Collagen mutations result in Osteogenesis imperfecta

Collagen is the most abundant protein in mammals and forms the main component of connective tissue. Over $28$‍ types of collagen have been identified and described in humans. However, type I collagen constitutes over $90\mathrm{\%}$‍ of collagen. Type I collagen is found in bone, tendon, ligament, and skin. This protein is composed of three polypeptide chains, which wind around one another into a triple-helix stabilized by covalent cross-linkages. Collagen is distinctive in that its subunits are composed of a regular arrangement of Gly-Pro-X, where X can be any amino acid. Each amino acid has a precise role, as the glycine side chain is the only one that is small enough to fit into the center of the triple helix. The angle of the proline residue enables the polypeptide chains to fold into the triple helical structure.

Osteogenesis imperfecta, which affects $6$‍ to $7$‍ per $100,000$‍ people worldwide, typically results from genetic mutations in the collagen genes. The most common form of osteogenesis imperfecta, type I, arises from a dominant mutation in the COL1A1 gene, which results in a deficiency in collagen production. Other types of osteogenesis imperfecta, including types II, III, and IV are caused by dominant mutations in the COL1A1 or COL1A2 genes that alter the structure of type I collagen. In all cases, people with osteogenesis imperfecta suffer from bones that fracture easily.

Where does the mutation that causes osteogenesis imperfecta first occur?