Haploinsufficiency of MSX1: a Mechanism for Selective Tooth Agenesis

ABSTRACT

Previously, we found that the cause of autosomal dominant selective tooth agenesis in one family is a missense mutation resulting in an arginine-to-proline substitution in the homeodomain of MSX1. To determine whether the tooth agenesis phenotype may result from haploinsufficiency or a dominant-negative mechanism, we have performed biochemical and functional analyses of the mutant protein Msx1(R31P). We show that Msx1(R31P) has perturbed structure and reduced thermostability compared with wild-type Msx1. As a consequence, the biochemical activities of Msx1(R31P) are severely impaired, since it exhibits little or no ability to interact with DNA or other protein factors or to function in transcriptional repression. We also show that Msx1(R31P) is inactive in vivo, since it does not display the activities of wild-type Msx1 in assays of ectopic expression in the limb. Furthermore, Msx1(R31P) does not antagonize the activity of wild-type Msx1 in any of these assays. Because Msx1(R31P) appears to be inactive and does not affect the action of wild-type Msx1, we propose that the phenotype of affected individuals with selective tooth agenesis is due to haploinsufficiency.

ACKNOWLEDGMENTS

G.H., H.V., and A.J.B. contributed equally to this work.

We are indebted to Cliff Tabin (Harvard Medical School) for generous assistance with the chick retroviral expression studies and for many helpful discussions and Lee Niswander (Memorial Sloan Kettering) for the gift of the GMsx1 probe. We thank Michael Shen and Aaron Shakin (CABM) for helpful comments on the manuscript and Qing Li for assistance with the CD analysis.

This work was supported by NIH grant HD29446-06 to C.A.-S.; HHMI funding to C.E.S., J.G.S., and H.V.; predoctoral grants from the American Heart Association to G.H. and H.Z.; and a grant from the American Association of Orthodontists Foundation to H.V. C.A.-S. is a recipient of an NSF Young Investigator award.