Page 1236 - Hematology_ Basic Principles and Practice ( PDFDrive )

1082 Part VII Hematologic Malignancies

non-Hodgkin lymphoma, suggesting involvement of a putative multicellular in origin or the consequence of excessive proliferation
environmental toxin that may have broad hematopoietic toxicity. The of normal HSCs.
possibility of environmental factors contributing to the development The clonality of blood cell production in PV has subsequently
of PV was recently raised by the identification of a cluster of patients been confirmed using restriction fragment length polymorphisms of
with JAK2V617F-positive PV in an area of Eastern Pennsylvania that the active X chromosome. A monoclonal pattern of X chromosome
contained numerous sources of hazardous materials including, waste inactivation has been defined in RBCs, granulocytes, monocytes, and
coal power plants and United States Environmental Protection platelets in female patients with PV.
Agency Superfund sites. What possible environmental factors might On the basis of the knowledge that RBC production in PV is not
contribute to this fourfold increase in the number of PV cases is associated with excessive EPO production, numerous investigators
currently the subject of intense investigation. hypothesized that the erythroid progenitor cell in this disorder was
In several large studies, 5% of patients with PV were younger than no longer subject to physiologic regulators. PV BM can form sub-
40 years of age, 1% were younger than 25 years at diagnosis, and stantial numbers of erythroid colonies in vitro in the absence of
0.1% were younger than 20 years. Small numbers of patients with exogenous EPO, but normal human BM is incapable of forming such
PV have been reported who presented during childhood. Interest- colonies without the addition of EPO. These erythroid colonies were
ingly, females comprise the majority of PV patients diagnosed before termed endogenous colonies. When erythroid PV and normal BM were
the age of 40 years, suggesting a unique risk factor for developing the subsequently assayed in the presence of EPO, PV BM was character-
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disease at early age. These young female patients tend to present ized by a higher cloning efficiency. Additional observations suggested
with unique complications such as abdominal venous thrombosis and that the altered response to cytokines was not restricted to EPO but
their risk of transformation to MF or AML is similar to older patients. included a variety of other cytokines and that this cytokine hyper-
The JAK2V617F mutation has a prevalence of 0.1–0.2% in the sensitivity was characteristic of progenitor cells committed to a variety
general population. These individuals usually have an allele burden of other lineages.
of less than 10%. Some these patients have an undiagnosed MPN or In addition, BM cells cloned from African–American female
represent an early molecular stage of a MPN, which will evolve over G6PD heterozygotes with PV in the presence and absence of exog-
time. Those normal individuals with an allele burden of <2% likely enous EPO revealed that the erythroid colonies that formed in the
have a latent form of an MPN. absence of exogenous EPO contained the same G6PD isoenzyme
type as that expressed by peripheral blood elements. Thus, the
so-called endogenous erythroid colonies arose from the abnormal clone
Pathobiology that was responsible for supplying RBCs, granulocytes, and platelets
to the peripheral blood. When exogenous EPO was added, increasing
Considerable speculation has centered on the pathobiology of the numbers of colonies were formed containing cellular elements
erythrocytosis that characterizes PV. The expanded RBC mass in PV expressing the other G6PD isoenzymes; presumably, these colonies
patients is due to a two- to threefold increase in the production of originated from cells not involved in the malignant process. Similarly,
RBCs by a hyperplastic BM and is not attributable to prolongation small numbers of granulocyte–macrophage colonies not originating
of the RBC lifespan. Granulocyte and platelet production are also from the PV clone were also observed in these assays. These data
increased in this disorder. This overly exuberant production of all collectively indicate the existence of malignant and nonmalignant
cellular elements of the blood suggests that the basic defect resides at populations of HPCs in PV BM. The relative frequency of the
the level of the pluripotent hematopoietic stem cell (HSC). neoplastic clone in relation to normal progenitor cells was further
Serum EPO levels have been shown to be subnormal in patients examined by Adamson and coworkers, who, by monitoring the
with PV, elevated in many but not all cases of secondary erythrocy- proportion of neoplastic erythroid clones and their numeric relation-
tosis, and normal in patients with relative polycythemia, indicating ship to normal clones over a period of several years, showed disease
that PV cannot be an abnormality of EPO production. progression to be associated with a significant decline in the frequency
In 1951, Dameshek postulated that chronic myeloid leukemia, of normal progenitor cells and increasing proportion of the neoplastic
PV, essential thrombocythemia, and PMF were related disorders, clone. Erythroid progenitor cells from PV patients are, in fact,
which he called myeloproliferative disorders, but are now termed MPNs abnormally sensitive to the actions of this EPO. This increased
because of the evidence that these disorders are hematologic responsiveness allows these cells to form colonies in the presence of
malignancies. small amounts of EPO. Most PV patients possess two distinct popu-
Since the mid-1970s, data have accumulated that conclusively lations of erythroid progenitor cells: a normally EPO-responsive
demonstrate PV to be the result of a neoplastic proliferation of population and a population of cells similar in proliferative and
hematopoietic cells. The cellular origin of the disorder was first maturational behavior in vitro but requiring little or no EPO. These
established by the analysis of glucose-6-phosphate dehydrogenase investigators suggested that the proliferation of the normal progenitor
(G6PD) isoenzymes in African–American women who were hetero- cells in vivo was at a disadvantage. A number of investigators have
zygous for this X-linked gene. This approach was based on the demonstrated that the increased responsiveness of these BM progeni-
random irreversible inactivation of one X chromosome in each female tor populations extends to their responses to other cytokines, includ-
somatic cell during embryogenesis. Inactivation of the same X chro- ing SCF, IL-3, GM-CSF, and IGF-1.
mosome occurs in the progeny of these cells. A normal African–
American female heterozygous for G6PD will therefore have
approximately equal populations of BM cells with a different G6PD Identification of JAK2V617F Mutation in PV
isoenzyme. The G6PD isoenzymes can be readily distinguished by
electrophoretic methods. For more than two decades a number of research groups searched
This approach was exploited in a seminal study by Adamson and for the genetic defect underlying PV. Most groups predicted that this
coworkers in an effort to determine the cellular origin of PV. They defect involved the signaling pathways downstream of the EPOR.
presumed that cells comprising a tumor that arises from a single cell These pathways include the tyrosine kinase JAK2 and the tran-
in a G6PD heterozygote would express a single isoenzyme type, but scriptional signal transducers and activators of transcription, STAT3
a neoplasm originating from multiple cells would express both iso- and STAT5. The initial discovery of a mutation was predicated on
enzyme types. These investigators found that circulating RBCs, a somewhat simplistic yet revealing set of experiments performed
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granulocytes, and platelets obtained from African–American female by Vainchenker and colleagues in France. They observed that the
patients who were G6PD heterozygotes express the same isoenzyme, inhibition of JAK2 by a small molecule (AG490) or by small inter-
but skin and cultured BM fibroblasts obtained from these same fering RNA (siRNA) reduced EPO-independent colony formation
patients demonstrate both isoenzymes. They concluded that PV by PV BM mononuclear cells. This observation prompted them to
represented a clonal proliferation of neoplastic HSCs and was not directly sequence JAK2 in the hematopoietic cells of PV patients and