Heterozygous pathogenic variants involving CBFB cause a new skeletal disorder resembling cleidocranial dysplasia.

BACKGROUND: Cleidocranial dysplasia (CCD) is a rare skeletal dysplasia with significant clinical variability. Patients with CCD typically present with delayed closure of fontanels and cranial sutures, dental anomalies, clavicular hypoplasia or aplasia and short stature. Runt-related transcription factor 2 (RUNX2) is currently the only known disease-causing gene for CCD, but several studies have suggested locus heterogeneity. METHODS: The cohort consists of eight subjects from five unrelated families partially identified through GeneMatcher. Exome or genome sequencing was applied and in two subjects the effect of the variant was investigated at RNA level. RESULTS: In each subject a heterozygous pathogenic variant in CBFB was detected, whereas no genomic alteration involving RUNX2 was found. Three CBFB variants (one splice site alteration, one nonsense variant, one 2 bp duplication) were shown to result in a premature stop codon. A large intragenic deletion was found to delete exon 4, without affecting CBFB expression. The effect of a second splice site variant could not be determined but most likely results in a shortened or absent protein. Affected individuals showed similarities with RUNX2-related CCD, including dental and clavicular abnormalities. Normal stature and neurocognitive problems were however distinguishing features. CBFB encodes the core-binding factor ß subunit, which can interact with all RUNX proteins (RUNX1, RUNX2, RUNX3) to form heterodimeric transcription factors. This may explain the phenotypic differences between CBFB-related and RUNX2-related CCD. CONCLUSION: We confirm the previously suggested locus heterogeneity for CCD by identifying five pathogenic variants in CBFB in a cohort of eight individuals with clinical and radiographic features reminiscent of CCD.

Steel syndrome: Report of three patients, including monozygotic twins and review of clinical and mutation profiles.

Steel syndrome (MIM# 615155) is an autosomal recessive skeletal disorder, characterized by dislocations of the hips and radial heads, carpal coalition, short stature, facial dysmorphism, and scoliosis. Until date 47 patients have been reported. However, disease causing variants have been identified only in twenty Puerto Rican and nine non-Puerto Rican families. Here we report two monozygotic twins and a boy from two families with novel missense variants, c.295G > A p.(Ala99 Thr), c.3056C > A p.(Pro1019His) and c.2521G > A p.(Gly841Arg) in COL27A1. We describe for the first time, cleft palate and delayed carpal bone ossification as features of Steel syndrome. We reviewed clinical features in all mutation-proven Steel syndrome patients. Short stature and dislocation/subluxation of hip joint are consistently observed. Other features include dislocated radial heads, scoliosis, lordosis, carpal coalition, facial dysmorphism, hearing loss, bilateral fifth finger clinodactyly, knee deformities and developmental delay. Seven missense variants and eight null variants are reported in COL27A1 until date. We also looked into the genotype-phenotype correlation in Puerto Rican and non-Puerto Rican patients.

A homozygous hypomorphic BNIP1 variant causes an increase in autophagosomes and reduced autophagic flux and results in a spondylo-epiphyseal dysplasia.

BNIP1 (BCL2 interacting protein 1) is a soluble N-ethylmaleimide-sensitive factor-attachment protein receptor involved in ER membrane fusion. We identified the homozygous BNIP1 intronic variant c.84+3A>T in the apparently unrelated patients 1 and 2 with disproportionate short stature. Radiographs showed abnormalities affecting both the axial and appendicular skeleton and spondylo-epiphyseal dysplasia. We detected ~80% aberrantly spliced BNIP1 pre-mRNAs, reduced BNIP1 mRNA level to ~80%, and BNIP1 protein level reduction by ~50% in patient 1 compared to control fibroblasts. The BNIP1 ortholog in Drosophila, Sec20, regulates autophagy and lysosomal degradation. We assessed lysosome positioning and identified a decrease in lysosomes in the perinuclear region and an increase in the cell periphery in patient 1 cells. Immunofluorescence microscopy and immunoblotting demonstrated an increase in LC3B-positive structures and LC3B-II levels, respectively, in patient 1 fibroblasts under steady-state condition. Treatment of serum-starved fibroblasts with or without bafilomycin A1 identified significantly decreased autophagic flux in patient 1 cells. Our data suggest a block at the terminal stage of autolysosome formation and/or clearance in patient fibroblasts. BNIP1 together with RAB33B and VPS16, disease genes for Smith-McCort dysplasia 2 and a multisystem disorder with short stature, respectively, highlight the importance of autophagy in skeletal development.

