The critical role of the TB5 domain of fibrillin-1 in endochondral ossification.

Mutations in the fibrillin-1 (FBN1) gene are responsible for the autosomal dominant form of geleophysic dysplasia (GD), which is characterized by short stature and extremities, thick skin and cardiovascular disease. All known FBN1 mutations in patients with GD are localized within the region encoding the transforming growth factor-ß binding protein-like 5 (TB5) domain of this protein. Herein, we generated a knock-in mouse model, Fbn1Y1698C by introducing the p.Tyr1696Cys mutation from a patient with GD into the TB5 domain of murine Fbn1 to elucidate the specific role of this domain in endochondral ossification. We found that both Fbn1Y1698C/+ and Fbn1Y1698C/Y1698C mice exhibited a reduced stature reminiscent of the human GD phenotype. The Fbn1 point mutation introduced in these mice affected the growth plate formation owing to abnormal chondrocyte differentiation such that mutant chondrocytes failed to establish a dense microfibrillar network composed of FBN1. This original Fbn1 mutant mouse model offers new insight into the pathogenic events underlying GD. Our findings suggest that the etiology of GD involves the dysregulation of the extracellular matrix composed of an abnormal FBN1 microfibril network impacting the differentiation of the chondrocytes.

RINT1 Bi-allelic Variations Cause Infantile-Onset Recurrent Acute Liver Failure and Skeletal Abnormalities.

Pediatric acute liver failure (ALF) is life threatening with genetic, immunologic, and environmental etiologies. Approximately half of all cases remain unexplained. Recurrent ALF (RALF) in infants describes repeated episodes of severe liver injury with recovery of hepatic function between crises. We describe bi-allelic RINT1 alterations as the cause of a multisystem disorder including RALF and skeletal abnormalities. Three unrelated individuals with RALF onset ≤3 years of age have splice alterations at the same position (c.1333+1G>A or G>T) in trans with a missense (p.Ala368Thr or p.Leu370Pro) or in-frame deletion (p.Val618_Lys619del) in RINT1. ALF episodes are concomitant with fever/infection and not all individuals have complete normalization of liver function testing between episodes. Liver biopsies revealed nonspecific liver damage including fibrosis, steatosis, or mild increases in Kupffer cells. Skeletal imaging revealed abnormalities affecting the vertebrae and pelvis. Dermal fibroblasts showed splice-variant mediated skipping of exon 9 leading to an out-of-frame product and nonsense-mediated transcript decay. Fibroblasts also revealed decreased RINT1 protein, abnormal Golgi morphology, and impaired autophagic flux compared to control. RINT1 interacts with NBAS, recently implicated in RALF, and UVRAG, to facilitate Golgi-to-ER retrograde vesicle transport. During nutrient depletion or infection, Golgi-to-ER transport is suppressed and autophagy is promoted through UVRAG regulation by mTOR. Aberrant autophagy has been associated with the development of similar skeletal abnormalities and also with liver disease, suggesting that disruption of these RINT1 functions may explain the liver and skeletal findings. Clarifying the pathomechanism underlying this gene-disease relationship may inform therapeutic opportunities.

Defects of cohesin loader lead to bone dysplasia associated with transcriptional disturbance.

