December 4, 2006
In the late 1990s, scientists identified in chick embryos a gene that cartilage producing chondrocyte cells expressed in an abnormal hypertrophic state. They named the gene Cartilage-Associated Protein, or CRTAP. The scientists soon identified the corresponding human CRTAP gene and protein and characterized their expression patterns in various human organs to find its possible biological functions. Then the CRTAP publication trail went cold for a few years. Now, in the October 20 issue of the journal Cell, the trail picks up again in a big way as NIDCR grantees and colleagues report that mutations in the human CRTAP gene can cause different types of osteogenesis imperfecta (OI), also called brittle bone disease. Until now OI has been thought to be due to dominant mutations in type I collagen, the major protein of bone. However, CRTAP mutations cause a recessive form that is characterized by low bone mass, bone fragility, and long-bone deformities. The scientists show that CRTAP complexes with the P3H1 enzyme to make needed post translational modifications to fibrillar forming collagen proteins, key structural components of mineralized and connective tissues. When the CRTAP gene is altered, the process breaks down to varying degrees. According to the authors the specific location and/or type of mutation in the CRTAP gene seem to determine the severity of the condition. “At the milder end of the spectrum, the clinical picture is one of bone fragility and short limbs seen in OI type VII; at the severe end, the phenotype is similar to lethal OI type II,” concluded the authors.