Assessment of Bones Deficient in Fibrillin-1 Microfibrils Reveals Pronounced Sex Differences

Defects in the extracellular matrix protein fibrillin-1 that perturb transforming growth factor beta (TGFβ) bioavailability lead to Marfan syndrome (MFS). MFS is an autosomal-dominant disorder, which is associated with connective tissue and skeletal defects, among others. To date, it is unclear how biological sex impacts the structural and functional properties of bone in MFS. The aim of this study was to investigate the effects of sex on bone microarchitecture and mechanical properties in mice with deficient fibrillin-1, a model of human MFS. Bones of 11-week-old male and female Fbn1^mgR/mgR mice were investigated. Three-dimensional micro-computed tomography of femora and vertebrae revealed a lower ratio of trabecular bone volume to tissue volume, reduced trabecular number and thickness, and greater trabecular separation in females vs. males. Three-point bending of femora revealed significantly lower post-yield displacement and work-to-fracture in females vs. males. Mechanistically, we found higher Smad2 and ERK1/2 phosphorylation in females vs. males, demonstrating a greater activation of TGFβ signaling in females. In summary, the present findings show pronounced sex differences in the matrix and function of bones deficient in fibrillin-1 microfibrils. Consequently, sex-specific analysis of bone characteristics in patients with MFS may prove useful in improving the clinical management and life quality of these patients, through the development of sex-specific therapeutic approaches.

Wnt1 is an Lrp5-independent bone-anabolic Wnt ligand

WNT1 mutations in humans are associated with a new form of osteogenesis imperfecta and with early-onset osteoporosis, suggesting a key role of WNT1 in bone mass regulation. However, the general mode of action and the therapeutic potential of Wnt1 in clinically relevant situations such as aging remain to be established. Here, we report the high prevalence of heterozygous WNT1 mutations in patients with early-onset osteoporosis. We show that inactivation of Wnt1 in osteoblasts causes severe osteoporosis and spontaneous bone fractures in mice. In contrast, conditional Wnt1 expression in osteoblasts promoted rapid bone mass increase in developing young, adult, and aged mice by rapidly increasing osteoblast numbers and function. Contrary to current mechanistic models, loss of Lrp5, the co-receptor thought to transmit extracellular WNT signals during bone mass regulation, did not reduce the bone-anabolic effect of Wnt1, providing direct evidence that Wnt1 function does not require the LRP5 co-receptor. The identification of Wnt1 as a regulator of bone formation and remodeling provides the basis for development of Wnt1-targeting drugs for the treatment of osteoporosis.

Four novel mutations in EFNB1 in Indian patients with craniofrontonasal syndrome

Craniofrontonasal syndrome (CFNS) (OMIM #304110) is a very rare, X-linked developmental disorder characterized by facial stigmata, including hypertelorism, frontonasal dysplasia, craniosynostosis, bifid nasal tip, and digital abnormalities. CFNS is caused by mutations in the Ephrin 1 gene (EFNB1) located at Xq13.1, which encodes the transmembrane protein Ephrin B1. Interestingly, heterozygous females are more severely affected than hemizygous males. We report on four individuals from four unrelated Indian families with mild-to-severe CFNS. All patients had variable degrees of hypertelorism and nasal bridge depression, which did not correlate with changes in other tissues. Although patients 3 and 4 showed the most severe facial dysmorphism and syndactyly, there were no structural CNS changes or developmental delay. In contrast, patient 1 displayed agenesis of corpus callosum and developmental delay, although facial and finger abnormalities were milder. Patients 1, 2, and 4 showed different degrees of clefting. DNA sequencing revealed four previously undescribed heterozygous mutations in exons 1 and 2 of EFNB1. Patient 1 carried the second single amino acid deletion reported up to date. The other three affected individuals harbored frameshift mutations, leading to premature termination codons. Our findings broaden the spectrum of EFNB1 mutations and illustrate the absence of an obvious correlation between mutation type, severity, and expression of symptoms.

