CRTAP mutations in lethal and severe osteogenesis imperfecta: the importance of combining biochemical and molecular genetic analysis

Zebrafish Models for Skeletal and Extraskeletal Osteogenesis Imperfecta Features: Unveiling Pathophysiology and Paving the Way for Drug Discovery

In the last decades, the easy genetic manipulation, the external fertilization, the high percentage of homology with human genes and the reduced husbandry costs compared to rodents, made zebrafish a valid model for studying human diseases and for developing new therapeutical strategies. Since zebrafish shares with mammals the same bone cells and ossification types, it became widely used to dissect mechanisms and possible new therapeutic approaches in the field of common and rare bone diseases, such as osteoporosis and osteogenesis imperfecta (OI), respectively. OI is a heritable skeletal disorder caused by defects in gene encoding collagen I or proteins/enzymes necessary for collagen I synthesis and secretion. Nevertheless, OI patients can be also characterized by extraskeletal manifestations such as dentinogenesis imperfecta, muscle weakness, cardiac valve and pulmonary abnormalities and skin laxity. In this review, we provide an overview of the available zebrafish models for both dominant and recessive forms of OI. An updated description of all the main similarities and differences between zebrafish and mammal skeleton, muscle, heart and skin, will be also discussed. Finally, a list of high- and low-throughput techniques available to exploit both larvae and adult OI zebrafish models as unique tools for the discovery of new therapeutic approaches will be presented.

The structural basis for the collagen processing by human P3H1/CRTAP/PPIB ternary complex

Collagen posttranslational processing is crucial for its proper assembly and function. Disruption of collagen processing leads to tissue development and structure disorders like osteogenesis imperfecta (OI). OI-related collagen processing machinery includes prolyl 3-hydroxylase 1 (P3H1), peptidyl-prolyl cis-trans isomerase B (PPIB), and cartilage-associated protein (CRTAP), with their structural organization and mechanism unclear. We determine cryo-EM structures of the P3H1/CRTAP/PPIB complex. The active sites of P3H1 and PPIB form a face-to-face bifunctional reaction center, indicating a coupled modification mechanism. The structure of the P3H1/CRTAP/PPIB/collagen peptide complex reveals multiple binding sites, suggesting a substrate interacting zone. Unexpectedly, a dual-ternary complex is observed, and the balance between ternary and dual-ternary states can be altered by mutations in the P3H1/PPIB active site and the addition of PPIB inhibitors. These findings provide insights into the structural basis of collagen processing by P3H1/CRTAP/PPIB and the molecular pathology of collagen-related disorders.

Genetic Analysis, Phenotypic Spectrum and Functional Study of Rare Osteogenesis Imperfecta Caused by CRTAP Variants

OBJECTIVE

Deficiency of cartilage-associated protein (CRTAP) can cause extremely rare autosomal recessive osteogenesis imperfecta (OI) type VII. We investigated the pathogenic mechanisms of CRTAP variants through functional studies on bones of patients with OI.

METHODS

Two nonconsanguineous families with CRTAP mutations were included and their phenotypes and genotypes were evaluated. Bone specimens were obtained from 1 patient with OI and a normal control during orthopedic surgery. The impacts of the novel variant on the CRTAP transcript were confirmed. The expression levels of CRTAP mRNA and CRTAP protein were analyzed. The quantification of prolyl 3-hydroxylation in the α1 chain of type I collagen was evaluated.

RESULTS

Patients with OI type VII had early-onset recurrent fractures, severe osteoporosis, and bone deformities. The c.621 + 1G > A and c.1153-3C > G mutations were identified in CRTAP in the patients with OI. The c.621 + 1G > A variant was a novel mutation that could impair mRNA transcription, leading to a truncated CRTAP protein. In a patient with c.621 + 1G > A and c.1153-3C > G mutations in CRTAP, the mRNA and protein levels of CRTAP in osteoblasts were significantly decreased and the osteoid volume and osteoblast numbers were markedly reduced compared with those in the normal control individual. This was simultaneously accompanied by significantly reduced prolyl 3-hydroxylation at Pro986 in the α1 chain of type I collagen and invisible active bone formation in bone.

CONCLUSION

The novel c.621 + 1G > A mutation in CRTAP expands the genotypic spectrum of type VII OI. Biallelic mutations of c.621 + 1G > A and c.1153-3C > G in CRTAP can lead to reduced CRTAP mRNA and deficient CRTAP protein in osteoblasts, which reduces 3-hydroxylation in Pro986 of the α1 chain of type I collagen and impairs bone formation, thus contributing to severe OI type VII.

