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Childhood-onset osteoporosis is a rare but clinically significant condition. Studies have shown pathogenic variants in more than 20 different genes as causative for childhood-onset primary osteoporosis. The X-chromosomal PLS3, encoding Plastin-3, is one of the more recently identified genes. In this study, we describe five new families from four different European countries with PLS3-related skeletal fragility. The index cases were all hemizygous males presenting with long bone and vertebral body compression fractures. All patients had low lumbar spine bone mineral density (BMD). The age at the first clinical fracture ranged from 1.5 to 13 years old. Three of the identified PLS3 variants were stop-gain variants and two were deletions involving either a part or all exons of the gene. In four families the variant was inherited from the mother. All heterozygous women reported here had normal BMD and no bone fractures. Four patients received bisphosphonate treatment with good results, showing a lumbar spine BMD increment and vertebral body reshaping after 10 months to 2 years of treatment. Our findings expand the genetic spectrum of PLS3-related osteoporosis. Our report also shows that early treatment with bisphosphonates may influence the disease course and reduce the progression of osteoporosis, highlighting the importance of early diagnosis for prompt intervention and appropriate genetic counseling.

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged

Spondyloepimetaphyseal dysplasias (SEMDs) are a heterogeneous group of disorders with variable growth failure and skeletal impairments affecting the spine and long bone epiphyses and metaphyses. Here we report on four unrelated families with SEMD in which we identified two monoallelic missense variants and one monoallelic splice site variant in RPL13, encoding the ribosomal protein eL13. In two out of four families, we observed autosomal dominant inheritance with incomplete penetrance and variable clinical expressivity; the phenotypes of the mutation-positive subjects ranged from normal height with or without hip dysplasia to severe SEMD with severe short stature and marked skeletal dysplasia. In vitro studies on patient-derived dermal fibroblasts harboring RPL13 missense mutations demonstrated normal eL13 expression, with proper subcellular localization but reduced colocalization with eL28 (p < 0.001). Cellular functional defects in fibroblasts from mutation-positive subjects indicated a significant increase in the ratio of 60S subunits to 80S ribosomes (p = 0.007) and attenuated global translation (p = 0.017). In line with the human phenotype, our rpl13 mutant zebrafish model, generated by CRISPR-Cas9 editing, showed cartilage deformities at embryonic and juvenile stages. These findings extend the genetic spectrum of RPL13 mutations causing this novel human ribosomopathy with variable skeletal features. Our study underscores for the first time incomplete penetrance and broad phenotypic variability in SEMD-RPL13 type and confirms impaired ribosomal function. Furthermore, the newly generated rpl13 mutant zebrafish model corroborates the role of eL13 in skeletogenesis. © 2020 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

BACKGROUND: Studies exploring molecular mechanisms underlying congenital skeletal disorders have revealed novel regulators of skeletal homeostasis and shown protein glycosylation to play an important role. OBJECTIVE: To identify the genetic cause of rhizomelic skeletal dysplasia in a consanguineous Pakistani family. METHODS: Clinical investigations were carried out for four affected individuals in the recruited family. Whole genome sequencing (WGS) was completed using DNA from two affected and two unaffected individuals from the family. Sequencing data were processed, filtered and analysed. In silico analyses were performed to predict the effects of the candidate variant on the protein structure and function. Small interfering RNAs (siRNAs) were used to study the effect of Gnpnat1 gene knockdown in primary rat chondrocytes. RESULTS: The patients presented with short stature due to extreme shortening of the proximal segments of the limbs. Radiographs of one individual showed hip dysplasia and severe platyspondyly. WGS data analyses identified a homozygous missense variant c.226G>A; p.(Glu76Lys) in GNPNAT1, segregating with the disease. Glucosamine 6-phosphate N-acetyltransferase, encoded by the highly conserved gene GNPNAT1, is one of the enzymes required for synthesis of uridine diphosphate N-acetylglucosamine, which participates in protein glycosylation. Knockdown of Gnpnat1 by siRNAs decreased cellular proliferation and expression of chondrocyte differentiation markers collagen type 2 and alkaline phosphatase, indicating that Gnpnat1 is important for growth plate chondrocyte proliferation and differentiation. CONCLUSIONS: This study describes a novel severe skeletal dysplasia associated with a biallelic, variant in GNPNAT1. Our data suggest that GNPNAT1 is important for growth plate chondrogenesis., Funding Agencies:International Research Support Program (IRSP) by HEC, Pakistan Foundation Blanceflor Boncompagni Ludovisi, nee Bild IngaBritt och Arne Lundbergs forskningsstiftelse Byggmästare Olle Engkvist Stiftelse Nyckelfonden Stiftelsen Frimurare Barnhuset i Stockholm Örebro University, Örebro, Sweden Sigrid Juselius FoundationKoshish foundation USA

