Page 1281 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 70 Primary Myelofibrosis 1127

progenitor or stem cells has been linked to the development of the hypersensitivity to cytokines that characterizes hematopoietic
myeloproliferation. These observations support a model where a progenitors from each of the MPN. In a mouse BM transplant model,
decrease in MPL mass in platelets and megakaryocytes results in BM cells transduced with JAK2V617F results in a clinical phenotype
increased thrombopoietin levels, which acts on the primitive stems that closely resembles PV, including erythrocytosis, EMH, and BM
cells contributing to the development of myeloproliferation fibrosis. Although >90% of patients with PV are JAK2V617F posi-
Vannucchi and colleagues studied mutant mice with reduced tive, approximately 50% of PMF patients harbor this mutation. The
expression of the transcription factor GATA1 to further define the JAK2V617F mutation is homozygous in 13% of patients with PMF
role of megakaryocytes and the development of BM fibrosis. Muta- but in 30% of patients with PV. Homozygosity has been attributed
tions in the GATA1 functional pathway in human PMF have not to homologous recombination. Homozygosity of JAK2V617F in
been described. However, at the protein level, a large number of the PMF patients is associated with a more frequent occurrence of
megakaryocytes in the BM of PMF patients are GATA1 negative, unfavorable cytogenetic abnormalities. There are conflicting data as
suggesting that whatever the genetic defect leading to PMF is, it to whether the clinical course of patients with JAK2V617F-positive
involves the pathway that affects the posttranscriptional or post- and JAK2V617F-negative PMF differ. Additional somatic mutations
translational regulation of GATA1 in megakaryocytes have been identified in patients with PMF that likely play a role in
Megakaryocytes are not the only cells capable of releasing cyto- the biogenesis of PMF. A mutation in the transmembrane domain of
kines that promote BM fibrosis. Levels of macrophage colony- the thrombopoietin receptor (cMPL) has been documented in 9% of
stimulating factor, a cytokine that regulates macrophage development patients with JAK2V617F-negative PMF (MPL W515L or MPL
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and proliferation, are elevated in the serum of PMF patients. Mono- W515K). MPL 515L, MPL 515K, and MPL wild-type (WT) alleles
cytes and macrophages from patients with PMF can produce greater can coexist in the same patient. Furthermore, 30% of PMF patients
quantities of TGF-β and interleukin-1 (IL-1) than those from normal with mutations in cMPL also have the JAK2V617F mutation. By
control participants. IL-1 and TGF-β are fibroblast mitogens that studying archival material, the burden of MPL 515L, MPK 515K,
induce extracellular matrix protein production. Monocyte adhesion and JAK2V617F in PMF patients has been shown to remain constant
to extracellular matrix proteins has been shown to lead to the over- throughout the clinical course of patients with PMF. In a murine BM
production of IL-1 and TGF-β by PMF monocytes. Monocyte transplant assay, expression of MPL W515L but not WT MPL
adhesion through the adhesion molecule, CD44, appears to be resulted in a rapidly progressive, fully penetrable, lethal MPN (18
involved in the induction of fibrogenic cytokines by mediating the days) characterized by marked thrombocytosis, leukocytosis, spleno-
interaction between monocytes and accumulated extracellular matrix megaly, hepatomegaly, BM megakaryocytic hyperplasia, and BM
protein deposits. The proinflammatory transcriptional factor nuclear fibrosis but not erythrocytosis. These data have suggested that the
factor kappa-B (NFκB) plays a pivotal role in the elaboration of IL-1 MPL mutation favors the development of thrombocytosis but the
and TGF-β in the activation of NFκB monocytes and that of PMF JAK2V617F mutation favors the development of erythrocytosis.
patients. These investigators suggest that NFκB stimulates TGF-β PMF patients with MPL 515L/K PMF compared with MPL WT
production by influencing intracellular IL-1 levels, and may serve as PMF are older, present with more severe anemia, and are more likely
another potential therapeutic target for the treatment of PMF. to require transfusional support. The genetic origins of PMF likely
Abnormal cytokine expression in PMF is thought to represent an represent the culmination of multiple genetic and possibly epigenetic
inflammatory response to the disease phenotype that contributes not events.
