Cbfβ interacts with Runx2 and has a critical role in bone development

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Abstract
Runx2 (runt-related transcription factor 2, also known as Cbfa1, Osf2 and AML3) is essential for bone development in mice, and mutations in RUNX2 are found in 65–80% of individuals with cleidocranial dysplasia1,2. Although all Runx family members can interact with Cbfβ (core-binding factor b, encoded by Cbfb), a role for Cbfβ in bone development has not been demonstrated owing to lethality in Cbfb−/− mouse embryos at 12.5 days post coitum (d.p.c.) from hemorrhages and lack of definitive hematopoiesis3,4. Using a 'knock-in' strategy, we generated mouse embryonic stem (ES) cells that express Cbfb fused in-frame to a cDNA encoding green fluorescent protein (GFP)5. Cbfb+/GFP mice had normal life spans and appeared normal, but CbfbGFP/GFP pups died within the first day after birth. The CbfbGFP/GFP mice exhibited a delay in endochondral and intramembranous ossification as well as in chondrocyte differentiation, similar to but less severe than delays observed in Runx2−/− mice6,7. We demonstrate that Cbfβ is expressed in developing bone and forms a functional interaction with Runx2, and that CbfbGFP is a hypomorphic allele. The fusion allele maintains sufficient function in hematopoietic cells to bypass the early embryonic lethality, and identifies a new role for Cbfb in bone development. Our findings raise the possibility that mutations in CBFB may be responsible for some cases of cleidocranial dysplasia that are not linked to mutations in RUNX2.