Defective Bone Mineralization Research Articles

New Classification and Management of Abetalipoproteinemia and Related Disorders

New Classification and Management of Abetalipoproteinemia and Related Disorders

A compound heterozygous mutation of the alkaline phosphatase ALPL gene causes hypophosphatasia in a Han Chinese family.

Hypophosphatasia (HPP) is a rare hereditary systemic disease that is characterized by defective bone and/or dental mineralization, and is caused by mutations in the alkaline phosphatase gene (ALPL). The present study investigated the ALPL mutation in a Chinese Han family with HPP and studied the pathogenesis of the mutations of the ALPL gene. DNA was extracted from peripheral venous blood of the family members. Sanger sequencing was used to screen the mutations. Associations between pathogenesis for both mutations were analyzed by bioinformatics, subcellular localization, measurement of enzyme activity and western blotting. Sanger sequencing revealed the compound heterozygous mutations c.203C>T (p.T68M) and c.571G>A (p.E191K). The mutations were located at exon 4 and 6 of the ALPL gene and were predicted by Polyphen-2 analysis to be harmful. Protein analysis indicated a decrease in mature protein production and lower enzyme activity in 293T cells transfected with plasmids carrying the mutations. The ALPL gene was cloned into the pcDNA3.1(+) vector and mutant plasmids ALPL-pT68M and ALPL-pE191K were constructed. Immunofluorescence observed in cells transfected with the ALPL-pE191K mutant plasmid was mainly located in the cell membrane. However, staining in the cytoplasm was increased compared with the wild type, and almost no fluorescence was identified in 293T cells transfected with the ALPL-pT68M mutant plasmid. The present findings demonstrated that the compound heterozygous c.571G>A and c.203C>T mutations may contribute to childhood HPP by resulting in mislocalization, decreased protein expression and loss of enzyme activity in a Han Chinese family. The results of the current study may provide insights into the potential molecular mechanism of HPP.

Vitamin D supplementation in pregnancy, lactation and infancy: why is it fundamental?

Vitamin D is mainly made in the skin following exposure to sunlight but can also be taken in via specific natural or fortified food sources. It is essential for intestinal calcium absorption and thereby bone mineralisation. Deficiency in vitamin D, or low dietary calcium intake, therefore results in defective bone mineralisation causing osteomalacia in children and adults and rickets in growing children. The leading causes of rickets and osteomalacia worldwide are solar vitamin D deficiency and/or nutritional calcium deficiency. Long-standing severe vitamin D deficiency can lead to craniotabes, large fontanelle, leg bowing, stunted growth and obstructed labour. The deficient state in the infant is almost always acquired from the mother. Therefore, robust antenatal and infant vitamin D supplementation programmes are obligatory to prevent complications. Conversely, lack of programme monitoring and conflicting recommendations can lead to confusion among healthcare professionals and poor uptake in the population. Factors shown to improve adherence to supplementation include universal supplementation of breast- and bottle-fed infants, monitoring supplementation at antenatal/postnatal healthcare visits, providing information to families at discharge from the neonatal unit and financial family support.

Osteoidosis leads to altered differentiation and function of osteoclasts.

In patients with osteomalacia, a defect in bone mineralization leads to changed characteristics of the bone surface. Considering that the properties of the surrounding matrix influence function and differentiation of cells, we aimed to investigate the effect of osteoidosis on differentiation and function of osteoclasts. Based on osteomalacic bone biopsies, a model for osteoidosis in vitro (OIV) was established. Peripheral blood mononuclear cells were differentiated to osteoclasts on mineralized surfaces (MS) as internal control and on OIV. We observed a significantly reduced number of osteoclasts and surface resorption on OIV. Atomic force microscopy revealed a significant effect of the altered degree of mineralization on surface mechanics and an unmasking of collagen fibres on the surface. Indeed, coating of MS with RGD peptides mimicked the resorption phenotype observed in OIV, suggesting that the altered differentiation of osteoclasts on OIV might be associated with an interaction of the cells with amino acid sequences of unmasked extracellular matrix proteins containing RGD sequences. Transcriptome analysis uncovered a strong significant up‐regulation of transmembrane glycoprotein TROP2 in osteoclastic cultures on OIV. TROP2 expression on OIV was also confirmed on the protein level and found on the bone surface of patients with osteomalacia. Taken together, our results show a direct influence of the mineralization state of the extracellular matrix surface on differentiation and function of osteoclasts on this surface which may be important for the pathophysiology of osteomalacia and other bone disorders with changed ratio of osteoid to bone.

