Further evidence that the failure to cleave the aminopropeptide of type I procollagen is the cause of Ehlers-Danlos syndrome type VII

Abstract

Dermal fibroblasts from a Chinese Ehlers-Danlos syndrome type VII patient synthesized approximately equal amounts of normal pro-α2(I) chains of type I procollagen and abnormal ones with electrophoretic mobility of pNα2(I) chains, in which the amino-propeptide (N-propeptide) was retained. Reverse- transcriptase PCR analysis of the proband's RNA showed outsplicing of the 54 base exon 6 in half of the pro-α2(I) mRNAs. Exon 6 encodes 18 amino acids of the N-telopeptide which contains the procollagen N-proteinase cleavage site and a cross-link precursor lysine. Loss of these sequences would result in failure to cleave the amino-propeptide of pro-α2(I) and the accumulation of pN-α2(I) chains. Nucleotide sequencing analyses of the proband's COL1A2 gene showed the presence of a T to C transition at position +2 of intron 6 in one allele and the proband is heterozygous for the defect. This mutation which destroyed the consensus GT dinucleotide at the 5' splice donor site of the intron is responsible for the loss of exon 6 by exon skipping. Electron microscopic analysis of the patient's dermis showed the presence of abnormal collagen I fibrils of irregular diameter and circularity. This mutation in COL1A2 in an EDS VII patient is the first reported case in the Chinese population and is identical to one reported for another EDSVII (Libyan) patient. The occurrence of an identical mutation in two probands of different ethnic origin is direct evidence that the mutant genotype is the cause of the EDS VII phenotype. Therefore the major molecular defect for EDS VIIA/B is outsplicing of exon 6 of either COL1A1 or COL1A2 as a result of single base mutations in the intron-exon junctions. The consequential failure to remove the N-propeptides of either pro-α1(I) or pro-α2(I) is therefore probably the cause of the EDS VII phenotype.