Deficient coacervation of two farms of human tropoelastin associated with supravalvular aortic stenosis

Human tropoelastin associates by coacervation and is subsequently cross-lin ked to make elastin. In Williams syndrome, defective elastin deposition is associated with hemizygous deletion of the tropoelastin gene in supravalvul ar aortic stenosis (SVAS). Remarkably, point-mutation forms of SVAS corresp ond to incomplete forms of tropoelastin which include in-frame termination by nonsense mutations, yet the resulting phenotype of these disorders is no t explained because expression variably occurs from both normal and mutant alleles. Proteins corresponding to two truncated tropoelastin mutants were expressed and purified to homogeneity. Coacervation of these proteins occur red as expected with increasing temperature, but substantially contrasted w ith that of the performance of a normal tropoelastin. Significantly, associ ation by coacervation of the truncated SVAS tropoelastin molecules was negl igible at 37 degrees C, which contrasted with the substantial coacervation seen for normal tropoelastin. Furthermore their midpoints of coacervation i ncreased and correlated with the extent of deletion, in accord with the los s of hydrophobic regions required for tropoelastin association. Their secon dary structures are similar, as evidenced by CD studies. We propose a model for point-mutation SVAS in which aberrant tropoelastin molecules are incom petent and are mainly excluded from participation in coacervation and conse quently in elastogenesis. These forms of SVAS may consequently be considere d functionally similar to a hemizygous deletion, and mark point-mutation SV AS as a disorder of defective coacervation.