XYLT1 mutations in Desbuquois dysplasia type 2

Advances in the discovery and analyses of human tandem repeats

Long-read sequencing platforms provide unparalleled access to the structure and composition of all classes of tandemly repeated DNA from STRs to satellite arrays. This review summarizes our current understanding of their organization within the human genome, their importance with respect to disease, as well as the advances and challenges in understanding their genetic diversity and functional effects. Novel computational methods are being developed to visualize and associate these complex patterns of human variation with disease, expression, and epigenetic differences. We predict accurate characterization of this repeat-rich form of human variation will become increasingly relevant to both basic and clinical human genetics.

Xylosyltransferase I mediates the synthesis of proteoglycans with long glycosaminoglycan chains and controls chondrocyte hypertrophy and collagen fibers organization of in the growth plate

Genetic mutations in the Xylt1 gene are associated with Desbuquois dysplasia type II syndrome characterized by sever prenatal and postnatal short stature. However, the specific role of XylT-I in the growth plate is not completely understood. Here, we show that XylT-I is expressed and critical for the synthesis of proteoglycans in resting and proliferative but not in hypertrophic chondrocytes in the growth plate. We found that loss of XylT-I induces hypertrophic phenotype-like of chondrocytes associated with reduced interterritorial matrix. Mechanistically, deletion of XylT-I impairs the synthesis of long glycosaminoglycan chains leading to the formation of proteoglycans with shorter glycosaminoglycan chains. Histological and Second Harmonic Generation microscopy analysis revealed that deletion of XylT-I accelerated chondrocyte maturation and prevents chondrocytes columnar organization and arrangement in parallel of collagen fibers in the growth plate, suggesting that XylT-I controls chondrocyte maturation and matrix organization. Intriguingly, loss of XylT-I induced at embryonic stage E18.5 the migration of progenitor cells from the perichondrium next to the groove of Ranvier into the central part of epiphysis of E18.5 embryos. These cells characterized by higher expression of glycosaminoglycans exhibit circular organization then undergo hypertrophy and death creating a circular structure at the secondary ossification center location. Our study revealed an uncovered role of XylT-I in the synthesis of proteoglycans and provides evidence that the structure of glycosaminoglycan chains of proteoglycans controls chondrocyte maturation and matrix organization.

Deficiency of Fam20b-Catalyzed Glycosaminoglycan Chain Synthesis in Neural Crest Leads to Cleft Palate

Cleft palate is one of the most common birth defects. Previous studies revealed that multiple factors, including impaired intracellular or intercellular signals, and incoordination of oral organs led to cleft palate, but were little concerned about the contribution of the extracellular matrix (ECM) during palatogenesis. Proteoglycans (PGs) are one of the important macromolecules in the ECM. They exert biological functions through one or more glycosaminoglycan (GAG) chains attached to core proteins. The family with sequence similarity 20 member b (Fam20b) are newly identified kinase-phosphorylating xylose residues that promote the correct assembly of the tetrasaccharide linkage region by creating a premise for GAG chain elongation. In this study, we explored the function of GAG chains in palate development through Wnt1-Cre; Fam20b^f/f mice, which exhibited complete cleft palate, malformed tongue, and micrognathia. In contrast, Osr2-Cre; Fam20b^f/f mice, in which Fam20b was deleted only in palatal mesenchyme, showed no abnormality, suggesting that failed palatal elevation in Wnt1-Cre; Fam20b^f/f mice was secondary to micrognathia. In addition, the reduced GAG chains promoted the apoptosis of palatal cells, primarily resulting in reduced cell density and decreased palatal volume. The suppressed BMP signaling and reduced mineralization indicated an impaired osteogenesis of palatine, which could be rescued partially by constitutively active Bmpr1a. Together, our study highlighted the key role of GAG chains in palate morphogenesis.

Family with sequence similarity 20 member B regulates osteogenic differentiation of bone marrow mesenchymal stem cells on titanium surfaces

Successful bone regeneration on titanium (Ti) surfaces is a key process in dental implant treatment. Bone marrow mesenchymal stem cells (BMSCs) are fundamental cellular components of this process, and their early recruitment, proliferation, and differentiation into bone-forming osteoblasts are crucial. A proteoglycan (PG)-rich layer has been reported to exist between Ti surfaces and bones; however, the molecules that could potentially affect the formation of this layer remain unknown. Family with sequence similarity 20 member B (FAM20B) is a newly identified kinase that regulates the synthesis of glycosaminoglycans, an important component of the PG-rich layer. Because FAM20B is also closely associated with bone development, in this study, we examined the function of FAM20B in osteogenic differentiation of BMSCs on Ti surfaces. For this, BMSC cell lines with knocked down FAM20B (shBMSCs) were cultured on Ti surfaces. The results showed that the depletion of FAM20B reduced the formation of a PG-rich layer between the Ti surfaces and cells. The shBMSCs exhibited downregulated expression of osteogenic marker genes (ALP and OCN) and decreased mineral deposition. Moreover, shBMSCs reduced the molecular levels of p-ERK1/2, which plays an important role in MSC osteogenesis. The nuclear translocation of RUNX2, an important transcription factor for osteogenic differentiation, on the Ti surfaces is inhibited by the depletion of FAM20B in BMSCs. Moreover, the depletion of FAM20B reduced the transcriptional activity of RUNX2, which is important in regulating the expression of osteogenic genes. STATEMENT OF SIGNIFICANCE: Bone healing and regeneration on implanted titanium surfaces is a cell-material interaction. Such an interaction is enabled by bone marrow mesenchymal stem cells (BMSCs), and their early recruitment, proliferation, and differentiation into bone-forming osteoblasts are essential for bone healing and osseointegration. In this study, we found that the family with sequence similarity 20-B influenced the formation of a proteoglycan rich layer between BMSCs and the titanium surface and regulated the differentiation of BMSCs into bone-forming osteoblasts. We believe that our study contributes significantly to the further exploration of bone healing and osseointegration mechanisms on implanted titanium surfaces.

Antenatal Phenotype of Desbuquois Dysplasia

Desbuquois dysplasia (DBQD) is an uncommon, autosomal recessive disorder with multiple joint dislocations. It is caused by pathogenic variants in CANT1 (calcium-activated nucleotidase 1) [NM_001159773.2]. This study adds to the scant data of nine reported antenatal phenotypes of DBQD. The present paper describes two unrelated consanguineous families with antenatal features of lethal skeletal dysplasia. The defining radiological changes were identified in only one patient who presented in the late second and third trimesters. Solo exome sequencing was performed and two previously reported homozygous variants c.896C>T (p.Pro299Leu) in patient 1 and c.902_906dup (p.Ser303fs*20) in patient 2 were identified. This study highlights the fetal presentations in DBQD and adds to its phenotypic spectrum. A complete clinical workup, including fetal autopsy and radiographs is essential to confirm the diagnosis of lethal skeletal dysplasia. Molecular diagnosis remains the diagnostic modality to define the causative variant. A definitive diagnosis is essential to inform management and offer reproductive care.