IFITM5-related (type V) osteogenesis imperfecta with evidence of perinatal involvement: A case report

Osteogenesis imperfecta (OI) is a rare hereditary disorder characterized by bone fragility and frequent fractures. While most cases are attributed to variations in collagen-coding genes COL1A1 and COL1A2, other genes such as IFITM5 have also been associated with the disease, accounting for up to 5 % of cases. Here, we report a case of a 3-month-old female with a femur fracture and limb deformity. X-rays revealed evidence of osteopenia and previous fractures in the arms, clavicle, ribs, and left limb, alongside prenatal bone deformities detected by ultrasound. Initial clinical evaluation suggested progressively deforming (Sillence's type III) osteogenesis imperfecta (OI). Molecular testing led to the diagnosis of IFITM5-related OI, identifying the c.-14C>T (rs587776916) variant. Although this variant has been previously reported in patients with IFITM5-related OI, prenatal involvement had not been associated with this variant.

Ventriculosubgaleal shunt placement for hydrocephalus in osteogenesis imperfecta with novel compound heterozygous CRTAP variants

Osteogenesis imperfecta is characterized by frequent fractures, bone deformities, and other systemic symptoms. Severe osteogenesis imperfecta may progress to hydrocephalus; however, treatment strategies for this complication remain unclear. Here, we describe severe osteogenesis imperfecta in an infant with symptomatic hydrocephalus treated with ventriculosubgaleal shunt placement. Targeted next-generation sequencing revealed novel compound heterozygous CRTAP variants, i.e., NM_006371.5, c.241 G > T, p.(Glu81*) and NM_006371.5, c.923-2_932del. We suggest that ventriculosubgaleal shunt placement is an effective and safe treatment for hydrocephalus in patients with severe osteogenesis imperfecta.

Deep intronic mutation in CRTAP results in unstable isoforms of the protein to induce type I collagen aggregation in a lethal type of osteogenesis imperfecta type VII

Genetic mutations are involved in Mendelian disorders. Unbuffered intronic mutations in gene variants can generate aberrant splice sites in mutant transcripts, resulting in mutant isoforms of proteins with modulated expression, stability, and function in diseased cells. Here, we identify a deep intronic variant, c.794_1403A>G, in CRTAP by genome sequencing of a male fetus with osteogenesis imperfecta (OI) type VII. The mutation introduces cryptic splice sites in intron-3 of CRTAP, resulting in two mature mutant transcripts with cryptic exons. While transcript-1 translates to a truncated isoform (277 amino acids) with thirteen C-terminal non-wild-type amino acids, transcript-2 translates to a wild-type protein sequence, except that this isoform contains an in-frame fusion of non-wild-type twenty-five amino acids in a tetratricopeptide repeat sequence. Both mutant isoforms of CRTAP are unstable due to the presence of a unique 'GWxxI' degron, which finally leads to loss of proline hydroxylation and aggregation of type I collagen. Although type I collagen aggregates undergo autophagy, the overall proteotoxicity resulted in death of the proband cells by senescence. In summary, we present a genetic disease pathomechanism by linking a novel deep intronic mutation in CRTAP to unstable mutant isoforms of the protein in lethal OI type VII.

Osteogenesis imperfecta in 140 Turkish families: Molecular spectrum and, comparison of long-term clinical outcome of those with COL1A1/A2 and biallelic variants

BACKGROUND

Osteogenesis imperfecta (OI) is a clinically and genetically heterogeneous group of diseases characterized by increased bone fragility and deformities. Although most patients with OI have heterozygous mutations in COL1A1 or COL1A2, 17 genes have been reported to cause OI, most of which are autosomal recessive (AR) inherited, during the last years. The aim of this study is to determine the mutation spectrum in Turkish OI cohort and to investigate the genotype-phenotype correlation.

METHODS

150 patients from 140 Turkish families with OI phenotype were included in this study. Mutations in OI-related genes were identified using targeted gene panel, MLPA analysis for COL1A1 and whole exome sequencing. 113 patients who had OI disease-causing variants were followed for 1-20 years.

RESULTS

OI disease-causing variants were detected in 117 families, of which 62.4% in COL1A1/A2, 35.9% in AR-related genes. A heterozygous variant in IFITM5 and a hemizygous in MBTPS2 were also described, one in each patient. Eighteen biallelic variants (13 novel) were identified in nine genes (FKBP10, P3H1, SERPINF1, TMEM38B, WNT1, BMP1, CRTAP, FAM46A, MESD) among which FKBP10, P3H1 and SERPINF1 were most common. The most severe phenotypes were in patients with FKBP10, SERPINF1, CRTAP, FAM46A and MESD variants. P3H1 patients had moderate, while BMP1 had the mild phenotype. Clinical phenotypes were variable in patients with WNT1 and TMEM38B mutations. We also found mutations in ten genes (PLS3, LRP5, ANO5, SLC34A1, EFEMP2, PRDM5, GORAB, OCRL1, TNFRSF11B, DPH1) associated with diseases presenting clinical features which overlap OI, in eleven families.

