A Mutation in the 5'-UTR of IFITM5 Creates an In-Frame Start Codon and Causes Autosomal-Dominant Osteogenesis Imperfecta Type V with Hyperplastic Callus

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous disorder associated with bone fragility and susceptibility to fractures after minimal trauma. OI type V has an autosomal-dominant pattern of inheritance and is not caused by mutations in the type I collagen genes COL1A1 and COL1A2. The most remarkable and pathognomonic feature, observed in ∼65% of affected individuals, is a predisposition to develop hyperplastic callus after fractures or surgical interventions. To identify the molecular cause of OI type V, we performed whole-exome sequencing in a female with OI type V and her unaffected parents and searched for de novo mutations. We found a heterozygous de novo mutation in the 5′-untranslated region of IFITM5 (the gene encoding Interferon induced transmembrane protein 5), 14 bp upstream of the annotated translation initiation codon (c.−14C>T). Subsequently, we identified an identical heterozygous de novo mutation in a second individual with OI type V by Sanger sequencing, thereby confirming that this is the causal mutation for the phenotype. IFITM5 is a protein that is highly enriched in osteoblasts and has a putative function in bone formation and osteoblast maturation. The mutation c.−14C>T introduces an upstream start codon that is in frame with the reference open-reading frame of IFITM5 and is embedded into a stronger Kozak consensus sequence for translation initiation than the annotated start codon. In vitro, eukaryotic cells were able to recognize this start codon, and they used it instead of the reference translation initiation signal. This suggests that five amino acids (Met-Ala-Leu-Glu-Pro) are added to the N terminus and alter IFITM5 function in individuals with the mutation.     

Phenotype and Genotype Analysis of Chinese Patients with Osteogenesis Imperfecta Type V

Osteogenesis imperfecta (OI) type V is an autosomal-dominant disease characterized by calcification of the forearm interosseous membrane, radial head dislocation, a subphyseal metaphyseal radiodense line, and hyperplastic callus formation. The causative mutation, c.-14C>T in the 5''-untranslated region of IFITM5, was recently discovered to be involved in this disease. However, in spite of the little genotypic variability, considerable phenotypic variability has been recognized in two cohorts of patients, the majority of whom were Caucasians. Using exome sequencing, we identified the same heterozygous mutation in four Chinese families with OI type V. This study confirms the molecular cause of OI type V and describes the phenotype of Chinese patients with this disorder. In conclusion, the phenotype of Chinese patients was generally similar to that of Caucasian patients.     

Osteoblast-enriched membrane protein IFITM5 regulates the association of CD9 with an FKBP11-CD81-FPRP complex and stimulates expression of interferon-induced genes

Osteoblasts are rich in interferon-inducible transmembrane protein 5 (IFITM5), the expression of which peaks around the early mineralization stage. This membrane protein directly associates with FK506 binding protein 11 (FKBP11). To examine the molecular function of IFITM5, we analyzed the protein interaction network around IFITM5–FKBP11. We found that FKBP11 was associated with CD81, which interacts with prostaglandin F2 receptor negative regulator (FPRP) and CD9; cumulatively, these associations result in the formation of a FKBP11–CD81–[FPRP/CD9] complex. However, CD9 dissociated from the complex following expression of Ifitm5, which also led to osteoblast-specific increased expression of 5 interferon-induced genes: bone marrow stromal cell antigen 2 (Bst2), interferon inducible protein 1 (Irgm), interferon-induced protein with tetratricopeptide repeats 3 (Ifit3), b(2)-microglobulin (B2m), and MHC (A.CA/J(H-2K-f) class I antigen gene. Induction of these genes likely resulted from dissociation of CD9 from the FKBP11–CD81–[FPRP/CD9] complex. Cumulatively, these results suggest that IFITM5 is involved not only in bone formation, but also in immune system activity.     

