EXPRESSION OF A MUTANT HUMAN FIBRILLIN ALLELE UPON A NORMAL HUMAN OR MURINE GENETIC BACKGROUND RECAPITULATES A MARFAN CELLULAR PHENOTYPE

The Marfan syndrome (MFS) is a connective tissue disorder inherited as an autosomal dominant trait and caused by mutations in the gene encod ing fibrillin, a 350-kD glycoprotein that multimerizes to form extrace llular microfibrils. It has been unclear whether disease results from a relative deficiency of wild-type fibrillin; from a dominant-negative effect, in which mutant fibrillin monomers disrupt the function of th e wild-type protein encoded by the normal allele; or from a dynamic an d variable interplay between these two pathogenetic mechanisms. We hav e now addressed this issue in a cell culture system. A mutant fibrilli n allele from a patient with severe MFS was expressed in normal human and murine fibroblasts by stable transfection. Immunohistochemical ana lysis of the resultant cell lines revealed markedly diminished fibrill in deposition and disorganized microfibrillar architecture. Pulse-chas e studies demonstrated normal levels of fibrillin synthesis but substa ntially reduced deposition into the extracellular matrix. These data i llustrate that expression of a mutant fibrillin allele, on a backgroun d of two normal alleles, is sufficient to disrupt normal microfibrilla r assembly and reproduce the MFS cellular phenotype. This underscores the importance of the fibrillin amino-terminus in normal microfibrilla r assembly and suggests that expression of the human extreme 5' fibril lin coding sequence may be sufficient, in isolation, to produce an ani mal model of MFS. Lastly, this substantiation of a dominant-negative e ffect offers mutant allele knockout as a potential strategy for gene t herapy.