An alpha2(I) glycine to aspartate substitution is responsible for the presence of a kink in type I collagen in a lethal case of osteogenesis imperfecta

NGS analysis of collagen type I genes in Polish patients with Osteogenesis imperfecta: a nationwide multicenter study

Osteogenesis imperfecta (OI) is a rare genetic disorder of the connective tissue. It presents with a wide spectrum of skeletal and extraskeletal features, and ranges in severity from mild to perinatal lethal. The disease is characterized by a heterogeneous genetic background, where approximately 85%-90% of cases have dominantly inherited heterozygous pathogenic variants located in the COL1A1 and COL1A2 genes. This paper presents the results of the first nationwide study, performed on a large cohort of 197 Polish OI patients. Variants were identified using a next-generation sequencing (NGS) custom gene panel and multiplex ligation probe amplification (MLPA) assay. The following OI types were observed: 1 (42%), 2 (3%), 3 (35%), and 4 (20%). Collagen type I pathogenic variants were reported in 108 families. Alterations were observed in α1 and α2 in 70% and 30% of cases, respectively. The presented paper reports 97 distinct causative variants and expands the OI database with 38 novel pathogenic changes. It also enabled the identification of the first glycine-to-tryptophan substitution in the COL1A1 gene and brought new insights into the clinical severity associated with variants localized in "lethal regions". Our results contribute to a better understanding of the clinical and genetic aspects of OI.

Intrafibrillar mineralization deficiency and osteogenesis imperfecta mouse bone fragility

Osteogenesis imperfecta (OI), a brittle bone disease, is known to result in severe bone fragility. However, its ultrastructural origins are still poorly understood. In this study, we hypothesized that deficient intrafibrillar mineralization is a key contributor to the OI induced bone brittleness. To test this hypothesis, we explored the mechanical and ultrastructural changes in OI bone using the osteogenesis imperfecta murine (oim) model. Synchrotron X-ray scattering experiments indicated that oim bone had much less intrafibrillar mineralization than wild type bone, thus verifying that the loss of mineral crystals indeed primarily occurred in the intrafibrillar space of oim bone. It was also found that the mineral crystals were organized from preferentially in longitudinal axis in wild type bone to more randomly in oim bone. Moreover, it revealed that the deformation of mineral crystals was more coordinated with collagen fibrils in wild type than in oim bone, suggesting that the load transfer deteriorated between the two phases in oim bone. The micropillar test revealed that the compression work to fracture of oim bone (8.2 ± 0.9 MJ/m³) was significantly smaller (p < 0.05) than that of wild type bone (13.9 ± 2.7 MJ/m³), while the bone strength was not statistically different (p > 0.05) between the two genotype groups. In contrast, the uniaxial tensile test showed that the ultimate strength of wild type bone (50 ± 4.5 MPa) was significantly greater (p < 0.05) than that of oim bone (38 ± 5.3 MPa). Furthermore, the nanoscratch test showed that the toughness of oim bone was much less than that of wild type bone (6.6 ± 2.2 GJ/m³ vs. 12.6 ± 1.4 GJ/m³). Finally, in silico simulations using a finite element model of sub-lamellar bone confirmed the links between the reduced intrafibrillar mineralization and the observed changes in the mechanical behavior of OI bone. Taken together, these results provide important mechanistic insights into the underlying cause of poor mechanical quality of OI bone, thus pave the way toward future treatments of this brittle bone disease.

Comprehensive Genetic Analysis of 128 Candidate Genes in a Cohort With Idiopathic, Severe, or Familial Osteoporosis

Context

The genetic bases of osteoporosis (OP), a disorder with high heritability, are poorly understood at an individual level. Cases of idiopathic or familial OP have long puzzled clinicians as to whether an actionable genetic cause could be identified.

Objective

We performed a genetic analysis of 28 cases of idiopathic, severe, or familial osteoporosis using targeted massively parallel sequencing.

Design

Targeted sequencing of 128 candidate genes was performed using Illumina NextSeq. Variants of interest were confirmed by Sanger sequencing or SNP array.

Patients and Setting

Thirty-seven patients in an academic tertiary hospital participated (54% male; median age, 44 years; 86% with fractures), corresponding to 28 sporadic or familial cases.

Main Outcome Measure

The identification of rare stop-gain, indel, splice site, copy-number, or nonsynonymous variants altering protein function.