Cohesin loader nipped-B-like protein (Nipbl) is increasingly recognized for its important role in development and cancer. Cornelia de Lange Syndrome (CdLS), mostly caused by heterozygous mutations of Nipbl, is an autosomal dominant disease characterized by multiorgan malformations. However, the regulatory role and underlying mechanism of Nipbl in skeletal development remain largely elusive. In this study, we constructed a Nipbl-a Cas9-knockout (KO) zebrafish, which displayed severe retardation of global growth and skeletal development. Deficiency of Nipbl remarkably compromised cell growth and survival, and osteogenic differentiation of mammalian osteoblast precursors. Furthermore, Nipbl depletion impaired the cell cycle process, and caused DNA damage accumulation and cellular senescence. In addition, nucleolar fibrillarin expression, global rRNA biogenesis, and protein translation were defective in the Nipbl-depleted osteoblast precursors. Interestingly, an integrated stress response inhibitor (ISRIB), partially rescued Nipbl depletion-induced cellular defects in proliferation and apoptosis, osteogenesis, and nucleolar function. Simultaneously, we performed transcriptome analysis of Nipbl deficiency on human neural crest cells and mouse embryonic fibroblasts in combination with Nipbl ChIP-Seq. We found that Nipbl deficiency caused thousands of differentially expressed genes including some important genes in bone and cartilage development. In conclusion, Nipbl deficiency compromised skeleton development through impairing osteoblast precursor cell proliferation and survival, and osteogenic differentiation, and also disturbing the expression of some osteogenesis-regulatory genes. Our study elucidated that Nipbl played a pivotal role in skeleton development, and supported the fact that treatment of ISRIB may provide an early intervention strategy to alleviate the bone dysplasia of CdLS.

Dynll1 is essential for development and promotes endochondral bone formation by regulating intraflagellar dynein function in primary cilia.

Mutations in subunits of the cilia-specific cytoplasmic dynein-2 (CD2) complex cause short-rib thoracic dystrophy syndromes (SRTDs), characterized by impaired bone growth and life-threatening perinatal respiratory complications. Different SRTD mutations result in varying disease severities. It remains unresolved whether this reflects the extent of retained hypomorphic protein functions or relative importance of the affected subunits for the activity of the CD2 holoenzyme. To define the contribution of the LC8-type dynein light chain subunit to the CD2 complex, we have generated Dynll1-deficient mouse strains, including the first-ever conditional knockout (KO) mutant for any CD2 subunit. Germline Dynll1 KO mice exhibit a severe ciliopathy-like phenotype similar to mice lacking another CD2 subunit, Dync2li1. Limb mesoderm-specific loss of Dynll1 results in severe bone shortening similar to human SRTD patients. Mechanistically, loss of Dynll1 leads to a partial depletion of other SRTD-related CD2 subunits, severely impaired retrograde intra-flagellar transport, significant thickening of primary cilia and cilia signaling defects. Interestingly, phenotypes of Dynll1-deficient mice are very similar to entirely cilia-deficient Kif3a/Ift88-null mice, except that they never present with polydactyly and retain relatively higher signaling outputs in parts of the hedgehog pathway. Compared to complete loss of Dynll1, maintaining very low DYNLL1 levels in mice lacking the Dynll1-transcription factor ASCIZ (ATMIN) results in significantly attenuated phenotypes and improved CD2 protein levels. The results suggest that primary cilia can maintain some functionality in the absence of intact CD2 complexes and provide a viable animal model for the analysis of the underlying bone development defects of SRTDs.

Integrating glycomics and genomics uncovers SLC10A7 as essential factor for bone mineralization by regulating post-Golgi protein transport and glycosylation.

Genomics methodologies have significantly improved elucidation of Mendelian disorders. The combination with high-throughput functional-omics technologies potentiates the identification and confirmation of causative genetic variants, especially in singleton families of recessive inheritance. In a cohort of 99 individuals with abnormal Golgi glycosylation, 47 of which being unsolved, glycomics profiling was performed of total plasma glycoproteins. Combination with whole-exome sequencing in 31 cases revealed a known genetic defect in 15 individuals. To identify additional genetic factors, hierarchical clustering of the plasma glycomics data was done, which indicated a subgroup of four patients that shared a unique glycomics signature of hybrid type N-glycans. In two siblings, compound heterozygous mutations were found in SLC10A7, a gene of unknown function in human. These included a missense mutation that disrupted transmembrane domain 4 and a mutation in a splice acceptor site resulting in skipping of exon 9. The two other individuals showed a complete loss of SLC10A7 mRNA. The patients' phenotype consisted of amelogenesis imperfecta, skeletal dysplasia, and decreased bone mineral density compatible with osteoporosis. The patients' phenotype was mirrored in SLC10A7 deficient zebrafish. Furthermore, alizarin red staining of calcium deposits in zebrafish morphants showed a strong reduction in bone mineralization. Cell biology studies in fibroblasts of affected individuals showed intracellular mislocalization of glycoproteins and a defect in post-Golgi transport of glycoproteins to the cell membrane. In contrast to yeast, human SLC10A7 localized to the Golgi. Our combined data indicate an important role for SLC10A7 in bone mineralization and transport of glycoproteins to the extracellular matrix.

