FURTHER EVIDENCE THAT THE FAILURE TO CLEAVE THE AMINOPROPEPTIDE OF TYPE-I PROCOLLAGEN IS THE CAUSE OF EHLERS-DANLOS SYNDROME TYPE-VII

Dermal fibroblasts from a Chinese Ehlers-Danlos syndrome type VII pati ent synthesized approximately equal amounts of normal pro-alpha2(I) ch ains of type I procollagen and abnormal ones with electrophoretic mobi lity of pNalpha2(I) chains, in which the amino-propeptide (N-propeptid e) was retained. Reverse-transcriptase PCR analysis of the proband's R NA showed outsplicing of the 54 base exon 6 in half of the pro-alpha2( I) mRNAs. Exon 6 encodes 18 amino acids of the N-telopeptide which con tains the procollagen N-proteinase cleavage site and a cross-link prec ursor lysine. Loss of these sequences would result in failure to cleav e the amino-propeptide of pro-alpha2(I) and the accumulation of pN-alp ha2(I) chains. Nucleotide sequencing analyses of the proband's COL1A2 gene showed the presence of a T to C transition at position + 2 of int ron 6 in one allele and the proband is heterozygous for the defect. Th is mutation which destroyed the consensus GT dinucleotide at the 5' sp lice 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 diamet er and circularity. This mutation in COL1A2 in an EDS VII patient is t he first reported case in the Chinese population and is identical to o ne reported for another EDSVII (Libyan) patient. The occurrence of an identical mutation in two probands of different ethnic origin is direc t evidence that the mutant genotype is the cause of the EDS VII phenot ype. Therefore the major molecular defect for EDS VIIA/B is outsplicin g of exon 6 of either COL1A1 or COL1A2 as a result of single base muta tions in the intron-exon junctions. The consequential failure to remov e the N-propeptides of either pro-alpha1(I) or pro-alpha2(I) is theref ore probably the cause of the EDS VII phenotype. (C) 1994 Wiley-Liss, Inc.