Page 1261 - Hematology_ Basic Principles and Practice ( PDFDrive )

Chapter 69 Essential Thrombocythemia 1107

proportion of nonclonally derived leukocytes exist in addition to the reduce erythroid differentiation, creating a differentiation pattern
clonally derived population of leukocytes in patients with ET. In one that resembles ET. Furthermore, inhibition of STAT1 signaling in
study of 42 patients with ET, 31 patients exhibited clonality of at ET hematopoietic progenitor cells led to enhanced erythropoiesis
least one hematopoietic lineage, but the remaining 11 patients had and reduced megakaryocytopoiesis. These studies suggest that in ET,
polyclonal origin of all lineages studied. The biogenesis of polyclonal JAK2V617F induces simultaneous activation of STAT5 and STAT1
ET remains ill defined. It is possible that small numbers of normal pathways, but in PV, the relative reduced levels of phospho-STAT1
hematopoietic stem cells account for this admixture of nonclonal reduces a brake, favoring more profound erythropoiesis.
populations. It has been reported by several groups that ET patients Thrombopoietin is the primary physiologic regulator of throm-
with polyclonal hematopoiesis have fewer thrombotic complications. bopoiesis. This growth factor acts by binding to its cell surface
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Interestingly, in some patients, monoclonality of hematopoiesis is receptor, MPL. MPL is expressed by CD34 hematopoietic stem and
restricted to platelets despite the polyclonal origin of the other lin- progenitor cells, MKs, and platelets. Normal or slightly elevated
eages. Other studies, however, have indicated a common origin of thrombopoietin levels have been observed in patients with ET. Fur-
granulocytes, platelets, and B lymphocytes in this disorder. Such thermore, expression of the thrombopoietin receptor and its mRNA
studies raise the possibility that the malignant transformation leading has been shown to be dramatically reduced in the platelets of patients
to ET occurs at a number of cellular stages along the hematopoietic with ET. Thrombopoietin serum levels are controlled by platelet mass
cellular hierarchy. through MPL-mediated thrombopoietin uptake and degradation.
Increased numbers of megakaryocyte progenitor cells are present The reduced platelet MPL expression occurs not only in ET but also
in the BM and the peripheral blood of patients with ET. These data in PV and PMF, and has been shown to be a downstream event of
support the concept that the principal abnormality is an expansion JAK2V617F, which promotes the proteasomal degradation of MPL.
of the progenitor cell pool. In addition, progenitor cells were noted The reduced MPL likely results in the decreased capacity of platelets
to either be hypersensitive or independent of the addition of exogenous to absorb thrombopoietin, contributing to the increased megakaryo-
cytokines, including interleukin (IL)-3, IL-6, and thrombopoietin. A cyte mass and thrombocytosis. The mutations in thrombopoietin
second subpopulation of colony-forming unit–megakaryocyte (CFU- receptor, MPLW515L and MPL515K, are present in approximately
MK) assayed from patients with ET remained dependent on the 3–5% of patients with ET. About 60% of patients with MPL muta-
addition of exogenous cytokines. tions have the W515L mutation and 40% the W515K mutation. The
This hypersensitivity of ET progenitor cells to a variety of cyto- mutant allele burden is greater than 50% in 50% of W515K patients
kines is due to the clonal acquisition of driver mutations (JAK2, MPL, compared with 17% of W515L patients. The most prevalent MPL
or CALR) that activate the JAK-STAT signaling pathway, allowing mutations in ET occur on tryptophan 515, an amino acid that
hematopoiesis to occur in the absence of exogenous cytokines. The maintains MPL in an inactive form in the absence of cytokines.