Impairment of chondrogenesis and microfibrillar network in Adamtsl2 deficiency.

Mutations in the a disintegrin and metalloproteinase with thrombospondin motif-like 2 ( ADAMTSL2) gene are responsible for the autosomal recessive form of geleophysic dysplasia, which is characterized by short stature, short extremities, and skeletal abnormalities. However, the exact function of ADAMTSL2 is unknown. To elucidate the role of this protein in skeletal development, we generated complementary knockout (KO) mouse models with either total or chondrocyte Adamtsl2 deficiency. We observed that the Adamtsl2 KO mice displayed skeletal abnormalities reminiscent of the human phenotype. Adamtsl2 deletion affected the growth plate formation with abnormal differentiation and proliferation of chondrocytes. In addition, a TGF-ß signaling impairment in limbs lacking Adamtsl2 was demonstrated. Further investigations revealed that Adamtsl2 KO chondrocytes failed to establish a microfibrillar network composed by fibrillin1 and latent TGF-ß binding protein 1 fibrils. Chondrocyte Adamtsl2 KO mice also exhibited dwarfism. These studies uncover the function of Adamtsl2 in the maintenance of the growth plate ECM by modulating the microfibrillar network.-Delhon, L., Mahaut, C., Goudin, N., Gaudas, E., Piquand, K., Le Goff, W., Cormier-Daire, V., Le Goff, C. Impairment of chondrogenesis and microfibrillar network in Adamtsl2 deficiency.

Skeletal Dysplasias Caused by Sulfation Defects.

Proteoglycans (PGs) are macromolecules present on the cell surface and in the extracellular matrix that confer specific mechanical, biochemical, and physical properties to tissues. Sulfate groups present on glycosaminoglycans, linear polysaccharide chains attached to PG core proteins, are fundamental for correct PG functions. Indeed, through the negative charge of sulfate groups, PGs interact with extracellular matrix molecules and bind growth factors regulating tissue structure and cell behavior. The maintenance of correct sulfate metabolism is important in tissue development and function, particularly in cartilage where PGs are fundamental and abundant components of the extracellular matrix. In chondrocytes, the main sulfate source is the extracellular space, then sulfate is taken up and activated in the cytosol to the universal sulfate donor to be used in sulfotransferase reactions. Alteration in each step of sulfate metabolism can affect macromolecular sulfation, leading to the onset of diseases that affect mainly cartilage and bone. This review presents a panoramic view of skeletal dysplasias caused by mutations in genes encoding for transporters or enzymes involved in macromolecular sulfation. Future research in this field will contribute to the understanding of the disease pathogenesis, allowing the development of targeted therapies aimed at alleviating, preventing, or modifying the disease progression.

FGFR2 mutations in bent bone dysplasia syndrome activate nucleolar stress and perturb cell fate determination.

