Polycythemia vera erythroblasts: too tough to die

Comment on Zeuner et al, page 3495

The analysis of death receptor pathways in polycythemia vera patients reveals the mechanisms used by erythroid precursor cells to elude homeostasis-regulating red cell production.

Polycythemia vera (PV) is a monoclonal myeloproliferative disorder characterized by overproduction of erythroid cells. In the majority of PV patients, loss of homeostatic regulation results from a unique V617F mutation in the gene that encodes Janus kinase 2 (JAK2). This mutation mimics the constitutive activation of the erythropoietin (EPO) receptor in erythroid precursors.^1-6  Yet, it has been unclear how constitutive JAK2 activation modulates the normal signaling pathways of erythrocyte development to achieve survival of the erythroid precursor cells.

In this issue of Blood, Zeuner and colleagues have elucidated the consequences of JAK2 mutation on the mechanism involved in the negative regulation of erythropoiesis. Using a powerful unilineage culture system, the same group has previously demonstrated that immature erythroblasts express functional death receptors, while mature erythroblasts produce the natural ligands of these death receptors, namely CD95 (Fas/Apo-1) ligand (CD95L) and TRAIL.⁷  When high numbers of mature erythroblasts are produced in the erythroblastic islands of healthy individuals' bone marrow, their production of CD95L and TRAIL is increased, generating a growth and differentiation arrest in immature erythroblasts and providing a feedback loop that regulates erythrocyte production.⁷  The mechanism for this inhibition was surprising, as it is mediated by caspases, proteases that are usually activated during apoptosis and cleave a defined set of target proteins. Two proteins important for the development of erythrocytes, the transcription factors GATA-1 and SCL, are such caspase targets, and the caspase-mediated cleavage and resultant inactivation of these erythroid transcription factors is not compatible with normal erythroid development and survival.⁸  Stimulation of the CD95 and TRAIL death receptors on immature erythroblasts by their cognate ligands expressed on mature erythroblasts results in caspase activation and consequently in the degradation of SCL and GATA-1, which in turn induces a growth and differentiation blockade that results in reduced red cell production.

The De Maria group has now revealed that erythroid precursors of PV patients are resistant to the erythroid development-inhibiting activity of TRAIL and CD95 ligand. However, this resistance is not due to altered expression of death receptors or intracellular signaling components required for propagation of the inhibitory signal; rather, it depends on an unnaturally high expression of the short form of the cellular FLICE-inhibitory protein (c-FLIP_short). c-FLIP_short interferes with caspase activation at the death-inducing signaling complex (DISC) that forms when death receptors are cross-linked by their respective ligands. According to Zeuner et al, this inhibits the cleavage of downstream substrates including SCL and GATA-1. The constitutive activation of JAK2 and its downstream pathways in immature erythroid cells allows for the expression of high levels of c-FLIP_short, even in the presence of limited amounts of EPO. The authors demonstrate that the presence of a biallelic mutation of JAK2 correlates with lower caspase activation, impaired GATA-1 degradation, and resistance of erythroid precursors to CD95 ligand and TRAIL. The next challenge will be to identify the molecular link between constitutive JAK2 activity and the induction of the observed high levels of c-FLIP_short.

The findings reported by Zeuner et al contribute to our understanding of the pathogenesis of PV and functionally link the JAK2 mutation with impaired regulation of erythrocyte production. The elucidation of the molecular mechanisms responsible for the development of myeloproliferative disorders will be essential for designing new and effective treatments. Studies like the one summarized here that involve primary human cells of a single lineage more closely resemble the specific alterations carried by individual patients and are particularly informative in this context. ▪