Sustained fetal hematopoiesis causes juvenile death from leukemia: evidence from a dual-age-specific mouse model.

It is not clear whether disrupted age-specific hematopoiesis contributes to the complex manifestations in leukemia patients who carry identical mutations, particularly in pediatric and adult patients with similar clinical characteristics. By studying a dual-age-specific mouse model, we demonstrate that (1) loss of Pten during the fetal-to-adult hematopoiesis switch (hematopoiesis switch) causes sustained fetal hematopoiesis, resulting in death in juvenile leukemia; (2) myeloid-biased hematopoiesis in juvenile mice is associated with the sustained fetal properties of hematopoietic stem cells (HSCs); (3) the age specificity of juvenile myelomonocytic leukemia depends on the copy number of Pten and Nf1; (4) single-allelic Pten deletion during the hematopoiesis switch causes constitutive activation of MAPK in juvenile mice with Nf1 loss of heterozygosity (LOH); and (5) Nf1 LOH causes monocytosis in juvenile mice with Pten haploinsufficiency but does not cause lethality until adulthood. Our data suggest that 1 copy of Pten is sufficient to maintain an intact negative-feedback loop of the Akt pathway and HSC function in reconstitution, despite MAPK being constitutively activated in juvenile Pten+/ΔNf1LOH mice. However, 2 copies of Pten are required to maintain the integrity of the MAPK pathway in juvenile mice with Nf1 haploinsufficiency. Our data indicate that previous investigations of Pten function in wild-type mice may not reflect the impact of Pten loss in mice with Nf1 mutations or other genetic defects. We provide a proof of concept that disassociated age-specific hematopoiesis contributes to leukemogenesis and pediatric demise.

PTEN is indispensable for cells to respond to MAPK inhibitors in myeloid leukemia.

Constitutively activated MAPK and AKT signaling pathways are often found in solid tumors and leukemias. PTEN is one of the tumor suppressors that are frequently found deficient in patients with late-stage cancers or leukemias. In this study we demonstrate that a MAPK inhibitor, PD98059, inhibits both AKT and ERK phosphorylation in a human myeloid leukemia cell line (TF-1), but not in PTEN-deficient leukemia cells (TF-1a). Ectopic expression of wild-type PTEN in myeloid leukemia cells restored cytokine responsiveness at physiological concentrations of GM-CSF (<0.02 ng/mL) and significantly improved cell sensitivity to MAPK inhibitor. We also found that Early Growth Response 1 (EGR1) was constitutively over-expressed in cytokine-independent TF-1a cells, and ectopic expression of PTEN down-regulated EGR1 expression and restored dynamics of EGR1 expression in response to GM-CSF stimulation. Data from primary bone marrow cells from mice with Pten deletion further supports that PTEN is indispensible for myeloid leukemia cells in response to MAPK inhibitors. Finally, We demonstrate that the absence of EGR1 expression dynamics in response to GM-CSF stimulation is one of the mechanisms underlying drug resistance to MAPK inhibitors in leukemia cells with PTEN deficiency. Our data suggest a novel mechanism of PTEN in regulating expression of EGR1 in hematopoietic cells in response to cytokine stimulation. In conclusion, this study demonstrates that PTEN is dispensable for myeloid leukemia cells in response to MAPK inhibitors, and PTEN regulates EGR1 expression and contributes to the cytokine sensitivity in leukemia cells.