A missense mutation in the human liver/bone/kidney alkaline phosphatase gene causing a lethal form of hypophosphatasia

Disease burden by ALPL variant number in patients with non-life-threatening hypophosphatasia in the Global HPP Registry

BACKGROUND

Hypophosphatasia (HPP) is a rare metabolic disease caused by autosomal dominant or recessive inheritance of ALPL variants resulting in low alkaline phosphatase activity. The objective of this analysis was to compare HPP disease burden between patients with non-life-threatening disease in the Global HPP Registry who have one ALPL variant versus two or more ALPL variants.

METHODS

Patients were included if they had one or more ALPL variants identified through genetic testing and first HPP manifestations after 6 months of age. Assessments included history of HPP manifestations, Brief Pain Inventory-Short Form (BPI-SF), Health Assessment Questionnaire-Disability Index (HAQ-DI), 6-Min Walk Test (6MWT), Paediatric Quality of Life Inventory (PedsQL) and 36-Item Short-Form Survey V.2 (SF-36v2).

RESULTS

Of 685 included patients, 568 (82.9%) had one ALPL variant, 116 (16.9%) had two variants, and one (0.1%) had three variants. Patients with two or more ALPL variants had higher proportions of skeletal (52.1% vs 32.6%), dental (73.5% vs 56.0%), muscular (36.8% vs 23.6%) and neurological (22.2% vs 8.8%) manifestations at last assessment. BPI-SF, HAQ-DI, PedsQL and SF-36v2 scores were similar between groups. Distances walked on the 6MWT were similar between groups for children. Distance walked was lower among adults with two or more variants (293 m (n=8)) than adults with one variant (466 m (n=103)), although the former group was very small.

CONCLUSION

HPP disease burden is high in patients with HPP, regardless of ALPL variant number. While prevalence of HPP-specific manifestations was higher in patients with two or more variants than those with one variant, patient-reported outcomes were similar between groups.

TRIAL REGISTRATION NUMBER

NCT02306720; EUPAS13514.

Genetic characterization of a large cohort of individuals with a clinical suspicion of hypophosphatasia in the United States

Hypophosphatasia (HPP) is a rare metabolic disease resulting from variants in ALPL, inherited in an either autosomal recessive or autosomal dominant manner. Sponsored clinical ALPL testing was offered in the US for individuals with a clinical suspicion of HPP. Gene variants were assessed to determine the likelihood of identifying disease-causing variants, uncover genotype-phenotype relationships, and for further understanding of ALPL variants in the US HPP population. Variants were detected by Sanger sequencing and classified as pathogenic or likely pathogenic (P/LP; positive test result), variant(s) of uncertain significance (indeterminate test result), benign or likely benign (negative test result), or no variants (negative test result). Clinical signs/symptoms, age, sex, and family history data were voluntarily reported by participating clinicians and were explored for possible association with the test result. Of 1103 individuals tested, results were positive in 40 %, indeterminate in 5 %, and negative in 55 %. Most positive tests were monoallelic P/LP variants (n = 413). The most frequently identified P/LP variants were c.1133A > T/p.Asp378Val (n = 61), c.571G > A/p.Glu191Lys (n = 47), and c.1250A > G/p.Asn417Ser (n = 44). In total, 23 novel ALPL variants were identified, of which 43 % were P/LP and the most frequent type was missense (74 %). Among the 25 % of participants for whom signs/symptoms were reported, a significant association was observed for those with a family history of HPP signs/symptoms and a positive test result. These data contribute important information on the likelihood of disease-causing ALPL variants in individuals with clinical signs/symptoms of HPP, the importance of family history in HPP testing, distribution of ALPL variants, and identification of novel ALPL variants.

Dental manifestations of hypophosphatasia: translational and clinical advances

Hypophosphatasia (HPP) is an inherited error in metabolism resulting from loss-of-function variants in the ALPL gene, which encodes tissue-nonspecific alkaline phosphatase (TNAP). TNAP plays a crucial role in biomineralization of bones and teeth, in part by reducing levels of inorganic pyrophosphate (PPi), an inhibitor of biomineralization. HPP onset in childhood contributes to rickets, including growth plate defects and impaired growth. In adulthood, osteomalacia from HPP contributes to increased fracture risk. HPP also affects oral health. The dentoalveolar complex, that is, the tooth and supporting connective tissues of the surrounding periodontia, include 4 unique hard tissues: enamel, dentin, cementum, and alveolar bone, and all can be affected by HPP. Premature tooth loss of fully rooted teeth is pathognomonic for HPP. Patients with HPP often have complex oral health issues that require multidisciplinary dental care, potentially involving general or pediatric dentists, periodontists, prosthodontists, and orthodontists. The scientific literature to date has relatively few reports on dental care of individuals with HPP. Animal models to study HPP included global Alpl knockout mice, Alpl mutation knock-in mice, and mice with tissue-specific conditional Alpl ablation, allowing for new studies on pathological mechanisms and treatment effects in dental and skeletal tissues. Enzyme replacement therapy (ERT) in the form of injected, recombinant mineralized tissue-targeted TNAP has been available for nearly a decade and changed the prognosis for those with HPP. However, effects of ERT on dental tissues remain poorly defined and limitations of the current ERT have prompted exploration of gene therapy approaches to treat HPP. Preclinical gene therapy studies are promising and may contribute to improved oral health in HPP.

New insights into the landscape of ALPL gene variants in patients with hypophosphatasia from the Global HPP Registry

Hypophosphatasia (HPP) is a rare, inherited metabolic disease characterized by low tissue-nonspecific alkaline phosphatase activity due to ALPL gene variants. We describe ALPL variants from the observational, prospective, multinational Global HPP Registry. Inclusion in the analysis required a diagnosis of HPP, low serum ALP activity, and ≥1 ALPL variant. Of 1176 patients enrolled as of September 2022, 814 met inclusion criteria in Europe (48.9%), North America (36.7%), Japan (10.2%), Australia (2.6%), and elsewhere (1.6%). Most patients (74.7%) had 1 ALPL variant; 25.3% had ≥2 variants. Nearly all patients (95.6%) had known disease-causing variants; 4.4% had variants of uncertain significance. Disease-causing variants were predominantly missense (770/1556 alleles). The most common variants were c.571G>A (102/1628 alleles), c.1250A>G (66/1628 alleles), and c.1559del (61/1628 alleles). Variant profiles were generally consistent, except in Japan, where a higher proportion of patients (68.7%) had ≥2 ALPL variants, likely because more had disease onset before age 6 months (53.0% vs. 10.1%-23.1% elsewhere). Frameshift mutations (61/164 alleles) and inframe deletions (7/164 alleles) were more common in Japan. Twenty-three novel variants were discovered, each in a single geographic region, predominantly Europe. Analyses confirmed previously known ALPL variants, identified novel variants, and characterized geographic variation in frequency and type of ALPL variants in a large population.

The Benefits of Whole-Exome Sequencing in the Differential Diagnosis of Hypophosphatasia

Hypophosphatasia (HPP) is a rare inherited disorder characterized by the decreased activity of tissue-nonspecific alkaline phosphatase (TNSALP), caused by mutations in the ALPL gene. The aim of this study was to conduct differential diagnostics in HPP patients using whole-exome sequencing (WES). The medical records of HPP patients and the genetic testing of the ALPL gene were reviewed. Seven patients were recruited and underwent WES using the Illumina or MGI sequencing platforms. All of the exome samples were matched onto a GRCh38.p13 reference genome assembly by using the Genome Analysis ToolKit (GATK) and the BWA MEM read aligner. We present the clinical and molecular findings of the seven patients referred for genetic analyses due to a clinical and biochemical suspicion of HPP. In two patients out of three (with identified heterozygous variants in the ALPL gene), we also identified c.682T>A in exon 3 of the WNT10A gene and c.3470del in exon 23 of the SMC1A gene variants for the first time. In four patients, variants in the ALPL gene were not detected, but WES allowed us to identify for the first time rare variants (c.5651A>C in exon 36 of the TRIO gene, c.880T>G in exon 6 of the TRPV4 gene, c.32078-1G>T in intron 159 of the TTN gene, c.47720_47721del in exon 235 of the TTN gene, and c.1946G>A in exon 15 of the SLC5A1 gene) and to conduct differential diagnostics with HPP. Using WES, for the first time, we demonstrate the possibility of early differential diagnostics in HPP patients with other rare genetic diseases.

Safety, pharmacokinetics, and pharmacodynamics of efzimfotase alfa, a second-generation enzyme replacement therapy: phase 1, dose-escalation study in adults with hypophosphatasia

Hypophosphatasia (HPP) is a rare, inherited metabolic disease caused by deficient activity of tissue-nonspecific alkaline phosphatase (TNSALP). Efzimfotase alfa (ALXN1850) is a second-generation TNSALP enzyme replacement therapy in development for HPP. This first-in-human open-label, dose-escalating phase 1 trial evaluated efzimfotase alfa safety, tolerability, pharmacokinetics, pharmacodynamics, and immunogenicity. Fifteen adults (5/cohort) with HPP received efzimfotase alfa in doses of 15 mg (cohort 1), 45 mg (cohort 2), or 90 mg (cohort 3) as one intravenous (i.v.) dose followed by 3 weekly subcutaneous (s.c.) doses. The primary objective was to assess safety and tolerability. Secondary objectives included pharmacokinetics, pharmacodynamics of ALP substrates known to be biomarkers of disease (inorganic pyrophosphate [PPi] and pyridoxal 5'-phosphate [PLP]) and immunogenicity. Treatment-emergent adverse events (TEAEs) occurred in 12 (80%) participants. Eight (53%) participants had injection site reactions (ISRs), observed after 10 of 41 (24%) s.c. injections. Most ISR TEAEs were mild and resolved within 1-2 d. Peak and total exposures of efzimfotase alfa increased in a greater-than-dose proportional manner over the range of 15-90 mg after i.v. and s.c. dosing. The arithmetic mean elimination half-life was approximately 6 d; absolute bioavailability was 28.6%-36.8% over the s.c. dose range of 15-90 mg. Dose-dependent reductions in plasma concentrations of PPi and PLP relative to baseline reached nadir in the first week after i.v. dosing and were sustained for 3-4 wk after the last s.c. dose. Four (27%) participants tested positive for antidrug antibodies (ADAs), 3 of whom were ADA positive before the first dose of efzimfotase alfa. ADAs had no apparent effect on efzimfotase alfa pharmacokinetics/pharmacodynamics. No participants had neutralizing antibodies. Efzimfotase alfa demonstrated acceptable safety, tolerability, and pharmacokinetic profiles and was associated with sustained reductions in biomarkers of disease in adults with HPP, supporting further evaluation in adult and pediatric patients. Registration: ClinicalTrials.gov NCT04980248 (https://clinicaltrials.gov/study/NCT04980248).

