Type I collagen triplet duplication mutation in lethal osteogenesis imperfecta shifts register of alpha chains throughout the helix and disrupts incorporation of mutant helices into fibrils and extracellular matrix

Update on the Genetics of Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous heritable skeletal dysplasia characterized by bone fragility and deformity, growth deficiency, and other secondary connective tissue defects. OI is now understood as a collagen-related disorder caused by defects of genes whose protein products interact with collagen for folding, post-translational modification, processing and trafficking, affecting bone mineralization and osteoblast differentiation. This review provides the latest updates on genetics of OI, including new developments in both dominant and rare OI forms, as well as the signaling pathways involved in OI pathophysiology. There is a special emphasis on discoveries of recessive mutations in TENT5A, MESD, KDELR2 and CCDC134 whose causality of OI types XIX, XX, XXI and XXI, respectively, is now established and expends the complexity of mechanisms underlying OI to overlap LRP5/6 and MAPK/ERK pathways. We also review in detail new discoveries connecting the known OI types to each other, which may underlie an eventual understanding of a final common pathway in OI cellular and bone biology.

Classification of osteogenesis imperfecta: Importance for prophylaxis and genetic counseling

Osteogenesis imperfecta (OI) is a genetically heterogeneous monogenic disease characterized by decreased bone mass, bone fragility, and recurrent fractures. The phenotypic spectrum varies considerably ranging from prenatal fractures with lethal outcomes to mild forms with few fractures and normal stature. The basic mechanism is a collagen-related defect, not only in synthesis but also in folding, processing, bone mineralization, or osteoblast function. In recent years, great progress has been made in identifying new genes and molecular mechanisms underlying OI. In this context, the classification of OI has been revised several times and different types are used. The Sillence classification, based on clinical and radiological characteristics, is currently used as a grading of clinical severity. Based on the metabolic pathway, the functional classification allows identifying regulatory elements and targeting specific therapeutic approaches. Genetic classification has the advantage of identifying the inheritance pattern, an essential element for genetic counseling and prophylaxis. Although genotype-phenotype correlations may sometimes be challenging, genetic diagnosis allows a personalized management strategy, accurate family planning, and pregnancy management decisions including options for mode of delivery, or early antenatal OI treatment. Future research on molecular pathways and pathogenic variants involved could lead to the development of genotype-based therapeutic approaches. This narrative review summarizes our current understanding of genes, molecular mechanisms involved in OI, classifications, and their utility in prophylaxis.

Dissecting the phenotypic variability of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a heterogeneous family of collagen type I-related diseases characterized by bone fragility. OI is most commonly caused by single-nucleotide substitutions that replace glycine residues or exon splicing defects in the COL1A1 and COL1A2 genes that encode the α1(I) and α2(I) collagen chains. Mutant collagen is partially retained intracellularly, impairing cell homeostasis. Upon secretion, it assembles in disorganized fibrils, altering mineralization. OI is characterized by a wide range of clinical outcomes, even in the presence of identical sequence variants. Given the heterotrimeric nature of collagen I, its amino acid composition and the peculiarity of its folding, several causes may underlie the phenotypic variability of OI. A deep analysis of entries regarding glycine and splice site collagen substitution of the largest publicly available patient database reveals a higher risk of lethal phenotype for carriers of variants in α1(I) than in α2(I) chain. However, splice site variants are predominantly associated with lethal phenotype when they occur in COL1A2. In addition, lethality is increased when mutations occur in regions of importance for extracellular matrix interactions. Both extracellular and intracellular determinants of OI clinical severity are discussed in light of the findings from in vitro and in vivo OI models. Combined with meticulous tracking of clinical cases via a publicly available database, the available OI animal models have proven to be a unique tool to shed light on new modulators of phenotype determination for this rare heterogeneous disease.

Evolutionary Genetic Signatures of Selection on Bone-Related Variation within Human and Chimpanzee Populations

Bone strength and the incidence and severity of skeletal disorders vary significantly among human populations, due in part to underlying genetic differentiation. While clinical models predict that this variation is largely deleterious, natural population variation unrelated to disease can go unnoticed, altering our perception of how natural selection has shaped bone morphologies over deep and recent time periods. Here, we conduct the first comparative population-based genetic analysis of the main bone structural protein gene, collagen type I α 1 (COL1A1), in clinical and 1000 Genomes Project datasets in humans, and in natural populations of chimpanzees. Contrary to predictions from clinical studies, we reveal abundant COL1A1 amino acid variation, predicted to have little association with disease in the natural population. We also find signatures of positive selection associated with intron haplotype structure, linkage disequilibrium, and population differentiation in regions of known gene expression regulation in humans and chimpanzees. These results recall how recent and deep evolutionary regimes can be linked, in that bone morphology differences that developed among vertebrates over 450 million years of evolution are the result of positive selection on subtle type I collagen functional variation segregating within populations over time.

