Hypophosphatasia

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.

Rare causes of osteoporosis

Osteoporosis is a metabolic bone disease characterized by loss of bone mass and strength, resulting in increased risk of fractures. It is classically divided into primary (post-menopausal or senile), secondary and idiopathic forms. There are many rare diseases, that cause directly or indirectly osteoporosis. The identification and classification of most of these rare causes of osteoporosis is crucial for the specialists in endocrinology and not, in order to prevent this bone complication and to provide for an early therapy. Several pathogenic mechanisms are involved, including various aspects of bone metabolism such as: decreased bone formation, increased bone resorption, altered calcium, phosphorus and/or vitamin D homeostasis, and abnormal collagen synthesis. In this review, less common forms of primary and secondary osteoporosis are described, specifying, if applicable: genetic causes, epidemiology, clinical features, and pathogenic mechanisms causing osteoporosis. A greater awareness of all rare causes of osteoporosis could reduce the number of cases classified as idiopathic osteoporosis and allow the introduction of appropriate and timely treatments.

Ex Vivo Expanded Allogeneic Mesenchymal Stem Cells with Bone Marrow Transplantation Improved Osteogenesis in Infants with Severe Hypophosphatasia

Patients with severe hypophosphatasia (HPP) develop osteogenic impairment with extremely low alkaline phosphatase (ALP) activity, resulting in a fatal course during infancy. Mesenchymal stem cells (MSCs) differentiate into various mesenchymal lineages, including bone and cartilage. The efficacy of allogeneic hematopoietic stem cell transplantation for congenital skeletal and storage disorders is limited, and therefore we focused on MSCs for the treatment of HPP. To determine the effect of MSCs on osteogenesis, we performed multiple infusions of ex vivo expanded allogeneic MSCs for two patients with severe HPP who had undergone bone marrow transplantation (BMT) from asymptomatic relatives harboring the heterozygous mutation. There were improvements in not only bone mineralization but also muscle mass, respiratory function, and mental development, resulting in the patients being alive at the age of 3. After the infusion of MSCs, chimerism analysis of the mesenchymal cell fraction isolated from bone marrow in the patients demonstrated that donor-derived DNA sequences existed. Adverse events of BMT were tolerated, whereas those of MSC infusion did not occur. However, restoration of ALP activity was limited, and normal bony architecture could not be achieved. Our data suggest that multiple MSC infusions, following BMT, were effective and brought about clinical benefits for patients with lethal HPP. Allogeneic MSC-based therapy would be useful for patients with other congenital bone diseases and tissue disorders if the curative strategy to restore clinically normal features, including bony architecture, can be established.

Reversible Deterioration in Hypophosphatasia Caused by Renal Failure With Bisphosphonate Treatment

Hypophosphatasia is an inborn error of metabolism caused by mutations in the ALPL gene. It is characterized by low serum alkaline phosphatase (ALP) activity and defective mineralization of bone, but the phenotype varies greatly in severity depending on the degree of residual enzyme activity. We describe a man with compound heterozygous mutations in ALPL, but no previous bone disease, who suffered numerous disabling fractures after he developed progressive renal failure (for which he eventually needed dialysis treatment) and was prescribed alendronate treatment. A bone biopsy showed marked osteomalacia with low osteoblast numbers and greatly elevated pyrophosphate concentrations at mineralizing surfaces. In vitro testing showed that one mutation, T117H, produced an ALP protein with almost no enzyme activity; the second, G438S, produced a protein with normal activity, but its activity was inhibited by raising the media phosphate concentration, suggesting that phosphate retention (attributable to uremia) could have contributed to the phenotypic change, although a pathogenic effect of bisphosphonate treatment is also likely. Alendronate treatment was discontinued and, while a suitable kidney donor was sought, the patient was treated for 6 months with teriparatide, which significantly reduced the osteomalacia. Eighteen months after successful renal transplantation, the patient was free of symptoms and the scintigraphic bone lesions had resolved. A third bone biopsy showed marked hyperosteoidosis but with plentiful new bone formation and a normal bone formation rate. This case illustrates how pharmacological (bisphosphonate treatment) and physiologic (renal failure) changes in the "environment" can dramatically affect the phenotype of a genetic disorder.

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.