Metabolic Bone Disease: An Overview

Metabolic bone diseases are a heterogenous group of conditions that all result in aberrant bone mineral homeostasis with resulting skeletal disease. The underlying causes are variable, ranging from nutritional deficiencies to pathogenic variants in skeletal genes. To properly diagnose and treat these conditions, a clinician needs to understand bone metabolism as well as recognize the signs of disease in a patient. This review will focus on three relatively common metabolic bone diseases (osteogenesis imperfecta, hypophosphatasia, and X-linked hypophosphatemic rickets) that are caused by genetic variants, not by nutritional deficiency. As molecular DNA sequencing has improved, the scientific community has been able to better understand the genetic basis of these conditions and create sophisticated medical treatments based on the genetic deficiency.

Genetic polymorphisms and their association with neurobiological and psychological factors in anorexia nervosa: a systematic review

Background and aims

Anorexia nervosa (AN) is a complex neuropsychiatric disorder. This systematic review synthesizes evidence from diverse studies to assess and investigate the association between gene polymorphisms and psychological and neurobiological factors in patients with AN.

Methods

A systematic search across PubMed, PsycINFO, Scopus, and Web of Science databases, along with manual searching, was conducted. The review protocol was approved by PROSPERO (CRD42023452548). Out of 1,250 articles, 11 met the inclusion criteria. The quality of eligible articles was assessed using the Newcastle-Ottawa Scale (NOS) tool. The systematic review followed the PRISMA guidelines.

Results

The serotoninergic system, particularly the 5-HTTLPR polymorphism, is consistently linked to altered connectivity in the ventral attention network, impaired inhibitory control, and increased susceptibility to AN. The 5-HTTLPR polymorphism affects reward processing, motivation, reasoning, working memory, inhibition, and outcome prediction in patients with AN. The dopaminergic system, involving genes like COMT, DRD2, DRD3, and DAT1, regulates reward, motivation, and decision-making. Genetic variations in these dopaminergic genes are associated with psychological manifestations and clinical severity in patients with AN. Across populations, the Val66Met polymorphism in the BDNF gene influences personality traits, eating behaviors, and emotional responses. Genes like OXTR, TFAP2B, and KCTD15 are linked to social cognition, emotional processing, body image concerns, and personality dimensions in patients with AN.

Conclusion

There was an association linking multiple genes to the susceptibly and/or severity of AN. This genetic factor contributes to the complexity of AN and leads to higher diversity of its clinical presentation. Therefore, conducting more extensive research to elucidate the underlying mechanisms of anorexia nervosa pathology is imperative for advancing our understanding and potentially developing targeted therapeutic interventions for the disorder.Systematic review registration: [https://clinicaltrials.gov/], identifier [CRD42023452548].

Pyridoxine challenge reflects pediatric hypophosphatasia severity and thereby examines tissue-nonspecific alkaline phosphatase's role in vitamin B6 metabolism

Alkaline phosphatase (ALP) is detected in most human tissues. However, ALP activity is routinely assayed using high concentrations of artificial colorimetric substrates in phosphate-free laboratory buffers at lethal pH. Hypophosphatasia (HPP) is the inborn-error-of-metabolism caused by loss-of-function mutation(s) of the ALPL gene that encodes the ALP isoenzyme expressed in bone, liver, kidney, and elsewhere and is therefore designated "tissue-nonspecific" ALP (TNSALP). Consequently, HPP harbors clues concerning the biological function of this phosphohydrolase that is anchored onto the surface of cells. The biochemical signature of HPP features low serum ALP activity (hypophosphatasemia) together with elevated plasma levels of three natural substrates of TNSALP: i) phosphoethanolamine (PEA), a component of the linkage apparatus that binds ALPs and other proteins to the plasma membrane surface; ii) inorganic pyrophosphate (PPi), an inhibitor of bone and tooth mineralization; and iii) pyridoxal 5'-phosphate (PLP), the principal circulating vitameric form of vitamin B6 (B6). Autosomal dominant and autosomal recessive inheritance involving several hundred ALPL mutations underlies the remarkably broad-ranging expressivity of HPP featuring tooth loss often with muscle weakness and rickets or osteomalacia. Thus, HPP associates the "bone" isoform of TNSALP with biomineralization, whereas the physiological role of the "liver", "kidney", and other isoforms of TNSALP remains uncertain. Herein, to examine HPP's broad-ranging severity and the function of TNSALP, we administered an oral challenge of pyridoxine (PN) hydrochloride to 116 children with HPP. We assayed both pre- and post-challenge serum ALP activity and plasma levels of PLP, the B6 degradation product pyridoxic acid (PA), and the B6 vitamer pyridoxal (PL) that can enter cells. Responses were validated by PN challenge of 14 healthy adults and 19 children with metabolic bone diseases other than HPP. HPP severity was assessed using our HPP clinical nosology and patient height Z-scores. PN challenge of all study groups did not alter serum ALP activity in our clinical laboratory. In HPP, both the post-challenge PLP level and the PLP increment correlated (Ps < 0.0001) with the clinical nosology and height Z-scores (Rs = +0.6009 and + 0.4886, and Rs = -0.4846 and - 0.5002, respectively). In contrast, the plasma levels and increments of PA and PL from the PN challenge became less pronounced with HPP severity. We discuss how our findings suggest extraskeletal TNSALP primarily conditioned the PN challenge responses, and explain why they caution against overzealous B6 supplementation of HPP.

Clinical and molecular description of the first Italian cohort of 33 subjects with hypophosphatasia

Introduction

Hypophosphatasia (HPP) is a rare genetic disease caused by inactivating variants of the ALPL gene. Few data are available on the clinical presentation in Italy and/or on Italian HPP surveys.

Methods

There were 30 suspected HPP patients recruited from different Italian tertiary cares. Biological samples and related clinical, biochemical, and anamnestic data were collected and the ALPL gene sequenced. Search for large genomic deletions at the ALPL locus (1p36) was done. Phylogenetic conservation and modeling were applied to infer the effect of the variants on the protein structure.

Results

There were 21 ALPL variants and one large genomic deletion found in 20 out of 30 patients. Unexpectedly, NGS-driven differential diagnosis allowed uncovering three hidden additional HPP cases, for a total of 33 HPP subjects. Eight out of 24 coding variants were novel and classified as "pathogenic", "likely pathogenic", and "variants of uncertain significance". Bioinformatic analysis confirmed that all the variants strongly destabilize the homodimer structure. There were 10 cases with low ALP and high VitB6 that resulted negative to genetic testing, whereas two positive cases have an unexpected normal ALP value. No association was evident with other biochemical/clinical parameters.

Discussion

We present the survey of HPP Italian patients with the highest ALPL mutation rate so far reported and confirm the complexity of a prompt recognition of the syndrome, mostly for HPP in adults. Low ALP and high VitB6 values are mandatory for the genetic screening, this latter remaining the gold standard not only to confirm the clinical diagnosis but also to make differential diagnosis, to identify carriers, to avoid likely dangerous therapy in unrecognized cases.

Pediatric reference values of alkaline phosphatase: Analysis from a German population-based cohort and influence of anthropometric and blood parameters

BACKGROUND

Due to different growth and metabolic processes, reference values of alkaline phosphatase (AP) for children aged 3 month to 18 years are dependent on age and sex. They are not constant and differ from those of adults due to the growth processes taking place. Accordingly, reference levels of AP continuous across these ages were generated for boys and girls based on of a large German health- and population-based study, LIFE Child. We considered AP at different growth and Tanner stages and additionally its association with other anthropometric parameters. The association between AP and BMI was of particulary great interest due to controversial literature on this topic. The role of AP in liver metabolism was investigated by examining ALAT, ASAT, and GGT.

METHODS

3976 healthy children (12,093 visits) were included from the LIFE Child study from 2011 to 2020. The subjects´ age ranged from 3 months to 18 years. Serum samples from 3704 subjects (10,272 cases, 1952 boys and 1753 girls) were analysed for AP after applying specific exclusion criteria. After calculating of reference percentiles, associations between AP and height-SDS, growth velocity, BMI-SDS, Tanner stage and the liver enzymes ALAT, ASAT and GGT were examined via linear regression models.

RESULTS

In the continuous reference levels, AP showed a first peak during the first year of life, followed by a plateau at a lower level until the start of puberty. In girls, AP increased beginning at the age 8, with a peak around 11 years, in boys beginning at the age 9, with a peak around age 13. Afterwards, AP values decreased continuously until age 18. In Tanner stages 1 and 2, AP levels did not differ between the two sexes. We found a strong positive association between AP-SDS and BMI-SDS. We also observed a significantly positive association between AP-SDS and height-SDS, which was stronger in boys than in girls. We found different intensities in the associations of AP with growth velocity depending on age group and sex. Furthermore, we found a significantly positive association between ALAT and AP in girls but not in boys, whereas ASAT-SDS and GGT-SDS were significantly positively associated with AP-SDS in both sexes.

CONCLUSION

Sex and age, but also BMI may act as confounding factors for AP reference ranges. Our data confirm the remarkable association between AP and growth velocity (or height-SDS, respectively) during infancy and puberty. In addition, we were able to specify the associations between AP and ALAT, ASAT, and GGT and their differences in both sexes. These relations should be considered when evaluating liver and bone metabolism markers, especially in infancy.