A novel FAM20C mutation causing hypophosphatemic osteomalacia with osteosclerosis (mild Raine syndrome) in an elderly man with spontaneous osteonecrosis of the knee

Raine syndrome is characterized by FGF23-mediated hypophosphatemic osteomalacia with osteosclerosis caused by mutations in the FAM20C gene. We report a case of a 72-year-old man who presented with rapid progressive spontaneous osteonecrosis of the knee (SONK). A full osteologic assessment including dual energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses revealed a high bone mass in the lumbar spine and hip (DXA T-score + 7.5 and + 4.7/+4.2) with increased bone microstructural parameters in the distal radius and tibia (BV/TV 127%, 140% of the age-matched mean, respectively), as well as a low bone turnover state. Phosphate levels were low due to renal phosphate wasting and high FGF23 levels (126.5 pg/ml, reference range 23.2-95.4 pg/ml). Using gene panel sequencing, we identified a novel FAM20C heterozygous missense mutation in combination with a homozygous duplication that potentially alters splicing. Taken together, this is the first case of mild Raine syndrome with spontaneous osteonecrosis of the knee, phosphate wasting, and a pronounced trabecular high bone mass phenotype.

Sclerosing bone dysplasias with hallmarks of dysosteosclerosis in four patients carrying mutations in SLC29A3 and TCIRG1

The osteopetroses and related sclerosing bone dysplasias can have a broad range of manifestations. Especially in the milder forms, sandwich vertebrae are an easily recognizable and reliable radiological hallmark. We report on four patients from three families presenting with sandwich vertebrae and platyspondyly. The long bone phenotypes were discordant with one patient showing modeling defects and patchy osteosclerosis, while the second displayed only metaphyseal sclerotic bands, and the third and fourth had extreme metaphyseal flaring with uniform osteosclerosis. Two of the four patients had experienced pathological fractures, two had developmental delay, but none showed cranial nerve damage, hepatosplenomegaly, or bone marrow failure. According to these clinical features the diagnoses ranged between intermediate autosomal recessive osteopetrosis and dysosteosclerosis. After exclusion of mutations in CLCN7 we performed gene panel and exome sequencing. Two novel mutations in SLC29A3 were found in the first two patients. In the third family a TCIRG1 C-terminal frameshift mutation in combination with a mutation at position +4 in intron 2 were detected. Our study adds two cases to the small group of individuals with SLC29A3 mutations diagnosed with dysosteosclerosis, and expands the phenotypic variability. The finding that intermediate autosomal recessive osteopetrosis due to TCIRG1 splice site mutations can also present with platyspondyly further increases the molecular heterogeneity of dysosteosclerosis-like sclerosing bone dysplasias.

GOPHER: Generator Of Probes for capture Hi-C Experiments at high Resolution

BACKGROUND

Target enrichment combined with chromosome conformation capturing methodologies such as capture Hi-C (CHC) can be used to investigate spatial layouts of genomic regions with high resolution and at scalable costs. A common application of CHC is the investigation of regulatory elements that are in contact with promoters, but CHC can be used for a range of other applications. Therefore, probe design for CHC needs to be adapted to experimental needs, but no flexible tool is currently available for this purpose.

RESULTS

We present a Java desktop application called GOPHER (Generator Of Probes for capture Hi-C Experiments at high Resolution) that implements three strategies for CHC probe design. GOPHER's simple approach is similar to the probe design of previous approaches that employ CHC to investigate all promoters, with one probe being placed at each margin of a single digest that overlaps the transcription start site (TSS) of each promoter. GOPHER's simple-patched approach extends this methodology with a heuristic that improves coverage of viewpoints in which the TSS is located near to one of the boundaries of the digest. GOPHER's extended approach is intended mainly for focused investigations of smaller gene sets. GOPHER can also be used to design probes for regions other than TSS such as GWAS hits or large blocks of genomic sequence. GOPHER additionally provides a number of features that allow users to visualize and edit viewpoints, and outputs a range of files useful for documentation, ordering probes, and downstream analysis.

CONCLUSION

GOPHER is an easy-to-use and robust desktop application for CHC probe design. Source code and a precompiled executable can be downloaded from the GOPHER GitHub page at https://github.com/TheJacksonLaboratory/Gopher .

