Ehlers Danlos syndrome type VIIB. Incomplete cleavage of abnormal type I procollagen by N-proteinase in vitro results in the formation of copolymers of collagen and partially cleaved pNcollagen that are near circular in cross-section.

We have shown that a child with Ehlers Danlos syndrome (EDS) type VII has a G to A transition at the first nucleotide of intron 6 in one of her COL1A2 alleles. Half of the cDNA clones prepared from the proband's pro alpha 2(I) mRNA lacked exon 6. The type I procollagen secreted by the proband's dermal fibroblasts in culture was purified, and collagen fibrils were generated in vitro by cleavage of the procollagen with the procollagen N- and C-proteinases. Incubation of the procollagen with N-proteinase resulted in a 1:1 mixture of pCcollagen and uncleaved procollagen. Incubation of this mixture with C-proteinase generated collagen and abnormal pNcollagen (pNcollagen-ex6) that readily copolymerized into fibrils. By electron microscopy these fibrils resembled the hieroglyphic fibrils seen in the N-proteinase-deficient skin of dermatosparactic animals and humans and were distinct from the near circular cross-section fibrils seen in the tissues of individuals with EDS type VII. Further incubation of the hieroglyphic fibrils with N-proteinase resulted in partial cleavage of the pNcollagen-ex6 in which the abnormal pN alpha 2(I) chains remained intact. These fibrils were not hieroglyphic but were near circular in cross-section. Fibrils formed from collagen and pNcollagen-ex6 that had been partially cleaved with elevated amounts of N-proteinase prior to fibril formation were also near circular in cross-section. The results are consistent with a model of collagen fibril formation in which the intact N-propeptides are located exclusively at the surface of the hieroglyphic fibrils. Partial cleavage of the pNcollagen-ex6 by N-proteinase allows the N-propeptides to be incorporated within the body of the fibrils. The model provides an explanation for the morphology and molecular composition of collagen fibrils in the tissues of patients with EDS type VII.