Pathogenesis

PV is a clonal stem cell disease with trilineage myeloid involvement.¹³ In addition, some studies suggest clonal heterogeneity, including clonal involvement of B lymphocytes,¹⁴ as well as polyclonal granulopoiesis in certain cases.¹⁵ In vitro, erythroid colony formation in patients with PV does not require the addition of exogenous erythropoietin (Epo).¹⁶ This phenomenon is called endogenous erythroid colony growth and does not occur in either normal controls or in patients with nonclonal polycythemia. In addition, erythroid progenitor cells in PV display growth factor hypersensitivity to Epo, insulin-like growth factor (IGF)–1, and other cytokines.^17,18 The consistently observed IGF1 hypersensitivity of erythroid cells in PV has been attributed to alterations in IGF1 binding proteins.¹⁹ However, several studies show that growth factor–independent or growth factor–hypersensitive colony formation is specific to neither PV nor erythroid progenitor cells.¹⁸

The Epo receptor gene as well as its protein are intact in PV.²⁰ On the other hand, several postreceptor molecular abnormalities have been reported and include increased baseline phosphorylation of the IGF1 receptor,²¹ decreased activity of SH-PTP1 (a tyrosine phosphatase),²² increased activity of membrane-associated SH-PTP,²³ constitutive activation of STAT3,²⁴ upregulation of negative control elements of the cell cycle (p16/p14),²⁵ and abundance, in erythroid precursors, of antiapoptotic proteins (Bcl-x_L).²⁶ However, these observations have not been always reproducible, and none of them have been shown to be specific to PV, as opposed to other MPNs.

In 2005, a novel Janus kinase 2 (JAK2) mutation (JAK2V617F) was described in association with PV, ET, and PMF.²⁷ JAK2V617F occurs in approximately 95% of PV patients but also in approximately 50% of those with ET or PMF.^28–31 In addition, the mutation is found in a small proportion of patients with other myeloid neoplasms including nonclassic MPNs and MDS.^32,33 Other JAK2 mutations (i.e., JAK2 exon 12 mutations) were subsequently described in the majority of patients with JAK2V617F⁻ PV or “idiopathic erythrocytosis,”³⁴ thus raising the possibility that a JAK2 mutation is essential for the PV phenotype. On the other hand, neither JAK2V617F nor JAK2 exon 12 mutations have so far been reported in lymphoid disorders,^35–38 solid tumor,^41–41 or secondary myeloproliferation including congenital or acquired polycythemia.^44–44

JAK2V617F is a G-to-T somatic mutation, at nucleotide 1849, in exon 14, resulting in the substitution of valine by phenylalanine at codon 617.28–31 JAK2 exon 12 mutations (F537-K539delinsL, H538QK539L, K539L, N542–E543del) include both in-frame deletions and tandem point mutations.³⁴ Both exon 14 and exon 12 JAK2 mutations induce cytokine-independent/hypersensitive proliferation in Epo receptor–expressing cell lines and a PV-like phenotype in mice.³⁴ Whereas homozygosity for JAK2V617F, which results from mitotic recombination, can be demonstrated in the majority of patients with PV,⁴⁵ exon 12 JAK2 mutations are often heterozygous.³⁴ The precise pathogenetic role of JAK2 mutations in PV and related MPNs continues to be investigated.

Diagnosis

In clinical practice, the term polycythemia is used to indicate the possible occurrence of an increased erythrocyte volume or red blood cell mass (RCM). Such a perception might be either real (true polycythemia) or spurious (apparent polycythemia; Fig. 100-1).¹⁸ True polycythemia may represent either PV or a nonclonal increase in RCM that is often, but not always, mediated by Epo (secondary polycythemia; Fig. 100-2). Apparent polycythemia may result from either a reduction in plasma volume (relative polycythemia) or an inaccurate perception of an elevated RCM that results from not appreciating high normal values of hemoglobin/hematocrit.⁴⁶ Inapparent polycythemia is the converse of apparent polycythemia and indicates a true increase in RCM that is masked by a normal hemoglobin/hematocrit value secondary to a concomitant increase in plasma volume (see Fig. 100-1).⁴⁷

In 1975, the PV study group published a set of “diagnostic criteria” that were primarily used to ensure the exclusion, to treatment protocols, of patients with secondary or apparent polycythemia.⁴⁸ These criteria required the demonstration of increased RCM by blood volume measurement using labeled erythrocytes as well as the demonstration of normal hemoglobin oxygen saturation. In 2001, the WHO published a refined diagnostic criteria for PV and related MPNs that recognized the value of both bone marrow histology and “MPN-specific” biological parameters.⁴⁹ The recent discovery of the almost invariable association between PV and a JAK2 mutation has led to revised WHO diagnostic criteria for PV (Table 100-2)⁵⁰ and a refined diagnostic algorithm (see Fig. 100-2),⁵¹ both of which incorporate JAK2 mutation screening.

