RUNX2/CBFA1 mutations in cleidocranial dysplasia: Phenotypic and structure/function correlations

RUNX2/CBFA1 is a required transcriptional determinant for osteoblast differentiation. Heterozygous mutations in RUNX2 cause cleidocranial dysplasia (CCD), a human skeletal dysplasia characterized by defective intramembranous ossification, i.e., hypoplastic clavicles and delayed closure of fontanel. Our natural history study of over 90 CCD patients and previous clinical studies together point to significant variable expressivity and an underlying general dysplasia involving endochondral ossification. The clinical spectrum spans from severe CCD with osteopenia, to classic and mild CCD, to patients with only isolated dental anomalies. Moreover, additional genetic loci contributing to the CCD spectrum may exist. Mutation analysis of CCD patients of different ethnic backgrounds have revealed over 50 missense, nonsense, deletion, splicing, and insertion mutations. Most of these mutations disrupt DNA binding in vitro and hence result in haploinsufficiency. Some mutations do not affect DNA binding, but instead localize to amino acid residues implicated in mediating protein-protein interaction by X-ray crystallographic studies. Few patients have been reported to have expansion of a unique polyalanine stretch which has been described to harbor a transactivation domain. We have found that another unique domain, the polyglutamine stretch, also adds to the transactivation by RUNX2. However, no expansion or contractions of this region have yet been found to date in humans. Together these data show that the clinical spectrum of CCD can be caused by loss of the RUNX2 protein, loss of its DNA binding activity, loss of its transactivation, and perhaps also loss of specific protein-protein interactions.