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Prepublished online as a Blood First Edition Paper on August 8, 2002; DOI 10.1182/blood-2001-12-0349.
REVIEW ARTICLE
From the Division of Hematology, Department of
Medicine, Johns Hopkins University School of Medicine, Baltimore, MD.
Polycythemia vera is a clonal disorder
arising in a multipotent hematopoietic progenitor cell that causes the
accumulation of morphologically normal red cells, white cells,
platelets, and their progenitors in the absence of a definable stimulus
and to the exclusion of nonclonal hematopoiesis.1,2 First
described in 1892,3 polycythemia vera is not a new disease
and while uncommon, with an incidence of at least 2 per
100 000,4-6 it is not a rare disease. Yet, after 10 decades of careful clinical and laboratory investigation, the etiology
of polycythemia vera remains unknown and there is no consensus as to
the optimal therapy for the disorder.7 There is, however,
no reason for this to be so. Although the molecular basis of
polycythemia vera remains elusive, it is the central thesis of this
review that the pathophysiology of polycythemia vera is sufficiently
well defined for the provision of a rational treatment program that
prolongs life, alleviates the specific morbidities associated with the
disease, and avoids complications related to the consequences of the
underlying molecular defect. However, for such an approach to be
successful, it is first necessary to recognize the contradictions
between what is actually known about this disease and how that
knowledge has been interpreted and applied clinically, and that is the
purpose of this review.
Erythropoiesis and growth factor hypersensitivity
These paradoxical results are due to the ability of polycythemia
vera erythroid progenitor cells to proliferate in vitro in the absence
of erythropoietin.15 This unusual behavior, however, does
not define the limits of the abnormal clone since not all polycythemia
vera erythroid progenitor cells exhibit erythropoietin-independence in
vitro.16,17 Importantly, primitive polycythemia vera
erythroid progenitor cells exhibiting either erythropoietin-dependence
or -independence could give rise to both erythropoietin-dependent and
-independent progeny, a characteristic that appeared fixed given that
it remained constant over time when these progenitor cells were
cultured in vitro. It is also worth noting that the in vitro
proliferation of polycythemia vera erythroid progenitor cells in the
absence of erythropoietin was not robust.17 This may be a
consequence of the tendency of these cells to differentiate faster in
vitro than normal erythroid progenitor cells in the absence of
erythropoietin.18 Thus, the dominance exerted by the
polycythemia vera erythroid clone over polyclonal erythroid precursors2 could be due in part to its ability
to complete its differentiation more efficiently in a low
erythropoietin milieu.
This dominance may also involve transforming growth factors produced by
polycythemia vera mononuclear cells19 and the
hypersensitivity of polycythemia erythroid progenitor cells to
interleukin 3 (IL-3), granulocyte macrophage-colony-stimulating factor
(GM-CSF),20 stem cell factor (SCF),21 and
insulinlike growth factor (IGF-1).22 Despite
claims to the contrary,23,24 this hypersensitivity does
not appear to be the consequence of an abnormality in the negative
regulatory hematopoietic phosphatase, SHP-1.25,26 Indeed, evidence has been obtained for increased activity of
a membrane-associated protein tyrosine phosphatase in polycythemia vera
erythroid progenitor cells27 and although
overexpression of INK4a and ARF has also been documented in these
cells28 and constitutive phosphorylation of STAT3 has been
observed in the granulocytes in a minority of patients,29
no consistent abnormality of signal transduction or cell cycle
regulation has been identified to date in polycythemia vera nor have
mutations been identified in p53 or Ras during the chronic phase of the
illness.30 There is evidence for both
gain31,32 and loss33,34 of suppressor genes in polycythemia vera but exactly which genes and how they might
affect erythroid progenitor cell behavior remain unknown.
