THE LEUKEMIC CORE BINDING-FACTOR BETA-SMOOTH MUSCLE MYOSIN HEAVY-CHAIN (CBF-BETA-SMMHC) CHIMERIC PROTEIN REQUIRES BOTH CBF-BETA AND MYOSIN HEAVY-CHAIN DOMAINS FOR TRANSFORMATION OF NIH 3T3 CELLS

An inversion of chromosome 16 associated with the M4Eo subtype of acut e myeloid leukemia produces a chimeric protein fusing the beta subunit of the transcription factor core binding factor (CBF beta) to the tai l region of smooth muscle myosin heavy chain (SMMHC). We investigated the oncogenic properties of this CBF beta-SMMHC chimeric protein using a 3T3 transformation assay. NIH 3T3 cells expressing CBF beta-SMMHC a cquired a transformed phenotype, as indicated by their ability to form foci, grow in soft agarose, and form tumors in nude mice. Cells expre ssing normal CBF beta or the SMMHC tail domain did not become transfor med. Electrophoretic mobility-shift assays showed that extracts from c ells transformed by CBF beta-SMMHC no longer formed the normal CBF/DNA complex but instead formed a much larger complex that did not migrate into the gel, Analysis of CBF beta-SMMHC deletion mutants demonstrate d that the chimeric protein was transforming only if two domains were both present: (i) CBF beta sequences necessary for association with th e CBF alpha subunit, and (ii) SMMHC sequences important for the format ion of multimeric filaments, These results are direct evidence that CB F beta-SMMHC can function as an oncoprotein.