The Human Myofibroblast Marker Xylosyltransferase-I: A New Indicator for Macrophage Polarization

Chronic inflammation and excessive synthesis of extracellular matrix components, such as proteoglycans (PG), by fibroblast- or macrophage-derived myofibroblasts are the hallmarks of fibrotic diseases, including systemic sclerosis (SSc). Human xylosyltransferase-I (XT-I), which is encoded by the gene XYLT1, is the key enzyme that is involved in PG biosynthesis. Increased cellular XYLT1 expression and serum XT-I activity were measured in SSc. Nothing is known so far about the regulation of XT-I in immune cells, and their contribution to the increase in measurable serum XT-I activity. We utilized an in vitro model, with primary human CD14+CD16+ monocyte-derived macrophages (MΦ), in order to investigate the role of macrophage polarization on XT-I regulation. The MΦ generated were polarized towards two macrophage phenotypes that were associated with SSc, which were classified as classical pro-inflammatory (M1-like), and alternative pro-fibrotic (M2-like) MΦ. The fully characterized M1- and M2-like MΦ cultures showed differential XT-I gene and protein expressions. The fibrotic M2-like MΦ cultures exhibited higher XT-I secretion, as well as increased expression of myofibroblast marker α-smooth muscle actin, indicating the onset of macrophage-to-myofibroblast transition (MMT). Thus, we identified XT-I as a novel macrophage polarization marker for in vitro generated M1- and M2-like MΦ subtypes, and broadened the view of XT-I as a myofibroblast marker in the process of MMT.

Pathogenic Roles of Heparan Sulfate and Its Use as a Biomarker in Mucopolysaccharidoses

Heparan sulfate (HS) is an essential glycosaminoglycan (GAG) as a component of proteoglycans, which are present on the cell surface and in the extracellular matrix. HS-containing proteoglycans not only function as structural constituents of the basal lamina but also play versatile roles in various physiological processes, including cell signaling and organ development. Thus, inherited mutations of genes associated with the biosynthesis or degradation of HS can cause various diseases, particularly those involving the bones and central nervous system (CNS). Mucopolysaccharidoses (MPSs) are a group of lysosomal storage disorders involving GAG accumulation throughout the body caused by a deficiency of GAG-degrading enzymes. GAGs are stored differently in different types of MPSs. Particularly, HS deposition is observed in patients with MPS types I, II, III, and VII, all which involve progressive neuropathy with multiple CNS system symptoms. While therapies are available for certain symptoms in some types of MPSs, significant unmet medical needs remain, such as neurocognitive impairment. This review presents recent knowledge on the pathophysiological roles of HS focusing on the pathogenesis of MPSs. We also discuss the possible use and significance of HS as a biomarker for disease severity and therapeutic response in MPSs.

Mutant B3GALT6 in a Multiplex Family: A Dominant Variant Co-Segregated With Moderate Malformations

B3GALT6 is a well-documented disease-related gene. Several B3GALT6-recessive variants have been reported to cause Ehlers–Danlos syndrome (EDS). To the best of our knowledge, no dominant B3GALT6 variant that causes human disease has been reported. In 2012, we reported on a three-generation, autosomal-dominant family with multiple members who suffered from radioulnar joint rotation limitation, scoliosis, thick vermilion of both lips, and others, but the genetic cause was unknown. Here, exome sequencing of the family identified mutant B3GALT6 as the cause of the multiplex affected family. We observed that, in the compound heterozygous pattern (i.e., c.883C>T:p.R295C and c.510_517del:p.L170fs*268), mutant B3GALT6 led to severe consequences, and in the dominant pattern, an elongated B3GALT6 variant co-segregated with moderate phenotypes. The functional experiments were performed in vitro. The R295C variant led to subcellular mislocalization, whereas the L170fs*268 showed normal subcellular localization, but it led to an elongated protein. Given that most of the catalytic galactosyltransferase domain was disrupted for the L170fs*268 (it is unlikely that such a protein has activity), we propose that the L170fs*268 occupies the normal B3GALT6 protein position in the Golgi and exerts a dominant-negative effect.

First Characterization of Human Dermal Fibroblasts Showing a Decreased Xylosyltransferase-I Expression Induced by the CRISPR/Cas9 System

BACKGROUND

Xylosyltransferases-I and II (XT-I and XT-II) catalyze the initial and rate limiting step of the proteoglycan (PG) biosynthesis and therefore have an import impact on the homeostasis of the extracellular matrix (ECM). The reason for the occurrence of two XT-isoforms in all higher organisms remains unknown and targeted genome-editing strategies could shed light on this issue.

METHODS

XT-I deficient neonatal normal human dermal fibroblasts were generated by using the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated proteins (Cas) 9 system. We analyzed if a reduced XT-I activity leads to abnormalities regarding ECM-composition, myofibroblast differentiation, cellular senescence and skeletal and cartilage tissue homeostasis.

RESULTS

We successfully introduced compound heterozygous deletions within exon 9 of the XYLT1 gene. Beside XYLT1, we detected altered gene-expression levels of further, inter alia ECM-related, genes. Our data further reveal a dramatically reduced XT-I protein activity. Abnormal myofibroblast-differentiation was demonstrated by elevated alpha-smooth muscle actin expression on both, mRNA- and protein level. In addition, wound-healing capability was slightly delayed. Furthermore, we observed an increased cellular-senescence of knockout cells and an altered expression of target genes knowing to be involved in skeletonization.

CONCLUSION

Our data show the tremendous relevance of the XT-I isoform concerning myofibroblast-differentiation and ECM-homeostasis as well as the pathophysiology of skeletal disorders.

Roles of Chondroitin Sulfate Proteoglycans as Regulators of Skeletal Development

The extracellular matrix (ECM) is critically important for most cellular processes including differentiation, morphogenesis, growth, survival and regeneration. The interplay between cells and the ECM often involves bidirectional signaling between ECM components and small molecules, i.e., growth factors, morphogens, hormones, etc., that regulate critical life processes. The ECM provides biochemical and contextual information by binding, storing, and releasing the bioactive signaling molecules, and/or mechanical information that signals from the cell membrane integrins through the cytoskeleton to the nucleus, thereby influencing cell phenotypes. Using these dynamic, reciprocal processes, cells can also remodel and reshape the ECM by degrading and re-assembling it, thereby sculpting their environments. In this review, we summarize the role of chondroitin sulfate proteoglycans as regulators of cell and tissue development using the skeletal growth plate model, with an emphasis on use of naturally occurring, or created mutants to decipher the role of proteoglycan components in signaling paradigms.

Short stature associated with a novel mutation in the aggrecan gene: A case report and literature review

BACKGROUND

Mutations in the aggrecan (ACAN) gene are identified in patients with: spondyloepiphyseal dysplasia, Kimberley type; short stature with advanced bone age (BA); in the presence or absence of heterozygous ACAN mutation-induced early-onset osteoarthritis and/or osteochondritis dissecans; and spondyloepimetaphyseal dysplasia, ACAN type. Heterozygous mutations contribute to spondyloepiphyseal dysplasia, Kimberley type (MIM#608361), which is a milder skeletal dysplasia. In contrast, homozygous mutations cause a critical skeletal dysplasia, which is called spondyloepimetaphyseal dysplasia, ACAN type (MIM#612813). Lately, investigations on exome and genome sequencing have shown that ACAN mutations can also lead to idiopathic short stature with or without an advanced BA, in the presence or absence of early-onset osteoarthritis and/or osteochondritis dissecans (MIM#165800). We herein reported a heterozygous defect of ACAN in a family with autosomal dominant short stature, BA acceleration, and premature growth cessation.

