Despite being an attractive molecular target for both lymphoid and myeloid leukemias characterized by activated tyrosine kinases, the molecular and physiological consequences of reduced signal transducer and activator of transcription-5 (Stat5) during leukemogenesis are not well known. Stat5 is a critical regulator of mouse hematopoietic stem cell (HSC) self-renewal and is essential for normal lymphocyte development. We report that pan-hematopoietic deletion in viable adult Vav1-Cre conditional knockout mice as well as Stat5abnull/null fetal liver transplant chimeras generated HSCs with reduced expression of quiescence regulating genes (Tie2, Mpl, Slamf1, Spi1, Cited2) and increased expression of B-cell development genes (Satb1, Dntt, Btla, Flk2). Using a classical murine B-cell acute lymphoblastic leukemia (B-ALL) model, we demonstrate that these HSCs were also poised to produce a burst of B-cell precursors upon expression of Bcl-2 combined with oncogenic Myc. This strong selective advantage for leukemic transformation in the background of Stat5 deficient hematopoiesis was permissive for faster initiation of Myc-induced transformation to B-ALL. However, once established, the B-ALL progression in secondary transplant recipients was Stat5-independent. Overall, these studies suggest that Stat5 can play multiple important roles that not only preserve the HSC compartment but can limit accumulation of potential pre-leukemic lymphoid populations.
Leucine carboxyl methyltransferase-1 (LCMT-1) methylates the C-terminal leucine -carboxyl group of the catalytic subunits of the protein phosphatase 2A (PP2A) subfamily of protein phosphatases, PP2Ac, PP4c, and PP6c. LCMT-1 differentially regulates the formation and function of a subset of the heterotrimeric complexes that PP2A and PP4 form with their regulatory subunits. Global LCMT-1 knockout causes embryonic lethality in mice, but LCMT-1 function in development is unknown. In this study, we analyzed the effects of global LCMT-1 loss on embryonic development. LCMT-1 knockout causes loss of PP2Ac methylation, indicating that LCMT-1 is the sole PP2Ac methyltransferase. PP2A heterotrimers containing the B and B B-type subunits are dramatically reduced in whole embryos, and the steady-state levels of PP2Ac and the PP2A structural A subunit are also down 30%. Strikingly, global loss of LCMT-1 causes severe defects in fetal hematopoiesis and usually death by embryonic day 16.5. Fetal livers of homozygous lcmt-1 knockout embryos display hypocellularity, elevated apoptosis, and greatly reduced numbers of hematopoietic stem and progenitor cell-enriched KitLinSca1 cells. The percent cycling cells and mitotic indices of WT and lcmt-1 knockout fetal liver cells are similar, suggesting that hypocellularity may be due to a combination of apoptosis and/or defects in specification, self-renewal, or survival of stem cells. Indicative of a possible intrinsic defect in stem cells, noncompetitive and competitive transplantation experiments reveal that lcmt-1 loss causes a severe multilineage hematopoietic repopulating defect. Therefore, this study reveals a novel role for LCMT-1 as a key player in fetal liver hematopoiesis.
Signal transducer and activator of transcription 5 (STAT5) regulates normal lympho-myeloid development through activation downstream of early-acting cytokines, their receptors, and Janus kinases (JAKs). Despite a general understanding of the role of STAT5 in hematopoietic stem cell (HSC) proliferation, survival, and self-renewal, the transcriptional targets and mechanisms of gene regulation that control multi-lineage engraftment following transplantation for the most part remain to be understood. In this review, we focus on the role of STAT5 in HSC transplantation and recent developments toward identifying the relevant downstream target genes and their role as part of a pleiotropic STAT5 mediated signaling response.