A case of FIP1L1-PDGFRA-positive chronic eosinophilic leukemia with a rare FIP1L1 breakpoint.

The idiopathic hypereosinophilic syndrome (HES) has remained for a long time a diagnosis of exclusion. Differential diagnosis between the HES and the related chronic eosinophilic leukemia (CEL) relied on the identification of signs of clonality that allowed, when present, the reclassification of patients as CEL. Recently, a new acquired mutation was described in approximately 50% of the HES/CEL patients: a cryptic deletion on chromosome band 4q12 generating a FIP1L1-PDGFRA fusion gene. According to the World Health Organization classification, this clonal abnormality has been proposed as a new surrogate marker for chronic eosinophilic leukemia diagnosis. Fluorescence in situ hybridization and reverse transcriptase-polymerase chain reaction protocols were developed for an accurate del(4)(q12q12) and FIP1L1-PDGFRA fusion gene detection. Here, we report a patient with a rare FIP1L1 intron 16 breakpoint located outside of the reported FIP1L1 breakpoint region (ie, from FIP1L1 introns 9 to 13). This case illustrates the risk of false-negative results with diagnostic procedures that do not take into account the occurrence of rare FIP1L1 breakpoints. As targeted therapy with tyrosine kinase inhibitors has dramatically changed the prognosis of FIP1L1-PDGFRA (+) CEL, false-negative results could hamper accurate diagnosis and treatment.