Elevated Leukocyte Alkaline Phosphatase Scores Induced by Jak2 V617F Mutation

Leukocyte alkaline phosphatase (LAP) enzymatic activity is a marker of the last stages of myeloid differentiation. The level of LAP is quantitated as the LAP score. Estimation of the LAP score has been useful for distinguishing chronic myelogenous leukemia (CML) from BCR-ABL–negative chronic myeloproliferative disorders (MPDs) and neutrophilic reactions in severe infections. CML patients usually have a low LAP score, whereas elevated LAP scores are seen in patients with polycythemia vera (PV), primary myelofibrosis (PMF), and leukocytosis caused by infections. An acquired Jak2 V617F mutation is seen in approximately 95% of patients with PV and in about 50% of patients with essential thrombocythemia or PMF. It has been shown that Jak2 V617F mutation induced constitutive activation of the JAK-STAT signaling pathway. We speculated that an elevated LAP score might be caused due to activation of JAK-STAT signaling through a Jak2 V617F mutation, and conducted this study to address this question. We analyzed the LAP scores in Jak2 V617F-positive and -negative MPD patients. Jak2 V617F-positive MPD patients had higher LAP scores than Jak2 V617F-negative patients. Moreover, patients carrying homozygous mutations had higher LAP scores than patients with heterozygous mutations. AG490, the Jak2 inhibitor, was shown to significantly decrease the LAP expression in neutrophils of Jak2 V617F-positive patients. We lentivirally transfected the acute promyelocytic leukemia cell line NB4 with the Jak2 V617F mutation and wild-type Jak2 V617F. The expression level of Jak2 was not significantly different between the Jak2 V617F mutation and wild-type Jak2 V617F. We then examined the LAP scores of transfected NB4 cells after these cells were differentiated by all-transretinoic acid and granulocyte colony stimulating factor. It was observed that the Jak2 V617F mutation and not the wild-type Jak2 induced elevated LAP scores. Furthermore, we showed that Jak2 followed the MAP kinase pathway and not the PI3 kinase pathway, as a downstream signaling pathway to elevate the LAP scores using MEK 1/2 (U0126) and PI3 kinase (LY294002) inhibitors. In conclusion, we obtained direct evidence that Jak2 V617F mutation induces elevated LAP scores via the MAP kinase pathway.