Development of Fluorescently Labeled, Functional Type I Collagen Molecules

While de novo collagen fibril formation is well-studied, there are few investigations into the growth and remodeling of extant fibrils, where molecular collagen incorporation into and erosion from the fibril surface must delicately balance during fibril growth and remodeling. Observing molecule/fibril interactions is difficult, requiring the tracking of molecular dynamics while, at the same time, minimizing the effect of the observation on fibril structure and assembly. To address the observation-interference problem, exogenous collagen molecules are tagged with small fluorophores and the fibrillogenesis kinetics of labeled collagen molecules as well as the structure and network morphology of assembled fibrils are examined. While excessive labeling significantly disturbs fibrillogenesis kinetics and network morphology of assembled fibrils, adding less than ≈1.2 labels per collagen molecule preserves these characteristics. Applications of the functional, labeled collagen probe are demonstrated in both cellular and acellular systems. The functional, labeled collagen associates strongly with native fibrils and when added to an in vitro model of corneal stromal development at low concentration, the labeled collagen is incorporated into a fine extracellular matrix (ECM) network associated with the cells within 24 h.

Genetic analysis in Japanese patients with osteogenesis imperfecta: Genotype and phenotype spectra in 96 probands

BACKGROUND

Osteogenesis imperfecta (OI) is a rare connective-tissue disorder characterized by bone fragility. Approximately 90% of all OI cases are caused by variants in COL1A1 or COL1A2. Additionally, IFITM5 variants are responsible for the unique OI type 5. We previously analyzed COL1A1/2 variants in 22 Japanese families with OI through denaturing high-performance liquid chromatography screening, but our detection rate was low (41%).

METHODS

To expand the genotype-phenotype correlations, we performed a genetic analysis of COL1A1/2 and IFITM5 in 96 non-consanguineous Japanese OI probands by Sanger sequencing.

RESULTS

Of these individuals, 54, 41, and 1 had type 1 (mild), type 2-4 (moderate-to-severe), and type 5 phenotypes, respectively. In the mild group, COL1A1 nonsense and splice-site variants were prevalent (n = 30 and 20, respectively), but there were also COL1A1 and COL1A2 triple-helical glycine substitutions (n = 2 and 1, respectively). In the moderate-to-severe group, although COL1A1 and COL1A2 glycine substitutions were common (n = 14 and 18, respectively), other variants were also detected. The single case of type 5 had the characteristic c.-14C>T variant in IFITM5.

CONCLUSION

These results increase our previous detection rate for COL1A1/2 variants to 99% and provide insight into the genotype-phenotype correlations in OI.

Targeting cellular stress in vitro improves osteoblast homeostasis, matrix collagen content and mineralization in two murine models of osteogenesis imperfecta

Most cases of dominantly inherited osteogenesis imperfecta (OI) are caused by glycine substitutions in the triple helical domain of type I collagen α chains, which delay collagen folding, and cause the synthesis of collagen triple helical molecules with abnormal structure and post-translational modification. A variable extent of mutant collagen ER retention and other secondary mutation effects perturb osteoblast homeostasis and impair bone matrix quality. Amelioration of OI osteoblast homeostasis could be beneficial both to osteoblast anabolic activity and to the content of the extracellular matrix they deposit. Therefore, the effect of the chemical chaperone 4-phenylbutyrate (4-PBA) on cell homeostasis, collagen trafficking, matrix production and mineralization was investigated in primary osteoblasts from two murine models of moderate OI, Col1a1+/G349C and Col1a2+/G610C. At the cellular level, 4-PBA prevented intracellular accumulation of collagen and increased protein secretion, reducing aggregates within the mutant cells and normalizing ER morphology. At the extracellular level, increased collagen incorporation into matrix, associated with more mature collagen fibrils, was observed in osteoblasts from both models. 4-PBA also promoted OI osteoblast mineral deposition by increasing alkaline phosphatase expression and activity. Targeting osteoblast stress with 4-PBA improved both cellular and matrix abnormalities in culture, supporting further in vivo studies of its effect on bone tissue composition, strength and mineralization as a potential treatment for classical OI.

