Concepts in collagen biochemistry: evidence that collagenopathies underlie osteogenesis imperfecta.

Collagen, the principal structural macromolecule of the body, is expressed as a family of genetically distinct protein molecules. Two main subfamilies are emerging, the interstitial, fibrillar collagens (Types I, II and III) and the cell-associated or basement membrane collagens (Types IV and V). All types are synthesized as precursor molecules of procollagen, which are trimmed by specific proteases to form the subunit of extracellular fibrils. Evidence is accumulating that the osteogenesis imperfecta group of diseases are inherited molecular pathologies of Type I collagen, the commonest collagen type. However, a variety of biochemical lesions are anticipated, in part because of the marked clinical heterogeneity of the disorder. For example, in one variant a defect is becoming evident in the primary structure of Type I procollagen which seems to inhibit its rate of secretion from the cell. In another, there is evidence of complete failure to express the collagen alpha 2 chain, a disorder perhaps analogous to the thalassemia syndromes of hemoglobin pathology.