Clinical features of Japanese polycythemia vera and essential thrombocythemia patients harboring CALR, JAK2V617F, JAK2Ex12del, and MPLW515L/K mutations

Highlights

• We assessed the clinical features and genetic mutations of Japanese MPN patients.

• The clinical features of JAK2V617F and CALR mutation ET differ.

• We did not find a correlation between CALR mutations and the 46/1 haplotype.

• CALR mutation ET had a lower incidence of thrombosis relative to JAK2V617F ET.

Abstract

The risk of complication of polycythemia vera (PV) and essential thrombocythemia (ET) by thrombosis in Japanese patients is clearly lower than in western populations, suggesting that genetic background such as race may influence the clinical features. This study aimed to clarify the relationship between genetic mutations and haplotypes and clinical features in Japanese patients with PV and ET.

Clinical features were assessed prospectively among 74 PV and 303 ET patients. There were no clinical differences, including JAK2V617F allele burden, between PV patients harboring the various genetic mutations. However, CALR mutation-positive ET patients had a significantly lower WBC count, Hb value, Ht value, and neutrophil alkaline phosphatase score (NAP), and significantly more platelets, relative to JAK2V617F-positive ET patients and ET patients with no mutations. Compared to normal controls, the frequency of the JAK246/1 haplotype was significantly higher among patients with JAK2V617F, JAK2Ex12del, or MPL mutations, whereas no significant difference was found among CALR mutation-positive patients. CALR mutation-positive patients had a lower incidence of thrombosis relative to JAK2V617F-positive patients.

Our findings suggest that JAK2V617F-positive ET patients and CALR mutation-positive patients have different mechanisms of occurrence and clinical features of ET, suggesting the potential need for therapy stratification in the future.

Introduction

Myeloproliferative neoplasms (MPNs) are characterized by the monoclonal proliferation of one or more lines of myeloid cells due to hematopoietic stem cell abnormalities [1]. MPNs involve hyperplasia of differentiated blood cells and an increased risk of thrombosis, and progression to leukemia [2].

Mutations in the JAK2 gene are thought to underlie the development of polycythemia vera (PV) [3], essential thrombocythemia (ET), and primary myelofibrosis (PMF) [4], [5], [6]. Indeed, ≥95% of PV patients, and half or more of ET and PMF patients, harbor the JAK2V617F mutation [7]. This mutation induces ligand-independent constitutive activation of JAK2-STAT5 signaling by erythropoietin, thrombopoietin, and granulocyte-colony stimulating factor, thereby inducing cell proliferation. The pathology of MPNs can be reproduced in transgenic mice and knock-in mice with the JAK2V617F mutation, suggesting that the mutation is the primary cause of MPNs [8], [9], [10]

Other than the JAK2V617F mutation, the JAK2 exon 12 deletion (JAK2Ex12del) mutation in PV patients, and the W515L/K (MPLW515L/K) mutation in the c-MPL gene, which encodes the thrombopoietin receptor, in ET and IMF patients, have been discovered and shown to be involved in the pathogenesis of these diseases [11], [12], [13], [14] These newly discovered mutations are included as components of the diagnostic criteria for MPNs [15], and detection of genetic mutations is now required for diagnosing these neoplasms.

In addition to genetic mutations, JAK2V617F-positive PV, ET, and PMF patients are significantly more likely to have the JAK2 46/1 haplotype compared to normal controls [16], [17]. Thus, the 46/1 haplotype-containing allele has a high rate of harboring the JAK2V617F mutation. In addition to those with the JAK2V617F mutation, those with the JAK2Ex12del mutation and those with the MPLW515L/K mutation, which is encoded on a different chromosome, are very likely to have the 46/1 haplotype [16], [17]. These findings suggest that the haplotype of the JAK2 gene is closely related to the risk of developing MPNs [18], [19], [20], [21], [22].

A number of studies have analyzed the relationship between the clinical features of MPNs and genetic mutations. These studies suggest that the JAK2V617F mutation is strongly associated with PV pathology. According to these reports, JAK2V617F-positive ET patients have a high incidence of progressing to PV, and those with PV have a higher rate of harboring a homozygous JAK2V617F mutation compared to ET patients [15]. Moreover, increases in the allele frequency of the JAK2V617F mutation correlates with higher Hb and Ht values [23], [24], JAK2Ex12del -positive PV patients tend to have higher Hb and Ht values than JAK2V617F-positive PV patients, and MPLW515L/K-positive ET patients have a higher platelet count relative to ET patients with no mutations and JAK2V617F -positive ET patients [25].

While some reports suggest that JAK2V617F-postive MPN patients have a high risk of developing thrombosis [26], there are also studies suggesting otherwise. Thus, there appears to be no consensus on the risk of developing thrombosis based on the presence or absence of the JAK2V617F mutation [27], [28]. Moreover, the relationship between these genetic mutations and progression to myelofibrosis and leukemia remains unclear. In our previous retrospective study targeting Japanese PV and ET patients, the risk of complication by thrombosis was found to be lower compared to that in western PV and ET patients, suggesting that genetic background such as race may influence the clinical features of MPNs [2], [29], [30]. To date, no large scale study has been conducted to address the relationship between genetic mutations and clinical features in Japanese MPN patients. With respect to haplotypes, results may largely vary due to race. Thus, the clinical features of Japanese MPN patients are likely to differ somewhat relative to their western counterparts, underscoring the need to conduct studies that target this population.

In 2013, next generation sequencing identified new genetic mutations in the Calreticulin (CALR) gene in ET and PMF patients who do not have JAK2mutations [31], [32]. Against this backdrop, the present study aimed to clarify the clinical features of Japanese MPN patients by haplotype and genetic mutations.