Homozygosity for TNSALP mutation 1348c>T (Arg433Cys) causes infantile hypophosphatasia manifesting transient disease correction and variably lethal outcome in a kindred of black ancestry

Precision diagnosis and treatment of vitamin metabolism-related epilepsy

Epilepsy is a chronic disorder of the nervous system caused by abnormal discharges from brain cells. Structural, infectious, metabolic, immunologic, and unknown causes can contribute to the development of seizures. In recent years, there has been increasing attention on epilepsy caused by genetic metabolic disorders. More than two hundred inherited metabolic disorders have been identified as potential cause of seizures, and they are mainly associated with energy deficiency in the brain, accumulation of toxic substances, abnormal neurotransmitter transmission, and deficiency of cofactors. Vitamins play a crucial role as components of several enzymes or coenzymes. Impaired metabolism of thiamine, biotin, vitamin B6, vitamin B12 and folic acid can contribute to early-onset seizures and developmental abnormalities in infants. However, timely supplementation therapy can significantly improve patient prognosis of affected patients. Therefore, a thorough understanding and investigation of the metabolic basis of epilepsy is essential for the development of precise therapeutic approaches, which could provide significant therapeutic benefits for patients.

Hypophosphatasia in Japan: ALPL Mutation Analysis in 98 Unrelated Patients

Hypophosphatasia (HPP) is highly variable in clinical expression and is generally classified into six subtypes. Although it would be beneficial to be able to predict the clinical course from the ALPL genotype, studies on this issue are limited. Here, we aimed to clarify the features of Japanese HPP and the relationships between genotype and clinical manifestations. We analyzed 98 unrelated Japanese patients to investigate the percentage of each clinical form, frequently detected mutations, and the relationship between the genotype and phenotype. Some of the identified mutants were characterized by transfection experiments. Perinatal severe form was the most frequent (45.9%), followed by perinatal benign form (22.4%). Among the 196 alleles, p.Leu520ArgfsX86 (c.1559delT) was detected in 89 alleles, and p.Phe327Leu (c.979T>C) was identified in 23 alleles. All of the homozygotes for p.Leu520ArgfsX86 were classified into perinatal severe form, and patients carrying p.Phe327Leu in one of the alleles were classified into perinatal benign or odonto HPP. Twenty of the 22 patients with perinatal benign HPP were compound heterozygous for p.Phe327Leu and another mutation. Most patients with odonto HPP were found to be monoallelic heterozygotes for dominant-negative mutations or compound heterozygotes with mutants having residual activity. The high prevalence of p.Leu520ArgfsX86 and p.Phe327Leu mutations might underlie the high rate of perinatal severe and perinatal benign forms, respectively, in Japanese HPP. Although ALPL genotyping would be beneficial for predicting the clinical course to an extent, the observed phenotypical variability among patients sharing the same genotypes suggests the presence of modifiers.

Hypophosphatasia: Biological and Clinical Aspects, Avenues for Therapy

Hypophosphatasia (HPP) is a rare inherited systemic metabolic disease caused by mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. TNSALP is expressed in the liver, kidney and bone, and its substrates include TNSALP inorganic pyrophosphate, pyridoxal-5'-phosphate (PLP)/vitamin B6 and phosphoethanolamine (PEA). Autosomal recessive and dominant forms of the disease result in a range of clinical entities. Major hallmarks are low alkaline phosphatase (ALP) and elevated PLP and PEA levels. Very severe infantile forms of HPP cause premature death as a result of respiratory insufficiency and also present with hypo-mineralisation leading to deformed limbs with, in some cases, the near-absence of bones and skull altogether. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency are indicative of a poor prognosis. Craniosynostosis is frequent. HPP leads to an unusual presentation of rickets with high levels of calcium and phosphorus, resulting in hypercalciuria, nephrocalcinosis and low ALP levels. Hypercalcaemic crisis, failure to thrive and growth retardation are concerns in infants. Fractures are common in both infantile and adult forms of the disease, concomitantly occurring with unexplained chronic pain and fatigue. Dental clinical presentations, which include the premature loss of teeth, are also commonly found in HPP and specifically manifest as odontohypophosphatasia. A novel enzyme therapy for human HPP, asfotase alfa, which is specifically targeted to mineralised tissues, has been developed in the past decades. While this treatment seems very promising, especially for infantile HPP, many questions regarding its long-term effects, the management of treatment, and any potential secondary adverse effects remain unresolved.