Osteopetrose

Unter der Diagnose Osteopetrose wird eine heterogene Gruppe von seltenen monogenetischen Knochenerkrankungen zusammengefasst, die sich durch eine reduzierte Osteoklasten-Aktivität, eine gesteigerte Knochenmasse und eine gesteigerte Knochenbrüchigkeit auszeichnet. Die Osteopetrosen werden verursacht durch Mutationen in verschiedenen Genen, die in der Osteoklastogenese oder Osteoklasten-Funktion involviert sind. Durch den Verlust der Knochenresorption werden Knochenumbau und -neubildung beeinträchtigt, wodurch sich die Qualität und Architektur des Knochengewebes verschlechtert. Häufige klinische Zeichen sind Osteosklerose, Knochendeformitäten, hämatologische Insuffizienzen durch Verengung der Knochenmarkräume, Sehstörungen durch Verengung der Foramina nervi optici und andere neurologische Störungen. Bis auf eine X-chromosomale Form sind alle infantilen schweren Osteopetrosen autosomal rezessiv vererbt. Die rezessiven Formen verlaufen gewöhnlich ohne Behandlung tödlich im Säuglings- oder Kindesalter, wobei allerdings selten mildere klinische Verläufe als sog. Intermediäre Osteopetrose vorkommen. Die dominante Form ist assoziiert mit Mutationen im CLCN7-Gen und verläuft in der Regel milder. Die meisten schweren Osteopetrosen können durch eine allogene hämatopoetische Stammzelltransplantation (HSZT) behandelt werden, sofern sie Osteoklasten-autonome Ursachen haben und nicht mit einer primären neurologischen Komponente verbunden sind.

GORAB scaffolds COPI at the trans-Golgi for efficient enzyme recycling and correct protein glycosylation

COPI is a key mediator of protein trafficking within the secretory pathway. COPI is recruited to the membrane primarily through binding to Arf GTPases, upon which it undergoes assembly to form coated transport intermediates responsible for trafficking numerous proteins, including Golgi-resident enzymes. Here, we identify GORAB, the protein mutated in the skin and bone disorder gerodermia osteodysplastica, as a component of the COPI machinery. GORAB forms stable domains at the trans-Golgi that, via interactions with the COPI-binding protein Scyl1, promote COPI recruitment to these domains. Pathogenic GORAB mutations perturb Scyl1 binding or GORAB assembly into domains, indicating the importance of these interactions. Loss of GORAB causes impairment of COPI-mediated retrieval of trans-Golgi enzymes, resulting in a deficit in glycosylation of secretory cargo proteins. Our results therefore identify GORAB as a COPI scaffolding factor, and support the view that defective protein glycosylation is a major disease mechanism in gerodermia osteodysplastica.

Generation of a human induced pluripotent stem cell line (BIHi002-A) from a patient with CLCN7-related infantile malignant autosomal recessive osteopetrosis

Autosomal recessive osteopetrosis (ARO) is a genetic bone disease that can be caused by mutations in the CLCN7 gene preventing osteoclast-mediated bone resorption. We generated a human induced pluripotent stem cell (hiPSC) line, BIHi002-A, from peripheral blood mononuclear cells of an ARO patient carrying the CLCN7 mutations c.875G>A and c.1208G>A using Sendai viral vectors. The pluripotent identity of the BIHi002-A line was confirmed by their expression of typical markers for undifferentiated hiPSCs, their capacity to differentiate into cells of the three germ layers and by PluriTest analysis. The BIHi002-A line provides a tool for disease modelling and therapy development.

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.

Correction: Arterial tortuosity syndrome: 40 new families and literature review

In the published version of this paper the author Neus Baena's name was incorrectly given as Neus Baena Diez. This has now been corrected in both the HTML and PDF versions of the paper.

A novel mutation in CDH11, encoding cadherin-11, cause Branchioskeletogenital (Elsahy-Waters) syndrome

Cadherins are cell-adhesion molecules that control morphogenesis, cell migration, and cell shape changes during multiple developmental processes. Until now four distinct cadherins have been implicated in human Mendelian disorders, mainly featuring skin, retinal and hearing manifestations. Branchio-skeleto-genital (or Elsahy-Waters) syndrome (BSGS) is an ultra-rare condition featuring a characteristic face, premature loss of teeth, vertebral and genital anomalies, and intellectual disability. We have studied two sibs with BSGS originally described by Castori et al. in 2010. Exome sequencing led to the identification of a novel homozygous nonsense variant in the first exon of the cadherin-11 gene (CDH11), which results in a prematurely truncated form of the protein. Recessive variants in CDH11 have been recently demonstrated in two other sporadic patients and a pair of sisters affected by BSGS. Although the function of this cadherin (also termed Osteoblast-Cadherin) is not completely understood, its prevalent expression in osteoblastic cell lines and up-regulation during differentiation suggest a specific function in bone formation and development. This study identifies a novel loss-of-function variant in CDH11 as a cause of BSGS and supports the role of cadherin-11 as a key player in axial and craniofacial malformations.

