Infantile hypophosphatasia in a Chinese patient: identification and characterization of novel compound heterozygous ALPL mutations

Hypophosphatasia (HPP) is a genetic disease caused by loss-of-function mutations in ALPL, which encodes tissue-nonspecific alkaline phosphatase (ALP). Early diagnosis and treatment of perinatal and infantile HPP are important because of their high mortality rates. Enzyme replacement therapy (ERT) using human recombinant tissue-nonspecific ALP asfotase alfa was introduced in Korea in 2016. We report the first experience of ERT over 6 years for perinatal lethal and infantile HPP in Korea. The first patient was a 6-week-old Korean boy with a failure to thrive. The second patient was an 8-day-old Korean-Uzbek body with generalized tonic-clonic seizure with cyanosis. HPP was suspected in both patients because of the very low level of ALP activity and rachitic findings on radiographs, and the disease was confirmed by Sanger sequencing of the ALPL gene. The first patient with infantile HPP started ERT at 21 months of age and the second patient with perinatal HPP started ERT at 30 days of age. Both patients received asfotase alfa (2 mg/kg 3 times per week subcutaneously, adjusted to 3 mg/kg 3 times per week if required) for 6 years. After 6 years of ERT, radiographic findings and growth standard deviation scores improved in both patients. The second patient showed no evidence of rickets after 3 years of ERT. Mechanical respiratory support and supplemental oxygen were not required after 4.5 years of treatment in the first patient and at 2 months after treatment in the second patient. Among the 2 patients, the patient who started ERT early had a much better prognosis despite a more severe initial clinical presentation. Our results suggest that early diagnosis and prompt treatment play an important role in improving long-term prognosis and avoiding morbidity and premature mortality in patients with perinatal and infantile HPP.

Bone-Targeted Replacement Therapy for Hypophosphatasia

Hypophosphatasia (HPP) is caused by mutations in the tissue nonspecific alkaline phosphatase (TNSALP) gene in an autosomal recessive or dominant manner and characterized by defective mineralization of bone and low serum ALP levels. In this report, we present a family with HPP mother (case 1) and HPP child (case 2) who have identical TNSALP gene mutation (c.1015G>A p.Gly339Arg heterozygous mutation) but distinct clinical phenotypes. Whereas case 1 appeared to be asymptomatic despite extremely low levels of serum ALP, case 2 had several HPP-related symptoms, such as tooth loss, fractures, short stature, with slightly decreased ALP levels. Upon the diagnosis of HPP, case 1 discontinued denosumab, which was used to treat her rheumatoid arthritis, concerning the risk of atypical femoral fractures. The clinical course of this family was suggestive in a genotype-phenotype imbalance in HPP, the underdiagnosis of HPP in adults, and the risk of atypical femoral fractures using bone resorption inhibitors.

Pyridoxine-responsive seizures as the first symptom of infantile hypophosphatasia caused by two novel missense mutations (c.677T > C, p.M226T; c.1112C > T, p.T371I) of the tissue-nonspecific alkaline phosphatase gene

ObjectiveWe examined the clinical and genetic features of hypophosphatasia (HPP) in Japanese patients. HPP is a rare metabolic bone disorder of bone mineralisation caused by mutations in the liver/bone/kidney alkaline...

Hypophosphatasia (HPP) is a rare disease caused by loss-of-function mutations in the tissue-nonspecific alkaline phosphatase (TNAP, TNSALP) gene. HPP causes a multisystemic syndrome with a predominant bone phenotype. The clinical spectrum ranges from high lethality in early onset (<6 months) HPP to mild late-onset syndromes. HPP management so far has been only supportive. Subcutaneous asfotase alfa, a first-in-class bone-targeted human TNAP enzyme replacement therapy, is the first compound to be approved for long-term treatment of bone manifestations in pediatric-onset HPP. In noncomparative clinical trials (treatment up to 7 years), this treatment was associated with skeletal, respiratory and functional improvement in perinatal, infantile and childhood-onset HPP. Compared with age-matched historical controls, patients with life-threatening perinatal and infantile HPP treated with asfotase alfa had substantially improved bone mineralization, survival and ventilation-free survival. In childhood HPP, asfotase alfa improved growth, gross motor function, strength and agility and decreased pain. The compound was well tolerated and most adverse events were of mild to moderate intensity. To date, data and experience concerning its efficacy and safety in long-term treatment are not yet available. Further studies to evaluate risks and benefits of enzyme replacement therapy with asfotase alfa in adults are in progress and are also strongly needed.