Clinical and Genetic Characteristics of Hypophosphatasia in Chinese Adults

Hypophosphatasia (HPP) is an inherited disease caused by ALPL mutation, resulting in decreased alkaline phosphatase (ALP) activity and damage to bone and tooth mineralization. The clinical symptoms of adult HPP are variable, making diagnosis challenging. This study aims to clarify the clinical and genetic characteristics of HPP in Chinese adults. There were 19 patients, including 1 with childhood-onset and 18 with adult-onset HPP. The median age was 62 (32-74) years and 16 female patients were involved. Common symptoms included musculoskeletal symptoms (12/19), dental problems (8/19), fractures (7/19), and fatigue (6/19). Nine patients (47.4%) were misdiagnosed with osteoporosis and six received anti-resorptive treatment. The average serum ALP level was 29.1 (14-53) U/L and 94.7% (18/19) of patients had ALP levels below 40 U/L. Genetic analysis found 14 ALPL mutations, including three novel mutations-c.511C>G (p.His171Ala), c.782C>A (p.Pro261Gln), and 1399A>G (p.Met467Val). The symptoms of two patients with compound heterozygous mutations were more severe than those with heterozygous mutations. Our study summarized the clinical characteristics of adult HPP patients in the Chinese population, expanded the spectrum of pathogenic mutations, and deepened clinicians' understanding of this neglected disease.

Effects of Infantile Hypophosphatasia on Human Dental Tissue

Hypophosphatasia (HPP) is an inherited, systemic disorder, caused by loss-of-function variants of the ALPL gene encoding the enzyme tissue non-specific alkaline phosphatase (TNSALP). HPP is characterized by low serum TNSALP concentrations associated with defective bone mineralization and increased fracture risk. Dental manifestations have been reported as the exclusive feature (odontohypophosphatasia) and in combination with skeletal complications. Enzyme replacement therapy (asfotase alfa) has been shown to improve respiratory insufficiency and skeletal complications in HPP patients, while its effects on dental status have been understudied to date. In this study, quantitative backscattered electron imaging (qBEI) and histological analysis were performed on teeth from two patients with infantile HPP before and during asfotase alfa treatment and compared to matched healthy control teeth. qBEI and histological methods revealed varying mineralization patterns in cementum and dentin with lower mineralization in HPP. Furthermore, a significantly higher repair cementum thickness was observed in HPP compared to control teeth. Comparison before and during treatment showed minor improvements in mineralization and histological parameters in the patient when normalized to matched control teeth. HPP induces heterogeneous effects on mineralization and morphology of the dental status. Short treatment with asfotase alfa slightly affects mineralization in cementum and dentin. Despite HPP being a rare disease, its mild form occurs at higher prevalence. This study is of high clinical relevance as it expands our knowledge of HPP and dental involvement. Furthermore, it contributes to the understanding of dental tissue treatment, which has hardly been studied so far.

Gene Therapy Using Recombinant AAV Type 8 Vector Encoding TNAP-D10 Improves the Skeletal Phenotypes in Murine Models of Osteomalacia

Hypophosphatasia (HPP), caused by loss-of-function mutations in the ALPL gene encoding tissue-nonspecific alkaline phosphatase (TNAP), is characterized by skeletal and dental hypomineralization that can vary in severity from life-threatening to milder manifestations only in adulthood. PHOSPHO1 deficiency leads to early-onset scoliosis, osteomalacia, and fractures that mimic pseudo-HPP. Asfotase alfa, a life-saving enzyme replacement therapy approved for pediatric-onset HPP, requires subcutaneous injections 3 to 6 times per week. We recently showed that a single injection of an adeno-associated virus vector serotype 8 harboring TNAP-D10 (AAV8-TNAP-D10) effectively prevented skeletal disease and prolonged life in Alpl -/- mice phenocopying infantile HPP. Here, we aimed to determine the efficacy of AAV8-TNAP-D10 in improving the skeletal and dental phenotype in the Alpl Prx1/Prx1 and Phospho1 -/- mouse models of late-onset (adult) HPP and pseudo-HPP, respectively. A single dose of 3 × 1011 vector genomes per body (vg/b) was injected intramuscularly into 8-week-old Alpl Prx1/Prx1 and wild-type (WT) littermates, or into 3-day-old Phospho1 -/- and WT mice, and treatment efficacy was evaluated after 60 days for late-onset HPP mice and after 90 days for Phospho1 -/- mice. Biochemical analysis showed sustained serum alkaline phosphatase activity and reduced plasma PPi levels, and radiographic images, micro-computed tomography (micro-CT) analysis, and hematoxylin and eosin (H&E) staining showed improvements in the long bones in the late-onset HPP mice and corrected scoliosis in the Phospho1 -/- mice. Micro-CT analysis of the dentoalveolar complex did not reveal significant changes in the phenotype of late-onset HPP and pseudo-HPP models. Moreover, alizarin red staining analysis showed that AAV8-TNAP-D10 treatment did not promote ectopic calcification of soft organs in adult HPP mice after 60 days of treatment, even after inducing chronic kidney disease. Overall, the AAV8-TNAP-D10 treatment improved the skeletal phenotype in both the adult HPP and pseudo-HPP mouse models. This preclinical study will contribute to the advancement of gene therapy for the improvement of skeletal disease in patients with heritable forms of osteomalacia. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Proposing a clinical algorithm for better diagnosis of hypophosphatasia in resource-limiting situations

Early diagnosis of hypophosphatasia (HPP) is challenging. Here, we propose to broaden the diagnostic criteria of HPP by reviewing published data on BMD and fractures in HPP patients. Non-osteoporotic fractures and higher than normal lumbar BMD were recurrent in HPP patients and could be included as diagnostic criteria. HPP is a genetic disorder caused by autosomal recessive or dominant loss-of-function mutations in the ALPL gene that encodes for tissue-nonspecific alkaline phosphatase (TNSALP). Expressive genetic heterogeneity and varying severity of TNSALP deficiency lead to a wide-ranging presentation of skeletal diseases at different ages that coupled with HPP's rarity and limitation of biochemical and mutational studies present serious hurdles to early diagnosis and management of HPP. To widen the scope of HPP diagnosis, we assessed the possibility of areal bone mineral density (BMD) as an additional clinical feature of this disease. PubMed, Web of Science, and ScienceDirect were searched with the following keywords: ("Hypophosphatasia OR HPP") AND ("Bone Mineral Density OR BMD") AND "Human". Studies and case reports of subjects with age ≥ 18 years and having BMD data were included. We pooled data from 25 publications comprising 356 subjects (90 males, 266 females). Only four studies had a control group. Biochemical hallmarks, pyridoxal 5'-phosphate (PLP) and phosphoethanolamine (PEA), were reported in fifteen and six studies, respectively. Twenty studies reported genetic data, nineteen studies reported non-vertebral fractures, all studies reported lumbar spine (LS) BMD, and nineteen reported non-vertebral BMD. Higher than normal and normal BMD at LS were reported in three and two studies, respectively. There was marked heterogeneity in BMD at the non-vertebral sites. Higher than normal or normal LS BMD in an adult with minimal or insufficient fractures, pseudofractures, non-healing fractures, fragility fractures, and stress fractures may be included in the diagnostic protocol of HPP. However, genetic testing is recommended for a definitive diagnosis.

Biochemical and clinical manifestations in adults with hypophosphatasia: a national cross-sectional study

Hypophosphatasia (HPP) is a rare disease affecting bone mineralization. Adults with HPP have an increased occurrence of low-energy fractures, which cannot be explained by reduced bone mass assessed by dual energy X-ray absorptiometry. The bone phenotype in adults with HPP requires further studies investigating bone strength and bone structural parameters.

INTRODUCTION

Hypophosphatasia (HPP) is a rare inherited disorder of bone and mineral metabolism, characterized by broad-ranging clinical manifestations and severity. However, studies investigating the clinical spectrum in adults with HPP compared to a control group are scarce. The aim of this study was to evaluate biochemical and clinical characteristics as well as bone health in a Danish cohort of adults with HPP.

METHODS

We conducted a cross-sectional study assessing biochemical parameters, fracture prevalence, bone mineral density (BMD), bone turnover markers, physical performance and pain characteristics in 40 adults with HPP and 40 sex-, age-, BMI- and menopausal status-matched healthy controls.

RESULTS

Patients with HPP had a significantly higher prevalence of non-vertebral, low-energy fractures (p =  < 0.001). BMD at the lumbar spine, total hip, femoral neck, forearm and whole body did not differ between the groups. Low levels of the bone-specific alkaline phosphatase correlated significantly with higher BMD at the lumbar spine and femoral neck in both groups. The bone formation marker N-terminal propeptide of type 1 procollagen was significantly lower in patients with HPP than healthy controls (p = 0.006). Adults with HPP had significantly reduced walking capability (p =  < 0.001) and lower body strength (p =  < 0.001). Chronic pain was significantly more prevalent in adults with HPP than the control group (p = 0.029).

CONCLUSIONS

The increased occurrence of low-energy fractures in adults with HPP is not explained by low BMD. Adults with HPP have reduced physical performance when compared with healthy controls.

Impact of discontinuing 5 years of enzyme replacement treatment in a cohort of 6 adults with hypophosphatasia: A case series

Asfotase alfa is a human recombinant enzyme replacement therapy for hypophosphatasia. We describe 6 adults who were treated with asfotase alfa for 61–68 months in a clinical trial (NCT01163149), after which asfotase alfa was discontinued for 15–48 months. The patients experienced clinical deterioration and, when treatment was restarted, showed improvement. Patients with hypophosphatasia should be closely monitored if asfotase alfa is stopped as clinical decline is likely. Clinical practice guidelines are needed.

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Discontinuation of asfotase alfa results in deterioration of hypophosphatasia.

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Discontinuation of asfotase alfa results in worse pain scores.

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Reinitiation of asfotase alfa is associated with clinical and biochemical improvement.

