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T cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer that is frequently associated with activating mutations in NOTCH1 and dysregulation of MYC. Here, we performed 2 complementary screens to identify FDA-approved drugs and drug-like small molecules with activity against T-ALL. We developed a zebrafish system to screen small molecules for toxic activity toward MYC-overexpressing thymocytes and used a human T-ALL cell line to screen for small molecules that synergize with Notch inhibitors. We identified the antipsychotic drug perphenazine in both screens due to its ability to induce apoptosis in fish, mouse, and human T-ALL cells. Using ligand-affinity chromatography coupled with mass spectrometry, we identified protein phosphatase 2A (PP2A) as a perphenazine target. T-ALL cell lines treated with perphenazine exhibited rapid dephosphorylation of multiple PP2A substrates and subsequent apoptosis. Moreover, shRNA knockdown of specific PP2A subunits attenuated perphenazine activity, indicating that PP2A mediates the drug's antileukemic activity. Finally, human T-ALLs treated with perphenazine exhibited suppressed cell growth and dephosphorylation of PP2A targets in vitro and in vivo. Our findings provide a mechanistic explanation for the recurring identification of phenothiazines as a class of drugs with anticancer effects. Furthermore, these data suggest that pharmacologic PP2A activation in T-ALL and other cancers driven by hyperphosphorylated PP2A substrates has therapeutic potential., Introduction T cell acute lymphoblastic leukemia (T-ALL) is an aggressive cancer affecting mainly adolescents and young adults. Intensified treatment regimens have improved outcomes, but patients who fail conventional therapy have [...]

The MYC oncogenic transcription factor is overexpressed in most human cases of T cell acute lymphoblastic leukemia (T-ALL), often downstream of mutational NOTCH1 activation. Genetic alterations in the PTEN-PI3K-AKT pathway are also common in T-ALL. We generated a conditional zebrafish model of T-ALL in which 4-hydroxytamoxifen (4HT) treatment induces MYC activation and disease, and withdrawal of 4HT results in T-ALL apoptosis and tumor regression. However, we found that loss-of-function mutations in zebrafish pten genes, or expression of a constitutively active Akt2 transgene, rendered tumors independent of the MYC oncogene and promoted disease progression after 4HT withdrawal. Moreover, MYC suppresses pten mRNA levels, suggesting that Akt pathway activation downstream of MYC promotes tumor progression. Our findings indicate that Akt pathway activation is sufficient for tumor maintenance in this model, even after loss of survival signals driven by the MYC oncogene.

Purpose: To identify children with T-cell acute lymphoblastic leukemia (T-ALL) at high risk of induction chemotherapy failure by using DNA copy number analysis of leukemic cells collected at diagnosis., Patients and Methods: Array comparative genomic hybridization (CGH) was performed on genomic DNA extracted from diagnostic lymphoblasts from 47 children with T-ALL treated on Children's Oncology Group Study P9404 or Dana-Farber Cancer Institute Protocol 00-01. These samples represented nine patients who did not achieve an initial complete remission, 13 who relapsed, and 25 who became long-term, event-free survivors. The findings were confirmed in an independent cohort of patients by quantitative DNA polymerase chain reaction (DNA-PCR), an assay that is well suited for clinical application., Results: Analysis of the CGH findings in patients in whom induction chemotherapy failed compared with those in whom induction chemotherapy was successful identified the absence of biallelic TCRgamma locus deletion (ABD), a characteristic of early thymocyte precursors before V(D)J recombination, as the most robust predictor of induction failure (P < .001). This feature was also associated with markedly inferior event-free (P = .002) and overall survival (P < .001) rates: 25% versus 58% and 25% versus 72%, respectively. Using a rapid and inexpensive quantitative DNA-PCR assay, we validated ABD as a predictor of a poor response to induction chemotherapy in an independent series of patients., Conclusion: Lymphoblasts from children with T-ALL should be evaluated at diagnosis for deletion within the TCRgamma locus. Patients lacking biallelic deletion, which confers a high probability of induction failure with contemporary therapy, should be assigned to alternative therapy in the context of a prospective clinical trial.