Molecular Consequences of the SERPINH1/HSP47 Mutation in the Dachshund Natural Model of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heritable connective tissue disease characterized by bone fragility and increased risk of fractures. Up to now, mutations in at least 18 genes have been associated with dominant and recessive forms of OI that affect the production or post-translational processing of procollagen or alter bone homeostasis. Among those, SERPINH1 encoding heat shock protein 47 (HSP47), a chaperone exclusive for collagen folding in the ER, was identified to cause a severe form of OI in dachshunds (L326P) as well as in humans (one single case with a L78P mutation). To elucidate the disease mechanism underlying OI in the dog model, we applied a range of biochemical assays to mutant and control skin fibroblasts as well as on bone samples. These experiments revealed that type I collagen synthesized by mutant cells had decreased electrophoretic mobility. Procollagen was retained intracellularly with concomitant dilation of ER cisternae and activation of the ER stress response markers GRP78 and phospho-eIF2α, thus suggesting a defect in procollagen processing. In line with the migration shift detected on SDS-PAGE of cell culture collagen, extracts of bone collagen from the OI dog showed a similar mobility shift, and on tandem mass spectrometry, the chains were post-translationally overmodified. The bone collagen had a higher content of pyridinoline than control dog bone. We conclude that the SERPINH1 mutation in this naturally occurring model of OI impairs how HSP47 acts as a chaperone in the ER. This results in abnormal post-translational modification and cross-linking of the bone collagen.     

Collagen defects in lethal perinatal osteogenesis imperfecta.

Quantitative and qualitative abnormalities of collagen were observed in tissues and fibroblast cultures from 17 consecutive cases of lethal perinatal osteogenesis imperfecta (OI). The content of type I collagen was reduced in OI dermis and bone and the content of type III collagen was also reduced in the dermis. Normal bone contained 99.3% type I and 0.7% type V collagen whereas OI bone contained a lower proportion of type I, a greater proportion of type V and a significant amount of type III collagen. The type III and V collagens appeared to be structurally normal. In contrast, abnormal type I collagen chains, which migrated slowly on electrophoresis, were observed in all babies with OI. Cultured fibroblasts from five babies produced a mixture of normal and abnormal type I collagens; the abnormal collagen was not secreted in two cases and was slowly secreted in the others. Fibroblasts from 12 babies produced only abnormal type I collagens and they were also secreted slowly. The slower electrophoretic migration of the abnormal chains was due to enzymic overmodification of the lysine residues. The distribution of the cyanogen bromide peptides containing the overmodified residues was used to localize the underlying structural abnormalities to three regions of the type I procollagen chains. These regions included the carboxy-propeptide of the pro alpha 1(I)-chain, the helical alpha 1(I) CB7 peptide and the helical alpha 1(I) CB8 and CB3 peptides. In one baby a basic charge mutation was observed in the alpha 1(I) CB7 peptide and in another baby a basic charge mutation was observed in the alpha 1(I) CB8 peptide. The primary defects in lethal perinatal OI appear to reside in the type I collagen chains. Type III and V collagens did not appear to compensate for the deficiency of type I collagen in the tissues.     

Mutations in type I collagen genes resulting in osteogenesis imperfecta in humans

Osteogenesis imperfecta (OI), commonly known as "brittle bone disease", is a dominant autosomal disorder characterized by bone fragility and abnormalities of connective tissue. Biochemical and molecular genetic studies have shown that the vast majority of affected individuals have mutations in either the COL1A1 or COL1A2 genes that encode the chains of type I procollagen. OI is associated with a wide spectrum of phenotypes varying from mild to severe and lethal conditions. The mild forms are usually caused by mutations which inactivate one allele of COL1A1 gene and result in a reduced amount of normal type I collagen, while the severe and lethal forms result from dominant negative mutations in COL1A1 or COL1A2 which produce structural defects in the collagen molecule. The most common mutations are substitutions of glycine residues, which are crucial to formation and function of the collagen triple helix, by larger amino acids. Although type I collagen is the major structural protein of both bone and skin, the mutations in type I collagen genes cause a bone disease. Some reports showed that the mutant collagen can be expressed differently in bone and in skin. Since most mutations identified in OI are dominant negative, the gene therapy requires a fundamentally different approach from that used for genetic-recessive disorders. The antisense therapy, by reducing the expression of mutant genes, is able to change a structural mutation into a null mutation, and thus convert severe forms of the disease into mild OI type I.     