Fetal bone mineralization defects following maternal inadvertent exposure to thiazolidinediones in pregnancy.

Data on the efficacy of thiazolidinediones in pregnancy is limited. The present study highlights two cases of fetal bone mineralization defects following maternal exposure to thiazolidinedione.

Magnesium as a Janus-faced inhibitor of calcification

Vascular calcification is a life-threatening complication in patients with chronic kidney disease. Magnesium is a potent inhibitor of calcification and attracting attention as a new therapeutic candidate. ter Braake and colleagues demonstrate that magnesium supplementation strikingly prevents vascular calcification in Klotho knockout mice. However, these mice also show osteomalacia, indicating that magnesium has a Janus face. Maximizing the beneficial effects of magnesium without causing bone mineralization defects is an important next challenge.

Early exfoliation as an indication for evaluation: A case report

Hypophosphatasia (HPP) is a rare genetic skeletal disorder that occurs due to mutation in the ALPL gene. It is characterized by defective bone and teeth mineralization. Odontohypophosphatasia is the mildest form of HPP that affects only the dental tissues, manifests as early exfoliation of teeth, defective cementum formation, reduced alveolar bone height, and enlarged pulp chamber. We report a case of an 8-year-old female who presented only with dental findings of HPP. HPP was suspected due to reduced serum alkaline phosphatase level, reduced alveolar bone height, and enlarged pulp chamber. The cemental defects were identified by the ground section of the exfoliated tooth and measured using Image J software. Thus, we arrived at the final diagnosis of Odontohypophosphatasia.

Clinical application experience of asfotase alfa for a young patient with childhood hypophosphatasia

Hypophosphatasia (HPP) is a rare hereditary metabolic disease characterized by defective bone and dental mineralization, systemic complications that lead to disability of patients. HPP is classified into six forms according to the age of onset and severity of its clinical picture. The disease is caused by a reduced activity of the tissue nonspecific alkaline phosphatase (TNSALP) and elevated concentrations of pyrophosphates. Asfotase alfa is the only pathogenetic enzyme replacement therapy with recombinant human bone-targeted TNSALP approved for treatment of patients with perinatal, infantile and juvenile‐onset HPP. This treatment is associated with improved skeletal mineralization, respiratory function and overall survival in infants and young children with life-threatening hypophosphatasia. The world experience in application of recombinant alkaline phosphatase in adults is very limited. We present a clinical case that describes the first Russian experience in the use of asfotase alfa in an 18-year-old patient with late diagnosis of childhood-onset HPP.