Fibroblast Growth Factor (FGF) signaling promotes self-renewal in progenitor cells by encouraging proliferation and inhibiting cellular senescence. Yet, these beneficial effects can be hijacked by disease-causing mutations in FGF receptor (FGFR) during embryogenesis. By studying dominant FGFR2 mutations that are germline in bent bone dysplasia syndrome (BBDS), we reveal a mechanistic connection between FGFR2, ribosome biogenesis, and cellular stress that links cell fate determination to disease pathology. We previously showed that FGFR2 mutations in BBDS, which amplify nucleolar targeting of FGFR2, activate ribosomal DNA (rDNA) transcription and delay differentiation in osteoprogenitor cells and patient-derived bone. Here we find that the BBDS mutations augment the ability of FGFR2 to recruit histone-remodeling factors that epigenetically activate transcriptionally silent rDNA. Nucleolar morphology is controlled by chromatin structure, and the high levels of euchromatic rDNA induced by the BBDS mutations direct nucleolar disorganization, alter ribosome biogenesis, and activate the Rpl11-Mdm2-p53 nucleolar stress response pathway. Inhibition of p53 in cells expressing the FGFR2 mutations in BBDS rescues delayed osteoblast differentiation, suggesting that p53 activation is an essential pathogenic factor in, and potential therapeutic target for, BBDS. This work establishes rDNA as developmentally regulated loci that receive direct input from FGF signaling to balance self-renewal and cell fate determination.

PIGQ glycosylphosphatidylinositol-anchored protein deficiency: Characterizing the phenotype.

PIGQ (OMIM *605754) encodes phosphatidylinositol glycan biosynthesis class Q (PIGQ) and is required for proper functioning of an N-acetylglucosamine transferase complex in a similar manner to the more established PIGA, PIGC, and PIGH. There are two previous patients reported with homozygous and apparently deleterious PIGQ mutations. Here, we provide the first detailed clinical report of a patient with heterozygous deleterious mutations associated with glycosylphosphatidylinositol-anchored protein (GPI-AP) biosynthesis deficiency. Our patient died at 10 months of age. The rare skeletal findings in this disorder expand the differential diagnosis of long bone radiolucent lesions and sphenoid wing dysplasia. This clinical report describes a new and rare disorder-PIGQ GPI-AP biosynthesis deficiency syndrome.

[Wnt Signaling and Skeletal Dysplasias.]

Identification of responsible genes for skeletal dysplasias evidences their critical roles in the skeletal development and maintenance. Mutations in the genes encoding the components of Wnt canonical pathway, which include WNT1, LRP5, LRP4, SOST and WTX, cause the disorders characterized by abnormal in bone mass. On the other hand, mutations in the genes for the components of Wnt non-canonical pathway such as WNT5A, ROR2, DVL1 and DVL3 are associated with dysmorphic skeletal disorders which manifest short limbs and facial anomalies. Thus, both canonical and non-canonical pathways of Wnt signaling play substantial roles in the human skeletons, and it is suggested that the former mainly controls bone mass while the latter regulates skeletal morphogenesis.

Skeletal overgrowth-causing mutations mimic an allosterically activated conformation of guanylyl cyclase-B that is inhibited by 2,4,6,-trinitrophenyl ATP.

Activating mutations in the receptor for C-type natriuretic peptide (CNP), guanylyl cyclase B (GC-B, also known as Npr2 or NPR-B), increase cellular cGMP and cause skeletal overgrowth, but how these mutations affect GTP catalysis is poorly understood. The A488P and R655C mutations were compared with the known mutation V883M. Neither mutation affected GC-B concentrations. The A488P mutation decreased the EC50 5-fold, increased Vmax 2.6-fold, and decreased the Km 13-fold, whereas the R655C mutation decreased the EC50 5-fold, increased the Vmax 2.1-fold, and decreased the Km 4.7-fold. Neither mutation affected maximum activity at saturating CNP concentrations. Activation by R655C did not require disulfide bond formation. Surprisingly, the A488P mutant only activated the receptor when it was phosphorylated. In contrast, the R655C mutation converted GC-B-7A from CNP-unresponsive to CNP-responsive. Interestingly, neither mutant was activated by ATP, and the Km and Hill coefficient of each mutant assayed in the absence of ATP were similar to those of wild-type GC-B assayed in the presence of ATP. Finally, 1 mm 2,4,6,-trinitrophenyl ATP inhibited all three mutants by as much as 80% but failed to inhibit WT-GC-B. We conclude that 1) the A488P and R655C missense mutations result in a GC-B conformation that mimics the allosterically activated conformation, 2) GC-B phosphorylation is required for CNP-dependent activation by the A488P mutation, 3) the R655C mutation abrogates the need for phosphorylation in receptor activation, and 4) an ATP analog selectively inhibits the GC-B mutants, indicating that a pharmacologic approach could reduce GC-B dependent human skeletal overgrowth.