CONCLUSION

We identified disease-causing mutations in 83.6% in a large Turkish pediatric OI cohort. 40 novel variants were described. Clinical features and long-term follow-up findings of AR inherited OI types and especially very rare biallelic variants were presented for the first time. Unlike previously reported studies, the mutations that we found in P3H1 were all missense, causing a moderate phenotype.

Collagen transport and related pathways in Osteogenesis Imperfecta

Osteogenesis Imperfecta (OI) comprises a heterogeneous group of patients who share bone fragility and deformities as the main characteristics, albeit with different degrees of severity. Phenotypic variation also exists in other connective tissue aspects of the disease, complicating disease classification and disease course prediction. Although collagen type I defects are long established as the primary cause of the bone pathology, we are still far from comprehending the complete mechanism. In the last years, the advent of next generation sequencing has triggered the discovery of many new genetic causes for OI, helping to draw its molecular landscape. It has become clear that, in addition to collagen type I genes, OI can be caused by multiple proteins connected to different parts of collagen biosynthesis. The production of collagen entails a complex process, starting from the production of the collagen Iα1 and collagen Iα2 chains in the endoplasmic reticulum, during and after which procollagen is subjected to a plethora of posttranslational modifications by chaperones. After reaching the Golgi organelle, procollagen is destined to the extracellular matrix where it forms collagen fibrils. Recently discovered mutations in components of the retrograde transport of chaperones highlight its emerging role as critical contributor of OI development. This review offers an overview of collagen regulation in the context of recent gene discoveries, emphasizing the significance of transport disruptions in the OI mechanism. We aim to motivate exploration of skeletal fragility in OI from the perspective of these pathways to identify regulatory points which can hint to therapeutic targets.

MBTPS2, a membrane bound protease, underlying several distinct skin and bone disorders

The MBTPS2 gene on the X-chromosome encodes the membrane-bound transcription factor protease, site-2 (MBTPS2) or site-2 protease (S2P) which cleaves and activates several signaling and regulatory proteins from the membrane. The MBTPS2 is critical for a myriad of cellular processes, ranging from the regulation of cholesterol homeostasis to unfolded protein responses. While its functional role has become much clearer in the recent years, how mutations in the MBTPS2 gene lead to several human disorders with different phenotypes including Ichthyosis Follicularis, Atrichia and Photophobia syndrome (IFAP) with or without BRESHECK syndrome, Keratosis Follicularis Spinulosa Decalvans (KFSD), Olmsted syndrome, and Osteogenesis Imperfecta type XIX remains obscure. This review presents the biological role of MBTPS2 in development, summarizes its mutations and implicated disorders, and discusses outstanding unanswered questions.

Type I and type V procollagen triple helix uses different subsets of the molecular ensemble for lysine posttranslational modifications in the rER

Collagen is the most abundant protein in humans. It has a characteristic triple-helix structure and is heavily posttranslationally modified. The complex biosynthesis of collagen involves processing by many enzymes and chaperones in the rough endoplasmic reticulum. Lysyl hydroxylase 1 (LH1) is required to hydroxylate lysine for cross-linking and carbohydrate attachment within collagen triple helical sequences. Additionally, a recent study of prolyl 3-hydroxylase 3 (P3H3) demonstrated that this enzyme may be critical for LH1 activity; however, the details surrounding its involvement remain unclear. If P3H3 is an LH1 chaperone that is critical for LH1 activity, P3H3 and LH1 null mice should display a similar deficiency in lysyl hydroxylation. To test this hypothesis, we compared the amount and location of hydroxylysine in the triple helical domains of type V and I collagen from P3H3 null, LH1 null, and wild-type mice. The amount of hydroxylysine in type V collagen was reduced in P3H3 null mice, but surprisingly type V collagen from LH1 null mice contained as much hydroxylysine as type V collagen from wild-type mice. In type I collagen, our results indicate that LH1 plays a global enzymatic role in lysyl hydroxylation. P3H3 is also involved in lysyl hydroxylation, particularly at cross-link formation sites, but is not required for all lysyl hydroxylation sites. In summary, our study suggests that LH1 and P3H3 likely have two distinct mechanisms to recognize different collagen types and to distinguish cross-link formation sites from other sites in type I collagen.

Over-Representation of Recessive Osteogenesis Imperfecta in Asian Indian Children

Several genes are implicated in the etiology of early onset osteogenesis imperfecta (OI). The various genes causing severe OI include WNT1 , SERPINF1 , P3H1 , CREB3L1 , and CRTAP , although glycine substitutions in COL1A1chains have also been predicted to cause perinatal lethal OI . Patients with early onset OI present decreased mobility, recurrent rib fractures, bony deformities, and chest infections that lead to an early death. We reported our experience in children with OI in Asian Indian families, which includes two patients with SERPINF1 pathogenic variants; and another two patients with severe OI and antenatal fractures caused by pathogenic variants in the CRTAP gene, identified by next generation sequencing (NGS). For one affected fetus, medical termination of pregnancy was done. The other baby was started on zoledronate therapy just after birth and is now 3 years old. Prenatal diagnosis was subsequently done on chorionic villus sample in the latter family.

