SEVERE (TYPE-III) OSTEOGENESIS IMPERFECTA DUE TO GLYCINE SUBSTITUTIONS IN THE CENTRAL DOMAIN OF THE COLLAGEN TRIPLE-HELIX

The molecular defects responsible for three cases of severe (type III) osteogenesis imperfecta (O1) were investigated. The mutation sites we re localized in pro alpha 1(I) and pro alpha 2(I) mRNA molecules, resp ectively, by chemical cleavage of mismatch in heteroduplex nucleic aci ds. Mutation identification was achieved by reverse transcription- pol ymerase chain reaction-DNA amplification, followed by cloning and sequ encing. Two unrelated patients were demonstrated to bear the same G-A transition at nucleotide 2418 of the pro alpha 1(1) coding region, lea ding to G589S substitution and resulting in very similar clinical mani festations. In the latter patient, a G-T transversion at nucleotide 21 66 was found in one pro alpha 2(I) allele, which caused a G586V substi tution and again severe O1. Presumably all three mutations occurred de novo in the probands, since they were not found in their parents' DNA , The biochemical findings on type I collagen were very similar in all the probands: the mutations here described had little destabilizing e ffects on triple helix formation, secretion and stability. The half-li fe of the collagen incorporated into the insoluble matrix was comparab le with that of controls, These mutations are localized in the gap zon e of the fibrils where mineral nucleation occurs. This fact suggests t hat they probably do not exert destabilizing effects on the individual collagen molecules, but rather on the mineralization process, once th e defective molecules are incorporated into the fibrils, hence causing severe phenotypes.