CASE SUMMARY

A 2-year-old male patient visited us due to growth retardation. The patient presented symmetrical short stature (height 79 cm, < -2 SD) without facial features and other congenital abnormalities. Whole-exome sequencing revealed a heterozygous pathogenic variant c. 871C>T (p. Gln291*) of ACAN, which was not yet reported in cases of short stature. This mutation was also detected in his father and paternal grandmother. According to the Human Gene Mutation Database, 67 ACAN mutations are registered. Most of these mutations are genetically inheritable, and very few children with short stature are associated with ACAN mutations. To date, heterozygous ACAN mutations have been reported in approximately 40 families worldwide, including a few individuals with a decelerated BA.

CONCLUSION

Heterozygous c. 871C>T (p. Gln291*) variation of the ACAN gene was the disease-causing variant in this family. Collectively, our newly discovered mutation expanded the spectrum of ACAN gene mutations.

Severe phenotypes of B3GAT3-related disorder caused by two heterozygous variants: a case report and literature review

Background

Linkeropathies refers to a series of extremely rare hereditary connective tissue diseases affected by various glycosyltransferases in the biosynthesis of proteoglycans. We report for the first time two heterozygous variants of B3GAT3 in a Chinese infant, in whom Marfan syndrome was suspected at birth.

Case presentation

A 2-month-old boy from a non-consanguineous Chinese family without a family history presented severe phenotypes of joint dislocation, obvious flexion contractures of the elbow, arachnodactyly with slightly adducted thumbs, cranial dysplasia, foot abnormalities and aortic root dilation; Marfan syndrome was suspected at birth. Our patient was the youngest, at the age of 2 months, to experience aortic root dilation. Two B3GAT3 variants, NM_012200.2, c.752T>C, p.V251A and c.47C>A, p.S16*, with heterozygosity were identified in the patient by whole-exome sequencing; the variants were inherited from his parents. During close follow-up, significant changes in the cranial profile and obvious external hydrocephalus were present at the age of 7 months, which differs from previously reported cases.

Conclusion

We diagnosed a patient with congenital heart defects at an early age with a B3GAT3-related disorder instead of Marfan syndrome and expanded the spectrum of B3GAT3-related disorders. We also provide a literature review of reported B3GAT3 cases; for at least one of the variants, this is the first report of genotype–phenotype correlations in individuals with cardiovascular defects being related to the acceptor substrate-binding subdomain of B3GAT3.

Novel compound heterozygous variants in XYLT1 gene caused Desbuquois dysplasia type 2 in an aborted fetus: a case report

Background

Desbuquois dysplasia type 2 (DBQD2) is an infrequent dysplasia with a wide range of symptoms, including facial deformities, growth retardation and short long bones. It is an autosomal recessive disorder caused by mutations in the XYLT1 gene that encodes xylosyltransferase-1.

Case presentation

We studied an aborted fetus from Iranian non-consanguineous parents who was therapeutically aborted at 19 weeks of gestation. Ultrasound examinations at 18 weeks of gestation revealed growth retardation in her long bones and some facial problems. Whole-exome sequencing was performed on the aborted fetus which revealed compound heterozygous XYLT1 mutations: c.742G>A; p.(Glu248Lys) and c.1537 C>A; p.(Leu513Met). Sanger sequencing and segregation analysis confirmed the compound heterozygosity of these variants in XYLT1.

Conclusion

The c.1537 C>A; p.(Leu513Met) variant has not been reported in any databases so far and therefore is novel. This is the third compound heterozygote report in XYLT1 and further supports the high heterogeneity of this disease.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12887-022-03132-5.

Supply chain logistics - the role of the Golgi complex in extracellular matrix production and maintenance

The biomechanical and biochemical properties of connective tissues are determined by the composition and quality of their extracellular matrix. This, in turn, is highly dependent on the function and organisation of the secretory pathway. The Golgi complex plays a vital role in directing matrix output by co-ordinating the post-translational modification and proteolytic processing of matrix components prior to their secretion. These modifications have broad impacts on the secretion and subsequent assembly of matrix components, as well as their function in the extracellular environment. In this Review, we highlight the role of the Golgi in the formation of an adaptable, healthy matrix, with a focus on proteoglycan and procollagen secretion as example cargoes. We then discuss the impact of Golgi dysfunction on connective tissue in the context of human disease and ageing.

Factors affecting prepubertal and pubertal bone age progression

Bone age (BA) is a clinical marker of bone maturation which indicates the developmental stage of endochondral ossification at the epiphysis and the growth plate. Hormones that promote the endochondral ossification process include growth hormone, insulin-like growth factor-1, thyroid hormone, estrogens, and androgens. In particular, estrogens are essential for growth plate fusion and closure in both sexes. Bone maturation in female children is more advanced than in male children of all ages. The promotion of bone maturation seen in females before the onset of puberty is thought to be an effect of estrogen because estrogen levels are higher in females than in males before puberty. Sex hormones are essential for bone maturation during puberty. Since females have their pubertal onset about two years earlier than males, bone maturation in females is more advanced than in males during puberty. In the present study, we aimed to review the factors affecting prepubertal and pubertal BA progression, BA progression in children with hypogonadism, and bone maturation and deformities in children with Turner syndrome.

An Overview of in vivo Functions of Chondroitin Sulfate and Dermatan Sulfate Revealed by Their Deficient Mice

Chondroitin sulfate (CS), dermatan sulfate (DS) and heparan sulfate (HS) are covalently attached to specific core proteins to form proteoglycans in their biosynthetic pathways. They are constructed through the stepwise addition of respective monosaccharides by various glycosyltransferases and maturated by epimerases as well as sulfotransferases. Structural diversities of CS/DS and HS are essential for their various biological activities including cell signaling, cell proliferation, tissue morphogenesis, and interactions with a variety of growth factors as well as cytokines. Studies using mice deficient in enzymes responsible for the biosynthesis of the CS/DS and HS chains of proteoglycans have demonstrated their essential functions. Chondroitin synthase 1-deficient mice are viable, but exhibit chondrodysplasia, progression of the bifurcation of digits, delayed endochondral ossification, and reduced bone density. DS-epimerase 1-deficient mice show thicker collagen fibrils in the dermis and hypodermis, and spina bifida. These observations suggest that CS/DS are essential for skeletal development as well as the assembly of collagen fibrils in the skin, and that their respective knockout mice can be utilized as models for human genetic disorders with mutations in chondroitin synthase 1 and DS-epimerase 1. This review provides a comprehensive overview of mice deficient in CS/DS biosyntheses.

Inherited Proteoglycan Biosynthesis Defects-Current Laboratory Tools and Bikunin as a Promising Blood Biomarker

Proteoglycans consist of proteins linked to sulfated glycosaminoglycan chains. They constitute a family of macromolecules mainly involved in the architecture of organs and tissues as major components of extracellular matrices. Some proteoglycans also act as signaling molecules involved in inflammatory response as well as cell proliferation, adhesion, and differentiation. Inborn errors of proteoglycan metabolism are a group of orphan diseases with severe and irreversible skeletal abnormalities associated with multiorgan impairments. Identifying the gene variants that cause these pathologies proves to be difficult because of unspecific clinical symptoms, hardly accessible functional laboratory tests, and a lack of convenient blood biomarkers. In this review, we summarize the molecular pathways of proteoglycan biosynthesis, the associated inherited syndromes, and the related biochemical screening techniques, and we focus especially on a circulating proteoglycan called bikunin and on its potential as a new biomarker of these diseases.

