Collagen crosslinks and mineral crystallinity in bone of patients with osteogenesis imperfecta

In cortical bone samples from patients with osteogenesis imperfecta (OI), the concentrations of hydroxypyridinium cross-linking amino acids in collagen were measured by reversed-phase HPLC and the x-axis crystallinity of the apatite mineral phase was determined by x-ray diffraction. Bone samples from three patients with type I, nine patients with type III, and eight patients with type IV OI were analyzed and compared with human bone from nine controls. The concentration of the two chemical forms of the mature collagen crosslinking amino acids, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP), and the ratio HP/LP were found to be alike in bone collagen of OI patients and healthy controls. However, the c-axis crystallinity of the apatite was found to be reduced in the type III and IV OI patients compared with controls. Regression analysis of crosslink concentrations and c-axis crystallinity in OI bones did not show any correlation. Therefore, collagen molecules deposited in the extracellular matrix of OI bone appear to fulfill the structural requirements for the action of the enzyme lysyl oxidase, such that a normal concentration of intermolecular crosslinks is formed compared with healthy bone. Consequently, crosslink formation and apatite crystal growth seem to be regulated independently in OI bone.