Mutations Preventing Regulated Exon Skipping in MET Cause Osteofibrous Dysplasia.

The periosteum contributes to bone repair and maintenance of cortical bone mass. In contrast to the understanding of bone development within the epiphyseal growth plate, factors that regulate periosteal osteogenesis have not been studied as intensively. Osteofibrous dysplasia (OFD) is a congenital disorder of osteogenesis and is typically sporadic and characterized by radiolucent lesions affecting the cortical bone immediately under the periosteum of the tibia and fibula. We identified germline mutations in MET, encoding a receptor tyrosine kinase, that segregate with an autosomal-dominant form of OFD in three families and a mutation in a fourth affected subject from a simplex family and with bilateral disease. Mutations identified in all families with dominant inheritance and in the one simplex subject with bilateral disease abolished the splice inclusion of exon 14 in MET transcripts, which resulted in a MET receptor (MET(Δ14)) lacking a cytoplasmic juxtamembrane domain. Splice exclusion of this domain occurs during normal embryonic development, and forced induction of this exon-exclusion event retarded osteoblastic differentiation in vitro and inhibited bone-matrix mineralization. In an additional subject with unilateral OFD, we identified a somatic MET mutation, also affecting exon 14, that substituted a tyrosine residue critical for MET receptor turnover and, as in the case of the MET(Δ14) mutations, had a stabilizing effect on the mature protein. Taken together, these data show that aberrant MET regulation via the juxtamembrane domain subverts core MET receptor functions that regulate osteogenesis within cortical diaphyseal bone.

Neonatal bone marrow transplantation prevents bone pathology in a mouse model of mucopolysaccharidosis type I.

Neonatal bone marrow transplantation (BMT) could offer a novel therapeutic opportunity for genetic disorders by providing sustainable levels of the missing protein at birth, thus preventing tissue damage. We tested this concept in mucopolysaccharidosis type I (MPS IH; Hurler syndrome), a lysosomal storage disorder caused by deficiency of α-l-iduronidase. MPS IH is characterized by a broad spectrum of clinical manifestations, including severe progressive skeletal abnormalities. Although BMT increases the life span of patients with MPS IH, musculoskeletal manifestations are only minimally responsive if the timing of BMT delays, suggesting already irreversible bone damage. In this study, we tested the hypothesis that transplanting normal BM into newborn MPS I mice soon after birth can prevent skeletal dysplasia. We observed that neonatal BMT was effective at restoring α-l-iduronidase activity and clearing elevated glycosaminoglycans in blood and multiple organs. At 37 weeks of age, we observed an almost complete normalization of all bone tissue parameters, using radiographic, microcomputed tomography, biochemical, and histological analyses. Overall, the magnitude of improvements correlated with the extent of hematopoietic engraftment. We conclude that BMT at a very early stage in life markedly reduces signs and symptoms of MPS I before they appear.

Osteoblast dysfunctions in bone diseases: from cellular and molecular mechanisms to therapeutic strategies.

