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. One‐third 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 fibril–matrix 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. Hum Mutat 28(3), 209–221, 2007. Published 2006 Wiley‐Liss, Inc.

Classic osteogenesis imperfecta, an autosomal dominant disorder associated with osteoporosis and bone fragility, is caused by mutations in the genes for type I collagen. A recessive form of the disorder has long been suspected. Since the loss of cartilage-associated protein (CRTAP), which is required for post-translational prolyl 3-hydroxylation of collagen, causes severe osteoporosis in mice, we investigated whether CRTAP deficiency is associated with recessive osteogenesis imperfecta. Three of 10 children with lethal or severe osteogenesis imperfecta, who did not have a primary collagen defect yet had excess post-translational modification of collagen, were found to have a recessive condition resulting in CRTAP deficiency, suggesting that prolyl 3-hydroxylation of type I collagen is important for bone formation.

In many CNS pathways, target innervation occurs by axon branching rather than extension of the primary growth cone into targets. To investigate mechanisms of branch formation, we studied the effects of attractive and inhibitory guidance cues on cortical axon branching. We found that netrin-1, which attracts cortical axons, and FGF-2 increased branching by >50%, whereas semaphorin 3A (Sema3A), which repels cortical axons, inhibited branching by 50%. Importantly, none of the factors affected axon length significantly. The increase in branching by FGF-2 and the inhibition of branching by Sema3A were mediated by opposing effects on the growth cone (expansion vs collapse) and on the cytoskeleton. FGF-2 increased actin polymerization and formation of microtubule loops in growth cones over many hours, whereas Sema3A depolymerized actin filaments, attenuated microtubule dynamics, and collapsed microtubule arrays within minutes. Netrin-1 promoted rapid axon branching, often without involving the growth cone. Branches formedde novoon the axon shaft within 30 min after local application of netrin-1, which induced rapid accumulation of actin filaments in filopodia. Importantly, increased actin polymerization and microtubule dynamics were necessary for axon branching to occur. Taken together, these results show that guidance factors influence the organization and dynamics of the cytoskeleton at the growth cone and the axon shaft to promote or inhibit axon branching. Independent of axon outgrowth, axon branching in response to guidance cues can occur over different time courses by different cellular mechanisms.

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 two-dimensional 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.