Osteoclasts recycle via osteomorphs during RANKL-stimulated bone resorption.

Osteoclasts are large multinucleated bone-resorbing cells formed by the fusion of monocyte/macrophage-derived precursors that are thought to undergo apoptosis once resorption is complete. Here, by intravital imaging, we reveal that RANKL-stimulated osteoclasts have an alternative cell fate in which they fission into daughter cells called osteomorphs. Inhibiting RANKL blocked this cellular recycling and resulted in osteomorph accumulation. Single-cell RNA sequencing showed that osteomorphs are transcriptionally distinct from osteoclasts and macrophages and express a number of non-canonical osteoclast genes that are associated with structural and functional bone phenotypes when deleted in mice. Furthermore, genetic variation in human orthologs of osteomorph genes causes monogenic skeletal disorders and associates with bone mineral density, a polygenetic skeletal trait. Thus, osteoclasts recycle via osteomorphs, a cell type involved in the regulation of bone resorption that may be targeted for the treatment of skeletal diseases.

Deletion of Trps1 regulatory elements recapitulates postnatal hip joint abnormalities and growth retardation of Trichorhinophalangeal syndrome in mice.

Trichorhinophalangeal syndrome (TRPS) is a genetic disorder caused by point mutations or deletions in the gene-encoding transcription factor TRPS1. TRPS patients display a range of skeletal dysplasias, including reduced jaw size, short stature, and a cone-shaped digit epiphysis. Certain TRPS patients experience early onset coxarthrosis that leads to a devastating drop in their daily activities. The etiologies of congenital skeletal abnormalities of TRPS were revealed through the analysis of Trps1 mutant mouse strains. However, early postnatal lethality in Trps1 knockout mice has hampered the study of postnatal TRPS pathology. Here, through epigenomic analysis we identified two previously uncharacterized candidate gene regulatory regions in the first intron of Trps1. We deleted these regions, either individually or simultaneously, and examined their effects on skeletal morphogenesis. Animals that were deleted individually for either region displayed only modest phenotypes. In contrast, the Trps1Δint/Δint mouse strain with simultaneous deletion of both genomic regions exhibit postnatal growth retardation. This strain displayed delayed secondary ossification center formation in the long bones and misshaped hip joint development that resulted in acetabular dysplasia. Reducing one allele of the Trps1 gene in Trps1Δint mice resulted in medial patellar dislocation that has been observed in some patients with TRPS. Our novel Trps1 hypomorphic strain recapitulates many postnatal pathologies observed in human TRPS patients, thus positioning this strain as a useful animal model to study postnatal TRPS pathogenesis. Our observations also suggest that Trps1 gene expression is regulated through several regulatory elements, thus guaranteeing robust expression maintenance in skeletal cells.

TGF-ß and BMP Signaling Pathways in Skeletal Dysplasia with Short and Tall Stature.

The transforming growth factor ß (TGF-ß) and bone morphogenetic protein (BMP) signaling pathways play a pivotal role in bone development and skeletal health. More than 30 different types of skeletal dysplasia are now known to be caused by pathogenic variants in genes that belong to the TGF-ß superfamily and/or regulate TGF-ß/BMP bioavailability. This review describes the latest advances in skeletal dysplasia that is due to impaired TGF-ß/BMP signaling and results in short stature (acromelic dysplasia and cardiospondylocarpofacial syndrome) or tall stature (Marfan syndrome). We thoroughly describe the clinical features of the patients, the underlying genetic findings, and the pathomolecular mechanisms leading to disease, which have been investigated mainly using patient-derived skin fibroblasts and mouse models. Although no pharmacological treatment is yet available for skeletal dysplasia due to impaired TGF-ß/BMP signaling, in recent years advances in the use of drugs targeting TGF-ß have been made, and we also discuss these advances.

Null and missense mutations of ERI1 cause a recessive phenotypic dichotomy in humans.

