BoneKEy-Osteovision | Commentary
Structural differences between runx1 and runx2 account for the hierarchy of function for Cbfa1beta
DOI:10.1138/2003067
Kundu M, Javed A, Jeon JP, Horner A, Shum L, Eckhaus M, Muenke M, Lian JB, Yang Y, Nuckolls GH, Stein GS, Liu PP. Cbfbeta interacts with Runx2 and has a critical role in bone development. Nat Genet. 2002 Dec;32(4):639-44. Miller J, Horner A, Stacy T, Lowrey C, Lian JB, Stein G, Nuckolls GH, Speck NA. The core-binding factor beta subunit is required for bone formation and hematopoietic maturation. Nat Genet. 2002 Dec;32(4):645-9. Yoshida CA, Furuichi T, Fujita T, Fukuyama R, Kanatani N, Kobayashi S, Satake M, Takada K, Komori T. Core-binding factor beta interacts with Runx2 and is required for skeletal development. Nat Genet. 2002 Dec;32(4):633-8.
Runx2, which encodes a protein better known as Cbfa1, is an important gene in the field of osteoblast differentiation. In a series of papers published six years ago, it was shown that Runx2 is the earliest and most specific marker of osteoblast differentiation, that it is necessary and sufficient for osteoblast differentiation in vitro and in vivo, that it regulates the expression of multiple genes expressed in the osteoblast and, more importantly, that its osteoblast function is dominant (). Indeed, haploinsufficiency at the Runx2 locus causes the same phenotypic abnormalities, called cleidocranial dysplasia, in mice and in humans. For all of these reasons, Runx2 is seen as a determining factor of osteoblast differentiation in vertebrates. Runx2 belongs to a family of transcription factors conserved between C. elegans and humans. All members of this family share a common DNA binding domain 128 amino acids long. It is called the Runt domain after the name of the Drosophila member of this family in which this DNA binding moiety was first described (). Like Runx2, many members of this family of transcription factors play essential roles in various programs of cell differentiation in Drosophila, mice, and humans. This is, for instance, the case for Runx1, which encodes a protein better known as AML1. Seven years ago, two groups demonstrated convincingly that Runx1 is essential for fetal liver hematopoiesis () and that it may be the earliest determinant of this process, just as Runx2 is for osteoblast differentiation. Runx1 was identified through its involvement in chromosomal translocation in human leukemia; in addition, Runx1 binds to an enhancer core motif present in multiple hematopoietic specific genes and in murine leukemia virus enhancers where another protein (CBFb) can also bind.
Two groups demonstrated unequivocally that CBFb, which is broadly if not ubiquitously expressed, is a dimerization partner in vitro for Runx1 (). Furthermore, disruption of Cbfb in mice results in an absence of fetal liver hematopoiesis identical to the one observed in the absence of Runx1, demonstrating genetically that it is an obligatory heterodimerization partner of Runx1 in vivo (). Given the critical role of Cbfb during hematopoiesis, its ability to heterodimerize with the Runx1 runt domain, and its nearly ubiquitous expression, a legitimate question was: Is CBFb like E12/E47 for bHLH protein, a heterodimerization partner for all Runx proteins? In vitro evidence shows that Runx2 transactivation was only modestly increased by cotransfection with a Cbfb expression vector (). These data would argue that CBFb is a preferential partner of Runx and at least an accessory partner of Runx2. However, the significance of these results was limited by their in vitro nature, and the question needs to be addressed in vivo.
In Nature Genetics (December 2002), three groups described an elegant and exhaustive exploration of the function of Cbfb during skeletal development (). Two of these groups used a transgenesis approach to rescue the early embryonic lethality of Cbfb-deficient mice, whereas the third group knocked a cDNA encoding green fluorescent protein in the Cbfb locus. The three groups presented similar results, with only minor differences in the analysis of the skeleton of these mutant mice. To compare the importance of the roles of Runx2 and Cbfb during skeletal development, the most accurate way is to compare Runx2-null mice to Cbfb-null mice (see Table 1). These two mutant mice have in common a delay in the latent events of skeletogenesis (i.e., osteoblast differentiation and chondrocyte hypertrophy). This is an important finding because it clearly enriches the spectrum of genes involved in osteoblast differentiation and does so unambiguously.
The next question is whether Cbfb-null mice are a phenocopy of Runx2-null mice, as they are of Runx1-null mice? This is a critical question because it is the best available tool to define whether Runx2 controls osteoblast differentiation as a heterodimer, in which Cbfb is an obligatory partner, or not. If this were the case it would have tremendous implications for the field. Because of their high quality, these studies provide a fairly clear answer. Indeed, unlike the case for Runx1-null mice, the detailed analyses provided show that Cbfb -null mice are not a phenocopy of Runx2-null mice. For instance, Cbfb+/- mice do not have any skeletal abnormalities (T. Komori personal communication), and osteoblast differentiation is noticeably delayed, but not absent, in Cbfb-null mice. These results obtained in vivo are in overall agreement with results obtained in vitro, where the strong transactivation function of Runx2 is only moderately increased by Cbfb overexpression. Again, these studies are extremely important because they establish firmly a hierarchy of function for Cbfb: it is paramount for hematopoiesis and important (but more modestly) for skeletal development. In doing so, they also indicate the existence of important structural differences between Runx1 and Runx2.
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