Subnuclear targeting of Runx/Cbfa/AML factors is essential for tissue-specific differentiation during embryonic development

Runx (Cbfa/AML) transcription factors are critical for tissue-specific gene expression. A unique targeting signal in the C terminus directs Runx facto rs to discrete foci within the nucleus. Using Runx2/CBFA1/AML3 and its esse ntial role in osteogenesis as a model, we investigated the fundamental impo rtance of fidelity of subnuclear localization for tissue differentiating ac tivity by deleting the intranuclear targeting signal via homologous recombi nation, Mice homozygous for the deletion (Runx2 DeltaC) do not form bone du e to maturational arrest of osteoblasts. Heterozygotes do not develop clavi cles, but are otherwise normal. These phenotypes are indistinguishable from those of the homozygous and heterozygous null mutants, indicating that the intranuclear targeting signal is a critical determinant for function. The expressed truncated Runx2 DeltaC protein enters the nucleus and retains nor mal DNA binding activity, but shows complete loss of intranuclear targeting . These results demonstrate that the multifunctional N-terminal region of t he Runx2 protein is not sufficient for biological activity. We conclude tha t subnuclear localization of Runx factors in specific foci together with as sociated regulatory functions is essential for control of Runx-dependent ge nes involved in tissue differentiation during embryonic development.