GLY85 TO VAL SUBSTITUTION IN PRO-ALPHA-1(I) CHAIN CAUSES MILD OSTEOGENESIS IMPERFECTA AND INTRODUCES A SUSCEPTIBILITY TO PROTEASE DIGESTION

In this paper we describe a mild moderate form of osteogenesis imperfe cta caused by a point mutation in COL1A1 which converted glycine 85 to valine. The valine substitution introduced into the triple-helical do main of type-I collagen a conformational perturbation causing suscepti bility to digestive proteases. In fact, SDS/PAGE of pepsin-treated col lagen showed the presence of a faint band, migrating between alpha1(I) and alpha2(I), both in the medium and in the cell layer. On trypsin d igestion the band, a shortened form of alpha1(I), had a melting temper ature of 39.5-degrees-C. If the triple-helical collagen was obtained a fter trypsin or chymotrypsin digestion of procollagen, two shortened b ands were identified; the enzymes cleaved about 40% of the trimers. Th e mutant procollagen was normally secreted and processed in the extrac ellular matrix at a normal rate. When native type-I collagen was forme d after dextran-sulfate incubation, only chains of normal length were found, suggesting that the fibroblast proteases did not recognize the alteration introduced by the mutation. The effects of glycine 85 to va line substitution are compared with those produced by a previously des cribed arginine substitution of the same residue (Deak et al., 1991).