Hypophosphatasia (HPP) is the rare inborn-error-of-metabolism that features impaired mineralization of the skeleton and teeth due to a deactivating mutation or mutations of the gene ALPL which encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). We report 17-year follow-up of twin sisters and a brother referred in middle-age for painful proximal femoral "stress fractures" and then diagnosed with HPP. They reported generalized muscle and bone pain, metatarsal fractures, arthropathy and, since childhood, tooth loss. Their concordant findings were explained by compound heterozygosity in ALPL for a rare maternal missense mutation (c.1403C > T, p.Ala468Val) in exon 12, together with a novel presumably paternal change (c.863-14G > A) predicting a cryptic mRNA splice site in intron 8. Fractures continued during follow-up until one sister received a three-and-one-half-year course of hydroxyapatite-targeted TNSALP supplementation therapy (asfotase alfa) during which substantial improvement occurred in her clinical, biochemical, and functional parameters as well as quality of life. Following subsequent unplanned treatment cessation she suffered significant clinical deterioration, including new fractures and loss of mobility. Her bone histopathology documented osteomalacia. Treatment resumption restored its benefits. Among ten asymptomatic family members evaluated in this four-generation kindred, eight were carriers heterozygous for either ALPL mutation. Those harboring the maternal missense defect manifested mild hypophosphatasemia, suggesting a dominant-negative mutation effect. This experience underscores the importance of in-depth phenotyping and then clinical follow-up to characterize ALPL variant combinations, and for maintaining effective asfotase alfa treatment.

Bone mineral molecular architecture is tightly regulated by the kinetics of calcium phosphate phase transformations. In the rare skeletal disease hypophosphatasia (HPP), caused by inactivating mutations in the ALPL gene encoding tissue-nonspecific alkaline phosphatase (TNSALP), accumulation of inorganic pyrophosphate (PPi) alters these phase dynamics. Using solid-state nuclear magnetic resonance spectroscopy and high-resolution electron microscopy, in patient samples we show that bone mineral from compound heterozygous HPP patients exhibits a loss of hydrated amorphous interfacial phases and instead contains highly crystalline hydroxyapatite (HAP), correlating with abnormally high bone mineral density and brittle atypical femoral fractures. Synthetic and cellular models demonstrate that elevated PPi impedes normal phase transitions from amorphous calcium phosphate precursors, bypassing intermediate states and driving ordered HAP nucleation. These findings link disrupted mineral phase kinetics to pathological bone mineral molecular structure, emphasising the critical importance of the hydrated amorphous shell around bone mineral for its material properties and redefining HPP as a molecular mineralization disorder. Our findings also illustrate how biochemical cues regulate non-equilibrium crystallization pathways in biomineralization. ### Competing Interest Statement The authors have declared no competing interest. European Research Council, https://ror.org/0472cxd90, 101019499 Engineering and Physical Sciences Research Council, https://ror.org/0439y7842, EP/P030467/1

Camurati-Engelmann disease, type 1 (CED1, OMIM # 131300) is the rare autosomal dominant skeletal dysplasia caused by select heterozygous loss-of-function defects within the gene TGFB1, which encodes transforming growth factor beta 1 (TGFB1). CED1 mutations are found in TGFB1 exons 1-4 that form the latency-associated peptide (LAP) of pro-TGFB1. Consequently, skeletal action of TGFB1 increases and thereby enhances bone formation manifest clinically as "progressive diaphyseal dysplasia". Beginning 24 years ago negative TGFB1 analysis suggested rare genetic heterogeneity for CED, and Online Mendelian Inheritance In Man designated, of unknown etiology, "CED2" (OMIM % 606631). In 2024, three sporadic occurrences considered CED2 were reported to harbor either of two mutations of TGFB2, which encodes the LAP of transforming growth factor beta 2 (TGFB2). Herein, three adults (father, son, daughter) having the CED2 phenotype in a Peruvian family revealed a novel missense variant (c.108G > T, p.R36S) within the TGFB2 LAP domain. Debilitating painful skeletal disease featuring hyperostosis of entire long bones, worse in the men, presented early in childhood. Aminobisphosphonate therapy seemed helpful. Their TGFB2 variant was within a highly conserved domain across species, absent in the gnomAD database, "possibly damaging" by Polyphen-2, not tolerated by SIFT, homologous with TGFB1 at the same amino acid position (R36) as one reported TGFB2 mutation, co-segregated as autosomal dominant, and "likely pathogenic" per ACMG guidelines.