ERI1 is a 3'-to-5' exoribonuclease involved in RNA metabolic pathways including 5.8S rRNA processing and turnover of histone mRNAs. Its biological and medical significance remain unclear. Here, we uncover a phenotypic dichotomy associated with bi-allelic ERI1 variants by reporting eight affected individuals from seven unrelated families. A severe spondyloepimetaphyseal dysplasia (SEMD) was identified in five affected individuals with missense variants but not in those with bi-allelic null variants, who showed mild intellectual disability and digital anomalies. The ERI1 missense variants cause a loss of the exoribonuclease activity, leading to defective trimming of the 5.8S rRNA 3' end and a decreased degradation of replication-dependent histone mRNAs. Affected-individual-derived induced pluripotent stem cells (iPSCs) showed impaired in vitro chondrogenesis with downregulation of genes regulating skeletal patterning. Our study establishes an entity previously unreported in OMIM and provides a model showing a more severe effect of missense alleles than null alleles within recessive genotypes, suggesting a key role of ERI1-mediated RNA metabolism in human skeletal patterning and chondrogenesis.

AXIN1 bi-allelic variants disrupting the C-terminal DIX domain cause craniometadiaphyseal osteosclerosis with hip dysplasia.

Sclerosing skeletal dysplasias result from an imbalance between bone formation and resorption. We identified three homozygous, C-terminally truncating AXIN1 variants in seven individuals from four families affected by macrocephaly, cranial hyperostosis, and vertebral endplate sclerosis. Other frequent findings included hip dysplasia, heart malformations, variable developmental delay, and hematological anomalies. In line with AXIN1 being a central component of the ß-catenin destruction complex, analyses of primary and genome-edited cells harboring the truncating variants revealed enhanced basal canonical Wnt pathway activity. All three AXIN1-truncating variants resulted in reduced protein levels and impaired AXIN1 polymerization mediated by its C-terminal DIX domain but partially retained Wnt-inhibitory function upon overexpression. Addition of a tankyrase inhibitor attenuated Wnt overactivity in the AXIN1-mutant model systems. Our data suggest that AXIN1 coordinates the action of osteoblasts and osteoclasts and that tankyrase inhibitors can attenuate the effects of AXIN1 hypomorphic variants.

Combined clinical, structural, and cellular studies discriminate pathogenic and benign TRPV4 variants.

Dominant mutations in the calcium-permeable ion channel TRPV4 (transient receptor potential vanilloid 4) cause diverse and largely distinct channelopathies, including inherited forms of neuromuscular disease, skeletal dysplasias, and arthropathy. Pathogenic TRPV4 mutations cause gain of ion channel function and toxicity that can be rescued by small molecule TRPV4 antagonists in cellular and animal models, suggesting that TRPV4 antagonism could be therapeutic for patients. Numerous variants in TRPV4 have been detected with targeted and whole exome/genome sequencing, but for the vast majority, their pathogenicity remains unclear. Here, we used a combination of clinical information and experimental structure-function analyses to evaluate 30 TRPV4 variants across various functional protein domains. We report clinical features of seven patients with TRPV4 variants of unknown significance and provide extensive functional characterization of these and an additional 17 variants, including structural position, ion channel function, subcellular localization, expression level, cytotoxicity, and protein-protein interactions. We find that gain-of-function mutations within the TRPV4 intracellular ankyrin repeat domain target charged amino acid residues important for RhoA interaction, whereas ankyrin repeat domain residues outside of the RhoA interface have normal or reduced ion channel activity. We further identify a cluster of gain-of-function variants within the intracellular intrinsically disordered region that may cause toxicity via altered interactions with membrane lipids. In contrast, assessed variants in the transmembrane domain and other regions of the intrinsically disordered region do not cause gain of function and are likely benign. Clinical features associated with gain of function and cytotoxicity include congenital onset of disease, vocal cord weakness, and motor predominant disease, whereas patients with likely benign variants often demonstrated late-onset and sensory-predominant disease. These results provide a framework for assessing additional TRPV4 variants with respect to likely pathogenicity, which will yield critical information to inform patient selection for future clinical trials for TRPV4 channelopathies.

Involvement of kinesins in skeletal dysplasia: a review.

