A molecular approach to dominance in hypophosphatasia

Diagnosis and initial management of children presenting with premature loss of primary teeth associated with a systemic condition: A scoping review and development of clinical aid

BACKGROUND

Premature loss of primary teeth (PLPT) can be a rare presentation of systemic medical conditions. Premature loss of primary teeth may present a diagnostic dilemma to paediatric dentists.

AIMS

To identify systemic conditions associated with PLPT and develop a clinical aid.

DESIGN

OVID Medline, Embase and Web of Science were searched up to March 2023. Citation searching of review publications occurred. Exclusion occurred for conference abstracts, absence of PLPT and absence of English-language full text.

RESULTS

Seven hundred and ninety-one publications were identified via databases and 476 by citation searching of review articles. Removal of 390 duplicates occurred. Following the exclusion of 466 records on abstract review, 411 publications were sought for retrieval, of which 142 met inclusion criteria. Thirty-one systemic conditions were identified. For 19 conditions, only one publication was identified. The majority of publications, 91% (n = 129), were case reports or series. Most publications, 44% (n = 62), were related to hypophosphatasia, and 25% (n = 35) were related to Papillon-Lefèvre. Diagnostic features were synthesised, and a clinical aid was produced by an iterative consensus approach.

CONCLUSIONS

A diverse range of systemic diseases are associated with PLPT. Evidence quality, however, is low, with most diseases having a low number of supporting cases. This clinical aid supports paediatric dentists in differential diagnosis and onward referral.

Systemic effects of hypophosphatasia characterization of two novel variants in the ALPL gene

Introduction

Hypophosphatasia (HPP) is an inborn metabolic error caused by mutations in the ALPL gene encoding tissue non-specific alkaline phosphatase (TNSALP) and leading to decreased alkaline phosphatase (ALP) activity. Although the main characteristic of this disease is bone involvement, it presents a great genetic and clinical variability, which makes it a systemic disease.

Methods

Patients were recruited based on biochemical assessments. Diagnosis was made by measuring serum ALP and pyridoxal 5-phosphate levels and finally by Sanger sequencing of the ALPL gene from peripheral blood mononuclear cells. Characterization of the new variants was performed by transfection of the variants into HEK293T cells, where ALP activity and cellular localization were measured by flow cytometry. The dominant negative effect was analyzed by co-transfection of each variant with the wild-type gene, measuring ALP activity and analyzing cellular localization by flow cytometry.

Results

Two previously undescribed variants were found in the ALPL gene: leucine 6 to serine missense mutation (c.17T>C, L6S) affecting the signal peptide and threonine 167 deletion (c.498_500delCAC, T167del) affecting the vicinity of the active site. These mutations lead mainly to non-pathognomonic symptoms of HPP. Structural prediction and modeling tools indicated the affected residues as critical residues with important roles in protein structure and function. In vitro results demonstrated low TNSALP activity and a dominant negative effect in both mutations. The results of the characterization of these variants suggest that the pleiotropic role of TNSALP could be involved in the systemic effects observed in these patients highlighting digestive and autoimmune disorders associated with TNSALP dysfunction.

Conclusions

The two new mutations have been classified as pathogenic. At the clinical level, this study suggests that both mutations not only lead to pathognomonic symptoms of the disease, but may also play a role at the systemic level.

The Global ALPL gene variant classification project: Dedicated to deciphering variants

BACKGROUND

Hypophosphatasia (HPP) is an inherited multisystem disorder predominantly affecting the mineralization of bones and teeth. HPP is caused by pathogenic variants in ALPL, which encodes tissue non-specific alkaline phosphatase (TNSALP). Variants of uncertain significance (VUS) cause diagnostic delay and uncertainty amongst patients and health care providers.

RESULTS

The ALPL gene variant database (https://alplmutationdatabase.jku.at/) is an open-access archive for interpretation of the clinical significance of variants reported in ALPL. The database contains coding and non-coding variants, including single nucleotide variants, insertions/deletions and structural variants affecting coding or non-coding sequences of ALPL. Each variant in the database is displayed with details explaining the corresponding pathogenicity, and all reported genotypes and phenotypes, including references. In 2021, the ALPL gene variant classification project was established to reclassify VUS and continuously assess and update genetic, phenotypic, and functional variant information in the database. For this purpose, the database provides a unique submission system for clinicians, geneticists, genetic counselors, and researchers to submit VUS within ALPL for classification. An international, multidisciplinary consortium of HPP experts has been established to reclassify the submitted VUS using a multi-step process adhering to the stringent ACMG/AMP variant classification guidelines. These steps include a clinical phenotype assessment, deep literature research including artificial intelligence technology, molecular genetic assessment, and in-vitro functional testing of variants in a co-transfection model to measure ALP residual activity.

CONCLUSION

This classification project and the ALPL gene variant database will serve the global medical community, widen the genotypic and phenotypic HPP spectrum by reporting and characterizing new ALPL variants based on ACMG/AMP criteria and thus facilitate improved genetic counseling and medical decision-making for affected patients and families. The project may also serve as a gold standard framework for multidisciplinary collaboration for variant interpretation in other rare diseases.

