Abnormal type I collagen metabolism by cultured fibroblasts in lethal perinatal osteogenesis imperfecta

Abstract

Cultured skin fibroblasts from seven consecutive cases of lethal perinatal osteogenesis imperfecta (OI) expressed defects of type I collagen metabolism. The secretion of [14C]proline-labelled collagen by the OI cells was specifically reduced (51-79% of control), and collagen degradation was increased to twice that of control cells in five cases and increased by approx. 30% in the other two cases. Sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that four of the OI cell lines produced two forms of type I collagen consisting of both normally and slowly migrating forms of the α1(I)- and α2(I)-chains. In the other three OI cell lines only the 'slow' α(I)'- and α2(I)'-chains were detected. In both groups inhibition of the post-translational modifications of proline and lysine resulted in the production of a single species of type I collagen with normal electrophoretic migration. Proline hydroxylation was normal, but the hydroxylysine contents of α1(I)'- and α2(I)'-chains purified by h.p.l.c. were greater than in control α-chains. The glucosylgalactosylhydroxylysine content was increased approx. 3-fold while the galactosylhydroxylysine content was only slightly increased in the α1(I)'-chains relative to control α1(I)-chains. Peptide mapping of the CNBr-cleavage peptides provided evidence that the increased post-translational modifications were distributed throughout the α1(I)'- and α2(I)'-chains. It is postulated that the greater modification of these chains was due to structural defects of the α-chains leading to delayed helix formation. The abnormal charge heterogeneity observed in the α1 CB8 peptide of one patient may reflect such a structural defect in the type I collagen molecule.