PRO132 DISEASE BURDEN OF X-LINKED HYPOPHOSPHATEMIA

X-linked hypophosphatemia (XLH) is a rare genetic disease, serious, debilitating and deforming condition, characterized by renal phosphate wasting. XLH is caused by loss of function of the PHEX gene resulting in enhanced secretion of the phosphaturic hormone FGF23 and leading defective bone mineralization, leading to rickets and impaired physical function.This disease, which affects 1:20.000 people, results in impaired quality of life (QoL) of patients, since, despite the treatments, suffer progressive and debilitating complications. The main objective of the study was to determine QoL of patients, children and adults suffering XLH, as well as their caregivers. A multicentre, observational and cross-sectional study has been carried out by a scientific committee consisted of 17 health professionals experts in XLH. QoL was assessed with the following EuroQol-5 Dimensions (EQ-5D) questionnaires: the EQ-5D-3L proxy (completed by caregivers or parents of children under 12 years), the EQ-5D-3L (completed by patients aged between 12 and 18 years), and the EQ-5D-5L (completed by adult patients and caregivers). The total number of participants for this project was 50 patients, 21 children and 29 adults, and their corresponding caregivers. Children had mobility problems due to difficulties walking, moderate pain or discomfort, problems doing their daily activities and felt anxious or depressed. In addition, children showed problems washing or dressing themselves. Similarly, a high percentage of adults were unable to walk, suffered extreme pain, showed difficulties to carry out their daily activities, and reported anxiety and depression. Moreover, the results of QoL for caregivers and parents were slightly lower than the general Spanish population. In conclusion, both children and adults, showed an important deterioration in QoL that were worse than the general Spanish population. These findings suggest the need for new treatments, as in many cases, the conventional treatment does not prevent disease complications and the impairment of QoL.

X-linked Hypophosphatemic Rickets: the Challenges of Treatment

X-linked dominant hypophosphatemic rickets (XLHR) is the most common inherited form of rickets due to a mutation in the phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX gene) expressed in bones and teeth. This leads to impaired renal reabsorption of phosphate and defective bone mineralization. Clinical presentation often occurs in childhood, where children mostly present with bow legs, delayed walking, or gait difficulties. Other clinical features may also be present and these are described in this review in addition to the classic laboratory findings. Focus is made on the management of XLHR and its challenges, highlighting the complications that may arise from medical treatment with reference to literature. Moreover, we also describe novel treatment in XLHR; the potential use of growth hormone and cinacalcet, and the newly approved human monoclonal antibody against FGF-23 as a more targeted therapy.

Multiple Fractures and Impaired Bone Fracture Healing in a Patient with Pycnodysostosis and Hypophosphatasia.

Pycnodysostosis (PYCD) is a rare recessive inherited skeletal disease, characterized by short stature, brittle bones, and recurrent fractures, caused by variants in the Cathepsin K encoding gene that leads to impaired osteoclast-mediated bone resorption. Hypophosphatasia (HPP) is a dominant or recessive inherited condition representing a heterogeneous phenotype with dental symptoms, recurrent fractures, and musculoskeletal problems. The disease results from mutation(s) in the tissue non-specific alkaline phosphate encoding gene with reduced activity of alkaline phosphatase and secondarily defective mineralization of bone and teeth. Here, we present the first report of a patient with the coexistence of PYCD and HPP. This patient presented typical clinical findings of PYCD, including short stature, maxillary hypoplasia, and sleep apnoea. However, the burden of disease was caused by over 30 fractures, whereupon most showed delayed healing and non-union. Biochemical analysis revealed suppressed bone resorption and low bone formation capacity. We suggest that the coexistence of impaired bone resorption and mineralization may explain the severe bone phenotype with poor fracture healing.

Burden of disease in pediatric patients with hypophosphatasia: results from the HPP Impact Patient Survey and the HPP Outcomes Study Telephone interview