Substitution of murine type I collagen A1 3-hydroxylation site alters matrix structure but does not recapitulate osteogenesis imperfecta bone dysplasia

Null mutations in CRTAP or P3H1, encoding cartilage-associated protein and prolyl 3-hydroxylase 1, cause the severe bone dysplasias, types VII and VIII osteogenesis imperfecta. Lack of either protein prevents formation of the ER prolyl 3-hydroxylation complex, which catalyzes 3Hyp modification of types I and II collagen and also acts as a collagen chaperone. To clarify the role of the A1 3Hyp substrate site in recessive bone dysplasia, we generated knock-in mice with an α1(I)P986A substitution that cannot be 3-hydroxylated. Mutant mice have normal survival, growth, femoral breaking strength and mean bone mineralization. However, the bone collagen HP/LP crosslink ratio is nearly doubled in mutant mice, while collagen fibril diameter and bone yield energy are decreased. Thus, 3-hydroxylation of the A1 site α1(I)P986 affects collagen crosslinking and structural organization, but its absence does not directly cause recessive bone dysplasia. Our study suggests that the functions of the modification complex as a collagen chaperone are thus distinct from its role as prolyl 3-hydroxylase.

Osteogenesis imperfecta

Skeletal deformity and bone fragility are the hallmarks of the brittle bone dysplasia osteogenesis imperfecta. The diagnosis of osteogenesis imperfecta usually depends on family history and clinical presentation characterized by a fracture (or fractures) during the prenatal period, at birth or in early childhood; genetic tests can confirm diagnosis. Osteogenesis imperfecta is caused by dominant autosomal mutations in the type I collagen coding genes (COL1A1 and COL1A2) in about 85% of individuals, affecting collagen quantity or structure. In the past decade, (mostly) recessive, dominant and X-linked defects in a wide variety of genes encoding proteins involved in type I collagen synthesis, processing, secretion and post-translational modification, as well as in proteins that regulate the differentiation and activity of bone-forming cells have been shown to cause osteogenesis imperfecta. The large number of causative genes has complicated the classic classification of the disease, and although a new genetic classification system is widely used, it is still debated. Phenotypic manifestations in many organs, in addition to bone, are reported, such as abnormalities in the cardiovascular and pulmonary systems, skin fragility, muscle weakness, hearing loss and dentinogenesis imperfecta. Management involves surgical and medical treatment of skeletal abnormalities, and treatment of other complications. More innovative approaches based on gene and cell therapy, and signalling pathway alterations, are under investigation.

Tissue-specific mosaicism for a lethal osteogenesis imperfecta COL1A1 mutation causes mild OI/EDS overlap syndrome

Type I collagen is the predominant protein of connective tissues such as skin and bone. Mutations in the type I collagen genes (COL1A1 and COL1A2) mainly cause osteogenesis imperfecta (OI). We describe a patient with clinical signs of Ehlers-Danlos syndrome (EDS), including fragile skin, easy bruising, recurrent luxations, and fractures resembling mild OI. Biochemical collagen analysis of the patients' dermal fibroblasts showed faint overmodification of the type I collagen bands, a finding specific for structural defects in type I collagen. Bidirectional Sanger sequencing detected an in-frame deletion in exon 44 of COL1A1 (c.3150_3158del), resulting in the deletion of three amino acids (p.Ala1053_Gly1055del) in the collagen triple helix. This COL1A1 mutation was hitherto identified in four probands with lethal OI, and never in EDS patients. As the peaks on the electropherogram corresponding to the mutant allele were decreased in intensity, we performed next generation sequencing of COL1A1 to study mosaicism in skin and blood. While approximately 9% of the reads originating from fibroblast gDNA harbored the COL1A1 deletion, the deletion was not detected in gDNA from blood. Most likely, the mild clinical symptoms observed in our patient can be explained by the mosaic state of the mutation.

Absence of the ER Cation Channel TMEM38B/TRIC-B Disrupts Intracellular Calcium Homeostasis and Dysregulates Collagen Synthesis in Recessive Osteogenesis Imperfecta