Update on the management of hypophosphatasia

Hypophosphatasia is a rare inherited disease caused by a loss of function mutations in the gene that codes for the tissue-nonspecific alkaline phosphatase enzyme. It is autosomally inherited and at least 388 different genetic defects have been identified. The clinical presentation is variable from a severe perinatal form, that is fatal if untreated, to adult-onset disease. This review covers the pathophysiology, diagnosis and current management option including the recently licensed enzyme replacement therapy asfotase alfa.

Genetic engineering a large animal model of human hypophosphatasia in sheep

The availability of tools to accurately replicate the clinical phenotype of rare human diseases is a key step toward improved understanding of disease progression and the development of more effective therapeutics. We successfully generated the first large animal model of a rare human bone disease, hypophosphatasia (HPP) using CRISPR/Cas9 to introduce a single point mutation in the tissue nonspecific alkaline phosphatase (TNSALP) gene (ALPL) (1077 C > G) in sheep. HPP is a rare inherited disorder of mineral metabolism that affects bone and tooth development, and is associated with muscle weakness. Compared to wild-type (WT) controls, HPP sheep have reduced serum alkaline phosphatase activity, decreased tail vertebral bone size, and metaphyseal flaring, consistent with the mineralization deficits observed in human HPP patients. Computed tomography revealed short roots and thin dentin in incisors, and reduced mandibular bone in HPP vs. WT sheep, accurately replicating odonto-HPP. Skeletal muscle biopsies revealed aberrant fiber size and disorganized mitochondrial cristae structure in HPP vs. WT sheep. These genetically engineered sheep accurately phenocopy human HPP and provide a novel large animal platform for the longitudinal study of HPP progression, as well as other rare human bone diseases.

Genetics of hypophosphatasia

Hypophosphatasia (HPP) is a rare inherited disorder primarily affecting bone and dental mineralization. Although there is a continuum in the severity of the disease, clinical forms may be arbitrarily distinguished on the basis of age at onset and the presence or absence of bone symptoms: perinatal, infantile, juvenile, adult, prenatal benign, and odontological. Severe forms (perinatal and infantile) are autosomally recessively inherited while less severe forms may be autosomally recessively or dominantly inherited. Genetic counseling is complicated by the coexistence of the two modes of inheritance, the incomplete penetrance of the dominant forms, the markedly variable expression of the disease, including intra-familial expression, and the existence of a benign prenatal form that may sometimes be difficult to distinguish from the severe prenatal form. The disease is due to loss-of-function mutations in the Alkaline Phosphatase-Liver (ALPL) gene encoding the tissue nonspecific alkaline phosphatase (TNSALP). The great variety of missence mutations and the dominant negative effect of some mutations largely explain the clinical heterogeneity. Directed mutagenesis studies allowed further elucidation of the cellular pathophysiology of HPP, classification of the alleles in terms of their severity and dominant negative effect, and molecular explanations of the dominant inheritance mode. Genetics significantly contributed to show that there are in fact two HPPs, rare, severe and recessive HPP, and mild recessive or mild dominant HPP, which is markedly more frequent and probably under-diagnosed. The prevalence of the severe forms of HPP has been estimated to be 1/300,000 in France and Northern Europe while the prevalence of the moderate forms of HPP may reach 1/6,370.

Brief Clinical Report: Hypophosphatasia-Diagnostic Considerations and Treatment Outcomes in an Infant

Hypophosphatasia (HPP) is a rare, inherited metabolic bone disorder characterized by low serum alkaline phosphatase activity and impaired bone mineralization. Clinical manifestations and severity of symptoms vary widely in HPP, ranging from in utero death to isolated dental manifestations in adults. Treatment with enzyme replacement therapy has been reported to improve outcomes in perinatal, infantile, and childhood forms of HPP. Here, we present a case of a boy with poor linear growth, mild limb bowing, and radiographic rickets who was diagnosed with HPP before 6 months of age. Treatment with enzyme replacement therapy was initiated at 7 months of age, after which significant improvements in radiographic findings and linear growth were demonstrated. This case highlights several important challenges in the diagnosis, classification, and management of HPP.

