HAPLOINSUFFICIENCY OF MSX1 - A MECHANISM FOR SELECTIVE TOOTH AGENESIS

Previously, we found that the cause of autosomal dominant selective to oth agenesis in one family is a missense mutation resulting in an argn ine-to-proline substitution in the homeodomain of MSX1. To determine w hether the tooth agenesis phenotype may result from haploinsufficiency or a dominant-negative mechanism, we have performed biochemical and f unctional analyses of the mutant protein Msx1(R31P). We show that Msx1 (R31P) has perturbed structure and reduced thermostability compared wi th wild-type Msx1. As a consequence, the biochemical activities of Msx 1(R31P) are severely impaired, since it exhibits little or no ability to interact with DNA or other protein factors or to function in transc riptional 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 a ntagonize the activity of wild-type Msx1 in any of these assays. Becau se Msx1(R31P) appears to he 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.