Rare diseases, defined by the 2002 Rare Disease Act, affect fewer than 5 in 10,000 individuals. Rare metabolic bone diseases (MBDs), such as osteogenesis imperfecta, hypophosphatasia, osteopetrosis, and other unclassified disorders, can disrupt bone development and remodeling, posing diagnostic and management challenges. This study analyzed data from the rarembd.in registry (2010-2024), a 15-year database documenting only rare MBDs. Clinical presentation and demographic data of patients with rare MBDs were collated. Common MBDs (osteoporosis, primary hyperparathyroidism) were excluded. Genetic testing was performed in a subset of patients. There was a total of 218 patients with an almost equal gender distribution (male-to-female ratio of 1:1.07) and a mean age of 29.1 ± 18.9 years. The registry identified 29 rare MBDs with three main disease categories: demineralization disorders (50.4%), disorders of bone matrix and cartilage formation (32.5%), and sclerotic disorders (13.7%); with a smaller proportion categorized as unclassified bone disorders (2.7%). Rickets/osteomalacia (27.1%) was the most common, followed by osteogenesis imperfecta (23.4%) and fibrous dysplasia/McCune-Albright syndrome (18.8%). Fractures affected 57.7% of patients, with 24.5% experiencing multiple fractures, while 31.1% exhibited skeletal deformities. Mutation analysis in our registry identified pathogenic variants in the SOST, TGFβ1, SLC34A3, ALPL, and VCP genes, confirming the genetic basis of sclerosteosis, Camurati-Engelmann disease, hypophosphatemic rickets, hypophosphatasia, and IBMPFD, respectively. Different management strategies were used that included teriparatide, bisphosphonates (zoledronate or alendronate) with total contact casting, intralesional zoledronate, denosumab, calcium, active vitamin D, and recombinant human growth hormone. Total parathyroidectomy was performed in specific cases. The registry classified RMBDs into four categories, with demineralization disorders being the most common, followed by bone matrix/cartilage formation disorders, sclerotic diseases, and unclassified cases. There were 29 RMBDs, and rickets/osteomalacia was the most prevalent subtype, tumor-induced osteomalacia followed by familial hypophosphatemic osteomalacia. Among the unclassified bone disorders, fragility fractures emerged as the most common presentation.

SOST encodes a secreted glycoprotein that is similar in sequence to the differential screening-selected gene aberrative in neuroblastoma (DAN) family of bone morphogenetic protein (BMP) antagonists. Pathogenic variants in the SOST gene result in sclerosteosis, van Buchem disease (VBD), or craniodiaphyseal dysplasia. SOST-related genetic disorders are very rare, and limited studies have reported variants associated with sclerosteosis. Clinical tests such as magnetic resonance imaging (MRI), computed tomography (CT), emission computed tomography (ECT), electromyogram (EMG), routine blood tests, and physical examinations were conducted for the proband. Trio-whole exome sequencing (Trio-WES) was performed, and the rare variants (allele frequency < 0.01) in the exon and splicing regions were selected for further pathogenic evaluation. Candidate pathogenic variants were validated through Sanger sequencing. The wild and mutant SOST sequences were cloned into the pcDNA3.1 expression vector, and the RNA and protein expression levels were investigated in the HEK293T cell line. In this study, we present a case study of a proband who displays abnormal facial expressions accompanied by numbness. The results of the brain MRI show thickening of the skull and disappearance of the diplopia signal. The temporal bone CT scan indicates diffuse osteosclerosis affecting the bilateral ossicular chains and internal auditory meatus, as well as stenosis of the bilateral internal auditory meatus. Trio-WES sequencing detected a novel homozygous variant in the proband: NM_025237.3(SOST): c.327C>A (p.Cys109*), which was also validated in his sister from the same family. According to the ACMG guidelines, the variant is classified as "likely pathogenic." The in vitro experiments demonstrated that the variant caused a decrease in SOST expression at RNA and protein level and produced a truncated protein. The report presents new evidence for the clinical diagnosis of SOST-related facial numbness and expands the variant spectrum of SOST.

