COL5A1 Exon 14 Splice Acceptor Mutation Causes a Functional Null Allele, Haploinsufficiency of α1 (V) and Abnormal Heterotypic Interstitial Fibrils in Ehlers-Danlos Syndrome II

Abstract

We studied four affected individuals from a family of three generations with Ehlers-Danlos Syndrome II. Type V collagen transcripts of affected individuals were screened by reverse transcriptase-polymerase chain reaction. Amplification of the exon 9-28 region of α1(V) yielded normal and larger products from the proband. Sequencing of cDNA revealed a 100-base pair insertion from the 3′-end of intron 13 between exons 13 and 14 in one allele. The genomic defect was identified as an A-2→G substitution at the exon 14 splice acceptor site. A cryptic acceptor site - 100 nucleotide within intron 13 is used instead of the mutant splice site. The insertion shifts the reading frame +1 and results in a stop codon within exon 17. The mutant transcript was much less abundant than normal allele product in untreated cultured fibroblasts but was approximately equimolar in cycloheximide-treated cells, suggesting that the mutation causes nonsense-mediated decay of mRNA. By RNase protection experiments, the level of mutant transcript was determined to be 8% that of the normal transcript in untreated proband fibroblasts. Relative to type I collagen, proband fibroblasts secreted only 65% of the amount of type V collagen secreted by normal controls. Selective salt precipitation of proband secreted collagen provided supportive evidence that the α chain composition of type V collagen remains α1(V)2α2(V) even in the context of α1(V) haploinsufficiency. Type V collagen incorporates into type I collagen fibrils in the extracellular matrix and is thought to regulate fibril diameter. Transmission electron micrographs of type I collagen fibrils in a proband dermal biopsy showed greater heterogeneity in fibril diameter than in a matched control. The proband had a greater proportion of both larger and smaller fibrils and occasional fibrils with a cauliflower configuration. Unlike the genotype/phenotype relationship seen for type I collagen defects and osteogenesis imperfecta, the null allele in this family appears to cause clinical features similar to those seen in cases with structural alterations in type V collagen.