Hypophosphatasia: An overview For 2017

Hypophosphatasia (HPP) is the inborn-error-of-metabolism that features low serum alkaline phosphatase (ALP) activity (hypophosphatasemia) caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of ALP (TNSALP). Autosomal recessive or autosomal dominant inheritance from among >300 TNSALP (ALPL) mutations largely explains HPP's remarkably broad-ranging severity. TNSALP is a cell-surface homodimeric phosphohydrolase richly expressed in the skeleton, liver, kidney, and developing teeth. In HPP, TNSALP substrates accumulate extracellularly. Among them is inorganic pyrophosphate (PPi), a potent inhibitor of mineralization. Superabundance of extracellular PPi explains the hard tissue complications of HPP that feature premature loss of deciduous teeth and often rickets or osteomalacia as well as calcific arthropathies in some affected adults. In infants with severe HPP, blocked entry of minerals into the skeleton can cause hypercalcemia, and insufficient hydrolysis of pyridoxal 5'-phosphate (PLP), the major circulating form of vitamin B₆, can cause pyridoxine-dependent seizures. Elevated circulating PLP is a sensitive and specific biochemical marker for HPP. Also, the TNSALP substrate phosphoethanolamine (PEA) is usually elevated in serum and urine in HPP, though less reliably for diagnosis. Pathognomonic radiographic changes occur in pediatric HPP when the skeletal disease is severe. TNSALP mutation analysis is essential for recurrence risk assessment for HPP in future pregnancies and for prenatal diagnosis. HPP was the final rickets/osteomalacia to have a medical treatment. Now, significant successes using asfotase alfa, a mineral-targeted recombinant TNSALP, are published concerning severely affected newborns, infants, and children. Asfotase alfa was approved by regulatory agencies multinationally in 2015 typically for pediatric-onset HPP.

Hypophosphatasia: Enzyme Replacement Therapy Brings New Opportunities and New Challenges

Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Autosomal inheritance (dominant or recessive) from among more than 300 predominantly missense defects of TNSALP (ALPL) explains HPP's broad-ranging severity, the greatest of all skeletal diseases. In health, TNSALP is linked to cell surfaces and richly expressed in the skeleton and developing teeth. In HPP,TNSALP substrates accumulate extracellularly, including inorganic pyrophosphate (PPi), an inhibitor of mineralization. The PPi excess can cause tooth loss, rickets or osteomalacia, calcific arthropathies, and perhaps muscle weakness. Severely affected infants may seize from insufficient hydrolysis of pyridoxal 5'-phosphate (PLP), the major extracellular vitamin B₆ . Now, significant successes are documented for newborns, infants, and children severely affected by HPP given asfotase alfa, a hydroxyapatite-targeted recombinant TNSALP. Since fall 2015, this biologic is approved by regulatory agencies multinationally typically for pediatric-onset HPP. Safe and effective treatment is now possible for this last rickets to have a medical therapy, but a number of challenges involving diagnosis, understanding prognosis, and providing this treatment are reviewed herein. © 2017 American Society for Bone and Mineral Research.

Current concepts in odontohypophosphatasia form of hypophosphatasia and report of two cases

BACKGROUND

Hypophosphatasia is a rare inherited disease derived from mutations in tissue non-specific alkaline phosphatase genes, with typical oral symptoms including short root anomaly and dysplasia of dentin or cementum.

CASE PRESENTATION

Two young female patients presented with short root anomaly with a history of premature loss of deciduous and/or permanent teeth. The laboratory and imaging investigations were performed. One case was diagnosed as odontohypophosphatasia concurrent with hyperthyroidism, the other was odontohypophosphatasia concurrent with multiple radicular cysts.