only to the development of BM fibrosis, osteosclerosis, and increased Further insight into the phenotypic heterogeneity of JAK2V617F-
BM microvessel density, but also PMF-associated constitutional positive and -negative MPNs has recently been provided. Immuno-
symptoms including weight loss, anorexia, pruritus, bone pain, and histochemical analyses of BM have shown that PV is characterized
night sweats. Elevated levels of IL-8 and IL-2R have been closely by increased expression of phosphorylated STAT3 and STAT5
correlated with the presence of constitutional symptoms, the require- protein, but PMF is characterized by reduced expression of STAT3
ment for red blood cell (RBC) transfusions, and leukocytosis, as well and STAT5. This expression pattern was independent of JAK2V617F
as inferior OS and leukemia-free survival. These observations raise status. Such observations suggest that additional or alternative
the possibility that mutational events leading to the malignant molecular events occur in PMF and PV that might play a role in the
transformation of hematopoietic cells in PMF may also lead to activa- development of their distinctive clinical phenotypes.
tion of transcriptional programs that promote hematopoietic cell In 2013, mutations in CALR were reported by two separate labo-
survival and disease progression. The elevation of a variety of cyto- ratory groups. CALR is located on chromosome 19p13.2 and encodes
kines may therefore not only be responsible for the numerous epi- for a calcium-binding protein with multiple functions including
phenomena that occur as a consequence of the presence of these protein folding/chaperoning, cell proliferation and motility, phago-
malignant cells, but also act on the malignant clone affecting the cytosis, apoptosis, and calcium homeostasis. CALR can be localized
proliferation and differentiation in differing microenvironments to the endoplasmic reticulum, nucleus, extracellular matrix and
characteristic of the BM and various extramedullary sites, including membrane. CALR exon 9 frameshift mutations result in a mutant
the spleen. Ultimately, this may increase the risk of disease progres- calreticulin protein with a novel calcium-binding/endoplasmic reticu-
sion or leukemic transformation. lum retention motif C-terminus. These insertion/deletion mutations
When PMF mononuclear cells are cloned in semisolid media, (>50 reported) occurring in exon 9 of CALR and are found at a
erythroid and megakaryocyte colony formation occurs in the absence frequency of approximately 25% in ET and PMF patients, and
of added exogenous cytokines, a finding common to other MPNs. appear to be mutually exclusive with JAK2 and MPL mutations
These findings suggest that possible genetic mutations activating (frequency of approximately 75% of JAK2/MPL WT). Type 1 (52-bp
several intracellular signaling pathways responsible for normal hema- deletion) and type 2 (5-bp insertion) mutations constitute 80% of
topoiesis might account for this autonomous in vitro hematopoiesis. the reported mutations in JAK2-negative MPN patients. Retrospec-
In addition to a population of autonomous proliferating megakaryo- tive studies implicate the presence of CALR mutations in PMF
cyte progenitor cells, a second and more common population remains patients with higher platelet counts, lower hemoglobin and leukocyte
dependent on the addition of exogenous growth factors. The search counts, less risk of thrombosis, and better overall prognosis than
for the genetic mutations that accounted for the autonomous hema- patients with JAK2V617F. Initial studies also appear to implicate a
topoiesis that characterizes each of the MPNs culminated in the worse prognosis for type 2 CALR mutants compared with type 1.
discovery of a gain-of-function mutation of an autoinhibitory domain Additionally, studies have now shown that CALR mutant MF patients
of the JAK family of protein tyrosine kinases, which is involved in also have upregulated JAK-STAT signaling, and are also responsive
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cytokine receptor signalling. The JAK2V617F mutation leads to to the effects of JAK2 inhibitor treatment.
ongoing phosphorylation activity, which can then bind to a cytokine The exact pathobiologic link between CALR mutation and the
receptor and promote signal transducer and activator of transcription dysregulated JAK-STAT pathway is the subject of intense investiga-
(STAT) recruitment. This mutation is the likely cause of tion. Chachoua and colleagues have recently shown that neither