Evidence for a de novo, dominant germ-line mutation causative of osteogenesis imperfecta in two Red Angus calves

A genetic disorder, osteogenesis imperfecta (OI) is broadly characterized by connective tissue abnormalities and bone fragility most commonly attributed to alterations in Type I collagen. Two Red Angus calves by the same sire presented with severe bone and dental fragility, blue sclera, and evidence of in utero fractures consistent with OI congenita. Comparative analyses with human cases suggested the OI in these calves most closely resembled that classified as OI Type II. Due to the phenotypic classification and shared paternity, a dominant, germ-line variant was hypothesized as causative although recessive genotypes were also considered due to a close relationship between the sire and dam of one calf. Whole-genome sequencing revealed the presence of a missense mutation in the alpha 1 chain of collagen Type I (COL1A1), for which both calves were heterozygous. The variant resulted in the substitution of a glycine residue with serine in the triple helical domain of the protein; in this region, glycine normally occupies every third position as is critical for correct formation of the Type I collagen molecule. Allele-specific amplification by droplet digital PCR further quantified the variant at a frequency of nearly 4.4% in the semen of the sire while it was absent in his blood, supporting the hypothesis of a de novo causative variant for which the germ line of the sire was mosaic. The identification of novel variants associated with unwanted phenotypes in livestock is critical as the high prolificacy of breeding stock has the potential to rapidly disseminate undesirable variation.

Comparison of Bone Microarchitecture Between Adult Osteogenesis Imperfecta and Early-Onset Osteoporosis

Diagnosis and management of adult individuals with low bone mass and increased bone fragility before the age of 50 can be challenging. A number of these patients are diagnosed with mild osteogenesis imperfecta (OI) through detection of COL1A1 or COL1A2 mutations; however, a clinical differentiation from early-onset osteoporosis (EOOP) may be difficult. The purpose of this study was to determine the bone microstructural differences between mild OI and EOOP patients. 29 patients showed mutations in COL1A1 or COL1A2 and were classified as OI. Skeletal assessment included dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and bone turnover serum analyses. Bone microstructure of 21/29 OI patients was assessed and compared to 23 age- and sex-matched patients clinically classified EOOP but without mutations in the known disease genes as well as to 20 healthy controls. In the OI patients, we did not observe an age-dependent decrease in DXA Z-scores. HR-pQCT revealed a significant reduction in volumetric BMD and microstructural parameters in the distal radius and tibia in both the OI and EOOP cohorts compared to the healthy controls. When comparing the bone microstructure of OI patients with the EOOP cohort, significant differences were found in terms of bone geometry in the radius, while no significant changes were detected in all other HR-pQCT parameters at the radius and tibia. Taken together, adult mild OI patients demonstrate a predominantly high bone turnover trabecular bone loss syndrome that shows minor microstructural differences compared to EOOP without mutation detection.

Comparative X-ray morphometry of prenatal osteogenesis imperfecta type 2 and thanatophoric dysplasia: a contribution to prenatal differential diagnosis

OBJECTIVE

The purpose of the paper was to assess the morphometric parameters to improve the specificity of the ultrasound (US) signs for the early differential diagnosis between two lethal dysplasias, as thanatophoric dysplasia (TD) and osteogenesis imperfecta type 2 (OI-2).

METHOD

The diaphyseal length and the bowed shape of long bones associated with vertebral body dimension assessment were investigated in a group of 14 pregnancy terminations carried out in the time period 2007-2013. The definitive diagnosis was established after pregnancy termination by means of skeletal standardized X-rays, histopathology and gene analysis.

RESULTS

TD and OI-2 long bones were significantly shorter than controls. No significant differences were observed between the two dysplasias. The bowing angle was higher in OI-2; a true angulation or eventually axial displacement was present only in the latter. Furthermore, they did not show any evidence of vertebral collapse. The thanatophoric dysplasia presented less bowed long bones, and never true angulation. The spine was steadily characterized by flattened anterior vertebral bodies.

CONCLUSION

Long bone shortening is not a sufficient and accurate sign for early sonographic differential diagnosis between TD and OI-2. Angled diaphysis, axial diaphyseal displacement and a conserved vertebral body height in the prenatal period support the diagnosis of osteogenesis imperfecta type 2, while moderately regular bowed diaphysis associated with platyspondyly that of thanatophoric dysplasia.