BackgroundHypophosphatasia (HPP) is a rare, inherited, metabolic bone disease caused by deficient tissue-non-specific isoenzyme of alkaline phosphatase activity that manifests as a broad range of signs/symptoms, including bone mineralization defects and systemic complications. The burden of disease is poorly characterized, particularly in children. This study aimed to characterize the patient-reported burden of disease among children with HPP using two survey instruments: the HPP Impact Patient Survey (HIPS) and the HPP Outcomes Study Telephone interview (HOST).MethodsBetween September 2009 and June 2011, pediatric patients (aged younger than 18 years) with HPP were recruited to participate in the study via patient advocacy groups or their medical provider. Survey questions were used to capture information on patient demographics, HPP-related medical history, mobility, and health-related quality of life (HRQoL; using the 10-item Short-Form Health Survey for Children [SF-10], HIPS only).ResultsCommon clinical features of the 59 pediatric survey respondents (mean [standard deviation] age: 7.6 [5.1] years; 51% male) included pain (86% of patients), muscle weakness (71%), difficulty gaining weight (64%), and delayed walking (59%). Fracture was reported by 36% of patients; multiple fractures were also reported (15% of patients). Use of assistive devices for mobility was frequent among the study population (51%). In response to the SF-10, patients reported a substantial impact of HPP on their HRQoL; physical function was the most severely impaired component relative to normative data. Of patients responding to the HOST, two-thirds experienced worsening of at least one of their HPP-related signs/symptoms over a 5-year period.ConclusionsIn pediatric patients, HPP is associated with a high burden of disease and a substantial negative impact on HRQoL. The burden of HPP may increase and HRQoL reduce further over time as signs/symptoms that affect HRQoL worsen or new signs/symptoms manifest.

Frequency and age at occurrence of clinical manifestations of disease in patients with hypophosphatasia: a systematic literature review

BackgroundHypophosphatasia (HPP) is a rare, inherited, metabolic disease caused by tissue-nonspecific alkaline phosphatase deficiency, characterized by bone mineralization defects and systemic complications. Understanding of the clinical course and burden of HPP is limited by its rarity. This systematic literature review and synthesis of case report data aimed to determine the frequency and timing of clinical HPP manifestations and events.MethodsCase reports and series of patients with HPP who had been followed longitudinally for ≥1 year were identified. Demographics and clinical data of interest, identified through consultation with clinical experts in HPP, were extracted. Occurrences of clinical manifestations/events of interest were categorized, classified by age at first reported occurrence of HPP manifestations and visualized over time. Clinical manifestations/events considered to contribute to the clinical burden of HPP were identified. Kaplan–Meier curves were used to estimate the median (range) age at first occurrence of the most frequently reported manifestations/events.ResultsFrom the 283 studies that met the inclusion criteria, 265 patients with HPP with ≥1 year of longitudinal follow-up were identified (median [interquartile range] age 4 [0–34] years; 45% male). The types of clinical manifestations/events of interest experienced by individuals with ≥1 such manifestation/event (n = 261) often differed between older and younger patients. Most (94%) of the 265 patients experienced ≥1 manifestation/event deemed to contribute to the clinical burden of HPP; premature tooth loss (53.5%), fractures (35.8%), pain (33.6%), and gross motor/ambulation difficulties (30.9%) were most frequently reported. The median (range) age at first reported occurrence of respiratory symptoms, cranial abnormalities, and premature tooth loss ranged from 0.3 to 10 years, whereas the median age at first reported occurrence of fractures, pain, gross motor/ambulation difficulties, and surgery ranged from 33 to 70 years.ConclusionsHPP is associated with a high clinical burden of disease, regardless of age at first reported occurrence of HPP manifestations. Over an individual’s lifetime, the types of manifestations/events experienced can change and multiple HPP-related clinical manifestations/events can accumulate. These observations may reflect evolution and progression of the disease.

MON-496 Bone Mineral Density improvement with Vitamin Dsupplementation in a Case Of Childhood Hypophosphatasia

