JAK2 V617F mutation is a rare event in juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a unique clonal myelodysplastic/myeloproliferative disorder of infancy and early childhood, characterized by an aggressive clinical course and an extremely poor prognosis for patients who cannot receive an allogeneic hematopoietic stem cell transplantation.1, 2 The disease, usually presenting with leukocytosis and monocytosis, anemia, thrombocytopenia, hepatomegaly and marked splenomegaly, is characterized by an uncontrolled proliferation of monocytic lineage cells. Spontaneous growth of monocyte–macrophage colonies in semisolid media cultures in the absence of added growth factors, as well as a striking hypersensitivity of JMML myeloid progenitors to granulocyte–macrophage colony-stimulating factor (GM-CSF), is a distinctive feature of the disease.1

A pathologic activation of the RAS–RAF–MAP (mitogen-activated protein) kinase signal-transduction pathway from GM-CSF receptor to the nucleus is the key point of the pathophysiology of this disease.3, 4 In about 70% of JMML cases, this activation is due to mutations in RAS (25% of cases), NF1 (clinical diagnosis in about 11% of cases) or PTPN11 (35% of cases).3, 4, 5 All these genetic aberrations have been demonstrated to be able to produce, in experimental models, the development of progressive myeloproliferative disorders.4, 6 However, even though one of these gene mutations can be observed in more than two-thirds of JMML patients, in the remaining affected children a specific genetic alteration has not been identified so far.

Janus kinase 2 (JAK2) is one of the four tyrosine kinases involved in the transduction of cellular growth stimuli. After activation of growth factor or cytokine receptors, JAK proteins are recruited and activated through trans-phosphorylation. Following activation, JAK proteins phosphorylate specific tyrosine residues on the receptor, which then bind to signal transducer and activator of transcription (STAT) proteins, which further activate downstream signaling events. A somatic activating mutation, 1849G>T, causing phenylalanine to be substituted for valine at position 617 of JAK2 (V617F) was recently described in patients with myeloproliferative syndromes, such as polycythemia vera (PV) and essential thrombocythemia.7 More recently, the V617F mutation was observed also in other acute and chronic myeloid malignancies (acute myeloid leukemia and chronic myelomonocytic leukemia (CMML)).8

We investigated a possible role of V617F mutation of JAK2 in the development of JMML, especially in the subgroup of patients with unknown genetic defects.

Sixty-two children with a diagnosis of JMML were enrolled in the present study. Thirty-nine were boys and 23 girls. The median age at diagnosis was 1.3 years, with a range from 0.2–9 years. Details on the clinical characteristics at disease presentation are reported in Table 1. Three additional patients with a diagnosis of CMML were evaluated and included in the present study. CMML was secondary after chemotherapy for neuroblastoma or acute promyelocytic leukemia in a 3- and a 14-year-old patient, and primary in an 18-year-old boy. All patients included in the study had been reported to the registry of the European working group on myelodysplastic syndrome in childhood (EWOG-MDS).

Table 1 Patient characteristics

Tissue samples (either bone marrow or peripheral blood) from patients with JMML and CMML were collected under Institutional Review Board-approved protocols at each Institution and after having obtained a written informed consent from the parents. DNA was extracted and analyzed for mutations in one of two laboratories located in Freiburg, Germany and Pavia, Italy.

All patients affected by JMML were studied for PTPN11, N-RAS or K-RAS mutations. Furthermore, all children were carefully evaluated for a possible clinical diagnosis of NF1.

As far as JAK2 analysis is concerned, in 26 children with JMML and in the three patients with CMML, genomic DNA was extracted from patients’ cells (either bone marrow or peripheral blood) and the human JAK2 exon 12 was amplified by polymerase chain reaction; the resulting 460 bp amplified fragment was digested with BsaXI. The mutant allele remained undigested, whereas the wild-type allele was digested into 241, 189 and 30 bp fragments. The mutant fragment was purified and its DNA sequence was determined. In the remaining 36 children with JMML, the search for JAK2 V617F mutation was performed by denaturating high-performance liquid chromatography.9

Among the 62 children with JMML, it was possible to demonstrate a mutation of PTPN11 in 16 patients (26% of the study population), whereas 15 children (24%) presented a mutation of N-RAS or K-RAS. Finally, in one child, it was possible to detect both a PTPN11 and a RAS mutation. In eight children (13%), a clinical diagnosis of NF1 was formulated before or at the time of JMML diagnosis. In the remaining 22 children (35% of patients affected by JMML), mutational analysis was normal for both PTPN11 and RAS and clinical examination was negative for a diagnosis of NF1.

