Protein phosphorylation is a ubiquitous mechanism for cellular signal propagation, and signaling network complexity presents a challenge to protein kinase substrate identification. Few targets of Polo-like kinases are known, despite their significant role in coordinating cell-cycle progression. Here, we combine chemical-genetic, bioinformatic, and proteomic tools for Polo-like kinase substrate identification. Specific pharmacological inhibition of budding yeast Polo-like kinase, Cdc5, resulted in a misaligned preanaphase spindle and subsequently delayed anaphase nuclear migration, revealing a Cdc5 function. A cellular screen for Cdc5 substrates identified Spc72, a spindle pole body (SPB) component and microtubule anchor required for nuclear positioning. Spc72 bound to the Cdc5 PBD in a mitosis-specific manner, was phosphorylated by Cdc5 in vitro, and demonstrated a loss of mitotic phosphorylation in vivo upon Cdc5 inhibition. Finally, an examination of Cdc5 binding by SPB-localized proteins expanded our knowledge of Cdc5 function at the SPB.

In chronic-phase chronic myeloid leukemia (CML) patients, the lack of a major cytogenetic response (< 36% Ph ؉ metaphases) to imatinib within 12 months indicates failure and mandates a change of therapy. To identify biomarkers predictive of imatinib failure, we performed gene expression array profiling of CD34 ؉ cells from 2 independent cohorts of imatinib-naive chronic-phase CML patients. The learning set consisted of retrospectively selected patients with a complete cytogenetic response or more than 65% Ph ؉ metaphases within 12 months of imatinib therapy. Based on analysis of variance P less than .1 and fold difference 1.5 or more, we identified 885 probe sets with differential expression between responders and nonresponders, from which we extracted a 75-probe set minimal signature (classifier) that separated the 2 groups. On application to a prospectively accrued validation set, the classifier correctly predicted 88% of responders and 83% of nonresponders. Bioinformatics analysis and comparison with published studies revealed overlap of classifier genes with CML progression signatures and implicated ␤-catenin in their regulation, suggesting that chronic-phase CML patients destined to fail imatinib have more advanced disease than evident by morphologic criteria. Our classifier may allow directing more aggressive therapy upfront to the patients most likely to benefit while sparing good-risk patients from unnecessary toxicity. (Blood. 2010;115:315-325)

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Pioneering work with the Bcr-Abl inhibitor, imatinib, demonstrated a requirement for constant Bcr-Abl inhibition to achieve maximal therapeutic benefit in treating chronic myeloid leukemia (CML), establishing a paradigm that has guided further drug development for this disease. Surprisingly, the second-generation BcrAbl inhibitor, dasatinib, was reported to be clinically effective with once-daily dosing, despite a short (3-to 5-hour) plasma half-life. Consistent with this observation, dasatinib treatment of progenitor cells from chronic-phase CML patients for 4 hours, followed by washout, or continuously for 72 hours both resulted in an induction of apoptosis and a reduction in the number of clonogenic cells. Such acute treatments with clinically achievable dasatinib concentrations also irreversibly committed Bcr-Abl ؉ CML cell lines to apoptotic cell death. Potent transient Bcr-Abl inhibition using the alternative inhibitor, nilotinib, also resulted in cell death. These findings demonstrate that in vitro assays designed to model in vivo pharmacokinetics can predict clinical efficacy. Furthermore, they challenge the widely held notion that continuous target inhibition is required for optimal efficacy of kinase inhibitors. (Blood. 2009; 114:3459-3463)

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