First Report of Two Egyptian Patients with Desbuquois Dysplasia due to Homozygous CANT1 Mutations

Desbuquois dysplasia type 1 (DBQD1) is a very rare skeletal dysplasia characterized by growth retardation, short stature, distinct hand features, and a characteristic radiological monkey wrench appearance at the proximal femur. We report on 2unrelated Egyptian patients having the characteristic features of DBQD1 with different expressivity. Patient 1 presented at the age of 45 days with respiratory distress, short limbs, faltering growth, and distinctive facies while patient 2 presented at 5 years of age with short stature and hypospadias. The 2 patients shared radiological features suggestive of DBQD1. Whole-exome sequencing revealed a homozygous frameshift mutation in the CANT1 gene (NM_001159772.1:c.277_278delCT; p.Leu93ValfsTer89) in patient 1 and a homozygous missense mutation (NM_138793.4:c.898C>T; p.Arg300Cys) in patient 2. Phenotypic variability and variable expressivity of DBQD was evident in our patients. Hypoplastic scrotum and hypospadias were additional unreported associated findings, thus expanding the phenotypic spectrum of the disorder. We reviewed the main features of skeletal dysplasias exhibiting similar radiological manifestations for differential diagnosis. We suggest that the variable severity in both patients could be due to the nature of the CANT1 gene mutations which necessitates the molecular study of more cases for phenotype-genotype correlations.

Congenital Disorders of Deficiency in Glycosaminoglycan Biosynthesis

Glycosaminoglycans (GAGs) including chondroitin sulfate, dermatan sulfate, and heparan sulfate are covalently attached to specific core proteins to form proteoglycans, which are distributed at the cell surface as well as in the extracellular matrix. Proteoglycans and GAGs have been demonstrated to exhibit a variety of physiological functions such as construction of the extracellular matrix, tissue development, and cell signaling through interactions with extracellular matrix components, morphogens, cytokines, and growth factors. Not only connective tissue disorders including skeletal dysplasia, chondrodysplasia, multiple exostoses, and Ehlers-Danlos syndrome, but also heart and kidney defects, immune deficiencies, and neurological abnormalities have been shown to be caused by defects in GAGs as well as core proteins of proteoglycans. These findings indicate that GAGs and proteoglycans are essential for human development in major organs. The glycobiological aspects of congenital disorders caused by defects in GAG-biosynthetic enzymes including specific glysocyltransferases, epimerases, and sulfotransferases, in addition to core proteins of proteoglycans will be comprehensively discussed based on the literature to date.

Comparative genomics provides insights into the aquatic adaptations of mammals

Divergent lineages can respond to common environmental factors through convergent processes involving shared genomic components or pathways, but the molecular mechanisms are poorly understood. Here, we provide genomic resources and insights into the evolution of mammalian lineages adapting to aquatic life. Our data suggest convergent evolution, for example, in association with thermoregulation through genes associated with a surface heat barrier (NFIA) and internal heat exchange (SEMA3E). Combined with the support of previous reports showing that the UCP1 locus has been lost in many marine mammals independently, our results suggest that the thermostatic strategy of marine mammals shifted from enhancing heat production to limiting heat loss.

The ancestors of marine mammals once roamed the land and independently committed to an aquatic lifestyle. These macroevolutionary transitions have intrigued scientists for centuries. Here, we generated high-quality genome assemblies of 17 marine mammals (11 cetaceans and six pinnipeds), including eight assemblies at the chromosome level. Incorporating previously published data, we reconstructed the marine mammal phylogeny and population histories and identified numerous idiosyncratic and convergent genomic variations that possibly contributed to the transition from land to water in marine mammal lineages. Genes associated with the formation of blubber (NFIA), vascular development (SEMA3E), and heat production by brown adipose tissue (UCP1) had unique changes that may contribute to marine mammal thermoregulation. We also observed many lineage-specific changes in the marine mammals, including genes associated with deep diving and navigation. Our study advances understanding of the timing, pattern, and molecular changes associated with the evolution of mammalian lineages adapting to aquatic life.

The evolving genetic landscape of congenital disorders of glycosylation

Congenital Disorders of Glycosylation (CDG) are an expanding and complex group of rare genetic disorders caused by defects in the glycosylation of proteins and lipids. The genetic spectrum of CDG is extremely broad with mutations in over 140 genes leading to a wide variety of symptoms ranging from mild to severe and life-threatening. There has been an expansion in the genetic complexity of CDG in recent years. More specifically several examples of alternate phenotypes in recessive forms of CDG and new types of CDG following an autosomal dominant inheritance pattern have been identified. In addition, novel genetic mechanisms such as expansion repeats have been reported and several already known disorders have been classified as CDG as their pathophysiology was better elucidated. Furthermore, we consider the future and outlook of CDG genetics, with a focus on exploration of the non-coding genome using whole genome sequencing, RNA-seq and multi-omics technology.

Germline Saturation Mutagenesis Induces Skeletal Phenotypes in Mice

Proper embryonic and postnatal skeletal development require coordination of myriad complex molecular mechanisms. Disruption of these processes, through genetic mutation, contributes to variation in skeletal development. We developed a high-throughput N-ethyl-N-nitrosourea (ENU)-induced saturation mutagenesis skeletal screening approach in mice to identify genes required for proper skeletal development. Here, we report initial results from live-animal X-ray and dual-energy X-ray absorptiometry (DXA) imaging of 27,607 G3 mice from 806 pedigrees, testing the effects of 32,198 coding/splicing mutations in 13,020 genes. A total of 39.7% of all autosomal genes were severely damaged or destroyed by mutations tested twice or more in the homozygous state. Results from our study demonstrate the feasibility of in vivo mutagenesis to identify mouse models of skeletal disease. Furthermore, our study demonstrates how ENU mutagenesis provides opportunities to create and characterize putative hypomorphic mutations in developmentally essential genes. Finally, we present a viable mouse model and case report of recessive skeletal disease caused by mutations in FAM20B. Results from this study, including engineered mouse models, are made publicly available via the online Mutagenetix database. © 2021 American Society for Bone and Mineral Research (ASBMR).

Chondrodysplasias With Multiple Dislocations Caused by Defects in Glycosaminoglycan Synthesis

Chondrodysplasias with multiple dislocations form a group of severe disorders characterized by joint laxity and multiple dislocations, severe short stature of pre- and post-natal onset, hand anomalies, and/or vertebral anomalies. The majority of chondrodysplasias with multiple dislocations have been associated with mutations in genes encoding glycosyltransferases, sulfotransferases, and transporters implicated in the synthesis or sulfation of glycosaminoglycans, long and unbranched polysaccharides composed of repeated disaccharide bond to protein core of proteoglycan. Glycosaminoglycan biosynthesis is a tightly regulated process that occurs mainly in the Golgi and that requires the coordinated action of numerous enzymes and transporters as well as an adequate Golgi environment. Any disturbances of this chain of reactions will lead to the incapacity of a cell to construct correct glycanic chains. This review focuses on genetic and glycobiological studies of chondrodysplasias with multiple dislocations associated with glycosaminoglycan biosynthesis defects and related animal models. Strong comprehension of the molecular mechanisms leading to those disorders, mostly through extensive phenotypic analyses of in vitro and/or in vivo models, is essential for the development of novel biomarkers for clinical screenings and innovative therapeutics for these diseases.