CONCLUSION

This report presents two cases of odontohypophosphatasia, a rare disease which is difficult to be diagnosed, and highlights that the history of premature loss of deciduous and/or permanent teeth, oral manifestation and laboratory tests are crucial for clinical diagnosis.

Hypophosphatasia - aetiology, nosology, pathogenesis, diagnosis and treatment

Hypophosphatasia is the inborn error of metabolism characterized by low serum alkaline phosphatase activity (hypophosphatasaemia). This biochemical hallmark reflects loss-of-function mutations within the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP is a cell-surface homodimeric phosphohydrolase that is richly expressed in the skeleton, liver, kidney and developing teeth. In hypophosphatasia, extracellular accumulation of TNSALP natural substrates includes inorganic pyrophosphate, an inhibitor of mineralization, which explains the dento-osseous and arthritic complications featuring tooth loss, rickets or osteomalacia, and calcific arthopathies. Severely affected infants sometimes also have hypercalcaemia and hyperphosphataemia due to the blocked entry of minerals into the skeleton, and pyridoxine-dependent seizures from insufficient extracellular hydrolysis of pyridoxal 5'-phosphate, the major circulating form of vitamin B6, required for neurotransmitter synthesis. Autosomal recessive or dominant inheritance from ~300 predominantly missense ALPL (also known as TNSALP) mutations largely accounts for the remarkably broad-ranging expressivity of hypophosphatasia. High serum concentrations of pyridoxal 5'-phosphate represent a sensitive and specific biochemical marker for hypophosphatasia. Also, phosphoethanolamine levels are usually elevated in serum and urine, though less reliably for diagnosis. TNSALP mutation detection is important for recurrence risk assessment and prenatal diagnosis. Diagnosing paediatric hypophosphatasia is aided by pathognomic radiographic changes when the skeletal disease is severe. Hypophosphatasia was the last type of rickets or osteomalacia to await a medical treatment. Now, significant successes for severely affected paediatric patients are recognized using asfotase alfa, a bone-targeted recombinant TNSALP.

Molecular Genetics of Hypophosphatasia and Phenotype-Genotype Correlations

Hypophosphatasia (HPP) is due to deficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNAP). This enzyme cleaves extracellular substrates inorganic pyrophosphates (PPi), pyridoxal-5'-phosphate (PLP), phosphoethanolamine (PEA) and nucleotides, and probably other substrates not yet identified. During the last 15 years the role of TNAP in mineralization, and to a less degree in brain, has been investigated, providing hypotheses and explanations for both bone and neuronal HPP phenotypes. ALPL, the gene encoding TNAP, is subject to many mutations, mostly missense mutations. A few number of mutations are recurrently found and may be quite frequent in particular populations. This reflects founder effects. The great variety of mutations results in a great number of compound heterozygous genotypes and in highly variable clinical expressivity. A good correlation was observed between the severity of the disease and in vitro enzymatic activity of the mutant protein measured after site-directed mutagenesis. Many missense mutations found in severe hypophosphatasia produced a mutant protein that failed to reach the cell membrane , was accumulated in the cis-Golgi and was subsequently degraded in the proteasome. Missense mutations located in the catalytic site or in the homodimer interface were often shown by site-directed mutagenesis to have a dominant negative effect. Currently molecular diagnosis of HPP is based on the sequencing of the coding sequence of ALPL that allows detection of approximately 95 % of mutations in severe cases. In addition, other genes, especially genes encoding proteins involved in the regulation of extracellular PPi concentration, could modify the phenotype (modifier genes).

Hypophosphatasia: an overview of the disease and its treatment

This review presents the current knowledge on hypophosphatasia, a rare genetic disease of very variable severity (from lethal to mild) and clinical presentation, caused by defective production of tissue-non-specific alkaline phosphatase (TNSALP). Hypophosphatasia can affect babies in utero as well as infants, children, and adults. The article first presents the genetics of TNSALP and its many known mutations underlying the disease. Then, it presents the epidemiology, classification, and clinical presentation of the six different forms of the disease (perinatal lethal, prenatal benign, infantile, childhood, adult, and odontohypophosphatasia) as well as the essential diagnostic clues. The last section on treatment presents a survey of the therapeutic approaches, up to the ongoing phase 2 studies of enzyme replacement therapy.