Hypophosphatasia is associated with defective mineralization of bone with possible teeth involvement with low activity of serum and bone alkaline phosphatase. It is caused by a mutations in the TNSALP gene. It is usually life-threatening when it presents within the first six months of life. It has six subtypes: perinatal lethal, prenatal benign, infantile, childhood, adulthood and odontohypophosphatasia. Childhood hypophosphatasia, defined as onset of symptoms between six months and eighteen years, can present as rickets, pain, decreased mobility, deficits of growth, and fractures. CASE REPORT: A 3-year-old child presented in the Endocrinology clinic due to premature loss of her four lower incisors and x-rays showed significant bone loss in the mandible. She also had reduced serum alkaline phosphatase activity. She was diagnosed with Childhood Hypophosphatasia, which was confirmed by genetic investigation. The molecular study showed that the patient is heterozygous for a c.1133A greater than T (p.Asp378Val) variation in exon 10 of the ALPL gene. The child also had a low Vitamin D level of 23 ng/ml (30-100), the Dexa-scan showed a low bone mineral density with a z-score of -2.4 SD below the mean in the distal femur area, which responded to Vitamin D supplementation and oral calcium therapy and normalized. The child did not have any fractures about 9 years after initial diagnosis. Bone turnover markers including Osteocalcin and N-telopeptide levels also normalized. The premature loss of teeth stopped. The child grew and gained weight as expected. CONCLUSION: Hypophosphatasia is suspected in a child with premature loss of deciduous teeth. Testing for a reduced serum alkaline phosphatase activity should be done. The children can be associated with rickets and low vitamin D. Improvement in low bone mineral density is possible with giving adequate doses of vitamin D, calcium and doing non-contact sports. Monitoring of bone mineral density can be done on a yearly basis.

SUN-523 XLH Outcome Data from One Centre Experience with Burosumab

X-linked hypophosphataemia (XLH) is a rare, genetic, chronically debilitating and deformative bone disease that profoundly impacts the affected individual’s day-to-day functioning and quality of life. High levels of circulating fibroblast growth factor 23 (FGF23) lead to excess urinary phosphate excretion and subsequent hypophosphataemia, resulting in defective bone mineralisation. Burosumab is an anti-FGF23 fully human monoclonal antibody, and the first treatment to target the underlying pathophysiology of XLH. Real world evidence has an important role in validating the findings of clinical studies and capturing clinically relevant outcomes. We report the initial experience of burosumab from one UK centre Evelina London Children’s Hospital (ELCH) participating in the burosumab early access program (EAP). Methods An EAP for burosumab was made available for children in the United Kingdom with XLH in 12 specialist centres. Inclusion criteria for the EAP included radiographic evidence of disease, XLH confirmed by genetic PHEX mutation, confirmed familial X-linked inheritance mutation or family history. Patients must have also had an unsatisfactory response to best available care and treatment. EAP enrolment was between January and March 2018. A total of 142 applications were received of which 135 were approved with 132 receiving treatment to date.1 Of the 7 declined, 4 failed to meet diagnostic criteria and 3 had insufficient radiological evidence.1 Treatment, including, dose was in accordance with the EMA marketing authorisation. Results Data are available on 9 ELCH patients who had completed the initial 12-week burosumab titration period and 7 patients with 24 week data. Mean age was 6.87 years (range 1.5-16.33 years (44% female)). The mean height and weight at week 0 was 105.7 cm (75-153 cm) and 20.92 kg (10.5-40.5 kg). Mean dose was 0.60 mg/kg at week 0 and 0.90 mg/kg at the end of the initial titration period at week 12 (0.91 mg/kg at week 24). Mean serum phosphorus was 0.67 mmol/L (0.5-0.8 mmol/L) in week 0 and 0.99 mmol/L at week 12 and 1.07 mmol/L at week 24 (0.9-1.3 mmol/L) representing a 60% increase in serum phosphorus levels at week 24. Mean serum ALP fell from 412.44 IU/L (269-495 IU/L) at week 0 to 364.44 IU/L at week 12 and 371.86 IU/L at week 24 (271-508 IU/L). 6MWT and TUG scores were available for 4 patients. A mean increase of 144.25 m (56%) was seen from 258 m at baseline to 402.25 by week 24. TUG scores increased by 32% from 5.74 (5.01-6.4) at baseline to 7.57 (5.39-11.43) by week 24. No patients had discontinued treatment to date due to adverse events.1Conclusions Early data from treating XLH patients with burosumab in a real-world UK setting from one centre demonstrates that key biochemical responses are in line with findings from the clinical study program. Improvements in clinically meaningful relevant outcomes such as 6MWT and TUG scores were also seen.