Skeletal dysplasias are group of rare genetic diseases resulting from mutations in genes encoding structural proteins of the cartilage extracellular matrix (ECM), signaling molecules, transcription factors, epigenetic modifiers, and several intracellular proteins. Cell division, organelle maintenance, and intracellular transport are all orchestrated by the cytoskeleton-associated proteins, and intracellular processes affected through microtubule-associated movement are important for the function of skeletal cells. Among microtubule-associated motor proteins, kinesins in particular have been shown to play a key role in cell cycle dynamics, including chromosome segregation, mitotic spindle formation, and ciliogenesis, in addition to cargo trafficking, receptor recycling, and endocytosis. Recent studies highlight the fundamental role of kinesins in embryonic development and morphogenesis and have shown that mutations in kinesin genes lead to several skeletal dysplasias. However, many questions concerning the specific functions of kinesins and their adaptor molecules as well as specific molecular mechanisms in which the kinesin proteins are involved during skeletal development remain unanswered. Here we present a review of the skeletal dysplasias resulting from defects in kinesins and discuss the involvement of kinesin proteins in the molecular mechanisms that are active during skeletal development.

Compound heterozygosity for two variants in BMP5 in human skeletal dysostosis with atrioventricular septal defect.

The growth and development of the skeleton is regulated by bone morphogenetic proteins of which several are linked to genetic skeletal disorders. So far, no human skeletal malformations have been associated with variants in BMP5. Here, we report a patient with biallelic loss of function variants in BMP5 and a syndromic phenotype including skeletal dysostosis, dysmorphic features, hypermobility, laryngo-tracheo-bronchomalacia and atrioventricular septal defect. We discuss the phenotype in relation to the known tissue-specific expression of Bmp5 and similar morphological abnormalities previously reported in experimental animal models. Our findings suggest a new association between BMP5 variants and a range of developmental anomalies, involving ears, heart and skeleton, thereby increasing understanding of BMP5's role in human development.

RUNX2-related metaphyseal dysplasia with maxillary hypoplasia: A rare skeletal disorder resembling SFRP4-related Pyle disease.

Metaphyseal dysplasia with maxillary hypoplasia with or without brachydactyly (MDMHB) is an ultra-rare skeletal dysplasia caused by heterozygous intragenic RUNX2 duplications, comprising either exons 3 to 5 or exons 3 to 6 of RUNX2. In this study, we describe a 14-year-old Belgian boy with metaphyseal dysplasia with maxillary hypoplasia but without brachydactyly. Clinical and radiographic examination revealed mild facial dysmorphism, dental anomalies, enlarged clavicles, genua valga and metaphyseal flaring and thin cortices with an osteoporotic skeletal appearance. Exome sequencing led to the identification of a de novo heterozygous tandem duplication within RUNX2, encompassing exons 3 to 7. This duplication is larger than the ones previously reported in MDMHB cases since it extends into the C-terminal activation domain of RUNX2. We review previously reported cases with MDMHB and highlight the resemblance of this disorder with Pyle disease, which may be explained by intersecting molecular pathways between RUNX2 and sFRP4. This study expands our knowledge on the genotypic and phenotypic characteristics of MDMHB and the role of RUNX2 in rare bone disorders.

A mesomelic skeletal dysplasia, Kantaputra-like, not related to HOXD cluster region, and with phenotypic gender differences.

Mesomelic skeletal dysplasia is a heterogeneous group of skeletal disorders that has grown since the molecular basis of these conditions is in the process of research and discovery. Here, we report a Brazilian family with eight affected members over three generations with a phenotype similar to mesomelic Kantaputra dysplasia. This family presents marked shortening of the upper limbs with hypotrophy of the lower limbs and clubfeet without synostosis. Array-based CNV analysis and exome sequencing of four family members failed to show any region or gene candidate. Interestingly, males were more severely affected than females in this family, suggesting that gender differences could play a role in the phenotypic expressivity of this condition.

Unexpected findings in cervical spine in spondylometaphyseal dysplasia Sutcliff type FN1-related.

The autosomal dominant spondylometaphyseal dysplasia Sutcliff type or corner fracture type FN1-related is characterized by a combination of metaphyseal irregularities simulating fractures ("corner fractures"), developmental coxa vara, and vertebral changes. It is linked to heterozygous mutations in FN1 and COL2A1. Vertebral changes as delayed vertebral ossification, ovoid vertebral bodies, anterior vertebral wedging, and platyspondyly have been observed in this condition, while odontoid abnormalities have not been reported. We report an odontoid anomaly in a girl with SMD-CF FN1-related showing the heterozygous variant c.505T>A; p.(Cys169Ser), presenting at 11.9 years of age with acute quadriparesis. Images showed spinal cord compression and injury associated with os odontoideum and C1-C2 instability. She required decompression and instrumented occipitocervical stabilization, suffering from residual paraparesis. This paper describes the first case of SMD-CF FN1-related accompanied by odontoid anomalies.