Erythropoietin receptor function
Alternatively spliced forms of the erythropoietin receptor have been identified, one of which has a truncated cytoplasmic domain and was expressed at high levels in immature erythroid progenitor cells in contrast to the full-length receptor, expression of which increased with progenitor cell maturation.42 The function of the truncated receptor splice variant has been a matter of controversy.43,44 It was incapable of transmitting a signal but interfered with the proliferative activity of the full-length erythropoietin receptor44 while promoting differentiation.45 The truncated erythropoietin receptor splice variant was either not expressed well or at all in various erythroleukemia cell lines42,46 and its expression was also decreased or absent in polycythemia vera mononuclear cells.46 Since polycythemia vera erythroid progenitor cells differentiate more quickly than their normal counterparts,18 the significance of these observations with respect to the pathophysiology of the disease remains to be established. Programmed cell death The diverse and often conflicting observations concerning the behavior of polycythemia vera erythroid progenitor cells can be reconciled by the recent recognition that polycythemia vera erythroid progenitors overexpressed the antiapoptotic protein Bcl-xl and were resistant to apoptosis in the absence of erythropoietin.47 Under normal circumstances, early erythroid progenitor cells are largely dormant and require erythropoietin as a mitogen to initiate their entry into cell cycle,48 while late erythroid progenitor cells which are largely cycling require only erythropoietin as a survival factor to allow completion of terminal differentiation.49 Erythropoietin deprivation results in cell cycle arrest in G0/G1,50 and down-regulation of expression of the antiapoptotic proteins Bcl-2 and Bcl-xl followed by programmed cell death.51 By contrast, overexpression of Bcl-2 or Bcl-xl allowed erythroid progenitor cells to maintain their viability while undergoing terminal differentiation in the absence of erythropoietin.51This behavior is not unique to erythroid progenitor cells, since
multipotent hematopoietic progenitor cells overexpressing Bcl-2 not
only remained viable in the absence of both growth factors and serum
but were also able to undergo unrestricted lineage-specific commitment
and terminal differentiation under these conditions as
well.52 A significant feature of Bcl-2 overexpression was prolongation of the G1 phase of the cell
cycle.52 Importantly in this regard, in vitro, the
duration of G1 in erythroid progenitor cells appeared to be
erythropoietin dependent; low concentrations of erythropoietin were
associated with G1 prolongation and initiation of
differentiation while high concentrations were associated with G1 shortening and cell proliferation.53 Thus,
Bcl-xl overexpression could explain the accelerated differentiation of
polycythemia vera erythroid progenitor cells in vitro in the absence of
erythropoietin in contrast to their normal counterparts, as well as
their survival advantage in vivo where erythropoietin production is
suppressed, since both conditions promote G1 arrest and
terminal differentiation. Therefore, the excess of erythroid cells that
defines this disorder is more likely a consequence of cell accumulation
than cell proliferation. This type of behavior is also consistent with
and explains the incremental nature of the erythrocytosis in
polycythemia vera (Figure 2) since only a
fraction (5%-25%) of the erythroid progenitor cell population
exhibited this behavior and its expression appeared to be
random.17 Whether resistance to apoptosis in the absence of growth factors is a feature of myelopoiesis or thrombopoiesis in
this disorder remains to be determined. One caveat with respect to this
mechanism is that Bcl-xl expression is normally up-regulated with
terminal erythroid differentiation54 and whether
accelerated terminal erythroid differentiation in polycythemia vera was
the cause of increased Bcl-xl expression or its consequence, and thus, whether another antiapoptotic mechanism might be involved, has not been
established.
IGF-1 Insight into the mechanism for the resistance of polycythemia vera erythroid progenitor cells to apoptosis was provided by the observation that in the absence of serum, polycythemia vera erythroid progenitor cells were not more sensitive to erythropoietin than their normal counterparts. Rather, they were more sensitive to the antiapoptotic growth factor IGF-1.22 Furthermore, IGF-1 receptors on polycythemia vera peripheral blood mononuclear cells were constitutively tyrosine phosphorylated in contrast to those of normal blood mononuclear cells and also more sensitive to IGF-1.55 The serum concentration of IGF-1 binding protein-1 was increased in polycythemia vera patients and this protein was capable of stimulating erythroid burst formation in vitro.56 Importantly in this regard, it was recently demonstrated that cells expressing truncated erythropoietin receptors lacking their negative regulatory cytoplasmic domain and thought to be hypersensitive to erythropoietin36 were actually hyposensitive to the hormone in the absence of serum and hypersensitive to IGF-1.57 Thus, growth factor hypersensitivity in polycythemia vera could be a result of compensation for defective receptor-mediated signal transduction.The thrombopoietin receptor, Mpl Based on studies to date, the erythropoietin receptor cannot be implicated in such a process but the thrombopoietin receptor, Mpl, is a candidate. Mpl is expressed not only by megakaryocytes and platelets but also by pluripotent hematopoietic progenitor cells,58 the survival of which is enhanced by the cognate ligand of Mpl, thrombopoietin.59 Thrombopoietin also acts synergistically with SCF and IL-3 to promote the proliferation of pluripotent hematopoietic stem cells,60 while impaired Mpl or thrombopoietin expression results in a reduction in the number of both multilineage and committed hematopoietic progenitor cells.61 Thrombopoietin in conjunction with SCF promotes the production of neutrophils from CD34+ cells62 and in conjunction with erythropoietin, the production of erythroid progenitor cells,63 an effect that may be due to abrogation of apoptosis.64 Thrombopoietin overexpression in mice caused granulocytosis, thrombocytosis, osteomyelofibrosis with extramedullary hematopoiesis, and death,65 while ectopic expression of Mpl caused fatal erythroblastosis.66 Importantly, the retrovirus, MPLV, which encodes an Mpl gene truncated in its extracellular domain and fused with a viral envelope protein gene, induced a syndrome in mice that mimicked polycythemia vera,67 while hematopoietic progenitor cells infected with MPLV were growth factor-independent in vitro and capable of terminal differentiation in the absence of growth factors.68 Finally, Mpl expression and, therefore, its responsiveness to thrombopoietin in the megakaryocytes and platelets of polycythemia vera patients, were defective due to an as-yet-undefined impairment of its posttranslational glycosylation that became more profound with disease duration and extent.69 These observations, taken together with the recent demonstration that removal of the Mpl distal extracellular cytokine receptor domain conferred growth factor-independent survival on hematopoietic cells expressing these truncated receptors,70 support the contentions that impaired hematopoietic growth factor receptor signaling has a fundamental role in the pathophysiology of polycythemia vera, that Mpl is a candidate receptor in this regard, and that polycythemia vera is a disorder of cell accumulation not cell proliferation.