Recessive osteogenesis imperfecta (OI) is caused by defects in proteins involved in post-translational interactions with type I collagen. Recently, a novel form of moderately severe OI caused by null mutations in TMEM38B was identified. TMEM38B encodes the ER membrane monovalent cation channel, TRIC-B, proposed to counterbalance IP₃R-mediated Ca²⁺ release from intracellular stores. The molecular mechanisms by which TMEM38B mutations cause OI are unknown. We identified 3 probands with recessive defects in TMEM38B. TRIC-B protein is undetectable in proband fibroblasts and osteoblasts, although reduced TMEM38B transcripts are present. TRIC-B deficiency causes impaired release of ER luminal Ca²⁺, associated with deficient store-operated calcium entry, although SERCA and IP₃R have normal stability. Notably, steady state ER Ca²⁺ is unchanged in TRIC-B deficiency, supporting a role for TRIC-B in the kinetics of ER calcium depletion and recovery. The disturbed Ca²⁺ flux causes ER stress and increased BiP, and dysregulates synthesis of proband type I collagen at multiple steps. Collagen helical lysine hydroxylation is reduced, while telopeptide hydroxylation is increased, despite increased LH1 and decreased Ca²⁺-dependent FKBP65, respectively. Although PDI levels are maintained, procollagen chain assembly is delayed in proband cells. The resulting misfolded collagen is substantially retained in TRIC-B null cells, consistent with a 50–70% reduction in secreted collagen. Lower-stability forms of collagen that elude proteasomal degradation are not incorporated into extracellular matrix, which contains only normal stability collagen, resulting in matrix insufficiency. These data support a role for TRIC-B in intracellular Ca²⁺ homeostasis, and demonstrate that absence of TMEM38B causes OI by dysregulation of calcium flux kinetics in the ER, impacting multiple collagen-specific chaperones and modifying enzymes.

Osteogenesis imperfecta (OI) is a heritable disorder of connective tissues characterized by fracture susceptibility and growth deficiency. Most OI cases are caused by autosomal dominant mutations in the genes encoding type I collagen, COL1A1 and COL1A2. Delineation of novel gene defects causing dominant and recessive forms of OI has led to the understanding that the bone pathology results not only from abnormalities in type I collagen quantity and primary structure, but also from defects in post-translational modification, folding, intracellular transport and extracellular matrix incorporation. Recently, mutations in TMEM38B, which encodes the integral ER membrane K⁺ channel TRIC-B, have been identified as causative for the OI phenotype. However, the mechanism by which absence of TRIC-B causes OI has not been reported. Using cell lines established from three independent probands, we have demonstrated that absence of TRIC-B leads to abnormal ER Ca²⁺ flux and store-operated calcium entry (SOCE), although ER steady state Ca²⁺ is normal. Disruption of intracellular calcium dynamics alters the expression and activity of multiple collagen interacting chaperones and modifying enzymes within the ER. Thus TRIC-B deficiency causes OI by dysregulation of collagen synthesis, through the impairment of calcium-dependent gene expression and protein-protein interactions within the ER.

MBTPS2 mutations cause defective regulated intramembrane proteolysis in X-linked osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a collagen-related bone dysplasia. We identified an X-linked recessive form of OI caused by defects in MBTPS2, which encodes site-2 metalloprotease (S2P). MBTPS2 missense mutations in two independent kindreds with moderate/severe OI cause substitutions at highly conserved S2P residues. Mutant S2P has normal stability, but impaired functioning in regulated intramembrane proteolysis (RIP) of OASIS, ATF6 and SREBP transcription factors, consistent with decreased proband secretion of type I collagen. Further, hydroxylation of the collagen lysine residue (K87) critical for crosslinking is reduced in proband bone tissue, consistent with decreased lysyl hydroxylase 1 in proband osteoblasts. Reduced collagen crosslinks presumptively undermine bone strength. Also, proband osteoblasts have broadly defective differentiation. These mutations provide evidence that RIP plays a fundamental role in normal bone development.

Osteogenesis imperfecta (OI) is genetically linked to autosomal dominant or autosomal recessive mutations. Here, Marini et al. describe two families with X-chromosome-linked OI with mutations in MBTPS2 that alter regulated intramembrane proteolysis and subsequent defects in collagen crosslinking and osteoblast function.

Abnormal type I collagen post-translational modification and crosslinking in a cyclophilin B KO mouse model of recessive osteogenesis imperfecta

