Cancer stem cells, which share many common properties and regulatory machineries with normal stem cells, have recently been proposed to be responsible for tumorigenesis and to contribute to cancer resistance 1 . The main challenges in cancer biology are to identify cancer stem cells and to define the molecular events required for transforming normal cells to cancer stem cells. Here we show that Pten deletion in mouse haematopoietic stem cells leads to a myeloproliferative disorder, followed by acute T-lymphoblastic leukaemia (T-ALL). Self-renewable leukaemia stem cells (LSCs) are enriched in the c-Kit mid CD3 + Lin − compartment, where unphosphorylated β-catenin is significantly increased. Conditional ablation of one allele of the β-catenin gene substantially decreases the incidence and delays the occurrence of T-ALL caused by Pten loss, indicating that activation of the β-catenin pathway may contribute to the formation or expansion of the LSC population. Moreover, a recurring chromosomal translocation, T(14;15), results in aberrant overexpression of the c-myc oncogene in c-Kit mid CD3 + Lin − LSCs and CD3 + leukaemic blasts,

Multiple genetic or molecular alterations are known to be associated with cancer stem cell formation and cancer development. Targeting such alterations, therefore, may lead to cancer prevention. By crossing our previously established phosphatase and tensin homolog (Pten)-null acute T-lymphoblastic leukemia (T-ALL) model onto the recombination-activating gene 1 −/− background, we show that the lack of variable, diversity and joining [V(D)J] recombination completely abolishes the Tcrα/δ-c-myc translocation and T-ALL development, regardless of β-catenin activation. We identify mammalian target of rapamycin (mTOR) as a regulator of β-selection. Rapamycin, an mTOR-specific inhibitor, alters nutrient sensing and blocks T-cell differentiation from CD4 − CD8 − to CD4 + CD8 + , the stage where the Tcrα/δ-c-myc translocation occurs. Long-term rapamycin treatment of preleukemic Pten-null mice prevents Tcrα/δ-c-myc translocation and leukemia stem cell (LSC) formation, and it halts T-ALL development. However, rapamycin alone fails to inhibit mTOR signaling in the c-Kit mid CD3 + Lin − population enriched for LSCs and eliminate these cells. Our results support the idea that preventing LSC formation and selectively targeting LSCs are promising approaches for antileukemia therapies. T -lymphoblastic leukemia (T-ALL) is a common hematological malignancy that is associated with poor prognosis compared with other ALLs and is often fatal without effective treatment (1). Activating mutations in Notch gene homolog 1 (NOTCH1) are reported in 34-71% of human T-ALL patients (2, 3), whereas deletion or mutations of the phosphatase and tensin homolog (PTEN). tumor suppressor gene have recently been detected in 8-63% of pediatric T-ALL patients (3-6). The mutation status of NOTCH1 and PTEN can divide pediatric T-ALL patients into three groups: (i) those with both NOTCH1 and PTEN mutations, (ii) those with NOTCH1/F-box and WD repeat domain containing 7 (FBXW7) mutations, and (iii) those with only PTEN mutations (3). Interestingly, constitutive activation of the NOTCH signaling pathway is known to down-regulate PTEN expression (5, 7), suggesting that PTEN and its controlled PI3K/v-akt murine thymoma viral oncogene homolog (AKT)/mTOR pathway are critical for the etiology of human T-ALL. Furthermore, PTEN deletion seems to be correlated with poor response to chemotherapy (6) and resistance to pharmacological inhibition of NOTCH1 (5). Therefore, understanding the molecular mechanisms of PTEN-mediated T-ALL pathogenesis and drug resistance is a critical step to improving T-ALL therapeutics.To investigate the molecular and cellular mechanisms associated with PTEN-controlled T-ALL pathogenesis and therapeutic resistance, we have recently developed a Pten loxP/loxP ;VE-CadherinCre + ;Rosa26 loxP-stop-loxP -LacZ + (Pten null) T-ALL mouse model by conditional deletion of Pten in a subset of fetal liver hematopoietic stem cells (8). The resulting animals develop a transient myeloproliferative disorder followed by T-ALL with 100% penetrance. Besides Pt...