A POINT MUTATION IN AN INTRONIC BRANCH SITE RESULTS IN ABERRANT SPLICING OF COL5A1 AND IN EHLERS-DANLOS SYNDROME TYPE-II IN 2 BRITISH FAMILIES

Ehlers-Danlos syndrome (EDS) is a heterogeneous group of connective-ti ssue disorders characterized by skin fragility, joint laxity, and skel etal deformities. Type V collagen appears to have a causal role in EDS types I and II, which show phenotypic overlap and may sometimes be al lelic. Type V collagen can exist as a heterotrimer, [alpha 1(V)](2)alp ha 2(V), and it both coassembles with and regulates type I collagen-fi bril diameter. Using an intragenic COL5A1 polymorphism, we have demons trated linkage, at zero recombination, to the sane allele in two large British EDS type II families (LOD scores 4.1 and 4.3), Affected membe rs from each family were heterozygous for a point mutation in intron 3 2 (IVS32:T-25G), causing the 45-bp exon 33 to be lost from the mRNA in similar to 60% of transcripts from the mutant gene. This mutation lie s only 2 bp upstream of a highly conserved adenosine in the consensus branch-site sequence, which is required for lariat formation. Although both families shared the same marker allele, we have been unable to i dentify a common genealogy. This is the first description of a mutatio n at the lariat branch site, which plays a pivotal role in the splicin g mechanism, in a collagen gene. Very probably, the resulting in-frame exon skip has a dominant-negative effect due to incorporation of the mutant pro alpha chain into the triple-helical molecule. These finding s further confirm the importance of type V collagen in the causation o f EDS type II, and the novel collagen mutation indicates the importanc e of the lariat branch site in splicing.