Cyclophilin B (CyPB), encoded by PPIB, is an ER-resident peptidyl-prolyl cis-trans isomerase (PPIase) that functions independently and as a component of the collagen prolyl 3-hydroxylation complex. CyPB is proposed to be the major PPIase catalyzing the rate-limiting step in collagen folding. Mutations in PPIB cause recessively inherited osteogenesis imperfecta type IX, a moderately severe to lethal bone dysplasia. To investigate the role of CyPB in collagen folding and post-translational modifications, we generated Ppib^−/− mice that recapitulate the OI phenotype. Knock-out (KO) mice are small, with reduced femoral areal bone mineral density (aBMD), bone volume per total volume (BV/TV) and mechanical properties, as well as increased femoral brittleness. Ppib transcripts are absent in skin, fibroblasts, femora and calvarial osteoblasts, and CyPB is absent from KO osteoblasts and fibroblasts on western blots. Only residual (2–11%) collagen prolyl 3-hydroxylation is detectable in KO cells and tissues. Collagen folds more slowly in the absence of CyPB, supporting its rate-limiting role in folding. However, treatment of KO cells with cyclosporine A causes further delay in folding, indicating the potential existence of another collagen PPIase. We confirmed and extended the reported role of CyPB in supporting collagen lysyl hydroxylase (LH1) activity. Ppib^−/− fibroblast and osteoblast collagen has normal total lysyl hydroxylation, while increased collagen diglycosylation is observed. Liquid chromatography/mass spectrometry (LC/MS) analysis of bone and osteoblast type I collagen revealed site-specific alterations of helical lysine hydroxylation, in particular, significantly reduced hydroxylation of helical crosslinking residue K87. Consequently, underhydroxylated forms of di- and trivalent crosslinks are strikingly increased in KO bone, leading to increased total crosslinks and decreased helical hydroxylysine- to lysine-derived crosslink ratios. The altered crosslink pattern was associated with decreased collagen deposition into matrix in culture, altered fibril structure in tissue, and reduced bone strength. These studies demonstrate novel consequences of the indirect regulatory effect of CyPB on collagen hydroxylation, impacting collagen glycosylation, crosslinking and fibrillogenesis, which contribute to maintaining bone mechanical properties.

Osteogenesis imperfecta (OI), or brittle bone disease, is characterized by susceptibility to fractures from minimal trauma and growth deficiency. Deficiency of components of the collagen prolyl 3-hydroxylation complex, CRTAP, P3H1 and CyPB, cause recessive types VII, VIII and IX OI, respectively. We have previously shown that mutual protection within the endoplasmic reticulum accounts for the overlapping severe phenotype of patients with CRTAP and P3H1 mutations. However, the bone dysplasia in patients with CyPB deficiency is distinct in terms of phenotype and type I collagen biochemistry. Using a knock-out mouse model of type IX OI, we have demonstrated that CyPB is the major, although not unique, peptidyl prolyl cis-trans isomerase that catalyzes the rate-limiting step in collagen folding. CyPB is also required for activity of the collagen prolyl 3-hydroxylation complex; collagen α1(I) P986 modification is lost in the absence of CyPB. Unexpectedly, CyPB was found to also influence collagen helical lysyl hydroxylation in a tissue-, cell- and residue-specific manner. Thus CyPB facilitates collagen folding directly, but also indirectly regulates collagen hydroxylation, glycosylation, crosslinking and fibrillogenesis through its interactions with other collagen modifying enzymes in the endoplasmic reticulum.

New perspectives on osteogenesis imperfecta

A new paradigm has emerged for osteogenesis imperfecta as a collagen-related disorder. The more prevalent autosomal dominant forms of osteogenesis imperfecta are caused by primary defects in type I collagen, whereas autosomal recessive forms are caused by deficiency of proteins which interact with type I procollagen for post-translational modification and/or folding. Factors that contribute to the mechanism of dominant osteogenesis imperfecta include intracellular stress, disruption of interactions between collagen and noncollagenous proteins, compromised matrix structure, abnormal cell-cell and cell-matrix interactions and tissue mineralization. Recessive osteogenesis imperfecta 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, consistent with delayed collagen folding. Other causes of recessive osteogenesis imperfecta include deficiency of the collagen chaperones FKBP10 or Serpin H1. Murine models are crucial to uncovering the common pathways in dominant and recessive osteogenesis imperfecta bone dysplasia. Clinical management of osteogenesis imperfecta is multidisciplinary, encompassing substantial progress in physical rehabilitation and surgical procedures, management of hearing, dental and pulmonary abnormalities, as well as drugs, such as bisphosphonates and recombinant human growth hormone. Novel treatments using cell therapy or new drug regimens hold promise for the future.

