Functional haploinsufficiency of the human homeobox gene MSX2 causes defects in skull ossification


The genetic analysis of congenital skull malformations provides insight into normal mechanisms of calvarial osteogenesis1. Enlarged parietal foramina (PFM) are oval defects of the parietal bones caused by deficient ossification around the parietal notch, which is normally obliterated during the fifth fetal month2. PFM are usually asymptomatic, but may be associated with headache, scalp defects and structural or vascular malformations of the brain3,4. Inheritance is frequently autosomal dominant, but no causative mutations have been identified in non-syndromic cases. We describe here heterozygous mutations of the homeobox gene MSX2 (located on 5q34–q35) in three unrelated families with PFM. One is a deletion of approximately 206 kb including the entire gene and the others are intragenic mutations of the DNA-binding homeodomain (RK159-160del and R172H) that predict disruption of critical intramolecular and DNA contacts. Mouse Msx2 protein with either of the homeodomain mutations exhibited more than 85% reduction in binding to an optimal Msx2 DNA-binding site. Our findings contrast with the only described MSX2 homeodomain mutation5 (P148H), associated with craniosynostosis, that binds with enhanced affinity to the same target6. This demonstrates that MSX2 dosage is critical for human skull development and suggests that PFM and craniosynostosis result, respectively, from loss and gain of activity in an MSX2-mediated pathway of calvarial osteogenic differentiation.

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Figure 1: Skull radiographs from family 1, showing evolution of the phenotype with age.
Figure 2: Identification of MSX2 deletion in family 1.
Figure 3: Mutations of MSX2 in families 2 (left) and 3 (right).
Figure 4: Schematic diagram of MSX2 homeodomain bound to DNA, based on the structure of antennapedia15 and homology modelling of Msx1 (ref. 16).
Figure 5: Binding of full-length mouse Msx2 proteins to an optimum DNA-binding site, analysed by gel shift.

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We thank family members for their participation; A. Baxova, S. Kutilek, A. Salamanca, L. Solymosi, H. Urbach and S. White for help with collection of samples; R. Jaju and L. Kearney for FISH analysis; P. Anslow for radiological advice, R. Maas for sharing unpublished data; and S. Robertson for comments on the manuscript. PAC clones were provided by the UK HGMP Resource Centre. This work was funded by a Wellcome Trust Senior Fellowship in Clinical Science (A.O.M.W.) and grants from the NIDCD (R.E.M.).

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Correspondence to Andrew O.M. Wilkie.

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