INTRODUCTION:Different length in-frame duplications within exon 1 of TNFRSF11A encoding receptor activator of nuclear factor-kappa B (RANK) increase osteoclast action and cause rapid turnover bone disease. Complications include deafness, fractures, immobilization hypercalcemia, tooth resorption, and painful skeletal deformities. We investigated two siblings and their father and an unrelated girl with this autosomal dominant dento-osseous phenotype who suffered tendon avulsions. PATIENTS:A 13-year-old boy, his 16-year-old sister, and their 43-year-old father, all with hearing loss and progressive tooth root resorption, have a heterozygous 27-bp duplication within exon 1 of TNFRSF11A. Within 3 years, the siblings suffered seven tendon avulsions with minimal trauma, including the distal Achilles, triceps, and quadriceps tendons. Alendronate was given weekly, which decreased bone turnover markers (BTMs) and treated mild immobilization hypercalcemia following tendon repairs. Their father suffered an Achilles tendon avulsion without a clear mechanism at 20 years of age. An unrelated 10-year-old girl with hearing loss and progressive tooth root resorption was heterozygous de novo for a 12-bp duplication within exon 1 of TNFRSF11A. Alendronate was given weekly. At age 15 years, she bilaterally avulsed her triceps while playing volleyball and then avulsed her Achilles tendon two months later. CONCLUSIONS:Tendon avulsion seems to be a complication of constitutive RANK activation from select duplications of TNFRSF11A, but its precise pathogenesis and the impact of bisphosphonate therapy remain uncertain. Bisphosphonate therapy can treat or prevent associated immobilization hypercalcemia and decrease BTMs in people with constitutive RANK activation from such mutations.

Hypophosphatasia (HPP) is the inborn-error-of-metabolism from deactivating mutation(s) of ALPL, the gene that encodes the cell surface "tissue-nonspecific" isoenzyme of alkaline phosphatase (TNSALP). HPP's "biochemical signature" comprises low serum alkaline phosphatase activity together with elevated plasma levels of the TNSALP natural substrates phosphoethanolamine (PEA), pyridoxal 5'-phosphate (PLP), and inorganic pyrophosphate (PPi). Excess extracellular PPi (ePPi) inhibits mineralization and affected children prematurely shed deciduous teeth and often suffer weakness and rickets. Yet, HPP severity is greatest among all dento-osseous disorders and not fully explained by autosomal dominant versus autosomal recessive inheritance involving >470 ALPL mutations. Discordance of HPP phenotype sometimes manifests even among full siblings sharing an identical ALPL genotype. Herein, a girl's markedly discordant HPP featured at presentation life-threatening hypercalcemia, failure-to-thrive, and renal compromise. Subsequent pseudotumor cerebri syndrome caused blindness, and then craniosynostosis required cranial vault reconstruction. However, she was not deformed, had moderate hypophosphatasemia, normal plasma PLP level, and mild radiographic features of HPP rickets. Elevated plasma N-terminal parathyroid hormone-related protein (PTHrP) suggested malignancy, but corrected after kidney transplantation. HPP was diagnosed when whole exome sequencing revealed heterozygous ALPL c.1034C>T, p.A345V found in mild pediatric HPP and transmitted by her mother who considered herself well. Genes conditioning ePPi formation and underlying other skeletal diseases were intact. Hypercalcemia, unresponsive to bone antiresorptive drugs, corrected promptly with asfotase alfa TNSALP supplementation therapy. Her markedly discordant findings highlight genotype/phenotype plasticity for pediatric HPP, and her clinical course importance for early diagnosis.