Neurological Symptoms of Hypophosphatasia

Hypophosphatasia (HPP) is a bone metabolic disorder caused by mutations in the liver/bone/kidney alkaline phosphatase gene (ALPL), which encodes tissue-nonspecific alkaline phosphatase (TNAP). This disease is characterized by disrupted bone and tooth mineralization, and reduced serum AP activity. Along with bone and tooth symptoms, many neurological symptoms, seizure, encephalopathy, intracranial hypertension, mental retardation, deafness, and growth hormone deficiency (GHD), are frequently found in HPP patients. Seizure occurs in severe HPP types soon after birth, and responds to pyridoxine, but is an indicator of lethal prognosis. Encephalopathy rarely presents in severe HPP types, but has severe sequelae. Intracranial hypertension complicated in mild HPP types develops after the age of 1 year and sometimes need neurosurgical intervention. Mental retardation, deafness and GHD are more frequently found in Japanese HPP patients. Mental retardation occurs in all HPP types. Deafness in perinatal lethal type is both conductive and sensorineural. GHD develops in all but perinatal lethal type and the diagnosis tends to delay. The pathogenesis of these neural features of HPP might be due to impairment of both vitamin B6 metabolism and central nervous system development by ALPL mutations.

Molecular evolution of the tissue-nonspecific alkaline phosphatase allows prediction and validation of missense mutations responsible for hypophosphatasia

ALPL encodes the tissue nonspecific alkaline phosphatase (TNSALP), which removes phosphate groups from various substrates. Its function is essential for bone and tooth mineralization. In humans, ALPL mutations lead to hypophosphatasia, a genetic disorder characterized by defective bone and/or tooth mineralization. To date, 275 ALPL mutations have been reported to cause hypophosphatasia, of which 204 were simple missense mutations. Molecular evolutionary analysis has proved to be an efficient method to highlight residues important for the protein function and to predict or validate sensitive positions for genetic disease. Here we analyzed 58 mammalian TNSALP to identify amino acids unchanged, or only substituted by residues sharing similar properties, through 220 millions years of mammalian evolution. We found 469 sensitive positions of the 524 residues of human TNSALP, which indicates a highly constrained protein. Any substitution occurring at one of these positions is predicted to lead to hypophosphatasia. We tested the 204 missense mutations resulting in hypophosphatasia against our predictive chart, and validated 99% of them. Most sensitive positions were located in functionally important regions of TNSALP (active site, homodimeric interface, crown domain, calcium site, …). However, some important positions are located in regions, the structure and/or biological function of which are still unknown. Our chart of sensitive positions in human TNSALP (i) enables to validate or invalidate at low cost any ALPL mutation, which would be suspected to be responsible for hypophosphatasia, by contrast with time consuming and expensive functional tests, and (ii) displays higher predictive power than in silico models of prediction.

TNAP activity is localized at critical sites of retinal neurotransmission across various vertebrate species

Evidence is emerging with regard to the role of tissue non-specific alkaline phosphatase (TNAP) in neural functions. As an ectophosphatase, this enzyme might influence neural activity and synaptic transmission in diverse ways. The localization of the enzyme in known neural circuits, such as the retina, might significantly advance an understanding of its role in normal and pathological functioning. However, the presence of TNAP in the retina is scarcely investigated. Our multispecies comparative study (zebrafish, cichlid, frog, chicken, mouse, rat, golden hamster, guinea pig, rabbit, sheep, cat, dog, ferret, squirrel monkey, human) using enzyme histochemistry and Western blots has shown the presence of TNAP activity in the retina of several mammalian species, including humans. Although the TNAP activity pattern varies across species, we have observed the following trends: (1) in all investigated species (except golden hamster), retinal vessels display TNAP activity; (2) TNAP activity consistently occurs in the photoreceptor layer; (3) in majority of the investigated species, marked TNAP activity is present in the outer and inner plexiform layers. In zebrafish, frog, chicken, guinea pig, and rat, TNAP histochemistry has revealed several sublayers of the inner plexiform layer. Frog, golden hamster, guinea pig, mouse, and human retinas possess a subpopulation of amacrine cells positively staining for TNAP activity. The expression of TNAP in critical sites of retinal signal transmission across a wide range of species suggests its fundamental, evolutionally conserved role in vision.

Clinical and genetic aspects of hypophosphatasia in Japanese patients

OBJECTIVE

We 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 phosphatase (ALPL) gene, which encodes tissue-non-specific alkaline phosphatase isoenzyme. 