Clinical and Genetic Characteristics of Hypophosphatasia in Chinese Adults

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

Childhood Hypophosphatasia Associated with a Novel Biallelic ALPL Variant at the TNSALP Dimer Interface

The goal of this study was to perform a clinical and molecular investigation in an eight-year-old female child diagnosed with hypophosphatasia (HPP). The proband and her family were evaluated by medical and dental histories, biochemical analyses, radiographic imaging, and genetic analysis of the tissue-nonspecific alkaline phosphatase (ALPL) gene. A bioinformatic analysis was performed to predict the structural and functional impact of the point mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) molecule and to define their potential contribution to the phenotype. We identified a novel combination of heterozygous ALPL missense variants in the proband, p.Ala33Val and p.Asn47His, compatible with an autosomal recessive mode of inheritance and resulting in skeletal and dental phenotypes. Computational modeling showed that the affected Asn47 residue is located in the coil structure close to the N-terminal α-helix, whereas the affected Ala33 residue is localized in the N-terminal α-helix. Both affected residues are located close to the homodimer interface, suggesting they may impair TNSALP dimer formation and stability. Clinical and biochemical follow-up revealed improvements after six years of ERT. Reporting this novel combination of ALPL variants in childhood HPP provides new insights into genotype-phenotype associations for HPP and specific sites within the TNSALP molecule potentially related to a childhood-onset HPP and skeletal and dental manifestations. Beneficial effects of ERT are implicated in skeletal and dental tissues.

Different Dental Manifestations in Sisters with the Same ALPL Gene Mutation: A Report of Two Cases

Hypophosphatasia (HPP) is an inherited disease caused by mutation of the alkaline phosphatase (ALPL) gene in an autosomal dominant or an autosomal recessive manner. The main symptoms of HPP are bone hypomineralization and early exfoliation of the primary teeth. Some of the mutations identified in autosomal dominant families are reported to have dominant negative effects. In addition, the penetrance can vary among patients with the same variant even within the same family, resulting in various phenotypes of systemic symptoms. However, differences in dental symptoms between patients with HPP and carriers with the same ALPL variant have not been reported. Herein, we report on two sisters who had the same heterozygous ALPL variant with dominant negative effects. The older sister had bone and dental symptoms and was diagnosed with childhood HPP. In contrast, the younger sister was a carrier with no bone and dental symptoms. It can be inferred that this phenomenon was caused by the difference in penetrance. This case revealed that carriers with the ALPL mutation may have no dental symptoms characteristic of HPP. Because HPP is sometimes progressive, it is very important to carefully monitor carriers to detect the possible onset of dental and systemic symptoms.

Disorders of phosphate homeostasis in children, part 1: primer on mineral ion homeostasis and the roles of phosphate in skeletal biology

Phosphate has extensive physiological roles including energy metabolism, genetic function, signal transduction and membrane integrity. Regarding the skeleton, not only do phosphate and calcium form the mineral component of the skeleton, but phosphate is also essential in regulating function of skeletal cells. Although our understanding of phosphate homeostasis has lagged behind and remains less than that for calcium, considerable advances have been made since the recognition of fibroblast growth factor-23 (FGF23) as a bone-derived phosphaturic hormone that is a major regulator of phosphate homeostasis. In this two-part review of disorders of phosphate homeostasis in children, part 1 covers the basics of mineral ion homeostasis and the roles of phosphate in skeletal biology. Part 1 includes phosphate-related disorders of mineralization for which overall circulating mineral ion homeostasis remains normal. Part 2 covers hypophosphatemic and hyperphosphatemic disorders, emphasizing, but not limited to, those related to increased and decreased FGF23 signaling, respectively.

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

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

ALPL Genotypes in Patients With Atypical Femur Fractures or Other Biochemical and Clinical Signs of Hypophosphatasia

CONTEXT

Hypophosphatasia (HPP) is a rare metabolic disorder caused by deficiency of alkaline phosphatase (ALP) enzyme activity, leading to defective mineralization, due to pathogenic variants of the ALPL gene, encoding the tissue nonspecific alkaline phosphatase (TNSALP) enzyme. Inheritance can be autosomal recessive or autosomal dominant. An abnormal ALPL genetic test enables accurate diagnosis, avoiding the administration of contraindicated antiresorptive drugs that, in patients with HPP, substantially increase the risk of atypical femur fractures (AFFs) and worsen the fracture healing process that is usually already compromised in these patients.

OBJECTIVE

Performing ALPL genetic testing to identify rare variants in suspected adult patients with HPP. Comparing frequencies of ALPL common variants in individuals with biochemical and/or clinical signs suggestive of adult HPP and non-HPP controls, and among different clinical subgroups of patients with a clinical suspicion of adult HPP.

METHODS

Patients with suspected adult HPP were retrospectively selected for the genetic testing of the ALPL gene. Patients included were from 3 main European Bone Units (Florence, Naples, and Geneva); 106 patients with biochemical and/or clinical signs suggestive of a mild form of HPP were included.