Further defining the molecular spectrum and long-term follow-up of 17 patients with Dyggve-Melchior-Clausen and Smith-McCort dysplasia type 2.

Dyggve-Melchior-Clausen dysplasia (DMC) and Smith-McCort dysplasia (SMC types 1 and 2) are rare spondylo-epi-metaphyseal dysplasias with identical radiological and clinical findings. DMC and SMC type 1 are allelic disorders caused by homozygous or compound heterozygous variants in DYM, while biallelic causative variants in RAB33B lead to SMC type 2. The terminology "skeletal golgipathies" has been recently used to describe these conditions, highlighting the pivotal role of these two genes in the organization and intracellular trafficking of the Golgi apparatus. In this study, we investigated 17 affected individuals (8 males, 9 females) from 10 unrelated consanguineous families, 10 diagnosed with DMC and seven with SMC type 2. The mean age at diagnosis was 9.61 ± 9.72 years, ranging from 20 months to 34 years, and the average height at diagnosis was 92.85 ± 15.50 cm. All patients exhibited variable degrees of short trunk with a barrel chest, protruding abdomen, hyperlordosis, and decreased joint mobility. A total of nine different biallelic variants were identified, with six being located in the DYM gene and the remaining three detected in RAB33B. Notably, five variants were classified as novel, four in the DYM gene and one in the RAB33B gene. This study aims to comprehensively assess clinical, radiological, and molecular findings along with the long-term follow-up findings in 17 patients with DMC and SMC type 2. Our results suggest that clinical symptoms of the disorder typically appear from infancy to early childhood. The central notches of the vertebral bodies were identified as early as 20 months and tended to become rectangular, particularly around 15 years of age. Pseudoepiphysis was observed in five patients; we believe this finding should be taken into consideration when evaluating hand radiographs in clinical assessments. Furthermore, our research contributes to an enhanced understanding of clinical and molecular aspects in these rare "skeletal golgipathies," expanding the mutational spectrum and offering insights into long-term disease outcomes.

Leukoencephalopathy with calcifications, developmental brain abnormalities and skeletal dysplasia due to homozygosity for a hypomorphic CSF1R variant: A report of three siblings.

We report three siblings homozygous for CSF1R variant c.1969 + 115_1969 + 116del to expand the phenotype of "brain abnormalities, neurodegeneration, and dysosteosclerosis" (BANDDOS) and discuss its link with "adult leukoencephalopathy with axonal spheroids and pigmented glia" (ALSP), caused by heterozygous CSF1R variants. We evaluated medical, radiological, and laboratory findings and reviewed the literature. Patients presented with developmental delay, therapy-resistant epilepsy, dysmorphic features, and skeletal abnormalities. Secondary neurological decline occurred from 23 years in sibling one and from 20 years in sibling two. Brain imaging revealed multifocal white matter abnormalities and calcifications during initial disease in siblings two and three. Developmental brain anomalies, seen in all three, were most severe in sibling two. During neurological decline in siblings one and two, the leukoencephalopathy was progressive and had the MRI appearance of ALSP. Skeletal survey revealed osteosclerosis, most severe in sibling three. Blood markers, monocytes, dendritic cell subsets, and T-cell proliferation capacity were normal. Literature review revealed variable initial disease and secondary neurological decline. BANDDOS presents with variable dysmorphic features, skeletal dysplasia, developmental delay, and epilepsy with on neuro-imaging developmental brain anomalies, multifocal white matter abnormalities, and calcifications. Secondary neurological decline occurs with a progressive leukoencephalopathy, in line with early onset ALSP. Despite the role of CSF1R signaling in myeloid development, immune deficiency is absent. Phenotype varies within families; skeletal and neurological manifestations may be disparate.

Evaluation of functionality-mobility in patients with skeletal dysplasias. Application of the STEMS tool ("everyday symptoms and mobility screening tool for skeletal dysplasias").