Clinical and Genetic Characteristics of Pediatric Patients with Hypophosphatasia in the Russian Population

(1) Hypophosphatasia (HPP) is a rare inherited disease caused by mutations (pathogenic variants) in the ALPL gene which encodes tissue-nonspecific alkaline phosphatase (TNSALP). HPP is characterized by impaired bone mineral metabolism due to the low enzymatic activity of TNSALP. Knowledge about the structure of the gene and the features and functions of various ALPL gene variants, taking into account population specificity, gives an understanding of the hereditary nature of the disease, and contributes to the diagnosis, prevention, and treatment of the disease. The purpose of the study was to describe the spectrum and analyze the functional features of the ALPL gene variants, considering various HPP subtypes and clinical symptoms in Russian children. (2) From 2014−2021, the study included the blood samples obtained from 1612 patients with reduced alkaline phosphatase activity. The patients underwent an examination with an assessment of their clinical symptoms and biochemical levels of TNSALP. DNA was isolated from dried blood spots (DBSs) or blood from the patients to search for mutations in the exons of the ALPL gene using Sanger sequencing. The PCR products were sequenced using a reagent BigDye Terminator 3.1 kit (Applied Biosystems). Statistical analysis was performed using the GraphPad Prism 8.01 software. (3) The most common clinical symptoms in Russian patients with HPP and two of its variants (n = 22) were bone disorders (75%), hypomyotonia (50%), and respiratory failure (50%). The heterozygous carriage of the causal variants of the ALPL gene was detected in 225 patients. A total of 2 variants were found in 27 patients. In this group (n = 27), we identified 28 unique variants of the ALPL gene, of which 75.0% were missense, 17.9% were frameshift, 3.6% were splicing variants, and 3.6% were duplications. A total of 39.3% (11/28) of the variants were pathogenic, with two variants being probably pathogenic, and 15 variants had unknown clinical significance (VUS). Among the VUS group, 28.6% of the variants (7/28) were discovered by us for the first time. The most common variants were c.571G > A (p.Glu191Lys) and c.1171del (Arg391Valfs*12), with frequencies of 48.2% (13/28) and 11% (3/28), respectively. It was found that the frequency of nonsense variants of the ALPL gene was higher (p < 0.0001) in patients with the perinatal form compared to the infantile and childhood forms of HPP. Additionally, the number of homozygotes in patients with the perinatal form exceeded (p < 0.01) the frequencies of these genotypes in children with infantile and childhood forms of HPP. On the contrary, the frequencies of the compound-heterozygous and heterozygous genotypes were higher (p < 0.01) in patients with infantile childhood HPP than in perinatal HPP. In the perinatal form, residual TNSALP activity was lower (p < 0.0005) in comparison to the infantile and childhood (p < 0.05) forms of HPP. At the same time, patients with the heterozygous and compound-heterozygous genotypes (mainly missense variants) of the ALPL gene had greater residual activity (of the TNSALP protein) regarding those homozygous patients who were carriers of the nonsense variants (deletions and duplications) of the ALPL gene. Residual TNSALP activity was lower (p < 0.0001) in patients with pathogenic variants encoding the amino acids from the active site and the calcium and crown domains in comparison with the nonspecific region of the protein.

Dentoalveolar Defects of Hypophosphatasia are Recapitulated in a Sheep Knock-In Model

Hypophosphatasia (HPP) is the inherited error-of-metabolism caused by mutations in ALPL, reducing the function of tissue-nonspecific alkaline phosphatase (TNAP/TNALP/TNSALP). HPP is characterized by defective skeletal and dental mineralization and is categorized into several clinical subtypes based on age of onset and severity of manifestations, though premature tooth loss from acellular cementum defects is common across most HPP subtypes. Genotype-phenotype associations and mechanisms underlying musculoskeletal, dental, and other defects remain poorly characterized. Murine models that have provided significant insights into HPP pathophysiology also carry limitations including monophyodont dentition, lack of osteonal remodeling of cortical bone, and differing patterns of skeletal growth. To address this, we generated the first gene-edited large-animal model of HPP in sheep via CRISPR/Cas9-mediated knock-in of a missense mutation (c.1077C>G; p.I359M) associated with skeletal and dental manifestations in humans. We hypothesized that this HPP sheep model would recapitulate the human dentoalveolar manifestations of HPP. Compared to wild-type (WT), compound heterozygous (cHet) sheep with one null allele and the other with the targeted mutant allele exhibited the most severe alveolar bone, acellular cementum, and dentin hypomineralization defects. Sheep homozygous for the mutant allele (Hom) showed alveolar bone and hypomineralization effects and trends in dentin and cementum, whereas sheep heterozygous (Het) for the mutation did not exhibit significant effects. Important insights gained include existence of early alveolar bone defects that may contribute to tooth loss in HPP, observation of severe mantle dentin hypomineralization in an HPP animal model, association of cementum hypoplasia with genotype, and correlation of dentoalveolar defects with alkaline phosphatase (ALP) levels. The sheep model of HPP faithfully recapitulated dentoalveolar defects reported in individuals with HPP, providing a new translational model for studies into etiopathology and novel therapies of this disorder, as well as proof-of-principle that genetically engineered large sheep models can replicate human dentoalveolar disorders. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Urine phosphoethanolamine is a specific biomarker for hypophosphatasia in adults

OBJECTIVES

We investigated the utility of urine phosphoethanolamine (PEA) as a marker to aid in diagnosing and/or confirming hypophosphatasia (HPP) in adults and for monitoring patients on enzyme replacement therapy (ERT).

METHODS

Data was collected from seventy-eight adults who were referred to the Vanderbilt Program for Metabolic Bone Disease for evaluation of a possible or confirmatory HPP diagnosis between July 2014 through December 2019. Fifty-nine patients were diagnosed with HPP and nineteen were excluded from a diagnosis of HPP. The urine PEA results of those patients with a confirmed diagnosis of HPP and those patients with a diagnosis of HPP excluded were captured and compared to other laboratory and clinical parameters consistent with HPP, including alkaline phosphatase (ALP) activity, plasma pyridoxal 5'-phosphate (PLP), the presence of musculoskeletal abnormalities, and genetic testing for pathogenic mutations in ALPL.

RESULTS

Initial urine PEA values in patients in our HPP cohort and not on ERT were significantly higher (median = 150.0 nmol/mg creatinine, IQR = 82.0-202.0) compared patients in our HPP negative group (median 18.0 nmol/mg creatinine, IQR = 14.0-30.0, p < 0.0001) and higher than patients on ERT (median 65.0 nmol/mg creatinine, IQR = 45.3-79.8). Patients who began ERT had a decline in urine PEA levels after treatment with a mean decrease of 68.1 %. Plasma ALP levels were significantly lower in the group of patients with HPP and not on ERT group (median = 24.0 U/L, IQR = 15.0-29.50) compared to the patients without HPP (median = 45.50 U/L, IQR = 34.0-62.0;) and plasma PLP levels were significantly higher in the HPP non-ERT group (median = 284.0 nmol/L, IQR = 141.0-469.4) compared to the patients without HPP (median = 97.5 nmol/L, IQR = 43.7-206.0;). The area under the curve (AUC) of urine PEA, ALP, and PLP to distinguish between HPP and non-HPP patients is 0.968, 0.927 and 0.781, respectively, in our cohort. Urine PEA had 100 % specificity (95 % CI of 83.2 % to 100.0 %) for diagnosing HPP at a value >53.50 nmol/mg creatinine with a sensitivity of 88.4 %; 95%CI 75.5 to 94.9 %. ALP had a 100 % specificity (95 % CI of 82.4 % to 100.0 %) for diagnosing HPP at a value <30.5 U/L with a sensitivity of 77.2 %; (95%CI 64.8 to 86.2 %). PLP had a 100 % specificity (95 % CI of 81.6 % to 100.0 %) for diagnosing HPP at a value >436 nmol/L with a sensitivity of 26.9 %; (95%CI 16.8 to 40.3 %). The most common pathogenic or likely pathogenic mutations in our cohort were c.1250A>G (p.Asn417Ser), c.1133A>T (p.Asp378Val), c.881A>C (p.Asp294Ala), c.1171C>T (p.Arg391Cys), and c.571G>A, (p.Glu191Lys).

CONCLUSIONS

Urine PEA is a promising diagnostic and confirmatory marker for HPP in patients undergoing investigation for HPP. Urine PEA also has potential use as a marker to monitor ERT compliance. Future studies are necessary to evaluate the association between PEA levels and clinical outcomes.

Early-Onset Osteoporosis: Rare Monogenic Forms Elucidate the Complexity of Disease Pathogenesis Beyond Type I Collagen

Early-onset osteoporosis (EOOP), characterized by low bone mineral density (BMD) and fractures, affects children, premenopausal women and men aged <50 years. EOOP may be secondary to a chronic illness, long-term medication, nutritional deficiencies, etc. If no such cause is identified, EOOP is regarded primary and may then be related to rare variants in genes playing a pivotal role in bone homeostasis. If the cause remains unknown, EOOP is considered idiopathic. The scope of this review is to guide through clinical and genetic diagnostics of EOOP, summarize the present knowledge on rare monogenic forms of EOOP, and describe how analysis of bone biopsy samples can lead to a better understanding of the disease pathogenesis. The diagnostic pathway of EOOP is often complicated and extensive assessments may be needed to reliably exclude secondary causes. Due to the genetic heterogeneity and overlapping features in the various genetic forms of EOOP and other bone fragility disorders, the genetic diagnosis usually requires the use of next-generation sequencing to investigate several genes simultaneously. Recent discoveries have elucidated the complexity of disease pathogenesis both regarding genetic architecture and bone tissue-level pathology. Two rare monogenic forms of EOOP are due to defects in genes partaking in the canonical WNT pathway: LRP5 and WNT1. Variants in the genes encoding plastin-3 (PLS3) and sphingomyelin synthase 2 (SGMS2) have also been found in children and young adults with skeletal fragility. The molecular mechanisms leading from gene defects to clinical manifestations are often not fully understood. Detailed analysis of patient-derived transiliac bone biopsies gives valuable information to understand disease pathogenesis, distinguishes EOOP from other bone fragility disorders, and guides in patient management, but is not widely available in clinical settings. Despite the great advances in this field, EOOP remains an insufficiently explored entity and further research is needed to optimize diagnostic and therapeutic approaches. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Skeletal and extraskeletal disorders of biomineralization

The physiological process of biomineralization is complex and deviation from it leads to a variety of diseases. Progress in the past 10 years has enhanced understanding of the genetic, molecular and cellular pathophysiology underlying these disorders; sometimes, this knowledge has both facilitated restoration of health and clarified the very nature of biomineralization as it occurs in humans. In this Review, we consider the principal regulators of mineralization and crystallization, and how dysregulation of these processes can lead to human disease. The knowledge acquired to date and gaps still to be filled are highlighted. The disorders of mineralization discussed comprise a broad spectrum of conditions that encompass bone disorders associated with alterations of mineral quantity and quality, as well as disorders of extraskeletal mineralization (hyperphosphataemic familial tumoural calcinosis). Included are disorders of alkaline phosphatase (hypophosphatasia) and phosphate homeostasis (X-linked hypophosphataemic rickets, fluorosis, rickets and osteomalacia). Furthermore, crystallopathies are covered as well as arterial and renal calcification. This Review discusses the current knowledge of biomineralization derived from basic and clinical research and points to future studies that will lead to new therapeutic approaches for biomineralization disorders.