Results

Altogether, we identified 28 variants of interest, but only 3 were classified as pathogenic or likely pathogenic variants: COL1A2 p.(Arg708Gln), WNT1 p.(Gly169Asp), and IDUA p.(His82Gln). An association of variants in different genes was found in 21% of cases, including a young woman with severe OP bearing WNT1, PLS3, and NOTCH2 variants. Among genes of uncertain significance analyzed, a potential additional line of evidence has arisen for GWAS candidates GPR68 and NBR1, warranting further studies.

Conclusions

While we hope that continuing efforts to identify genetic predisposition to OP will lead to improved and personalized care in the future, the likelihood of identifying actionable pathogenic variants in intriguing cases of idiopathic or familial osteoporosis is seemingly low.

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.

Morphology of Osteogenesis Imperfecta Collagen Mimetic Peptide Assemblies Correlates with the Identity of Glycine-Substituting Residue

Osteogenesis imperfecta (OI) is a hereditary bone disorder with various phenotypes ranging from mild multiple fractures to perinatal lethal cases, and it mainly results from the substitution of Gly by a bulkier residue in type I collagen. Triple-helical peptide models of Gly mutations have been widely utilized to decipher the etiology of OI, although these studies are mainly limited to characterizing the peptide features, such as stability and conformation in the solution state. Herein, we have constructed a new series of triple-helical peptides DD(GPO)₅ ZPO(GPO)₄ DD (Z=Ala, Arg, Asp, Cys, Glu, Ser, and Val) mimicking the most common types of observed OI cases. The inclusion of special terminal aspartic acids enables these collagen mimetic peptides to self-assemble to form nanomaterials upon the trigger of lanthanide ions. We have for the first time systematically evaluated the effect of different OI mutations on the aggregated state of collagen mimetic peptides. We have revealed that the identity of the Gly-substituting residue plays a determinant role in the morphology and secondary structure of the collagen peptide assemblies, showing that bulkier residues tend to result in a disruptive secondary structure and defective morphology, which lead to more severe OI phenotypes. These findings of osteogenesis imperfecta collagen mimetic peptides in the aggregation state provide novel perspectives on the molecular mechanism of osteogenesis imperfecta, and may aid the development of new therapeutic strategies.

A self-assembling collagen mimetic peptide system to simultaneously characterize the effects of osteogenesis imperfecta mutations on conformation, assembly and activity

We have created a self-assembling collagen mimetic peptide system which for the first time facilitates simultaneous characterization of the effects of osteogenesis imperfecta mutations on stability, conformation, assembly and activity.

Non-linearity of the collagen triple helix in solution and implications for collagen function

Collagen adopts a characteristic supercoiled triple helical conformation which requires a repeating (Xaa-Yaa-Gly)_n sequence. Despite the abundance of collagen, a combined experimental and atomistic modelling approach has not so far quantitated the degree of flexibility seen experimentally in the solution structures of collagen triple helices. To address this question, we report an experimental study on the flexibility of varying lengths of collagen triple helical peptides, composed of six, eight, ten and twelve repeats of the most stable Pro-Hyp-Gly (POG) units. In addition, one unblocked peptide, (POG)_10unblocked, was compared with the blocked (POG)₁₀ as a control for the significance of end effects. Complementary analytical ultracentrifugation and synchrotron small angle X-ray scattering data showed that the conformations of the longer triple helical peptides were not well explained by a linear structure derived from crystallography. To interpret these data, molecular dynamics simulations were used to generate 50 000 physically realistic collagen structures for each of the helices. These structures were fitted against their respective scattering data to reveal the best fitting structures from this large ensemble of possible helix structures. This curve fitting confirmed a small degree of non-linearity to exist in these best fit triple helices, with the degree of bending approximated as 4–17° from linearity. Our results open the way for further studies of other collagen triple helices with different sequences and stabilities in order to clarify the role of molecular rigidity and flexibility in collagen extracellular and immune function and disease.

Identification of a p.Arg708Gln variant in COL1A2 in atypical femoral fractures

OBJECTIVES

Long-term bisphosphonates exposure is a proven risk factor for atypical femoral fractures (AFF) but several cases occur in untreated patients. The identification of other risk factors for AFF is critical for the management of osteoporosis. We here assessed the genetic factors associated with AFF regardless of the treatment.

METHODS

Cases were identified through ICD-10 codes in 3 academic centers. Medical records were analyzed by 2 investigators that adjudicated X-rays for typical or atypical fractures. Genetic screening for ALPL, SOX9, COL1A1 and COL1A2 variants was performed after patient's information and consent.