Diagnostic criteria William Osler was not the first to describe a patient with polycythemia vera.3 He was, however, the first to emphasize that phenotypic mimicry could confound the diagnostic process and to propose clinical criteria to distinguish polycythemia vera from other disorders causing erythrocytosis71 (Table 1). Remarkably, the Osler diagnostic criteria omit leukocytosis or thrombocytosis, an omission that may have diagnostic connotations as discussed below, but are otherwise similar to the major diagnostic criteria proposed by the Polycythemia Vera Study Group (PVSG) 6 decades later72 (Table 1). These diagnostic criteria are as important for what they do not specify as for what they do. First, bone marrow examination is not part of the diagnostic criteria. This is appropriate because bone marrow abnormalities, such as an increase in megakaryocytes and cellular hyperplasia with a loss of the fat spaces, while characteristic, can never alone be diagnostic for polycythemia vera,73 and if the other major PVSG diagnostic criteria are met, bone marrow examination is unnecessary. Furthermore, while nonrandom cytogenetic abnormalities and myelofibrosis have diagnostic implications, neither has prognostic implications74,75 and their frequency is too low to make bone marrow examination cost-effective. Even if bone marrow examination were diagnostic, that would still not abrogate the need for red cell mass and plasma volume determinations, tests that are justifiably cost-effective. Consequently, marrow examination is useful only when there is a change in the clinical course of the disease, for research purposes or for clinical trials. Yet, bone marrow aspiration and biopsy are frequently performed in the initial evaluation for polycythemia vera.7 This only confirms or generates an unexpressed fear of leukemia on the part of patients; a fear often validated by their physicians with respect to prognosis when in fact, with appropriate management, such an occurrence is only a remote possibility. Second, assays for erythropoietin and erythroid colony-forming cells are not part of the PVSG diagnostic criteria. While it can be argued that 30 years ago these tests were either insensitive or unavailable, it is equally easy to argue for the latter test that availability is still restricted clinically and that for both, specificity and sensitivity are unsatisfactory.
Serum erythropoietin The development of a sensitive and specific assay for circulating erythropoietin for the purpose of distinguishing autonomous from secondary erythrocytosis was long a holy grail of hematologists. Although we now have such an assay, it is an irony that due to the unique physiology of erythropoietin, measurement of the hormone in the circulation cannot be relied on to distinguish between autonomous and erythropoietin-driven erythrocytosis. This is because as the red cell mass expands, both improved tissue oxygenation and the associated increase in blood viscosity serve to depress erythropoietin production,76 while the plasma residence of erythropoietin is simultaneously reduced through increased catabolism by the expanded erythroid progenitor cell pool.77 The net result is that while the lowest erythropoietin levels occur in polycythemia vera14 (Figure 1), this is not absolute and a "normal" erythropoietin level is common in hypoxic erythrocytosis unless the hypoxia is extreme.78 Indeed, given the wide range of normal for serum erythropoietin (4 mU/mL-26 mU/mL), unless the premorbid serum erythropoietin level is known, a 6-fold elevation could occur without exceeding the normal range. Thus, while an elevated serum erythropoietin level suggests tissue hypoxia as a cause for erythrocytosis, a normal serum erythropoietin level does not exclude an hypoxic cause. Only an arterial oxygen saturation determination, as stipulated by the PVSG criteria, will suffice for this purpose.Erythroid progenitor cell assay Similarly, although erythropoietin-independent erythroid colony formation in vitro is characteristic of polycythemia vera15 and has been widely embraced as a diagnostic test for the disease, it cannot be recommended for this purpose. The reasons are many and cogent. Erythropoietin-independent erythroid colony formation is not specific for polycythemia vera. Although it is most obvious in this disorder, it can be seen in nonclonal causes of erythrocytosis79,80 and in healthy controls79,81 as well as in essential thrombocytosis.81-84 Furthermore, endogenous erythroid colony formation has been absent in some patients who meet the PVSG criteria for polycythemia vera.85 Part of the problem may be methodologic since the greatest specificity and sensitivity has been observed when bone marrow was used as the source for erythroid progenitor cells as opposed to peripheral blood and when erythroid colony-forming units (CFU-Es) were analyzed as opposed to erythroid burst-forming units (BFU-Es).86 The in vitro clonal assay for erythroid progenitor cells is also neither standardized nor widely available. For these reasons and because this assay does not establish clonality, it cannot be recommended as a routine clinical diagnostic test for polycythemia vera.PVSG minor criteria In an effort to improve diagnostic accuracy, particularly in the absence of splenomegaly, the PVSG added additional criteria such as the presence of leukocytosis and thrombocytosis. Approximately 60% of patients will not have both of these abnormalities initially4,87,88 nor will leukocyte alkaline phosphatase expression, serum vitamin B12, and unbound serum vitamin B12 binding capacity be uniformly elevated.88,89Impaired platelet Mpl expression A number of other phenotypic abnormalities of the red cells (eg, microcytic erythrocytosis,90 increased hemoglobin