Several metabolic, genetic and oncogenic bone diseases are characterized by defective or excessive bone formation. These abnormalities are caused by dysfunctions in the commitment, differentiation or survival of cells of the osteoblast lineage. During the recent years, significant advances have been made in our understanding of the cellular and molecular mechanisms underlying the osteoblast dysfunctions in osteoporosis, skeletal dysplasias and primary bone tumors. This led to suggest novel therapeutic approaches to correct these abnormalities such as the modulation of WNT signaling, the pharmacological modulation of proteasome-mediated protein degradation, the induction of osteoprogenitor cell differentiation, the repression of cancer cell proliferation and the manipulation of epigenetic mechanisms. This article reviews our current understanding of the major cellular and molecular mechanisms inducing osteoblastic cell abnormalities in age-related bone loss, genetic skeletal dysplasias and primary bone tumors, and discusses emerging therapeutic strategies to counteract the osteoblast abnormalities in these disorders of bone formation.

Ribosome biogenesis in skeletal development and the pathogenesis of skeletal disorders.

The skeleton affords a framework and structural support for vertebrates, while also facilitating movement, protecting vital organs, and providing a reservoir of minerals and cells for immune system and vascular homeostasis. The mechanical and biological functions of the skeleton are inextricably linked to the size and shape of individual bones, the diversity of which is dependent in part upon differential growth and proliferation. Perturbation of bone development, growth and proliferation, can result in congenital skeletal anomalies, which affect approximately 1 in 3000 live births [1]. Ribosome biogenesis is integral to all cell growth and proliferation through its roles in translating mRNAs and building proteins. Disruption of any steps in the process of ribosome biogenesis can lead to congenital disorders termed ribosomopathies. In this review, we discuss the role of ribosome biogenesis in skeletal development and in the pathogenesis of congenital skeletal anomalies. This article is part of a Special Issue entitled: Role of the Nucleolus in Human Disease.

Characterization of ANKRD11 mutations in humans and mice related to KBG syndrome.

Mutations in ANKRD11 have recently been reported to cause KBG syndrome, an autosomal dominant condition characterized by intellectual disability (ID), behavioral problems, and macrodontia. To understand the pathogenic mechanism that relates ANKRD11 mutations with the phenotype of KBG syndrome, we studied the cellular characteristics of wild-type ANKRD11 and the effects of mutations in humans and mice. We show that the abundance of wild-type ANKRD11 is tightly regulated during the cell cycle, and that the ANKRD11 C-terminus is required for the degradation of the protein. Analysis of 11 pathogenic ANKRD11 variants in humans, including six reported in this study, and one reported in the Ankrd11 (Yod/+) mouse, shows that all mutations affect the C-terminal regions and that the mutant proteins accumulate aberrantly. In silico analysis shows the presence of D-box sequences that are signals for proteasome degradation. We suggest that ANKRD11 C-terminus plays an important role in regulating the abundance of the protein, and a disturbance of the protein abundance due to the mutations leads to KBG syndrome.

Bent bone dysplasia-FGFR2 type, a distinct skeletal disorder, has deficient canonical FGF signaling.

Fibroblast growth factor receptor 2 (FGFR2) is a crucial regulator of bone formation during embryonic development. Both gain and loss-of-function studies in mice have shown that FGFR2 maintains a critical balance between the proliferation and differentiation of osteoprogenitor cells. We have identified de novo FGFR2 mutations in a sporadically occurring perinatal lethal skeletal dysplasia characterized by poor mineralization of the calvarium, craniosynostosis, dysmorphic facial features, prenatal teeth, hypoplastic pubis and clavicles, osteopenia, and bent long bones. Histological analysis of the long bones revealed that the growth plate contained smaller hypertrophic chondrocytes and a thickened hypercellular periosteum. Four unrelated affected individuals were found to be heterozygous for missense mutations that introduce a polar amino acid into the hydrophobic transmembrane domain of FGFR2. Using diseased chondrocytes and a cell-based assay, we determined that these mutations selectively reduced plasma-membrane levels of FGFR2 and markedly diminished the receptor's responsiveness to extracellular FGF. All together, these clinical and molecular findings are separate from previously characterized FGFR2 disorders and represent a distinct skeletal dysplasia.