A spectrum of recessiveness among Mendelian disease variants in UK Biobank

Recent work has found increasing evidence of mitigated, incompletely penetrant phenotypes in heterozygous carriers of recessive Mendelian disease variants. We leveraged whole-exome imputation within the full UK Biobank cohort (n ∼ 500K) to extend such analyses to 3,475 rare variants curated from ClinVar and OMIM. Testing these variants for association with 58 quantitative traits yielded 102 significant associations involving variants previously implicated in 34 different diseases. Notable examples included a POR missense variant implicated in Antley-Bixler syndrome that associated with a 1.76 (SE 0.27) cm increase in height and an ABCA3 missense variant implicated in interstitial lung disease that associated with reduced FEV1/FVC ratio. Association analyses with 1,134 disease traits yielded five additional variant-disease associations. We also observed contrasting levels of recessiveness between two more-common, classical Mendelian diseases. Carriers of cystic fibrosis variants exhibited increased risk of several mitigated disease phenotypes, whereas carriers of spinal muscular atrophy alleles showed no evidence of altered phenotypes. Incomplete penetrance of cystic fibrosis carrier phenotypes did not appear to be mediated by common allelic variation on the functional haplotype. Our results show that many disease-associated recessive variants can produce mitigated phenotypes in heterozygous carriers and motivate further work exploring penetrance mechanisms.

Bone-microarchitecture and bone-strength in a sample of adults with hypophosphatasia and a matched reference population assessed by HR-pQCT and impact microindentation

BACKGROUND

Hypophosphatasia (HPP) is an autosomal recessive or dominate disease affecting bone mineralization, and adults with HPP are in risk to develop metatarsal stress fractures and femoral pseudofractures. Given to the scarce data on the bone quality and its association to the fracture risk in adults with HPP, this study aimed to evaluate bone turnover, bone strength and structure in adults with HPP.

METHODS

In this cross-sectional study, we included 14 adults with genetically verified HPP and 14 sex-, age-, BMI-, and menopausal status-matched reference individuals. We analyzed bone turnover markers, and measured bone material strength index (BMSi) by impact microindentation. Bone geometry, volumetric density and bone microarchitecture as well as failure load at the distal radius and tibia were evaluated using a second-generation high-resolution peripheral quantitative computed tomography system.

RESULTS

Bone turnover markers did not differ between patients with HPP and reference individuals. BMSi did not differ between the groups (67.90 [63.75-76.00] vs 65.45 [58.43-69.55], p = 0.149). Parameters of bone geometry and volumetric density did not differ between adults with HPP and the reference group. Patients with HPP had a tendency toward higher trabecular separation (0.664 [0.613-0.724] mm vs 0.620 [0.578-0.659] mm, p = 0.054) and inhomogeneity of trabecular network (0.253 [0.235-0.283] mm vs 0.229 [0.208-0.252] mm, p = 0.056) as well as lower trabecular bone volume fraction (18.8 [16.4-22.7] % vs 22.8 [20.6-24.7] %, p = 0.054) at the distal radius. In addition, compound heterozygous adults with HPP had a significantly higher cortical porosity at the distal radius than reference individuals (1.5 [0.9-2.2] % vs 0.7 [0.6-0.7] %, p = 0.041).

CONCLUSIONS

BMSi is not reduced in adults with HPP. Increased cortical porosity may contribute to the occurrence of femoral pseudofractures in compound heterozygous adults with HPP. However, further studies investigating larger cohorts of adults with HPP using methods of bone histomorphometry are recommended to adequately assess the bone quality in adults with HPP.

Normal Mid-Gestation Fetal Ultrasonography Cannot Reliably Exclude Severe Perinatal Hypophosphatasia

INTRODUCTION

Hypophosphatasia is a systemic bone disease characterized by inhibition of bone mineralization due to mutations in the ALPL gene that results in a deficiency of tissue nonspecific alkaline phosphatase. The perinatal form is the most severe. In the past, this form was lethal, although human recombinant enzyme replacement therapy has now been developed and licensed, which improves survival. Perinatal hypophosphatasia is usually suggested on antenatal ultrasonography with undermineralization of the long bones, skull, and thoracic cavity. In the UK, antenatal ultrasonography for fetal anomalies is conducted at mid-gestation (i.e., 18-21 weeks gestational age), and if normal, no further routine scans are performed. Usually, this would identify abnormalities in bone mineralization suggestive of perinatal hypophosphatasia.

CASES

We describe 2 cases of perinatal hypophosphatasia where mid-gestation ultrasonography was normal. In the first case, where a previous pregnancy had been terminated for perinatal hypophosphatasia, third trimester ultrasonography revealed skeletal features of hypophosphatasia. In the second case, the diagnosis of perinatal hypophosphatasia was made only immediately after birth.

CONCLUSION

We conclude that serial antenatal ultrasonography or antenatal genetic testing should be considered in all pregnancies with a positive family history of hypophosphatasia, as mid-gestation ultrasonography cannot reliably exclude perinatal hypophosphatasia. This is especially important given that effective enzyme replacement therapy is now available.

Young woman with hypophosphatasia: A case report

Hypophosphatasia is a rare inherited disease defined by teeth and bone mineralization impairment leading to depletion of tissue non-specific alkaline phosphatase. We define a young woman diagnosed with hypophosphatasia (after several times alkaline phosphatase levels were low) was discovered following femoral fracture. A 30-year-old woman who presented for a history of early permanent teeth loss during the last 5 years and HPP-like symptoms in family history and bone radiograph verified bowing, deficient mineralization, and symmetrical subtrochanteric stress fractures of femurs was referred to our clinic for further management. Blood test findings defined raised phosphorus levels on two occasions at 6.2 and 5.7 mg/dl and insufficient 25-hydroxy vitamin D level. HPP early diagnosis and adequate treatment, depending on the clinical symptoms along with laboratory tests, could be effective in decreasing the suffering of the disease and side effects.

Rare Causes of Hypercalcemia

This article discusses rare causes of hypercalcemia. Hypercalcemia can rarely be associated with immobilization, genetic diseases in children such as Williams-Beuren syndrome, Hypophosphatasia, Jansen Metaphyseal Chondrodysplasia (JMC), cosmetic injection, milk-alkali syndrome (MAS), calcium sulfate beads administration, manganese intoxication, postacute kidney failure recovery, and Paget's disease.

Case Report: Variations in the ALPL Gene in Chinese Patients With Hypophosphatasia

Background: Hypophosphatasia (HPP) is an autosomal genetic disorder characterized biochemically by abnormal of bone parameters and serum alkaline phosphatase (ALP) activity as well as clinically by deficiency of teeth and bone mineralization. The clinical presentation is a continuum ranging from a prenatal lethal form with no skeletal mineralization to a mild form with late adult onset presenting with non-pathognomonic symptoms. ALP deficiency is the key to the pathogenesis of abnormal metabolism and skeletal system damage in HPP patients.

Methods: We investigated five patients with skeletal dysplasia in the clinic. Whole-exome sequencing was performed in order to aid diagnosis of the patients.

Results: Eight variants in the ALPL gene in the five unrelated Chinese patients (PA-1: c.649_650insC and c.707A > G; PA2: c.98C > T and c.707A > G; PA3: c.407G > A and c.650delTinsCTAA; PA4: c.1247G > T (homozygous); PA5: c.406C > T and c.1178A > G; NM_000478.5) were found. These variations caused two types of HPP: perinatal HPP and Odonto HPP. All cases reported in this study were autosomal recessive. Among the variants, c.1247G > T/p.Gly416Val (PA-4); c.1178A > G/p.Asn393Ser (PA-5) and c.707A > G/p.Tyr236Cys (PA-1, PA-2) have never been reported before.

Conclusion: Clinical phenotypes of perinatal HPP (PA-1,PA-2,PA-3 and PA-4) include skeletal dysplasia, shorter long bones, bowing of long bones, tetraphocomelia, abnormal posturing and abnormal bone ossification. Odonto HPP (PA-5) only presents as dental abnormality with severe dental caries and decreased ALP activity. Our study extends the pool of ALPL variants in different populations.

Biallelic variants in MESD, which encodes a WNT-signaling-related protein, in four new families with recessively inherited osteogenesis imperfecta

The bone disorder osteogenesis imperfecta (OI) is genetically heterogeneous. Most affected individuals have an autosomal dominant disorder caused by heterozygous variants in either of the type I collagen genes (COL1A1 or COL1A2). To date, two reports have linked Mesoderm Development LRP Chaperone (MESD) to autosomal recessive OI type XX. Four different biallelic pathogenic variants in MESD were shown to cause a progressively deforming phenotype, associated with recurrent fractures and oligodontia in five individuals in five families. Recently, compound heterozygosity for a frameshift predicted to lead to a premature termination codon in exon 2 of the 3-exon gene and a second frameshift in the terminal exon in MESD were detected in three stillbirths in one family with severe OI consistent with the neonatal lethal phenotype. We have identified four additional individuals from four independent families with biallelic variants in MESD: the earlier reported c.632dupA (p.Lys212Glufs∗19) and c.676C>T (p.Arg226∗)-which are associated with a severe form of OI-and one new pathogenic variant, c.603-606delTAAA (p.Asn201Lysfs∗15), which causes a neonatal lethal form of OI. MESD acts in the WNT signaling pathway, where it is thought to play a role in the folding of the WNT co-receptors low-density lipoprotein receptor-related proteins 5 and 6 (LRP5/LRP6) and in chaperoning their transit to the cell surface. Our report broadens the phenotypic and genetic spectrum of MESD-related OI, provides additional insight into the pathogenic pathways, and underscores the necessity of MESD for normal WNT signaling in bone formation.

