Marfan Syndrome Mutations Predominantly Alter Fibrillin Domain Folding

Abstract

Fibrillin is a 350kDa calcium-binding glycoprotein that is vital for the formation of elastic and non-elastic fibres in connective tissue. It is secreted into the extracellular matrix by fibroblasts and becomes incorporated into insoluble microfibrils, which provide a scaffold for deposition of elastin. Fibrillin-1 (FBN1) also interacts with latent transforming growth factor-β binding proteins and controls TGF-β bioavailability Mutations in fibrillin are associated with several different connective tissue diseases, especially Marfan syndrome (MFS). Fibrillin mutations may result in defective microfibrils, but also may cause dysregulation of TGF-β activation and signalling. FBN1 consists of repeating EGF (epidermal growth factor) and TB (transforming growth factor β-binding protein) domains. The majority of EGF domains are paired, are stabilised by multiple disulfides and contain a Ca2+ binding consensus sequence (cbEGF), which confers structural rigidity to the fragment, producing a rod-like conformation by structural and dynamic studies of tandem repeats of cbEGF domains. TB domains exist uniquely in the microfibril protein family, locating in extracellular matrix fibrils; the major function is involved in extracellular matrix construction and storage of latent TGF-β. We have examined the locations of over 600 mutations in fibrillin-1 that cause MFS, then correlated and classified the mutations in terms of their structural and functional consequences. We have shown that a large majority of missense mutations alter cbEGF tandem repeat structural rigidity, by either altering disulfide formation, Ca2+ binding or domain interactions. In other cases, storage of latent TGF-β is predicted to be altered due to TB domain structural perturbations.