Dominantly Inherited Osteogenesis Imperfecta in Man: An Examination of Collagen Biosynthesis

Abstract

We have examined control subjects and patients in an effor to discover a metabolic basis for dominantly inherited osterogenesis imperfecta (OI). Studies were carried out in vitro with cultured skin fibroblasts obtained from OI patients, and in vivo on peptide-bound hydroxyproline excretion in urine. Urinary hydroxyproline excretion (milligrams/24 hr) adjusted for age is essentially normal in OI patients, although the mean excretion rate is below average. The latter finding is presumably a reflection of the smaller body mass of OI patients. The OI skin fibroblasts, matched for age of donor, site of biopsy, phase of growth, and generation number in culture, incorporated L-proline into hot trichloroacetic acid (TCA)-soluble protein (collagen) at normal rates. The rate of conversion of proline to hydroxyproline in the nascent polypeptides is also normal in OI. Incorporation of L-lysine was also normal in OI. These findings indicate that peptide synthesis of collagen is not impaired in OI. Rates of galactose incorporation into collagen and the extractability of collagen into normal saline or 0.2 M citric acid were all normal both in OI cells and in the culture medium recovered from the monolayer. These findings, in combination with the urinary data on hydroxyproline excretion in vivo reveal that cross-linking and export of collagen in OI is essentially normal. The elution profile after ion exchange chromatography of fibroblast collagen on carboxymethyl (CM)--Sephadex was also examined. The normal 2/1 ratio of peak 1 (largely alpha 1(1) chains) to peak 2 ) largely alpha 2 chains) was found in OI fibroblast extracts, which implies that synthesis and initial aggregation of the two types of polypeptide to yield (alpha1(1))-2 alpha 2 collagen composition is not abnormal in OI. Despite the negative biochemical findings, a consistent defect in the morphology of OI cells was identified in the log phase and the confluent phase of monolayer cultures. The finding is characterized by irregular packing of the aggregated cells and by an irregular tessellated appearance of the individual OI fibroblast. This observation reassures us that the inherited defect is expressed in vitro.