Identification of a novel COL1A1 frameshift mutation, c.700delG,in a Chinese osteogenesis imperfecta family

Osteogenesis imperfecta (OI) is a family of genetic disorders associated with bone loss and fragility. Mutations associated with OI have been found in genes encoding the type I collagen chains. People with OI type I often produce insufficient α1-chain type I collagen because of frameshift, nonsense, or splice site mutations in COL1A1 or COL1A2. This report is of a Chinese daughter and mother who had both experienced two bone fractures. Because skeletal fragility is predominantly inherited, we focused on identifying mutations in COL1A1 and COL1A2 genes. A novel mutation in COL1A1, c.700delG, was detected by genomic DNA sequencing in the mother and daughter, but not in their relatives. The identification of this mutation led to the conclusion that they were affected by mild OI type I. Open reading frame analysis indicated that this frameshift mutation would truncate α1-chain type I collagen at residue p263 (p.E234KfsX264), while the wild-type protein would contain 1,464 residues. The clinical data were consistent with the patients’ diagnosis of mild OI type I caused by haploinsufficiency of α1-chain type I collagen. Combined with previous reports, identification of the novel mutation COL1A1-c.700delG in these patients suggests that additional genetic and environmental factors may influence the severity of OI.     

Six novel missense mutations in the LDL receptor-related protein 5 (LRP5) gene in different conditions with an increased bone density

Bone is a dynamic tissue that is subject to the balanced processes of bone formation and bone resorption. Imbalance can give rise to skeletal pathologies with increased bone density. In recent years, several genes underlying such sclerosing bone disorders have been identified. The LDL receptor-related protein 5 (LRP5) gene has been shown to be involved in both osteoporosis-pseudoglioma syndrome and the high-bone-mass phenotype and turned out to be an important regulator of peak bone mass in vertebrates. We performed mutation analysis of the LRP5 gene in 10 families or isolated patients with different conditions with an increased bone density, including endosteal hyperostosis, Van Buchem disease, autosomal dominant osteosclerosis, and osteopetrosis type I. Direct sequencing of the LRP5 gene revealed 19 sequence variants. Thirteen of these were confirmed as polymorphisms, but six novel missense mutations (D111Y, G171R, A214T, A214V, A242T, and T253I) are most likely disease causing. Like the previously reported mutation (G171V) that causes the high-bone-mass phenotype, all mutations are located in the aminoterminal part of the gene, before the first epidermal growth factor-like domain. These results indicate that, despite the different diagnoses that can be made, conditions with an increased bone density affecting mainly the cortices of the long bones and the skull are often caused by mutations in the LRP5 gene. Functional analysis of the effects of the various mutations will be of interest, to evaluate whether all the mutations give rise to the same pathogenic mechanism.     

In vitro expression of osteoblastic markers in cells isolated from normal fetal and postnatal human bone and from bone of patients with osteogenesis imperfecta

We studied the expression of osteoblastic markers in cultured cells isolated from the bone of 15 patients with different clinical forms of osteogenesis imperfecta (OI) and of seven fetal and postnatal controls. Cultured bone cells of ten OI patients produced abnormal collagen type I. Similar to controls, OI bone cells produced predominantly collagen type I with traces of collagen types III and V. The 1,25(OH)₂ vitamin D₃-stimulated synthesis of osteocalcin, a specific osteoblastic marker protein, was similar in OI bone cells and age-matched controls. Bone cells from fetal controls and from patients with the perinatal lethal OI type II produced less osteocalcin than bone cells from postnatal controls and surviving OI patients. OI bone cells responded to parath.yroid hormone (PTH) by increased production of cAMP similar to controls. Bone cells from fetal controls and from OI type II donors showed a decreased response to PTH. Activity of the bone-liver-kidney isoenzyme alkaline phosphatase (AP) was detected in all control and OI bone cells. The expression of all osteoblastic markers was similar in bone cells producing abnormal collagen type I. These observations show that OI bone cells in vitro express a pattern of osteoblastic markers similar to age-matched control bone cells indicating that osteoblastic differentiation is not altered by the underlying defects of collagen type I metabolism in OI bone cells. © 1993 Wiley-Liss, Inc.     

