BACKGROUNDThe FIP1L1-PDGFRalpha fusion gene plays an important role in the pathogenesis of chronic eosinophilic leukemia (CEL) and is a direct therapeutic target of the tyrosine kinase inhibitor imatinib mesylate.

METHODSIn 24 hypereosinophilic syndromes (HES) patients, using reverse transcriptase-polymerase chain reaction (RT-PCR), nested PCR and sequence analysis, we investigated the frequency of FIP1L1-PDGFRalpha and other abnormalities of tyrosine kinase family genes like PDGFRalpha, PDGFRbeta, C-KIT, FGFR1, ABL and FLT3 as well as gene mutation "hotspots", like MPL515 and JAK2V617F, frequently involved in myeloproliferative diseases. Fluorescence in situ hybridization was used to confirm the 4q12 deletion.

RESULTSThe FIP1L1-PDGFRalpha fusion transcript was found in 8 (33%) of 24 patients with HES, corresponding to the chromosome 4q12 deletion identified by FISH. The FIP1L1-PDGFRalpha-associated patients diagnosed with CEL, frequently had hepatosplenomegaly, eosinophil-related tissue damage, anemia, thrombocytopenia, myelofibrosis and a short overall survival time. Nevertheless, imatinib mesylate induced rapid and complete hematological responses in treated FIP1L1-PDGFRalpha cases, followed by molecular remission and reversal of myelofibrosis. FIP1L1-PDGFRalpha fusion could co-exist with other mutations of tyrosine kinase family genes, like FLT3 or PDGFRbeta. We also demonstrated that the SNPs of PDGFRbeta were associated with selective splicing of exon 19 in case 20.

CONCLUSIONSCorrelating the CEL genotype with phenotype, FIP1L1-PDGFRalpha emerges as a relatively homogeneous clinicobiological entity that co-exists with other abnormalities of tyrosine kinase family genes. It reflects the disease progression and there is a good response to imatinib. Detection of the FIP1L1-PDGFRalpha fusion gene is valid for both CEL diagnosis and therapy surveillance.