International Consensus Statement on the diagnosis, multidisciplinary management and lifelong care of individuals with achondroplasia.

Achondroplasia, the most common skeletal dysplasia, is characterized by a variety of medical, functional and psychosocial challenges across the lifespan. The condition is caused by a common, recurring, gain-of-function mutation in FGFR3, the gene that encodes fibroblast growth factor receptor 3. This mutation leads to impaired endochondral ossification of the human skeleton. The clinical and radiographic hallmarks of achondroplasia make accurate diagnosis possible in most patients. However, marked variability exists in the clinical care pathways and protocols practised by clinicians who manage children and adults with this condition. A group of 55 international experts from 16 countries and 5 continents have developed consensus statements and recommendations that aim to capture the key challenges and optimal management of achondroplasia across each major life stage and sub-specialty area, using a modified Delphi process. The primary purpose of this first International Consensus Statement is to facilitate the improvement and standardization of care for children and adults with achondroplasia worldwide in order to optimize their clinical outcomes and quality of life.

Corrigendum: Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium.

[This corrects the article DOI: 10.3389/fendo.2021.641543.].

Consensus Recommendations for the Diagnosis and Management of X-Linked Hypophosphatemia in Belgium.

X-linked hypophosphatemia (XLH) is the most common genetic form of hypophosphatemic rickets and osteomalacia. In this disease, mutations in the PHEX gene lead to elevated levels of the hormone fibroblast growth factor 23 (FGF23), resulting in renal phosphate wasting and impaired skeletal and dental mineralization. Recently, international guidelines for the diagnosis and treatment of this condition have been published. However, more specific recommendations are needed to provide guidance at the national level, considering resource availability and health economic aspects. A national multidisciplinary group of Belgian experts convened to discuss translation of international best available evidence into locally feasible consensus recommendations. Patients with XLH may present to a wide array of primary, secondary and tertiary care physicians, among whom awareness of the disease should be raised. XLH has a very broad differential-diagnosis for which clinical features, biochemical and genetic testing in centers of expertise are recommended. Optimal care requires a multidisciplinary approach, guided by an expert in metabolic bone diseases and involving (according to the individual patient's needs) pediatric and adult medical specialties and paramedical caregivers, including but not limited to general practitioners, dentists, radiologists and orthopedic surgeons. In children with severe or refractory symptoms, FGF23 inhibition using burosumab may provide superior outcomes compared to conventional medical therapy with phosphate supplements and active vitamin D analogues. Burosumab has also demonstrated promising results in adults on certain clinical outcomes such as pseudofractures. In summary, this work outlines recommendations for clinicians and policymakers, with a vision for improving the diagnostic and therapeutic landscape for XLH patients in Belgium.

Fibrous Dysplasia, Paget's Disease of Bone, and Other Uncommon Sclerotic Bone Lesions of the Craniofacial Bones.

Imaging studies of the brain, head and neck, sinuses, and dental computed tomography are among the most frequently performed procedures in radiologic departments. Systematic evaluation in the bone window may reveal common and uncommon sclerotic osseous abnormalities of the craniofacial skeleton.Most of these findings are incidental and unrelated to the initial clinical indications. Sporadically symptoms may arise due to lesional mass effect with compression on adjacent structures and neuroforaminal encroachment, resulting in proptosis, vision, or hearing loss. Other symptoms include craniofacial deformity, mandibular occlusion deformity, and local pain.This article reviews the most common disorders characterized by an increased bone density involving the craniofacial bones including fibrous dysplasia, Paget's disease of bone, meningioma with associated hyperostosis, and osteoma. Finally, typical examples of rarer sclerosing bone dysplasias are discussed as well.Emphasis is placed on imaging features and the differential diagnosis.

Biallelic variants p.Arg1133Cys and p.Arg1379Cys in COL2A1: Further delineation of phenotypic spectrum of recessive Type 2 collagenopathies.

The phenotypic spectrum of Type 2 collagenopathies ranges from lethal achondrogenesis Type 2 to milder osteoarthritis with mild chondrodysplasia. All of them are monoallelic except for the two recent reports showing that biallelic variants in COL2A1 can cause spondyloepiphyseal dysplasia congenita in two children. Here we report two additional families with homozygous variants, c.4135C>T (p.Arg1379Cys) and c.3190C>T (p.Arg1133Cys) in COL2A1 resulting in two distinct skeletal dysplasia phenotypes of intermediate severity. Though all six patients from four families exhibit a spondylo-epimetaphyseal dysplasia, they demonstrate a wide variation in severity of short stature and involvement of epiphyses, metaphyses, and vertebrae. We hypothesize that the variants are likely to be hypomorphic, given the underlying mechanisms of disease causation for known heterozygous variants in COL2A1. With this report, we provide further evidence to the existence of autosomal recessive Type 2 collagenopathy.