METHODS

We retrospectively investigate the incidence and clinical features of 52 patients with paediatric HPP who were born between 1999 and 2010. Mutations of the ALPL gene were analysed in 31 patients.

RESULTS

The annual incidence of perinatal lethal HPP (PLH) was estimated to be 2-3/1 000 000 births. The most frequent clinical type was PLH followed by prenatal benign. In addition to bone symptoms, cerebral manifestations were frequently observed including convulsion, mental retardation, deafness and short stature with growth hormone deficiency. Respiratory failure was the most significant predictor of a poor prognosis for PLH. The first and second most frequent mutations in the ALPL gene were c.1559delT and c.T979C (p.F327L), respectively. The c.1559delT homozygous mutation was lethal with respiratory failure. Patients with the p.F327L compound heterozygous mutation had the different non-lethal type with short stature and a gradual improvement in ALP level and bone mineralisation.

CONCLUSIONS

The most frequent clinical type was the PLH type with prognosis related to respiratory failure, biochemical/radiological changes and ALPL mutations. Cerebral manifestations frequently occurred. Genotype-phenotype correlations were associated with specific outcomes in the PLH type, whereas different clinical features were associated with the same genotype in the non-lethal type.

Infantile loss of teeth: odontohypophosphatasia or childhood hypophosphatasia

Hypophosphatasia is a hereditary disorder characterized by a deficiency of serum and bone alkaline phosphatase (ALP) activity and defective skeletal mineralization. It is caused by a loss of function mutations in the tissue nonspecific ALP gene (TNSALP) encoding the tissue nonspecific alkaline phosphatase. A 4-year-and-8-month-old girl presented with premature exfoliation of the anterior incisors and canines. Very low ALP level (27 IU/ml) suggested the diagnosis of hypophosphatasia, which was supported by an elevated urine phosphoethanolamine/Cr of 84 μmol/mmol (reference range, <25 μmol/mmol) and serum pyridoxal-5'-phosphate of 393 μg/L (reference range, 3.6-18 μg/L). The phenotype of the patient was subsequently classified as mild childhood hypophosphatasia. TNSALP gene sequencing revealed the homozygous c.382 G > A (p.V128M) mutation. This mutation was previously observed in a series of patients with severe hypophosphatasia, pointing out the possible role of other genetic or environmental factors in the modulation of the hypophosphatasia phenotype.

Infantile hypophosphatasia secondary to a novel compound heterozygous mutation presenting with pyridoxine-responsive seizures

Hypophosphatasia (HPP) is a rare metabolic disease with the hallmark finding of deficient serum tissue nonspecific alkaline phosphatase (TNSALP) activity. TNSALP is primarily known for its role in mineralization; hence, HPP is characterized by defective mineralization of bone and/or teeth. TNSALP is also necessary for proper vitamin B6 metabolism and its participation as a cofactor for neurotransmitters in the central nervous system. Defective TNSALP activity in the brain can result in intractable seizures responsive to pyridoxine. The pathophysiology of pyridoxine-responsive seizures (PRS) in severe HPP remains to be clearly defined. We review the case of a 2-month-old Caucasian boy presenting with seizures refractory to conventional antiepileptic medications. Empiric treatment with favorable response to pyridoxine in conjunction with severe metabolic bone disease, extremely low serum alkaline phosphatase, elevated phosphoethanolamine, hypercalcemia, hypercalciuria, and nephrocalcinosis led to a clinical diagnosis of infantile HPP. Sequence analysis revealed compound heterozygosity of the TNSALP gene with a novel mutation in exon 9 and a previously reported mutation in exon 12. This case reminds the physician that severe infantile HPP can present with PRS as its major initial manifestation and should alert clinicians to consider HPP in their differential of PRS. In addition, despite this severe genotype, the clinical diagnosis of our patient was delayed because of minimal phenotypic features initially. This highlights that the phenotype-genotype correlation could be variable even in severe disease. This case also demonstrates that HPP should be classified as PRS and not a form of pyridoxine-dependent epilepsy (PDE) as our patient was able to stop the pyridoxine supplementation without seizure recurrence once enzyme replacement was initiated. With the advent of enzyme replacement therapy, this once fatal disease may have improved morbidity and mortality.