In vivo cellular adaptation to ER stress: survival strategies with double-edged consequences

Disturbances to the balance of protein synthesis, folding and secretion in the endoplasmic reticulum (ER) induce stress and thereby the ER stress signaling (ERSS) response, which alleviates this stress. In this Commentary, we review the emerging idea that ER stress caused by abnormal physiological conditions and/or mutations in genes that encode client proteins of the ER is a key factor underlying different developmental processes and the pathology of diverse diseases, including diabetes, neurodegeneration and skeletal dysplasias. Recent studies in mouse models indicate that the effect of ERSS in vivo and the nature of the cellular strategies induced to ameliorate pathological ER stress are crucial factors in determining cell fate and clinical disease features. Importantly, ERSS can affect cellular proliferation and the differentiation program; cells that survive the stress can become 'reprogrammed' or dysfunctional. These cell-autonomous adaptation strategies can generate a spectrum of context-dependent cellular consequences, ranging from recovery to death. Secondary effects can include altered cell-extracellular-matrix interactions and non-cell-autonomous alteration of paracrine signaling, which contribute to the final phenotypic outcome. Recent reports showing that ER stress can be alleviated by chemical compounds suggest the potential for novel therapeutic approaches.

Procollagen triple helix assembly: an unconventional chaperone-assisted folding paradigm

Fibers composed of type I collagen triple helices form the organic scaffold of bone and many other tissues, yet the energetically preferred conformation of type I collagen at body temperature is a random coil. In fibers, the triple helix is stabilized by neighbors, but how does it fold? The observations reported here reveal surprising features that may represent a new paradigm for folding of marginally stable proteins. We find that human procollagen triple helix spontaneously folds into its native conformation at 30-34 degrees C but not at higher temperatures, even in an environment emulating Endoplasmic Reticulum (ER). ER-like molecular crowding by nonspecific proteins does not affect triple helix folding or aggregation of unfolded chains. Common ER chaperones may prevent aggregation and misfolding of procollagen C-propeptide in their traditional role of binding unfolded polypeptide chains. However, such binding only further destabilizes the triple helix. We argue that folding of the triple helix requires stabilization by preferential binding of chaperones to its folded, native conformation. Based on the triple helix folding temperature measured here and published binding constants, we deduce that HSP47 is likely to do just that. It takes over 20 HSP47 molecules to stabilize a single triple helix at body temperature. The required 50-200 microM concentration of free HSP47 is not unusual for heat-shock chaperones in ER, but it is 100 times higher than used in reported in vitro experiments, which did not reveal such stabilization.

Y-position cysteine substitution in type I collagen (α1(I) R888C/p.R1066C) is associated with osteogenesis imperfecta/Ehlers-Danlos syndrome phenotype

The most common mutations in type I collagen causing types II-IV osteogenesis imperfecta (OI) result in substitution for glycine in a Gly-Xaa-Yaa triplet by another amino acid. We delineated a Y-position substitution in a small pedigree with a combined OI/Ehlers-Danlos Syndrome (EDS) phenotype, characterized by moderately decreased DEXA z-score (-1.3 to -2.6), long bone fractures, and large-joint hyperextensibility. Affected individuals have an alpha1(I)R888C (p.R1066C) substitution in one COL1A1 allele. Polyacrylamide gel electrophoresis (PAGE) of [(3)H]-proline labeled steady-state collagen reveals slight overmodification of the alpha1(I) monomer band, much less than expected for a substitution of a neighboring glycine residue, and a faint alpha1(I) dimer. Dimers form in about 10% of proband type I collagen. Dimer formation is inefficient compared to a possible 25%, probably because the SH-side chains have less proximity in this Y-position than when substituting for a glycine. Theoretical stability calculations, differential scanning calorimetry (DSC) thermograms, and thermal denaturation curves showed only weak local destabilization from the Y-position substitution in one or two chains of a collagen helix, but greater destabilization is seen in collagen containing dimers. Y-position collagen dimers cause kinking of the helix, resulting in a register shift that is propagated the full length of the helix and causes resistance to procollagen processing by N-proteinase. Collagen containing the Y-position substitution is incorporated into matrix deposited in culture, including immaturely and maturely cross-linked fractions. In vivo, proband dermal fibrils have decreased density and increased diameter compared to controls, with occasional aggregate formation. This report on Y-position substitutions in type I collagen extends the range of phenotypes caused by nonglycine substitutions and shows that, similar to X- and Y-position substitutions in types II and III collagen, the phenotypes resulting from nonglycine substitutions in type I collagen are distinct from those caused by glycine substitutions.

Prolyl 3-hydroxylase 1 deficiency causes a recessive metabolic bone disorder resembling lethal/severe osteogenesis imperfecta

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.