Recent advancements in genetic research have uncovered new forms of monogenic osteoporosis, expanding our understanding of the molecular pathways regulating bone health. Despite active research, knowledge on the pathomechanisms, disease-specific biomarkers, and optimal treatment in these disorders is still limited. Mutations in WNT1, encoding a WNT/beta-catenin pathway ligand WNT1, and PLS3, encoding X chromosomally inherited plastin 3 (PLS3), both result in early-onset osteoporosis with prevalent fractures and disrupted bone metabolism. However, despite marked skeletal pathology, conventional bone markers are usually normal in both diseases. Our study aimed to identify novel bone markers in PLS3 and WNT1 osteoporosis that could offer diagnostic potential and shed light on the mechanisms behind these skeletal pathologies. We measured several parameters of bone metabolism, including serum dickkopf-1 (DKK1), sclerostin, and intact and C-terminal fibroblast growth factor 23 (FGF23) concentrations in 17 WNT1 and 14 PLS3 mutation-positive subjects. Findings were compared with 34 healthy mutation-negative subjects from the same families. Results confirmed normal concentrations of conventional metabolic bone markers in both groups. DKK1 concentrations were significantly elevated in PLS3 mutation-positive subjects compared with WNT1 mutation-positive subjects (p amp;lt; .001) or the mutation-negative subjects (p = .002). Similar differences were not seen in WNT1 subjects. Sclerostin concentrations did not differ between any groups. Both intact and C-terminal FGF23 were significantly elevated in WNT1 mutation-positive subjects (p = .039 and p = .027, respectively) and normal in PLS3 subjects. Our results indicate a link between PLS3 and DKK1 and WNT1 and FGF23 in bone metabolism. The normal sclerostin and DKK1 levels in patients with impaired WNT signaling suggest another parallel regulatory mechanism. These findings provide novel information on the molecular networks in bon, Funding Agencies|Sigrid Juselius FoundationSigrid Juselius Foundation; Folkhalsan Research Foundation; Academy of FinlandAcademy of Finland; Foundation for Pediatric Research; Helsinki University Research Funds; Swedish Research CouncilSwedish Research Council; Novo Nordisk FoundationNovo Nordisk Foundation; Swedish Childhood Cancer Foundation; Stockholm County Council (ALF project)Stockholm County Council; ALF grants from Region Ostergotland; Finnish Cultural Foundation through the Foundations post-doctorate pool; HRH Crown Princess Lovisas Foundation; Helsinki University Hospital through the Doctoral Programme in Clinical Research; Finnish Medical Foundation; Paivikki and Sakari Sohlberg Foundation; Helsinki University; Finnish Norwegian Research Foundation; Orion Research Foundation; Finnish Otolaryngology-Head and Neck Surgery (ORL-HNS) Foundation

The skeletal system provides support for the body, enables movement and protects inner organs. Moreover, it supplies blood cells and acts as a reservoir for minerals and fat. Several external factors, including nutrition and long-term illness, influence bone health but genetic factors also play an important role. More than 400 different rare skeletal diseases, collectively called skeletal dysplasias, have thus far been delineated and mutations in over 350 genes have been identified as underlying causes in these conditions. Although the recent evolution of the sequencing technologies and molecular methods has increased diagnostic yield of rare skeletal diseases, knowledge on the genetic and phenotypic features in some of these conditions is still limited and novel forms of skeletal dysplasia still remain to be characterized. This thesis focused on rare skeletal diseases primarily affecting the major component of the skeleton, the bone. In paper I and III Sanger sequencing was used. In paper I this method excluded the presence of rare variants in CRTAP, encoding the cartilage associated protein, in patients with mild-to-severe skeletal fragility. In paper III two novel mutations in two components of the WNT signaling pathway, LRP5 and AMER1, were identified in two patients affected by high bone mass. In paper II a custom designed highresolution array-CGH, targeting all the genes thus far linked to skeletal diseases and the cilia genes, enabled the identification of two novel copy number variants (CNVs) affecting COL1A2 and PLS3 in two index patients with primary osteoporosis. Other rare CNVs in genes not yet related to bone homeostasis were detected and regarded as variants of unknown significance. In papers IV and V massively-parallel sequencing was applied. In paper IV five novel variants in the fibronectin gene (FN1), which was recently linked to spondylometaphyseal dysplasia with “corner fractures”, were revealed in five patients affected by this disease. Finally