Getting to know JAK

Neoplastic clone dominance is key to disease progression. In this issue of Blood, Fleischman et al suggest that in myeloproliferative neoplasms (MPNs), neoplastic clone dominance is linked to JAK2^V617F effects on responsiveness to and production of tumor necrosis factor (TNF)–α.¹ 

This proposal is convincingly backed by the following experimental evidence: (1) plasma TNF-α levels are significantly higher in patients with myelofibrosis, polycythemia vera, and essential thrombocythemia than in normal donors; (2) TNF-α levels in plasma correlate with JAK2^V617F allele burden, and TNF-α mRNA levels in HEL cells, homozygous for JAK2^V617F with multiple copies of this gene, are increased compared with wild-type JAK2 lines; (3) myeloid progenitors from JAK2^V617F patients are less sensitive to (or even, insensitive to) TNF-α inhibition of proliferation in vitro than are progenitors from normal individuals, and non-JAK2^V617F progenitors are suppressed in the same patient samples containing JAK2^V617F-expressing progenitor clones not suppressed by TNF-α; in fact, JAK2^V617F granulocyte-macrophage progenitors are enhanced and erythroid progenitors are relatively resistant to TNF-α suppression in vitro; and (4) Fancc^−/− mouse bone marrow progenitor cells, known to be hyper-responsive to inhibition of proliferation in vitro by TNF-α,²  are protected from this effect by ectopic expression of JAK2^V617F in Fancc^−/− progenitors; moreover, cells harvested from 5-FU–treated TNF-α^+/+, but not TNF-α^−/−, mice transduced with a JAK2^V617F-GFP retroviral vector and then transplanted into lethally irradiated syngeneic recipients, manifest disease burden increases.

This interesting paper by Fleischman and colleagues suggests an intriguing set of questions, including possible roles for negative regulation in disease progression of this and other malignant disorders, whether such negative regulation of “normal” clones is mediated at only the progenitor level, or also at a stem and/or precursor (eg, myeloblast to myelocyte) cell level, and whether or not one can treat the symptoms (eg, enhanced production of negative regulators and/or lack of response of the neoplastic clone(s) to these regulators), to slow or stabilize disease progression without removing the neoplastic clone(s), or after decreasing neoplastic clone burden. It would be important to know exactly how JAK2^V617F mediates its effect(s) on TNF-α production and progenitor cell responsiveness.

In addition to TNF-α there are a number of other cytokines/molecules with negative regulatory activities, including TNF-β, transforming growth factor-β, interferon-α, -β, and -γ, selected members of the chemokine family, the iron binding proteins H-ferritin and lactoferrin, and prostaglandins (PGs), notably PGE₁ and PGE₂.^3-5  Molecules with negative regulatory activities can work in synergy such that concentrations of each that are too low to suppress colony formation in vitro and in some cases in vivo when used alone, will when put together at these low concentrations mediate significant suppressive activity. Thus, additional suppressor mechanisms may manifest, alone or in combination, for disease progression. If synergy in cytokine suppression is involved, down-modulating production of one of these cytokines may be enough to remove single or synergistic suppressive effect(s). A number of cytokines have multiple actions. Under different situations they may act as negative or positive regulators entailing direct (eg, suppressive) and/or indirect (eg, stimulation of production of other cytokine) effects.⁵  However, down-modulating one molecule or its function may cause unwanted effects on another function of the molecule.

The concept that molecules with suppressive activity may mediate growth advantage of neoplastic cells through over-production of a molecule that inhibits normal progenitor cells, but not neoplastic progenitors from the patient, has previously been reported. A leukemia inhibitory activity (LIA), found in extracts from and medium conditioned by cells from patients with acute and chronic myeloid and lymphoid leukemia, and myelodysplasia (MDS/MPN)^3,6  and later identified as H-ferritin,³  and a colony inhibiting activity (CIA),⁷  later identified as lactoferrin,^3,5  were implicated in the selective growth advantage of leukemia and MDS cells, although the intracellular mechanisms mediating these effects have yet to be elucidated. Of interest in early studies on LIA/H-ferritin was that LIA could also be found in cells from patients with acute leukemia in full remission,^6,8  although not at the activity level of that of patients not in remission, perhaps not unsurprising when LIA was identified as H-ferritin that was found in cells from normal donors.³  Moreover, progenitors from patients with acute leukemia in remission were relatively insensitive to inhibition by LIA.^6,8  This latter finding brings up the question of what normal cells mean in the context of leukemia and MPN with regards to progenitor cell function. Are progenitor cells from patients that manifest no chromosomal or molecular defects normal? More effort needs to go into characterizing cells from patients with leukemias (eg, in remission) and MPNs as to their dose-responsiveness to both positive and negative soluble regulators, and to cells in the microenvironmental niche.

The work by Fleischman et al should revitalize interest in how cytokines may influence a proliferative advantage of neoplastic clones. Exactly how JAK2^V617F induces increased production of TNF-α and decreased JAK2^V617F progenitor cell responsiveness to TNF-α inhibition remains to be determined; such mechanistic knowledge will be important for attempts to clinically modulate TNF-α production and/or action. Of potential relevance, mice deficient in Tristetraprolin (TTP), a member of a small family of CCCH tandem zinc finger proteins capable of binding to RNA, and encoded by the immediate early response gene Zfp-36, were linked to overproduction of TNF-α, which led to enhanced medullary and extramedullary myeloid hyperplasia associated with a number of other disorders, including cachexia.⁹  Treatment of young TTP-deficient mice with antibodies to TNF-α prevented development of essentially all phenotypic aspects of TTP deficiency.⁹  Thus, it might be of interest to see if TTP plays a role in the phenotypes noted with JAK2^V617F progenitors. Targeted disruption of Zfp36l2, encoding another CCCH tandem zinc finger RNA-binding protein, also results in defective (in this case decreased) hematopoiesis,¹⁰  suggesting the possibility that this family of zinc finger genes may be involved in abnormalities of progenitor and stem-cell function associated with leukemia and MPN. This remains to be determined.