Molecular diagnosis of hypophosphatasia and differential diagnosis by targeted Next Generation Sequencing

Hypophosphatasia (HPP) is a rare inherited skeletal dysplasia due to loss of function mutations in the ALPL gene. The disease is subject to an extremely high clinical heterogeneity ranging from a perinatal lethal form to odontohypophosphatasia affecting only teeth. Up to now genetic diagnosis of HPP is performed by sequencing the ALPL gene by Sanger methodology. Osteogenesis imperfecta (OI) and campomelic dysplasia (CD) are the main differential diagnoses of severe HPP, so that in case of negative result for ALPL mutations, OI and CD genes had often to be analyzed, lengthening the time before diagnosis. We report here our 18-month experience in testing 46 patients for HPP and differential diagnosis by targeted NGS and show that this strategy is efficient and useful. We used an array including ALPL gene, genes of differential diagnosis COL1A1 and COL1A2 that represent 90% of OI cases, SOX9, responsible for CD, and 8 potentially modifier genes of HPP. Seventeen patients were found to carry a mutation in one of these genes. Among them, only 10 out of 15 cases referred for HPP carried a mutation in ALPL and 5 carried a mutation in COL1A1 or COL1A2. Interestingly, three of these patients were adults with fractures and/or low BMD. Our results indicate that HPP and OI may be easily misdiagnosed in the prenatal stage but also in adults with mild symptoms for these diseases.

Hypophosphatasia: validation and expansion of the clinical nosology for children from 25 years experience with 173 pediatric patients

Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) within the gene TNSALP that encodes the "tissue-nonspecific" isoenzyme of alkaline phosphatase (TNSALP). In HPP, inorganic pyrophosphate, an inhibitor of mineralization and substrate for TNSALP, accumulates extracellularly often leading to rickets or osteomalacia and tooth loss, and sometimes to craniosynostosis and calcium crystal arthropathies. HPP's remarkably broad-ranging expressivity spans stillbirth from profound skeletal hypomineralization to adult-onset dental problems or arthropathies without bone disease, which is largely explained by autosomal recessive versus autosomal dominant transmission from among several hundred, usually missense, TNSALP mutations. For clinical purposes, this expressivity has been codified according to absence or presence of skeletal disease and then patient age at presentation and diagnosis. Pediatric patients are reported principally with "odonto", "childhood", "infantile", or "perinatal" HPP. However, this nosology has not been tested using a cohort of patients, and the ranges of the clinical and laboratory findings have not been defined and contrasted among these patient groups. To evaluate the extant nosology for HPP in children, we assessed our 25 years experience with 173 pediatric HPP patients. Data were exclusively from inpatient studies. The childhood form of HPP was further designated "mild" or "severe". Here, we focused on demographic, clinical, and dual-energy X-ray absorptiometry parameters compared to data from healthy American children. The 173-patient cohort comprised 64 individuals with odonto HPP, 38 with mild childhood HPP, 58 with severe childhood HPP, and 13 with infantile HPP. None was a survivor of perinatal HPP. TNSALP analysis revealed a mutation(s) in all 105 probands tested. Thirteen mutations were unique. Most patients represented autosomal dominant inheritance of HPP. Mutant allele dosage generally indicated the disorder's severity. Gender discordance was found for severe childhood HPP; 42 boys versus 16 girls (p=0.006), perhaps reflecting parental concern about stature and strength. Key disease parameters (e.g., height, weight, numbers of teeth lost prematurely, grip strength, spine and hip bone mineral density) were increasingly compromised as HPP was designated more severe. Although data overlapped successively between the four patient groups, body size (height and weight) differed significantly. Thus, our expanded nosology for HPP in children organizes the disorder's broad-ranging expressivity and should improve understanding of HPP presentation, natural history, complications, and prognosis.

Clinical Forms and Animal Models of Hypophosphatasia

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.

