The Behavior of Cytogenetic/Molecular Markers Associated with JAK2 Mutation Status Before and After Primary Myelofibrosis Progression Supports the Hypothesis of the Leukemic Clone's Independence From the JAK2 Mutation

Abstract 5058

Leukemic transformation has been reported in 15–30% of primary myelofibrosis (PMF). However, very little is known about the molecular bases responsible for acute myeloid leukemia (AML) transformation. JAK2 status analysis of paired chronic myeloproliferative neoplasms (MPN) and AML samples showed that during AML transformation the JAK2 mutation could be lost. Two probable models have been proposed to explain the MPN evolution to AML: a) JAK2-mutated AML usually arose from PMF or myelofibrotic transformation of ET/PV as a consequence of genetic instability conferred by the presence of JAK2 mutations; b) JAK2 wild-type AML generally developed in patients with chronic-phase ET or PV, frequently as a consequence of a clone selection driven by the therapy (Thoennissen NH et al. Blood 2010, 115:2882; Beer PA et al., Blood 2010, 115:2891; Spivak JL, Blood 2010, 115:2727). In this report we describe two PMF cases with the JAK2V617F mutation associated with molecular/cytogenetic abnormalities who developed JAK2-wild type AML characterized by a leukemic clone bearing a new cytogenetic aberration.

At the PMF diagnosis, Case #1 showed a normal karyotype 46, XY[20] and resulted JAK2V617F positive. Molecular analyses performed at the time of AML transformation revealed the presence of a JAK2V617F negative clone bearing a novel t(12;18)(p13;q12) rearrangement. As the t(12;18) breakpoints were located centromerically to SETBP1 (18q12. 3), quantitative real-time PCR (qRT-PCR) experiments were made, showing SETBP1 overexpression. To investigate the occurrence of SETBP1 dysregulation and the presence of t(12;18) at PMF onset, qRT-PCR and Fluorescence in situ hybridization were performed, revealing gene overexpression and absence of the chromosomal translocation, respectively.

At PMF onset, Case #2 harbored a novel t(3;5)(q27. 1;q31. 1) in addition to the JAK2V617F mutation. At the time of AML evolution, disappearance of the t(3;5)(q27. 1;q31. 1) and leukemic clone expansion of t(3;3)(q21. 3;q26. 2) was associated with disappearance of the JAK2V617F mutation.

In contrast to literature data showing that JAK2 mutation loss is commonly associated with AML transformation after PV and ET, our findings suggest that evolution to JAK2-wild type AML could also occur in JAK2V617F PMF patients. The presence of different cytogenetic abnormalities associated with PMF and AML allowed us to follow the sequence of molecular events that lead to JAK2V617F disappearance, indicating that MPN and AML are clonally unrelated and probably generated by the transformation of different stem cell levels. Moreover, the well-documented clonal heterogeneity landscape in our cases demonstrated that the genomic instability responsible for AML transformation already existed at PMF onset and was not generated either by JAK2V617F mutation expression or by the therapy.