Transcriptional profiling of murine osteoblast differentiation based on RNA-seq expression analyses

Osteoblastic differentiation is a multistep process characterized by osteogenic induction of mesenchymal stem cells, which then differentiate into proliferative pre-osteoblasts that produce copious amounts of extracellular matrix, followed by stiffening of the extracellular matrix, and matrix mineralization by hydroxylapatite deposition. Although these processes have been well characterized biologically, a detailed transcriptional analysis of murine primary calvaria osteoblast differentiation based on RNA sequencing (RNA-seq) analyses has not previously been reported. Here, we used RNA-seq to obtain expression values of 29,148 genes at four time points as murine primary calvaria osteoblasts differentiate in vitro until onset of mineralization was clearly detectable by microscopic inspection. Expression of marker genes confirmed osteogenic differentiation. We explored differential expression of 1386 protein-coding genes using unsupervised clustering and GO analyses. 100 differentially expressed lncRNAs were investigated by co-expression with protein-coding genes that are localized within the same topologically associated domain. Additionally, we monitored expression of 237 genes that are silent or active at distinct time points and compared differential exon usage. Our data represent an in-depth profiling of murine primary calvaria osteoblast differentiation by RNA-seq and contribute to our understanding of genetic regulation of this key process in osteoblast biology.

Mutational analysis uncovers monogenic bone disorders in women with pregnancy-associated osteoporosis: three novel mutations in LRP5, COL1A1, and COL1A2

Pregnancy was found to be a skeletal risk factor promoting the initial onset of previously unrecognized monogenic bone disorders, thus explaining a proportion of cases with pregnancy-associated osteoporosis. Therapeutic measures should focus in particular on the normalization of the disturbed calcium homeostasis in order to enable the partial skeletal recovery.

INTRODUCTION

Pregnancy-associated osteoporosis (PAO) is a rare skeletal condition, which is characterized by a reduction in bone mineral density (BMD) in the course of pregnancy and lactation. Typical symptoms include vertebral compression fractures and transient osteoporosis of the hip. Since the etiology is not well understood, this prospective study was conducted in order to elucidate the relevance of pathogenic gene variants for the development of PAO.

METHODS

Seven consecutive cases with the diagnosis of PAO underwent a skeletal assessment (blood tests, DXA, HR-pQCT) and a comprehensive genetic analysis using a custom-designed gene panel.

RESULTS

All cases showed a reduced BMD (DXA T-score, lumbar spine - 3.2 ± 1.0; left femur - 2.2 ± 0.5; right femur - 1.9 ± 0.5), while the spine was affected more severely (p < 0.05). The trabecular and cortical thickness was overall reduced in HR-pQCT, while the trabecular number showed no alterations in most cases. The genetic analysis revealed three novel mutations in LRP5, COL1A1, and COL1A2.

CONCLUSION

Our data show that previously unrecognized monogenic bone disorders play an important role in PAO. Pregnancy should be considered a skeletal risk factor, which can promote the initial clinical onset of such skeletal disorders. The underlying increased calcium demand is essential in terms of prophylactic and therapeutic measures, which are especially required in individuals with a genetically determined low bone mass. The implementation of this knowledge in clinical practice can enable the partial recovery of the skeleton. Consistent genetic studies are needed to analyze the frequency of pathogenic variants in women with PAO.

Genetic assessment and folate receptor autoantibodies in infantile-onset cerebral folate deficiency (CFD) syndrome

INTRODUCTION

Cerebral folate deficiency (CFD) syndromes are defined as neuro-psychiatric conditions with low CSF folate and attributed to different causes such as autoantibodies against the folate receptor-alpha (FR) protein that can block folate transport across the choroid plexus, FOLR1 gene mutations or mitochondrial disorders. High-dose folinic acid treatment restores many neurologic deficits.