Hypophosphatasia: nonlethal disease despite skeletal presentation in utero (17 new cases and literature review)

Hypophosphatasia (HPP) is caused by deactivating mutation(s) within the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). Patients manifest rickets or osteomalacia and dental disease ranging from absence of skeletal mineralization in utero to only loss of adult dentition. Until recently, HPP skeletal disease in utero was thought to always predict a lethal outcome. However, several reports beginning in 1999 emphasized a benign prenatal form of HPP (BP-HPP) where skeletal disease detected in utero had a mild postnatal course. Here we describe prenatal and postnatal findings of 17 additional BP-HPP patients among our 178 pediatric HPP patients. Their findings are compared with those of their siblings with HPP, carrier parents, and others with identical TNSALP mutations. New information concerning 7 previously published BP-HPP patients accompanies a review of the HPP literature. Among our 17 BP-HPP patients, prenatal ultrasound showed normal chest or abdominal circumferences where recorded. Sometimes, poor skeletal mineralization, fetal crowding, and third-trimester improvement were observed. Postnatally, extremity bowing further improved (13 patients). BP-HPP severity postnatally spanned the "infantile" to "odonto" HPP phenotypes, resembling our patients who harbored identical TNSALP mutation(s). Eight had autosomal dominant (AD) and 9 had autosomal recessive (AR) BP-HPP. Fourteen of our 15 mothers were HPP carriers or affected. Of the 41 cumulative BP-HPP patients (24 literature cases meriting a BP-HPP diagnosis since 1996 plus our 17 patients), 63% had AR BP-HPP. Maternally transmitted HPP involved 11 of the 13 total AD BP-HPP probands (p = 0.01), supporting a maternal in utero effect on the baby. Fetal crowding, normal fetal mineralization and chest size, and TNSALP heterozygosity seem to identify BP-HPP. However, bowed fetal long bones with AR HPP, specific TNSALP mutations, or poor skeletal mineralization before the third trimester do not reliably diagnose HPP lethality.

Perinatal hypophosphatasia presenting as neonatal epileptic encephalopathy with abnormal neurotransmitter metabolism secondary to reduced co-factor pyridoxal-5'-phosphate availability

We describe two neonates presenting with perinatal hypophosphatasia and severe epileptic encephalopathy resulting in death. Both had increased levels of urinary vanillactate, indicating functional deficiency of aromatic amino acid decarboxylase, a pyridoxal-5-phosphate (PLP)-dependent enzyme required for dopamine and serotonin biosynthesis. Clinical findings and results of subsequent metabolic investigations were consistent with secondary pyridoxine-deficient encephalopathy. These patients highlight the importance of tissue non-specific alkaline phosphatase in the neuronal PLP-dependent metabolism of neurotransmitters. In addition, the disturbance of PLP metabolism appears to underlie the predominant neurological presentation in our patients. We recommend the measurement of serum alkaline phosphatase (ALP) during the assessment of perinatal seizures.

Hypophosphatasia: molecular testing of 19 prenatal cases and discussion about genetic counseling

OBJECTIVE

We studied hypophosphatasia (HP) mutations in 19 cases prenatally detected by ultrasonography without familial history of HP. We correlated the mutations with the reported ultrasound signs, and discussed genetic counseling with regard to the particular dominantly inherited prenatal benign form of HP.

METHOD

The coding sequence of the tissue nonspecific alkaline phosphatase (TNSALP) gene was analyzed by DNA sequencing, and 3D modeling was used to locate the mutated amino acids with regard to the functional domains of TNSALP.

RESULTS

Although reported ultrasound signs were heterogeneous, two mutated alleles were found in 18 of the 19 cases studied, indicating recessive transmission of the disease. Functional domains of TNSALP were affected by 74% of missense mutations. In all the cases, including one with only a heterozygous mutation, molecular, biological, and familial data do not corroborate the hypothesis of prenatal benign HP. The mutation c.1133A>T observed in the prenatal benign form of HP and common in USA was not found in this series.

CONCLUSION

The results point out the prenatally detectable allelic heterogeneity of HP. The nature of the detected mutations and the evidence of recessive inheritance do not support these cases being affected with prenatal benign HP.

