A combination of cytomorphology, cytogenetic analysis, fluorescence in situ hybridization and reverse transcriptase polymerase chain reaction for establishing clonality in cases of persisting hypereosinophilia.

To evaluate the frequency of clonal abnormalities in patients with unexplained persisting eosinophilia we analyzed 40 patients (27 males, 13 females) using cytomorphology, cytogenetic analysis, interphase fluorescence in situ hybridization (FISH), and reverse transcriptase polymerase chain reaction (RT-PCR). Cytogenetic analysis revealed clonal abnormalities in five patients (four of whom were males) including t(8;9)(p21;p24), ins(9;4)(q34;q12q31), del(6)(q24), and trisomy 8 (n=2). RT-PCR confirmed a PCM1-JAK2 fusion underlying the t(8;9). FISH analysis suggested a rearrangement involving PDGFRA in the ins(9;4). A FIP1L1-PDGFRA fusion gene was identified in four male patients by interphase FISH and RT-PCR. These methods in combination demonstrated clonality in 8/40 patients (20%) with a male predominance (6/8; 75%).