Cysteine substitutions in epidermal growth factor-like domains of fibrillin-1: Distinct effects on biochemical and clinical phenotypes

Fibrillin-1 (FBN1) contains 47 epidermal growth factor (EGF)-like domains c haracterized by six conserved cysteine residues. Cysteine substitutions tha t disrupt one of the three disulfide bonds are frequent causes of Marfan sy ndrome (MFS). We identified 19 new substitutions involving cysteine residue s in each of the six positions of EGF-like domains. Allele-specific mRNA as says revealed equal abundance of mutant and normal FBN1 transcripts in all 10 individuals studied. Quantitative pulse-chase analysis of fibrillin prot ein was performed on 25 mutant fibroblast strains with substitutions of 22 different cysteine residues in 18 different EGF-like domains spanning the e ntire gene. Normal synthesis and stability of mutant fibrillin molecules wa s seen in 20/25 individuals, 11 of whom showed delayed intracellular proces sing and/or secretion. In the remaining five cases, the mutant protein was apparently unstable. In four of these five cases, the second or third disul fide bond of EGF-like domains immediately preceding an 8-cysteine or hybrid domain was affected. All but two mutations caused severe reduction of matr ix deposition, which was attributed to a dominant-negative effect of mutant molecules. For genotype/phenotype comparisons, clinical data on 25 proband s and 19 mutation-positive family members were analyzed. Ocular manifestati ons were among the most consistent features (ectopia lentis in 86%, myopia in 80%). Nine mutations encoded by exons 26-32 resulted in early-onset clas sic MFS and, in one case, neonatal-lethal MFS. Mutations outside this regio n were associated with variable clinical phenotypes, including individuals with fibrillinopathies not meeting diagnostic criteria for MFS.