MON-498 When to Sweat the Details: A Case of Adult Hypophosphatasia

Introduction: Hypophosphatasia (HPP) is characterized by defective mineralization of bone due to low activity of serum bone alkaline phosphatase. Adult forms are characterized by stress fractures and pseudofractures of the lower extremities and early loss of adult dentition. Here we present a case of adult HPP with a novel variant of the ALPL gene (p.Ala105Asp). Case Description: A 59-year old male with past medical history of chronic pain, polyarthralgia, multiple tendon/muscle tears, severe periodontal disease, requiring multiple fillings and crowns since his early 20s, and calcium pyrophosphate deposition disease (CPPD) presents with progressively worsening peripheral neuropathy in his feet. He has a left fifth metatarsal small avulsion non-healing fracture and old left non-union tibial sesamoid fracture with loss of physical function impacting ADLs, requiring assistance with mobility. Vitamin B6 was 127.7 ng/mL [2.1-21.7 ng/mL] and alkaline phosphatase was 18 U/L [53-128 U/L]. Genetic testing was done which revealed he has a heterozygous variant of ALPL gene (p.Ala105Asp). He has 3 family members with variants of ALPL gene and have since been diagnosed with hypophosphatasia. Rest of the family are asymptomatic. The patient is currently undergoing evaluation for treatment with recombinant alkaline phosphatase, asfotase alfa. Discussion: Adult HPP may be inherited in an autosomal recessive or autosomal dominant manner, depending on the effect that the ALPL pathogenic variant has on TNSALP activity. Heterozygotes (carriers) are either asymptomatic (may manifest biochemically but not clinically) or symptomatic. This patient had a heterozygous variant in the ALPL gene (p.Ala105Asp), which, to our knowledge, has not been reported in association with ALPL-related conditions, however other variants flanking this position in the gene (p. Asn102_Asn104del, p.G1n106His) have been associated with adult, perinatal, and infantile HPP, suggesting a change at this position adversely affects protein structure and/or function. Furthermore, this variant has not been reported in the broad dataset (individuals without severe childhood onset disease). Conclusion: Adult HPP is a rare disorder that is often underdiagnosed due to varying nonspecific symptoms. Awareness of the disease and identification of patients who have findings such as elevated vitamin B6 and low alkaline phosphatase is essential prior to confirming diagnosis with genetic testing. Diagnosis is paramount as patients with this disorder can now be treated with recombinant alkaline phosphatase. This case was remarkable as this patient has a heterozygous novel variant in the ALPL gene (p.Ala105Asp).

Lethal perinatal hypophosphatasia caused by a novel compound heterozygous mutation: a case report

BackgroundHypophosphatasia (HPP) is a rare hereditary disorder characterized by defective bone and tooth mineralization and deficiency of tissue non-specific alkaline phosphatase (TNAP) activity. The clinical presentation of HPP is highly variable, and the prognosis for the infantile form is poor.Case presentationThis study reports a male infant diagnosed with lethal perinatal HPP. His gene analysis showed two heterozygous missense variants c.406C > T (p.R136C) and c.461C > T (p.A154V). The two mutations originated separately from his parents, consistent with autosomal recessive perinatal HPP, and the c.461C > T (p.A154V) was the novel mutation. Three-level structure model provide an explanation of the two mutated alleles correlating with the lethal phenotype of our patient. Results of SIFT, PolyPhen_2, and REVEL showed two mutations were pathogenic.ConclusionsWe demonstrated a case of perinatal lethal HPP caused by two heterozygous mutations, and one of which was novel. This finding will prove relevant for genetic counseling and perinatal gene testing for affected families.