Because more than 95% of patients with PV carry the JAK2V617F mutation, which is absent in both secondary and apparent polycythemia, it is most effective to initiate the workup of a patient with suspected PV with peripheral blood mutation screening for JAK2V617F (see Fig. 100-2). Furthermore, to minimize the consequences of false-positive or false-negative molecular test results, as well as capture the few cases of PV that are JAK2V617F⁻, concomitant measurement of serum Epo level is recommended.⁵² If the results of both tests are suggestive of PV (i.e., mutation-positive and low serum Epo), then the diagnosis is likely and bone marrow examination is encouraged but not essential for making the diagnosis. If the JAK2V617F and serum Epo test results are both not consistent with the diagnosis of PV (i.e., mutation-negative and either normal or increased Epo), then further investigation for PV is not advised unless dictated otherwise by the clinical scenario.

If there is discrepancy between the molecular test result and serum Epo level, one should first repeat both tests and then proceed with bone marrow examination if the results are unchanged (see Fig. 100-2). In a patient with acquired erythrocytosis and a subnormal serum Epo level, the possibility of exon 12 JAK2 mutations should be entertained. In this regard it should be noted that some cases with JAK2 exon 12 mutation–associated PV might not display overt PV-characteristic features, even at the level of bone marrow histology.³⁴

When congenital polycythemia is suspected, initial laboratory testing should include measurement of the oxygen tension at which hemoglobin is 50% saturated (i.e., p50).⁵¹ Left-shifted oxygen dissociation curve, suggested by decreased p50, suggests the presence of either high oxygen-affinity hemoglobinopathy (autosomal dominant)⁵³ or 2,3-bisphosphoglycerate (2,3-BPG) deficiency, usually a consequence of BPG mutase mutation (autosomal recessive).⁵⁴ If the p50 is normal, then the possibility of germline mutations of molecules that either enhance Epo effect (e.g., EPOR mutations)⁵⁵ or govern intracellular oxygen sensing (e.g., mutations involving the von Hippel-Lindau [VHL] tumor suppressor or hypoxia-inducible factor 1a [HIF-1a] prolyl hydroxylase genes)^56,57 should be considered (see Fig. 100-2). VHL mutations are usually associated with increased serum Epo level and constitute the most frequent mutations in congenital polycythemia (e.g., Chuvash polycythemia).⁵⁸ In contrast, serum Epo level is often subnormal in patients with EPOR mutations.⁵⁹

Bone marrow histology, to the experienced hematopathologist, is often revealing of characteristic changes of a MPN that include hypercellularity, increased number of megakaryocytes including cluster formation, the presence of giant megakaryocytes and pleomorphism in megakaryocyte morphology, mild reticulin fibrosis, and decreased bone marrow iron stores.⁶⁰ In contrast, cytogenetic studies in PV disclose abnormalities (trisomies of chromosomes 9 and 8 and deletions of the long arms of chromosomes 13 and 20) in only 13% to 18% of patients at diagnosis, and hence have limited diagnostic value.⁶¹

Treatment

The natural history of PV is characterized by a lifelong propensity for thrombohemorrhagic complications, late-onset disease transformation into post-PV myelofibrosis and/or AML, and a shortened life expectancy.⁶¹ Specific treatment has been shown to positively influence the risk of both macrovascular and microvascular complications but not that of clonal evolution to post-PV myelofibrosis or AML.