Reduced type I collagen utilization: a pathogenic mechanism in COL5A1 haplo-insufficient Ehlers-Danlos syndrome

To examine mechanisms by which reduced type V collagen causes weakened connective tissues in the Ehlers-Danlos syndrome (EDS), we examined matrix deposition and collagen fibril morphology in long-term dermal fibroblast cultures. EDS cells with COL5A1 haplo-insufficiency deposited less than one-half of hydroxyproline as collagen compared to control fibroblasts, though total collagen synthesis rates are near-normal because type V collagen represents a small fraction of collagen synthesized. Cells from patients with osteogenesis imperfecta (OI) and haplo-insufficiency for proalpha1(I) chains of type I collagen also incorporated about one-half the collagen as controls, but this amount was proportional to their reduced rates of total collagen synthesis. Collagen fibril diameter was inversely proportional to type V/type I collagen ratios (EDS > control > OI). However, a reduction of type V collagen, in the EDS derived cells, was associated with the assembly of significantly fewer fibrils compared to control and OI cells. These data indicate that in cell culture, the quantity of collagen fibrils deposited in matrix is highly sensitive to reduction in type V collagen, far out of proportion to type V collagen's contribution to collagen mass.     

A novel splicing mutation in COL1A1 gene caused type I osteogenesis imperfecta in a Chinese family

Osteogenesis imperfect (OI) is a heritable connective tissue disorder with bone fragility as a cardinal manifestation, accompanied by short stature, dentinogenesis imperfecta, hyperlaxity of ligaments and skin, blue sclerae and hearing loss. Dominant form of OI is caused by mutations in the type I procollagen genes, COL1A1/A2. Here we identified a novel splicing mutation c.3207+1G>A (GenBank ID: JQ236861) in the COL1A1 gene that caused type I OI in a Chinese family. RNA splicing analysis proved that this mutation created a new splicing site at c.3200, and then led to frameshift. This result further enriched the mutation spectrum of type I procollagen genes.     

Evolutionary Dynamics of the Interferon-Induced Transmembrane Gene Family in Vertebrates

Vertebrate interferon-induced transmembrane (IFITM) genes have been demonstrated to have extensive and diverse functions, playing important roles in the evolution of vertebrates. Despite observance of their functionality, the evolutionary dynamics of this gene family are complex and currently unknown. Here, we performed detailed evolutionary analyses to unravel the evolutionary history of the vertebrate IFITM family. A total of 174 IFITM orthologous genes and 112 pseudogenes were identified from 27 vertebrate genome sequences. The vertebrate IFITM family can be divided into immunity-related IFITM (IR-IFITM), IFITM5 and IFITM10 sub-families in phylogeny, implying origins from three different progenitors. In general, vertebrate IFITM genes are located in two loci, one containing the IFITM10 gene, and the other locus containing IFITM5 and various numbers of IR-IFITM genes. Conservation of evolutionary synteny was observed in these IFITM genes. Significant functional divergence was detected among the three IFITM sub-families. No gene duplication or positive selection was found in IFITM5 sub-family, implying the functional conservation of IFITM5 in vertebrate evolution, which is involved in bone formation. No IFITM5 locus was identified in the marmoset genome, suggesting a potential association with the tiny size of this monkey. The IFITM10 sub-family was divided into two groups: aquatic and terrestrial types. Functional divergence was detected between the two groups, and five IFITM10-like genes from frog were dispersed into the two groups. Both gene duplication and positive selection were observed in aquatic vertebrate IFITM10-like genes, indicating that IFITM10 might be associated with the adaptation to aquatic environments. A large number of lineage- and species-specific gene duplications were observed in IR-IFITM sub-family and positive selection was detected in IR-IFITM of primates and rodents. Because primates have experienced a long history of viral infection, such rapid expansion and positive selection suggests that the evolution of primate IR-IFITM genes is associated with broad-spectrum antiviral activity.     

Incorporation of type I collagen molecules that contain a mutant alpha 2(I) chain (Gly580-->Asp) into bone matrix in a lethal case of osteogenesis imperfecta.

