Prolyl hydroxylation is a critical posttranslational modification that affects structure, function, and turnover of target proteins. Prolyl 3-hydroxylation occurs at only one position in the triple-helical domain of fibrillar collagen chains, and its biological significance is unknown. CRTAP shares homology with a family of putative prolyl 3-hydroxylases (P3Hs), but it does not contain their common dioxygenase domain. Loss of Crtap in mice causes an osteochondrodysplasia characterized by severe osteoporosis and decreased osteoid production. CRTAP can form a complex with P3H1 and cyclophilin B (CYPB), and Crtap-/- bone and cartilage collagens show decreased prolyl 3-hydroxylation. Moreover, mutant collagen shows evidence of overmodification, and collagen fibrils in mutant skin have increased diameter consistent with altered fibrillogenesis. In humans, CRTAP mutations are associated with the clinical spectrum of recessive osteogenesis imperfecta, including the type II and VII forms. Hence, dysregulation of prolyl 3-hydroxylation is a mechanism for connective tissue disease.

Osteogenesis Imperfecta (OI) is a heritable disorder of connective tissue characterized by brittle bones, fractures and extraskeletal manifestations1. How structural mutations of type I collagen (dominant OI) or of its post-translational modification machinery (recessive OI) can cause abnormal quality and quantity of bone is poorly understood. Notably, the clinical overlap between dominant and recessive forms of OI suggests common molecular pathomechanisms2. Here, we show that excessive transforming growth factor-beta (TGFβ) signaling is a mechanism of OI in both recessive (Crtap−/−) and dominant (Col1a2tm1.1Mcbr) OI mouse models. In the skeleton, we find higher expression of TGFβ target genes, ratio of pSmad2/Smad2 protein, and in vivo Smad2 reporter activity. Anti-TGFβ treatment using the neutralizing antibody 1D11 corrects the bone phenotype in both forms of OI, and improves the lung abnormalities in Crtap−/− mice. Moreover, type I collagen of Crtap−/− mice shows reduced binding to the small leucine rich proteoglycan decorin, a known regulator of TGFβ activity3–4. Hence, altered TGFβ matrix-cell signaling is a primary mechanism in the pathogenesis of OI, and could be a promising target for the treatment of OI.

Osteogenesis imperfecta (OI) is characterized by bone fragility and fractures that may be accompanied by bone deformity, dentinogenesis imperfecta, short stature, and shortened life span. About 90% of individuals with OI have dominant mutations in the type I collagen genes COL1A1 and COL1A2. Recessive forms of OI resulting from mutations in collagen-modifying enzymes and chaperones CRTAP, LEPRE1, PPIB, and FKBP10 have recently been identified. We have identified an autosomal-recessive missense mutation (c.233T>C, p.Leu78Pro) in SERPINH1, which encodes the collagen chaperone-like protein HSP47, that leads to a severe OI phenotype. The mutation results in degradation of the endoplasmic reticulum resident HSP47 via the proteasome. Type I procollagen accumulates in the Golgi of fibroblasts from the affected individual and a population of the secreted type I procollagen is protease sensitive. These findings suggest that HSP47 monitors the integrity of the triple helix of type I procollagen at the ER/cis-Golgi boundary and, when absent, the rate of transit from the ER to the Golgi is increased and helical structure is compromised. The normal 3-hydroxylation of the prolyl residue at position 986 of the triple helical domain of proalpha1(I) chains places the role of HSP47 downstream from the CRTAP/P3H1/CyPB complex that is involved in prolyl 3-hydroxylation. Identification of this mutation in SERPINH1 gives further insight into critical steps of the collagen biosynthetic pathway and the molecular pathogenesis of OI.

Collagen triple helices are stabilized by 4-hydroxyproline residues. No function is known for the much less common 3-hydroxyproline (3Hyp), although genetic defects inhibiting its formation cause recessive osteogenesis imperfecta. To help understand the pathogenesis, we used mass spectrometry to identify the sites and local sequence motifs of 3Hyp residues in fibril-forming collagens from normal human and bovine tissues. The results confirm a single, essentially fully occupied 3Hyp site (A1) at Pro 986 in A-clade chains ␣1(I), ␣1(II), and ␣2(V). Two partially modified sites (A2 and A3) were found at Pro 944 in ␣1(II) and ␣2(V) and Pro 707 in ␣2(I) and ␣2(V), which differed from A1 in sequence motif. Significantly, the distance between sites 2 and 3, 237 residues, is close to the collagen D-period (234 residues). A search for additional D-periodic 3Hyp sites revealed a fourth site (A4) at Pro 470 in ␣2(V), 237 residues N-terminal to site 3. In contrast, human and bovine type III collagen contained no 3Hyp at any site, despite a candidate proline residue and recognizable A1 sequence motif. A conserved histidine in mammalian ␣1(III) at A1 may have prevented 3-hydroxylation because this site in chicken type III was fully hydroxylated, and tyrosine replaced histidine. All three B-clade type V/XI collagen chains revealed the same three sites of 3Hyp but at different loci and sequence contexts from those in A-clade collagen chains. Two of these B-clade sites were spaced apart by 231 residues. From these and other observations we propose a fundamental role for 3Hyp residues in the ordered self-assembly of collagen supramolecular structures.Collagens are the most abundant and ubiquitous proteins in multi-cellular animals. It is well established that 4-hydroxyproline (4Hyp) 2 residues stabilize the collagen triple helix through water-bridged intramolecular hydrogen bonding (1). However, the function of the much less abundant 3-hydroxyproline (3Hyp), although discovered 50 years ago, is unknown (2). Only 1-2 residues of 3Hyp occur per chain in collagen types I and II and 3-6 residues occur per chain in collagen types V and XI. The content is highest in type IV collagens of basement membranes in which 10% of the total hydroxyproline can be 3Hyp (3).Specific prolyl 3-hydroxylases (P3Hs) are responsible for 3Hyp synthesis. Three different genes encoding P3H1, P3H2, and P3H3 are present in the human genome, which show tissue specificity in their expression (4, 5). Substrate proline residues occur in a prerequisite sequence -Pro-4Hyp-Gly. The ␣1(I) chain has only one established 3Hyp site at Pro 986 in a motif conserved across vertebrate species (human GLPGPIGPPGPR) a close variant of which also occurs in type II collagen (human GIPGPIGPPGPR).Renewed interest in 3Hyp was recently sparked by the discovery that a recessive form of osteogenesis imperfecta (OI) is caused by mutations in CRTAP. This gene encodes a protein (cartilage-associated protein) that is bound to P3H1 and cyclophilin B in the endoplasmic reticulum and is requi...