Recent advances in the diagnosis and management of childhood hypophosphatasia

The article provides current information on the clinical forms of hypophosphatasia. The OMIM catalog lists 5 forms of hypophosphatasia: perinatal (lethal), infantile, childhood, adult, and odontohypophosphatasia. The ORPHA portal identifies 6 subtypes of the disorder, including adult, childhood, infantile, perinatal (lethal), and prenatal (benign) hypophosphatasia. M.E. Nunes (2023) identifies 7 forms of hypophosphatasia. International studies have established the pathogenesis, phenotypic variability, and severity of hypophosphatasia. A global consortium provides information on 446 mutation variants of the ALPL gene and 797 genotypes in pediatric and adult patients. The review presents updated diagnostic criteria for hypophosphatasia in children and adults with low alkaline phosphatase activity in the blood. Ten years of experience in 40 countries have proven the safety and efficacy of enzyme replacement therapy with Asfotase Alfa in children with perinatal, infantile, childhood hypophosphatasia, and odontohypophosphatasia. In the Russian Federation, enzyme replacement therapy with Asfotase Alfa for children with hypophosphatasia has been funded by the Circle of Kindness Foundation, established by the Ministry of Health, since 2021.

Abstract Disclosure: J. Ocampo Mascaro: None. R. Tran: None. S. Kallish: Consulting Fee; Self; Sanofi, Amicus. Research Investigator; Self; Ipsen, Sanofi, Incyte, Idorsia. Speaker; Self; Sanofi, Amicus. J. Berman: None. M. Al Mukaddam: Grant Recipient; Self; Ipsen, Incyte Research Funding. Hypophosphatasia (HPP) is an inborn error of metabolism resulting in low activity of the enzyme tissue non-specific alkaline phosphatase (ALP). It primarily involves bone and teeth mineralization. HPP has a wide clinical spectrum. It may present with rickets, fractures, failure to thrive, weakness, respiratory complications, and seizures in its more severe perinatal/infantile forms. Childhood forms tend to be milder but may have substantial musculoskeletal complications and early teeth loss. The clinical picture in adults is extremely variable too. It is usually mild with stress fractures and/or teeth loss in most individuals but can be debilitating in others due to musculoskeletal pain and recurrent fractures. This retrospective study describes the characteristics of 35 adult patients (ages 22-74 years) with HPP based on clinical presentation +/- molecular testing at the Penn Bone Center, located at a US-based large academic hospital. Out of 35 patients, 26 had an ALPL variant in genetic testing (12 pathogenic, 5 likely pathogenic, and 9 of uncertain significance), 3 were negative, and 6 did not undergo testing yet. ALP values ranged from 11 to 39 U/L (N: 38-126). Serum fasting vitamin B6 levels (off supplements) ranged from normal range to 18 times the upper limit of normal. Main symptoms reported were fractures (74%), arthralgias/myalgias (51%), diffuse bone pain (29%), atraumatic teeth loss (20%), muscle weakness (14%), and nephrolithiasis (9%). We found no significant negative correlation between ALP levels and number of symptoms (rs -0.026) or age of first symptom occurrence (rs 0.205). We also found no significant positive correlation between B6 elevation and number of symptoms (rs 0.236) or age of first symptom occurrence (rs -0.175). Six patients within our cohort had previous or current treatment with asfotase alfa. This is an FDA-approved therapy for the treatment of perinatal, infantile, and childhood HPP and has been used in adult patients if one of the presenting symptoms was before age 18 years. Response to therapy in our patient cohort was mixed. Three out of six patients discontinued treatment due to lack of clinical benefit. Continued research of the clinical characteristics of adult HPP is needed to better understand its natural history, markers of severity of disease, and how to determine who may potentially benefit from asfostase alfa therapy. Presentation: 6/1/2024

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

Objective: To analyze the oral phenotype and gene variation of children with hypophosphatasia (HPP), and explore the genotype-phenotype correlations. Methods: Eight children diagnosed with HPP from January 2008 to January 2023 in The Children's Hospital, Zhejiang University School of Medicine were recruited in this study. The pathogenic genes of 5 of them were sequentially analyzed and all of their oral manifestations, laboratory tests and genetic variation types were retrospectively analyzed. Results: A total of 8 children were recruited in the study, 3 males and 5 females, aged from 20 to 104 months, whose main complaints were premature deciduous tooth loss. Among them, 3 children were diagnosed with odonto HPP, and the other 5 children were diagnosed with childhood HPP, including 2 children was odonto HPP at the first diagnosis and modified as childhood HPP at the age of 5. The age range of first deciduous tooth loss is 9 to 18 months, and the age range of diagnosis was 20 to 104 months. The patients of odonto HPP only showed premature loss of deciduous anterior tooth, while the patients with childhood HPP also showed premature loss of multiple deciduous molars. Panoramic radiographic film revealed enlarged pulp chambers and radicular canals in some primary and permanent teeth. The enamel hypoplasia, hypoplastic short roots, and alveolar resorption of deciduous molar were observed in some cases. The serum alkaline phosphatase (ALP) (30-107 U/L) levels of all the patients were lower than that in the normal children of same age and gender, and the ALP value of the 1-3 years old girls with childhood HPP (30-33 U/L) was lower than that of the three children with odonto HPP (61-107 U/L), but there was no significant difference in statistical analysis. There were 8 variation sites of ALP liver/bone/kidney (ALPL) gene detected in 5 children and their families, all of which were missense variation, including the new variants in the mutations of c.1334C>G (p.Ser445Cys) and c.1259G>T (p.Gly420Val) that were not reported in the literature. One case was autosomal dominant inheritance and other 4 cases were complex heterozygous variation with autosomal recessive inheritance. Conclusions: Pediatric stomatologists are often the first doctors to detect childhood and odonto HPP. Diagnosis of mild HPP is often delayed. The severity of HPP is related to serum ALP level and ALPL gene mutation sites.