RESULTS

Genetic testing led to the identification of a heterozygote rare variant in 2.8% of cases who were initially referred as suspected osteoporosis. The analysis of frequencies of ALPL common variants showed a high prevalence (30.8%) of homozygosity in subjects who developed an AFF, in association with normal serum total ALP activity.

CONCLUSION

The results suggest homozygosity of common ALPL variants as a possible genetic mark of risk for these fractures.

Lethal Encephalopathy in an Infant with Hypophosphatasia despite Enzyme Replacement Therapy

Hypophosphatasia (HPP) is an inborn error of metabolism caused by loss-of-function mutations in the biomineralization-associated alkaline phosphatase gene, encoding tissue-nonspecific alkaline phosphatase (TNSALP). Symptoms include skeletal hypomineralization and extra-skeletal manifestations such as pyridoxine (B6)-responsive seizures due to impaired cerebral B6 passage. Since the introduction of enzyme replacement therapy (ERT), skeletal manifestations and B6-responsive seizures were reported to improve significantly. Nevertheless, there is an increasing evidence of B6-independent neurological manifestation of HPP including HPP-associated encephalopathy. Here, we present for the first time the brain alterations of an infant with neonatal HPP who died of neurological complications at the age of 5 months despite early initiation of ERT. CSF analysis showed normal concentrations of biogenic amines reflecting sufficient intracellular B6 availability. Postmortem histopathology revealed severe, localized affection of the cerebral cortex including cortical lesions in layers 2 and 3 in direct proximity to TNSALP-expressing neurons and hippocampal sclerosis. Our findings confirm that TNSALP deficiency may lead to a severe encephalopathy. We hypothesize that HPP-associated encephalopathy resistant to currently available ERT may develop in addition and probably independently of typical B6-responsive seizures in some patients. Prospective, controlled studies with close neurological follow-up including brain imaging are needed to identify patients at risk for severe neurological symptoms despite ERT.

Screening for hypophosphatasia: does biochemistry lead the way?

OBJECTIVES

Patients with childhood hypophosphatasia (HPP) often have unspecific symptoms. It was our aim to identify patients with mild forms of HPP by laboratory data screening for decreased alkaline phosphatase (AP) within a pediatric population.

METHODS

We conducted a retrospective hospital-based data screening for AP activity below the following limits: Girls: ≤12 years: <125 U/L; >12 years: <50 U/L Boys: ≤14 years: <125 U/L; >14 years: <70 U/L. Screening positive patients with otherwise unexplained hypophosphatasemia were invited for further diagnostics: Re-test of AP activity, pyridoxal 5'-phosphate (PLP) in hemolyzed whole blood, phosphoethanolamine (PEA) in serum and urine, and inorganic pyrophosphate in urine. Sequencing of the ALPL gene was performed in patients with clinical and/or laboratory abnormalities suspicious for HPP.

RESULTS

We assessed a total of 14,913 samples of 6,731 patients and identified 393 screening-positive patients. The majority of patients were excluded due to known underlying diseases causing AP depression. Of the 30 patients who participated in the study, three had a decrease in AP activity in combination with an increase in PLP and PEA. A heterozygous ALPL mutation was detected in each of them: One patient with a short stature was diagnosed with childhood-HPP and started with enzyme replacement therapy. The remaining two are considered as mutation carriers without osseous manifestation of the disease.

CONCLUSIONS

A diagnostic algorithm based on decreased AP is able to identify patients with ALPL mutation after exclusion of the differential diagnoses of hypophosphatasemia and with additional evidence of increased AP substrates.

Rare Causes of Hypercalcemia

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

New structural variations responsible for Charcot-Marie-Tooth disease: The first two large KIF5A deletions detected by CovCopCan software

Next-generation sequencing (NGS) allows the detection of mutations in inherited genetic diseases, like the Charcot-Marie-Tooth disease (CMT) which is the most common hereditary peripheral neuropathy. The majority of mutations detected by NGS are single nucleotide variants (SNVs) or small indels, while structural variants (SVs) are often underdiagnosed. PMP22 was the first gene described as being involved in CMT via a SV of duplication type. To date, more than 90 genes are known to be involved in CMT, with mainly SNVs and short indels described. Herein targeted NGS and the CovCopCan bioinformatic tool were used in two unrelated families, both presenting with typical CMT symptoms with pyramidal involvement. We have discovered two large SVs in KIF5A, a gene known to cause axonal forms of CMT (CMT2) in which no SVs have yet been described. In the first family, the patient presented with a large deletion of 12 kb in KIF5A from Chr12:57,956,278 to Chr12:57,968,335 including exons 2–15, that could lead to mutation c.(130-943_c.1717-533del), p.(Gly44_Leu572del). In the second family, two cases presented with a large deletion of 3 kb in KIF5A from Chr12:57,974,133 to Chr12:57,977,210 including exons 24–28, that could lead to mutation c.(2539-605_*36 + 211del), p.(Leu847_Ser1032delins33). In addition, bioinformatic sequence analysis revealed that a NAHR (Non-Allelic-Homologous-Recombination) mechanism, such as those in the PMP22 duplication, could be responsible for one of the KIF5A SVs and could potentially be present in a number of other patients. This study reveals that large KIF5A deletions can cause CMT2 and highlights the importance of analyzing not only the SNVs but also the SVs during diagnosis of neuropathies.