Embryonic ablation of osteoblast Smad4 interrupts matrix synthesis in response to canonical Wnt signaling and causes an osteogenesis-imperfecta-like phenotype.

To examine interactions between bone morphogenic protein (BMP) and canonical Wnt signaling during skeletal growth, we ablated Smad4, a key component of the TGF-ß-BMP pathway, in Osx1(+) cells in mice. We show that loss of Smad4 causes stunted growth, spontaneous fractures and a combination of features seen in osteogenesis imperfecta, cleidocranial dysplasia and Wnt-deficiency syndromes. Bones of Smad4 mutant mice exhibited markers of fully differentiated osteoblasts but lacked multiple collagen-processing enzymes, including lysyl oxidase (Lox), a BMP2-responsive gene regulated by Smad4 and Runx2. Accordingly, the collagen matrix in Smad4 mutants was disorganized, but also hypomineralized. Primary osteoblasts from these mutants did not mineralize in vitro in the presence of BMP2 or Wnt3a, and Smad4 mutant mice failed to accrue new bone following systemic inhibition of the Dickkopf homolog Dkk1. Consistent with impaired biological responses to canonical Wnt, ablation of Smad4 causes cleavage of ß-catenin and depletion of the low density lipoprotein receptor Lrp5, subsequent to increased caspase-3 activity and apoptosis. In summary, Smad4 regulates maturation of skeletal collagen and osteoblast survival, and is required for matrix-forming responses to both BMP2 and canonical Wnt.

Taxonomy of rare genetic metabolic bone disorders.

UNLABELLED: This article reports a taxonomic classification of rare skeletal diseases based on metabolic phenotypes. It was prepared by The Skeletal Rare Diseases Working Group of the International Osteoporosis Foundation (IOF) and includes 116 OMIM phenotypes with 86 affected genes. INTRODUCTION: Rare skeletal metabolic diseases comprise a group of diseases commonly associated with severe clinical consequences. In recent years, the description of the clinical phenotypes and radiographic features of several genetic bone disorders was paralleled by the discovery of key molecular pathways involved in the regulation of bone and mineral metabolism. Including this information in the description and classification of rare skeletal diseases may improve the recognition and management of affected patients. METHODS: IOF recognized this need and formed a Skeletal Rare Diseases Working Group (SRD-WG) of basic and clinical scientists who developed a taxonomy of rare skeletal diseases based on their metabolic pathogenesis. RESULTS: This taxonomy of rare genetic metabolic bone disorders (RGMBDs) comprises 116 OMIM phenotypes, with 86 affected genes related to bone and mineral homeostasis. The diseases were divided into four major groups, namely, disorders due to altered osteoclast, osteoblast, or osteocyte activity; disorders due to altered bone matrix proteins; disorders due to altered bone microenvironmental regulators; and disorders due to deranged calciotropic hormonal activity. CONCLUSIONS: This article provides the first comprehensive taxonomy of rare metabolic skeletal diseases based on deranged metabolic activity. This classification will help in the development of common and shared diagnostic and therapeutic pathways for these patients and also in the creation of international registries of rare skeletal diseases, the first step for the development of genetic tests based on next generation sequencing and for performing large intervention trials to assess efficacy of orphan drugs.

The puzzle of TRPV4 channelopathies.

Hereditary channelopathies, that is, mutations in channel genes that alter channel function and are causal for the pathogenesis of the disease, have been described for several members of the transient receptor potential channel family. Mutations in the TRPV4 gene, encoding a polymodal Ca(2+) permeable channel, are causative for several human diseases, which affect the skeletal system and the peripheral nervous system, with highly variable phenotypes. In this review, we describe the phenotypes of TRPV4 channelopathies and overlapping symptoms. Putative mechanisms to explain the puzzle, and how mutations in the same region of the channel cause different diseases, are discussed and experimental approaches to tackle this surprising problem are suggested.