STUDY AIMS AND METHODS

Among 36 patients from 33 families the infantile-onset CFD syndrome was diagnosed based on typical clinical features and low CSF folate. All parents were healthy. Three families had 2 affected siblings, while parents from 4 families were first cousins. We analysed serum FR autoantibodies and the FOLR1 and FOLR2 genes. Among three consanguineous families homozygosity mapping attempted to identify a monogenetic cause. Whole exome sequencing (WES) was performed in the fourth consanguineous family, where two siblings also suffered from polyneuropathy as an atypical finding.

RESULTS

Boys (72%) outnumbered girls (28%). Most patients (89%) had serum FR autoantibodies fluctuating over 5-6 weeks. Two children had a genetic FOLR1 variant without pathological significance. Homozygosity mapping failed to detect a single autosomal recessive gene. WES revealed an autosomal recessive polynucleotide kinase 3´phosphatase (PNKP) gene abnormality in the siblings with polyneuropathy.

DISCUSSION

Infantile-onset CFD was characterized by serum FR autoantibodies as its predominant pathology whereas pathogenic FOLR1 gene mutations were absent. Homozygosity mapping excluded autosomal recessive inheritance of any single responsible gene. WES in one consanguineous family identified a PNKP gene abnormality that explained the polyneuropathy and also its contribution to the infantile CFD syndrome because the PNKP gene plays a dual role in both neurodevelopment and immune-regulatory function. Further research for candidate genes predisposing to FRα-autoimmunity is suggested to include X-chromosomal and non-coding DNA regions.

Diagnostic strategies and genotype-phenotype correlation in a large Indian cohort of osteogenesis imperfecta

Osteogenesis Imperfecta (OI) is a clinically and genetically heterogeneous disorder. Although differential diagnosis is greatly facilitated by next generation sequencing, its availability can vary considerably. In this study, we compared targeted gene panel or exome sequencing with clinical scoring and grouping in a cohort of 50 OI index patients recruited by a single Indian clinical center in an unselected fashion. In 48 patients we observed a total of 24 novel mutations and 24 known OI mutations, of which several were recurrent. In one patient neither gene panel nor exome sequencing revealed any significant mutation and another patient harbored a class III COL1A1 intronic variant. The percentage of autosomal recessive forms due to mutations in BMP1, FKBP10, LEPRE1, SERPINF1, and WNT1 was unusually high (48%). Grouping according to phenotypic and radiographic features revealed four individuals with Bruck syndrome due to FKBP10 mutations, three patients with hypertrophic callus caused by IFITM5 mutations, and twenty with pronounced bone bowing, of which eight carried WNT1 mutations. There was a clear correlation between genotype and phenotype severity: IFITM5=LEPRE1>WNT1>SERPINF1>COL1A1 (qualitative)>BMP1>FKBP10>COL1A2 (qualitative)>COL1A1 (quantitative)>COL1A2 (quantitative). In one patient we found heterozygous variants in COL1A1 and COL1A2 inherited from parents without an obvious bone phenotype indicating that both variants might contribute to the phenotype. Our findings demonstrate the clinical utility of gene panel testing for OI, but in cases with contractures, hypertrophic callus formation, or - to some extent - extensive bowing single gene analysis might still be more cost-effective.

Impaired proteoglycan glycosylation, elevated TGF-β signaling, and abnormal osteoblast differentiation as the basis for bone fragility in a mouse model for gerodermia osteodysplastica

Gerodermia osteodysplastica (GO) is characterized by skin laxity and early-onset osteoporosis. GORAB, the responsible disease gene, encodes a small Golgi protein of poorly characterized function. To circumvent neonatal lethality of the Gorab^Null full knockout, Gorab was conditionally inactivated in mesenchymal progenitor cells (Prx1-cre), pre-osteoblasts (Runx2-cre), and late osteoblasts/osteocytes (Dmp1-cre), respectively. While in all three lines a reduction in trabecular bone density was evident, only Gorab^Prx1 and Gorab^Runx2 mutants showed dramatically thinned, porous cortical bone and spontaneous fractures. Collagen fibrils in the skin of Gorab^Null mutants and in bone of Gorab^Prx1 mutants were disorganized, which was also seen in a bone biopsy from a GO patient. Measurement of glycosaminoglycan contents revealed a reduction of dermatan sulfate levels in skin and cartilage from Gorab^Null mutants. In bone from Gorab^Prx1 mutants total glycosaminoglycan levels and the relative percentage of dermatan sulfate were both strongly diminished. Accordingly, the proteoglycans biglycan and decorin showed reduced glycanation. Also in cultured GORAB-deficient fibroblasts reduced decorin glycanation was evident. The Golgi compartment of these cells showed an accumulation of decorin, but reduced signals for dermatan sulfate. Moreover, we found elevated activation of TGF-β in Gorab^Prx1 bone tissue leading to enhanced downstream signalling, which was reproduced in GORAB-deficient fibroblasts. Our data suggest that the loss of Gorab primarily perturbs pre-osteoblasts. GO may be regarded as a congenital disorder of glycosylation affecting proteoglycan synthesis due to delayed transport and impaired posttranslational modification in the Golgi compartment.

