[Contribution of genetics to pathogenicity and diagnosis of Marfan syndrome]

The anatomical substrate of Marfan's syndrome is a degeneration of elastic fibres and disorganization of the collagen. It is now known that these lesions are due to mutation of genes localised on chromosome 15. The first of them (FBN1) codes for the main constitutive protein of the elastic tissue: fibrillin 1, present mainly in structures which must resist load and stress (aortic adventitia, the suspending ligament of the lens, skin); the second (FBN2) codes for fibrillin 2: responsible for the orientation of the elastin and mainly present in cartilage, the aortic media, the bronchi, and all tissues rich in elastin. Mutations of FBN1 are very common and are associated not only with Marfan's syndrome but also fibrillinopathies: incomplete forms, neonatal forms, ectopic lens, isolated aneurysms of the thoracic aorta. The widespread distribution of fibrillin explains the pleiotropic nature of Marfan's syndrome and its clinical presentation. The variability of interfamilial expression is due to genetic heterogeneity (at least two genes) and alletic differences (different mutations of FBN1 from one family to another), also explaining mild forms due to quantitative reduction in normal fibrillin and severe forms by "negative dominance" where the fibrillin is structurally abnormal because of alteration of the polymerisation mechanism. The biologic diagnosis of fibrillopathy can be made by a protein test analysing fibrillin on a culture of the patient's fibroblast obtained by skin biopsy. At present, molecular diagnosis of the mutation within the FBN1 gene is not feasible as a routine procedure.