A Gain of Function Mutation in JAK2 Is Frequently Found in Patients with AML-M2 and Normal Karyotype.

AML is the consequence of collaboration between at least two classes of mutations: class I, exemplified by constitutively activated tyrosine kinases or their downstream effectors, which confer a proliferative and/or survival advantage to hematopoietic cells; and class II mutations, that result in loss of function of transcription factors that are important for normal hematopoietic differentiation. The activating V617F mutation of JAK2 tyrosine kinase has been recently described as a common event in MPD. This mutation seems to disrupt the JH2 inhibitory regulation of JAK2, leaving the enzyme constitutively active. The same mutation was also found in a small number of patients with either atypical MPD or MDS. However, its incidence in AML is unclear. We investigated the incidence of the tyrosine kinase JAK2 mutation V617F in patients with AML, and its association with other factors with a prognosis meaning in this disease. We focused in cases with normal karyotype, which account for more than 40% of all AML, and that represent a therapeutic challenge because of lack of established therapy protocols. Material and methods. We analyzed 7 cell lines and 218 samples of AML patients at diagnosis: 38 included in the favorable prognosis group, 143 in the intermediate (137 with normal karyotype), and 45 in the unfavorable. DNA was extracted by standard procedures from bone marrow. V617F genotyping was performed by amplification refractory mutation system (ARMS) as previously described (Jones el al., 2005). Mutations of FLT3 (ITD and D835) were also analyzed. Results. We found the mutation V617F in 4.5% of overall AML cases (10/225), and in 4.4% of AML patients with normal karyotype (6/137). A high prevalence was observed in patients with the M2 subtype (7/61, 11.5%): 5 cases had a normal karyotype, one t(8;21), and another inv(16). Taking into account cases with AML-M2 and normal karyotype, the incidence of the V617F mutation was 19% (5/26). The mutation was heterozygous in most cases; we only detected homozygous mutation in 2 cell lines with AML-M6 and M7, both presenting a complex karyotype and 3q rearrangements. One of the cell lines had an addition on 9p24, supporting previous data that imply that 9pLOH, caused by mitotic recombination, could be responsible for the transition from heterozygous to homozygous V617F. Interestingly, one of the cases with M2 had both FLT3 and JAK2 mutations. FLT3 mutations were no detected in the other cases. Genetic evidence in vivo and studies on proliferation and phosphorylation in vitro indicate that the V617F mutation confers a proliferative and survival advantage. Patients carrying the V617F mutation had significantly longer disease duration and more complications in MPD, suggesting a possible prognosis meaning in AML patients. In conclusion, we have identified an acquired JAK2 mutation, which induces a constitutive JAK2 signaling, in 19% of patients with AML-M2 and a normal karyotype. Our data suggest that this activating JAK2 mutation may play an important role in the leukemogenesis mechanism of AML, and could contribute to define a subgroup within patients with AML and normal karyotype. Nevertheless, the presence of the V617F mutation opens the possibility for new targeted therapies in AML. Further studies of the clinical significance of the JAK2 mutation in AML-M2 will be of interest.