Osteogenesis imperfecta (OI) is a generalized disorder of connective tissue characterized by fragile bones and easy susceptibility to fracture. Most cases of OI are caused by mutations in type I collagen. We have identified and assembled structural mutations in type I collagen genes (COL1A1 and COL1A2, encoding the proα1(I) and proα2(I) chains, respectively) that result in OI. Quantitative defects causing type I OI were not included. Of these 832 independent mutations, 682 result in substitution for glycine residues in the triple helical domain of the encoded protein and 150 alter splice sites. Distinct genotype-phenotype relationships emerge for each chain. Onethird of the mutations that result in glycine substitutions in α1(I) are lethal, especially when the substituting residues are charged or have a branched side chain. Substitutions in the first 200 residues are nonlethal and have variable outcome thereafter, unrelated to folding or helix stability domains. Two exclusively lethal regions (helix positions 691-823 and 910-964) align with major ligand binding regions (MLBRs), suggesting crucial interactions of collagen monomers or fibrils with integrins, matrix metalloproteinases (MMPs), fibronectin, and cartilage oligomeric matrix protein (COMP). Mutations in COL1A2 are predominantly nonlethal (80%). Lethal substitutions are located in eight regularly spaced clusters along the chain, supporting a regional model. The lethal regions align with proteoglycan binding sites along the fibril, suggesting a role in fibrilmatrix interactions. Recurrences at the same site in α2(I) are generally concordant for outcome, unlike α1(I). Splice site mutations comprise 20% of helical mutations identified in OI patients, and may lead to exon skipping, intron inclusion, or the activation of cryptic splice sites. Splice site mutations in COL1A1 are rarely lethal; they often lead to frameshifts and the mild type I phenotype. In α2(I), lethal exon skipping events are located in the carboxyl half of the chain. Our data on genotype-phenotype relationships indicate that the two collagen chains play very different roles in matrix integrity and that phenotype depends on intracellular and extracellular events.

A new paradigm has emerged for osteogenesis imperfecta (OI) as a collagen-related disorder. The more prevalent autosomal dominant forms of OI are caused by primary defects in type I collagen, while autosomal recessive forms are caused by deficiency of proteins which interact with type I procollagen for post-translational modification and/or folding. Factors contributing to the mechanism of dominant OI include intracellular stress, disruption of interactions between collagen and non-collagenous proteins, compromised matrix structure, abnormal cell-cell and cell-matrix interactions and tissue mineralization. Recessive OI is caused by deficiency of any of the three components of the collagen prolyl 3-hydroxylation complex; absence of 3-hydroxylation is associated with increased modification of the collagen helix, supporting delayed collagen folding. Other causes of recessive OI include deficiency of collagen chaperones, FKBP65 or HSP47. Murine models are crucial to uncovering the common pathways in dominant and recessive OI bone dysplasia. Clinical management of OI is multidiscipinary, encompassing substantial progress in physical rehabilitation and surgical procedures, managment of hearing, dental and pulmonary abnormalities, as well as drugs such as bisphosphonates and rGH. Novel treatments using cell therapy or new drug regimens hold promise for the future.

A recessive form of severe osteogenesis imperfecta that is not caused by mutations in type I collagen has long been suspected. Mutations in human CRTAP (cartilage-associated protein) causing recessive bone disease have been reported. CRTAP forms a complex with cyclophilin B and prolyl 3-hydroxylase 1, which is encoded by LEPRE1 and hydroxylates one residue in type I collagen, alpha1(I)Pro986. We present the first five cases of a new recessive bone disorder resulting from null LEPRE1 alleles; its phenotype overlaps with lethal/severe osteogenesis imperfecta but has distinctive features. Furthermore, a mutant allele from West Africa, also found in African Americans, occurs in four of five cases. All proband LEPRE1 mutations led to premature termination codons and minimal mRNA and protein. Proband collagen had minimal 3-hydroxylation of alpha1(I)Pro986 but excess lysyl hydroxylation and glycosylation along the collagen helix. Proband collagen secretion was moderately delayed, but total collagen secretion was increased. Prolyl 3-hydroxylase 1 is therefore crucial for bone development and collagen helix formation.

Type I collagen, the predominant protein of vertebrates, polymerizes with type III and V collagens and non-collagenous molecules into large cable-like fibrils, yet how the fibril interacts with cells and other binding partners remains poorly understood. To help reveal insights into the collagen structure-function relationship, a data base was assembled including hundreds of type I collagen ligand binding sites and mutations on a twodimensional model of the fibril. Visual examination of the distribution of functional sites, and statistical analysis of mutation distributions on the fibril suggest it is organized into two domains. The "cell interaction domain" is proposed to regulate dynamic aspects of collagen biology, including integrin-mediated cell interactions and fibril remodeling. The "matrix interaction domain" may assume a structural role, mediating collagen cross-linking, proteoglycan interactions, and tissue mineralization. Molecular modeling was used to superimpose the positions of functional sites and mutations from the two-dimensional fibril map onto a three-dimensional x-ray diffraction structure of the collagen microfibril in situ, indicating the existence of domains in the native fibril. Sequence searches revealed that major fibril domain elements are conserved in type I collagens through evolution and in the type II/XI collagen fibril predominant in cartilage. Moreover, the fibril domain model provides potential insights into the genotype-phenotype relationship for several classes of human connective tissue diseases, mechanisms of integrin clustering by fibrils, the polarity of fibril assembly, heterotypic fibril function, and connective tissue pathology in diabetes and aging.Type I collagen is the most abundant protein in humans and other vertebrates, comprising much of the fibrous extracellular matrix scaffold of bones, tendons, skin, and many other tissues (1-4). In general, type I collagen and its binding partners are proposed to provide mechanical strength and form to tissues. Collagenous scaffolds are laid down and remodeled by cells and are also a predominant substrate for cell interactions, migration, and differentiation. Consequently, various debilitating human diseases are associated with type I collagen mutations, including osteogenesis imperfecta (OI, 2 brittle bone disease), Ehlers Danlos syndrome, vascular disorders, and others (3, 5). Type I collagen is also employed in human medicine as hemostatic sponges and implants to repair wounds and in tissue engineering applications as scaffolds (6).Type I collagen is synthesized in the endoplasmic reticulum as ␣1 and ␣2 procollagen chains, each encoded by separate genes that are translated into proteins somewhat longer than 1000 amino acid residues (3, 7). Nucleation domains on the C-terminal propeptide promote the polymerization of two ␣1 and one ␣2 chains into the procollagen triple helical monomer (Fig. 1, A and B). The triple helical domain of procollagen is composed of contiguous glycine-X-Y tri-peptide repeats, with the obligate glyci...