Parietal aplasia and hypophosphatasia in a child harboring a novel mutation in RUNX2 and a likely pathogenic variant in TNSALP

Cleidocranial dysplasia is a dominantly inherited skeletal dysplasia resulting from inherited or spontaneous mutations of Runt-related transcription factor 2 gene (RUNX2). It represents a clinical continuum typically characterized by wide calvarial sutures, clavicular hypoplasia and dental abnormalities. CDD has been rarely associated with skeletal and biochemical features that mimic hypophosphatasia. We report clinical, biochemical and molecular profile of a 3-year-old female with CCD, presented in utero with large cranial defects. She displayed severe parietal dysplasia, wide cranial sutures, clavicular abnormalities and biochemical features of hypophospatasia (HHP). She was preliminary diagnosed with benign perinatal HHP, harboring a likely pathogenic heterozygous TNSALP variant (p.Ser181Leu) inherited by the mother, who also displayed low levels of ALP. Asfotase alfa was introduced for a six-month-period with rather positive impact on cranial ossification. Nevertheless, focal skeletal disease (cranium and clavicles) and absence of clinical symptoms in the mother, carrier of the same genetic variant, posed diagnosis into question and further genetic analysis detected the novel spontaneous frameshift mutation c.1191delC (p.Phe398Leufs*86) in RUNX2 gene, establishing the CCD diagnosis. Although genotype-phenotype correlations are difficult, p.Phe398Leufs*86 appears to be associated with a severe cranial phenotype and absence of parietal bones, similarly to other adjacent frameshift/splicing mutations. The TNSALP variant (p.Ser181Leu) may contributed to patient's final phenotype, as well as to maternal low ALP levels. However, since low ALP levels have been also reported in few CCD patients with no alterations in TNSALP gene, studies to elucidate RUNX2 and TNSALP interactions could shed more light on differential diagnosis between CCD and HHP, CCD appropriate therapy and genetic counselling. ACCESSION NUMBER: (SUB8185506).

Excellent response to asfotase alfa treatment in an adolescent patient with hypophosphatasia

Hypophosphatasia (HPP) is a rare inherited metabolic disorder characterized by deficient activity of alkaline phosphatase, causing defective mineralization of bones and teeth. The symptoms vary from no symptoms to stillbirth or skeletal manifestations. Since 2015, asfotase alfa, an enzyme replacement treatment, has been approved for pediatric use in some jurisdictions. We describe the clinical outcome of asfotase alfa therapy in an adolescent patient with childhood HPP. The patient was diagnosed with HPP at 13 months. She had a history of hypertonia and failure to thrive from age 3 months. During childhood the patient experienced chronic skeletal pain, requiring daily use of analgesics and school absences. Her plasma pyridoxal-5-phosphate was elevated at >2500 mmol/L, phosphoethanolamine at 11 μM, and ALP decreased at 25 U/L. On the visual analog scale (VAS), a scale used to determine pain intensity, she stated an average of 7 (maximum 10) at age 13. She had no abnormalities on radiography. At age 13 the patient was started on asfotase alfa 1 mg/kg given subcutaneously 6 times weekly. Three months after treatment the patient had a decreased P-pyridoxal-5-phosphate level of 41 mmol/L, used fewer analgesics, and a lower average VAS-score. At every follow-up, she continued to exhibit improved biochemical values, along with lower VAS-scores. In conclusion, asfotase alfa significantly improved the patient's quality of life. This case suggests an association between children with HPP without radiographic abnormalities, but a debilitating pain phenotype, and a significant pain reduction on enzyme replacement therapy. Thus, this therapy should be considered in such patients.

Investigation of ALPL variant states and clinical outcomes: An analysis of adults and adolescents with hypophosphatasia treated with asfotase alfa

BACKGROUND

Hypophosphatasia (HPP), a rare metabolic disease, can be inherited in an autosomal recessive (biallelic) or an autosomal dominant (monoallelic) manner. Most of the severe, early-onset, frequently lethal HPP in infants is acquired through recessive inheritance; less severe, later-onset, typically nonlethal HPP phenotypes are acquired through either dominant or recessive inheritance. HPP's variable clinical presentation arises from >400 identified ALPL pathogenic variants with likely variable penetrance, especially with autosomal dominant inheritance. This post hoc analysis investigated the relationship between ALPL variant state (biallelic and monoallelic) and clinical outcomes with asfotase alfa in HPP.

METHODS

Data were pooled from two phase 2, randomized, open-label studies in adolescents and adults with HPP; one study evaluated the efficacy and safety of different doses of asfotase alfa (n = 25), and the other assessed the pharmacodynamics and safety of asfotase alfa (n = 19). Patients were grouped by ALPL variant state (biallelic or monoallelic). Available data from both studies included ALPL pathogenic variant state, Baseline characteristics, HPP-specific medical history, and Baseline TNSALP substrate levels (inorganic pyrophosphate [PPi] and pyridoxal 5'-phosphate [PLP]) concentrations). Clinical outcomes over 5 years of treatment were available from only the efficacy and safety study.