Individuals with differing forms of skeletal dysplasias (SD) frequently report impaired mobility and symptoms. With the objetive to evaluate mobility and associated symptoms in people with SD at an Argentinian pediatric hospital, using an Argentinian version of the Screening Tool for Everyday Mobility and Symptoms (STEMS), a simple questionnaire that allows clinicians to quickly identify the presence of symptoms associated with mobility in people with SD, while considering different environmental settings and the use of assistive devices, an analytical study of a consecutive sample of patients older than 5 years with SD and their affected relatives was carried out.Diagnosis, comorbidities, socioenvironmental, therapeutic, auxological and mobility variables were recorded. The presence and intensity of symptoms was noted through use of both the STEMS and validated scales. Descriptive, association and correlation analyzes were performed. One hundred and nineteen individuals with SD were enrolled in the study and divided into groups: Osteogenesis Imperfecta (OI, n = 55), Achondroplasia (ACH, n = 36) and Other SD resulting in disproportionate short stature (n = 28). Mobility assistive devices were almost exclusively used by individuals with OI. They were more frequently used by individuals with overweight and obesity, more severe form of the disease and in the outdoor settings. Two thirds (66.4%) of the individuals assessed in this study reported pain, 87.4% reported fatigue, and 58.8% reported both pain and fatigue. The intensity of symptoms was similar between groups and correlated with age and auxological variables. The STEMS was clear, easy and quick to use for identifying presence of pain and fatigue in this population group. The STEMS proved to be a simple and useful tool for evaluating functional mobility and associated symptoms in our population of individuals with SD.

Clinical utility of comprehensive gene panel testing for common and rare causes of skeletal dysplasia and other skeletal disorders: Results from the largest cohort to date.

Molecular genetics enables more precise diagnoses of skeletal dysplasia and other skeletal disorders (SDs). We investigated the clinical utility of multigene panel testing for 5011 unrelated individuals with SD in the United States (December 2019-April 2022). Median (range) age was 8 (0-90) years, 70.5% had short stature and/or disproportionate growth, 27.4% had a positive molecular diagnosis (MDx), and 30 individuals received two MDx. Genes most commonly contributing to MDx were FGFR3 (16.9%), ALPL (13.0%), and COL1A1 (10.3%). Most of the 112 genes associated with ≥1 MDx were primarily involved in signal transduction (n = 35), metabolism (n = 23), or extracellular matrix organization (n = 17). There were implications associated with specific care/treatment options for 84.4% (1158/1372) of MDx-positive individuals; >50% were linked to conditions with targeted therapy approved or in clinical development, including osteogenesis imperfecta, achondroplasia, hypophosphatasia, and mucopolysaccharidosis. Forty individuals with initially inconclusive results became MDx-positive following family testing. Follow-up mucopolysaccharidosis enzyme activity testing was positive in 14 individuals (10 of these were not MDx-positive). Our findings showed that inclusion of metabolic genes associated with SD increased the clinical utility of a gene panel and confirmed that integrated use of comprehensive gene panel testing with orthogonal testing reduced the burden of inconclusive results.

NANS-CDG: Expanding clinical insights with a novel patient with novel variants.

N-acetyl-d-neuraminic acid synthase-congenital disorder of glycosylation (NANS-CDG) is a rare autosomal recessive defect in the N-acetyl-neuraminic acid biosynthesis pathway. Herein, we report the first Korean NANS-CDG patient. A 10-year-old boy was referred to our clinic because of incidental radiographic findings indicating spondyloepimetaphyseal dysplasia. The patient had microcephaly, cavum septum pellucidum, and ventriculomegaly at birth, and at 10 years, a very short stature. He had a history of idiopathic chronic immune thrombocytopenia, central adrenal insufficiency, and hypothyroidism since infancy. The first unprovoked seizure occurred at the age of 2 years, and he was subsequently admitted to the hospital frequently because of respiratory infections and intractable seizures. Exome sequencing identified unreported biallelic variants of the NANS gene. Clinical and genetic confirmation of NANS-CDG highlights its expanding phenotypic and genotypic diversity.

A further case of chondrodysplasia with growth failure occurring after hematopoietic stem cell transplantation (HSCT).