Nosology and classification of genetic skeletal disorders: 2019 revision.

The application of massively parallel sequencing technology to the field of skeletal disorders has boosted the discovery of the underlying genetic defect for many of these diseases. It has also resulted in the delineation of new clinical entities and the identification of genes and pathways that had not previously been associated with skeletal disorders. These rapid advances have prompted the Nosology Committee of the International Skeletal Dysplasia Society to revise and update the last (2015) version of the Nosology and Classification of Genetic Skeletal Disorders. This newest and tenth version of the Nosology comprises 461 different diseases that are classified into 42 groups based on their clinical, radiographic, and/or molecular phenotypes. Remarkably, pathogenic variants affecting 437 different genes have been found in 425/461 (92%) of these disorders. By providing a reference list of recognized entities and their causal genes, the Nosology should help clinicians achieve accurate diagnoses for their patients and help scientists advance research in skeletal biology.

The SMAD-binding domain of SKI: a hotspot for de novo mutations causing Shprintzen-Goldberg syndrome.

Shprintzen-Goldberg syndrome (SGS) is a rare, systemic connective tissue disorder characterized by craniofacial, skeletal, and cardiovascular manifestations that show a significant overlap with the features observed in the Marfan (MFS) and Loeys-Dietz syndrome (LDS). A distinguishing observation in SGS patients is the presence of intellectual disability, although not all patients in this series present this finding. Recently, SGS was shown to be due to mutations in the SKI gene, encoding the oncoprotein SKI, a repressor of TGFß activity. Here, we report eight recurrent and three novel SKI mutations in eleven SGS patients. All were heterozygous missense mutations located in the R-SMAD binding domain, except for one novel in-frame deletion affecting the DHD domain. Adding our new findings to the existing data clearly reveals a mutational hotspot, with 73% (24 out of 33) of the hitherto described unrelated patients having mutations in a stretch of five SKI residues (from p.(Ser31) to p.(Pro35)). This implicates that the initial molecular testing could be focused on mutation analysis of the first half of exon 1 of SKI. As the majority of the known mutations are located in the R-SMAD binding domain of SKI, our study further emphasizes the importance of TGFß signaling in the pathogenesis of SGS.

Mutation-based growth charts for SEDC and other COL2A1 related dysplasias.

From data collected via a large international collaborative study, we have constructed a growth chart for patients with molecularly confirmed congenital spondylo-epiphyseal dysplasia (SEDC) and other COL2A1 related dysplasias. The growth chart is based on longitudinal height measurements of 79 patients with glycine substitutions in the triple-helical domain of COL2A1. In addition, measurements of 27 patients with other molecular defects, such as arginine to cysteine substitutions, splice mutations, and mutations in the C-terminal propeptide have been plotted on the chart. Height of the patients progressively deviate from that of normal children: compared to normal WHO charts, the mean length/height is -2.6 SD at birth, -4.2 SD at 5 years, and -5.8 SD in adulthood. The mean adult height (male and female combined) of patients with glycine substitutions in the triple-helical region is 138.2 cm but there is a large variation. Patients with glycine to cysteine substitutions tend to cluster within the upper part of the chart, while patients with glycine to serine or valine substitutions are situated between +1 SD and -1 SD. Patients with carboxy-terminal glycine substitutions tend to be shorter than patients with amino-terminal substitutions, while patients with splice mutations are relatively tall. However, there are exceptions and specific mutations can have a strong or a relatively mild negative effect on growth. The observation of significant difference in adult height between affected members of the same family indicates that height remains a multifactorial trait even in the presence of a mutation with a strong dominant effect.

Loss-of-function mutations in TGFB2 cause a syndromic presentation of thoracic aortic aneurysm.

