Molecular characterization of the idiopathic hypereosinophilic syndrome (HES) in 35 French patients with normal conventional cytogenetics

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Abstract
Idiopathic hypereosinophilic syndrome (HES) characterized by unexplained and persistent hypereosinophilia is heterogeneous and comprises several entities: a myeloproliferative form where myeloid lineages are involved with the interstitial chromosome 4q12 deletion leading to fusion between FIP1L1 and PDGFRA genes, the latter acquiring increased tyrosine kinase activity. And a lymphocytic variant, where hypereosinophilia is secondary to a primitive T lymphoid disorder demonstrated by the presence of a circulating T-cell clone. We performed molecular characterization of HES in 35 patients with normal karyotype by conventional cytogenetic analysis. TCRγ gene rearrangements suggesting T clonality were seen in 11 (31%) patients, and FIP1L1–PDGFRA by RT-PCR in six (17%) of 35 patients, who showed no evidence of T-cell clonality. An elevated serum tryptase level was observed in FIP1L1–PDGFRA-positive patients responding to imatinib, whereas serum IL-5 levels were not elevated in T-cell associated hypereosinophilia. Sequencing FIP1L1–PDGFRA revealed scattered breakpoints in FIP1L1-exons (10–13), whereas breakpoints were restricted to exon 12 of PDGFRA. In the 29 patients without FIP1L1–PDGFRA, no activating mutation of PDGFRA/PDGFRB was detected; however; one patient responded to imatinib. FISH analysis of the 4q12 deletion was concordant with FIP1L1–PDGFRA RT-PCR data. Further investigation of the nature of FIP1L1–PDGFRA affected cells will improve the classification of HES.