Phospholipases of mineralization competent cells and matrix vesicles: roles in physiological and pathological mineralizations

The present review aims to systematically and critically analyze the current knowledge on phospholipases and their role in physiological and pathological mineralization undertaken by mineralization competent cells. Cellular lipid metabolism plays an important role in biological mineralization. The physiological mechanisms of mineralization are likely to take place in tissues other than in bones and teeth under specific pathological conditions. For instance, vascular calcification in arteries of patients with renal failure, diabetes mellitus or atherosclerosis recapitulates the mechanisms of bone formation. Osteoporosis—a bone resorbing disease—and rheumatoid arthritis originating from the inflammation in the synovium are also affected by cellular lipid metabolism. The focus is on the lipid metabolism due to the effects of dietary lipids on bone health. These and other phenomena indicate that phospholipases may participate in bone remodelling as evidenced by their expression in smooth muscle cells, in bone forming osteoblasts, chondrocytes and in bone resorbing osteoclasts. Among various enzymes involved, phospholipases A₁ or A₂, phospholipase C, phospholipase D, autotaxin and sphingomyelinase are engaged in membrane lipid remodelling during early stages of mineralization and cell maturation in mineralization-competent cells. Numerous experimental evidences suggested that phospholipases exert their action at various stages of mineralization by affecting intracellular signaling and cell differentiation. The lipid metabolites—such as arachidonic acid, lysophospholipids, and sphingosine-1-phosphate are involved in cell signaling and inflammation reactions. Phospholipases are also important members of the cellular machinery engaged in matrix vesicle (MV) biogenesis and exocytosis. They may favour mineral formation inside MVs, may catalyse MV membrane breakdown necessary for the release of mineral deposits into extracellular matrix (ECM), or participate in hydrolysis of ECM. The biological functions of phospholipases are discussed from the perspective of animal and cellular knockout models, as well as disease implications, development of potent inhibitors and therapeutic interventions.

Rickets: Part II

This is the continuation of a two-part review of rickets. This part emphasizes the specific pathophysiology, clinical features, pathoanatomy and radiographic findings of vitamin D deficiency rickets. Other forms of rickets, differential diagnostic considerations and the potential relationship between low levels of vitamin D metabolites and unexplained fractures in infants are also discussed.

Respiratory mechanics in an infant with perinatal lethal hypophosphatasia treated with human recombinant enzyme replacement therapy

Hypophosphatasia is a rare autosomal recessive disorder caused by deficient activity of tissue nonspecific alkaline phosphatase (TNSALP) and characterized by defective bone mineralization. In the perinatal lethal form, respiratory complications due to rachitic deformities of the thoracic cage and associated hypoplastic lungs are present. ENB-0040 is a bone-targeted human recombinant TNSALP fusion protein that aims to restore skeletal mineralization. The goal of this study was to characterize pulmonary and thoracic cage mechanics in an infant with the perinatal lethal form of hypophosphatasia under enzyme replacement therapy. Pulmonary function testing was performed on a preterm, 8-week-old patient with hypophosphatasia who was mechanically ventilated since birth because of severe chest wall insufficiency. The measurements consisted of respiratory impulse oscillation measurements (resistance and reactance), ventilatory mechanics (compliance and resistance), and thoracoabdominal motion (TAM) analysis. At baseline, chest wall compliance was 50% of normal, and the TAM indicated predominantly abdominal displacement. After 12 weeks of treatment, a consistent decrease in ventilator requirements and improvement in lung function and chest wall mechanics were observed and correlated with thoracic cage radiologic findings. Measurable changes in chest wall dynamics and respiratory mechanics using noninvasive technology were useful for respiratory management and therapeutic guidance of ENB-0040 treatment in this patient.

Systems genetics of metabolism: the use of the BXD murine reference panel for multiscalar integration of traits

Metabolic homeostasis is achieved by complex molecular and cellular networks that differ significantly among individuals and are difficult to model with genetically engineered lines of mice optimized to study single gene function. Here, we systematically acquired metabolic phenotypes by using the EUMODIC EMPReSS protocols across a large panel of isogenic but diverse strains of mice (BXD type) to study the genetic control of metabolism. We generated and analyzed 140 classical phenotypes and deposited these in an open-access web service for systems genetics (www.genenetwork.org). Heritability, influence of sex, and genetic modifiers of traits were examined singly and jointly by using quantitative-trait locus (QTL) and expression QTL-mapping methods. Traits and networks were linked to loci encompassing both known variants and novel candidate genes, including alkaline phosphatase (ALPL), here linked to hypophosphatasia. The assembled and curated phenotypes provide key resources and exemplars that can be used to dissect complex metabolic traits and disorders.