(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.

Childhood hypophosphatasia (HPP) presents with bowing of the limbs, poor mobility, chronic pain, short stature, fractures, and motor impairment. Enzyme replacement therapy (ERT) provides improved pulmonary and physical function in life-threatening perinatal and infantile forms of HPP. However, treatment of those patients without life-threatening HPP is limited. This report describes the results of asfotase alfa (Strensiq®, Alexion Pharmaceuticals, Inc.) treatment in a 6-year-old girl with childhood HPP, who presented with premature loss of primary teeth, low mobility, and chronic pain in the legs. Sequence analysis of the TNSALP gene revealed three heterozygous variants; c.526G>A (reported previously), c.1051G>C (novel), c.787T>C (reported previously). After a four-year follow-up under ERT, a marked reduction in leg pain and restlessness was observed and physical therapy assessments showed remarkable improvements in motor function, pain score, and quality of life. The treatment decision in childhood HPP is not as clear as in infantile and perinatal forms and it is mostly based on the clinical and radiological condition of the patient. In patients with childhood HPP without severe skeletal involvement but accompanying motor retardation, ERT may improve quality of life, motor functions, and daily activities.

Patients with childhood hypophosphatasia (HPP) often have unspecific symptoms. It was our aim to identify patients with mild forms of HPP by laboratory data screening for decreased alkaline phosphatase (AP) within a pediatric population. We conducted a retrospective hospital-based data screening for AP activity below the following limits: Girls:≤12 years: <125U/L; >12 years: <50U/L Boys:≤14 years: <125U/L; >14 years: <70U/L. Screening positive patients with otherwise unexplained hypophosphatasemia were invited for further diagnostics: Re-test of AP activity, pyridoxal 5'-phosphate (PLP) in hemolyzed whole blood, phosphoethanolamine (PEA) in serum and urine, and inorganic pyrophosphate in urine. Sequencing of the ALPL gene was performed in patients with clinical and/or laboratory abnormalities suspicious for HPP. We assessed a total of 14,913 samples of 6,731 patients and identified 393 screening-positive patients. The majority of patients were excluded due to known underlying diseases causing AP depression. Of the 30 patients who participated in the study, three had a decrease in AP activity in combination with an increase in PLP and PEA. A heterozygous ALPL mutation was detected in each of them: One patient with a short stature was diagnosed with childhood-HPP and started with enzyme replacement therapy. The remaining two are considered as mutation carriers without osseous manifestation of the disease. A diagnostic algorithm based on decreased AP is able to identify patients with ALPL mutation after exclusion of the differential diagnoses of hypophosphatasemia and with additional evidence of increased AP substrates.

Objective: To analyze the clinical, genetic characteristics and follow-up data of Chinese patients with hypophosphatasia (HPP). Methods: A retrospective analysis was conducted on six children with HPP admitted to the Department of Endocrinology, Genetics and Metabolism in Beijing Children's Hospital from October 2010 to January 2019. Summarized the clinical and follow-up data of all six patients, as well as the pathogenic variants of five children. Results: The serum alkaline phosphatase levels of all six children (five males and one female) were significantly reduced (2-49 U/L). The 6 patients aged from 2 months to 6 years and 4 months, 4 infantile HPP, 1 childhood HIP and 1 odonto HPP. The four patients with infantile HPP presented with anorexia, slow weight gain and hypercalcemia, whereas the one patient with childhood HPP and the other patient with odonto HPP had tooth loss. The patient with childhood HPP also manifested with motor dysfunction. Genetic testing was conducted for five patients and 4 unrelated Chinese families and revealed 10 variations in ALPL gene, including 7 missense variation, 1 insertion variation, 1 frameshift variation, 1 deletion variation.Of which 3 were novel (p.Y28C, p.268, F>L, p.A176V).One of the infantile patients lost follow-up and the other three deceased. The clinical conditions were much improved with medical intervention for patients with childhood, orodonto HPP. Conclusions: While HPP patients with different ages of onset present with common features, the prognosis differ significantly. The prognosis is good for patients with childhood, orodonto HPP and poor for patients with infantile HPP. Genetic testing is the main method for definitive diagnosis.

