Fibrillin-1 is a large modular glycoprotein that assembles to form 10-12 nm 
microfibrils in the extracellular matrix. Mutations in the fibrillin-1 gene 
(FBN1) cause Marfan syndrome and related connective tissue disorders 
(fibrillinopathies) that show autosomal dominant inheritance. The pathogenic 
mechanism is thought to be a dominant negative effect of a mutant protein on 
microfibril assembly, although direct evidence is lacking. A significant group 
of disease-causing FBN1 mutations are cysteine substitutions within EGF domains 
that are predicted to cause misfolding by removal of disulphide bonds that 
stabilize the native domain fold. We have studied three missense mutations 
(C1117Y, C1129Y and G1127S) to investigate the effect of misfolding on the 
trafficking of fibrillin-1 from fibroblast cells. We demonstrate that both 
C1117Y and C1129Y, expressed as recombinant fragments of fibrillin-1, are 
retained and accumulate within the cell. Both undergo core glycosylation but 
lack the complex glycosylation observed in the secreted wild-type fragment, 
suggesting retention in the endoplasmic reticulum (ER). In addition, 
co-immunoprecipitation experiments show association with the ER chaperone 
calreticulin, but not calnexin, 78 kDa glucose-regulated protein (Grp78/BiP) or 
protein disulfide isomerase. In contrast, G1127S, which causes a moderate change 
in the EGF domain fold, shows a pattern of glycosylation and trafficking profile 
indistinguishable from the wild-type fragment. Since expression of the 
recombinant fragments does not disrupt the secretion of endogenous fibrillin-1 
by the cell, we propose that G1127S causes disease via an extracellular dominant 
negative effect. In contrast, the observed ER retention of C1117Y and C1129Y 
suggests that disease associated with these missense mutations is caused either 
by an intracellular dominant negative effect or haploinsufficiency.