Genetic Heterogeneity

The clinical variability of MFS is only partly explained by the great number of mutations identified in the FBN1 gene. In effect, we have demonstrated the existence of genetic heterogeneity (i.e., the involvement) in certain cases of MFS of mutations located in another gene named

MFS2 (for MFS type 2) through the study of a very large French family in which affected individuals display an incomplete form of the syndrome: typical skeletal and cardiovascular features as well as involvement of the skin and integument. No ocular manifestations were observed until one of the children in the fourth generation developed ectopia lentis. We showed that fibrillin-1 was normal in several affected family members and excluded linkage between the FBN1 gene and the disease in the family.[20] By exclusion mapping, we located the MFS2 gene on the short arm of chromosome 3.[21] We are now identifying MFS2 through positional cloning. Several families comparable to the French family in that they are not linked to, or do not carry, a mutation in the FBN1 gene have been identified. Other teams, through protein studies, have identified between 7% and 16% of MFS patients with normal fibrillin metabolism.[22,23] The precise determination of this percentage is important for laboratories involved in the diagnosis of MFS because it will give the risk associated with investigation of only the FBN1 gene.

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