MAPK inhibition and growth hormone: a promising therapy in XLH.

X-linked hypophosphatemia (XLH) leads to growth retardation and bone deformities, which are not fully avoided by conventional treatment with phosphate and vitamin D analogs. Pediatric patients have been treated with growth hormone (GH), and recent findings suggest that blocking fibroblast growth factor 23 actions may be the most effective therapy, but its effects on growth are not known. We here report the effect of MAPK inhibition alone or associated with GH on growth and growth plate and bone structure of young Hyp (the XLH animal model) mice. Untreated Hyp mice were severely growth retarded and had marked alterations in both growth plate structure and dynamics as well as defective bone mineralization. GH accelerated growth and improved mineralization and the cortical bone, but it failed in normalizing growth plate and trabecular bone structures. MAPK inhibition improved growth and rickets and, notably, almost normalized the growth plate organization. The administration of a MAPK pathway inhibitor plus GH was the most beneficial treatment because of the positive synergistic effect on growth plate and bone structures. Thus, the growth-promoting effect of GH is likely linked to increased risk of bone deformities, whereas the association of GH and MAPK inhibition emerges as a promising new therapy for children with XLH.-Fuente, R., Gil-Peña, H., Claramunt-Taberner, D., Hernández-Frías, O., Fernández-Iglesias, Á., Alonso-Durán, L., Rodríguez-Rubio, E., Hermida-Prado, F., Anes-González, G., Rubio-Aliaga, I., Wagner, C., Santos, F. MAPK inhibition and growth hormone: a promising therapy in XLH.

Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2.

Mechanisms leading to osteoporosis are incompletely understood. Genetic disorders with skeletal fragility provide insight into metabolic pathways contributing to bone strength. We evaluated 6 families with rare skeletal phenotypes and osteoporosis by next-generation sequencing. In all the families, we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane–resident sphingomyelin synthase SMS2. Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg). Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis. Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia. Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations. Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization. Osteoclast formation and function in vitro was normal. While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum. SGMS2 pathogenic variants underlie a spectrum of skeletal conditions, ranging from isolated osteoporosis to complex skeletal dysplasia, suggesting a critical role for plasma membrane–bound sphingomyelin metabolism in skeletal homeostasis.

DMP1 Ablation in the Rabbit Results in Mineralization Defects and Abnormalities in Haversian Canal/Osteon Microarchitecture.

DMP1 (dentin matrix protein 1) is an extracellular matrix protein highly expressed in bones. Studies of Dmp1 knockout (KO) mice led to the discovery of a rare autosomal recessive form of hypophosphatemic rickets (ARHR) caused by DMP1 mutations. However, there are limitations for using this mouse model to study ARHR, including a lack of Haversian canals and osteons (that occurs only in large mammalian bones), high levels of fibroblast growth factor 23 (FGF23), and PTH, in comparison with a moderate elevation of FGF23 and unchanged PTH in human ARHR patients. To better understand this rare disease, we deleted the DMP1 gene in rabbit using CRISPR/Cas9. This rabbit model recapitulated many features of human ARHR, such as the rachitic rosary (expansion of the anterior rib ends at the costochondral junctions), moderately increased FGF23, and normal PTH levels, as well as severe defects in bone mineralization. Unexpectedly, all DMP1 KO rabbits died by postnatal week 8. They developed a severe bone microarchitecture defect: a major increase in the central canal areas of osteons, concurrent with massive accumulation of osteoid throughout all bone matrix (a defect in mineralization), suggesting a new paradigm, where rickets is caused by a combination of a defect in bone microarchitecture and a failure in mineralization. Furthermore, a study of DMP1 KO bones found accelerated chondrogenesis, whereas ARHR has commonly been thought to be involved in reduced chondrogenesis. Our findings with newly developed DMP1 KO rabbits suggest a revised understanding of the mechanism underlying ARHR. © 2019 American Society for Bone and Mineral Research.