Identification of the Chimeric Protein Product of the CBFB-MYHII Fusion Gene in Inv(16) Leukemia Cells

An expressed gene formed by fusion between the CBFB transcription factor gene and the smooth muscle myosin heavy chain gene MYHII is consistently detected by reverse transcription polymerase chain reaction (RT-PCR) in patients who have acute myeloid leukemia (AML) subtype M4Eo with an inversion of chromosome 16. We have previously shown that a CBFB-MYHII cDNA construct can produce a chimeric protein and transform NIH 3T3 cells. However, the presence of the chimeric protein in patient cells has not been demonstrated previously. Here, we show that such chimeric proteins can be identified in vivo, primarily in the nuclei of the leukemic cells, by use of antibodies against the C-terminus of the smooth muscle myosin heavy chain and the fusion junction peptide. A very high molecular weight protein/DNA complex is generated when nuclear extracts from patient cells are used in electrophoretic mobility shift assays, as seen in NIH 3T3 cells transfected with the CBFB-MYHII cDNA. Immunofluorescence staining shows that the proteins are organized in vivo into novel structures within cell nuclei. One isoform of the transcript of the CBFB-MYHII fusion gene, containing the MHC₂₀₄ C-terminus, was the predominant form in all five cases studied.