RESULTS

In total, 44 patients with known variant status were included in the pooled analysis (biallelic, n = 30; monoallelic, n = 14). The most common pathogenic variant was c.571G > A (p.Glu191Lys) in biallelic patients (allele frequency: 19/60) and c.1133A > T (p.Asp378Val) in monoallelic patients (allele frequency: 7/28). Median (min, max) Baseline PPi concentrations were significantly higher in patients with a biallelic vs monoallelic variant state (5.3 [2.2, 12.1] vs 4.3 [3.5, 7.4] μM; P = 0.0113), as were Baseline PLP concentrations (221.4 [62.4, 1590.0] vs 75.1 [28.8, 577.0] ng/mL; P= 0.0022). HPP-specific medical history was generally similar between biallelic and monoallelic patients in terms of incidence and type of manifestations; notable exceptions included fractures, which were more common among monoallelic patients, and delayed walking and bone deformities such as abnormally shaped chest and head and bowing of arms or legs, which were more common among biallelic patients. Data from the efficacy and safety study (n = 19) showed that median PPi and PLP concentrations were normalized over 5 years of treatment in patients with both variant states. Median % predicted distance walked on the 6-Minute Walk Test remained within the normal range for monoallelic patients over 4 years of treatment, and improved from below normal (<84%) to normal in biallelic patients.

CONCLUSIONS

Although patients with biallelic variants had significantly higher Baseline PPi and PLP levels than monoallelic variants, both groups generally showed similar pretreatment Baseline clinical characteristics. Treatment with asfotase alfa for up to 5 years normalized TNSALP substrate concentrations and improved functional outcomes, with no clear differences between biallelic and monoallelic variant states. This study suggests that patients with HPP have significant disease burden, regardless of ALPL variant state.

Hypophosphatasia: A Unique Disorder of Bone Mineralization

Hypophosphatasia (HPP) is a rare genetic disease characterized by a decrease in the activity of tissue non-specific alkaline phosphatase (TNSALP). TNSALP is encoded by the ALPL gene, which is abundantly expressed in the skeleton, liver, kidney, and developing teeth. HPP exhibits high clinical variability largely due to the high allelic heterogeneity of the ALPL gene. HPP is characterized by multisystemic complications, although the most common clinical manifestations are those that occur in the skeleton, muscles, and teeth. These complications are mainly due to the accumulation of inorganic pyrophosphate (PPi) and pyridoxal-5′-phosphate (PLP). It has been observed that the prevalence of mild forms of the disease is more than 40 times the prevalence of severe forms. Patients with HPP present at least one mutation in the ALPL gene. However, it is known that there are other causes that lead to decreased alkaline phosphatase (ALP) levels without mutations in the ALPL gene. Although the phenotype can be correlated with the genotype in HPP, the prediction of the phenotype from the genotype cannot be made with complete certainty. The availability of a specific enzyme replacement therapy for HPP undoubtedly represents an advance in therapeutic strategy, especially in severe forms of the disease in pediatric patients.

Clinical and molecular findings in children and young adults with persistent low alkaline phosphatase concentrations

BACKGROUND

Hypophosphatasia is a rare inherited metabolic disease resulted by ALPL gene mutations. It is characterized by defective bone and teeth mineralization. The phenotypic spectrum is highly variable ranging from lethal perinatal form to mild forms which are only diagnosed in adulthood or remain undiagnosed despite persistently low concentrations of ALP. The aim of this study is to evaluate the clinical phenotype and frequency of ALPL mutations in a group of patient with hypophosphatasaemia.

METHODS

Thirty individuals with alkaline phosphatase values below 40 IU/L in at least two assessments and having no alternative explanation for their low ALP concentrations were included in the study. The clinical features and radiological data of the study group were re-investigated for hypophosphatasia-related findings. ALPL sequence analysis was performed using Sanger sequencing.

RESULTS

No patient in the study group had severe symptoms, nor had they initially been diagnosed as having hypophosphatasia. Four different heterozygous ALPL mutations (c.542C>T, c.648 + 1G>A, c.657G>T and c.862 + 1G>C) were found in four patients. One splice site mutation (c.862 + 1G>C) was reported for the first time in this study.

CONCLUSION

ALPL sequence analysis may help to diagnosing genetic defects in individuals with persistently low ALP concentrations and provide to take preventive measures before symptoms appear. As in the other populations, HPP displays allelic heterogeneity in our population.

