Abbreviations
aCML	Atypical chronic myeloid leukemia
AML	Acute myeloid leukemia
ASCT	Allogeneic stem cell transplant
ASXL1	Additional sex combs-like 1
ATP	Adenosine triphosphate
CMML	Chronic myelomonocytic leukemia
CR	Complete response
ESA	Erythroid stimulating agent
ET	Essential thrombocythemia
HDAC	Histone deacetylase
HPI	Hedgehog pathway inhibitors
HSCT	Hematopoietic stem cell transplant
ICSBP	Interferon consensus sequence binding protein
IDH	Isocitrate dehydrogenase
IKZF1	IKAROS family zinc finger 1
IPSS	International Prognostic Scoring System
IWG-MRT	International Working Group for Myelofibrosis Research and Treatment

MPL	Myeloproliferative leukemia
MPN	Myeloproliferative neoplasm
MPN-BP	Myeloproliferative neoplasm blast phase
mTOR	Mammalian target of rapamycin
NHEJ	Nonhomologous end joining
Ph	Philadelphia (chromosome)
PMV	Primary myelofibrosis
PV	Polycythemia vera
ROS	Reactive oxygen species
SNP-A	Single nucleotide polymorphism analysis
WHO	World Health Organization

Introduction

The myeloproliferative neoplasms (MPNs) are a distinct group of hematological disorders which exhibit terminal myeloid cell expansion in the peripheral blood. Based on the 2008 WHO Classification System, they are categorized into "classic" and "atypical" MPNs (Vardiman et al. 2009). The classic MPNs are further functionally classified based on the presence or absence of the t(9:22) chromosomal translocation in the Philadelphia (Ph) chromosome resulting in the BCR-ABL 1 fusion protein.

Although first described in 1951 by William Dameshek purely based on clinical and bone marrow similarities (Dameshek 1951), the BCR-ABL-negative MPNs (polycythemia vera [PV], essential thrombocythemia [ET], and primary myelofibrosis [PMF]) are currently in a period of rapid discovery regarding their pathogenetic mechanisms. The defining moment occurred for MPNs in 2005 with the discovery of JAK2 V617F mutation (Baxter et al. 2005; James et al. 2005; Kralovics et al. 2005; Levine et al. 2005). This gain-of-function mutation in the pseudoki-nase domain of the Janus kinase 2 gene (a key component of the cell growth and differentiation in the JAK-STAT pathway) results in the constitutive activation of the pathway. Since then, several additional genetic mutations with potential pathogenetic implications have been described; however, what leads to disease progression remains unclear.

The MPNs have a variable period of risk of vascular events and a long-term risk of transformation to an overt myelofibrotic phase, acute leukemia, or death (Fig. 12.1). Current available therapies have rarely been able to impact this natural history beyond palliating symptoms or decreasing the risk of vascular events. In this topic, we will focus on the most advanced clinical scenario for MPN patients, the biology and consequence of blastic transformation.

Phenotype of Leukemic Transformation in the MPNs

Disease progression in MPNs is variable and based on risk factors; however, eventually most patients develop overt acute leukemia or most appropriately what is called a blast phase (Mesa et al. 2007b) (see Fig. 12.1). Clinically, as patients progress, they tend to experience a decrease in the efficacy of intramedullary hematopoiesis as manifested by worsening thrombocytopenia, worsening constitutional symptoms, and the potential development of functional neutropenia (Mesa et al. 2005). Patients most commonly will reach a blast phase after first having gone through a myelofibrotic phase whether PMF or post-ET/PV MF (Mesa et al. 2005) . However, patients with PV or ET have been known to develop a blast phase without a clearly distinct prodrome of myelofibrosis developing (Finazzi et al. 2005).

Pathogenesis of Blastic Transformation in MPNs

The pathogenetic mechanisms of transformation to MPN-BP remain unclear. There are a growing number of MPN-associated mutations including JAK2, MPL, TET2, ASXL1, IDH1, IDH2, CBL, IKZF1, LNK, and EZH2.The mechanisms by which these mutations can lead to widely varying disease pheno-types or what leads to disease progression (Fig. 12.1) remains unclear. Although most of these mutations originate at a progenitor cell level, they neither represent the primary clono-genic event nor are they mutually exclusive.