Loeys-Dietz syndrome (LDS) associates with a tissue signature for high transforming growth factor (TGF)-ß signaling but is often caused by heterozygous mutations in genes encoding positive effectors of TGF-ß signaling, including either subunit of the TGF-ß receptor or SMAD3, thereby engendering controversy regarding the mechanism of disease. Here, we report heterozygous mutations or deletions in the gene encoding the TGF-ß2 ligand for a phenotype within the LDS spectrum and show upregulation of TGF-ß signaling in aortic tissue from affected individuals. Furthermore, haploinsufficient Tgfb2(+/-) mice have aortic root aneurysm and biochemical evidence of increased canonical and noncanonical TGF-ß signaling. Mice that harbor both a mutant Marfan syndrome (MFS) allele (Fbn1(C1039G/+)) and Tgfb2 haploinsufficiency show increased TGF-ß signaling and phenotypic worsening in association with normalization of TGF-ß2 expression and high expression of TGF-ß1. Taken together, these data support the hypothesis that compensatory autocrine and/or paracrine events contribute to the pathogenesis of TGF-ß-mediated vasculopathies.

Multicentric carpotarsal osteolysis is caused by mutations clustering in the amino-terminal transcriptional activation domain of MAFB.

Multicentric carpotarsal osteolysis (MCTO) is a rare skeletal dysplasia characterized by aggressive osteolysis, particularly affecting the carpal and tarsal bones, and is frequently associated with progressive renal failure. Using exome capture and next-generation sequencing in five unrelated simplex cases of MCTO, we identified previously unreported missense mutations clustering within a 51 base pair region of the single exon of MAFB, validated by Sanger sequencing. A further six unrelated simplex cases with MCTO were also heterozygous for previously unreported mutations within this same region, as were affected members of two families with autosomal-dominant MCTO. MAFB encodes a transcription factor that negatively regulates RANKL-induced osteoclastogenesis and is essential for normal renal development. Identification of this gene paves the way for development of novel therapeutic approaches for this crippling disease and provides insight into normal bone and kidney development.

Pseudoachondroplasia and multiple epiphyseal dysplasia: a 7-year comprehensive analysis of the known disease genes identify novel and recurrent mutations and provides an accurate assessment of their relative contribution.

Pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED) are relatively common skeletal dysplasias resulting in short-limbed dwarfism, joint pain, and stiffness. PSACH and the largest proportion of autosomal dominant MED (AD-MED) results from mutations in cartilage oligomeric matrix protein (COMP); however, AD-MED is genetically heterogenous and can also result from mutations in matrilin-3 (MATN3) and type IX collagen (COL9A1, COL9A2, and COL9A3). In contrast, autosomal recessive MED (rMED) appears to result exclusively from mutations in sulphate transporter solute carrier family 26 (SLC26A2). The diagnosis of PSACH and MED can be difficult for the nonexpert due to various complications and similarities with other related diseases and often mutation analysis is requested to either confirm or exclude the diagnosis. Since 2003, the European Skeletal Dysplasia Network (ESDN) has used an on-line review system to efficiently diagnose cases referred to the network prior to mutation analysis. In this study, we present the molecular findings in 130 patients referred to ESDN, which includes the identification of novel and recurrent mutations in over 100 patients. Furthermore, this study provides the first indication of the relative contribution of each gene and confirms that they account for the majority of PSACH and MED.

Genotype-phenotype analysis of the branchio-oculo-facial syndrome.

Branchio-oculo-facial syndrome (BOFS; OMIM#113620) is a rare autosomal dominant craniofacial disorder with variable expression. Major features include cutaneous and ocular abnormalities, characteristic facies, renal, ectodermal, and temporal bone anomalies. Having determined that mutations involving TFAP2A result in BOFS, we studied a total of 30 families (41 affected individuals); 26/30 (87%) fulfilled our cardinal diagnostic criteria. The original family with the 3.2 Mb deletion including the TFAP2A gene remains the only BOFS family without the typical CL/P and the only family with a deletion. We have identified a hotspot region in the highly conserved exons 4 and 5 of TFAP2A that harbors missense mutations in 27/30 (90%) families. Several of these mutations are recurrent. Mosaicism was detected in one family. To date, genetic heterogeneity has not been observed. Although the cardinal criteria for BOFS have been based on the presence of each of the core defects, an affected family member or thymic remnant, we documented TFAP2A mutations in three (10%) probands in our series without a classic cervical cutaneous defect or ectopic thymus. Temporal bone anomalies were identified in 3/5 patients investigated. The occurrence of CL/P, premature graying, coloboma, heterochromia irides, and ectopic thymus, are evidence for BOFS as a neurocristopathy. Intrafamilial clinical variability can be marked. Although there does not appear to be mutation-specific genotype-phenotype correlations at this time, more patients need to be studied. Clinical testing for TFAP2A mutations is now available and will assist geneticists in confirming the typical cases or excluding the diagnosis in atypical cases.

Mesomelia-synostoses syndrome results from deletion of SULF1 and SLCO5A1 genes at 8q13.