Utility of genetic testing for prenatal presentations of hypophosphatasia

Hypophosphatasia (HPP) is a rare inherited disease affecting bone and dental mineralization due to loss-of-function mutations in the ALPL gene encoding the tissue nonspecific alkaline phosphatase (TNSALP). Prenatal benign HPP (PB HPP) is a rare form of HPP characterized by in utero skeletal manifestations that progressively improve during pregnancy but often still leave symptoms after birth. Because the prenatal context limits the diagnostic tools, the main difficulty for clinicians is to distinguish PB HPP from perinatal lethal HPP, the most severe form of HPP. We previously attempted to improve genotype phenotype correlation with the help of a new classification of variants based on functional testing. Among 46 perinatal cases detected in utero or in the neonatal period for whose ALPL variants could be classified, imaging alone was thought to clearly diagnose severe lethal HPP in 35 cases, while in 11 cases, imaging abnormalities could not distinguish between perinatal lethal and BP HPP. We show here that our classification of ALPL variants may improve the ability to distinguish between perinatal lethal and PB HPP in utero.

Dental defects in the primary dentition associated with hypophosphatasia from biallelic ALPL mutations

ALPL encodes tissue-nonspecific alkaline phosphatase (TNAP), an enzyme expressed in bone, teeth, liver, and kidney. ALPL loss-of-function mutations cause hypophosphatasia (HPP), an inborn error-of-metabolism that produces skeletal and dental mineralization defects. Case reports describe widely varying dental phenotypes, making it unclear how HPP comparatively affects the three unique dental mineralized tissues: enamel, dentin, and cementum. We hypothesized that HPP affected all dental mineralized tissues and aimed to establish quantitative measurements of dental tissues in a subject with HPP. The female proband was diagnosed with HPP during childhood based on reduced alkaline phosphatase activity (ALP), mild rachitic skeletal effects, and premature primary tooth loss. The diagnosis was subsequently confirmed genetically by the presence of compound heterozygous ALPL mutations (exon 5: c.346G>A, p.A116T; exon 10: c.1077C>G, p.I359M). Dental defects in 8 prematurely exfoliated primary teeth were analyzed by high resolution micro-computed tomography (micro-CT) and histology. Similarities to the Alpl^-/- mouse model of HPP were identified by additional analyses of murine dentoalveolar tissues. Primary teeth from the proband exhibited substantial remaining root structure compared to healthy control teeth. Enamel and dentin densities were not adversely affected in HPP vs. control teeth. However, analysis of discrete dentin regions revealed an approximate 10% reduction in the density of outer mantle dentin of HPP vs. control teeth. All 4 incisors and the molar lacked acellular cementum by micro-CT and histology, but surprisingly, 2 of 3 prematurely exfoliated canines exhibited apparently normal acellular cementum. Based on dentin findings in the proband's teeth, we examined dentoalveolar tissues in a mouse model of HPP, revealing that the delayed initiation of mineralization in the incisor mantle dentin was associated with a broader lack of circumpulpal dentin mineralization. This study describes a quantitative approach to measure effects of HPP on dental tissues. This approach has uncovered a previously unrecognized novel mantle dentin defect in HPP, as well as a surprising and variable cementum phenotype within the teeth from the same HPP subject.

Multigenerational case examples of hypophosphatasia: Challenges in genetic counseling and disease management

Hypophosphatasia (HPP) is an inherited metabolic condition caused by pathogenic mutations in the ALPL gene. This leads to deficiency of tissue non-specific alkaline phosphatase (TNSALP), resulting in decreased mineralization of the bones and/or teeth and multi-systemic complications. Inheritance may be autosomal dominant or recessive, and the phenotypic spectrum, including age of onset, varies widely. We present four families demonstrating both modes of inheritance of HPP and phenotypic variability and discuss the resultant challenges in disease management, genetic counseling, and risk assessment. Failure to consider different modes of inheritance in a family with HPP may lead to an inaccurate risk assessment upon which medical and reproductive decisions may be made. We highlight the essential role of high-quality genetic counseling and meaningful biochemical and molecular testing strategies in the evaluation and management of families with HPP.

Hypophosphatasia: a genetic-based nosology and new insights in genotype-phenotype correlation

Hypophosphatasia (HPP) is caused by pathogenic variants in the ALPL gene. There is a large continuum in the severity, ranging from a lethal perinatal form to dental issues. We analyzed a cohort of 424 HPP patients from European geographic origin or ancestry. Using 3D modeling and results of functional tests we classified ALPL pathogenic variants according to their dominant negative effect (DNE) and their severity. The cohort was described by the genotypes resulting from alleles s (severe recessive), Sd (severe dominant), and m (moderate). Many recurrent variants showed a regional anchor pointing out founder effects rather than multiple mutational events. Homozygosity was an aggravating factor of the severity and moderate alleles were rare both in number and frequency. Pathogenic variants with DNE were found in both recessive and dominant HPP. Sixty percent of the adults tested were heterozygous for a variant showing no DNE, suggesting another mechanism of dominance like haploinsufficiency. Adults with dominant HPP without DNE were found statistically less severely affected than adults with DNE variants. Adults with dominant HPP without DNE represent a new clinical entity mostly diagnosed from 2010s, characterized by nonspecific signs of HPP and low alkaline phosphatase, and for which a high prevalence is expected. In conclusion, the genetic composition of our cohort suggests a nosology with 3 clinical forms: severe HPP is recessive and rare, moderate HPP is recessive or dominant and more common, and mild HPP, characterized by low alkaline phosphatase and unspecific clinical signs, is dominantly inherited and very common.

