The COL1A1 gene provides instructions for making part of a large molecule called Collagen Type I. Collagens are a family of proteins that strengthen and support many tissues in the body, including cartilage, bones, tendons, skin, and the white part of the eye (sclera). Type I collagen is the most abundant form of collagen in the human body. A lack of this critical protein is at the root of bone fragility. The gene variants probably affect the production of type I collagen, but not the structure of the molecule. Variants of changes in the gene can cause an imbalance between the expression of COL1A1 and COL1A2 proteins, which leads to instability of collagen fibers. The resulting abnormal type I collagen fibrils weaken the connective tissue, which increases the risk of tendon and ligament injuries.
Variations in the COL1A1 gene may be associated with low bone mineral density, an increased risk of osteoporosis, fractures, and degeneration of the intervertebral discs.