Mesomelia-synostoses syndrome (MSS) or mesomelic dysplasia with acral synostoses Verloes-David-Pfeiffer type is a rare autosomal-dominant disorder characterized by mesomelic limb shortening, acral synostoses, and multiple congenital malformations. So far, five patients in four unrelated families have been reported worldwide with MMS. By using whole-genome oligonucleotide array CGH, we have identified an interstitial deletion at 8q13 in all patients. The deletions vary from 582 Kb to 738 Kb in size, but invariably encompass only two genes: SULF1, encoding the heparan sulfate 6-O-endosulfatase 1, and SLCO5A1, encoding the solute carrier organic anion transporter family member 5A1. SULF1 acts as a regulator of numerous growth factors in skeletal embryonic development whereas the function of SLCO5A1 is yet unknown. Breakpoint sequence analyses performed in two families showed nonrecurrent deletions. Real-time quantitative RT-PCR analysis showed the highest levels of SULF1 transcripts in human osteoblasts and cartilage whereas SLCO5A1 was highly expressed in human fetal and adult brain and heart. Our results strongly suggest that haploinsufficiency of SULF1 contributes to this mesomelic chondrodysplasia, highlighting the critical role of endosulfatase in human skeletal development. Codeletion of SULF1 and SLCO5A1--which does not result from a low-copy repeats (LCRs)-mediated recombination event in at least two families--was found in all patients, so we suggest that haploinsufficiency of SULF1 combined with haploinsufficiency of SLCO5A1 (or the altered expression of a neighboring gene through position effect) could be necessary in the pathogenesis of MSS.

Mesomelic dysplasia with acral synostoses Verloes-David-Pfeiffer type: follow-up study documents progressive clinical course.

Verloes-David-Pfeiffer mesomelia-synostoses syndrome is an autosomal-dominant form of mesomelic dysplasia comprising typical acral synostoses combined with ptosis, hypertelorism, palatal abnormality, CHD, and ureteral anomalies. Since the original reports in 1995, two other patients have been described with this syndrome, one of them the patient reported in 1998 by Day-Salvatore. In this article, we report on the follow-up of some of the original cases and review the literature. We confirm that the Verloes-David-Pfeiffer syndrome (VDPS) is a progressive skeletal disorder that despite repeated corrective surgical intervention leads to severe limb deformities. No mutations were detected in the FLNB gene. To date, the cause and the pathogenesis of VDPS remain unknown. The latter is characterized in this study as a syndromic type of skeletal dysplasia because besides congenital malformations and multiple acromelic synostoses arising prenatally, VDPS manifests in postnatal life as a severe osteochondrodysplasia.

Encephalocraniocutaneous lipomatosis accompanied by the formation of bone cysts: Harboring clues to pathogenesis?

Encephalocraniocutaneous lipomatosis (ECCL) is a sporadically occurring neurocutaneous disorder characterized by ocular anomalies, mainly choristomas; by skin lesions consisting of hairless fatty tissue nevi (nevus psiloliparus), focal dermal hypoplasia, alopecia, and periocular skin tags; and by CNS anomalies, including intracranial and spinal lipomas and often mental retardation and seizures. Here, we report on three boys with ECCL with typical abnormalities of the eyes, skin and brain and, in addition, coarctation of the aorta. All three children developed multiple cystic bone lesions, which progressively spread throughout the skeleton in Patient 1 and was shown histologically to be non-ossifying fibromas in Patient 2. We hypothesize that ECCL may be caused by mosaicism for a mutated gene involved in benign mesenchymal tumors and in vasculogenesis.

Acanthosis nigricans in a child with mild osteochondrodysplasia and K650Q mutation in the FGFR3 gene.

A girl with a mild sporadic osteochondrodysplasia (OCD) similar to hypochondroplasia but with significant short stature is reported. She has been followed clinically between the ages of 9 months and 14 years. Growth remained normal throughout childhood with stature evolving about 3.5 SDs under the mean for age. By 8 years of age gradually appearing acanthosis nigricans (AN) in the neck and flanks was histopathologically confirmed. It provided the new incentive to search for specific FGFR3 mutations associated with this dermatologic abnormality. This resulted in the identification of the 1948A > C transversion predicting the K650Q missense substitution in the FGFR3 protein. Besides the expansion of the phenotypic spectrum of FGFR3-related OCDs to HCH with AN, this observation underscores the continuing adverse effect of this specific mutation upon the normal inhibitory signaling of the receptor at least in epidermal cells.