Heparan Sulfate Biosynthesis in Zebrafish

The biosynthesis of heparan sulfate (HS) proteoglycans occurs in the Golgi compartment of cells and will determine the sulfation pattern of HS chains, which in turn will have a large impact on the biological activity of the proteoglycans. Earlier studies in mice have demonstrated the importance of HS for embryonic development. In this review, the enzymes participating in zebrafish HS biosynthesis, along with a description of enzyme mutants available for functional studies, are presented. The consequences of the zebrafish genome duplication and maternal transcript contribution are briefly discussed as are the possibilities of CRISPR/Cas9 methodologies to use the zebrafish model system for studies of biosynthesis as well as proteoglycan biology.

Congenital disorders of glycosylation: A multi-genetic disease family with multiple subcellular locations

This review discusses a selection of congenital disorders of glycosylation that show peculiar features, such as an unusual presentation, different phenotypes, a novel biochemical/genetic mechanism, a relatively high frequency or a relatively efficient treatment.

Cloning, expression and enzyme activity delineation of two novel CANT1 mutations: the disappearance of dimerization may indicate the change of protein conformation and even function

Background

Desbuquois dysplasia (DBQD) was a rare autosomal recessive skeletal dysplasia. Calcium activated nucleotidase 1 (CANT1) mutation was identified as a common pathogenic change for DBQD type 1 and Kim variant but not for DBQD type 2. To our knowledge, all patients with DBQD type 1 currently found could be explained by mutations in the CANT1 gene, but mutations in the CANT1 gene might not be directly diagnosed as DBQD type 1.

Results

We have identified two novel CANT1 mutations (mut1: c.594G > A [p.Trp198*], mut2: c.734C > T [p.Pro245Leu]) in three children from a family of Chinese origin for the first time. Two of the three children could be diagnosed as typical DBQD type 1 and one child could not be diagnosed as DBQD type 1 based on the clinical data we had. To further clarify the effect of the two mutations of the CANT1 gene, we studied the CANT1 gene expression and detected the protein secretion and nucleotide enzyme activity through cDNA cloning and expression vectors construction for wild and mutant types. The mut1 was a nonsense mutation which could lead to premature termination and produced the truncated bodies; The CANT1 dimer of mut2 was significantly reduced and even undetectable. The extracellular secretion of mut1 was extremely high while mut2 was significantly reduced compared with the wild type. And mut1 and mut2 also could result in a significant reduction in the activity of CANT1 nucleotidease. From the results we could deduce that the two mutations of the CANT1 gene were the causes of the two cases in this study.

Conclusions

Regarding the particularity of the cases reported in this study, the pathogenesis of CANT1 might be more complicated. The genetic and phenotype of three children with the same genetic background need to be further studied. Larger cohort of patients was needed to establish genotype–phenotype correlations in DBQD.

Advances in repeat expansion diseases and a new concept of repeat motif-phenotype correlation

Recently repeat expansions have been found in more than 10 diseases in the past two years. Because the same repeat motifs are found in similar disease (as exemplified by benign adult familial myoclonic epilepsy) or in diseases with overlapping phenotype (as exemplified by fragile X tremor/ataxia syndrome, neuronal intranuclear inclusion disease, oculopharyngeal myopathy with leukoencephalopathy, and oculopharyngodistal myopathy), we propose a new concept of 'repeat motif-phenotype correlation', which argue for toxic gain-of-function mechanism caused by expanded repeats, rather than altered functions of genes harboring expanded repeats. The concept is expected to help identify repeat expansions taking the similar or overlapping clinical presentations as the clues. Although repeat expansions have been identified predominantly in autosomal dominant diseases, recent progresses have demonstrated that they are also observed in autosomal recessive diseases. Furthermore, repeat expansions are not infrequently observed in patients without family histories, which urges us to pay attention to sporadic diseases. We should expand our views toward repeat expansion diseases to accelerate discovery of diseases caused by repeat expansions, better understanding the disease mechanisms, and development of therapeutic measures.

A novel homozygous variant in CANT1 causes Desbuquois dysplasia type 1 in a Chinese family and review of literatures

Desbuquois dysplasia (DBQD) is a severe chondrodysplasia characterized by short stature, retarded development, multiple joint dislocations, and a distinct radiological appearance of the proximal femur. Pathogenic variants in the calcium-activated nucleotidase 1 (CANT1) or xylosyltransferase 1 (XYLT1) gene have been previously reported to cause DBQD. Here we present a 12-year-old boy manifesting the typical features of DBQD type 1 caused by a homozygous intronic variant c.836-9G>A of CANT1. To our knowledge, this is the first DBQD case described in China revealing that a CANT1 variant was also responsible for DBQD in the Chinese population and further emphasizing the role of CANT1 variants in the etiology of DBQD type 1. Our finding provides certainty for the DBQD clinical diagnosis of this patient and expands the spectrum of known DBQD genetic risk factors. On the basis of this study, amniocentesis-based prenatal diagnosis or preimplantation genetic diagnosis (PGD)-based assisted reproduction could be a helpful aristogenesis strategy to avoid the birth of a DBQD affected child.

CANT1 deficiency in a mouse model of Desbuquois dysplasia impairs glycosaminoglycan synthesis and chondrocyte differentiation in growth plate cartilage

Desbuquois dysplasia (DD) type 1 is a rare skeletal dysplasia characterized by a short stature, round face, progressive scoliosis, and joint laxity. The causative gene has been identified as calcium‐activated nucleotidase 1 (CANT1), which encodes a nucleotidase that preferentially hydrolyzes UDP to UMP and phosphate. In this study, we generated Cant1 KO mice using CRISPR/Cas9‐mediated genome editing. All F0 mice possessing frameshift deletions at both Cant1 alleles exhibited a dwarf phenotype. Germline transmission of the edited allele was confirmed in an F0 heterozygous mouse, and KO mice were generated by crossing of the heterozygous breeding pairs. Cant1 KO mice exhibited skeletal defects, including short stature, thoracic kyphosis, and delta phalanx, all of which are observed in DD type 1 patients. The glycosaminoglycan (GAG) content and extracellular matrix space were reduced in the growth plate cartilage of mutants, and proliferating chondrocytes lost their typical flat shape and became round. Chondrocyte differentiation, especially terminal differentiation to hypertrophic chondrocytes, was impaired in Cant1 KO mice. These findings indicate that CANT1 is involved in the synthesis of GAG and regulation of chondrocyte differentiation in the cartilage and contribute to a better understanding of the pathogenesis of DD type 1.

