MOLECULAR BIOLOGY OF THE MARFAN SYNDROME

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Marfan syndrome (MFS) is a dominant disorder of connective tissue with a prevalence of about 1 in 5,000. Manifestations involve the ocular, skeletal, cardiovascular and pulmonary systems. Mutations in the gene encoding fibrillin-1 (FBN1), the major constituent of extracellular microfibrils, cause MFS. The prevailing view has been that microfibrils regulate the deposition of tropoelastin in late fetal life. A genetically determined deficiency of microfibrils would result in failed elastogenesis and hence inherent weakness of the aortic wall. Characterization of fibrillin-1 deficient mouse lines that were created by targeted gene disruption revealed intact and extended elastic lamellae in the aortic media despite a severe quantitative and qualitative abnormality of microfibrils. Fragmentation of elastic fibers was preceded by a predictable pathogenetic sequence that included an intense fibroproliferative response of vascular smooth muscle cells (VSMC) and inflammation-mediated recruitment and activation of elastases. We hypothesize that loss of matrix-cell attachments that are normally mediated by microfibrils trigger abnormal cellular differentiation and behavior. Alternatively, this abnormal cellular phenotype is predicted by dysregulation of the multipotential cytokine TGFbeta. The microfibrillar matrix is believed to sequester latent TGFbeta, thus regulating its activation by cell surface-associated factors. A pathologic upregulation of TGFbeta activity is also consistent with our observed primary failure of alveolar branching morphogenesis in Fbn1 (-/-) mice. We propose comprehensive characterization of SMC phenotype through in situ analysis of expressed transcripts, secreted proteins and surface markers. We will scrutinize the role of TGFbeta in vascular and pulmonary disease using a sensitive and specific TGFbeta-responsive reporter transgene and antibodies specific to the active and latent forms. Analysis of the phenotypic consequences of targeted overexpression of TGFbeta in differentiated VSMC and pulmonary myofibroblasts will determine whether this dysregulation is sufficient for disease pathogenesis. We will assess the ability of TGFbeta-neutralizing Ab to rescue lung branching morphogenesis in organ culture. Characterization of a fibrillin-1 deficient line that overexpresses a potent tissue inhibitor of metalloproteinases (TIMP) will determine whether pharmacologic elastase inhibitors hold promise for the modulation of the natural history of MFS. Results will be correlated with those observed in mice heterozygous for a dominant-negative Fbn1 allele, a mechanism that recapitulates the human condition.