Mouthguards for a childhood hypophosphatasia patient to protect periodontal tissue of immature permanent teeth – Case report

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.

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

Hypophosphatasia is a rare hereditary metabolic bone disease caused by ALPL gene mutation. This papaer report the genetic diagnosis of a child with childhood hypophosphatasia, and the prenatal diagnosis of his sibling. We hope it can provide reference for clinical diagnosis and prenatal diagnosis of this disease. Key words: Hypophosphatasia; ALPL; Gene test; Prenatal diagnosis

Genetic correction of induced pluripotent stem cells mediated by transcription activator-like effector nucleases targeting ALPL recovers enzyme activity and calcification in vitro

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

Introduction: Hypophosphatasia (HPP) is a rare inherited metabolic bone disorder, caused by loss-of-function mutations within the gene that encodes the tissue nonspecific alkaline phosphatase (TNSALP). Extracellular accumulation of TNSALP natural substrates leads to inhibition of teeth and bone mineralization. Based on age of presentation and presence of skeletal disease, the clinical forms of HPP are perinatal, infantile, childhood, adult and odonto-HPP. The use of enzyme replacement therapy has shown to be highly beneficial specially in the most severe forms. Objective: Report a novel mutation. Clinical Case: A seven-year-old boy with short stature, postnatal growth retardation, development delay and learning disabilities. Background: normal delivery, BW: 2860 g, BL: 47 cm. GA: 39 weeks. Mother: short stature 143.0 cm (-2.9 SDS), bone pain and ALP: 26 UI/L (NR: 40-150) PLP (Pyridoxal 5ˊphosphate): 40 µg/L (NR: 5-50). Physical exam: weight: 15.3 kg (-2.64 SDS), height: 100.5 cm (-3.99 SDS), head circumference: 49.0 cm (-2 SDS), upper segment: 56.2 cm (pc 25). His midparental target height: 162.2cm (-1.55 SDS). Cleft palate, micrognathia, low set ears, wide spaced nipples, mild hyperlaxity, short and wide fingers, clinodactyly of the 5th finger. Tanner stage 1. Laboratory tests showed low serum ALP: 62 UI/L (NR: 135-537 for age and sex), PLP/vitamin B6: 41 µg/L (NR: 5-50), 4 Pyridoxic acid: < 3 µg/L (NR: 3-30). Radiological imaging: hypomineralization in proximal ulna, radius and distal humerus, widening of the first metacarpals with radiolucent zones in all metacarpals and widened metaphysis in lower long bones. Analysis ALPL gene: exon 5 heterozygous variation c.317A>Gp. (Gln106Arg), sequencing Sanger. No ALPL gene deletion or duplication was found. Mother carries the same mutation. Conclusion: This is a novel heterozygous mutation with dominant effect that probably generates a mild form of hypophosphatasia. Unless otherwise noted, all abstracts presented at ENDO are embargoed until the date and time of presentation. For oral presentations, the abstracts are embargoed until the session begins. s presented at a news conference are embargoed until the date and time of the news conference. The Endocrine Society reserves the right to lift the embargo on specific abstracts that are selected for promotion prior to or during ENDO.

PURPOSE: To modify the Performance-Oriented Mobility Assessment-Gait (POMA-G) subtest and validate this modified POMA-G (mPOMA-G) in children with hypophosphatasia (HPP), a rare metabolic disorder that can manifest with musculoskeletal symptoms that impair mobility and ambulation.METHODS: Based on feedback from an expert panel, the POMA-G was modified by removing gait initiation/path assessments and expanding the rating scale for step length/continuity to capture aspects of observational gait analysis relevant to children with HPP. Three trained physical therapists used the mPOMA-G for video-based assessments of gait in 14 children with childhood HPP who participated in a clinical study of asfotase alfa or in a natural history study. Intraclass correlation coefficients (ICCs) were calculated to determine interrater and intrarater agreement. Concurrent validity was evaluated by correlations with other validated assessment tools.RESULTS: Across 192 observations from available videos, interrater and intrarater agreement of mPOMA-G scores was significant (ICCs: 0.76 for both; 0.001). mPOMA-G scores had strong concurrent validity with the Childhood Health Assessment Questionnaire, Pediatric Outcomes Data Collection Instrument Transfer and Mobility Scale, Sports and Physical Function subscale, and 6-Minute Walk Test (all 0.0002).CONCLUSION: The mPOMA-G is a reliable and valid measure for detecting clinically significant impairments in children with HPP.

Hypophosphatasia: Clinical manifestations, diagnostic recommendations and therapeutic options

Hypophosphatasia: Clinical manifestations, diagnostic recommendations and therapeutic options

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