F synthesis18), white cells (eg, increased surface expression of IgG receptors,91 impaired responsiveness to chemotactants92), and platelets (eg, reduced PGD2 receptor expression,93 impaired lipoxygenase generation94,95) have been observed in polycythemia vera but none of these abnormalities are either specific for the disease or suitable clinically for diagnostic use. Recently, impaired expression of the thrombopoietin receptor, Mpl, by the platelets of polycythemia vera and idiopathic myelofibrosis patients was documented96 and the severity of this defect correlated with the duration and extent of disease.69 A similar abnormality was not present in secondary erythrocytosis or chronic myelogenous leukemia (CML) but was present in some patients with essential thrombocytosis.97 Interestingly, recently the development of polycythemia vera or idiopathic myelofibrosis was observed in 3 of 4 essential thrombocytosis patients with reduced platelet Mpl expression,98 suggesting that the abnormality could have prognostic significance in this disorder. Whether impaired platelet Mpl expression will prove clinically useful for distinguishing polycythemia vera from other types of erythrocytosis or to identify polycythemia vera in the absence of erythrocytosis remains to be established. However, the consistency with which impaired platelet and megakaryocyte Mpl expression was observed in polycythemia vera patients96,99 suggests that it may be useful diagnostically.PRV-1 expression Recently, overexpression of the mRNA of a novel member of the uPAR receptor superfamily, designated PRV-1, was identified in polycythemia vera granulocytes by subtractive hybridization.89 This GPI-linked surface membrane receptor was detected only in normal granulocytes after exposure to granulocyte-colony-stimulating factor (G-CSF), was not expressed in the mononuclear cells from patients with secondary erythrocytosis, CML, or acute myelogenous leukemia (AML) and did not correlate with the expression of leukocyte alkaline phosphatase, another GPI-linked protein.89 Interestingly, like impaired Mpl expression, PRV-1 expression has also been detected in certain patients with essential thrombocytosis who also demonstrated in vitro erythropoietin-independent colony formation.100 Quantitation of PRV-1 mRNA expression may also prove useful diagnostically in distinguishing polycythemia vera from other disorders causing erythrocytosis.Clonality assays The most important omission from the PVSG diagnostic criteria for polycythemia vera was a requirement for the establishment of clonality. Unfortunately, however, in contrast to CML, there has been no clinically applicable clonal assay for polycythemia vera or its companion myeloproliferative disorders, idiopathic myelofibrosis and essential thrombocytosis. Nonrandom chromosome abnormalities are not uncommon in these disorders and in polycythemia vera the most frequent are trisomies of 1q, 8, 9 or 9p, del13q, del20q, or interstitial deletions of 13 or 20; occasionally multiple defects are present.31,74,101,102 However, there is no consistent or unique cytogenetic abnormality associated with any of these disorders and at the time of diagnosis, using conventional cytogenetic techniques, less than 20% of polycythemia vera patients exhibit a cytogenetic abnormality and many patients never develop one.74 In this regard, the use of interphase fluorescence in situ hybridization (FISH) may increase the diagnostic yield.32 However, bone marrow aspiration is still required for cytogenetic analysis unless there are numerous primitive cells in the circulation.In the absence of a specific and consistent cytogenetic marker, clonality assays in the chronic myeloproliferative disorders other than CML have been limited to genes on the X chromosome whose expression in women is subject to random inactivation. It was on this basis that the clonality of polycythemia vera was first established.1 However, this type of clonal analysis is limited to women who are informative with respect to the expression of G-6PD isoenzymes or specific DNA polymorphisms. Expression of the former is restricted while the latter analysis is not informative in up to 30% of female patients103 and its utility is further limited by nonrandom, age-associated X-linked gene inactivation.104-106 Thus, although it has been clearly established as a general principle that polycythemia vera is a clonal disorder and represents the consequences of transformation of a multipotent hematopoietic progenitor cell, there is currently no way to establish clonality in the majority of patients considered to have the disease. While this is not an issue in patients exhibiting trilineage hematopoietic cell hyperplasia and extramedullary hematopoiesis, it is a critical issue in patients with erythrocytosis alone since in the absence of proof of clonality, it is difficult to justify the use of mutagenic drugs. Osler was well aware of the phenotypic mimicry that complicated the diagnosis of polycythemia vera71 and given the well-documented existence of unidentified and newly recognized nonclonal forms of erythrocytosis107,108 we would do well to follow his lead. Measurement of the red cell mass and plasma volume With respect to what the PVSG diagnostic criteria do stipulate, the most important is elevation of the red cell mass. Admittedly, this criterion does not establish clonality or even distinguish polycythemia vera from other disorders that cause erythrocytosis. However, it makes a vitally important point with respect to the evaluation of a high hematocrit level in general and polycythemia vera in particular that
only direct measurement of the red cell mass and plasma volume will
suffice to distinguish absolute erythrocytosis from so-called spurious
