A RARE BRANCH-POINT MUTATION IS ASSOCIATED WITH MISSPLICING OF FIBRILLIN-2 IN A LARGE FAMILY WITH CONGENITAL CONTRACTURAL ARACHNODACTYLY

Congenital contractural arachnodactyly (CCA) is an autosomal dominant disorder that is phenotypically similar to but genetically distinct fr om Marfan syndrome. Genetic-linkage analysis has implicated the fibril lin-2 gene (FBN2) as the CCA locus. Mutation analysis of two isolated CCA patients revealed missense mutations, indicating that defects in F BN2 may be responsible for this disorder. However, cosegregation of a mutant allele with the disease phenotype has not yet been established. We have investigated the primary cause of CCA in a large well-charact erized kindred with five generations comprising 18 affected individual s. Previous studies demonstrated linkage of this family's CCA phenotyp e to FBN2. Mutation analysis of cDNA derived from the proband and her affected brother, using a nonisotopic RNase cleavage assay, revealed t he partial skipping of exon 31. Approximately 25% mutant transcript is produced, which is apparently sufficient to cause a CCA phenotype. Se quence analysis of genomic DNA revealed an unusual base composition fo r intron 30 and identified the mutation, a g-26t transversion, in the vicinity of the splicing branch-point site in intron 30. Genomic DNA f rom 30 additional family members, both affected and unaffected, then w as analyzed for the mutation. The results clearly demonstrate cosegreg ation of the branchpoint mutation with the CCA phenotype. This is the first report of a CCA mutation in a multiplex family, unequivocally es tablishing that mutations in FBN2 are responsible for the CCA phenotyp e. In addition, branchpoint mutations only very rarely have been assoc iated with human disease, suggesting that the unusual composition of t his intron influences splicing stability.