Molecular Mechanism of α1(I)-Osteogenesis Imperfecta/Ehlers-Danlos Syndrome

We demonstrate that 85 N-terminal amino acids of the alpha1(I) chain participate in a highly stable folding domain, acting as the stabilizing anchor for the amino end of the type I collagen triple helix. This anchor region is bordered by a microunfolding region, 15 amino acids in each chain, which include no proline or hydroxyproline residues and contain a chymotrypsin cleavage site. Glycine substitutions and amino acid deletions within the N-anchor domain induce its reversible unfolding above 34 degrees C. The overall triple helix denaturation temperature is reduced by 5-6 degrees C, similar to complete N-anchor removal. N-propeptide partially restores the stability of mutant procollagen but not sufficiently to prevent N-anchor unfolding and a conformational change at the N-propeptide cleavage site. The ensuing failure of N-proteinase to cleave at the misfolded site leads to incorporation of pN-collagen into fibrils. Similar, but weaker, effects are caused by G88E substitution in the adjacent triplet, which appears to alter N-anchor structure as well. As in Ehlers-Danlos syndrome (EDS) VIIA/B, fibrils containing pN-collagen are thinner and weaker causing EDS-like laxity of large and small joints and paraspinal ligaments. However, distinct structural consequences of N-anchor destabilization result in a distinct alpha1(I)-osteogenesis imperfecta (OI)/EDS phenotype.

Selection of cell source for ligament tissue engineering

Use of appropriate types of cells could potentially improve the functionality and structure of tissue engineered constructs, but little is known about the optimal cell source for ligament tissue engineering. The object of this study was to determine the optimal cell source for anterior cruciate ligament (ACL) tissue engineering. Fibroblasts isolated from anterior cruciate ligament, medial collateral ligament (MCL), as well as bone marrow mesenchymal stem cells (MSC) were compared using the following parameters: proliferation rate, collagen excretion, expression of collagen type I, II, and III, as well as alpha-smooth muscle actin. Green fluorescent protein (GFP) transfected MSCs were used to trace their fate in the knee joints. MSC, ACL, and MCL fibroblasts were all highly stained with antibodies for collagen types I and III and alpha-smooth muscle actin while negatively stained with collagen type II. Proliferation rate and collagen excretion of MSCs were higher than ACL and MCL fibroblasts (p < 0.05), and MSCs could survive for at least 6 weeks in knee joints. In summary, MSC is potentially a better cell source than ACL and MCL fibroblasts for anterior cruciate ligament tissue engineering.

Domains and Maturation Processes That Regulate the Activity of ADAMTS-2, a Metalloproteinase Cleaving the Aminopropeptide of Fibrillar Procollagens Types I–III and V

Processing of fibrillar collagens is required to generate collagen monomers able to self-assemble into elongated and cylindrical collagen fibrils. ADAMTS-2 belongs to the "A disintegrin and metalloproteinase with thrombospondin type 1 motifs" (ADAMTS) family. It is responsible for most of the processing of the aminopropeptide of type I procollagen in the skin, and it also cleaves type II and type III procollagens. ADAMTS are complex secreted enzymes that are implicated in various physiological and pathological processes. Despite accumulating evidence indicating that their activity is regulated by ancillary domains, additional information is required for a better understanding of the specific function of each domain. We have generated 17 different recombinant forms of bovine ADAMTS-2 and characterized their processing, activity, and cleavage specificity. The results indicated the following: (i) activation of the ADAMTS-2 zymogen involves several cleavages, by proprotein convertases and C-terminal processing, and generates at least seven distinct processed forms; (ii) the C-terminal domain negatively regulates enzyme activity, whereas two thrombospondin type 1 repeats are enhancer regulators; (iii) the 104-kDa form displays the highest aminoprocollagen peptidase activity on procollagen type I; (iv) ADAMTS-2 processes the aminopropeptide of alpha1 type V procollagen homotrimer at the end of the variable domain; and (v) the cleaved sequence (PA) is different from the previously described sites ((P/A)Q) for ADAMTS-2, redefining its cleavage specificity. This finding and the existence of multiple processed forms of ADAMTS-2 strongly suggest that ADAMTS-2 may be involved in function(s) other than processing of fibrillar procollagen types I-III.