Fundus Albipunctatus Associated with Compound Heterozygous Mutations in RPE65

Hypophosphatasia (HPP) is due to mutations of the tissue non-specific alkaline phosphatase (TNAP) gene expressed in the liver, kidney, and bone. TNAP substrates include inorganic pyrophosphate cleaved into inorganic phosphate (Pi) in bone, pyridoxal-5'-phosphate (PLP), the circulating form of vitamin B6, and phosphoethanolamine (PEA). As an autosomal recessive or dominant disease, HPP results in a range of clinical forms. Its hallmarks are low alkaline phosphatase (AP) and elevated PLP and PEA levels. Perinatal HPP may cause early death with respiratory insufficiency and hypomineralization resulting in deformed limbs and sometimes near-absence of bones and skull. Infantile HPP is diagnosed before 6 months of life. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency in the brain indicate poor prognosis. Craniosynostosis is frequent. Unlike in other forms of rickets, calcium and phosphorus are not decreased, resulting in hypercalciuria and nephrocalcinosis. Hypercalcemic crisis may occur. Failure to thrive and growth retardation are concerns. In infantile and adult forms of HPP, non-traumatic fractures may be the prominent manifestation, with otherwise unexplained chronic pain. Progressive myopathy has been described. Dental manifestations with early loss of teeth are usual in HPP and in a specific form, odontohypophosphatasia. HPP has been studied in knock-out mice models which mimic its severe form. Animal models have made a major contribution to the development of an original enzyme therapy for human infantile HPP, which is however essentially targeted at mineralized tissues. Better knowledge of its extraskeletal manifestations, including pain and neurological symptoms, is therefore required.

Objective To explore the clinical and genetic characteristics of a Chinese baby with perinatal hypophosphatasia (HPP) and his parents for better understanding of the disease. Methods The clinical data of the patient with HPP was carefully collected. The laboratory and radiographic examination data were taken for this baby patient. Sequencing for all the twelve tissue-nonspecific alkaline phosphatase(ALPL) exons and the flanking exon-intron junctions were performed in the proband and his parents with their genomic DNA from peripheral blood. Results The blood level of alkaline phosphatase was decreased in this patient while serum calcium level was increased. His bone revealed chondrodysplasia-like change. Compound heterozygous mutations were found in the proband, with c. 346G>A(p.A116T) in exon 5 and c. 1171C>T(p.R391C) in exon 10. His father and mother were without clinical manifestation while respectively carried c. 346G>A(p.A116T and c. 1171C>T(p.R391C) missense mutations, suggesting an autosomal recessive inheritance in this family. Conclusion Perinatal HPP has a high mortality rate. Skeletal deformities, hypercalcemia, and low level of ALP are important in the differential diagnosis of perinatal HPP. Key words: Mutation; Hypophosphatasia; Alkaline phosphatase gene

Deactivating mutations in the TNSALP gene cause HPP. Akp2(-/-) mice model severe infantile HPP, but there is no model for the relatively mild adult form. Here we report on mice with an induced mutation in Akp2 that affects splicing. The phenotype of homozygotes mirror aspects of the adult form of HPP. Hypophosphatasia (HPP) is a clinically varied skeletal disorder resulting from deficiency of tissue nonspecific alkaline phosphatase (TNSALP). Mice lacking Akp2 model infantile HPP characterized by skeletal hypomineralization, impaired growth, seizures, and perinatal mortality. No animal model exists to study the less severe forms of the disease that typically present in later life. N-ethyl-N-nitrosourea (ENU) mutagenesis was used to generate mouse models of human disease. A mouse with low plasma alkaline phosphatase (ALP) activity was identified by our clinical chemistry screen. Its offspring were used for inheritance studies and subjected to biochemical, histological, and radiological phenotyping. DNA was extracted for mapping and osteoblasts harvested for functional studies. We showed semidominant inheritance of the low ALP phenotype and mapped the underlying point mutation to Akp2. Affected offspring bear the splice site mutation 862 + 5G>A-a hypomorphic allele named Akp2(Hpp). The same mutation has been reported in a patient. Akp2(Hpp/+) mice have approximately 50% of normal plasma ALP but display no other biochemical or skeletal abnormalities. Unlike Akp2(-/-) mice, Akp2(Hpp/Hpp) mice have normal initial skeletal development and growth, a normal lifespan and do not have seizures. TNSALP is low but detectable in Akp2(Hpp/Hpp) plasma. Osteoblasts display approximately 10% of normal ALP activity and reduced intracellular inorganic phosphate levels, yet are capable of normal mineralization in vitro. TNSALP substrates are significantly elevated in urine (inorganic pyrophosphate and phosphoethanolamine) and plasma (pyridoxal 5'-phosphate), whereas plasma inorganic pyrophosphate levels are normal. Akp2(Hpp/Hpp) mice develop late-onset skeletal disease, notably defective endochondral ossification and bone mineralization that leads to arthropathies of knees and shoulders. Akp2(Hpp/Hpp) mice mirror a number of clinical features of the human adult form of HPP. These mice provide for the first time an animal model of late onset HPP that will be valuable in future mechanistic studies and for the evaluation of therapies such as those aimed at HPP.