Autosomal recessive hypophosphatasia manifesting in utero with long bone deformity but showing spontaneous postnatal improvement

CONTEXT

Hypophosphatasia (HPP) is a heritable metabolic disorder of the skeleton that includes variable expressivity conditioned by gene dosage effect and the variety of mutations in the tissue nonspecific alkaline phosphatase (TNSALP) gene. Patient age when skeletal problems first manifest generally predicts the clinical course, with perinatal HPP causing bone disease in utero with postnatal lethality.

OBJECTIVE

Our objective was to identify TNSALP mutations and characterize the inheritance pattern of a family with clinically variable HPP with one child manifesting in utero with long bone deformity but showing spontaneous prenatal and postnatal improvement.

DESIGN

TNSALP enzyme and substrate analysis and TNSALP mutation analysis were performed on all family members.

PATIENTS

A boy with HPP showing long bone deformity that spontaneously improved in utero and after birth is described. His older brother has the childhood form of HPP without findings until after infancy. His parents and twin sister are clinically unaffected.

RESULTS

Both boys are compound heterozygotes for the same missense mutations in TNSALP, documenting autosomal recessive inheritance for their HPP. The parents each carry one defective allele.

CONCLUSIONS

The patient is an autosomal recessive case of HPP with prenatal long bone deformity but with spontaneous prenatal and postnatal improvement. Thus, prenatal detection by sonography of bowing of long bones from HPP, even with autosomal recessive inheritance, does not necessarily predict lethality but can represent variable expressivity or the effects of modifiers on the TNSALP defect(s).

Hypophosphatasia

Hypophosphatasia is a rare inherited disorder characterized by defective bone and tooth mineralization, and deficiency of serum and bone alkaline phosphatase activity. The frequency of the disease has been estimated to be one in 100 000 for severe forms, but mild forms of hypophosphatasia may be more common. The symptoms are highly variable in their clinical expression, which ranges from stillbirth without mineralized bone to early tooth loss without bone symptoms. The transmission of severe forms is autosomal recessive, while milder forms may be transmitted as dominant or recessive autosomal traits. The diagnosis is based on serum alkaline phosphatase assay and molecular analysis of the liver/bone/kidney alkaline phosphatase gene (ALPL). Currently, there is no treatment for the disease. Over the past 10 years, great progress has been made in understanding the structure of tissue non-specific alkaline phosphatase, its function in bone mineralization, and the effect of ALPL mutations responsible for hypophosphatasia.

Adult hypophosphatasia treated with teriparatide

INTRODUCTION

Hypophosphatasia (HPP) features low serum alkaline phosphatase (ALP) activity (hypophosphatasemia) due to loss-of-function mutation within TNSALP, the gene that encodes "tissue-nonspecific" ALP (TNSALP). Consequently, inorganic pyrophosphate accumulates extracellularly and impairs skeletal mineralization. Affected adults manifest osteomalacia, often with slowly healing metatarsal stress fractures (MTSFs) and proximal femur pseudofractures. Pharmacotherapy remains elusive.

PATIENT AND METHODS

A middle-aged woman sustained a slowly healing MTSF and then two enlarging MTSFs and a spontaneous proximal femur fracture. Pain persisted at all fracture sites. HPP was diagnosed as a result of low ALP activity (10-24 IU/liter; normal, 40-150 IU/liter) and elevated inorganic phosphate and pyridoxal 5'-phosphate concentrations in serum. Teriparatide (TPTD) (recombinant human PTH 1-34), 20 microg, was injected sc daily in an attempt to enhance osteoblast synthesis of TNSALP.

RESULTS

Six weeks later, all fracture pain improved, and it resolved after 4 months. Radiographs of the enlarging MTSFs showed repair after 2-4 months. The femur fracture partially mended after 2 months and then healed. Additionally, hypophosphatasemia and hyperphosphatemia corrected, and biochemical markers of bone remodeling increased as long as TPTD (given for 18 months) was continued. The patient carried a heterozygous TNSALP missense mutation, p.D378V, which is common in the United States.

CONCLUSION

This first HPP patient given TPTD demonstrated fracture repair accompanying correction of hypophosphatasemia and hyperphosphatemia and bone marker responses indicating enhanced skeletal remodeling. Increased TNSALP synthesis in bone together with lowered extracellular concentrations of inorganic phosphate (a competitive inhibitor of ALPs) seemed to improve her skeletal mineralization. Further evaluation of TPTD for HPP is warranted.