Rare coding variants in ALPL are associated with low serum alkaline phosphatase and low bone mineral density

Alkaline phosphatase (ALP) plays an essential role in the regulation of tissue mineralization, and its activity is highly heritable. Guided by genetic associations discovered in a murine model, we hypothesized a role for rare coding variants in determining serum ALP level and bone mineral density (BMD) in humans. We sequenced the coding regions of the ALP gene (ALPL) in men with low and normal serum ALP activity levels. Single-nucleotide ALPL variants, including 19 rare nonsynonymous variants (minor allele frequency <1%), were much more frequent among the low ALP group (33.8%) than the normal group (1.4%, p = 1 × 10(-11)). Within the low ALP group, men with a rare, nonsynonymous variant had 11.2% lower mean serum ALP (p = 3.9 × 10(-4)), 6.7% lower BMD (p = 0.03), and 11.1% higher serum phosphate (p = 0.002) than those without. In contrast, common nonsynonymous variants had no association with serum ALP, phosphate, or BMD. Multiple rare ALPL coding variants are present in the general population, and nonsynonymous coding variants may be responsible for heritable differences in mineralization and thus BMD.

Novel heterozygous tissue-nonspecific alkaline phosphatase (TNAP) gene mutations causing lethal perinatal hypophosphatasia

Hypophosphatasia is a rare inherited disorder characterized by poor bone mineralization and deficiency of alkaline phosphatase activity. It is caused by mutations in the liver/bone/kidney alkaline phosphatase gene encoding the tissue-nonspecific isoenzyme of alkaline phosphatase (TNAP), which displays many allelic heterogeneities, leading to different clinical phenotypes. This study reports the case of a patient diagnosed with lethal perinatal hypophosphatasia. His gene analysis showed compound heterozygocity of two novel mutations: c.650delTinsCTAA and c.984_986delCTT, which led to p.217delVinsAK and p.328delF, respectively. The two mutations originated separately from his parents, consistent with autosomal recessive perinatal hypophosphatasia. For these two novel mutations, we analyzed their functions through three-dimensional structural analysis. This revealed that V217 is located in the β-sheet area, V217 is deleted, and insertion of alanine and lysine alter the secondary structure, causing instability in the hydrophobic region, which may influence the metal-binding vicinity. This mutant structure loses its catalytic activity. Deletion of 328F also results in protein structural alteration and affects TNAP functions. These results may provide an explanation of the two novel mutated alleles correlating with the lethal phenotype of our patient. In conclusion, we demonstrated the case of a patient with lethal perinatal hypophosphatasia caused by two novel heterozygous mutations.

Skeletal mineralization defects in adult hypophosphatasia--a clinical and histological analysis

Histomorphometry and quantitative backscattered electron microscopy of iliac crest biopsies from patients with adult hypophosphatasia not only confirmed the expected enrichment of non-mineralized osteoid, but also demonstrated an altered trabecular microarchitecture, an increased number of osteoblasts, and an impaired calcium distribution within the mineralized bone matrix.

INTRODUCTION

Adult hypophosphatasia is an inherited disorder of bone metabolism caused by inactivating mutations of the ALPL gene, encoding tissue non-specific alkaline phosphatase. While it is commonly accepted that the increased fracture risk of the patients is the consequence of osteomalacia, there are only few studies describing a complete histomorphometric analysis of bone biopsies from affected individuals. Therefore, we analyzed iliac crest biopsies from eight patients and set them in direct comparison to biopsies from healthy donors or from individuals with other types of osteomalacia.

METHODS

Histomorphometric analysis was performed on non-decalcified sections stained either after von Kossa/van Gieson or with toluidine blue. Bone mineral density distribution was quantified by backscattered electron microscopy.

RESULTS

Besides the well-documented enrichment of non-mineralized bone matrix in individuals suffering from adult hypophosphatasia, our histomorphometric analysis revealed alterations of the trabecular microarchitecture and an increased number of osteoblasts compared to healthy controls or to individuals with other types of osteomalacia. Moreover, the analysis of the mineralized bone matrix revealed significantly decreased calcium content in patients with adult hypophosphatasia.

