Osteogenesis imperfecta type IV: evidence of abnormal triple helical structure of type I collagen

Skin fibroblasts from a patient with mild osteogenesis imperfecta (OI) type IV synthesize two populations of type I procollagen molecules. One population contains pro alpha 1(I) and pro alpha 2(I) chains that migrate normally in sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and a second population contains only slower migrating pro alpha 1(I) and pro alpha 2(I) chains. The total amount of type I procollagen made by OI cells and the ratio of pro alpha 1(I):pro alpha 2(I) is normal. When labeled under conditions that inhibit post-translational modification of pro alpha chains, the OI cells produce only single populations of pro alpha 1(I) and pro alpha 2(I) chains indicating that the apparent increased molecular weight of some OI pro alpha chains is due to excessive post-translational modification rather than peptidyl insertions. Peptide maps indicate that excessive post-translational modification occurs along the entire triple helical segment of some alpha 1(I) and alpha 2(I) chains produced by OI cells. The effect of the mutation is to lower the melting temperature of the molecules containing slow migrating alpha 1(I) and alpha 2(I) chains to 39.5 degrees C (compared to 41.5 degrees C for control), and to delay secretion of the over-modified type I procollagen from OI cells. These data are consistent with a mutation near the carboxyl-terminal end of the triple helical domain which delays triple helical formation and renders all chains available for further post-translational modification amino-terminal to the mutation. Such alterations in triple helical structure, thermal stability, and secretion previously associated only with the lethal OI type II phenotype are thus also seen in the mild OI type IV phenotype.