Clonal Analysis of Agnogenic Myeloid Metaplasia

Does primary myelofibrosis involve a defective stem cell niche? From concept to evidence

Primary myelofibrosis (PMF) is the rarest and the most severe Philadelphia-negative chronic myeloproliferative syndrome. By associating a clonal proliferation and a mobilization of hematopoietic stem cells from bone marrow to spleen with profound alterations of the stroma, PMF is a remarkable model in which deregulation of the stem cell niche is of utmost importance for the disease development. This paper reviews key data suggesting that an imbalance between endosteal and vascular niches participates in the development of clonal stem cell proliferation. Mechanisms by which bone marrow niches are altered with ensuing mobilization and homing of neoplastic hematopoietic stem cells in new or reinitialized niches in the spleen and liver are examined. Differences between signals delivered by both endosteal and vascular niches in the bone marrow and spleen of patients as well as the responsiveness of PMF stem cells to their specific signals are discussed. A proposal for integrating a potential role for the JAK2 mutation in their altered sensitivity is made. A better understanding of the cross talk between stem cells and their niche should imply new therapeutic strategies targeting not only intrinsic defects in stem cell signaling but also regulatory hematopoietic niche-derived signals and, consequently, stem cell proliferation.

Chromosomal aberrations in human lymphocytes exposed to the anticholinesterase pesticide isofenphos with mechanisms of leukemogenesis

Human lymphocytes were exposed to the leukemogenic pesticide isofenphos (IFP) to investigate its effects on chromosomal DNA and cholinergic homeostasis using cholinesterase activity as a marker. Isolated peripheral lymphocytes were administered concentrations of IFP ranging from 0.1 ng/ml to 10 microg/ml. The absence (Group 1) and presence (Group 2) of DNA repair inhibitors 4 mM hydroxyurea (HU), 40 microM cytosine arabinoside (ARA-C) and an NADPH regenerating system (NRS) (Group 3) were analyzed at 1, 6 and 24 h by single cell gel electrophoresis using the comet assay. Significant damage to DNA directly from IFP at 1 h by remarkably low concentrations was observed in Group 1, escalating in Group 2 with DNA repair inhibition, while Group 3 disruptions were highest due to the presence of the NRS P-450 microsomal fraction conducive to producing reactive IFP-oxon and N-desalkyl metabolites. The extent of DNA aberrations increased further in parallel within the groups at 6 and 24 h. Male and female chemical sensitivities were similar on average (P < 0.01). Cholinesterase activity measured in a satellite group was inhibited with 0.1 microg/ml IFP by 69, 62, and 48% at 1, 6, and 24 h, respectively, indicating gradual induction of compensatory synthesis. Restoration of cholinergic homeostasis may be exceptionally impaired at higher IFP concentrations from acetyl-CoA depletion [Leuk. Res. 25 (2001) 883]. In summary, these studies reveal that exposure to the organophosphate pesticide isofenphos induces human DNA mutation beyond endogenous repair capacity and disrupts cholinergic nuclear signaling affectively constructing the mutator phenotype of leukemogenesis.

Spontaneous STAT5 activation induces growth factor independence in idiopathic myelofibrosis: possible relationship with FKBP51 overexpression

Spontaneous growth of megakaryocyte progenitors is one of the biologic hallmarks of idiopathic myelofibrosis (IMF). The molecular mechanisms underlying this hypersensitivity to cytokines are poorly understood. Using a differential display approach, we previously observed FK506 binding protein 51 (FKBP51) overexpression in pathologic megakaryocytes from IMF. Using an FKBP51-overexpressing cell line, we found sustained STAT5 activation associated with JAK2 phosphorylation. We subsequently tested whether this transcription factor was activated in patient samples. We detected a STAT5 nuclear translocation and activation in spontaneously grown megakaryocytes and in circulating CD34(+) cells from the majority of patients studied. The biologic role of this JAK/STAT pathway activation was demonstrated by inhibiting both the anti-apoptotic phenotype mediated by FKBP51 overexpression in UT7 cells and the spontaneous megakaryocytic growth by addition in culture of the JAK2 inhibitor AG490 or overexpression of a STAT5b dominant negative or SOCS-1. These results demonstrate that a constitutive STAT5 activation in IMF is indispensable for spontaneous growth of megakaryocytes. They also suggest that FKBP51 overexpression could be involved in STAT5 activation in IMF cells and in subsequent abnormal growth.