Collagen structure: the Madras triple helix and the current scenario

This year marks the 50th anniversary of the coiled-coil triple helical structure of collagen, first proposed by Ramachandran's group from Madras. The structure is unique among the protein secondary structures in that it requires a very specific tripeptide sequence repeat, with glycine being mandatory at every third position and readily accommodates the imino acids proline/hydroxyproline, at the other two positions. The original structure was postulated to be stabilized by two interchain hydrogen bonds, per tripeptide. Subsequent modeling studies suggested that the triple helix is stabilized by one direct inter chain hydrogen bond as well as water mediated hydrogen bonds. The hydroxyproline residues were also implicated to play an important role in stabilizing the collagen fibres. Several high resolution crystal structures of oligopeptides related to collagen have been determined in the last ten years. Stability of synthetic mimics of collagen has also been extensively studied. These have confirmed the essential correctness of the coiled-coil triple helical structure of collagen, as well as the role of water and hydroxyproline residues, but also indicated additional sequence-dependent features. This review discusses some of these recent results and their implications for collagen fiber formation.

Mutations Near Amino End of α1(I) Collagen Cause Combined Osteogenesis Imperfecta/Ehlers-Danlos Syndrome by Interference with N-propeptide Processing

Patients with OI/EDS form a distinct subset of osteogenesis imperfecta (OI) patients. In addition to skeletal fragility, they have characteristics of Ehlers-Danlos syndrome (EDS). We identified 7 children with types III or IV OI, plus severe large and small joint laxity and early progressive scoliosis. In each child with OI/EDS, we identified a mutation in the first 90 residues of the helical region of alpha1(I) collagen. These mutations prevent or delay removal of the procollagen N-propeptide by purified N-proteinase (ADAMTS-2) in vitro and in pericellular assays. The mutant pN-collagen which results is efficiently incorporated into matrix by cultured fibroblasts and osteoblasts and is prominently present in newly incorporated and immaturely cross-linked collagen. Dermal collagen fibrils have significantly reduced cross-sectional diameters, corroborating incorporation of pN-collagen into fibrils in vivo. Differential scanning calorimetry revealed that these mutant collagens are less stable than the corresponding procollagens, which is not seen with other type I collagen helical mutations. These mutations disrupt a distinct folding region of high thermal stability in the first 90 residues at the amino end of type I collagen and alter the secondary structure of the adjacent N-proteinase cleavage site. Thus, these OI/EDS collagen mutations are directly responsible for the bone fragility of OI and indirectly responsible for EDS symptoms, by interference with N-propeptide removal.

A large tandem duplication within the COL4A5 gene is responsible for the high prevalence of Alport syndrome in French Polynesia

A large tandem duplication within the COL4A5 gene is responsible for the high prevalence of Alport syndrome in French Polynesia. Background. The prevalence of X-linked Alport syndrome, a progressive inherited nephropathy associated with mutations in the type IV collagen gene COL4A5, is remarkably high in French Polynesia. Methods. A vast clinical, genealogic, and molecular study was undertaken in Polynesia, based on public records, patients' interviews, linkage analysis, and mutation screening. Results and Conclusions. We show that the high frequency of Alport syndrome in this region is due to a founder mutation that occurred onto a common haplotype shared by affected and unaffected individuals, the presence of which precludes indirect molecular diagnosis. We have characterized the mutation as a tandem duplication of 35 COL4A5 exons, resulting in a approximately 65% increase in the length of the collagenous domain of the alpha 5(IV) chain, which is still able to assemble into type IV collagen network as shown by immunofluorescence analysis. That mutation is associated with severe and highly penetrant ocular symptoms and with uniformly thin glomerular basement membrane (GBM) in male adult patients. However, the rate of progression of the renal disease is very variable from one male patient to another, demonstrating the importance of strong modifier factors. Our results suggest that the 20% to 50% of "missing"COL4A5 mutations in X-linked Alport syndrome may be rearrangements similar to that reported here, which was not detectable by sequencing of either individual COL4A5 exons or overlapping cDNA fragments. Finally, we provide the basis for a polymerase chain reaction (PCR) assay that accurately identifies female carriers and allows adequate genetic counseling in this population.

Hereditary elliptocytosis: spectrin and protein 4.1R

Hereditary elliptocytosis (HE) is a common disorder of erythrocyte shape, occurring especially in individuals of African and Mediterranean ancestry, presumably because elliptocytes confer some resistance to malaria. The principle lesion in HE is mechanical weakness or fragility of the erythrocyte membrane skeleton due to defects in alpha-spectrin, beta-spectrin, or protein 4.1. Numerous mutations have been described in the genes encoding these proteins, including point mutations, gene deletions and insertions, and mRNA processing defects. Several mutations have been identified in a number of individuals on the same genetic background, suggesting a "founder effect." The majority of HE patients are asymptomatic, but some may experience hemolytic anemia, splenomegaly, and intermittent jaundice.