RESULTS

A total of 389 cases were identified and 268 were ruled out according to the ASBMR Task Force recommendations. On the remaining 121, 14 (11.6%) were AFF. Anti-osteoporotic drugs were more frequent in the AFF group compared to the typical fracture group (35% vs 5%, P<0.001) but only 4 (28.6%) patients with AFF had been exposed to bisphosphonates. Genetic analysis performed in 5 patients found one with a heterozygous mutation in COL1A2 (rs72658163, NM_000089.3:c.2123G>A, p.Arg708Gln). This rare variant (Minor Allele Frequency=0.0008) causes a missense mutation that alters collagen fibrillogenesis. Eight heterozygous polymorphisms for ALPL were also found in 3 patients.

CONCLUSION

Genetic screening for variants in only 4 genes and 5 patients with AFF resulted in the identification of genetic variants in 3 patients including a rare variant in COL1A2, suggesting a possible genetic susceptibility to AFF. This finding should encourage clinician to further genotype patients with AFF in a collaborative multicentric project.

R992C (p.R1192C) Substitution in collagen II alters the structure of mutant molecules and induces the unfolded protein response

We investigated the molecular bases of spondyloepiphyseal dysplasia (SED) associated with the R992C (p.R1192C) substitution in collagen II. At the protein level, we analyzed the structure and integrity of mutant molecules, and at the cellular level, we specifically studied the effects of the presence of the R992C collagen II on the biological processes taking place in host cells. Our studies demonstrated that mutant collagen II molecules were characterized by altered electrophoretic mobility, relatively low thermostability, the presence of atypical disulfide bonds, and slow rates of secretion into the extracellular space. Analyses of cellular responses to the presence of the mutant molecules showed that excessive accumulation of thermolabile collagen II was associated with the activation of an "unfolded protein response" and an increase in apoptosis of host cells. Collectively, these data suggest that molecular mechanisms of SED may be driven not only by structural changes in the architecture of extracellular collagenous matrices, but also by intracellular processes activated by the presence of mutant collagen II molecules.

Mapping of SPARC/BM-40/osteonectin-binding sites on fibrillar collagens

The 33-kDa matrix protein SPARC (BM-40, osteonectin) binds several collagen types with moderate affinity. The collagen-binding site resides in helix alphaA of the extracellular calcium-binding domain of SPARC and is partially masked by helix alphaC. Previously, we found that the removal of helix alphaC caused a 10-fold increase in the affinity of SPARC for collagen, and we identified amino acids crucial for binding by site-directed mutagenesis. In this study, we used rotary shadowing, CNBr peptides, and synthetic peptides to map binding sites of SPARC onto collagens I, II, and III. Rotary shadowing and electron microscopy of SPARC-collagen complexes identified a major binding site approximately 180 nm from the C terminus of collagen. SPARC binding was also detected with lower frequency near the matrix metalloproteinase cleavage site. These data fit well with our analysis of SPARC binding to CNBr peptides, denaturation of which abolished binding, indicating triple-helical conformation of collagen to be essential. SPARC binding was substantially decreased in two of seven alpha2(I) mutant procollagen I samples and after N-acetylation of Lys/Hyl side chains in wild-type collagen. Synthetic peptides of collagen III were used to locate the binding sites, and we found SPARC binding activity in a synthetic triple-helical peptide containing the sequence GPOGPSGPRGQOGVMGFOGPKGNDGAO (where O indicates 4-hydroxyproline), with affinity for SPARC comparable with that of procollagen III. This sequence is conserved among alpha chains of collagens I, II, III, and V. In vitro collagen fibrillogenesis was delayed in the presence of SPARC, suggesting that SPARC might modulate collagen fibril assembly in vivo.

