Temporal Changes in PTEN and mTORC2 Regulation of Hematopoietic Stem Cell Self-Renewal and Leukemia Suppression

Pten deletion from adult mouse hematopoietic cells activates the PI3-kinase pathway, inducing hematopoietic stem cell (HSC) proliferation, HSC depletion, and leukemogenesis. Pten is also mutated in human leukemias, but rarely in early childhood leukemias. We hypothesized that this reflects developmental changes in PI3-kinase pathway regulation. Here we show that Rictor deletion prevents leukemogenesis and HSC depletion after Pten deletion in adult mice, implicating mTORC2 activation in these processes. However, Rictor deletion had little effect on the function of normal HSCs. Moreover, Pten deletion from neonatal HSCs did not activate the PI3-kinase pathway or promote HSC proliferation, HSC depletion, or leukemogenesis. Pten is therefore required in adult, but not neonatal, HSCs to negatively regulate mTORC2 signaling. This demonstrates that some critical tumor suppressor mechanisms in adult cells are not required by neonatal cells. Developmental changes in key signaling pathways therefore confer temporal changes upon stem cell self-renewal and tumor suppressor mechanisms. º Levels of phosphorylated AKT do not necessarily increase in dividing HSCs º Pten deletion promotes HSC proliferation and leukemogenesis by activating mTORC2 º RICTOR, and therefore mTORC2, deficiency has little effect on normal HSC function º PTEN maintains HSCs and suppresses leukemia in adult, but not neonatal, mice Myeloid leukemias driven by PTEN mutations are common in adults but rare in young children. Adult HSCs require PTEN to suppress the PI3-kinase pathway via TORC2 signaling whereas neonatal HSCs lack this tumor suppressor mechanism.