PDGFRBis fused toPDE4DIPin an MPD associated with eosinophilia and t(1;5)(q23;q33). (A) Southern blot analysis of the t(1;5) and control DNA with a HindIII-XhoI PDGFRB genomic probe spanning exon 11. Rearranged bands are indicated by a star. Cloning of the genomic breakpoints for this fusion confirmed that the wild-type PDGFRB and PDE4DIP-PDGFRB HindIII restriction fragments are of very similar size and, therefore, comigrated in the Southern blot. At DNA level, the fusion of these genes occurs at IVS 12-767 (PDE4DIP) and IVS 10+289 (PDGFRB). (B) Single-step RT-PCR confirms the expression of the PDE4DIP-PDGFRB fusion. The reciprocal fusion is not expressed. PDGFRB RT-PCR confirms the integrity of the cDNAs. (C) Top: Isoform-specific nested RT-PCR showing that the PDE4DIP isoform lacking the LZ domain (KIAA 0477) is fused to PDGFRB in the t(1;5). Below: Diagram of the primary protein structure of human myomegalin, putative oligomerization domains, the breakpoint in the t(1;5), and the location of the primers (horizontal arrows) used in the isoform-specific nested RT-PCR. (D) Diagrammatic representation of the myomegalin (MM)-PDGFRB protein fusion and contributing nucleotides, amino acids, and domains. (E) Left panel: Western blot analysis showing lower levels of phosphorylated AKT (top subpanel) in the patient's primary cells 2 months into imatinib treatment. Total AKT is shown in the lower subpanel. Right panel: Densitometry of relevant bands showed a 60% reduction in the expression of phospho-AKT after imatinib.