Infrequent V 617 F mutation of the JAK 2 gene in myeloid leukemia and its absence in lymphoid malignancies in Japan

A unique mutation of the JAK2 gene, V617F, has recently been identified in polycythemia vera, essential thrombocythemia and myeloid metaplasia with myelofibrosis. To determine the relevance of this mutation in other types of hematological neoplasms in Japan, we performed allele-specific polymerase chain reaction analysis on the JAK2 gene. The V617F mutation was detected in one out of 130 myeloid neoplasms, but in none of 114 lymphoid malignancies and four biphenotypic acute leukemias. Although a favorable chromosomal alteration t(8;21)(q22;q22) was observed in one acute myeloid leukemia (AML) patient with the mutation, two courses of chemotherapy resulted in induction failure and short survival. Sequencing of JAK2 cDNA revealed expression of the mutant allele in the patient. The V617F mutation might play a role in the pathogenesis of certain AML cases.

Myeloproliferative disorders (MPD) comprise a heterogeneous group of diseases including chronic myelocytic leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET), and myeloid metaplasia with myelofibrosis (MMM).With the exception of CML, characterized by the Philadelphia chromosome and the BCR-ABL fusion gene, genetic events causing these disorders have remained unidentified for a long time.The Janus kinase 2 (JAK2) gene encodes a tyrosine kinase involved in cytokine signal transduction.JAK phosphorylates cytoplasmic targets including signal transducers and activators of transcription (STAT).In hematological neoplasms, several chromosomal translocations involving the JAK2 gene locus have been identified (Khwaja, 2006).Recently, a unique mutation of the JAK2 gene, G to T transversion at nucleotide 1849 resulting in valine to phenylalanine substitution at amino acid position 617 (V617F), has been identified in MPD (Baxter et al., 2005;James et al., 2005;Kralovics et al., 2005;Levine et al., 2005a).This mutation was present in most patients with PV and in half of the patients with ET and MMM.Since the V617F mutation is located in the JH2 negative regulatory domain of the JAK2 gene, it disrupts the auto-inhibitory activity of JAK2.In consequence, the mutation leads to constitutive tyrosine phosphorylation activity, promoting cytokine hypersensi-tivity and inducing erythrocytosis in a mouse model (James et al., 2005).
Recent studies have shown the V617F mutation to be present in some cases of acute myeloid leukemia (AML), myelodysplastic syndrome (MDS) and chronic myelomonocytic leukemia (CMMoL) (Jelinek et al., 2005;Levine et al., 2005b).However, limited information is available to determine whether the mutation is specifically associated with myeloid neoplasms, and most of the studies are from Europe and North America.In order to assess the range and frequency of the mutation in Japanese patients with hematological neoplasms, we performed allelespecific polymerase chain reaction (PCR) analysis on the JAK2 gene.
ALLF-S is specific for the mutant allele and contains an intentional mismatch at the third nucleotide from the 3' end, to improve specificity.After 5 min at 94 °C, 36 amplification cycles of 60 s at 94 °C, 60 s at 58 °C and 60 s at 72 °C were performed, with a subsequent 7 min extension at 72 °C.Electrophoresis was repeated two or three times in each sample, using independent PCR products.Primers ALLF-S and ALLR-S amplify 203-bp (base pair) products, while the size of products using primers ALLF-IC and ALLR-S is 364 bp.The 203-bp products indicated the V617F mutation, while the 364-bp products indicated the internal control.PV samples with the V617F mutation were used as positive control.Aberrant bands were detected in one of the 248 samples (AML02, Figure 1 lane 5).
To confirm the aberrant bands detected by allelespecific PCR, DNA from AML02 was subsequently sequenced in both directions on a MegaBase sequence system (Amersham, Buckingham, UK).PCR products were purified and ligated into pGEM-T vector (Promega, Madison, WI, USA).Sequencing revealed G to T transversion at nucleotide 1849, resulting in the V617F mutation (Figure 2).Several samples without aberrant bands by allele-specific PCR were also sequenced, but only wild-type sequences were obtained.
The V617F mutation of the JAK2 gene was found in one of the 130 myeloid neoplasms, but not in the 114 lymphoid malignancies or in the four biphenotypic acute leukemias.The mutation was detected in the majority of Japanese patients with PV and ET (K Yoshinaga, N Mori, Y Wang, M Shiseki, T Motoji, unpublished data).In contrast to classic MPD, the V617F mutation was infrequent in myeloid leukemia and absent in lymphoid malignancies.Recent reports show that the JAK2 V617F mutation was not identified in either acute lymphoblastic leukemia (ALL) or chronic lymphocytic leukemia (CLL) (Levine et al., 2005b).Our study showed that also in Japan the V617F mutation is absent in ALL and CLL.Furthermore, it was also undetectable in adult T-cell leukemia (ATL), macroglobulinemia, multiple myeloma (MM), and malignant lymphoma (ML).
AML02 was obtained from a patient with AML showing M2 morphology according to the French-American-British (FAB) classification at diagnosis.This patient was a 70-year-old woman with no history of preceding MPD or MDS.Peripheral blood tests revealed 50.1 x 10 9 /L leukocytes with 76.5% blast cells, 7.8 g/dL hemoglobin, and 55 x10 9 /L platelets.Although the favorable chromosomal alteration t(8;21)(q22;q22) accompanied by AML1/ETO transcripts was the only chromosomal abnormality observed in the patient, two courses of chemotherapy resulted in induction failure and short survival.The mutation was found in nine out of 14 AML02 clones.Since this sample was expected to contain more than 90% blast cells after mononuclear cell isolation, this case may have been heterozygous for the V617F mutation.Another possibility is that a substantial proportion of the leukemic cells from AML02 did not harbor the mutation.
Our study revealed that the V617F mutation is infrequent in myeloid leukemia and absent in lymphoid malignancies in Japan.Nevertheless, it was detected in AML02, along with the favorable cytogenetic alteration t(8;21)(q22;q22).Previous studies have demonstrated that additional event(s) are required for the development of AML in the presence of t(8;21)(q22;q22) (Yuan et al., 2001;Kuchenbauer et al., 2006).Tyrosine kinases have been involved in several disorders including BCR-ABL in CML, PDGFRβ in CMMoL, and PDGFRα in chronic eosinophilic leukemia.Furthermore, activating mutations in the FLT3 receptor tyrosine kinase gene are the most common genetic events in AML.Patients with FLT3 mutations have a poor prognosis, suggesting the important role of tyrosine kinase gene mutations in leukemogenesis.The relation between the V617F mutation and short survival in AML02 is unclear.The V617F mutation might confer a proliferative advantage to blast cells and play a role in the pathogenesis of certain AML cases.As suggested for FLT3 mutations, constitutive signaling in the absence of ligand may result in reduced apoptosis of the leukemic cells or may grant increased repair capacity following cell damage; either of these mechanisms could be considered as inducing chemoresistance (Kottaridis et al., 2001).Further studies will clarify the incidence and significance of the V617F mutation in AML with t(8;21)(q22;q22).

Figure 2 -
Figure 2 -Sequence analysis of the JAK2 gene in AML.Sequencing identified G to T transversion at nucleotide 1849, resulting in the V617F mutation in the AML sample (A, AML02).Wild-type sequences are shown as control (B, MM01).