or relative erythrocytosis due to plasma volume contraction or red cell
redistribution.109-111 Indeed, it is not hyperbole to
state that failure to appreciate this basic concept has been
responsible for more diagnostic and therapeutic failures in patients
with polycythemia vera than any other single factor. The reason appears
to be a lack of understanding of the pathophysiology of the red mass
and plasma volume in disorders causing erythrocytosis.
Normally, the hematocrit or hemoglobin is maintained at a constant level that is characteristic for each individual but varies between individuals of the same sex by as much as 15% and between sexes by approximately 10%. As a corollary, erythropoietin production as represented by the serum erythropoietin level also remains constant.112 With tissue hypoxia, regardless of cause, as the red cell mass increases there is usually a concomitant reduction in plasma volume. Given the other factors involved in plasma volume regulation as well as in its measurement, the magnitude of plasma volume reduction reported with hypoxic erythrocytosis due to right-to-left cardiac shunts,113 impaired pulmonary gas exchange,114 carbon monoxide poisoning,115 and low ambient oxygen tension116 varies but the downward trend is unmistakable. It should not be surprising, therefore, that therapy with recombinant erythropoietin,117 androgenic steroids,118 and even blood transfusion119 also leads to a reduction in plasma volume; the mechanisms involved are unknown but may in part be protective since increasing whole blood viscosity suppresses endogenous erythropoietin production.76,120 By contrast, in polycythemia vera where erythropoiesis is autonomous and erythropoietin production is suppressed, as the red cell mass increases the plasma volume may be unchanged or increase121 until the hematocrit level is more than 60%.122 Unfortunately, the hematocrit level, whether directly determined by centrifugation or calculated from the mean corpuscular volume (MCV) and the red cell count, will not reflect these changes because even under normal circumstances, the distribution of red cells and plasma is not uniform throughout the circulatory system.123-125 Rather, as demonstrated by independent determinations of the red cell mass and plasma volume, the ratio of red cells to plasma is higher in the peripheral vessels, venous or arterial, than it is in the body as a whole (ie, whole body hematocrit derived from independent measurements of red cell mass and plasma volume/peripheral venous hematocrit = 0.92).126,127 This is a consequence of both the slower flow of the peripherally displaced plasma compared with the axial red cells and plasma skimming in the smaller vessels.123,125,128,129 With disease, changes occur in the plasma volume and red cell mass or in their distribution, particularly if there is splenomegaly121,130-132 that will not be evident by hematocrit level determination alone.109,133-136 Indeed, the assumption that the hematocrit accurately reflects the red cell mass or, stated differently, that the red cell mass:plasma volume ratio is constant in health and disease or even for all parts of the circulation, is simply incorrect and nowhere more so than when erythrocytosis is present.114,127,134,136 Remarkably, although it was established in 1921123 and
repeatedly confirmed125,126,137,138 that only by
independently measuring the red cell mass (currently with
51chromium) and the plasma volume (with
125I-albumin) could an accurate assessment of each as well
as the total blood volume be obtained, there is still resistance to
this standard of practice. The reasons for this are several. First, the
range of normal for such measurements, not unlike the range of the
hematocrit, is wide, which affects their sensitivity. Consequently, only values more than 2 standard deviations (25%) above the mean are
considered abnormal.139 Second, since adipose tissue has a
low vascularity, red cell mass correlates better with lean body mass
than body weight127 but, unfortunately, there is no simple method for clinically assessing lean body mass.111
Therefore, if body weight in mL/kg is used as standard, the red cell
mass will be underestimated in the obese,140 requiring
correction factors based on weight.139 For these reasons
and because of the need to use 2 radioisotopes, it has been variously
advocated that the red cell mass be calculated from the plasma volume
and the hematocrit141 by a mathematical formula based on
the regression of the red cell mass on the hematocrit,142
or dispensed with altogether on the grounds that the diagnosis of
polycythemia vera is usually clinically apparent on the basis of other
findings.143 None of these approaches are either
physiologically sound or clinically acceptable because they rely on
assumptions about the red cell mass:plasma volume ratio derived from
measurements in healthy individuals that are not valid in
disease.109,127 Simply stated, in disease, the red cell
mass and plasma volume can vary independently of each other and it is
no more possible to assess the red cell mass from the hematocrit than
it is to determine total body sodium from the serum sodium
concentration. Splenomegaly, of course, just compounds the
problem.130-132,144 Stated differently, it is the
author's opinion that whenever the diagnosis of polycythemia vera is
considered, due to the potential of plasma volume expansion, a
"normal" hematocrit value can never be considered normal and an
independent determination of both the red cell mass and plasma volume
by isotope dilution is mandatory for diagnostic and therapeutic purposes. Examples of the usefulness of red cell mass and plasma volume
measurements in the evaluation of the cause of a high hematocrit level
are shown in Table 2.