With expanded carrier screening, founder populations run the risk of being overlooked

Genetically isolated populations exist worldwide. Specific genetic disorders, including rare autosomal recessive disorders may have high prevalences in these populations. We searched for Dutch genetically isolated populations and their autosomal recessive founder mutations. We investigated whether these founder mutations are covered in the (preconception) expanded carrier screening tests of five carrier screening providers. Our results show that the great majority of founder mutations are not covered in these screening panels, and these panels may thus not be appropriate for use in founder populations. It is therefore important to be aware of founder mutations in a population when offering carrier tests.

Targeted carrier screening for four recessive disorders: high detection rate within a founder population

In a genetically isolated community in the Netherlands four severe recessive genetic disorders occur at relatively high frequency (pontocerebellar hypoplasia type 2 (PCH2), fetal akinesia deformation sequence (FADS), rhizomelic chondrodysplasia punctata type 1 (RCDP1), and osteogenesis imperfecta (OI) type IIB/III. Over the past decades multiple patients with these disorders have been identified. This warranted the start of a preconception outpatient clinic, in 2012, aimed at couples planning a pregnancy. The aim of our study was to evaluate the offer of targeted genetic carrier screening as a method to identify high-risk couples for having affected offspring in this high-risk subpopulation. In one year, 203 individuals (92 couples and 19 individuals) were counseled. In total, 65 of 196 (33.2%) tested individuals were carriers of at least one disease, five (7.7%) of them being carriers of two diseases. Carrier frequencies of PCH2, FADS, RCDP1, and OI were 14.3%, 11.2%, 6.1%, and 4.1% respectively. In individuals with a positive family history for one of the diseases, the carrier frequency was 57.8%; for those with a negative family history this was 25.8%. Four PCH2 carrier-couples were identified. Thus, targeted (preconception) carrier screening in this genetically isolated population in which a high prevalence of specific disorders occurs detects a high number of carriers, and is likely to be more effective compared to cascade genetic testing. Our findings and set-up can be seen as a model for carrier screening in other high-risk subpopulations and contributes to the discussion about the way carrier screening can be offered and organized in the general population.

An additional function of the rough endoplasmic reticulum protein complex prolyl 3-hydroxylase 1·cartilage-associated protein·cyclophilin B: the CXXXC motif reveals disulfide isomerase activity in vitro

Collagen biosynthesis occurs in the rough endoplasmic reticulum, and many molecular chaperones and folding enzymes are involved in this process. The folding mechanism of type I procollagen has been well characterized, and protein disulfide isomerase (PDI) has been suggested as a key player in the formation of the correct disulfide bonds in the noncollagenous carboxyl-terminal and amino-terminal propeptides. Prolyl 3-hydroxylase 1 (P3H1) forms a hetero-trimeric complex with cartilage-associated protein and cyclophilin B (CypB). This complex is a multifunctional complex acting as a prolyl 3-hydroxylase, a peptidyl prolyl cis-trans isomerase, and a molecular chaperone. Two major domains are predicted from the primary sequence of P3H1: an amino-terminal domain and a carboxyl-terminal domain corresponding to the 2-oxoglutarate- and iron-dependent dioxygenase domains similar to the α-subunit of prolyl 4-hydroxylase and lysyl hydroxylases. The amino-terminal domain contains four CXXXC sequence repeats. The primary sequence of cartilage-associated protein is homologous to the amino-terminal domain of P3H1 and also contains four CXXXC sequence repeats. However, the function of the CXXXC sequence repeats is not known. Several publications have reported that short peptides containing a CXC or a CXXC sequence show oxido-reductase activity similar to PDI in vitro. We hypothesize that CXXXC motifs have oxido-reductase activity similar to the CXXC motif in PDI. We have tested the enzyme activities on model substrates in vitro using a GCRALCG peptide and the P3H1 complex. Our results suggest that this complex could function as a disulfide isomerase in the rough endoplasmic reticulum.

Mutations in FKBP10, which result in Bruck syndrome and recessive forms of osteogenesis imperfecta, inhibit the hydroxylation of telopeptide lysines in bone collagen

Although biallelic mutations in non-collagen genes account for <10% of individuals with osteogenesis imperfecta, the characterization of these genes has identified new pathways and potential interventions that could benefit even those with mutations in type I collagen genes. We identified mutations in FKBP10, which encodes the 65 kDa prolyl cis-trans isomerase, FKBP65, in 38 members of 21 families with OI. These include 10 families from the Samoan Islands who share a founder mutation. Of the mutations, three are missense; the remainder either introduce premature termination codons or create frameshifts both of which result in mRNA instability. In four families missense mutations result in loss of most of the protein. The clinical effects of these mutations are short stature, a high incidence of joint contractures at birth and progressive scoliosis and fractures, but there is remarkable variability in phenotype even within families. The loss of the activity of FKBP65 has several effects: type I procollagen secretion is slightly delayed, the stabilization of the intact trimer is incomplete and there is diminished hydroxylation of the telopeptide lysyl residues involved in intermolecular cross-link formation in bone. The phenotype overlaps with that seen with mutations in PLOD2 (Bruck syndrome II), which encodes LH2, the enzyme that hydroxylates the telopeptide lysyl residues. These findings define a set of genes, FKBP10, PLOD2 and SERPINH1, that act during procollagen maturation to contribute to molecular stability and post-translational modification of type I procollagen, without which bone mass and quality are abnormal and fractures and contractures result.

