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The following case study focuses on a 59-year-old man who was well until approximately six months ago when he developed increasing abdominal discomfort, early satiety, and weight loss of 15 pounds due to poor appetite. Test your knowledge by reading the background information below and making the proper selection.

The patient’s white blood cell count was 1.8 x 10³ cells/mm³ with an absolute neutrophil count of 1.3 x 10³ cells/mm³, hemoglobin 9.2 gm/dl, and a platelet count of 77,000 cells/mm³. A bone marrow biopsy resulted in a "dry" aspirate, but the core biopsy was markedly fibrotic (MF-3 according to the European consensus grading system).¹ An estimate of the blast count based upon touch preps as well as a CD34⁺/CD117⁺ immunostain of the core biopsy demonstrated only 1.2 percent blasts. Qualitative molecular testing demonstrated the JAK2-V617F mutation.

Which of the following statements is most accurate?

1. The presence of the JAK2-V617F mutation establishes the diagnosis of a myeloproliferative disorder.
2. The absence of the JAK2-V617F mutation rules out the diagnosis of primary polycythemia.
3. Quantitative mutant allele frequency testing indicates that a higher proportion of the JAK2-V617F alleles are identified in patients with primary polycythemia vera (PV) compared with patients with essential thrombocythemia (ET).
4. Treatment of patients with myelofibrosis with JAK2 inhibitors can improve symptoms, reduce spleen size, and eliminate the presence of the JAK2-V617F mutation.

Answer

3. Quantitative mutant allele frequency testing indicates that a higher proportion of the JAK2-V617F alleles are identified in patients with primary polycythemia vera (PV) compared with patients with essential thrombocythemia (ET).

Explanation

The discovery of the Philadelphia chromosome resulting from the translocation of the Abl gene on chromosome 9 to chromosome 22 adjacent to the Bcr gene and the subsequent development of molecularly targeted therapies with tyrosine kinase inhibitors (TKI) have revolutionized the treatment of chronic myeloid leukemia (CML). Subsequently, characterization of the somatic mutation in the JAK2 gene, which results in a valine to phenylalanine substitution at position 617 (JAK2-V617F) in a significant number of patients with Bcr-Abl negative myeloproliferative diseases (MPDs) has initiated a search for effective targeted therapies for these conditions as well.^2-4

Unlike the Bcr-Abl transgene’s association with CML, the absence of the JAK2-V617F mutation does not rule out the presence of primary PV, ET, or primary myelofibrosis (PMF). Moreover, its presence does not establish the diagnosis of any of the MPDs. This somatic alteration can also be found infrequently in atypical CML, chronic myelomonocytic leukemia (CMML), chronic neutrophilic leukemia (CNL), hypereosinophilic syndrome (HES), and myelodysplastic syndrome (MDS).^5,6

Gene dosage plays a role in a number of human diseases including the amplification of the her-2/neu kinase in breast cancer and c-met amplification in colon and lung cancers. However, the phenomenon that the degree of expression of a single gene alteration can modulate the phenotype of a disease is relatively unique to JAK2. Recently, investigators demonstrated that the ratio of mutant to normal JAK2 alleles was higher in patients with PV and PMF compared with patients with ET.⁷ This led to the possibility that the allele frequency, and thus gene dosage, may alter the phenotype of the disease. Through the utilization of a Cre-recombinase system that allowed for varied expression of the human V617F mutant JAK2 allele in mice, researchers demonstrated that mice expressing higher levels of the mutant JAK2 displayed a phenotype more consistent with PV, while mice harboring lower levels of expression of the mutant JAK2 allele had an ET phenotype.

Because of the purported role that the Janus kinase plays in the development of the JAK2⁺ MPDs, researchers have begun to develop and test a number of JAK2 inhibitors in these disorders. CEP701 is a small molecule Flt-3 inhibitor that has completed phase I/II testing in AML, but it is also capable of binding the ATP-binding site on JAK2. In vitro, CEP701 leads to reduced proliferative capacity of erythroid progenitor cells from patients with MPDs, but the suppression is independent of the JAK2 phenotype.⁸ Verstovsek and colleagues recently presented their phase I/II experience with INCB18242 in patients with PMF or post-PV/ET myelofibrosis at the ASCO annual meeting.⁹ Patients treated with INCB18242 had a marked reduction in splenomegaly as well as a modest, but statistically significant, reduction in the JAK2-V617F allele burden. Interestingly, a reduction in the proinflammatory cytokine IL-6 was also noted, perhaps contributing to the reduction in constitutional symptoms. Recently, researchers began phase I testing of TG101348, a relatively selective JAK2 inhibitor, in patients with PMF, but the data on its clinical activity are too immature to report.

The field of MPDs was permanently altered with the development of the molecularly targeted TKIs for CML. With the recent advent of molecular testing and the identification of the JAK2-V617F mutation, we are hopefully beginning to unravel the mysteries of some of the remaining MPDs. As with CML, molecularly targeted treatments are emerging for these conditions, but they are still very early in their clinical development.

References

1. Thiele J, Kvasnicka HM, Facchetti F, et al. European consensus on grading bone marrow fibrosis and assessment of cellularity. Haematologica. 2005;90:1128-32.
2. James C, Ugo V, Le Couédic JP, et al. A unique clonal JAK2 mutation leading to constitutive signalling causes polycythaemia vera. Nature. 2005;434:1144-8.
3. Levine RL, Wadleigh M, Cools J, et al. Activating mutation in the tyrosine kinase JAK2 in polycythemia vera, essential thrombocythemia, and myeloid metaplasia with myelofibrosis. Cancer Cell. 2005;7:387-97.
4. Baxter EJ, Scott LM, Campbell PJ, et al. Acquired mutation of the tyrosine kinase JAK2 in human myeloproliferative disorders. Lancet. 2005;365:1054-61.
5. Ingram W, Lea NC, Cervera J, et al. The JAK2 V617F mutation identifies a subgroup of MDS patients with isolated deletion 5q and a proliferative bone marrow. Leukemia. 2006;20:1319-21.
6. Steensma DP, Dewald GW, Lasho TL, et al. . Blood. 2005;106:1207-9.
7. Tiedt R, Hao-Shen H, Sobas MA, et al. . Blood. 2008;111:3931-40..
8. Hexner EO, Serdikoff C, Jan M, et al. . Blood. 2008;111:5663-71.
9. Verstovsek S, Kantarjian HM, Pardanani A, et al. A phase I/II study of INCB018424, an oral, selective JAK inhibitor, in patients with primary myelofibrosis (PMF) and post polycythemia vera/essential thrombocythemia myelofibrosis (Post-PV/ET MF). J Clin Oncol. 2008;26:abstract 7004.

Case study submitted by Dale Bixby, MD, PhD, of the University of Michigan.