The description of the natural history of polycythemia vera in current hematology textbooks disguises the fact that little more is known today about its clinical course than was known 80 years ago. Attempts to define the natural history of polycythemia vera then as now have been frustrated not only by the low incidence of the disorder but also its chronic nature which precludes most physicians from seeing more than a few of these patients or even following them for a sufficient duration to encounter the full scope of the disease. These factors coupled with an initial lack of appreciation of the effect of radiation or chemotherapy on bone marrow function led to the acceptance of anecdotal case studies as representative of the natural history of polycythemia vera. The natural history hypothesis In 1954, based on such anecdotal reports, a hypothetical description of the natural history of polycythemia vera was proposed by Wasserman according to which the disease evolved through a series of clinical stages beginning with an asymptomatic phase and proceeding through erythrocytotic, compensated or inactive, and spent or postpolycythemic myeloid metaplasia phases before terminating in acute leukemia if death from another cause did not intervene first.145 Given the similarity of this proposed natural history to that of CML, it is not surprising that without either testing or debate, the status of dogma was promptly conferred on this hypothetical description, and for the past 46 years it has been reiterated unchanged in greater or lesser detail in virtually every major hematology textbook when in fact the clinical data from which it was derived fail to meet the lowest standard of evidence-based medicine. Indeed, the basis for this concept of the natural history of polycythemia vera was a 1938 publication by Rosenthal and Bassen describing 13 patients from a series of 75 who were thought to be representative of the various hematologic manifestations of the disease.146 Of the 13 patients. 2 were anemic and thereby thought to manifest the so-called "spent" phase of the disorder, a concept first proposed by Minot and Buckman.147 Both, however, had previously been treated with radiation therapy.Dameshek had earlier proposed a similar natural history scheme148 but qualified his proposal as follows: "it is difficult to state what the `normal' course of the disease would be without the various therapeutic methods which undoubtedly influence it." Unfortunately, this caveat was disregarded and only rarely have polycythemia vera patients been followed long enough without exposure to therapeutic interventions such as radiation or 32P that significantly alter bone marrow function for any meaningful appreciation of the natural history of the disorder to be obtained. When this has been achieved, a different picture of the disease emerges.149,150 Additionally, it is not widely appreciated that the disease may have different manifestations depending on the age at onset and the patient's sex.151 Finally, adding to the confusion has been the uncritical acceptance or misuse of terms such as "spent phase" and "postpolycythemic myeloid metaplasia" together with the assumption that the various chronic myeloproliferative disorders are interrelated152 when in fact proof is lacking that they are.153 The spent phase of polycythemia vera While it has been clearly established that polycythemia vera can be complicated by anemia, myelofibrosis, and myeloid metaplasia, the frequency with which these complications occur and their clinical significance in the absence of cytotoxic therapy has rarely been ascertained prospectively. For example, the development of anemia due to bone marrow exhaustion has been considered an inevitable event in the natural history of polycythemia vera and possibly a forerunner of leukemia.145,154 As mentioned above, it was in this context that the term "spent phase" was initially used.147 However, in a large series of polycythemia vera patients managed without radiation therapy, anemia was most often due to chemotherapy, hemorrhage, deficiency of iron, folic acid or vitamin B12, or another disease and not to intrinsic marrow failure.149 Importantly, the role of iron deficiency as a cause for anemia was not recognized in early descriptions of the disease,147 nor was it appreciated that the plasma volume expansion associated with splenomegaly could mask the true red cell mass. Furthermore, in some patients, the "spent phase" proved to be reversible with therapy.150 When these facts, together with ferrokinetic studies in patients with advanced disease155 are taken into account, it is clear that that there is not necessarily an irreversible exhaustion of erythropoiesis with disease progression or duration and that while the frequency with which refractory anemia is a direct consequence of polycythemia vera remains to be established, from the available data, it appears to be uncommon.156,157 More important than the so-called spent phase is the peculiar wasting syndrome that can develop late in the course of polycythemia vera in which intractable weight loss is associated with extramedullary hematopoiesis and substantial elevation of the white cell and platelet counts.Finally, the term "spent phase" as used today also appears to mean different things to different authors. Initially and still employed to denote the progression