Recessive osteogenesis imperfecta: clinical, radiological, and molecular findings

Osteogenesis imperfecta (OI) or "brittle bone disease" is currently best described as a group of hereditary connective tissue disorders related to primary defects in type I procollagen, and to alterations in type I procollagen biosynthesis, both associated with osteoporosis and increased susceptibility to bone fractures. Initially, the autosomal dominant forms of OI, caused by mutations in either COL1A1 or COL1A2, were described. However, for decades, the molecular defect of a small percentage of patients clinically diagnosed with OI has remained elusive. It has been in the last 6 years that the genetic causes of several forms of OI with autosomal recessive inheritance have been characterized. These comprise defects of collagen chaperones, and proteins involved in type I procollagen assembly, processing and maturation, as well as proteins involved in the formation and homeostasis of bone tissue. This article reviews the recently characterized forms of recessive OI, focusing in particular on their clinical and molecular findings, and on their radiological characterisation. Clinical management and treatment of OI in general will be discussed, too.

Deficiency of CRTAP in non-lethal recessive osteogenesis imperfecta reduces collagen deposition into matrix

Deficiency of any component of the ER-resident collagen prolyl 3-hydroxylation complex causes recessive osteogenesis imperfecta (OI). The complex modifies the α1(I)Pro986 residue and contains cartilage-associated protein (CRTAP), prolyl 3-hydroxylase 1 (P3H1) and cyclophilin B (CyPB). Fibroblasts normally secrete about 10% of CRTAP. Most CRTAP mutations cause a null allele and lethal type VII OI. We identified a 7-year-old Egyptian boy with non-lethal type VII OI and investigated the effects of his null CRTAP mutation on collagen biochemistry, the prolyl 3-hydroxylation complex, and collagen in extracellular matrix. The proband is homozygous for an insertion/deletion in CRTAP (c.118_133del16insTACCC). His dermal fibroblasts synthesize fully overmodified type I collagen, and 3-hydroxylate only 5% of α1(I)Pro986. CRTAP transcripts are 10% of control. CRTAP protein is absent from proband cells, with residual P3H1 and normal CyPB levels. Dermal collagen fibril diameters are significantly increased. By immunofluorescence of long-term cultures, we identified a severe deficiency (10-15% of control) of collagen deposited in extracellular matrix, with disorganization of the minimal fibrillar network. Quantitative pulse-chase experiments corroborate deficiency of matrix deposition, rather than increased matrix turnover. We conclude that defects of extracellular matrix, as well as intracellular defects in collagen modification, contribute to the pathology of type VII OI.

Osteogenesis Imperfecta: A Review with Clinical Examples

Osteogenesis imperfecta (OI) is characterized by susceptibility to bone fractures, with a severity ranging from subtle increase in fracture frequency to prenatal fractures. The first scientific description of OI dates from 1788. Since then, important milestones in OI research and treatment have, among others, been the classification of OI into 4 types (the 'Sillence classification'), the discovery of defects in collagen type I biosynthesis as a cause of most cases of OI and the use of bisphosphonate therapy. Furthermore, in the past 5 years, it has become clear that OI comprises a group of heterogeneous disorders, with an estimated 90% of cases due to a causative variant in the COL1A1 or COL1A2 genes and with the remaining 10% due to causative recessive variants in the 8 genes known so far, or in other currently unknown genes. This review aims to highlight the current knowledge around the history, epidemiology, pathogenesis, clinical/radiological features, management, and future prospects of OI. The text will be illustrated with clinical descriptions, including radiographs and, where possible, photographs of patients with OI.

Severe osteogenesis imperfecta caused by a small in-frame deletion in CRTAP

Mutations of proteins involved in posttranslational modification of collagen type I can cause osteogenesis imperfecta (OI) inherited in a recessive pattern. The cartilage-associated protein (CRTAP) is part of a heterotrimeric complex (together with prolyl-3-hydroxylase-1 [P3H1] and cyclophilin B) that 3-hydroxylates the alpha 1 chain of collagen type I at proline residue 986 and plays a collagen chaperon role. CRTAP mutations usually cause severe OI. We report on a patient with OI and a homozygous in-frame deletion in CRTAP and a severe form of OI. The girl was born with markedly deformed long bones. Despite intravenous bisphosphonate treatment, she developed multiple vertebral compression fractures and severe scoliosis and at 4 years of age was able to sit only with support. Although CRTAP transcript levels were normal in the patient's fibroblasts, protein levels of both CRTAP and P3H1 were severely reduced. The degree of 3-hydroxylation at proline residue 986 was also decreased. This report characterizes a patient with a CRTAP small in-frame deletion. We are unaware of prior reports of this finding. We suggest that this deletion affects crucial amino acids that are important for the interaction and/or stabilization of CRTAP and P3H1.

