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Collagen Abnormalities in Osteogenesis Imperfecta

A N U M B E R of previous reports have suggested that the collagen of connective tissues is abnormal in osteogenesis imperfecta. Examples are an increased proportion of hydroxy proline and hydrolysine in collagen in bone and teeth, a decreased amount of collagen in skin, or a decrease in the amount of matrix collagen with an increased amount as woven bone or 'reticulin' in skin. All these are rather non-specific but a little more specific is the evidence from studies with fibroblast cultures that decreased amounts of type 1 collagen was formed in two patients with osteogenesis imperfecta. Among the patients studied in Oxford, there has been little evidence of abnormality in plasma levels or urinary excretion of hydroxyproline. Interpretation of results in this field is difficult because the patients are often very small for their age. Studies of biopsy samples of skin have suggested that the amount of collagen may be reduced in patients with the 'mild' disorder but there was a considerable overlap in the results between the normal and osteogenesis imperfecta samples. The amount of collagen was generally normal in severe cases with white sclerae. Collagen in these samples was studied in greater detail by extracting the soluble collagen and the polymeric collagen which can be precipitated from the 'soluble' collagen fraction by mild alkalis. This fraction includes collagen molecules with covalent cross-links. The polymeric collagen has the usual banded structure by electron microscopy. The amount of true soluble collagen is not abnormal in osteogenesis imperfecta. However the cohesiveness (as assessed by electron microscopic appearances) of polymeric collagen appeared to be abnormal in homocystinuria and in some patients with the severe disease