Lineage-specific STAT5 target gene activation in hematopoietic progenitor cells predicts the FLT3(+)-mediated leukemic phenotype.

Mutations that activate FMS-like tyrosine kinase 3 (FLT3) are frequent occurrences in acute myeloid leukemia. Two distinct types of mutations have been described: internal duplication of the juxtamembranous domain (ITD) and point mutations of the tyrosine kinase domain (TKD). Although both mutations lead to constitutive FLT3 signaling, only FLT3-ITD strongly activates signal transducer and activator of transcription 5 (STAT5). In a murine transplantation model, FLT3-ITD induces a myeloproliferative neoplasm, whereas FLT3-TKD leads to a lymphoid malignancy with significantly longer latency. Here we report that the presence of STAT5 is critical for the development of a myeloproliferative disease by FLT3-ITD in mice. Deletion of Stat5 in FLT3-ITD-induced leukemogenesis leads not only to a significantly longer survival (82 vs 27 days) of the diseased mice, but also to an immunophenotype switch with expansion of the lymphoid cell compartment. Interestingly, we were able to show differential STAT5 activation in FLT3-ITD(+) myeloid and lymphoid murine progenitors. STAT5 target genes such as Oncostatin M were highly expressed in FLT3-ITD(+) myeloid but not in FLT3-ITD(+) lymphoid progenitor cells. Strikingly, FLT3-TKD expression in combination with Oncostatin M is sufficient to reverse the phenotype to a myeloproliferative disease in FLT3-TKD mice. Thus, lineage-specific STAT5 activation in hematopoietic progenitor cells predicts the FLT3(+)-mediated leukemic phenotype in mice.

Cks1 is a critical regulator of hematopoietic stem cell quiescence and cycling, operating upstream of Cdk inhibitors.

Cyclin-dependent kinase subunit 1 (Cks1) is a critical rate-limiting component of the Skp1-Cullin1-Skp2 (SCF(Skp2)) ubiquitin ligase that controls cell cycle inhibitor abundance. Cyclin-dependent kinase (Cdk) inhibitors (CKIs) regulate hematopoietic stem cell (HSC) self-renewal, regeneration after cytotoxic stress and tumor cell proliferation. We thus studied the role of Cks1 in HSC and in a prototypic stem cell disorder, chronic myeloid leukemia (CML). Cks1 transcript was highly expressed in Lin-Sca-1+Kit+ (LSK) HSC, and the loss resulted in accumulation of the SCF(Skp2)/Cks1 substrates p21, p27, p57 and p130 particularly in CD150+ LSK cells. This accumulation correlated with decreased proliferation and accumulation of Cks1(-/-) HSC, slower regeneration after stress and prolonged HSC quiescence. At the hematopoietic progenitor (HPC) level, loss of Cks1 sensitized towards apoptosis. In CML, Cks1 expression was increased, and treatment with the Abl kinase inhibitor, imatinib, reduced Cks1 expression. Also, we found that Cks1 is critical for Bcr-Abl-induced cytokine-independent clonogenic activity. In conclusion, our study presents a novel function of Cks1 in maintaining HSC/HPC homeostasis and shows that Cks1 is a possible target in therapies aimed at the SCF(Skp2)/Cks1 complex that controls CKI abundance and cancer cell proliferation.

Expression of neurochondrin in the developing and adult mouse brain.

Here we describe a detailed analysis of the expression of neurochondrin ( ncdn) in the developing and adult mouse brain. Ncdn is first expressed in the hindbrain and spinal cord at embryonic day 10.5 (E10.5) followed by expression in the midbrain at E11.5. By E18 ncdn is also expressed in the diencephalon and telencephalon. However, strongest expression is still observed in the hindbrain. In adults, the expression in the forebrain is as strong as in the hindbrain. Ncdn is highly expressed in the hippocampus, piriform cortex, septum, amygdaloid complex, medial geniculate nucleus, inferior colliculus, cerebellar nuclei and the nuclei of the Vth, VIIth, and XIIth cranial nerves.