Lethal/severe osteogenesis imperfecta in a large family: a novel homozygous LEPRE1 mutation and bone histological findings

We report a large consanguineous Turkish family in which multiple individuals are affected with autosomal recessive lethal or severe osteogenesis imperfecta (OI) due to a novel homozygous LEPRE1 mutation. In one affected individual histological studies of bone tissue were performed, which may indicate that the histology of LEPRE1 -associated OI is indistinguishable from COL1A1/2 -, CRTAP -, and PPIB -related OI.

Wormian bones in osteogenesis imperfecta: Correlation to clinical findings and genotype

The presence of a larger than usual number of Wormian bones (accessory skull bones completely surrounded by a suture line) is a well-known radiographic sign of osteogenesis imperfecta (OI), but the phenotypic and genotypic correlates are not well characterized. In the present study we retrospectively analyzed skull radiographs of 195 OI patients (median age 11.8 years, range 0.4-48 years; 100 female). A significant number of Wormian bones (SNWB, defined as the presence of 10 or more Wormian bones) were found in at least one patient in all of the OI types studied (I, III to VII). SNWB were observed in 35% of patients with OI type I, in 96% of patients with OI type III and 78% of patients with OI type IV. SNWB were present in 28% of patients with haploinsufficiency (nonsense and frameshift) mutations in COL1A1, in 96% of patients with helical glycine substitutions in the alpha 1 chain of collagen type I and in 72% of patients with helical glycine substitutions in the alpha 2 chain of collagen type I. Stepwise multivariate logistic regression analysis showed that height z-score, an indicator of disease severity, was inversely related with the prevalence of SNWB. SNWB were visible in 19 of the 26 patients who had skull radiographs in the first year of life, including a 2-week-old newborn. Thus, it appears that SNWB occur more frequently in more severely affected OI patients and seem to develop mostly in utero.

The skeletal dysplasias

The skeletal dysplasias (osteochondrodysplasias) are a heterogeneous group of more than 350 disorders frequently associated with orthopedic complications and varying degrees of dwarfism or short stature. These disorders are diagnosed based on radiographic, clinical, and molecular criteria. The molecular mechanisms have been elucidated in many of these disorders providing for improved clinical diagnosis and reproductive choices for affected individuals and their families. An increasing variety of medical and surgical treatment options can be offered to affected individuals to try to improve their quality of life and lifespan.

Role of cartilage-associated protein in skeletal development

The past 3 years have been exciting for collagen biologists and human geneticists studying the disease known as osteogenesis imperfecta (OI or brittle bone disease). Functional studies on cartilage-associated protein (Crtap) have identified it as an essential component of a heterotrimeric, endoplasmic reticulum resident complex responsible for collagen prolyl 3-hydroxylation and chaperone function. Importantly, human mutations in the CRTAP gene have been associated with recessive forms of OI. Although the function and in vivo biological significance of the 3-hydroxyproline modification are still poorly understood, studies on Crtap have led to the identification of additional genes in which mutations also cause recessive forms of OI. These discoveries have now focused the interest of geneticists on the endoplasmic reticulum that will require the help of biochemists to unravel the molecular dynamics and complexities of collagen folding.

Generalized connective tissue disease in Crtap-/- mouse

Mutations in CRTAP (coding for cartilage-associated protein), LEPRE1 (coding for prolyl 3-hydroxylase 1 [P3H1]) or PPIB (coding for Cyclophilin B [CYPB]) cause recessive forms of osteogenesis imperfecta and loss or decrease of type I collagen prolyl 3-hydroxylation. A comprehensive analysis of the phenotype of the Crtap-/- mice revealed multiple abnormalities of connective tissue, including in the lungs, kidneys, and skin, consistent with systemic dysregulation of collagen homeostasis within the extracellular matrix. Both Crtap-/- lung and kidney glomeruli showed increased cellular proliferation. Histologically, the lungs showed increased alveolar spacing, while the kidneys showed evidence of segmental glomerulosclerosis, with abnormal collagen deposition. The Crtap-/- skin had decreased mechanical integrity. In addition to the expected loss of proline 986 3-hydroxylation in alpha1(I) and alpha1(II) chains, there was also loss of 3Hyp at proline 986 in alpha2(V) chains. In contrast, at two of the known 3Hyp sites in alpha1(IV) chains from Crtap-/- kidneys there were normal levels of 3-hydroxylation. On a cellular level, loss of CRTAP in human OI fibroblasts led to a secondary loss of P3H1, and vice versa. These data suggest that both CRTAP and P3H1 are required to maintain a stable complex that 3-hydroxylates canonical proline sites within clade A (types I, II, and V) collagen chains. Loss of this activity leads to a multi-systemic connective tissue disease that affects bone, cartilage, lung, kidney, and skin.