To understand more directly the tissue defect in osteogenesis imperfecta (OI), bone matrix was analyzed from an infant with lethal OI (type II) of defined mutation (collagen alpha 2(I)Gly580-->Asp). Pepsin-solubilized alpha 1(I) and alpha 2(I) chains and derived CNBr-peptides migrated more slowly on sodium dodecyl sulfate-polyacrylamide gel electrophoresis compared with normal human controls. The peptide alpha 2(I)CB3,5, predicted to contain the mutation site, ran as a retarded doublet band and was purified by high performance liquid chromatography and digested with V8 protease. Two peptides with amino-terminal sequences beginning at residue 576 of the alpha 2(I) chain were isolated. One had the normal sequence. The other differed in that aspartic acid replaced glycine at residue 580 as predicted from cDNA analysis, and in having an unhydroxylated proline at residue 579. From yields on microsequencing and the relative intensities of the two forms of alpha 2(I)CB3,5 on SDS-polyacrylamide gel electrophoresis, the ratio of mutant to normal alpha 2(I) chains in the infant's bone matrix was 0.7/1. Although the effects of an efficient incorporation of mutant chains on the properties of the bone matrix are unknown, it may be that in this OI case the tissue abnormalities result more from the presence of mutant protein than from an underexpression of matrix.     

The Interferon-induced Transmembrane Proteins,IFITM1, IFITM2, and IFITM3 Inhibit Hepatitis C Virus Entry

The interferon-induced transmembrane (IFITM) family of proteins have recently been identified as important host effector molecules of the type I interferon response against viruses. IFITM1 has been identified as a potent antiviral effector against hepatitis C virus (HCV), whereas the related family members IFITM2 and IFITM3 have been described to have antiviral effects against a broad range of RNA viruses. Here, we demonstrate that IFITM2 and IFITM3 play an integral role in the interferon response against HCV and act at the level of late entry stages of HCV infection. We have established that in hepatocytes, IFITM2 and IFITM3 localize to the late and early endosomes, respectively, as well as the lysosome. Furthermore, we have demonstrated that S-palmitoylation of all three IFITM proteins is essential for anti-HCV activity, whereas the conserved tyrosine residue in the N-terminal domain of IFITM2 and IFITM3 plays a significant role in protein localization. However, this tyrosine was found to be dispensable for anti-HCV activity, with mutation of the tyrosine resulting in an IFITM1-like phenotype with the retention of anti-HCV activity and co-localization of IFITM2 and IFITM3 with CD81. In conclusion, we propose that the IFITM proteins act in a coordinated manner to restrict HCV infection by targeting the endocytosed HCV virion for lysosomal degradation and demonstrate that the actions of the IFITM proteins are indeed virus and cell-type specific.     

Analysis of the COL1A1 and COL1A2 genes by PCR amplification and scanning by conformation-sensitive gel electrophoresis identifies only COL1A1 mutations in 15 patients with osteogenesis imperfecta type I: identification of common sequences of null-allele mutations.

Although >90% of patients with osteogenesis imperfecta (OI) have been estimated to have mutations in the COL1A1 and COL1A2 genes for type I procollagen, mutations have been difficult to detect in all patients with the mildest forms of the disease (i.e., type I). In this study, we first searched for mutations in type I procollagen by analyses of protein and mRNA in fibroblasts from 10 patients with mild OI; no evidence of a mutation was found in 2 of the patients by the protein analyses, and no evidence of a mutation was found in 5 of the patients by the RNA analyses. We then searched for mutations in the original 10 patients and in 5 additional patients with mild OI, by analysis of genomic DNA. To assay the genomic DNA, we established a consensus sequence for the first 12 kb of the COL1A1 gene and for 30 kb of new sequences of the 38-kb COL1A2 gene. The sequences were then used to develop primers for PCR for the 103 exons and exon boundaries of the two genes. The PCR products were first scanned for heteroduplexes by conformation-sensitive gel electrophoresis, and then products containing heteroduplexes were sequenced. The results detected disease-causing mutations in 13 of the 15 patients and detected two additional probable disease-causing mutations in the remaining 2 patients. Analysis of the data developed in this study and elsewhere revealed common sequences for mutations causing null alleles.     