CSGALNACT1-congenital disorder of glycosylation: A mild skeletal dysplasia with advanced bone age

Congenital disorders of glycosylation (CDGs) comprise a large number of inherited metabolic defects that affect the biosynthesis and attachment of glycans. CDGs manifest as a broad spectrum of disease, most often including neurodevelopmental and skeletal abnormalities and skin laxity. Two patients with biallelic CSGALNACT1 variants and a mild skeletal dysplasia have been described previously. We investigated two unrelated patients presenting with short stature with advanced bone age, facial dysmorphism, and mild language delay, in whom trio-exome sequencing identified novel biallelic CSGALNACT1 variants: compound heterozygosity for c.1294G>T (p.Asp432Tyr) and the deletion of exon 4 that includes the start codon in one patient, and homozygosity for c.791A>G (p.Asn264Ser) in the other patient. CSGALNACT1 encodes CSGalNAcT-1, a key enzyme in the biosynthesis of sulfated glycosaminoglycans chondroitin and dermatan sulfate. Biochemical studies demonstrated significantly reduced CSGalNAcT-1 activity of the novel missense variants, as reported previously for the p.Pro384Arg variant. Altered levels of chondroitin, dermatan, and heparan sulfate moieties were observed in patients' fibroblasts compared to controls. Our data indicate that biallelic loss-of-function mutations in CSGALNACT1 disturb glycosaminoglycan synthesis and cause a mild skeletal dysplasia with advanced bone age, CSGALNACT1-CDG.

Posterior vertebral column resection for rigid proximal thoracic kyphoscoliosis with broken growing rods in a patient with Desbuquois dysplasia

STUDY DESIGN

Case report.

OBJECTIVE

To describe the importance of preoperative halo-gravity traction and posterior vertebral column resection (PVCR) for severe proximal thoracic kyphoscoliosis associated with Desbuquois dysplasia, after breakage of a growing rod construct. Desbuquois dysplasia is a rare, autosomal recessive chondrodysplasia characterized by short stature, joint laxity, kyphoscoliosis, and characteristic facial dysmorphism. Our 8-year-old patient developed severe, progressive, infantile-onset kyphoscoliosis and had been initially treated with Vertical Expandable Prosthetic Titanium Rib (VEPTR) rods. She subsequently underwent growing rod placement, but the eventual rod fracture resulted in a severe angular kyphosis.

METHODS

Clinical and radiographic case review.

RESULTS

The broken implants were removed, and she was treated with 2.5 months of preoperative halo-gravity traction. She then underwent a T4 PVCR and C7-L4 instrumented posterior spinal fusion. The patient had an uneventful postoperative course without any neurologic problems. Two years postoperatively, correction was well maintained with appropriate alignment and balance without implant breakage.

CONCLUSION

To our knowledge, this is the first report of treatment of spinal deformity associated with Desbuquois dysplasia. Our results suggest that preoperative halo-gravity traction and PVCR are safe and efficacious techniques for severe rigid kyphoscoliosis in the cervicothoracic region associated with broken growing rods in a patient with Desbuquois dysplasia.

LEVEL OF EVIDENCE

Level IV.

Xylosyltransferase-deficient human HEK293 cells show a strongly reduced proliferation capacity and viability

Human xylosyltransferases-I and -II (XT-I and XT-II) catalyze the initial and rate-limiting step in proteoglycan (PG)-biosynthesis. Because PG are major components of the extracellular matrix (ECM), an alternated XT expression is associated with the manifestation of ECM-related diseases. While Drosophila melanogaster and Caenorhabditis elegans only harbor one XT-isoform, all higher organisms contain two isoforms, which are expressed in a tissue-specific manner. The reason for the appearance of two isoenzymes remains unexplained and remarkable, as all other enzymes involved in the synthesis of the tetrasaccharid linker, which connects the PG core protein with attached glycosaminoglycans, only show one isoform. In human, mutations in the XYLT genes cause diseases affecting the homeostasis of the ECM, such as skeletal dysplasias. We investigated for the first time whether already XT-I-deficient human embryonic kidney (HEK293) cells can compensate for decreased expression levels of both XT-isoforms. A siRNA-mediated XYLT2 mRNA knockdown led to reduced cellular proliferation rates and a partially increased cellular senescence of treated HEK293 cells. These results were verified by conducting a stable CRISPR/Cas9-mediated XYLT2 knockout, which revealed that only cells expressing at least partially functional XT-II proteins remain proliferative. Our study, therefore, shows for the first time that cells lacking both XT-isoforms are not viable and clearly indicates the importance of the XT concerning the cellular metabolism.

Severe Peripheral Joint Laxity is a Distinctive Clinical Feature of Spondylodysplastic-Ehlers-Danlos Syndrome (EDS)-B4GALT7 and Spondylodysplastic-EDS-B3GALT6

Variations in genes encoding for the enzymes responsible for synthesizing the linker region of proteoglycans may result in recessive conditions known as "linkeropathies". The two phenotypes related to mutations in genes B4GALT7 and B3GALT6 (encoding for galactosyltransferase I and II respectively) are similar, characterized by short stature, hypotonia, joint hypermobility, skeletal features and a suggestive face with prominent forehead, thin soft tissue and prominent eyes. The most outstanding feature of these disorders is the combination of severe connective tissue involvement, often manifesting in newborns and infants, and skeletal dysplasia that becomes apparent during childhood. Here, we intend to more accurately define some of the clinical features of B4GALT7 and B3GALT6-related conditions and underline the extreme hypermobility of distal joints and the soft, doughy skin on the hands and feet as features that may be useful as the first clues for a correct diagnosis.

Prenatal diagnosis of Desbuquois dysplasia Type 1: Utilization of high-density SNP array to map homozygosity and identify the gene

Desbuquois dysplasia (DBQD1 [MIM 251450]) is an autosomal recessive chondrodysplasia with micromelia, severe joint laxity and dislocations, and a characteristic radiographic "monkey wrench" appearance at the proximal femur. Type 1 Desbuquois dysplasia is caused by mutations in CANT1 and is distinct from Type 2, caused by mutations in XYLT1, in that the former has unique hand anomalies including accessory phalangeal ossification centers, advanced carpal bone maturation, and/or axial phalangeal deviation. Severe prenatal presentations have been rarely reported. We report a Pakistani female in a consanguineous relationship with a diagnosis of Type 1 Desbuquois dysplasia in three consecutive pregnancies. Multiple similar severe fetal limb anomalies were detected by ultrasound in Pregnancy 1 and 2. Regions of homozygosity within the single nucleotide polymorphism (SNP)-microarray from both terminated fetuses were compared, revealing CANT1 as a likely disease-causing candidate gene. Insufficient fetal DNA precluded confirmatory testing, therefore parents were tested; both had a previously reported heterozygous CANT1 mutation (c.643G>T; Glu215Term). The patient presented with a third pregnancy revealing similarly abnormal limb position and probable polysyndactyly by ultrasound. Targeted testing of CANT1 revealed homozygous c.643G>T CANT1 mutations and this pregnancy was terminated. In clinical situations in which ample DNA is not available or more expensive testing (e.g., whole exome sequencing) with a longer turnaround time is not feasible, utilization of SNP-microarray in consanguineous families at risk for rare autosomal recessive disorders may dramatically narrow the list of candidate genes.