Osteosclerotic bone diseases include more than 30 rare diseases characterized by excessive bone formation. The aim of this study is to compare the molecular pathogenesis and prognostic features of 12 different osteosclerotic diseases. Thirty-four patients from 23 families were included, 25 of whom were followed for a period of one to 22 years. Exome sequencing was performed in 20 families. Primary hypertrophic osteoarthropathy (PHOAR1/2) was found in 12 patients, followed by juvenile Paget's disease (JPD)-5 in five, craniometaphyseal dysplasia (CMD) and Camurati-Engelmann disease (CED) in four, Ghosal hematodiaphyseal dysplasia (GHDD) in three patients, sclerosteosis-1 in two patients, and ultra-rare diseases including trichothiodystrophy-1, prenatal Caffey disease, melorheosteosis, and Lenz-Majewski hyperostotic dwarfism in one patient each. Patients with CMD and sclerosteosis-1 had severe cranial sclerosis leading to facial dysmorphism. CMD was characterized by metaphyseal widening, radiolucency, and diaphyseal sclerosis of the long bones in early childhood and later developed Erlenmeyer flask deformity sparing the vertebrae and pelvis, whereas sclerosteosis-1 manifested as generalized sclerosis. CED and GHDD share bone pain, difficulty in walking, and diaphyseal sclerosis, with some patients also having bone marrow involvement. Interestingly, patients with CED and JPD-5 showed osteopenia in early childhood, followed by the development of osteosclerosis in late childhood. Clinical and radiologic findings improved over time in PHOAR1 patients, whereas they progressed in JPD-5 and trichothiodystrophy-1 patients. Intra- and interfamilial clinical differences were observed in CMD, CED, JPD-5, and GHDD. The knowledge gained about the natural history of osteosclerotic diseases will make an important contribution to their diagnosis and management.

Sclerosteosis, an ultra-rare disorder characterised by high bone mass (HBM) and skeletal overgrowth, leads to facial paralysis, hearing loss and raised intracranial pressure, which is currently managed only through high-risk surgery. Sclerosteosis is caused by SOST mutations and loss of functional sclerostin, a protein that suppresses osteogenesis by antagonising Wnt/β-catenin signalling. Herein, using in vitro and in vivo approaches, we explore whether LGK974, another potent Wnt inhibitor that targets porcupine (PORCN, Wnt-specific acyltransferase), is a promising sclerosteosis therapeutic. In vitro assays showed that 100 nmol/L LGK974 significantly reduced osteoblast alkaline phosphatase (ALP) activity/mineralisation, decreased Wnt/osteoblast marker (Axin2, Runx2 and Ocn) expression, and downregulated ossification and the Wnt signalling pathway, without affecting osteoclast numbers/resorption. To assess in vivo effects, 6-week-old male and female Sost deficient (Sost-/-) mice received LGK974 for 4 weeks and right hindlimbs were subjected to 20 N peak loading to assess mechanoadaptive interactions. µCT revealed significant reductions in vertebral trabecular number and lower cortical bone volume in loaded and non-loaded tibiae in male and female LGK974-treated Sost-/- mice. Interestingly, the target engagement biomarker Axin2 was only significantly reduced in male vertebrae, which may indicate differences in male and female response to LGK974. This study also shows that PORCN inhibition may effectively limit characteristic HBM and skeletal overgrowth in sclerosteosis patients at sites with severe pathology.

Van Buchem disease (VBD) is an inherited rare sclerosing bone disorder, due to defective synthesis of sclerostin, a negative regulator of bone formation. Our earlier cross-sectional studies of patients with VBD and the closely related sclerosteosis suggested that the accrual of bone mass does not continue after puberty but longitudinal studies of patients with sclerostin deficiency are not available. The aim, therefore, of the present study was the long-term assessment of adult patients with VBD. Fifteen previously evaluated patients with genetically confirmed VBD were invited to participate in the study and 11 (4 women) consented. Mean follow-up time was 8.9 ± 1.1 yr and median age at follow-up was 47 yr (range 20-60). Seven patients developed permanent facial paresis, 9 had progressing hearing loss, and 2 developed had increased intracranial pressure requiring cranial surgery. Dental problems were common, and 3 patients developed osteoarthritis during follow-up. None experienced a cardiovascular event. BMD did not change at the LS or the left FN; Z-scores were 10.2 ± 1.3 SD vs 9.4 ± 2.3 SD, p=.62, and 8.9 ± 2.2 SD vs 7.7 ± 2.2 SD, p=.15, respectively. The variability of the clinical expression and progression of the disease, despite the stabilization of BMD but with progressive cranial nerve compression, requires continuous monitoring of these patients for whom no disease-specific therapy is currently available.