Hyperplastic callus formation in osteogenesis imperfecta.

Osteogenesis imperfecta, an inherited disorder of connective tissues, affects roughly (OI) 4000 people in Germany (11). The main clinical symptoms are fragile bones, progressing skeletal deformities, generalized osteoporosis and short stature. Incidentally, the clinical manifestations can range from perinatal lethal forms to phenotypical normal adults. In many instances the underlying causes of the disease are mutations in gene coding for collagen I, the predominant protein in most connective tissues. Fracture healing is usually not impaired, although in a unique group of OI-patients, a tumor-like hyperplastic callus occurs with excessive deposition of extracellular matrix constituents. Biochemical analysis of the callus is reminiscent of bone from early stages of human development and normal fracture healing (e.g. collagen type composition, degree of posttranslational modification). This underlines that, besides collagen mutations, the regulation of collagen synthesis and their posttranslational processing might be disturbed in patients with hyperplastic callus formation.