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-trans retinoic 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.

Leukocyte Alkaline Phosphatase Score as a Marker of Severity and Progression of Myelodysplastic Syndrome.

Leukocyte Alkaline Phosphatase (LAP) Score is valuable in the work-up of certain hematological diseases. Most notably, the score is decreased in Chronic Myelogenous Leukemia and Paraoxysmal Nocturnal Hemaglobinurea but increased in leukemoid reaction to infection and Polycythemia Vera. Last year we reported the LAP scores of 14 patients with Myelodysplastic Syndrome (MDS). Our results showed that patients with less than 5% bone marrow blasts had significantly higher LAP scores than patients with 5–19% bone marrow blasts. We raised the possibility that LAP scores decrease as MDS progresses (Blood, Nov 2006; 108: 4865). In the present study we attempt to further evaluate the utility of LAP in MDS. In addition to our original cohort, bone marrow aspirate results and LAP scores were reviewed from 14 more patients with MDS, for a total of 28 patients. We again assessed the relationship of LAP to bone marrow blast percentage. Furthermore, we recorded a second LAP score, taken at a later date, from 16 of the 28 patients. For those patients with two LAP scores we compared the trend of LAP score to the interval activity of MDS, using transfusion requirement, complete blood cell count (CBC) and clinical assessment as markers of disease activity. In our analysis of LAP score relative to bone marrow blast percentage we again found a significant difference between patients with less than 5% blasts (n=8) and those with 5% to19% blasts (n=20). Patients with less than 5% blasts had significantly higher LAP scores (90.25 ± 18.27) than those with 5 to19% blasts (44.35 ± 52.09) (p<0.0048) (see charts 1 and 2). In our analysis of LAP score in relation to disease progression we found that among patients for whom LAP score decreased, 42.9% (3/7) had disease progression. In patients whose LAP score increased, 11.1% (1/9) had disease progression (p<0.2615) (chart 3). Overall, our results confirm that LAP scores do tend to be lower in patients with more severe disease, as assessed by bone marrow blast percentage. However, although a trend was observed toward change in LAP score correlating with disease activity this was not statistically significant, and larger prospective studies are necessary to assess whether LAP is an accurate marker of MDS progression. Chart 1: LAP scores of patients 1 through 8 with bone marrow blasts less than 5% (mean 90.25, median 96) Chart 2: LAP scores for patients 1 through 20 with bone marrow blasts of 5% to 19% (mean 44.35, median 30) Chart 1: LAP scores of patients 1 through 8 with bone marrow blasts less than 5% (mean 90.25, median 96) . / Chart 2: LAP scores for patients 1 through 20 with bone marrow blasts of 5% to 19% (mean 44.35, median 30) Chart 3: Percent of patients with disease progression among those with decrease in LAP score (white) and those with increase in LAP score (gray) (p<0.2615). Chart 3: Percent of patients with disease progression among those with decrease in LAP score (white) and those with increase in LAP score (gray) (p<0.2615).

Platelet vWF, ADAMTS-13, and Neutrophil Activation in Patients with Essential Thrombocythemia (ET)

ET is a chronic myeloproliferative disorder, the benign clinical course of which can be complicated by both thromboses and hemorrhages. The levels of the largest multimers of plasma vWF, are relevant for the hemostatic balance in this disease. vWF is stored in the cellular granules of both endothelial cells and platelets, from which is released upon activation. Neutrophils and platelets circulate in an activation status in ET patients, leading to an increased formation of platelet/neutrophil aggregates. In this study we evaluated in ET patients, the platelet vWF content and its relation to neutrophil activation. In addition, the activity of ADAMTS13, the enzyme responsible for maintaining the normal size distribution of vWF multimers, was studied. A group of 61 consecutive ET patients (50% carrying the JAK2 V617F mutation) and 30 healthy controls were included into the study. Total vWF antigen (vWF:Ag) and activity (vWF:Act) levels were determined in both plasma and in isolated washed platelets by ELISA and collagen binding assay (CBA), respectively. ADAMTS13 activity was measured in plasma according to Gerritsen’s method. CD11b and leukocyte alkaline phosphatase (LAP) were measured on neutrophil membrane as activation markers. Plasma vWF:Ag levels resulted significantly increased in ET patients compared to controls (p&lt;0.05), the subgroup of patients with the JAK2 mutation showing the highest levels; whereas vWF:Act was not statistically different. The vWF Act/Ag ratio was significantly lower in ET compared to controls (0.85±0.15 vs 0.95±0.23; p&lt;0.01). A significant inverse relation between platelet count and plasma vWF:Act existed. On the other hand, platelets from these patients had a decreased content of both vWF:Ag and vWF:Act than platelets form controls (p&lt;0.01), suggesting an increased platelet release of vWF. In addition, platelets from ET JAK2 mutation carriers had significantly lower vWF:Act compared to wild-type subjects. Accordingly, the platelet vWF Act/Ag ratio of JAK2 mutation carriers was significantly (p&lt;0.01) lower vs wild-type patients and controls. Neutrophil confirmed to be activated in these patients, as previously described. Indeed, surface activation markers were increased in ET patients compared to controls, and, in particular, CD11b levels were inversely related to platelet vWF:Act (p&lt;0.01). The measurement of plasma ADAMTS13 showed a reduction in the activity of this protease in ET patients (68±16%) compared to normal control mean (p&lt;0.01). In conclusion, in ET patients and particularly in JAK2 mutation carriers, a decrease in platelet vWF:Ag and Act is found and correlates with neutrophil activation, suggesting a role of neutrophil in platelet degranulation. Increased vWF release and reduced ADAMTS13 activity can represent a mechanism of hypercoagulation in these patients.