ABNORMAL TYPE-III COLLAGEN PRODUCED BY AN EXON-17-SKIPPING MUTATION OF THE COL3A1 GENE IN EHLERS-DANLOS SYNDROME TYPE-IV IS NOT INCORPORATED INTO THE EXTRACELLULAR-MATRIX

A novel heterozygous mutation of the COL3Al gene that encodes the alpha 1(III) chains of type III collagen was identified in a family with the: acrogeric form of Ehlers-Danlos syndrome type IV (EDS-IV). Cultured dermal fibroblasts produced normal and shortened alpha 1(III) chains. The triple helix of the latter chain was shortened owing to a 33 amino acid deletion of Gly-184 to Pro-216. The corresponding region of cDNA lacked 99 base pairs from nucleotides 1051 to 1149. The deletions corresponded exactly to the normal sequence encoded by exon 17 of the COL3Al gene. The proband was heterozygous for a T to G transversion at position +2 of intron 17, which resulted in skipping of exon 17, The splicing defect was not corrected by growing the fibroblasts at 33 degrees C and no other splicing variants were identified at 33 or 37 degrees C, The affected brother had the same mutation but his unaffected mother did not. Heterotrimeric type III collagen molecules containing normal and mutant chains were retained within the cell. The mutant homotrimeric molecules were modified and secreted normally and were thermally stable. These normal characteristics of the mutant homotrimers suggested that the loss of ten Gly-Xaa-Yaa triplets (where Gly-Xaa-Yaa is a repetitive amino acid triplet structure in which Xaa and Yaa are other amino acids, proline and hydroxyproline being more common in the Yaa position) did not adversely affect the formation and stability of the triple helix or the structural requirements for secretion. However, the mutant homotrimers were not incorporated into the extracellular matrix of an in vitro model of EDS-IV dermis. The EDS-IV phenotype in this family was probably due to a deficiency in the amount of normal type III collagen available for formation of the heterotypic collagen fibrils of the extracellular matrix. Intracellular and extracellular quality-control mechanisms prevented the incorporation of heterotrimeric and homotrimeric mutant type III collagen molecules into the cross-linked extracellular matrix.