development of multicellular eukaryotes. Disturbed signaling cascades evoking uncontrolled expression of STAT5 target genes are associated with cancer and immunological failure. Here, we summarize how STAT5 acetylation is integrated into posttranslational modification networks within cells. Moreover, we focus on how inhibitors of deacetylases and tyrosine kinases can correct leukemogenic signaling nodes involving STAT5. Such small molecules can be exploited in the fight against neoplastic diseases and immunological disorders.

Selective small molecule inhibitors of p110α and δ isoforms of phosphoinosityl-3-kinase are cytotoxic to human acute myeloid leukemia progenitors

The phosphoinosityl-3-kinase (PI3K) pathway is frequently constitutively active in blast cells from acute myeloid leukemia (AML) patients. RNA and protein from all four catalytic isoforms of PI3K (p110α, β, γ, and δ) were expressed in 38 AML samples, which also showed expression of phosphorylated Akt Ser473, indicating PI3K activation. Initial treatment of 12 AML samples with inhibitors targeting each of the four isoforms demonstrated that p110α and δ inhibition are more effective in killing AML blast colony-forming cells (CFC) than p110β or γ inhibition. In subsequent experiments, AML CFC from 46 patient samples were treated with the p110α and δ selective inhibitors, PI3Kα inhibitor 2 or PCN5603, and dose-dependent progenitor kill and inhibition of phosphorylated Akt Ser473 expression was observed. AML samples were more sensitive to PI3Kα inhibitor 2 and PCN5603 killing than normal bone marrow or normal peripheral blood CFC (median IC(50) for AML and normal CFCs treated with PI3Kα inhibitor 2, 1.8 and 4.3 μM, respectively, and for PCN5603, 1.9 and 6.2 μM, respectively). Furthermore, treatment of AML cells with PCN5603 also decreased survival of more primitive leukemia progenitors identified in long-term culture (AML long-term culture initiating cells), while less toxicity toward normal bone marrow long-term culture initiating cells was observed. Selective inhibition of the p110α and δ isoforms of PI3K kills AML progenitors while causing relative sparing of analogous normal cells.

JAK2 Inhibition: Reviewing a New Therapeutical Option in Myeloproliferative Neoplasms

JAK2 is a tyrosine kinase gene that plays an essential role in the development of normal haematopoiesis. Hyperactivation of JAK2 occurs in myeloproliferative neoplasms by different mechanisms. As a consequence, JAK2 inhibitors have been designed to suppress the cytokine signalling cascade caused by the constitutive activation of JAK2. In clinical trials, JAK2 inhibitors are efficient in decreasing spleen size, controlling clinical symptoms, and improving quality of life in patients with myeloproliferative neoplasms. However, JAK2 inhibitors are unable to target uncommitted hematopoietic progenitors responsible of the initiation of the myeloproliferative disease. It is expected that, in order to cure the myeloproliferative disease, JAK2 inhibitors should be combined with other drugs to target simultaneously different pathways and to target the initiator hematopoietic cell population in myeloproliferative disorders. Taking advantage of the inhibition of the cytokine cascade of JAK2 inhibitors, these compounds are going to be used not only to treat patients with hematological neoplasms but may also be beneficial to treat patients with rheumatoid arthritis or other inflammatory diseases.

Characterization and response of newly developed high-grade glioma cultures to the tyrosine kinase inhibitors, erlotinib, gefitinib and imatinib

High-grade gliomas (HGG), are the most common aggressive brain tumours in adults. Inhibitors targeting growth factor signalling pathways in glioma have shown a low clinical response rate. To accurately evaluate response to targeted therapies further in vitro studies are necessary. Growth factor pathway expression using epidermal growth factor receptor (EGFR), mutant EGFR (EGFRvIII), platelet derived growth factor receptor (PDGFR), C-Kit and C-Abl together with phosphatase and tensin homolog (PTEN) expression and downstream activation of AKT and phosphorylated ribosomal protein S6 (P70S6K) was analysed in 26 primary glioma cultures treated with the tyrosine kinase inhibitors (TKIs) erlotinib, gefitinib and imatinib. Response to TKIs was assessed using 50% inhibitory concentrations (IC(50)). Response for each culture was compared with the EGFR/PDGFR immunocytochemical pathway profile using hierarchical cluster analysis (HCA) and principal component analysis (PCA). Erlotinib response was not strongly associated with high expression of the growth factor pathway components. PTEN expression did not correlate with response to any of the three TKIs. Increased EGFR expression was associated with gefitinib response; increased PDGFR-α expression was associated with imatinib response. The results of this in vitro study suggest gefitinib and imatinib may have therapeutic potential in HGG tumours with a corresponding growth factor receptor expression profile.

Downstream molecular pathways of FLT3 in the pathogenesis of acute myeloid leukemia: biology and therapeutic implications

FLT3 is a type III receptor tyrosine kinase. Mutations of FLT3 comprise one of the most frequently identified types of genetic alterations in acute myeloid leukemia. One-third of acute myeloid leukemia patients have mutations of this gene, and the majority of these mutations involve an internal tandem duplication in the juxtamembrane region of FLT3, leading to constitutive activation of downstream signaling pathways and aberrant cell growth. This review summarizes the current understanding of the effects of the downstream molecular signaling pathways after FLT3 activation, with a particular focus on the effects on transcription factors. Moreover, this review describes novel FLT3-targeted therapies, as well as efficient combination therapies for FLT3-mutated leukemia cells.