Altered collagen metabolism in osteogenesis imperfecta fibroblasts: a study on 33 patients with diverse forms

The pattern of collagen metabolism was analysed in fibroblast cultures from patients with diverse forms of osteogenesis imperfecta (OI). Generally, OI fibroblasts show an insufficient collagen synthesis which is most obvious in patients between 2 and 9 years of age during which period control fibroblasts have an elevated collagen synthesis. OI fibroblasts remain on a basal level except for fibroblasts from OI type IV patients which seem to approach normal levels. In addition, OI fibroblasts generally show a slightly increased degradation of newly synthesized collagen which again is most obvious between 2 and 9 years. These differences in collagen degradation, however, only contribute to a minor extent to the lack of net collagen synthesis during early childhood. No correlation could be found between the degree of overmodification of collagen and its degradation since fibroblasts of both OI type I and OI type II have an elevated degradation though only the latter ones produce overmodified collagen molecules. Pulse labelling of collagen with radioactivity labelled sugars was used to distinguish between normal collagen chains or CNBr-derived peptides and those which were overmodified. In all three cases studied (OI II, OI III, OI IV) the entire triple helical domain of alpha 1(I) and alpha 2(I) was overglycosylated. The amount of overmodification, however, was not uniform but rather unique for each patient studied. We assume that the molecular defects in the majority of OI cases may be located in the mechanisms operating on the control of both the age appropriate synthesis of collagen and its degree of post-translational modification.