of polycythemia vera to a state of bone marrow failure and possibly a prelude to leukemia145,146 it has also been used to describe a phase of polycythemia vera with splenomegaly without either myelofibrosis or myeloid metaplasia in which erythrocytosis continues unabated with its extent masked by plasma volume expansion.136 As indicated above, the former use of the term has never been authenticated in a scientifically acceptable fashion and the latter use, of course, is actually contradictory since in the type of patients described, bone marrow function was actually robust.155,158 Furthermore, such patients have also been described by others as being in the "stationary phase" of the disease,148 a designation that is equally improbable since no evidence has been provided that disease activity has changed. All of this only serves to illustrate the confusion that results when anecdotal experiences are uncritically elevated to the status of paradigm and medical jargon is substituted for medical judgment. In a disease such as polycythemia vera, anecdotal case reports are invaluable for defining what is possible but only prospective longitudinal studies of well-defined patient cohorts can establish what is probable. Myeloid metaplasia This conclusion also applies to 2 other complications of polycythemia vera, myeloid metaplasia and myelofibrosis. Since splenomegaly in polycythemia vera is initially due to red cell congestion,13,159-161 without a tissue biopsy, the assumption that splenic enlargement represents extramedullary hematopoiesis could be erroneous.133,162 It is probably for this reason that estimates of the frequency of myeloid metaplasia, based on the presence of immature myeloid and erythroid cells in the circulation or organomegaly, have varied from 7% to 14%.75,145,157,163 Furthermore, the widely used term "postpolycythemic myeloid metaplasia"164 is more than imprecise; it is an oxymoron since it implies that myeloid metaplasia is a terminal complication or conversion of polycythemia vera to another disorder rather than being an integral component of the disease itself165 that may not shorten survival.75,166,167 It also implies that bone marrow activity is impaired when in fact, in the absence of radiation or chemotherapy, there is no correlation between the extent of extramedullary hematopoiesis and bone marrow hematopoietic activity in polycythemia vera.168As originally used, "postpolycythemic myeloid metaplasia" was a synonym for the spent phase of polycythemia vera in which the predominant features were myeloid metaplasia, splenomegaly, myelofibrosis, and anemia.164 However, as indicated above with respect to anemia, there have been few attempts to analyze the influence of radiation or chemotherapy on the development of "postpolycythemic myeloid metaplasia" but when this has been done, the majority of patients fitting this description have been exposed to agents toxic to the bone marrow.163,164,169 Thus, a century after the description of polycythemia vera, judgment must remain suspended as to the frequency with which bone marrow failure is a terminal feature of this disease in the absence of an exogenous cause but published data suggest that it is not high.157 Myelofibrosis Given the above, it should not be surprising, therefore, that substantial misconceptions also exist with respect to significance of myelofibrosis in polycythemia vera. This is because the mechanisms for myelofibrosis are poorly understood170-172 and the means for quantitating it imprecise due to the patchy nature of the process.168,173-175 Furthermore, with respect to diagnosis, it has generally been assumed that polycythemia vera and idiopathic myelofibrosis are closely related disorders,152 although apart from phenotypic similarities, this assumption has never been substantiated scientifically. Indeed, some investigators have considered the development of myeloid metaplasia and myelofibrosis in polycythemia vera as indicating the onset of a "transitional myeloproliferative disorder"135 rather than part of the natural history of the disease, while others have considered erythrocytosis to be a complication of idiopathic myelofibrosis.134 The development of myelofibrosis per se in the setting of polycythemia vera has also been considered to have adverse prognostic implications,176 another erroneous assumption.75,150,156,174,177Because there is no widely accepted standard for the quantitation of marrow reticulin158 nor a universally acknowledged definition for the term "myelofibrosis" outside the setting of the disease, idiopathic myelofibrosis,178 it is difficult to obtain a precise frequency for its occurrence in polycythemia vera as well as the impact of therapy on this. Within the limitations of these data, it appears that increased marrow reticulin is part of the natural history of the disease, presumably reflecting in part the increase in marrow cellularity158,179 and possibly disease duration.180 However, an increase in marrow reticulin, or even osteosclerosis, is certainly not synonymous with a "spent" phase75,134,135,155,158,181-183 and can be modified to a certain extent by therapy,75 particularly with busulfan.182,184 Spontaneous regression of myelofibrosis has also been observed,168,175,182,185,186 although the patchy nature of the process makes this observation as difficult to substantiate as the contention