Craniotubular dysplasia, Ikegawa type (OMIM #619727) denotes the autosomal recessive skeletal disease identified in 2021 featuring blindness acquired in childhood. Five young members of four Indian families harbored a homozygous indel within TMEM53 (OMIM *619722), the gene that encodes transmembrane protein 53 (TMEM53). When intact, TMEM53 spans the nuclear envelope of osteoprogenitor cells, dampens BMP-SMAD signaling, and thereby slows bone formation. Consequently, defective TMEM53 accelerates osteogenesis. Herein, an American boy is compound heterozygous for a novel deletion and a novel missense mutation within TMEM53. His vision and sensorineural hearing became impaired. Radiographic survey revealed diploic thickening of his skull, broad calvarial and facial bones, skeletal modeling errors, vertebral body flattening, wide ribs, and osteopenia of expanded bones. DXA areal bone density (gm/cm(2)) Z-scores were low. His optic, auditory, and spinal canals were narrow. Mineral metabolism was intact. Serum alkaline phosphatase and osteocalcin levels were normal yet CTX was high. Iliac crest histomorphometry documented accelerated bone formation. His acute vision loss briefly improved following prednisone administration, optic canal decompression, and optic nerve sheath fenestration, but then progressed despite further surgeries and zoledronate treatment aimed to suppress bone turnover. Next generation sequencing of genes associated with elevated skeletal mass, including from high bone turnover, did not suggest an etiology. Whole genome sequencing then revealed within TMEM53: i) a paternally transmitted 54-base deletion, which included the mRNA splice acceptor site for exon 2 as well as 31 bases of exonic sequence (c. 62-23_92del), and ii) a maternally transmitted missense variant (c.650C > T, p.Ser217Leu: NM_024587.4/NP_078863.2) which is extremely rare in gnomAD (frequency = 0.000036), replaces Ser217 highly conserved across species, and is scored as damaging by SIFT and Mutation Taster. We call this new osteopathy TMEM53 craniotubular dysplasia.

Hypophosphatasia (HPP) is the dento-osseous disorder caused by deactivating mutation(s) of ALPL, the gene that encodes the "tissue-nonspecific" isoenzyme of alkaline phosphatase (TNSALP). In HPP, 3 natural substrates of cell-surface TNSALP accumulate extracellularly; phosphoethanolamine (PEA), inorganic pyrophosphate (PPi), and pyridoxal 5'-phosphate (PLP). Hypophosphatasemia together with elevated plasma levels of PEA, PPi, and PLP comprise its biochemical signature. PPi can inhibit mineralization and in extracellular excess can impair bone and tooth hardening and perhaps explain weak muscle. Autosomal dominant or autosomal recessive inheritance from among more than 400 mutations of ALPL largely accounts for HPP's broad-ranging severity, greatest among all skeletal diseases. Pediatric HPP spans life-threatening perinatal and infantile forms, childhood forms, and odonto-HPP selectively featuring premature loss of deciduous teeth. ALPL gene testing and TNSALP supplementation therapy have bolstered familiarity with HPP, but there are new considerations for diagnosis. Herein, diagnosis of a boy's mild childhood HPP was delayed by missteps involving his medical and dental history, physical examination, radiographic findings, and clinical laboratory studies. We review how pediatric HPP is now identified. Prompt diagnosis while appreciating the broad-ranging severity of HPP underlies the safe and effective management of this inborn-error-of-metabolism.