Hypophosphatasia in adolescents and adults: overview of diagnosis and treatment

This article provides an overview of the current knowledge on hypophosphatasia-a rare genetic disease of very variable presentation and severity-with a special focus on adolescents and adults. It summarizes the available information on the many known mutations of tissue-nonspecific alkaline phosphatase (TNSALP), the epidemiology and clinical presentation of the disease in adolescents and adults, and the essential diagnostic clues. The last section reviews the therapeutic approaches, including recent reports on enzyme replacement therapy (EnzRT).

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

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

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.

Clinical and genetic aspects of mild hypophosphatasia in Japanese patients

Background

Hypophosphatasia (HPP) is a rare inborn error of metabolism that results from a dysfunctional tissue non-specific alkaline phosphatase enzyme (TNSALP). Although genotype-phenotype correlations have been described in HPP patients, only sparse information is currently available on the genetics of mild type HPP.

Methods

We investigated 5 Japanese patients from 3 families with mild HPP (patients 1 and 2 are siblings; patient 4 is a daughter of patient 5) who were referred to Fujita Health University due to the premature loss of deciduous teeth. Physical and dental examinations, and blood, urine and bone density tests were conducted. Genetic analysis of the ALPL gene was performed in all patients with their informed consent.

Results

After a detailed interview and examination, we found characteristic symptoms of HPP in some of the study cases. Mobile teeth or the loss of permanent teeth were observed in 2 patients, and 3 out of 5 patients had a history of asthma. The serum ALP levels of all patients were 30% below the lower limit of the age equivalent normal range. ALPL gene analysis revealed compound heterozygous mutations, including Ile395Val and Leu520Argfs in family 1, Val95Met and Gly491Arg in family 2, and a dominant missense mutation (Gly456Arg) in family 3. The 3D-modeling of human TNSALP revealed three mutations (Val95Met, Ile395Val and Gly456Arg) at the homodimer interface. Severe collisions between the side chains were predicted for the Gly456Arg variant.

Discussion

One of the characteristic findings of this present study was a high prevalence of coexisting asthma and a high level serum IgE level. These characteristics may account for the fragility of tracheal tissues and a predisposition to asthma in patients with mild HPP. The genotypes of the five mild HPP patients in our present study series included 1) compound heterozygous for severe and hypomorphic mutations, and 2) dominant-negative mutations. All of these mutations were at the homodimer interface, but only the dominant-negative mutation was predicted to cause a severe collision effect between the side chains. This may account for varying mechanisms leading to different effects on TNSALP function.

Japanese nationwide survey of hypophosphatasia reveals prominent differences in genetic and dental findings between odonto and non-odonto types

Hypophosphatasia (HPP) is a rare and intractable metabolic bone disease caused by mutations in the ALPL gene. Here, we undertook a nationwide survey of HPP in Japan, specifically regarding the prominent genetic and dental manifestations of odonto (n = 16 cases) and other (termed “non-odonto”) (n = 36 cases) types. Mean serum alkaline phosphatase (ALP) values in odonto-type patients were significantly greater than those of non-odonto-type patients (P<0.05). Autosomal dominant and autosomal recessive inheritance patterns were detected, respectively, in 89% of odonto-type and 96% of non-odonto-type patients. The ALPL “c.1559delT” mutation, associated with extremely low ALP activity, was found in approximately 70% of cases. Regarding dental manifestations, all patients classified as odonto-type showed early exfoliation of the primary teeth significantly more frequently than patients classified as non-odonto-type (100% vs. 56%; P<0.05). Tooth hypomineralisation was detected in 42% of non-odonto-type patients, but not in any odonto-type patients (0%; P<0.05). Collectively, these results suggest that genetic and dental manifestations of patients with odonto-type and non-odonto-type HPP are significantly different, and these differences should be considered during clinical treatment of patients with HPP.

A novel combination of biallelic ALPL mutations associated with adult hypophosphatasia: A phenotype-genotype association and computational analysis study

Hypophosphatasia (HPP) is an inherited metabolic disorder that causes defective skeletal and dental mineralization. HPP exhibits a markedly heterogeneous range of clinical manifestations caused by dysfunction of the tissue-nonspecific isozyme of alkaline phosphatase (TNSALP), resulting from loss-of-function mutations in the ALPL gene. HPP has been associated with predominantly missense mutations in ALPL, and a number of compound heterozygous genotypes have been identified. Here, we describe a case of a subject with adult-onset HPP caused by a novel combination of missense mutations p.Gly473Ser and p.Ala487Val, resulting in chronic musculoskeletal pain, myopathy, persistent fatigue, vomiting, and an uncommon dental phenotype of short-rooted permanent teeth. Pedigree and biochemical analysis indicated that severity of symptoms was correlated with levels of residual ALP activity, and co-segregated with the p.Gly473Ser missense mutation. Bioinformatic analysis to predict the structural and functional impact of each of the point mutations in the TNSALP molecule, and its potential contribution to the clinical symptoms, revealed that the affected Gly473 residue is localized in the homodimer interface and predicted to have a dominant negative effect. The affected Ala487 residue was predicted to bind to Tyr479, which is closely located the N-terminal α-helix of TNSALP monomer 2, suggesting that both changes may impair dimer stability and catalytic functions. In conclusion, these findings assist in defining genotype-phenotype associations for HPP, and further define specific sites within the TNSALP molecule potentially related to neuromuscular manifestations in adult HPP, allowing for a better understanding of HPP pathophysiology.