Lack of cyclophilin B in osteogenesis imperfecta with normal collagen folding

Osteogenesis imperfecta is a heritable disorder that causes bone fragility. Mutations in type I collagen result in autosomal dominant osteogenesis imperfecta, whereas mutations in either of two components of the collagen prolyl 3-hydroxylation complex (cartilage-associated protein [CRTAP] and prolyl 3-hydroxylase 1 [P3H1]) cause autosomal recessive osteogenesis imperfecta with rhizomelia (shortening of proximal segments of upper and lower limbs) and delayed collagen folding. We identified two siblings who had recessive osteogenesis imperfecta without rhizomelia. They had a homozygous start-codon mutation in the peptidyl-prolyl isomerase B gene (PPIB), which results in a lack of cyclophilin B (CyPB), the third component of the complex. The proband's collagen had normal collagen folding and normal prolyl 3-hydroxylation, suggesting that CyPB is not the exclusive peptidyl-prolyl cis-trans isomerase that catalyzes the rate-limiting step in collagen folding, as is currently thought.

Null mutations in LEPRE1 and CRTAP cause severe recessive osteogenesis imperfecta

Classical osteogenesis imperfecta (OI) is a dominant genetic disorder of connective tissue caused by mutations in either of the two genes encoding type I collagen, COL1A1 and COL1A2. Recent investigations, however, have generated a new paradigm for OI incorporating many of the prototypical features that distinguish dominant and recessive conditions, within a type I collagen framework. We and others have shown that the long-sought cause of the recessive form of OI, first postulated in the Sillence classification, lies in defects in the genes encoding cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1 (P3H1/LEPRE1). Together with cyclophilin B (PPIB), CRTAP and P3H1 comprise the collagen prolyl 3-hydroxylation complex, which catalyzes a specific posttranslational modification of types I, II, and V collagen, and may act as a general chaperone. Patients with mutations in CRTAP or LEPRE1 have a lethal to severe osteochondrodystrophy that overlaps with Sillence types II and III OI but has distinctive features. Infants with recessive OI have white sclerae, undertubulation of the long bones, gracile ribs without beading, and a small to normal head circumference. Those who survive to childhood or the teen years have severe growth deficiency and extreme bone fragility. Most causative mutations result in null alleles, with the absence or severe reduction of gene transcripts and proteins. As expected, 3-hydroxylation of the Pro986 residue is absent or severly reduced, but bone severity and survival length do not correlate with the extent of residual hydroxylation. Surprisingly, the collagen produced by cells with an absence of Pro986 hydroxylation has helical overmodification by lysyl hydroxylase and prolyl 4-hydroxylase, indicating that the folding of the collagen helix has been substantially delayed.

PPIB mutations cause severe osteogenesis imperfecta

Deficiency of cartilage-associated protein (CRTAP) or prolyl 3-hydroxylase 1(P3H1) has been reported in autosomal-recessive lethal or severe osteogenesis imperfecta (OI). CRTAP, P3H1, and cyclophilin B (CyPB) form an intracellular collagen-modifying complex that 3-hydroxylates proline at position 986 (P986) in the alpha1 chains of collagen type I. This 3-prolyl hydroxylation is decreased in patients with CRTAP and P3H1 deficiency. It was suspected that mutations in the PPIB gene encoding CyPB would also cause OI with decreased collagen 3-prolyl hydroxylation. To our knowledge we present the first two families with recessive OI caused by PPIB gene mutations. The clinical phenotype is compatible with OI Sillence type II-B/III as seen with COL1A1/2, CRTAP, and LEPRE1 mutations. The percentage of 3-hydroxylated P986 residues in patients with PPIB mutations is decreased in comparison to normal, but it is higher than in patients with CRTAP and LEPRE1 mutations. This result and the fact that CyPB is demonstrable independent of CRTAP and P3H1, along with reported decreased 3-prolyl hydroxylation due to deficiency of CRTAP lacking the catalytic hydroxylation domain and the known function of CyPB as a cis-trans isomerase, suggest that recessive OI is caused by a dysfunctional P3H1/CRTAP/CyPB complex rather than by the lack of 3-prolyl hydroxylation of a single proline residue in the alpha1 chains of collagen type I.