Aberrant interchain disulfide bridge of tissue-nonspecific alkaline phosphatase with an Arg433→Cys substitution associated with severe hypophosphatasia

Various mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene are responsible for hypophosphatasia characterized by defective bone and tooth mineralization; however, the underlying molecular mechanisms remain largely to be elucidated. Substitution of an arginine at position 433 with a histidine [TNSALP(R433H)] or a cysteine [TNSALP(R433C)] was reported in patients diagnosed with the mild or severe form of hypophosphatasia, respectively. To define the molecular phenotype of the two TNSALP mutants, we sought to examine them in transient (COS-1) and conditional (CHO-K1 Tet-On) heterologous expression systems. In contrast to an 80 kDa mature form of the wild-type and TNSALP(R433H), a unique disulfide-bonded 160 kDa molecular species appeared on the cell surface of the cells expressing TNSALP(R433C). Sucrose density gradient centrifugation demonstrated that TNSALP(R433C) forms a disulfide-bonded dimer, instead of being noncovalently assembled like the wild-type. Of the five cysteine residues per subunit of the wild-type, only Cys102 is thought to be present in a free form. Replacement of Cys102 with serine did not affect the dimerization state of TNSALP(R433C), implying that TNSALP(R433C) forms a disulfide bridge between the cysteine residues at position 433 on each subunit. Although the cross-linking did not significantly interfere with the intracellular transport and cell surface expression of TNSALP(R433C), it strongly inhibited its alkaline phosphatase activity. This is in contrast to TNSALP(R433H), which shows enzyme activity comparable to that of the wild-type. Importantly, addition of dithiothreitol to the culture medium was found to partially reduce the amount of the cross-linked form in the cells expressing TNSALP(R433C), concomitantly with a significant increase in enzyme activity, suggesting that the cross-link between two subunits distorts the overall structure of the enzyme such that it no longer efficiently carries out its catalytic function. Increased susceptibility to proteases confirmed a gross conformational change of TNSALP(R433C) compared with the wild-type. Thus, loss of function resulting from the interchain disulfide bridge is the molecular basis for the lethal hypophosphatasia associated with TNSALP(R433C).

Low Serum Alkaline Phosphatase Activity and Pathologic Fracture: Case Report and Brief Review of Hypophosphatasia Diagnosed in Adulthood

OBJECTIVE

To describe an elderly patient with low serum alkaline phosphatase (ALP) activity detected after a pathologic fracture and to characterize hypophosphatasia in adult patients.

METHODS

We present a case report of a 64-year-old woman, who was referred after sustaining an atraumatic femoral fracture treated successfully with intramedullary nailing. Clinical, biochemical, radiologic, and molecular studies explore the differential diagnosis of her hypophosphatasemia, and the features, diagnosis, and management of the adult form of hypophosphatasia are reviewed.

RESULTS

Physical examination of our patient was revealing only for short stature. Bone mineral density evaluated by dual-energy x-ray absorptiometry was unremarkable. Biochemical investigations showed normal calcium, elevated inorganic phosphate, and low ALP levels in serum. In light of the hypophosphatasemia and pathologic fracture, the serum pyridoxal 5'-phosphate concentration was measured and found to be considerably elevated, a substantiation of the diagnosis of hypophosphatasia. Analysis of the gene encoding the "tissue-nonspecific" isoenzyme of ALP (TNSALP) demonstrated a novel, heterozygous, missense mutation causing her disorder.

CONCLUSION

Hypophosphatasia is a rare inborn error of metabolism due to a deactivating mutation (or mutations) of the gene encoding TNSALP, in turn leading to global deficiency of TNSALP activity and inadequate skeletal mineralization and fractures. Our patient illustrates the importance of low serum ALP activity in the assessment of patients with fractures. No established treatment exists for hypophosphatasia, but the correct diagnosis should help to avoid the use of traditional therapies for osteoporosis or osteomalacia, which would be ineffective or potentially harmful.