In Vivo Studies of Mutant Fibrillin-1 Microfibrils

Abstract

In humans, mutations in fibrillin-1 result in a variety of genetic disorders with distinct clinical phenotypes. While most of the known mutations in fibrillin-1 cause Marfan syndrome, a number of other mutations lead to clinical features unrelated to Marfan syndrome. Pathogenesis of Marfan syndrome is currently thought to be driven by mechanisms due to haploinsufficiency of wild-type fibrillin-1. However, haploinsufficiencydriven mechanisms cannot explain the distinct phenotypes found in other fibrillinopathies. To test the hypothesis that mutations in fibrillin-1 cause disorders through primary effects on microfibril structure, two different mutations were generated in Fbn1 in mice. One mutation leads to a truncated fibrillin-1 molecule that is tagged with green fluorescent protein, allowing visualization of mutant fibrillin-1 incorporated into microfibrils. In heterozygosity, these mutant mice demonstrate progressive fragmentation of the aortic elastic lamellae and also display fragmentation of microfibrils in other tissues. Fibrillin-2 epitopes are also progressively revealed in these mice, suggesting that fibrillin-2 immunoreactivity can serve as a marker for microfibril degradation. In contrast, a second mutation (inframe deletion of the first hybrid domain) in fibrillin-1 results in stable microfibrils, demonstrating that fibrillin-1 molecules are not required to be in perfect register for microfibril structure and function and that the first hybrid domain is dispensable for microfibril assembly. Taken together, these results suggest that perturbation of microfibril structure may underlie one of the major features of the Marfan syndrome: fragmentation of aortic elastic lamellae.

TGF-β and the TGF-β Family: Context-Dependent Roles in Cell and Tissue Physiology

“…The absence of fibrillin-1 in Fbn1-deficient mice results in excessive TGF-b activity in vivo, which could be at the basis of the pathogenesis of emphysema and other manifestations of Marfan syndrome (Neptune et al 2003). Additionally, mutant fibrillin-1 disturbs the microfibril structure (Judge et al 2004;Charbonneau et al 2010) and activates TGF-b signaling, which may then over time generate the pathological features of Marfan syndrome.…”

Homotypic Versican G1 Domain Interactions Enhance Hyaluronan Incorporation into Fibrillin Microfibrils

“…The molecular organization and, presumably, the function of microfibrils differ between tissues, depending on the age of the tissue and on the predominant fibrillin subtype (11,12). Because the localization of versican, as determined by light and electron microscopy, is not completely identical to that of fibrillin microfibrils, versican may impart tissue-specific functions to microfibrils (8,13).…”

Skeletogenic phenotype of human Marfan embryonic stem cells faithfully phenocopied by patient-specific induced-pluripotent stem cells

“…Although the molecular pathogenesis of MFS was initially attributed to a structural weakness of the fibrillin-rich microfibrils within the ECM, more recent results have documented that many of the pathogenic abnormalities in MFS are the result of alterations in TGF-β signaling (18,19). Mutations in other genes have been reported to cause MFS-related disorders, such as TGF-β receptor-I and -II in MFS type 2 and Loeys-Dietz syndrome, and myosin heavy chain (MYH)11 and actin/alpha2 smooth muscle/aorta (ACTA2) in familial thoracic aortianeurysms and dissections (21,22).To date, by necessity most knowledge of MFS has been obtained by extrapolation of studies in the mouse Fbn1 null/ transgenic models (2,(23)(24)(25)(26)(27). However, with the derivation of human embryonic stem cells carrying a common FBN1 mutation, as well as human induced pluripotent-stem (iPS) cells from MFS patients, we now have a unique opportunity to examine key features of this syndrome on a human genome background.…”