Precision Medicine Approaches for Infantile-Onset Developmental and Epileptic Encephalopathies

Epilepsy is an etiologically heterogeneous condition; however, genetic factors are thought to play a role in most patients. For those with infantile-onset developmental and epileptic encephalopathy (DEE), a genetic diagnosis is now obtained in more than 50% of patients. There is considerable motivation to utilize these molecular diagnostic data to help guide treatment, as children with DEEs often have drug-resistant seizures as well as developmental impairment related to cerebral epileptiform activity. Precision medicine approaches have the potential to dramatically improve the quality of life for these children and their families. At present, treatment can be targeted for patients with diagnoses in many genetic causes of infantile-onset DEE, including genes encoding sodium or potassium channel subunits, tuberous sclerosis, and congenital metabolic diseases. Precision medicine may refer to more intelligent choices of conventional antiseizure medications, repurposed agents previously used for other indications, novel compounds, enzyme replacement, or gene therapy approaches. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Gene Therapy Using Adeno-Associated Virus Serotype 8 Encoding TNAP-D10 Improves the Skeletal and Dentoalveolar Phenotypes in Alpl-/- Mice

Hypophosphatasia (HPP) is caused by loss-of-function mutations in the ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP), whose deficiency results in the accumulation of extracellular inorganic pyrophosphate (PP_i ), a potent mineralization inhibitor. Skeletal and dental hypomineralization characterizes HPP, with disease severity varying from life-threatening perinatal or infantile forms to milder forms that manifest in adulthood or only affect the dentition. Enzyme replacement therapy (ERT) using mineral-targeted recombinant TNAP (Strensiq/asfotase alfa) markedly improves the life span, skeletal phenotype, motor function, and quality of life of patients with HPP, though limitations of ERT include frequent injections due to a short elimination half-life of 2.28 days and injection site reactions. We tested the efficacy of a single intramuscular administration of adeno-associated virus 8 (AAV8) encoding TNAP-D₁₀ to increase the life span and improve the skeletal and dentoalveolar phenotypes in TNAP knockout (Alpl^-/- ) mice, a murine model for severe infantile HPP. Alpl^-/- mice received 3 × 10¹¹ vector genomes/body of AAV8-TNAP-D₁₀ within 5 days postnatal (dpn). AAV8-TNAP-D₁₀ elevated serum ALP activity and suppressed plasma PP_i . Treatment extended life span of Alpl^-/- mice, and no ectopic calcifications were observed in the kidneys, aorta, coronary arteries, or brain in the 70 dpn observational window. Treated Alpl^-/- mice did not show signs of rickets, including bowing of long bones, enlargement of epiphyses, or fractures. Bone microstructure of treated Alpl^-/- mice was similar to wild type, with a few persistent small cortical and trabecular defects. Histology showed no measurable osteoid accumulation but reduced bone volume fraction in treated Alpl^-/- mice versus controls. Treated Alpl^-/- mice featured normal molar and incisor dentoalveolar tissues, with the exceptions of slightly reduced molar enamel and alveolar bone density. Histology showed the presence of cementum and normal periodontal ligament attachment. These results support gene therapy as a promising alternative to ERT for the treatment of HPP. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Hypophosphatasia: is it an underdiagnosed disease even by expert physicians?

INTRODUCTION

Hypophosphatasia (HPP) is caused by mutations in the ALPL that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (ALP). Clinical manifestations range from extreme life-threatening lethal forms to no signs or symptoms at all.

MATERIALS AND METHODS

Consecutive 30,000 outpatients and inpatients with ALP data were screened retrospectively, out of which 1000 patients were found to have low levels of ALP more than once. Then, patients were evaluated for the symptoms and signs of HPP with further biochemical and genetic analyses.

RESULTS

Thirty-seven patients who had severe musculoskeletal pain, recurrent fractures, and tooth anomalies were then screened with substrate and DNA sequencing analyses for HPP. It was determined that eight patients had variants in the ALPL gene. A total of eight different ALPL variants were identified in eight patients. The variants, namely c.244G > C (p.Gly82Arg), c.1444C > T (p.His482Tyr), c.1487A > G (p.Asn493Ser), and c.675_676insCA (p.Met226GlnfsTer52), had not been previously reported.

DISCUSSION

Considering the wide spectrum of clinical signs and symptoms, HPP should be among the differential lists of bone, muscle, and tooth abnormalities at any age.

Biochemical, clinical and genetic characteristics in adults with persistent hypophosphatasaemia; Data from an endocrinological outpatient clinic in Denmark

Background

Hypophosphatasia (HPP) is an inborn disease caused by pathogenic variants in ALPL. Low levels of alkaline phosphatase (ALP) are a biochemical hallmark of the disease. Scarce knowledge about the prevalence of HPP in Scandinavia exists, and the variable clinical presentations make diagnostics challenging. The aim of this study was to investigate the prevalence of ALPL variants as well as the clinical and biochemical features among adults with endocrinological diagnoses and persistent hypophosphatasaemia.

Methods

A biochemical database containing ALP measurements of 26,121 individuals was reviewed to identify adults above 18 years of age with persistently low levels of ALP beneath range (≤ 35 ± 2.7 U/L). ALPL genetic testing, biochemical evaluations and assessment of clinical features by a systematic questionnaire among included patients, were performed.

Results

Among 24 participants, thirteen subjects (54.2%) revealed a disease-causing variant in ALPL and reported mild clinical features of HPP, of which musculoskeletal pain was the most frequently reported (n = 9). The variant c. 571G > A; p.(Glu191Lys) was identified in six subjects, and an unreported missense variant (c.1019A > C; p.(His340Pro)) as well as a deletion of exon 2 were detected by genetic screening. Biochemical analyses showed no significant differences in ALP (p = 0.059), the bone specific alkaline phosphatase (BALP) (p = 0.056) and pyridoxal-5′-phosphate (PLP) (p = 0.085) between patients with an ALPL variant and negative genetic screening. Patients with a variant in ALPL had significantly higher PLP levels than healthy controls (p = 0.002). We observed normal ALP activity in some patients classified as mild HPP, and slightly increased levels of PLP in two subjects with normal genetic screening and four healthy controls. Among 51 patients with persistent hypophosphatasaemia, fifteen subjects (29.4%) received antiresorptive treatment. Two patients with unrecognized HPP were treated with bisphosphonates and did not show complications due to the treatment.

Conclusions

Pathogenic variants in ALPL are common among patients with endocrinological diagnoses and low ALP. Regarding diagnostics, genetic testing is necessary to identify mild HPP due to fluctuating biochemical findings. Antiresorptive treatment is a frequent reason for hypophosphatasaemia and effects of these agents in adults with a variant in ALPL and osteoporosis remain unclear and require further studies.

•

Mild HPP is overlooked among adults with endocrinological diagnoses.

•

Biochemical hallmarks of HPP can fluctuate to normal among mild forms.

•

Genetic screening of ALPL including MLPA is necessary to identify mild HPP.

•

Musculoskeletal pain is the most common symptom in adult HPP.

•

Effects of bisphosphonates in mild HPP need further studies.

Pharmacokinetics of Asfotase Alfa in Adult Patients With Pediatric-Onset Hypophosphatasia

Hypophosphatasia is a rare metabolic disease resulting from variant(s) in the gene‐encoding tissue‐nonspecific isozyme of alkaline phosphatase. In this 13‐week, phase 2a, multicenter, randomized, open‐label, dose‐response study (ClinicalTrials.gov: NCT02797821), the pharmacokinetics of asfotase alfa, an enzyme replacement therapy approved for the treatment of hypophosphatasia, was assessed in adult patients with pediatric‐onset hypophosphatasia. In total, 27 adults were randomly assigned 1:1:1 to a single subcutaneous dose of asfotase alfa (0.5, 2.0, or 3.0 mg/kg) during week 1. From week 3 to week 9, patients received 0.5, 2.0, or 3.0 mg/kg subcutaneously 3 times per week (equivalent to 1.5, 6.0, or 9.0 mg/kg/wk, respectively). Noncompartmental analysis revealed exposure (maximum concentration in the dosing interval and area under the concentration‐time curve from time 0 to infinity) to asfotase alfa increased between single‐ and multiple‐dose administration and with increasing doses; however, extensive interindividual variability was observed in the concentration‐time profiles within each dose cohort. Median terminal elimination half‐life was ≈5 days following multiple‐dose administration, with steady state achieved by approximately day 29. Dose‐normalized exposure data indicated that asfotase alfa activity was approximately dose‐proportional within the studied dose range. Additionally, dose‐normalized exposure was comparable across body mass index categories of <25, ≥25 to <30, and ≥30 kg/m², indicating that asfotase alfa dosing bioavailability was consistent in these patients, including those who were obese. These data, together with previously published pharmacodynamic results in this study population, support the use of asfotase alfa at the recommended dose of 6 mg/kg/wk in adults with pediatric‐onset hypophosphatasia.

Serum alkaline phosphatase levels are associated with coronary artery calcification patterns and plaque vulnerability

OBJECTIVES

This study aimed to investigate the association of serum alkaline phosphatase (ALP) with calcification patterns and plaque morphology detected by intravascular ultrasound (IVUS) in acute coronary syndrome (ACS) patients.

BACKGROUND

ALP has been shown to predict vascular calcification and long-term cardiovascular events. However, the relationship between ALP and vascular calcification patterns or plaque morphology remains unclear.

METHODS

In total, 328 ACS patients who underwent IVUS examinations were screened from January 2017 to December 2018; among them, 234 eligible participants were grouped according to the tertiles of ALP levels (<68, 68-80, and >80 IU/L). Demographic data and IVUS parameters were documented and analyzed.