In all JMML patients, the analysis of JAK2 gene showed the wild-type genotype. Likewise, in bone marrow samples from the three patients affected by CMML, it was possible to demonstrate the wild-type JAK2 genotype in all cases.

Activating mutations in genes encoding for signaling molecules are common events in human malignancies. In acute leukemias, these mutations may act as cooperating events, conferring a proliferative advantage to leukemic hematopoietic progenitor cells, as compared to normal hematopoietic progenitors. Some of these genetic lesions, such as PTPN11 or RAS mutations, as well as the BCR/ABL fusion gene, can be observed in both acute myeloid and lymphoblastic leukemia, as well as in myeloproliferative/myelodysplastic disorders.

As far as myeloproliferative diseases are concerned, a recurrent clonal mutation, V617F, in a highly conserved residue of the pseudokinase domain of the JAK2 tyrosine kinase, was recently reported in most patients with PV, as well as in one-half of the patients with ET or myelofibrosis with myeloid metaplasia.7, 10

JAK2 V617F mutation results in the constitutive activation of tyrosine kinase, in the subsequent phosphorylation of SAT5 and, ultimately, in growth factor-independent growth of hematopoietic cells. Moreover, the fact that the mutation has never been observed either in a large series of healthy individuals or in buccal cells from patients with PV carrying the mutation in hematopoietic cells strongly supports its pathophysiologic relevance for the development of myeloproliferative disorders.10

Experimental data show that JAK2 is physically associated with the GM-CSF receptor β-chain, becoming activated upon exposure of myeloid cells to GM-CSF.11, 12 For this reason, we hypothesized that a mutation in this gene could be involved in the development of JMML, at least in the subgroup of patients with unknown genetic defects.

To our knowledge, only two studies have evaluated so far the incidence of V617F mutation in children with JMML, even though in very small cohorts, demonstrating the presence of the mutation in one of the patients studied.13, 14 The child with V617F mutation presented also a 45, XY, −7 karyotype and did not show the PTPN11 mutation. Our study, performed on a significantly larger cohort of patients affected by this rare disease, did not confirm the preliminary observation by Tono et al.13 In fact, none of the 62 children affected by JMML and the three patients with CMML whom we analyzed presented the V617F mutation. On the contrary, our data are in agreement with recently published reports suggesting that the JAK2 V617F mutation is uncommon in myeloid malignancies other than the classical BCR/ABL-negative myeloproliferative disorders.11 On this basis, the V617F mutation observed in one child with JMML by Tono et al.13 could be considered as a second event that occurred in an already transformed malignant cell, rather than a mutation sufficient for leukemogenesis by itself, whereas other genetic aberrations, such as mutations in RAS, PTPN11 and the loss of heterozygosity in NF1, are the transforming events in JMML cells.

In conclusion, our study demonstrates that the JAK2 V617F mutation is an extremely rare event in patients with JMML and, reasonably, it is not involved in the development of this disease. Further studies are warranted in order to identify other genetic events responsible for development of JMML, especially in cases in which a genetic aberration has not been identified so far.

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Acknowledgements

This work was supported in part by grants from Ministero della Salute–IRCCS Policlinico San Matteo (Ricerca Corrente grants 80458 and 80125) to MZ and FL and by a private Italian grant in memory of Sofia Luce Rebuffat, to FL

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Zecca, M., Bergamaschi, G., Kratz, C. et al. JAK2 V617F mutation is a rare event in juvenile myelomonocytic leukemia. Leukemia 21, 367–369 (2007). https://doi.org/10.1038/sj.leu.2404484

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