Gerodermia osteodysplastica (GO) is segmental progeroid disorder affecting connective tissues and bone, leading to extreme bone fragility. The cause are loss-of-function mutations in the Golgi protein GORAB, whose function has been only partially unravelled. Using several mouse models and patient-derived primary cells we elucidate that loss of Gorab elicits a defect in proteoglycan glycanation, which is associated with collagen disorganization in dermis and bone. We also found evidence for TGF-β upregulation and enhanced downstream signalling. If these changes occur in mesenchymal stem cells or early osteoblasts they impair osteoblast differentiation resulting in cortical thinning and spontaneous fractures. We thus match GO mechanistically with also phenotypically overlapping progeroid connective tissue disorders with glycanation defects.

Cutis laxa and excessive bone growth due to de novo mutations in PTDSS1

The cutis laxa syndromes are multisystem disorders that share loose redundant inelastic and wrinkled skin as a common hallmark clinical feature. The underlying molecular defects are heterogeneous and 13 different genes have been involved until now, all of them being implicated in elastic fiber assembly. We provide here molecular and clinical characterization of three unrelated patients with a very rare phenotype associating cutis laxa, facial dysmorphism, severe growth retardation, hyperostotic skeletal dysplasia, and intellectual disability. This disorder called Lenz–Majewski syndrome (LMS) is associated with gain of function mutations in PTDSS1, encoding an enzyme involved in phospholipid biosynthesis. This report illustrates that LMS is an unequivocal cutis laxa syndrome and expands the clinical and molecular spectrum of this group of disorders. In the neonatal period, brachydactyly and facial dysmorphism are two early distinctive signs, later followed by intellectual disability and hyperostotic skeletal dysplasia with severe dwarfism allowing differentiation of this condition from other cutis laxa phenotypes. Further studies are needed to understand the link between PTDSS1 and extra cellular matrix assembly.

A novel COL1A2 C-propeptide cleavage site mutation causing high bone mass osteogenesis imperfecta with a regional distribution pattern

Osteogenesis imperfecta (OI) is typically characterized by low bone mass and increased bone fragility caused by heterozygous mutations in the type I procollagen genes (COL1A1/COL1A2). We report two cases of a 56-year-old woman and her 80-year-old mother who suffered from multiple vertebral and non-vertebral fractures with onset in early childhood. A full osteologic assessment including dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses pointed to a high bone mineral density (BMD) in the hip (DXA Z-score + 3.7 and + 3.9) but low to normal bone mass in the spine and preserved bone microstructure in the distal tibia. Serum markers of bone formation and bone resorption were elevated. Using whole exome sequencing, we identified a novel mutation in the COL1A2 gene causing a p. (Asp1120Gly) substitution at the protein level and affecting the type I procollagen C-propeptide cleavage site. In line with previously reported cases, our data independently prove the existence of an unusual phenotype of high bone mass OI caused by a mutation in the procollagen C-propeptide cleavage with a clinically persistent phenotype through adulthood.

Clinical Significance of DXA and HR-pQCT in Autosomal Dominant Osteopetrosis (ADO II)