RESULTS

After adjusting for potential confounders, independent associations were observed between ALP and the presence of coronary calcification, spotty calcification, minimum lumen area (MLA) ≤ 4.0 mm2 , and plaque burden (PB) > 70%. Compared with the lowest ALP tertile group, the highest ALP group had higher risks of calcification (odds ratio [OR], 2.85; 95% confidence interval [95%CI], 1.38-5.90; p = .005), spotty calcification (OR, 1.86; 95%CI, 1.09-3.84; p = .012), MLA≤4.0 mm2 (OR, 3.32; 95%CI, 1.51-7.28; p = .003), and PB > 70% (OR, 4.59; 95%CI, 1.83-11.50; p = .001). Similar results were found when ALP was analyzed as a continuous variable or a category variate according to the cut-off value determined by the receiver operating characteristic curve analysis. Furthermore, the model including clinical factors and ALP significantly improved the predictive power for coronary calcification, spotty calcification, MLA≤4.0 mm2 , and PB > 70%.

CONCLUSION

Our findings suggest that ALP may be a potential predictive biomarker for calcification and plaque vulnerability.

Glucose Metabolism in Osteoblasts in Healthy and Pathophysiological Conditions

Bone tissue in vertebrates is essential to performing movements, to protecting internal organs and to regulating calcium homeostasis. Moreover, bone has also been suggested to contribute to whole-body physiology as an endocrine organ, affecting male fertility; brain development and cognition; and glucose metabolism. A main determinant of bone quality is the constant remodeling carried out by osteoblasts and osteoclasts, a process consuming vast amounts of energy. In turn, clinical conditions associated with impaired glucose metabolism, including type I and type II diabetes and anorexia nervosa, are associated with impaired bone turnover. As osteoblasts are required for collagen synthesis and matrix mineralization, they represent one of the most important targets for pharmacological augmentation of bone mass. To fulfill their function, osteoblasts primarily utilize glucose through aerobic glycolysis, a process which is regulated by various molecular switches and generates adenosine triphosphate rapidly. In this regard, researchers have been investigating the complex processes of energy utilization in osteoblasts in recent years, not only to improve bone turnover in metabolic disease, but also to identify novel treatment options for primary bone diseases. This review focuses on the metabolism of glucose in osteoblasts in physiological and pathophysiological conditions.

Bone healing and reactivation of remodeling under asfotase alfa therapy in adult patients with pediatric-onset hypophosphatasia

Hypophosphatasia (HPP) is a hereditary musculoskeletal disorder caused by inactivating variants in the ALPL gene and subsequently reduced serum tissue-nonspecific alkaline phosphatase (TNSALP) activity. This inborn error of metabolism results in decreased bone quality, accumulations of osteoid, and reduced bone mineralization. Increased incidence of fractures and prolonged bone healing are characteristic features for HPP. Available enzyme replacement therapy (asfotase alfa), was reported to recover bone mineralization and bone quality in adult HPP patients. Moreover, it was shown that asfotase alfa improved fracture healing of former nonunions in two adult HPP patients. We hypothesized that the nonunions are filled partially with osteoid, offering great potential to benefit from the treatment with asfotase alfa to promote bone healing. In the present study, we report three adult patients with pediatric-onset HPP and detected ALPL-mutations with prolonged bone healing after arthrodesis, tibial stress fracture, and osteotomy. After the initiation of asfotase alfa, immediately increased levels of alkaline phosphatase (ALP) and bone-specific ALP, as well as decreased levels of pyridoxal-5-phosphate (PLP), were detected in biochemical analysis. Importantly, even after up to 5 years of non-healing, a progredient consolidation was shown, assessed by a custom three-dimensional evaluation of repeated cone-beam computed tomography (CBCT) images, characterized by rapidly increasing levels of bone volume per tissue volume (BV/TV) within the volume of interest (i.e., the region of the non-healing bone). These radiographical findings were in line with the reported restoration of functional ability and pain-free full weight-bearing, as well as increased neuromuscular parameters (e.g., improved muscle strength). Taken together, our findings indicate that asfotase alfa improves the osseous consolidation of nonunions likely due to re-mineralization of osteoid tissue filling the former gap and improving the functional ability in adult HPP patients, characterized by increasing levels of BV/TV assessed via an innovative three-dimensional evaluation of CBCT images.

Congenital Hyperphosphatemic Conditions Caused by the Deficient Activity of FGF23

Congenital diseases that could result in hyperphosphatemia at an early age include hyperphosphatemic familial tumoral calcinosis (HFTC)/hyperostosis-hyperphosphatemia syndrome (HHS) and congenital hypoparathyroidism/pseudohypoparathyroidism due to the insufficient activity of fibroblast growth factor (FGF) 23 and parathyroid hormone. HFTC/HHS is a rare autosomal recessive disease caused by inactivating mutations in the FGF23, UDP-N-acetyl-alpha-D-galactosamine:polypeptide N-acetylgalactosaminyltransferase 3 (GALNT3), or Klotho (KL) genes, resulting in the excessive cleavage of active intact FGF23 (FGF23, GALNT3) or increased resistance to the action of FGF23 (KL). Massive ectopic calcification, known as tumoral calcinosis (TC), is seen in periarticular soft tissues, typically in the hip, elbow, and shoulder in HFTC/HHS, reducing the range of motion. However, other regions, such as the eye, intestine, vasculature, and testis, are also targets of ectopic calcification. The other symptoms of HFTC/HHS are painful hyperostosis of the lower legs, dental abnormalities, and systemic inflammation. Low phosphate diets, phosphate binders, and phosphaturic reagents such as acetazolamide are the treatment options for HFTC/HHS and have various consequences, which warrant the development of novel therapeutics involving recombinant FGF23.

Pharmacodynamics of asfotase alfa in adults with pediatric-onset hypophosphatasia

BACKGROUND

Hypophosphatasia (HPP) is the rare, inherited, metabolic bone disease characterized by low activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) leading to excess extracellular inorganic pyrophosphate (PPi) and pyridoxal 5'-phosphate (PLP). Asfotase alfa is the human recombinant enzyme-replacement therapy that replaces deficient TNSALP. However, there is limited information concerning the appropriate dose of asfotase alfa for adult patients with pediatric-onset HPP. Thus, we evaluated the pharmacodynamics and safety/tolerability of different doses of asfotase alfa in such patients.

METHODS

This 13-week, Phase 2a, open-label study enrolled adults (aged ≥18 years) with pediatric-onset HPP. They were randomized 1:1:1 to receive a single subcutaneous dose of asfotase alfa (0.5, 2.0, or 3.0 mg/kg) at Week 1, then 3 times per week (ie, 1.5, 6.0, or 9.0 mg/kg/wk) starting at Week 3 for 7 weeks. Key outcome measures included change from Baseline to before the third dose during Week 9 (trough) in plasma PPi (primary outcome measure) and PLP (secondary outcome measure).

RESULTS

Twenty-seven adults received asfotase alfa 0.5 (n = 8), 2.0 (n = 10), and 3.0 (n = 9) mg/kg; all completed the study. Median (range) age was 45 (18-77) years; most patients were white (96%) and female (59%). Median plasma PPi and PLP concentrations decreased from Baseline to Week 9 in all 3 cohorts. Differences in least squares mean (LSM) changes in PPi were significant with 2.0 mg/kg (p = 0.0008) and 3.0 mg/kg (p < 0.0001) vs. 0.5 mg/kg. Differences in LSM changes in PLP were also significant for 2.0 mg/kg (p = 0.0239) and 3.0 mg/kg (p = 0.0128) vs. 0.5 mg/kg. Injection site reactions were the most frequent treatment-emergent adverse event (78%), showing increasing frequency with increasing dose.

CONCLUSIONS

Adults with pediatric-onset HPP receiving asfotase alfa at 6.0 mg/kg/wk (the recommended dose) or 9.0 mg/kg/wk had greater reductions in circulating PPi and PLP concentrations compared with a lower dose of 1.5 mg/kg/wk.

TRIAL REGISTRATION

Clinicaltrials.gov identifier NCT02797821.

Tissue-Nonspecific Alkaline Phosphatase-A Gatekeeper of Physiological Conditions in Health and a Modulator of Biological Environments in Disease

Tissue-nonspecific alkaline phosphatase (TNAP) is a ubiquitously expressed enzyme that is best known for its role during mineralization processes in bones and skeleton. The enzyme metabolizes phosphate compounds like inorganic pyrophosphate and pyridoxal-5′-phosphate to provide, among others, inorganic phosphate for the mineralization and transportable vitamin B6 molecules. Patients with inherited loss of function mutations in the ALPL gene and consequently altered TNAP activity are suffering from the rare metabolic disease hypophosphatasia (HPP). This systemic disease is mainly characterized by impaired bone and dental mineralization but may also be accompanied by neurological symptoms, like anxiety disorders, seizures, and depression. HPP characteristically affects all ages and shows a wide range of clinical symptoms and disease severity, which results in the classification into different clinical subtypes. This review describes the molecular function of TNAP during the mineralization of bones and teeth, further discusses the current knowledge on the enzyme’s role in the nervous system and in sensory perception. An additional focus is set on the molecular role of TNAP in health and on functional observations reported in common laboratory vertebrate disease models, like rodents and zebrafish.

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.

Visualization of Mineral-Targeted Alkaline Phosphatase Binding to Sites of Calcification In Vivo

A mineral-targeted form of recombinant tissue-nonspecific alkaline phosphatase (TNAP), asfotase alfa, was approved multinationally as an enzyme replacement therapy for hypophosphatasia in 2015. Two reports to date have shown evidence of binding of this drug to mineralizing tissues using histochemistry and immunohistochemistry. Here, we sought to expand on those earlier studies by directly visualizing the in vivo binding of asfotase alfa conjugated with AnaTag HiLyte Fluor 750 or Alexa Fluor 647 fluorescent dye to sites of skeletal/dental mineralization and ectopic calcification. We utilized 40-day-old Tagln-Cre; Hprt^ALPL/Y mice, a model of severe medial vascular calcification; Tie2-Cre; Hprt^ALPL/Y mice, a model of severe intimal calcification; and sibling WT Hprt^ALPL/Y mice, devoid of soft-tissue calcification. A single dose of 8 mg/kg labeled asfotase alfa was injected via the retro-orbital route. Skeletal tissues and soft organs were imaged ex vivo 2 days after the injection. Strong fluorescence signal was observed in all skeletal tissues (calvaria, vertebra, long bones, jaw, and mandibles) from mutant and WT mice. Fluorescence analysis of histological sections from bones revealed strong binding of asfotase alfa. Asfotase alfa binding to sites of ectopic calcification in the heart, aorta, and renal artery were found in both the Tagln-Cre; Hprt^ALPL/Y and Tie2-Cre; Hprt^ALPL/Y mice but not in WT mice. In addition, asfotase alfa binding was also found in the kidney stroma and brain of the Tie2-Cre; Hprt^ALPL/Y mice. Our results show that fluorescence-labeled asfotase alfa administered in vivo binds not only to sites of skeletal and dental mineralization but also to sites of ectopic calcification in these animal models. © 2020 American Society for Bone and Mineral Research.