Defective C-propeptides of the proalpha2(I) chain of type I procollagen impede molecular assembly and result in osteogenesis imperfecta

Type I procollagen is a heterotrimer composed of two proalpha1(I) chains and one proalpha2(I) chain, encoded by the COL1A1 and COL1A2 genes, respectively. Mutations in these genes usually lead to dominantly inherited forms of osteogenesis imperfecta (OI) by altering the triple helical domains, but a few affect sequences in the proalpha1(I) C-terminal propeptide (C-propeptide), and one, which has a phenotype only in homozygotes, alters the proalpha2(I) C-propeptide. Here we describe four dominant mutations in the COL1A2 gene that alter sequences of the proalpha2(I) C-propeptide in individuals with clinical features of a milder form of the disease, OI type IV. Three of the four appear to interfere with disulfide bonds that stabilize the C-propeptide conformation and its interaction with other chains in the trimer. Cultured cells synthesized proalpha2(I) chains that were slow to assemble with proalpha1(I) chains to form heterotrimers and that were retained intracellularly. Some alterations led to the uncharacteristic formation of proalpha1(I) homotrimers. These findings show that the C-propeptide of proalpha2(I), like that of the proalpha1(I) C-propeptide, is essential for efficient assembly of type I procollagen heterotrimers. The milder OI phenotypes likely reflect a diminished amount of normal type I procollagen, small populations of overmodified heterotrimers, and proalpha1(I) homotrimers that are compatible with normal skeletal growth.

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.

Three arginine to cysteine substitutions in the pro-alpha (I)-collagen chain cause Ehlers-Danlos syndrome with a propensity to arterial rupture in early adulthood

Mutations in the COL1A1 and COL1A2 genes, encoding the proalpha1 and 2 chains of type I collagen, cause osteogenesis imperfecta (OI) or Ehlers-Danlos syndrome (EDS) arthrochalasis type. Although the majority of missense mutations in the collagen type I triple helix affect glycine residues in the Gly-Xaa-Yaa repeat, few nonglycine substitutions have been reported. Two arginine-to-cysteine substitutions in the alpha1(I)-collagen chain are associated with classic EDS [R134C (p.R312C)] or autosomal dominant Caffey disease with mild EDS features [R836C (p.R1014C)]. Here we show alpha1(I) R-to-C substitutions in three unrelated patients who developed iliac or femoral dissection in early adulthood. In addition, manifestations of classic EDS in Patient 1 [c.1053C>T; R134C (p.R312C); X-position] or osteopenia in Patients 2 [c.1839C>T; R396C (p.R574C); Y-position] and 3 [c.3396C>T; R915C (p.R1093C); Y-position] are seen. Dermal fibroblasts from the patients produced disulfide-bonded alpha1(I)-dimers in approximately 20% of type I collagen, which were efficiently secreted into the medium in case of the R396C and R915C substitution. Theoretical stability calculations of the collagen type I heterotrimer and thermal denaturation curves of monomeric mutant alpha1(I)-collagen chains showed minor destabilization of the collagen helix. However, dimers were shown to be highly unstable. The R134C and R396C caused delayed procollagen processing by N-proteinase. Ultrastructural findings showed collagen fibrils with variable diameter and irregular interfibrillar spaces, suggesting disturbed collagen fibrillogenesis. Our findings demonstrate that R-to-C substitutions in the alpha1(I) chain may result in a phenotype with propensity to arterial rupture in early adulthood. This broadens the phenotypic range of nonglycine substitutions in collagen type I and has important implications for genetic counseling and follow-up of patients carrying this type of mutation.

Potential modifier role of the R618Q variant of proalpha2(I)collagen in type I collagen fibrillogenesis: in vitro assembly analysis

An arginine to glutamine substitution in the triple helix of proalpha2(I)collagen (R618Q) was first reported in a patient with a variant of Marfan syndrome and later identified in conjunction with a second mutation in a patient with osteogenesis imperfecta (OI). The presence of the R618Q proalpha2(I)collagen allele in unaffected or mildly affected family members suggests that the R618Q allele is either a non-affecting polymorphism or a potential genetic modifier. Conservation of arginine618 across species and fibrillar collagen types suggests it is functionally significant. To investigate the functional significance of the R618Q proalpha2(I)collagen allele, we isolated type I collagen from cultured dermal fibroblasts of control and two unrelated individuals heterozygous for the R618Q proalpha2(I)collagen allele and evaluated helical stability and fibrillar assembly. Type I collagen thermal stability analyzed by protease susceptibility and CD spectroscopy demonstrated no statistical difference between control and R618Q containing collagen molecules. In vitro fibril assembly analyses demonstrated that R618Q containing collagen exhibits rapid fibrillar growth with minimal fibril nucleation phase. Further, electron microscopy demonstrated that the diameter of assembled R618Q containing collagen fibrils was approximately 20% of control collagen fibrils. These findings suggest the R618Q variant does not impact triple helical stability but has a role in collagen fibril assembly, supporting the hypothesis that the R618Q proalpha2(I)collagen variant is a modifier of connective tissue structure/function and is potentially involved in disease pathogenesis.