JAK2V617F mutation occurs in 50–60% of ET patients while recur- Rarely in ET patients another MPL mutation, S505N, located in the
rent CALR mutations occur in 25% of patients and 3–5% have MPL exon 10 domain that encodes the transmembrane domain of MPL
mutations. The patients with ET who lack such driver mutations are and induces dimerization of the transmembrane helix in an active
said to be “triple negative”. confirmation, serves as a driver mutation. The MPL mutations occur-
JAK2 is a cytoplasmic tyrosine kinase that plays a key role in ring in ET trigger conformational changes in the receptor, bringing
mediating intracellular signaling from a variety of growth factors, in close proximity two molecules of bound JAK2 for transphosphory-
including IL-3, erythropoietin, granulocyte-macrophage colony- lation and activation of the JAK-STAT signaling cascade. The loss of
stimulating factor (GM-CSF), granulocyte colony-stimulating factor tryptophan but not the acquisition of a particular residue induces the
(G-CSF), and thrombopoietin. Coexpression of JAK2V617F with a constitutive activation of MPL. More than 50% of patients with MPL
homodimeric type 1 cytokine receptor (including erythropoietin, mutant alleles are also JAK2V617F positive. In ET, both JAK2V617F
thrombopoietin, or G-CSF) is necessary for hormone activation of and MPL mutations arise preferentially on a specific constitutional
JAK-STAT (signal transducer and activator of transcription) signaling JAK2 46/1 haplotype. Two hypotheses have been proposed to account
pathways and for hematopoietic cell proliferation to become growth for this predilection: 46/1 is inherently genetically more unstable
factor independent. The JAK2V617F mutation is present in ET (hypermutability hypothesis) or 46/1 confers a growth advantage that
patients with both clonal and polyclonal hematopoiesis. Patients with favors the predominance of JAK2V617F hematopoiesis (fertile
clonal hematopoiesis have a higher JAK2V617F allele burden (26%) ground hypothesis). The association of MPL mutations with the
than patients with polyclonal hematopoiesis (16%). The relative size JAK2 46/1 haplotype strongly favors the hypermutability hypothesis
of the JAK2V617F clone is often small and remains stable over time rather than the fertile ground hypothesis. The presence of MPLW515K
in patients with both clonal and polyclonal hematopoiesis. Although mutations in ET patients are associated with lower hemoglobin levels
an allele burden higher than 50% indicating the presence of granu- and higher platelet counts, as well as preferential expansion of the
locytes homozygous for JAK2V617F has been found in 70% of PV numbers of megakaryocytes at the expense of erythroid precursors,
patients, it has been observed less frequently in ET patients. All PV as observed in BM biopsy specimens.
patients have assayable erythroid colonies that are homozygous for Mutations in the CALR gene occur in 25% of ET patients and
JAK2V617F, even in PV patients with a low burden of JAK2V617F. rarely occur together with mutations of JAK2 or MPL. CALR muta-
By contrast, hematopoietic colonies cloned from ET patients are only tions have been observed exclusively in ET and MF patients but not
occasionally JAK2V617F homozygous, unlike the case of PV. The PV. The wild-type CALR gene encodes for an evolutionarily con-
transition from JAK2 heterozygous to homozygous progenitors is a served, multifunctional protein involved in multiple cellular processes
consequence of homologous recombination. These studies suggest ranging from calcium homeostasis and protein folding in the endo-
that such an event is characteristic of PV but rarely occurs in ET. If plasmic reticulum, to apoptotic cell death clearance and cellular
such an event occurred in ET, it would likely lead to a transition from adhesion. The CALR mutations identified in MPN mainly consist of
an ET phenotype to a PV phenotype. deletions (i.e., type I) or insertions (i.e., type II) occurring within the
STATs are activated downstream to JAK2V617F. STAT3 is a exon 9, which create a novel epitope in the C-terminal domain of
pivotal regulator of megakaryocytopoiesis, which might provide an the protein. Despite the heterogeneity of these mutations, the new
explanation for its exclusive upregulation in ET. To further examine C-terminus sequence is identical and results in the loss of the KDEL
the differences between hematopoiesis in JAK2V617F ET and PV, domain, which is critical for CALR retention in the endoplasmic
the gene expression changes in JAK2V617F-heterozygous erythroid reticulum and its ability to regulate calcium homeostasis. The original
colonies have been examined. Erythroblasts from ET patients were studies describing CALR mutations in MPN indicated that such a
characterized by enhanced expression of genes associated with inter- uniform defect may confer a proliferative advantage to the malignant
feron (IFN) signaling and phosho-STAT1 compared with PV eryth- cells via activation of the JAK-STAT pathway. Mutations of CALR
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roblasts. STAT1 is essential for IFN-γ signaling. Increased STAT1 are found almost exclusively in patients with MF and ET, the two
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in normal CD34 cells has been shown to favor megakaryocytic but MPN entities in which MK hyperplasia is a hallmark of the disease.