Fig. 12.1 Clinical and pathogenetic changes occurring during myeloproliferative disorder progression. ET essential thrombocythemia, PV polycythemia vera, PMF primary myelofibrosis, Post-ET/PV MF post-essential thrombocythemia/polycythemia vera myelofibrosis 

JAK2 Mutations

JAK2 located on chromosome 9p24 is one of the members of the Janus family of nonreceptor protein tyrosine kinases. JAK2 is ubiquitously expressed in mammalian cells and is an integral part of signal transduction and activation of the JAK-STAT pathway (which is involved in cell growth, proliferation, and survival) (Aaronson and Horvath 2002) .

JAK2V617F

JAK2 V617F is the most common mutation in BCR-ABL1-negative MPNs (Mesa et al. 2007b) and is seen in ~95% of patients with PV, in ~50% of patients with ET, in ~65% of patients with PMF, and in ~50% of patients in blast phase MPN. This gain-of-function mutation (G to T) involving exon 14 leading to the substitution of valine with phenylalanine (V to F) at codon 617 results in the constitutive activation of the JAK-STAT pathway (Lu et al. 2005) . Although preclinical data suggested the likely possibility of the JAK2 V617F mutation in disease progression (Tiedt et al. 2008), there is evidence that neither the presence of a JAK2 V617F mutation (Mesa et al. 2006b) nor an increased allele burden is more common in those who transform to MPN-BP. In addition, the majority (Theocharides et al. 2007), but not all (Swierczek et al. 2007), isolated acute leukemia clones obtained from previously JAK2-V167F mutant patients will have reverted to a JAK-wild type state. Clinical observations from series of MPN-BP patients further support that JAK2 V617F allele burden does not increase (or likely decreases) after transformation (Tam et al. 2008). In a recent observation of 778 patients with BCR-ABL1-negative MPNs of whom 7 transformed to BP, the JAK2 V617F mutation was noted in approximately 50% of those cases. However, although all JAK2 V617F-positive patients remained positive for this mutation after leukemic transformation, the mutation itself did not appear to alter the course of the disease, suggesting that JAK2 V617F is not essential for transformation in these cases (Lopes da Silva et al. 2011).

Other recent observations further demonstrate that perhaps different pathways for leukemic transformation occur in MPNs (a MF phenotypic step in JAK2 V617F-mutated patients, a direct MPN-BP from ET/PV in JAK2 wild type patients) (Beer et al. 2010). To further complicate this picture, there have been reports of multiple mutations occurring in the same patient, reiterating the fact that these mutations are neither mutually exclusive nor confined to a predictable pattern of occurrence (Schaub et al. 2010; Kralovics 2008).

JAK2 Exon 12 Mutations

The JAK2 exon 12 mutations are usually specific to the JAK2 V617F-negative PV patients. Initially identified in 2007 (Scott et al. 2007b) till date, >10 JAK2 exon 12 mutations have been described in the literature (Pietra et al. 2008). The muta-tional frequency in blast phase MPN is unknown, and the clinical course appears to be similar to that of JAK2 V617F mutated patients.

Myeloproliferative Leukemia Virus Oncogene (MPL) Mutations

MPL, found at chromosome 1p34, encodes the thrombopoietin receptor that works in concert with thrombopoietin for platelet production. Acquired MPL mutations (e.g., W515L and W515K) are associated with severe anemia and have been detected in patients with ET or MF, but not in patients with PV (Pancrazzi et al. 2008; Pikman et al. 2006). The incidence of MPL mutations in blast phase MPN is unknown and so is its pathogenetic relevance.

TET2 Mutations

TET2 mutations found on chromosome 4q24 are thought to play a pivotal role in epigenetic regulation of transcription (Tahiliani et al. 2009). The incidence of TET2 mutations in blast phase MPN has been reported to be approximately 17%. In a recent report using single nucleotide polymorphism analysis (SNP-A), all patients with TET2 mutations had additional chromosomal lesions. In addition, no TET2 mutations were noted in chronic phase MPN (Makishima et al. 2011) . However, from a prognostic standpoint in patients with PV and PMF, the presence of a mutant TET2 did not affect leukemic transformation or survival.

Additional Sex Combs-Like 1 (ASXL1) Mutations

ASXL1 mutations are found on chromosome 20q11.1 and belong to the enhancer of trithorax and polycomb gene family (Carbuccia et al. 2009). They are believed to affect regulation of transcription and RAR-mediated signaling (Carbuccia et al. 2009). The incidence of ASXL1 mutation in blast phase MPN is noted to be between 10% and 19% (Boultwood et al. 2010; Makishima et al. 2011; Abdel-Wahab et al. 2011). Currently, it is unclear whether ASXL1 mutations are an early or secondary event, and recent data are conflicting regarding its role with clinical outcome (Abdel-Wahab et al. 2011).