Optimization of a capillary electrophoretic method to detect and quantify the Gly-Pro dipeptide in complex matrices from long term cultured prolidase deficiency fibroblasts

A capillary electrophoresis (CE) method has been developed and optimized for the detection of Gly-Pro dipeptide in complex biological samples: medium, cell layer and matrix obtained from long term cultured human fibroblasts of control and prolidase deficiency patients. The influence of different detergents in the sample preparation and electrophoretic conditions were investigated. The method was validated for cellular extracts with respect to limits of detection and quantitation, precision, linearity, accuracy and robustness. The optimized method was applied to real samples and revealed a significant increase of intracellular Gly-Pro dipeptide in prolidase deficiency fibroblasts with respect to the control.

Type XIX collagen purified from human umbilical cord is characterized by multiple sharp kinks delineating collagenous subdomains and by intermolecular aggregates via globular, disulfide-linked, and heparin-binding amino termini

Type XIX collagen was discovered from the sequence of rhabdomyosarcoma cDNA clones. The chain is composed of a 268-residue amino terminus, an 832-residue discontinuous collagenous region, and a 19-residue carboxyl peptide. Light microscopy immunohistochemistry of adult human tissues demonstrated that type XIX is localized in vascular, neuronal, mesenchymal, and some epithelial basement membrane zones. It also appears to be involved in events linked to skeletal myogenesis. In this report, we have presented the first direct evidence for the molecular structure of type XIX collagen. Using human umbilical cord, native type XIX was purified by neutral salt extraction and by ion exchange and antibody affinity chromatography. Type XIX was found to represent only approximately 10(-6)% of the dry weight of tissue, making it by far the least abundant collagen ever isolated. Transmission electron microscopy after rotary shadowing revealed the appearance of rodlike structures with multiple sharp bends, a small nodule at one end of the molecule, and a total length of 240 nm. Domain-specific antibodies were used to identify the nodule as the noncollagenous amino terminus, whereas the location of most kinks corresponds to major interruptions separating the five collagenous subdomains. More than half of the type XIX molecules observed were present in oligomers of different size and complexity, resulting from association of the amino-terminal domains. Biochemical analysis demonstrated that these supramolecular aggregates are dependent upon and/or stabilized by intermolecular disulfide cross-links and that the globular amino terminus contains a high affinity, heparin-binding site. The polymorphic conformational states of this rare collagen, and its ability to self-assemble into a higher order structure provide focal points for future determination of biologically significant functions in cell-cell and/or cell-matrix interactions.

Destabilization of osteogenesis imperfecta collagen-like model peptides correlates with the identity of the residue replacing glycine

Mutations resulting in replacement of one obligate Gly residue within the repeating (Gly-Xaa-Yaa)(n) triplet pattern of the collagen type I triple helix are the major cause of osteogenesis imperfecta (OI). Phenotypes of OI involve fragile bones and range from mild to perinatal lethal. In this study, host-guest triple-helical peptides of the form acetyl-(Gly-Pro-Hyp)(3)-Zaa-Pro-Hyp-(Gly-Pro-Hyp)(4)-Gly-Gly-amide are used to isolate the influence of the residue replacing Gly on triple-helix stability, with Zaa = Gly, Ala, Arg, Asp, Glu, Cys, Ser, or Val. Any substitution for Zaa = Gly (melting temperature, T(m) = 45 degrees C) results in a dramatic destabilization of the triple helix. For Ala and Ser, T(m) decreases to approximately 10 degrees C, and for the Arg-, Val-, Glu-, and Asp-containing peptides, T(m) < 0 degrees C. A Gly --> Cys replacement results in T(m) < 0 degrees C under reducing conditions but shows a broad transition (T(m) approximately 19 degrees C) in an oxidizing environment. Addition of trimethylamine N-oxide increases T(m) by approximately 5 degrees C per 1 M trimethylamine N-oxide, resulting in stable triple-helix formation for all peptides and allowing comparison of relative stabilities. The order of disruption of different Gly replacements in these peptides can be represented as Ala </= Ser < CPO(red) < Arg < Val < Glu </= Asp. The rank of destabilization of substitutions for Gly in these Gly-Pro-Hyp-rich homotrimeric peptides shows a significant correlation with the severity of natural OI mutations in the alpha1 chain of type I collagen.