The main hallmark of high bone mass (HBM) disorders is increased bone mineral density, potentially visible in conventional radiographs and quantifiable by other radiographic methods. While one of the most common forms of HBM is CLCN7-related autosomal dominant osteopetrosis type II (ADO II), there is no consensus on diagnostic thresholds. We therefore wanted to assess whether CLCN7-osteopetrosis patients differ from benign HBM cases in terms of (1) bone mineral density, (2) bone structure, and (3) microarchitectural abnormalities. 16 patients meeting the criteria of HBM (DXA T/Z-score ≥ 2.5 at all sites) were included in this retrospective study. Osteologic assessment using dual-energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT), and serum analyses was performed. The presence of CLCN7 and/or other HBM gene mutations affecting bone mass were tested using a custom designed bone panel. While a DXA threshold for ADO II could be implemented (DXA Z-score ≥ + 6.0), the differences in bone microarchitecture were of lesser extent compared to the benign HBM group. All adult patients with ADO II suffered from elevated fracture rates independent from Z-score. In HR-pQCT, structural alterations, such as bone islets were found only inconsistently. In cases of HBM, a DXA Z-score ≥ 6 may be indicative for an inheritable HBM disorder, such as ADO II. Microarchitectural bone alterations might represent local microfracture repair or accumulation of cartilage remnants due to impaired osteoclast function, but seem not to be correlated with fracture risk.

De Novo Mutations in SLC25A24 Cause a Craniosynostosis Syndrome with Hypertrichosis, Progeroid Appearance, and Mitochondrial Dysfunction

Gorlin-Chaudhry-Moss syndrome (GCMS) is a dysmorphic syndrome characterized by coronal craniosynostosis and severe midface hypoplasia, body and facial hypertrichosis, microphthalmia, short stature, and short distal phalanges. Variable lipoatrophy and cutis laxa are the basis for a progeroid appearance. Using exome and genome sequencing, we identified the recurrent de novo mutations c.650G>A (p.Arg217His) and c.649C>T (p.Arg217Cys) in SLC25A24 in five unrelated girls diagnosed with GCMS. Two of the girls had pronounced neonatal progeroid features and were initially diagnosed with Wiedemann-Rautenstrauch syndrome. SLC25A24 encodes a mitochondrial inner membrane ATP-Mg/P_i carrier. In fibroblasts from affected individuals, the mutated SLC25A24 showed normal stability. In contrast to control cells, the probands' cells showed mitochondrial swelling, which was exacerbated upon treatment with hydrogen peroxide (H₂O₂). The same effect was observed after overexpression of the mutant cDNA. Under normal culture conditions, the mitochondrial membrane potential of the probands' fibroblasts was intact, whereas ATP content in the mitochondrial matrix was lower than that in control cells. However, upon H₂O₂ exposure, the membrane potential was significantly elevated in cells harboring the mutated SLC25A24. No reduction of mitochondrial DNA copy number was observed. These findings demonstrate that mitochondrial dysfunction with increased sensitivity to oxidative stress is due to the SLC25A24 mutations. Our results suggest that the SLC25A24 mutations induce a gain of pathological function and link mitochondrial ATP-Mg/P_i transport to the development of skeletal and connective tissue.

Examining tissue composition, whole-bone morphology and mechanical behavior of GorabPrx1 mice tibiae: A mouse model of premature aging

Gerodermia osteodysplastica (GO) is a segmental progeroid disorder caused by loss-of-function mutations in the GORAB gene, associated with early onset osteoporosis and bone fragility. A conditional mouse model of GO (Gorab^Prx1) was generated in which the Gorab gene was deleted in long bones. We examined the biomechanical/functional relevance of the Gorab^Prx1 mutants as a premature aging model by characterizing bone composition, tissue-level strains, and whole-bone morphology and mechanical properties of the tibia. MicroCT imaging showed that Gorab^Prx1 tibiae had an increased anterior convex curvature and decreased cortical cross-sectional area, cortical thickness and moments of inertia, compared to littermate control (LC) tibiae. Fourier transform infrared (FTIR) imaging indicated a 34% decrease in mineral/matrix ratio and a 27% increase in acid phosphate content in the posterior metaphyseal cortex of the Gorab^Prx1 tibiae (p < .05), suggesting delayed mineralization. In vivo strain gauge measurement and finite element analysis showed ∼two times higher tissue-level strains within the Gorab^Prx1 tibiae relative to LC tibiae when subjected to axial compressive loads of the same magnitude. Three-point bending tests suggested that Gorab^Prx1 tibiae were weaker and more brittle, as indicated by decreasing whole-bone strength (46%), stiffness (55%), work-to-fracture (61%) and post-yield displacement (47%). Many of these morphological and biomechanical characteristics of the Gorab^Prx1 tibia recapitulated changes in other animal models of skeletal aging. Future studies are necessary to confirm how our observations might guide the way to a better understanding and treatment of GO.