Detection of the FIP1L1-PDGFRA fusion in PBMCs from patients with HES and elevated tryptase. (A) RT-PCR analysis of RNA isolated from PBMCs from patients with HES and elevated serum tryptase (lanes 1-5), patients with HES and normal tryptase levels (lanes 6-9), and patients with a familial form of eosinophilia linked to chromosome 5q (lanes 10-11). The water control is indicated by the letter w. Only patients with elevated tryptase levels (1-5) were positive for the FIP1L1-PDGFRA fusion. Different PCR products are observed in the different cases, due to different breakpoints in the FIP1L1 gene (B). The different bands observed in each case represent splice variants (the sequence of 1 splice variant is shown in panel B), as observed previously.7 GAPDH was amplified as a control for cDNA quality. (B) cDNA and deduced amino acid sequence of FIP1L1-PDGFRA fusions from patients 1 to 5. Sequences derived from FIP1L1 are shown in lowercase; sequences derived from introns of FIP1L1 that are present in the fusion cDNA are underlined; sequences derived from PDGFRA are shown in uppercase in bold. Breakpoints in FIP1L1 are variable (in introns 8, 8a, 9, or 10), but all breakpoints in PDGFRA occur in exon 12. All fusions are in-frame. Exon numbering is according to Cools et al.7