Phlebotomy is the cornerstone of therapy in PV and is the only treatment modality that has improved survival in affected patients (Box 100-1). It is reported that median survival may be as low as 2 years in the absence of such treatment.⁶³ Based on limited retrospective studies in PV that showed a progressive increase in the incidence of vascular occlusive episodes above a hematocrit level of 44%,⁶⁴ as well as other studies that showed suboptimal cerebral blood flow in ranges of hematocrit values between 46% and 52%,⁶⁵ the therapeutic target hematocrit level has, for a long time, been set at or below 45%. This particular practice is now further supported by the results of a recent randomized study where 365 adults with PV were treated with a target hematocrit, less than 45% or 45% to 50%.⁶⁶ After a median follow-up of 31 months, the primary end point of thrombotic events or deaths from cardiovascular causes was recorded in 5 of 182 patients in the low-hematocrit group (2.7%) and 18 of 183 patients in the high-hematocrit group (9.8%) (P = 0.007). Furthermore, because of the physiological difference in hematocrit values between the two genders as well as among different races, it is reasonable, though not evidence-based, to target an even lower hematocrit level (i.e., 42%) in women and African-Americans.⁶⁷

Pathogenesis

Trilineage clonal myeloproliferation has been demonstrated in the majority of patients with ET using X chromosome–linked DNA or gene product analysis.104,105 However, X-linked clonal assays have revealed both polyclonal hematopoiesis in a substantial minority of patients with ET¹⁰⁶ and “monoclonal” hematopoiesis in normal elderly controls.¹⁰⁷ Furthermore, in some cases, the clonal process in ET was shown to include lymphocytes¹⁴ or be restricted to megakaryocytes.¹⁰⁵ The clonal basis of ET was recently underlined by JAK2V617F mutation analysis that revealed the presence of the mutation even in those patients who feature “polyclonal” hematopoiesis by X-linked clonality studies.^106,108 However, the primary clonogenic event in ET remains undefined despite the recent descriptions of two gain-of-function mutations that occur in 50% (JAK2V617F)¹⁰⁹ and 1% (MPLW515L/K)¹¹⁰ of patients with ET, respectively.

Other biological features in ET include in vitro growth factor independence/hypersensitivity of both erythroid and megakaryocyte progenitor cells,111,112 low serum Epo level,¹¹³ altered megakaryocyte/platelet Mpl expression,^114,115 increased neutrophil PRV-1 expression,^116,117 and decreased platelet serotonin content.¹¹⁸ Laboratory studies in ET have demonstrated myeloid growth factor hypersensitivity to interleukin-3¹¹⁹ as well as thrombopoietin (Tpo).¹¹² Growth factor independence of myeloid progenitor cells in ET and related myeloproliferative disorders has now been attributed to mutations involving molecules of the JAK-STAT pathway including the aforementioned JAK2V617F and MPLW515L/K. In patients with ET,¹²⁰ PV,¹²¹ and myelofibrosis,¹²² serum Tpo levels are usually normal or elevated despite an increased megakaryocyte mass. This has been attributed to the markedly decreased megakaryocyte/platelet expression of myeloproliferative leukemia (MPL), although the extent of this particular phenotypic abnormality is significantly less in ET as compared with PV or PMF.^{114,115,123,124}

In addition to clonal myeloproliferation, ET is also characterized by microvascular symptoms (e.g., headaches and erythromelalgia) as well as an increased risk of both thrombosis and bleeding.¹²⁵ Pathogenesis in the former might involve abnormal thromboxane A₂ generation and small vessel–based platelet-endothelial interactions that are inhibited by aspirin therapy.^126,127 On the other hand, there is increasing information that implicates granulocytes but not platelets as the thrombogenic culprit in ET.^88,128–130 Consistent with these observations, a recent study has identified leukocytosis as an independent risk factor for thrombosis in ET.¹³¹ Bleeding diathesis in ET is currently believed to involve an acquired von Willebrand syndrome that becomes apparent in the presence of extreme thrombocytosis.⁹⁰ The mechanism of acquired von Willebrand syndrome in ET is currently believed to involve a platelet count-dependent increased proteolysis of high-molecular-weight von Willebrand protein.⁹⁰ Other qualitative platelet defects in ET are believed to play a minor role in disease-associated hemorrhage and include defects in epinephrine-, collagen-, and adenosine diphosphate–induced platelet aggregation, decreased adenosine triphosphate secretion, and acquired storage pool deficiency that results from abnormal in vivo platelet activation.¹²⁵

Diagnosis

Although thrombocytosis is the hallmark of ET, more than 85% of cases with thrombocytosis seen in routine clinical practice are reactive (secondary thrombocytosis) and associated with other comorbid conditions (Table 100-6).¹³² The degree of thrombocytosis is a poor discriminator of ET from secondary thrombocytosis. However, the clinical scenario is often helpful in distinguishing ET from secondary thrombocytosis. In routine clinical practice, it is important to exclude the contribution of either iron-deficiency anemia or a hyposplenic state as possible causes of an otherwise unexplained thrombocytosis (Box 100-2