Phosphorylation of the Antiviral Protein Interferon-inducible Transmembrane Protein 3 (IFITM3) Dually Regulates Its Endocytosis and Ubiquitination

Interferon-inducible transmembrane protein 3 (IFITM3) is essential for innate defense against influenza virus in mice and humans. IFITM3 localizes to endolysosomes where it prevents virus fusion, although mechanisms controlling its trafficking to this cellular compartment are not fully understood. We determined that both mouse and human IFITM3 are phosphorylated by the protein-tyrosine kinase FYN on tyrosine 20 (Tyr²⁰) and that mouse IFITM3 is also phosphorylated on the non-conserved Tyr²⁷. Phosphorylation led to a cellular redistribution of IFITM3, including plasma membrane accumulation. Mutation of Tyr²⁰ caused a similar redistribution of IFITM3 and resulted in decreased antiviral activity against influenza virus, whereas Tyr²⁷ mutation of mouse IFITM3 showed minimal effects on localization or activity. Using FYN knockout cells, we also found that IFITM3 phosphorylation is not a requirement for its antiviral activity. Together, these results indicate that Tyr²⁰ is part of an endocytosis signal that can be blocked by phosphorylation or by mutation of this residue. Further mutagenesis narrowed this endocytosis-controlling region to four residues conforming to a YXXΦ (where X is any amino acid and Φ is Val, Leu, or Ile) endocytic motif that, when transferred to CD4, resulted in its internalization from the cell surface. Additionally, we found that phosphorylation of IFITM3 by FYN and mutagenesis of Tyr²⁰ both resulted in decreased IFITM3 ubiquitination. Overall, these results suggest that modification of Tyr²⁰ may serve in a cellular checkpoint controlling IFITM3 trafficking and degradation and demonstrate the complexity of posttranslational regulation of IFITM3.     

Identification and molecular characterization of two novel mutations in COL1A2 in two Chinese families with osteogenesis imperfecta

Osteogenesis imperfecta (OI, also known as brittle bone disease) is caused mostly by mutations in two type Ⅰ collagen genes, COL1A1 and COL1A2 encoding the pro-α1 (Ⅰ) and pro-α2 (Ⅰ) chains of type Ⅰ collagen, respectively. Two Chinese families with autosomal dominant OI were identified and characterized. Linkage analysis revealed linkage of both families to COL1A2 on chromosome 7q21.3-q22.1. Mutational analysis was carried out using direct DNA sequence analysis. Two novel missense mutations, c.3350A＞G and c.3305G＞C, were identified in exon 49 of COL1A2 in the two families, respectively. The c.3305G＞C mutation resulted in substitution of a glycine residue (G) by an alanine residue (A) at codon 1102 (p.G1102A), which was found to be mutated into serine (S), argine (R), aspartic acid (D), or valine (V) in other families. The c.3350A＞G variant may be a de novo mutation resulting in p.Y1117C. Both mutations co-segregated with OI in respective families, and were not found in 100 normal controls. The G1102 and Y1117 residues were evolutionarily highly conserved from zebrafish to humans. Mutational analysis did not identify any mutation in the COX-2 gene (a modifier gene of OI). This study identifies two novel mutations p.G1102A and p.Y1117C that cause OI, significantly expands the spectrum of COL1A2 mutations causing OI, and has a significant implication in prenatal diagnosis of OI.     

A Missense Mutation in the SERPINH1 Gene in Dachshunds with Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a hereditary disease occurring in humans and dogs. It is characterized by extremely fragile bones and teeth. Most human and some canine OI cases are caused by mutations in the COL1A1 and COL1A2 genes encoding the subunits of collagen I. Recently, mutations in the CRTAP and LEPRE1 genes were found to cause some rare forms of human OI. Many OI cases exist where the causative mutation has not yet been found. We investigated Dachshunds with an autosomal recessive form of OI. Genotyping only five affected dogs on the 50 k canine SNP chip allowed us to localize the causative mutation to a 5.82 Mb interval on chromosome 21 by homozygosity mapping. Haplotype analysis of five additional carriers narrowed the interval further down to 4.74 Mb. The SERPINH1 gene is located within this interval and encodes an essential chaperone involved in the correct folding of the collagen triple helix. Therefore, we considered SERPINH1 a positional and functional candidate gene and performed mutation analysis in affected and control Dachshunds. A missense mutation (c.977C>T, p.L326P) located in an evolutionary conserved domain was perfectly associated with the OI phenotype. We thus have identified a candidate causative mutation for OI in Dachshunds and identified a fifth OI gene.     

Homozygosity for a Missense Mutation in SERPINH1, which Encodes the Collagen Chaperone Protein HSP47, Results in Severe Recessive Osteogenesis Imperfecta

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 proα1(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.