Core-binding factor beta (CBF beta), but not CBF beta-smooth muscle myosinheavy chain, rescues definitive hematopoiesis in CBF beta-deficient embryonic stem

Core-binding factor beta (CBF beta) is the non-DNA-binding subunit of the h eterodimeric CBFs. Genes encoding CBF beta (CBFB), and one of the DNA-bindi ng CBF alpha subunits, Runx1 (also known as CBF alpha2, AML1, and PEBP2 alp haB), are required for normal hematopoiesis and are also frequent targets o f chromosomal translocations in acute leukemias in humans. Homozygous disru ption of either the Runx1 or Cbfb gene in mice results in embryonic lethali ty at midgestation due to hemorrhaging in the central nervous system, and s everely impairs fetal liver hematopoiesis. Results of this study show that Cbfb-deficient mouse embryonic stem (ES) cells can differentiate into primi tive erythroid colonies in vitro, but are impaired in their ability to prod uce definitive erythroid and myeloid colonies, mimicking the in vivo defect . Definitive hematopoiesis is restored by ectopic expression of full-length Cbfb transgenes, as well as by a transgene encoding only the heterodimeriz ation domain of CBF beta. In contrast, the CBF beta -smooth muscle myosin h eavy chain (SMMHC) fusion protein generated by the inv(16) associated with acute myeloid leukemias (M4Eo) cannot rescue definitive hematopoiesis by Cb fb-deficient ES cells. Sequences responsible for the inability of CBF beta -SMMHC to rescue definitive hematopoiesis reside in the SMMHC portion of th e fusion protein. Results also show that the CBF beta -SMMHC fusion protein transdominantly inhibits definitive hematopoiesis, but not to the same ext ent as homozygous loss of Runx1 or Cbfb. CBF beta -SMMHC preferentially inh ibits the differentiation of myeloid lineage cells, while increasing the nu mber of blastlike cells in culture. (Blood. 2001;97: 2248-2256) (C) 2001 by The American Society of Hematology.