Biallelic CSGALNACT1-mutations cause a mild skeletal dysplasia

Genetic causes of skeletal disorders are manifold and affect, among others, enzymes of bone and connective tissue synthesis pathways. We present a twelve-year-old boy with a mild skeletal dysplasia, hypermobility of joints and axial malalignment of lower limbs and feet. Exome sequencing revealed a biallelic loss of function mutation in CSGALNACT1, which encodes chondroitin sulfate N-acetylgalactosaminyltransferase 1 and plays a major role in the chondroitin sulfate chain biosynthesis and therefore in the synthesis of glycosaminoglycans. Recently, the first case of a pediatric patient with a mild skeletal dysplasia due to a compound heterozygous large intragenic deletion and a damaging missense variant in CSGALNACT1 was reported. We here identify a second case and the first juvenile patient with a homozygous frameshift variant in CSGALNACT1 which corroborates its role in mild and non-progressive skeletal dysplasia with joint laxity.

Prenatal diagnosis of Desbuquois dysplasia type 1 by whole exome sequencing before the occurrence of specific ultrasound signs

Desbuquois dysplasia is a very severe and sometimes lethal form of osteochondrodysplasia characterized by prenatal onset of severe micromelic short stature, joint laxity with multiple joint dislocations, specific radiographic features, and facial dysmorphism. Here, we report a case for which whole exome sequencing allowed early prenatal diagnosis of Desbuquois dysplasia before the detection of characteristic ultrasound signs of the disease.

Further Defining the Phenotypic Spectrum of B3GAT3 Mutations and Literature Review on Linkeropathy Syndromes

The term linkeropathies (LKs) refers to a group of rare heritable connective tissue disorders, characterized by a variable degree of short stature, skeletal dysplasia, joint laxity, cutaneous anomalies, dysmorphism, heart malformation, and developmental delay. The LK genes encode for enzymes that add glycosaminoglycan chains onto proteoglycans via a common tetrasaccharide linker region. Biallelic variants in XYLT1 and XYLT2, encoding xylosyltransferases, are associated with Desbuquois dysplasia type 2 and spondylo-ocular syndrome, respectively. Defects in B4GALT7 and B3GALT6, encoding galactosyltransferases, lead to spondylodysplastic Ehlers-Danlos syndrome (spEDS). Mutations in B3GAT3, encoding a glucuronyltransferase, were described in 25 patients from 12 families with variable phenotypes resembling Larsen, Antley-Bixler, Shprintzen-Goldberg, and Geroderma osteodysplastica syndromes. Herein, we report on a 13-year-old girl with a clinical presentation suggestive of spEDS, according to the 2017 EDS nosology, in whom compound heterozygosity for two B3GAT3 likely pathogenic variants was identified. We review the spectrum of B3GAT3-related disorders and provide a comparison of all LK patients reported up to now, highlighting that LKs are a phenotypic continuum bridging EDS and skeletal disorders, hence offering future nosologic perspectives.

Bone and connective tissue disorders caused by defects in glycosaminoglycan biosynthesis: a panoramic view

Glycosaminoglycans (GAGs) are a heterogeneous family of linear polysaccharides that constitute the carbohydrate moiety covalently attached to the protein core of proteoglycans, macromolecules present on the cell surface and in the extracellular matrix. Several genetic disorders of bone and connective tissue are caused by mutations in genes encoding for glycosyltransferases, sulfotransferases and transporters that are responsible for the synthesis of sulfated GAGs. Phenotypically, these disorders all reflect alterations in crucial biological functions of GAGs in the development, growth and homoeostasis of cartilage and bone. To date, up to 27 different skeletal phenotypes have been linked to mutations in 23 genes encoding for proteins involved in GAG biosynthesis. This review focuses on recent genetic, molecular and biochemical studies of bone and connective tissue disorders caused by GAG synthesis defects. These insights and future research in the field will provide a deeper understanding of the molecular pathogenesis of these disorders and will pave the way for developing common therapeutic strategies that might be targeted to a range of individual phenotypes.

The clinical and mutational spectrum of B3GAT3 linkeropathy: two case reports and literature review

Background

Proteoglycans are large and structurally complex macromolecules which can be found in abundancy in the extracellular matrix and on the surface of all animal cells. Mutations in the genes encoding the enzymes responsible for the formation of the tetrasaccharide linker region between the proteoglycan core protein and the glycosaminoglycan side chains lead to a spectrum of severe and overlapping autosomal recessive connective tissue disorders, collectively coined the ‘glycosaminoglycan linkeropathies’.

Results

We report the clinical findings of two novel patients with a complex linkeropathy due to biallelic mutations in B3GAT3, the gene that encodes glucuronosyltransferase I, which catalyzes the addition of the ultimate saccharide to the linker region. We identified a previously reported c.667G > A missense mutation and an unreported homozygous c.416C > T missense mutation. We also performed a genotype and phenotype-oriented literature overview of all hitherto reported patients harbouring B3GAT3 mutations. A total of 23 patients from 10 families harbouring bi-allelic mutations and one patient with a heterozygeous splice-site mutation in B3GAT3 have been reported. They all display a complex phenotype characterized by consistent presence of skeletal dysplasia (including short stature, kyphosis, scoliosis and deformity of the long bones), facial dysmorphology, and spatulate distal phalanges. More variably present are cardiac defects, joint hypermobility, joint dislocations/contractures and fractures. Seven different B3GAT3 mutations have been reported, and although the number of patients is still limited, some phenotype-genotype correlations start to emerge. The more severe phenotypes seem to have mutations located in the substrate acceptor subdomain of the catalytic domain of the glucuronosyltransferase I protein while more mildly affected phenotypes seem to have mutations in the NTP-sugar donor substrate binding subdomain.

Conclusions

Loss-of-function mutations in B3GAT3 are associated with a complex connective tissue phenotype characterized by disproportionate short stature, skeletal dysplasia, facial dysmorphism, spatulate distal phalanges and -to a lesser extent- joint contractures, joint hypermobility with dislocations, cardiac defects and bone fragility. Based on the limited number of reported patients, some genotype-phenotype correlations start to emerge.

Biallelic B3GALT6 mutations cause spondylodysplastic Ehlers-Danlos syndrome

Proteoglycans are among the most abundant and structurally complex biomacromolecules and play critical roles in connective tissues. They are composed of a core protein onto which glycosaminoglycan (GAG) side chains are attached via a linker region. Biallelic mutations in B3GALT6, encoding one of the linker region glycosyltransferases, are known to cause either spondyloepimetaphyseal dysplasia (SEMD) or a severe pleiotropic form of Ehlers-Danlos syndromes (EDS). This study provides clinical, molecular and biochemical data on 12 patients with biallelic B3GALT6 mutations. Notably, all patients have features of both EDS and SEMD. In addition, some patients have severe and potential life-threatening complications such as aortic dilatation and aneurysm, cervical spine instability and respiratory insufficiency. Whole-exome sequencing, next generation panel sequencing and direct sequencing identified biallelic B3GALT6 mutations in all patients. We show that these mutations reduce the amount of β3GalT6 protein and lead to a complete loss of galactosyltransferase activity. In turn, this leads to deficient GAG synthesis, and ultrastructural abnormalities in collagen fibril organization. In conclusion, this study redefines the phenotype associated with B3GALT6 mutations on the basis of clinical, molecular and biochemical data in 12 patients, and provides an in-depth assessment of β3GalT6 activity and GAG synthesis to better understand this rare condition.