Single-Molecule Analysis Determines Isozymes of Human Alkaline Phosphatase in Serum

Alkaline phosphatase (ALP) is an important biomarker, as high levels of ALP in blood can indicate liver disease or bone disorders. However, current clinical blood tests only measure the total concentration of ALP but are unable to distinguish enzyme isotypes. Here, we demonstrate a novel and rapid approach to profile various ALP isozymes in blood via a single-molecule-analysis platform. The microarray platform provides enzyme kinetics of hundreds of individual molecules at high throughput. Using these single molecule kinetics, we characterize the different activity profiles of ALP isotypes. By analyzing both healthy and disease samples, we found the single molecule activity distribution of ALP in serum reflects the health status of patients. This result demonstrates the potential utility of the method for improving the conventional ALP test, as well as for analyzing other enzymatic biomarkers, including enzyme isotypes.

Genetic contributions to the etiology of anorexia nervosa: New perspectives in molecular diagnosis and treatment

BACKGROUND

Anorexia nervosa is a multifactorial eating disorder that manifests with self-starvation, extreme anxiety, hyperactivity, and amenorrhea. Long-term effects include organ failure, disability, and in extreme cases, even death.

METHODS

Through a literature search, here we summarize what is known about the molecular etiology of anorexia nervosa and propose genetic testing for this condition.

RESULTS

Anorexia nervosa often has a familial background and shows strong heritability. Various genetic studies along with genome-wide association studies have identified several genetic loci involved in molecular pathways that might lead to anorexia.

CONCLUSION

Anorexia nervosa is an eating disorder with a strong genetic component that contributes to its etiology. Various genetic approaches might help in the molecular diagnosis of this disease and in devising novel therapeutic options.

Skeletal abnormalities secondary to antenatal etidronate treatment for suspected generalised arterial calcification of infancy

Background

Generalised arterial calcification of infancy (GACI) is a rare disorder characterised by the deposition of hydroxyapatite crystals within the vessel walls. It is associated with a high mortality rate. Bisphosphonates have been used with some success in the treatment of GACI. However, there is a paucity of data on the antenatal use of bisphosphonates for GACI. In this paper, we report development of the skeletal changes suggestive of hypophosphatasia (HPP) in an infant with GACI, whose mother was treated with etidronate during pregnancy.

Case report

A Caucasian infant boy had a suspected antenatal diagnosis of GACI based on the findings suggestive of calcification of the annulus of the tricuspid valve and wall of the right ventricular (RV) outflow tract and main pulmonary artery on foetal echocardiography and the genetic analysis which showed a pathogenic heterozygous mutation in ABCC6. Based on these findings, mother was started on etidronate treatment from 26 weeks of gestation. A healthy male baby was delivered at 38 weeks of gestation. Initial postnatal echocardiogram on day 1 of life was normal with good biventricular function; subtle changes suggestive of microcalcifications were detected on the CT angiography. Serum calcium, phosphate, alkaline phosphatase and renal profile were normal. Further, the serum inorganic pyrophosphate (PPi) level was significantly low. Skeletal changes suggestive of HPP were seen on the radiographs. The baby developed cardiac dysfunction on day 4 of life with evidence of ischaemic changes on electrocardiogram (ECG).Treatment with etidronate was started in view of probable evolving coronary calcifications. Despite treatment with cardiac supportive measures and bisphosphonate, he succumbed to death in the third week of life.

Discussion

We believe, this is the first report of skeletal changes suggestive of HPP, arising secondary to antenatal etidronate (first generation bisphosphonate) used for the treatment of suspected GACI due to a heterozygous ABCC6 mutation.

Dysregulation of FOXO1 (Forkhead Box O1 Protein) Drives Calcification in Arterial Calcification due to Deficiency of CD73 and Is Present in Peripheral Artery Disease

Supplemental Digital Content is available in the text.

Objective:

The recessive disease arterial calcification due to deficiency of CD73 (ACDC) presents with extensive nonatherosclerotic medial layer calcification in lower extremity arteries. Lack of CD73 induces a concomitant increase in TNAP (tissue nonspecific alkaline phosphatase; ALPL), a key enzyme in ectopic mineralization. Our aim was to investigate how loss of CD73 activity leads to increased ALPL expression and calcification in CD73-deficient patients and assess whether this mechanism may apply to peripheral artery disease calcification.

Approach and Results:

We previously developed a patient-specific disease model using ACDC primary dermal fibroblasts that recapitulates the calcification phenotype in vitro. We found that lack of CD73-mediated adenosine signaling reduced cAMP production and resulted in increased activation of AKT. The AKT/mTOR (mammalian target of rapamycin) axis blocks autophagy and inducing autophagy prevented calcification; however, we did not observe autophagy defects in ACDC cells. In silico analysis identified a putative FOXO1 (forkhead box O1 protein) binding site in the human ALPL promoter. Exogenous AMP induced FOXO1 nuclear localization in ACDC but not in control cells, and this was prevented with a cAMP analogue or activation of A2a/2b adenosine receptors. Inhibiting FOXO1 reduced ALPL expression and TNAP activity and prevented calcification. Mutating the FOXO1 binding site reduced ALPL promoter activation. Importantly, we provide evidence that non-ACDC calcified femoropopliteal arteries exhibit decreased CD73 and increased FOXO1 levels compared with control arteries.

Conclusions:

These data show that lack of CD73-mediated cAMP signaling promotes expression of the human ALPL gene via a FOXO1-dependent mechanism. Decreased CD73 and increased FOXO1 was also observed in more common peripheral artery disease calcification.

Perinatal Hypophosphatasia in a Premature Infant

A premature male infant was delivered at 32 weeks' gestation due to category-2 fetal tracing after preterm labor. The physical exam showed shortened and bowed long bones, with calvarium felt in small area of the head. Serum alkaline phosphatase was very low on admission. Extensive metaphyseal abnormalities, bowing of long bones, and poor ossification of all bones were noted on skeletal survey (radiography). Based on ultrasound evidence of "bowing" and long bone fractures at 26 weeks, amniocentesis was performed that later diagnosed hypophosphatasia by genetic testing while ruling out osteogenesis imperfecta. Although ventilated initially, the infant gradually improved with enzyme replacement therapy (ERT) and was extubated to noninvasive ventilation for 6 weeks. Following clinical deterioration with hypoxic respiratory failure secondary to sepsis at 4 months of age, he succumbed to severe pulmonary hypertension, likely secondary to chronic lung disease and prolonged ventilation. Early diagnosis allowed timely initiation of appropriate therapy. Radiological improvement with the therapy showed promising results in this rare disease. Despite specific novel therapy being available, variability in presentation dictates prognosis in this previously universally fatal condition. The potential unknown effects of ERT on pulmonary vascular remodeling need further investigation.

Loss-of-Function Mutations in the ALPL Gene Presenting with Adult Onset Osteoporosis and Low Serum Concentrations of Total Alkaline Phosphatase

Hypophosphatasia (HPP) is a rare inherited disorder characterized by rickets and low circulating concentrations of total alkaline phosphatase (ALP) caused by mutations in ALPL. Severe HPP presents in childhood but milder forms can present in adulthood. The prevalence and clinical features of adult HPP are poorly defined. The aim of this study was to evaluate the prevalence and clinical significance of low serum total alkaline phosphatase (ALP) levels in a clinic-based population of adult osteoporotic patients. We searched for patients with low ALP in a cohort of 3285 patients referred to an osteoporosis clinic over a 10-year period and performed mutation screening of ALPL in those with low ALP (≤40 U/L) on two or more occasions. These individuals were matched with four clinic controls with a normal ALP. We also evaluated the prevalence of low ALP and ALPL mutations in 639 individuals from the general population from the same region. We identified 16/3285 (0.49%) clinic patients with low ALP and 14 (87.5%) had potentially pathogenic variants in ALPL. Eight of these individuals were heterozygous for mutations previously described in HPP and 2 were heterozygous for novel mutations (p.Arg301Trp and p.Tyr101X). These mutations were not found in clinic controls or in the general population. Eight patients with low ALP, including 4 with ALPL mutations, were treated with bisphosphonates for an average of 6.5 years. In these individuals, the rate of fractures during treatment was comparable to that in normal ALP clinic controls who were treated with bisphosphonates. We conclude that heterozygous loss-of-function mutations in ALPL are common in osteoporosis patients with low ALP. Further studies are required to determine how best these individuals should be treated. © 2019 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Large-scale in vitro functional testing and novel variant scoring via protein modeling provide insights into alkaline phosphatase activity in hypophosphatasia

Hypophosphatasia (HPP) is a rare metabolic disorder characterized by low tissue‐nonspecific alkaline phosphatase (TNSALP) typically caused by ALPL gene mutations. HPP is heterogeneous, with clinical presentation correlating with residual TNSALP activity and/or dominant‐negative effects (DNE). We measured residual activity and DNE for 155 ALPL variants by transient transfection and TNSALP enzymatic activity measurement. Ninety variants showed low residual activity and 24 showed DNE. These results encompass all missense variants with carrier frequencies above 1/25,000 from the Genome Aggregation Database. We used resulting data as a reference to develop a new computational algorithm that scores ALPL missense variants and predicts high/low TNSALP enzymatic activity. Our approach measures the effects of amino acid changes on TNSALP dimer stability with a physics‐based implicit solvent energy model. We predict mutation deleteriousness with high specificity, achieving a true‐positive rate of 0.63 with false‐positive rate of 0, with an area under receiver operating curve (AUC) of 0.9, better than all in silico predictors tested. Combining this algorithm with other in silico approaches can further increase performance, reaching an AUC of 0.94. This study expands our understanding of HPP heterogeneity and genotype/phenotype relationships with the aim of improving clinical ALPL variant interpretation.