CONCLUSIONS

Taken together, our data show that adult hypophosphatasia does not solely result in an enrichment of osteoid, but also in a considerable degradation of bone quality, which might contribute to the increased fracture risk of the affected individuals.

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.

[Diagnosis and treatment of osteomalacia by the rheumatologist]

Osteomalacia is a metabolic bone disease characterized by an alteration of bone mineralization, frequently related to alterations in vitamin D or phosphate metabolism. The clinical symptoms, radiological and analytical findings of this process vary and may be confounded with other processes, especially osteoporosis. This article reviews the most frequent causes of osteomalacia, its diagnosis and therapeutic approaches.

Hypophosphatasia in a child with widened anterior fontanelle: lessons learned from late diagnosis and incorrect treatment

Hypophosphatasia is characterized by deficiency of serum alkaline phosphatase with defective bone and teeth mineralization. We report on an 11-month-old boy who developed a complex clinical picture characterized by bulging anterior fontanelle, growth failure, nephrocalcinosis and impaired bone mineralization during high-dose calcium and vitamin D supplementation. This therapy had been started 5 months earlier for a presumed diagnosis of nutritional rickets established on the grounds of isolated widened anterior fontanelle. However, laboratory investigations revealed reduced alkaline phosphatase levels associated with hypercalcemia, hypercalciuria, low PTH and normal 25-hydroxy vitamin D levels. Genetic testing detected a compound heterozygote for the novel mutation (c.262G>A) and the described mutation (c.920C>T) in the ALPL gene.

CONCLUSION

  High calcium and vitamin D supplementation should not be started in the presence of isolated signs of nutritional rickets without assessing calcium-phosphate metabolism. In fact, in rare bone-mineralizing disorders, this combined therapy might induce severe clinical complications.

Parathyroid hormone treatment improves pain and fracture healing in adult hypophosphatasia

INTRODUCTION

Adult hypophosphatasia (HPP) is characterized by low serum alkaline phosphatase (S-ALP) and poorly healing fractures due to ALPL gene mutations. Increased S-ALP and fracture repair were reported in two patients treated with teriparatide, PTH 1-34. The effects of full-length PTH 1-84 have not been studied.

METHODS

Two 56- and 64-yr-old sisters (patients 1 and 2) with HPP and with long-standing, painful femur fractures received PTH 1-84 (Preotact, 100 μg/d sc) for 7 and 18 months, respectively. Patient 1 had another treatment 8 months later because of new femur fractures. We characterized the underlying mutation(s) and treatment effects according to S-ALP, bone markers, serum ionized calcium, plasma phosphate (P-Pi), pain, mobility, and fracture healing.

RESULTS

Both patients were compound heterozygotes for a p.G339R and p.E191K ALPL mutation. S-ALP increased significantly, 4.9- and 6.8-fold in patient 1 and 2.7-fold in patient 2. Responses decreased at 6 months but remained higher than basal activity. Serum N-terminal propeptide of type I procollagen and urinary N-telopeptide of type I collagen increased 14- to 19-fold and 9-5-fold in patient 1, respectively, and 9- and 3-fold in patient 2. P-Pi fluctuated in patient 1 and increased in patient 2. Pain and mobility improved promptly. Fractures healed after 7-8 months of treatment in patient 1 and at 15 months in patient 2.

CONCLUSION

PTH 1-84 improves pain, mobility, and fracture repair in adult HPP, even after repeat treatment. Residual activity of the p.E191K ALPL gene mutation could explain why PTH can stimulate S-ALP. P-Pi concentrations may modulate the response.

Genetic Etiology and Dental Pulp Cell Deficiency of Hypophosphatasia

Hypophosphatasia is caused by mutations of the tissue-non-specific alkaline phosphatase ( TNSALP) gene with deficiency of dentin structure. The aim of this study was to examine whether TNSALP mutation in dental pulp cells contributes to dentin dysplasia in hypophosphatasia. Mutation analysis showed that compound heterozygous mutations of TNSALP were identified in three hypophosphatasia patients, including 3 novel mutation sites. Exfoliated teeth from the patients showed abnormal dentin mineralization and loss of cementum, as assessed by ground sections and scanning electron microscope analysis. Dental pulp cells isolated from one of the patients showed a significantly reduced TNSALP activity and mineralization capacity when compared with those in dental pulp cells from the unaffected individuals. Our results suggested that dentin dysplasia in hypophosphatasia may be associated with the decreased mineralization ability of dental pulp cells.

