The V617F mutation in Jak2 is not found in childhood acute lymphoblastic leukaemia (2005)

Members of the Jak kinase family, including Tyk2, Jak1, Jak2 and Jak3, play an important role in mediating cellular signals between cytokine/hormone receptors and downstream effector proteins by virtue of their tyrosine phosphorylation activities. They share structural homology and have two kinase domains, JH1 and JH2. While only the JH1 domain has kinase functionality, JH2 plays an important negative-regulatory role on JH1 activity. Recently a somatic, gain of function, mutation has been identified in JAK2 in a range of Philadelphia chromosome-negative myeloproliferative diseases including polycythaemia vera, essential thrombocythaemia and chronic idiopathic myelofibrosis (Baxter et al, 2005; Kralovics et al, 2005; Levine et al, 2005). A point mutation in exon 12 of the JAK2 gene results in substitution of valine for phenylalanine at amino acid position 617 in the JH2 domain, leading to constitutive tyrosine phosphorylation activity and cytokine hypersensitivity. Neoplastic cells can be heterozygous for the mutation or hemizygous if they are associated with loss of heterozygosity (LOH) of 9p, where JAK2 is situated. The explanation for V617F yielding different chronic myeloproliferative disease phenotypes is not clear but may relate to the target cell for transformation or the presence of other co-operating genetic events. Jak2 activation is also implicated in the pathology of childhood acute lymphoblastic leukaemia (ALL). Two studies showed constitutive activation of Jak2 in both primary samples and leukaemic cell lines, with inhibition blocking leukaemic cell growth selectively, in vitro and in vivo, by inducing programmed cell death (Meydan et al, 1996). In addition, LOH of 9p is a relatively common event, with JAK2 invariably being present in the minimally deleted region. One known mechanism of JAK2 activation in ALL is chromosomal translocation with the t(9;12)(p24;p13) yielding a chimaeric Tel–Jak2 fusion protein that has deregulated Jak2 kinase activity, confers cytokine-independent proliferation and results in the formation of ALL in a mouse model system (Lacronique et al, 1997; Peeters et al, 1997; Carron et al, 2000). However, this is a rare occurrence. Taken together, these observations provide a rationale for investigating the presence of V617F in childhood ALL. To detect the V617 mutation, we performed allele-specific polymerase chain reaction using genomic DNA isolated from bone marrow samples from children with ALL treated at our institution (Baxter et al, 2005). Eighty-six samples were collected during diagnosis and 42 samples during relapse. All samples were collected with appropriate consent. Amplicons were size fractioned by standard agarose gel electrophoresis. DNA from HEL92.1.7, an erythroleukaemic cell line, served as a positive control. None of the 128 ALL samples examined were positive for the 203-bp product, indicative of V617F mutation, but all were positive for the internal control (representative gel shown in Fig 1). Using HEL92.1.7 cells spiked into normal peripheral blood, the sensitivity of the assay was determined to be between 1% and 5%. Allele-specific polymerase chain reaction (PCR) for the V617F mutation. The 364-bp band serves as an internal control. The 203-bp band indicates the presence of the mutation. Lanes 1–7, acute lymphoblastic leukaemia samples; lane 8, HEL92.1.7 positive control; lane 9, PCR negative control with no DNA template; M, marker lanes. Thus the V617F mutation is absent in childhood ALL and this finding concords with a mutational screening study of the JH2 domain in a small cohort of ALL patients (Cools et al, 1999). Given the importance of the Jak2 activation in primary ALL, a comprehensive mutational screen of all of its coding exons is warranted.