A novel gene (FAM20B encoding glycosaminoglycan xylosylkinase) for neonatal short limb dysplasia resembling Desbuquois dysplasia

Desbuquois dysplasia (DBQD) is an autosomal recessive heterogeneous disorder characterized by joint laxity and skeletal changes, including a distinctive monkey-wrench appearance of the femora, advanced carpal ossification, and abnormal patterning of the preaxial digits. Two genes for DBQD (CANT1 encoding calcium-activated nucleotidase-1 and XYLT1 encoding xylosyltransferase-1) have been reported. We propose a novel gene for neonatal short limb dysplasia resembling DBQD, based on the phenotype and genotype of two affected siblings. The affected boy and girl died in early infancy and shortly after birth, respectively. The clinical hallmarks included mid-face hypoplasia, thoracic hypoplasia with respiratory failure, very short stature (approximately -7 SD of birth length) with mesomelic shortening of the limbs, and multiple dislocations of the large joints. Radiological examinations showed prominent lesser trochanter, flared metaphyses of the long bones, and joint dislocations. The affected boy had preaxial digital hypoplasia, and the affected girl showed overlapping and syndactyly of the preaxial digits. Molecular analyses of the girl showed compound heterozygous variants in FAM20B (NM_014864: c.174_178delTACCT p.T59Afs*19/c.1038delG p.N347Mfs*4). FAM20B encodes glycosaminoglycan xylosylkinase, which acts downstream of xylosyltransferase-1. Given the fact that FAM20B deficiency causes skeletal phenotypes in mice and zebrafish, these variants are highly probable to be pathogenic.

GGC Repeat Expansion and Exon 1 Methylation of XYLT1 Is a Common Pathogenic Variant in Baratela-Scott Syndrome

Baratela-Scott syndrome (BSS) is a rare, autosomal-recessive disorder characterized by short stature, facial dysmorphisms, developmental delay, and skeletal dysplasia caused by pathogenic variants in XYLT1. We report clinical and molecular investigation of 10 families (12 individuals) with BSS. Standard sequencing methods identified biallelic pathogenic variants in XYLT1 in only two families. Of the remaining cohort, two probands had no variants and six probands had only a single variant, including four with a heterozygous 3.1 Mb 16p13 deletion encompassing XYLT1 and two with a heterozygous truncating variant. Bisulfite sequencing revealed aberrant hypermethylation in exon 1 of XYLT1, always in trans with the sequence variant or deletion when present; both alleles were methylated in those with no identified variant. Expression of the methylated XYLT1 allele was severely reduced in fibroblasts from two probands. Southern blot studies combined with repeat expansion analysis of genome sequence data showed that the hypermethylation is associated with expansion of a GGC repeat in the XYLT1 promoter region that is not present in the reference genome, confirming that BSS is a trinucleotide repeat expansion disorder. The hypermethylated allele accounts for 50% of disease alleles in our cohort and is not present in 130 control subjects. Our study highlights the importance of investigating non-sequence-based alterations, including epigenetic changes, to identify the missing heritability in genetic disorders.

Calcium activated nucleotidase 1 (CANT1) is critical for glycosaminoglycan biosynthesis in cartilage and endochondral ossification

Desbuquois dysplasia type 1 (DBQD1) is a chondrodysplasia caused by mutations in CANT1 gene encoding an ER/Golgi calcium activated nucleotidase 1 that hydrolyses UDP. Here, using Cant1 knock-in and knock-out mice recapitulating DBQD1 phenotype, we report that CANT1 plays a crucial role in cartilage proteoglycan synthesis and in endochondral ossification. Specifically, the glycosaminoglycan synthesis was decreased in chondrocytes from Cant1 knock-out mice and their hydrodynamic size was reduced, whilst the sulfation was increased and the overall proteoglycan secretion was delayed. Interestingly, knock-out chondrocytes had dilated ER cisternae suggesting delayed protein secretion and cellular stress; however, no canonical ER stress response was detected using microarray analysis, Xbp1 splicing and protein levels of BiP and ATF4. The observed proteoglycan defects caused deregulated chondrocyte proliferation and maturation in the growth plate resulting in the reduced skeletal growth. In conclusion, the pathogenic mechanism of DBQD1 comprises deregulated chondrocyte performance due to defective intracellular proteoglycan synthesis and altered proteoglycan properties in the extracellular matrix.

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Desbuquois dysplasia type 1 (DBQD1) is a recessive skeletal dysplasia caused by mutations in CANT1 gene, a Calcium activated nucleotidase of the ER/Golgi.

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The Cant1 knock-out mouse recapitulates human DBQD1.

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Cant1 is critical for different steps of proteoglycan biosynthesis including glycosaminoglycan chain synthesis, length and sulfation.

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The intracellular GAG synthesis defects cause delayed proteoglycan secretion with ER enlargement.

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In Cant1 knock-out chondrocytes ER enlargement is not linked to canonical ER stress.

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The proteoglycan defects cause deregulated chondrocyte proliferation and maturation in the growth plate resulting in reduced skeletal growth.

Effect of Polarization and Chronic Inflammation on Macrophage Expression of Heparan Sulfate Proteoglycans and Biosynthesis Enzymes

Heparan sulfate (HS) proteoglycans on immune cells have the ability to bind to and regulate the bioactivity more than 400 bioactive protein ligands, including many chemokines, cytokines, and growth factors. This makes them important regulators of the phenotype and behavior of immune cells. Here we review how HS biosynthesis in macrophages is regulated during polarization and in chronic inflammatory diseases such as rheumatoid arthritis, atherosclerosis, asthma, chronic obstructive pulmonary disease and obesity, by analyzing published micro-array data and mechanistic studies in this area. We describe that macrophage expression of many HS biosynthesis and core proteins is strongly regulated by macrophage polarization, and that these expression patterns are recapitulated in chronic inflammation. Such changes in HS biosynthetic enzyme expression are likely to have a significant impact on the phenotype of macrophages in chronic inflammatory diseases by altering their interactions with chemokines, cytokines, and growth factors.

SLC10A7 mutations cause a skeletal dysplasia with amelogenesis imperfecta mediated by GAG biosynthesis defects

Skeletal dysplasia with multiple dislocations are severe disorders characterized by dislocations of large joints and short stature. The majority of them have been linked to pathogenic variants in genes encoding glycosyltransferases, sulfotransferases or epimerases required for glycosaminoglycan synthesis. Using exome sequencing, we identify homozygous mutations in SLC10A7 in six individuals with skeletal dysplasia with multiple dislocations and amelogenesis imperfecta. SLC10A7 encodes a 10-transmembrane-domain transporter located at the plasma membrane. Functional studies in vitro demonstrate that SLC10A7 mutations reduce SLC10A7 protein expression. We generate a Slc10a7^−/− mouse model, which displays shortened long bones, growth plate disorganization and tooth enamel anomalies, recapitulating the human phenotype. Furthermore, we identify decreased heparan sulfate levels in Slc10a7^−/− mouse cartilage and patient fibroblasts. Finally, we find an abnormal N-glycoprotein electrophoretic profile in patient blood samples. Together, our findings support the involvement of SLC10A7 in glycosaminoglycan synthesis and specifically in skeletal development.

The majority of skeletal dysplasia are caused by pathogenic variants in genes required for glycosaminoglycan (GAG) metabolism. Here, Dubail et al. identify genetic variants in the solute carrier family protein SLC10A7 in families with skeletal dysplasia and amelogenesis imperfecta that disrupt GAG synthesis.