Biological basis for the use of autologous bone marrow stromal cells in the treatment of congenital pseudarthrosis of the tibia

The study was designed to establish the biological basis for the use of autologous bone-marrow stromal cells (MSC) in order to improve the curing opportunities of congenital pseudarthrosis of the tibia (CPT). The investigation was planned by taking into account that the pathophysiology of bone healing mainly depends on the osteogenic potential of the resident cells, although several factors play a crucial role in restoring the normal bone structure. Bone marrow samples were collected from the lesion site (P) and the iliac crest (IC) of 7 patients affected by CPT and type 1 neurofibromatosis (NF1+) and 6 patients affected by CPT without NF1 (NF1-). Four patients without CPT served as control group. Biochemical, functional and molecular assays showed that the ability to generate bone-forming cells was higher in IC-MSC than in P-MSC, but lower in CPT patients than in control group. We evaluated whether host factors, such as autologous serum and the microenvironment surrounding the pseudarthrosis lesion, could impair the osteogenic differentiation of IC-MSC. Autologous serum was less effective than FBS in promoting the IC-MSC differentiation, but the damage was more evident in NF1- than in NF1+ patients. Additionally, the supernatant of osteoblast cultures obtained from bone fragments close to the lesion site favoured the differentiation of IC-MSC in NF1- patients. In summary, our results suggest that MSC transplantation could be a promising strategy for the therapy of CPT. Further studies are warranted to confirm the clinical effectiveness in comparison to standard surgical treatment.

Mild forms of hypophosphatasia mostly result from dominant negative effect of severe alleles or from compound heterozygosity for severe and moderate alleles

Background

Mild hypophosphatasia (HPP) phenotype may result from ALPL gene mutations exhibiting residual alkaline phosphatase activity or from severe heterozygous mutations exhibiting a dominant negative effect. In order to determine the cause of our failure to detect a second mutation by sequencing in patients with mild HPP and carrying on a single heterozygous mutation, we tested the possible dominant effect of 35 mutations carried by these patients.

Methods

We tested the mutations by site-directed mutagenesis. We also genotyped 8 exonic and intronic ALPL gene polymorphisms in the patients and in a control group in order to detect the possible existence of a recurrent intronic mild mutation.

Results

We found that most of the tested mutations exhibit a dominant negative effect that may account for the mild HPP phenotype, and that for at least some of the patients, a second mutation in linkage disequilibrium with a particular haplotype could not be ruled out.

Conclusion

Mild HPP results in part from compound heterozygosity for severe and moderate mutations, but also in a large part from heterozygous mutations with a dominant negative effect.

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.

Orodental phenotype and genotype findings in all subtypes of hypophosphatasia

Background

Hypophosphatasia (HP) is a rare inherited disorder characterized by a wide spectrum of defects in mineralized tissues and caused by deficiency in the tissue non-specific alkaline phosphatase gene (ALPL). The symptoms are highly variable in their clinical expression, and relate to numerous mutations in this gene. The first clinical sign of the disease is often a premature loss of deciduous teeth, mostly in the moderate forms.

Aim

The purpose of this study was to document the oral features of HP patients and to relate theses features to the six recognized forms of HP in 5 patients with known genotype and to investigate the genotype-phenotype correlations.

Methods

Clinical and radiographic examinations were carried out. We collected medical and dental history in the kindred and biochemical data. Finally, mutations in the ALPL gene were tested by DNA sequencing in SESEP laboratory.

Results

We have for the first time related the known dental anomalies which occur as integral features of HP to the recognized clinical forms of HP. We also pointed out striking dental abnormalities which were never described in association with this rare disease. Accurate genotype-phenotype severity correlations were observed.

Conclusion

This work allowed us to compare orodental manifestations in all the clinical forms of HP within the patient's sample. According to the severity of the disorder, some dental defects were infrequent, while other were always present. The long term prognosis of the permanent teeth varies from a patient to another. As premature loss of primary teeth is often the first, and sometimes the only visible symptom of the milder forms, the paediatric dentist plays a critical role in the detection and diagnosis of the disease.