Introduction: Ultra-rare mendelian osteolytic disorders caused by different length in-frame activating duplications within exon 1 of TNFRSF11A encoding receptor activator of nuclear factor-kappa B (RANK) comprise familial expansile osteolysis (FEO), expansile skeletal hyperphosphatasia (ESH), early-onset familial Paget's disease of bone (PDB2), juvenile Paget's disease 2 (JPD2), and panostotic expansile bone disease (PEBD). FEO typically presents with childhood-onset deafness followed by resorption of permanent dentition, and then appendicular bone pain, fractures, and deformities from progressive focal expansile osteolytic lesions emerging from a background of generalized high bone turnover. An 18-bp duplication in TNFRSF11A has been reported in all kindreds with FEO, whereas a 12-bp duplication was found in the young man with PEBD complicated by a massive jaw tumor. We report the clinical course and successful treatment with bisphosphonates of a girl with the 12-bp duplication yet a skeletal phenotype seemingly milder than PEBD.Case presentation and discussion: This 10-year-old girl presented for dental and orthodontic treatment and was found to have progressive external tooth root resorption. Speech delay was identified at age 18 months, and audiological evaluation showed both conductive and sensorineural hearing loss subsequently treated with a cochlear implant at age 3 years. Biochemical studies indicated increased bone turnover with elevated urinary N-telopeptide levels and serum alkaline phosphatase in the upper normal range. Low lumbar spine bone mineral density (BMD) was revealed by dual-energy X-ray absorptiometry, but whole-body Technetium-99 m bone scintigraphy was normal. Genetic testing identified the identical de novo 12-bp duplication within exon 1 of TNFRSF11A harbored by the young man with PEBD and massive jaw tumor. Bisphosphonate treatment, initiated with one dose of intravenous zoledronic acid that caused prolonged hypocalcemia, then comprised weekly oral alendronate that decreased bone turnover markers and normalized her BMD.Conclusion: Constitutive activation of RANK signaling should be considered a possible cause in any young person with rapid bone turnover, particularly in the context of early-onset deafness and/or root resorption of permanent

Osteoblast Wnt/β-catenin signaling conditions skeletal development and health. Bone formation is stimulated when on the osteoblast surface a Wnt binds to low-density lipoprotein receptor-related protein 5 (LRP5) or 6 (LRP6), in turn coupled to a frizzled receptor. Sclerostin and dickkopf1 inhibit osteogenesis if either links selectively to the first β-propeller of LRP5 or LRP6, thereby disassociating these cognate co-receptors from the frizzled receptor. Sixteen heterozygous mutations identified since 2002 within LRP5 and three heterozygous mutations identified since 2019 within LRP6 prevent this binding of sclerostin or dickkopf1 and account for the exceptionally rare, but highly instructive, autosomal dominant disorders called LRP5 and LRP6 high bone mass (HBM). Herein, we characterize LRP6 HBM in the first large affected family. Their novel heterozygous LRP6 missense mutation (c.719C>T, p.Thr240Ile) was present in two middle-aged sisters and three of their sons. They considered themselves healthy. Their broad jaw and torus palatinus developed during childhood and, contrary to the two previous reports of LRP6 HBM, the appearance of their adult dentition was unremarkable. Skeletal modeling, defined radiographically, supported classification as an endosteal hyperostosis. Areal bone mineral density (g/cm2) of the lumbar spine and total hip featured accelerated increases reaching Z-scores of ~ +8 and +6, respectively, although biochemical markers of bone formation were normal. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Inhalant use disorder is a psychiatric condition characterized by repeated deliberate inhalation from among a broad range of household and industrial chemical products with the intention of producing psychoactive effects. In addition to acute intoxication, prolonged inhalation of fluorinated compounds can cause skeletal fluorosis (SF). We report a young woman referred for hypophosphatasemia and carrying a heterozygous ALPL gene variant (c.457T>C, p.Trp153Arg) associated with hypophosphatasia, the heritable metabolic bone disease featuring impaired skeletal mineralization, who instead suffered from SF. Manifestations of her SF included recurrent articular pain, axial osteosclerosis, elevated bone mineral density, maxillary exostoses, and multifocal periarticular calcifications. SF was suspected when a long history was discovered of 'huffing' a computer cleaner containing 1,1-difluoroethane. Investigation revealed markedly elevated serum and urine levels of F-. Histopathology and imaging techniques including backscattered electron mode scanning electron microscopy, X-ray microtomography, energy dispersive and wavelength dispersive X-ray emission microanalysis, and polarized light microscopy revealed that her periarticular calcifications were dystrophic deposition of giant pseudo-crystals of francolite, a carbonate-rich fluorapatite. Identifying unusual circumstances of F- exposure is key for diagnosing non-endemic SF. Increased awareness of the disorder can be lifesaving.