Analysis of JAK2 Mutation in Essential Thrombocythemia (ET) Patients with Monoclonal and Polyclonal X-Chromosome Inactivation Patterns (XCIPs).

Abstract Background: ET is considered as a hematopoietic stem cell disorder, closely related to polycythemia vera (PV). Several studies in female ET patients, using XCIPs analysis, have shown the persistence of polyclonal patterns in a subset of patients, the significance of this finding being unclear (Blood: 1997;89:128 and 1999;93:417). Patients with polyclonal patterns were suggested to be at lower risk of thrombosis. The mechanism of sustained megakaryocytic proliferation, in the absence of a reactive cause, is still to be found in this disease. The recent description of an activating V617F JAK2 mutation in PV prompted us to study its relevance in ET, especially with respect to XCIPs analysis results. Methods: We looked for V617F JAK2 mutation in whole blood DNA in a series of 43 ET female patients for whom XCIPs had previously been reported by our group (Blood: 1997;89:128). Monoclonal pattern was defined by greater than 75% expression of one allele of an X-chromosome polymorphic gene. ET diagnosis was made according to PVSG criteria, after careful exclusion of PV by red cell mass measurement, CML by karyotype and/or BCR-ABL RT-PCR, and idiopathic myelofibrosis by bone marrow biopsy. V617F JAK2 mutation was screened by direct DNA sequencing (sensitivity of the assay: about 5%). Results: V617F JAK2 mutation was found in 12 of the 43 samples (28%), in agreement with recent reports. No significant differences were found between JAK2 mutated and non-mutated patients for age (43 vs 57 years), clinical symptoms at presentation (33% vs 35%), platelet count (866 vs 907 G/l), or cytoreductive therapy (41% vs 61%), respectively. Of the 35 patients with a monoclonal pattern in granulocytes (and polyclonal T lymphocytes), 8 (23%) had JAK2 mutation, as compared to 4 of the 8 patients (50%) with polyclonal pattern in granulocytes (p=0.185, Fisher’s exact test). Conclusion: The 50% incidence of JAK2 mutation we found in ET patients with a polyclonal pattern of hematopoiesis may be due to a better sensitivity of the JAK2 mutation assay by comparison to XCIPs clonality assays. It shows that many ET with a XCIPs polyclonal pattern are in fact clonal disorders. On the other hand, the absence of JAK2 mutation in about 75% of patients with a monoclonal pattern suggests that other molecular events can lead to clonal megakaryocytic proliferation in a large proportion of ET patients.