MISSENSE MUTATIONS IMPAIR INTRACELLULAR PROCESSING OF FIBRILLIN AND MICROFIBRIL ASSEMBLY IN MARFAN-SYNDROME

Dermal fibroblasts from nine Marfan syndrome patients with missense mu tations in the fibrillin-1 gene (FBN1) produced nearly normal amounts of fibrillin as determined by quantitative pulse-chase experiments. Ho wever, six of the seven mutations involving substitutions of highly co nserved cysteine residues exhibited lower rates of intracellular trans port and secretion. This effect is likely due to improper folding, sin ce intracellular fibrillin processing was also affected by the reducin g agent dithiothreitol. Normal secretion patterns were seen in three m utations that either change the conformation of EGF-like domains or ch ange consensus amino acids required for Ca++-binding. In all nine fibr oblast strains, however, the deposition of fibrillin in the extracellu lar matrix was reduced to 50% of normal in two and to less than 30% in seven of the nine samples studied. The protein alterations caused by these missense mutations are associated with moderate to severe featur es of Marfan syndrome and a dominant negative mechanism is suggested t o play a major role in their pathogenesis.