that myelofibrosis is a progressive and destructive process.173-175,187 If studies of idiopathic myelofibrosis patients can be used as a guide, myelofibrosis and osteosclerosis are not usually progressive processes.168,174,188 There does, however, appear to be a higher incidence of "myelofibrosis" in polycythemia vera patients exposed to either radiation or chemotherapy than those treated by phlebotomy alone, 8.4%156,163,164,173,189 versus 4.5%,149,156,163 but there is not absolute agreement on this point.75,176 As a corollary, in studies of idiopathic myelofibrosis, polycythemia vera patients comprised 16% (range, 7%-38%) of the patients and most had been treated with radiation or chemotherapy.173,175,177,190-193 To the extent that it has been evaluated, marrow reticulin was increased in 8% to 15% of polycythemia vera patients at the time of diagnosis,73,181,194,195 a finding that appeared to have no prognostic significance.75,150,167 This is in contrast to development of myelofibrosis in CML, which is generally associated with disease acceleration.196 Taking everything together, while it is clear that the hematologic manifestations of polycythemia vera include myeloid metaplasia and myelofibrosis, it is not yet established that these processes indicate terminal bone marrow failure or that marrow failure is an inevitable consequence of the disease in the absence of cytotoxic therapy. Certainly, given the limitations of the data upon which the current concept of the natural history of polycythemia vera is based, it is time to abandon uncritical and unphysiologic terms such as "stationary," "spent," "transitional," and "postpolycythemic myeloid metaplasia" in favor of descriptions based on quantitative, prospective, and biologically sound observations. Acute leukemia: relationship to therapy The relationship between polycythemia vera and acute leukemia also requires clarification because it is central to the management of polycythemia vera. In 1950, Schwartz and Ehrlich reviewed the published evidence that acute leukemia was a feature of polycythemia vera in the absence of exposure to mutagenic therapy.197 Of 83 published cases, only 30 had unequivocal evidence of polycythemia vera preceding the development of acute leukemia and 25 of these patients had been irradiated. Of the remaining 5 patients, data supporting the absence of radiation exposure were available for only one. Since the criteria employed by Schwartz and Ehrlich for the diagnosis of polycythemia vera and the diagnosis of acute leukemia were stringent, it is ironic that in the one case of polycythemia vera that they accepted as spontaneously developing acute leukemia, the diagnosis was not histologically confirmed.146 They also emphasized the association of the newly introduced 32P therapy with acute leukemia in polycythemia vera patients and correctly predicted that this complication would increase in frequency. By contrast, the advocates of 32P therapy thought that the development of acute leukemia was an inevitable consequence of extended patient survival.133,198Modan and Lilienfeld's landmark 1965 paper unequivocally demonstrated that the leukemogenic effect of irradiation or 32P was not a consequence of prolonged survival,163 but the impact of their observations was blunted by the report of Halnan and Russell in which the incidence of acute leukemia in 32P-treated polycythemia vera patients was negligible.199 Any doubt about this issue, however, was eliminated by the PVSG-01 trial which established that therapy with either 32P or the alkylating agent, chlorambucil, resulted in an incidence of acute leukemia far exceeding that observed with phlebotomy alone without prolonging survival.200 The apparent conflict between the Modan and Lilienfeld data and that of Halnan and Russell was in part due to study design, since the mean time of onset of 32P-induced acute leukemia is approximately 8.5 years201 while most of Halnan and Russell's patients were followed for only 5 years, and in part to differences in radiation dose. Parenthetically, this episode serves to remind us that the absence of evidence is not proof of its absence. Polycythemia vera has the dubious distinction of being the first
hematologic malignancy and only the third disorder
temporally Spontaneous acute leukemia: Richter syndrome revisited In contrast to the well-established body of data on treatment-induced acute leukemia in polycythemia vera,163,180,197,200,207,212-214 few data exist other than anecdotal case reports with respect to the spontaneous occurrence of acute leukemia. As mentioned above, up to 1950, Schwartz and Ehrlich accepted as valid only one published report describing such an event.197 In the same year, Dameshek reported that one of his 50 patients and 2 of 100 Mayo Clinic patients treated only by phlebotomy had developed acute leukemia.148 Since then there have been 12 individually published case reports215-217 and an additional 11 patients identified by questionnaire215 in whom acute leukemia complicated polycythemia vera in the absence of irradiation or chemotherapy, while in 4 published series comprising 505 patients, 3 cases of spontaneous acute leukemia were observed.149, | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Introduction
The pathogenesis of...
), secondary erythrocytosis
(
) or relative erythrocytosis (x).
Adapted from Handin
et al