Aortic Wall Mechanics and Composition in a Transgenic Mouse Model of Marfan Syndrome

In Marfan syndrome, mutations of the fibrillin gene (FBN1) lead to aneurysm of the thoracic aorta, making the aortic wall more susceptible to dissection, but the precise sequence of events underlying aneurysm formation is unknown. We used a rodent model of Marfan syndrome, the mgR/mgR mouse (with mgR: hypomorphic FBN1 mutation), which underexpresses FBN1, to distinguish between a defect in the early formation of elastic fibers and the later disruption of elastic fibers. The content of desmosine plus isodesmosine was used as an index of early elastogenesis; disruption of elastic fibers was analyzed by histomorphometry. Because disruption of the medial elastic fibers may produce aortic stiffening, so amplifying the aneurysmal process, we measured thoracoabdominal pulse wave velocity as an indicator of aortic wall stiffness. Both mgR/mgR and wild-type (C57BL/6J-129SV) strains were normotensive, and wall stress was not significantly modified because the increase in internal diameter (0.80±0.06 vs 0.63±0.03 mm in wild type, P <0.05) was accompanied by increased medial cross-sectional area. The aortic wall stiffened (4-fold increase in the elastic modulus-to-wall stress ratio). Desmosine content was not modified (mgR/mgR 432±31 vs wild type 492±42 &mgr;g/mg wet weight, P >0.05). Elastic fibers showed severe fragmentation: the percentage of the media occupied by elastic fibers was 18±3% in mgR/mgR mice vs 30±1% in wild-type mice, with the number of elastic segments being 1.9±0.2 vs 1.4±0.1×10−6/mm2 in the wild type (both P <0.05). In conclusion, underexpression of FBN1 in mice leads to severe elastic network fragmentation but no change in cross-linking, together with aortic dilatation. This result suggests that fragmentation of the medial elastic network and not a defect in early elastogenesis is 1 of the determinants of aortic dilatation in Marfan syndrome.