HintergrundDie Hypophosphatasie (HPP) ist eine genetische Erkrankung, die durch eine oder mehrere Mutationen im Gen für alkalische Phosphatase (ALP) verursacht wird, verantwortlich für die Kodierung der gewebespezifischen ALP und für den Mineralisierungsprozess.Ziel der ArbeitBestimmung der Prävalenz der HPP bei rheumatologischen Patienten.Material und MethodenRetrospektive Analyse der Krankenakten aller erwachsener Patienten mit pathologisch erniedrigten gesamt ALP-Werten (<35 U/l), die zwischen Januar 2017 und Juni 2019 in der Rheumatologie der Medizinischen Klinik III am Universitätsklinikum Bonn behandelt wurden. Die Analyse wurde in Bezug auf klinische Zeichen sowie auf die Ergebnisse der Gentests für HPP untersucht.ErgebnisseBei 60 von 2289 Patienten (2,62 %) zeigten sich pathologisch niedrige ALP-Werte, bei 30 von ihnen (1,31 %) wurden persistierend niedrige ALP-Werte festgestellt. Bei 19 dieser 30 Patienten wurde ein Gentest für ALP-Genmutationen durchgeführt. Sieben der 19 Patienten (36,84 %) hatten HPP-Zeichen (Insuffizienzfrakturen oder schlechter Zahnstatus seit der Kindheit), alle mit pathologischer ALP-Mutation. Drei dieser Patienten (15,78 %) hatten jeweils eine Insuffizienzfraktur mit normwertiger Knochendichtemessung in der Vorgeschichte. Insgesamt 13 von 19 Patienten wiesen (68,42 %) Mutationen im ALP-Gen auf. Interessanterweise wurde keine Assoziation mit einer Chondrokalzinose festgestellt.DiskussionDie HPP scheint eine unterdiagnostizierte Erkrankung mit einem höheren Anteil betroffener Patienten, welche in der Rheumatologie vorstellig werden, zu sein. Daher sollten zukünftige Studien darauf abzielen, ein Diagnostikprotokoll in der klinischen Praxis zu entwickeln.

This article reported the clinical features of one child with infantile hypophosphatasia (HPP) and his pedigree information. The proband was a 5-month-old boy with multiple skeletal dysplasia (koilosternia, bending deformity of both radii, and knock-knee deformity of both knees), feeding difficulty, reduction in body weight, developmental delay, recurrent pneumonia and respiratory failure, and a significant reduction in blood alkaline phosphatase. Among his parents, sister, uncle, and aunt (other family members did not cooperate with us in the examination), his parents and aunt had a slight reduction in alkaline phosphatase and his aunt had scoliosis; there were no other clinical phenotypes or abnormal laboratory testing results. His ALPL gene mutation came from c.228delG mutation in his mother and c.407G>A compound heterozygous mutation in his father. His aunt carried c.228delG mutation. The c.407G>A mutation had been reported as the pathogenic mutation of HPP, and c.228delG mutation was a novel pathogenic mutation. Hypophosphatasia is caused by ALPL gene mutation, and ALPL gene detection is an effective diagnostic method. This study expands the mutation spectrum of ALPL gene and provides a theoretical basis for genetic diagnosis of this disease.

Hypophosphatasia (HPP) is a rare, potentially life-threatening disease characterised by hypomineralisation of bones and teeth.1 The fundamental defect is reduced functional activity of the enzyme tissue-nonspecific alkaline phosphatase (usually just...

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.

Whole-exome resequencing reveals recessive mutations in TRAP1 in individuals with CAKUT and VACTERL association