There is an emerging body of evidence showing that young patients, post haematopoietic stem cell transplantation (HSCT), can develop skeletal changes that mimic an osteochondrodysplasia process. The key discriminator is that these children have had otherwise normal growth and skeletal development before the therapeutic intervention (HSCT), typically for a haematological malignancy. Herein we present that case of a boy who underwent HSCT for Haemophagocytic Lymphohistiocytosis (HLH) aged 2 years. Following Intervention with HSCT this boy's growth has severely decelerated (stature less than 1st centile matched for age) and he has developed a spondyloepiphyseal dysplasia.

Natural history of Wolcott-Rallison syndrome: A systematic review and follow-up study.

BACKGROUND AND AIMS: To systematically review the literature for reports on Wolcott-Rallison syndrome, focusing on the spectrum and natural history, genotype-phenotype correlations, patient and native liver survival, and long-term outcomes. METHODS: PubMed, Livio, Google Scholar, Scopus and Web of Science databases were searched. Data on genotype, phenotype, therapy, cause of death and follow-up were extracted. Survival and correlation analyses were performed. RESULTS: Sixty-two studies with 159 patients met the inclusion criteria and additional 30 WRS individuals were collected by personal contact. The median age of presentation was 2.5 months (IQR 2) and of death was 36 months (IQR 50.75). The most frequent clinical feature was neonatal diabetes in all patients, followed by liver impairment in 73%, impaired growth in 72%, skeletal abnormalities in 59.8%, the nervous system in 37.6%, the kidney in 35.4%, insufficient haematopoiesis in 34.4%, hypothyroidism in 14.8% and exocrine pancreas insufficiency in 10.6%. Episodes of acute liver failure were frequently reported. Liver transplantation was performed in six, combined liver-pancreas in one and combined liver-pancreas-kidney transplantation in two individuals. Patient survival was significantly better in the transplant cohort (p = .0057). One-, five- and ten-year patient survival rates were 89.4%, 65.5% and 53.1%, respectively. Liver failure was reported as the leading cause of death in 17.9% of cases. Overall survival was better in individuals with missense mutations (p = .013). CONCLUSION: Wolcott-Rallison syndrome has variable clinical courses. Overall survival is better in individuals with missense mutations. Liver- or multi-organ transplantation is a feasible treatment option to improve survival.

The Radiological and Histological Phenotype of Skeletal Abnormalities in Fetal ARCN1-Related Syndrome.

Mutations in ARCN1 give rise to a syndromic disorder with rhizomelic short stature with microretrognathia and developmental delay. ARCN1 encodes the delta subunit of the coat protein I complex, which is required for intracellular trafficking of collagen 1 and which may also be involved in the endoplasmic reticulum (ER) stress response. In this paper we describe for the first time the skeletal histological abnormalities in an 18-week-old fetus with an ARCN1 mutation, and we suggest that the skeletal phenotype in ARCN1-related syndrome has more resemblance with ER stress than with a defect in collagen 1 metabolism.

Renal Pathology of Ciliopathies.

Renal ciliopathies are a group of genetic disorders that affect the function of the primary cilium in the kidney, as well as other organs. Since primary cilia are important for regulation of cell signaling pathways, ciliary dysfunction results in a range of clinical manifestations, including renal failure, cyst formation, and hypertension. We summarize the current understanding of the pathophysiological and pathological features of renal ciliopathies in childhood, including autosomal dominant and recessive polycystic kidney disease, nephronophthisis, and Bardet-Biedl syndrome, as well as skeletal dysplasia associated renal ciliopathies. The genetic basis of these disorders is now well-established in many cases, with mutations in a large number of cilia-related genes such as PKD1, PKD2, BBS, MKS, and NPHP being responsible for the majority of cases. Renal ciliopathies are broadly characterized by development of interstitial fibrosis and formation of multiple renal cysts which gradually enlarge and replace normal renal tissue, with each condition demonstrating subtle differences in the degree, location, and age-related development of cysts and fibrosis. Presentation varies from prenatal diagnosis of congenital multisystem syndromes to an asymptomatic childhood with development of complications in later adulthood and therefore clinicopathological correlation is important, including increasing use of targeted genetic testing or whole genome sequencing, allowing greater understanding of genetic pathophysiological mechanisms.