Msx2 gene dosage influences the number of proliferative osteogenic cells in growth centers of the developing murine skull: A possible mechanism for MSX2-mediated craniosynostosis in humans

Throughout its complex morphogenesis, the vertebrate skull must at once pro tect the brain and expand to accommodate its growth. A key structural adapt ation that allows this dual role is the separation of the bony plates of th e skull with sutures, fibrous joints that serve as growth centers and allow the calvarial bones to expand as the brain enlarges. Craniosynostosis, the premature fusion of one or more calvarial bones with consequent abnormalit ies in skull shape, is a common developmental anomaly that disrupts this pr ocess. We found previously that a single amino acid substitution in the hom eodomain of the human MSX2 gene is associated with the autosomal dominant d isorder craniosynostosis, Boston type. This mutation enhances the affinity of Msx2 for its target sequence, suggesting that the mutation acts by a dom inant positive mechanism. Consistent with this prediction, we showed that g eneral overexpression of Msx2 under the control of the broadly expressed CM V promoter causes the calvarial bones to invade the sagittal suture. Here w e use tissue-specific overexpression of Msx2 within the calvarial sutures t o address the developmental mechanisms of craniosynostosis and skull morpho genesis. We demonstrate that a segment of the Msx2 promoter directs reporte r gene expression to subsets of cells within the sutures. In late embryonic and neonatal stages, this promoter is expressed in undifferentiated mesenc hymal cells medial to the growing bone. By PS, promoter activity is reduced in the suture, exhibiting a punctate pattern in undifferentiated osteoblas tic cells in the outer margin of the osteogenic front. Overexpression of Ms x2 under the control of this promoter is sufficient to enhance parietal bon e growth into the sagittal suture by Pb. This phenotype is preceded by an i ncrease in both the number and the BrdU labeling of osteoblastic cells in t he osteogenic fronts of the calvarial bones. These findings suggest that an important early event in MSX2-mediated craniosynostosis in humans is a tra nsient retardation of osteogenic cell differentiation in the suture and a c onsequent increase in the pool of osteogenic cells, (C) 1999 Academic Press .