Isocitrate Dehydrogenase (IDH1 and IDH2) Mutations

IDH1 and IDH2 are located on chromosomes 2q33.3 and 15q26.1, respectively (Dang et al. 2009; Gross et al. 2010). These mutations confer an enzymatic gain of function that increases 2-hydroxyglutarate, eventually leading to malignant transformation (Dang et al. 2009; Gross et al. 2010) . They were originally described in gliomas (Parsons et al. 2008), and the presence of the mutations was associated with superior survival (Weller et al. 2009). The mutational frequency of IDH in patients with blast phase MPN is approximately 21% (Pardanani et al. 2010). In a recent report using SNP-A, Makishima et al. (2011) detected canonical IDH mutations (R132 [IDH1] and R 172 [IDH2]) in blast phase CML, suggesting its role to predict a more malignant phenotype.

IKAROS Family Zinc Finger 1 (IKZF1) Mutations

IKZF1 mutations located on chromosome 7p12 encode for Ikaros transcription factors, which are key regulators of lymphoid differentiation. In a single study, IKZF1 mutational frequency in blast phase MPN has been reported as 19% (Jager et al. 2010). Given the low mutational frequency (<0.5%) that has been observed in chronic phase disease, it is certainly enticing to assume its relevant role in blast phase MPN; however, more studies will be required to establish this.

Based on our current knowledge of the patho-genesis of blast phase MPN, it is clear that we have made significant strides in identifying mutations in just the last couple of years. Unlike CML, patho-genetic mechanisms in BP-MPL are far more complex than originally estimated when the JAK2 V617F mutation was diagnosed and targeted. The transformation of CML from CP to BP is typically associated with additional karyotypic abnormalities which are independent of the BCR-ABL translocation (Vardiman et al. 2001). However, as we have previously demonstrated, patients with MPNs who eventually transform are more likely to have karyotypic abnormalities at diagnosis and develop new abnormalities prior to MPN-BP (Mesa et al. 2005, 2007b). The presence of chromosomal instability and acquisition of additional mutations are crucial for blastic transformation.

Defining Blastic Transformation from the MPNs

Historically, it has been noted that a better definition of BP-MPN (based on clinical and hematological characteristics) was required for determining therapeutic interventions (Karanas and Silver 1968) . The majority of these patients (~75%) died within 6 months, and they were noted to have >30% myeloblasts and promyelo-cytes in the peripheral blood or bone marrow. Approximately four decades later and with the advent of newer molecular technology, we still continue to strive toward developing a clinically meaningful classification.

The reason why this is so difficult is because BP in patients with a prior MPN in part arises from lack of clear diagnostic guidance as to what constitutes acute leukemia in these patients. Patients with all chronic myeloid neoplasms exist in a spectrum of disease severity from the point of their diagnosis to acute leukemia. What constitutes this latter threshold in between is an arbitrary set point in a biological continuum. The World Health Organization’s (WHO) updated classification in 2008 of myeloid neoplasms classified MPN-BP in an attempt to address some of these issues (Vardiman et al. 2009) .

WHO Definition of Acute Leukemia

The World Health Organization’s (WHO) updated classification in 2008 of myeloid neoplasms classified MPN-BP as acute myeloid leukemia with multilineage dysplasia (Vardiman et al. 2009) . This subgroup was further divided into those who had a prior case of MDS or an MPN/ MDS overlap disorder (Vardiman et al. 2009). This definition is most pertinent to those with an MPN/MDS overlap disorder (chronic myel-omonocytic leukemia [CMML], atypical chronic myeloid leukemia [aCML], or MPN/MDS unclassifiable) yet does not really address those with prior PMF, post-ET/PV MF, or prior ET/PV.

The threshold for a diagnosis of achieving BP was either 20% blood or marrow blasts or the presence of an acute leukemia defining karyotypic lesions despite blast percentage (t[8;21][q22:q22], inv[16][p13q22], t[16;16] [p13;q22], or t[15;17][q22;q12]) (Vardiman et al. 2009) . However, it does not address the issue that although karyotypic abnormalities are quite common among patients with MPN-BP (Mesa et al. 2005), it is less clear whether these defining mutations play any role in a transformed MPN (Mesa et al. 2005) as opposed to de novo AML.