NOTCH1 promotes T cell leukemia-initiating activity by RUNX-mediated regulation of PKC-θ and reactive oxygen species

The authors uncover a new mechanism for the regulation of the activity of leukemia-initiating cells in T-ALL. A subpopulation of stem cells with low amounts of reactive oxygen species (ROS) is enriched in their ability to reconstitute disease in mouse models, and this effect is regulated by repression of PKC-θ, which increases ROS production. Moreover, oncogenic NOTCH, a common T-ALL–driving alteration, regulates stem cell activity by increasing RUNX3 expression, which represses RUNX1, a PCK-θ activator, a pathway that is conserved in human patients. Reactive oxygen species (ROS), a byproduct of cellular metabolism, damage intracellular macromolecules and, when present in excess, can promote normal hematopoietic stem cell differentiation and exhaustion1,2,3. However, mechanisms that regulate the amount of ROS in leukemia-initiating cells (LICs) and the biological role of ROS in these cells are largely unknown. We show here that the ROSlow subset of CD44+ cells in T cell acute lymphoblastic leukemia (T-ALL), a malignancy of immature T cell progenitors, is highly enriched in the most aggressive LICs and that ROS accumulation is restrained by downregulation of protein kinase C θ (PKC-θ). Notably, primary mouse T-ALLs lacking PKC-θ show improved LIC activity, whereas enforced PKC-θ expression in both mouse and human primary T-ALLs compromised LIC activity. We also show that PKC-θ is regulated by a new pathway in which NOTCH1 induces runt-related transcription factor 3 (RUNX3), RUNX3 represses RUNX1 and RUNX1 induces PKC-θ. NOTCH1, which is frequently activated by mutation in T-ALL4,5,6 and required for LIC activity in both mouse and human models7,8, thus acts to repress PKC-θ. These results reveal key functional roles for PKC-θ and ROS in T-ALL and suggest that aggressive biological behavior in vivo could be limited by therapeutic strategies that promote PKC-θ expression or activity, or the accumulation of ROS.