Dental-craniofacial manifestation and treatment of rare diseases

Rare diseases are usually genetic, chronic and incurable disorders with a relatively low incidence. Developments in the diagnosis and management of rare diseases have been relatively slow due to a lack of sufficient profit motivation and market to attract research by companies. However, due to the attention of government and society as well as economic development, rare diseases have been gradually become an increasing concern. As several dental-craniofacial manifestations are associated with rare diseases, we summarize them in this study to help dentists and oral maxillofacial surgeons provide an early diagnosis and subsequent management for patients with these rare diseases.

Genetic analysis of adults heterozygous for ALPL mutations

Hypophosphatasia (HPP) is a rare inherited metabolic bone disease due to a deficiency of the tissue nonspecific alkaline phosphatase isoenzyme (TNSALP) encoded by the ALPL gene. Patients have consistently low serum alkaline phosphatase (AP), so that this parameter is a good hallmark of the disease. Adult HPP is heterogeneous, and some patients present only mild nonpathognomonic symptoms which are also common in the general population such as joint pain, osteomalacia and osteopenia, chondrocalcinosis, arthropathy and musculoskeletal pain. Adult HPP may be recessively or dominantly inherited; the latter case is assumed to be due to the dominant negative effect (DNE) of missense mutations derived from the functional homodimeric structure of TNSALP. However, there is no biological argument excluding the possibility of other causes of dominant HPP. Rheumatologists and endocrinologists are increasingly solicited for patients with low AP and nonpathognomonic symptoms of HPP. Many of these patients are heterozygous for an ALPL mutation and a challenging question is to determine if these symptoms, which are also common in the general population, are attributable to their heterozygous ALPL mutation or not. In an attempt to address this question, we reviewed a cohort of 61 adult patients heterozygous for an ALPL mutation. Mutations were distinguished according to their statistical likelihood to show a DNE. One-half of the patients carried mutations predicted with no DNE and were slightly less severely affected by the age of onset, serum AP activity and history of fractures. We hypothesized that these mutations result in another mechanism of dominance or are recessive alleles. To identify other genetic factors that could trigger the disease phenotype in heterozygotes for potential recessive mutations, we examined the next-generation sequencing results of 32 of these patients for a panel of 12 genes involved in the differential diagnosis of HPP or candidate modifier genes of HPP. The heterozygous genotype G/C of the COL1A2 coding SNP rs42524 c.1645C > G (p.Pro549Ala) was associated with the severity of the phenotype in patients carrying mutations with a DNE whereas the homozygous genotype G/G was over-represented in patients carrying mutations without a DNE, suggesting a possible role of this variant in the disease phenotype. These preliminary results support COL1A2 as a modifier gene of HPP and suggest that a significant proportion of adult heterozygotes for ALPL mutations may have unspecific symptoms not attributable to their heterozygosity.

Hypophosphatasia

We review here clinical, pathophysiological, diagnostic, genetic and molecular aspects of Hypophosphatasia (HPP), a rare inherited metabolic disorder. The clinical presentation is a continuum ranging from a prenatal lethal form with no skeletal mineralization to a mild form with late adult onset presenting with nonpathognomonic symptoms. The prevalence of severe forms is low, whereas less severe forms are more frequently observed. The disease is caused by loss-of-function mutations in the ALPL gene encoding the Tissue Nonspecific Alkaline Phosphatase (TNSALP), a central regulator of mineralization. Severe forms are recessively inherited, whereas moderate forms are either recessively or dominantly inherited, and the more severe the disease is, the more often it is subject to recessive inheritance. The diagnosis is based on a constantly low alkaline phosphatase (AP) activity in serum and genetic testing that identifies ALPL mutations. More than 340 mutations have been identified and are responsible for the extraordinary clinical heterogeneity. A clear but imperfect genotype-phenotype correlation has been observed, suggesting that other genetic or environmental factors modulate the phenotype. Enzyme replacement therapy is now available for HPP, and other approaches, such as gene therapy, are currently being investigated.

Genetic evaluations of Chinese patients with odontohypophosphatasia resulting from heterozygosity for mutations in the tissue-non-specific alkaline phosphatase gene

Background

Hypophosphatasia is a rare heritable metabolic disorder characterized by defective bone and tooth mineralization accompanied by a deficiency of tissue-non-specific (liver/bone/kidney) isoenzyme of alkaline phosphatase activity, caused by a number of loss-of-function mutations in the alkaline phosphatase liver type gene. We seek to explore the clinical manifestations and identify the mutations associated with the disease in a Chinese odonto- hypophosphatasia family.

Results

The proband and his younger brother affected with premature loss of primary teeth at their 2-year-old. They have mild abnormal serum alkaline phosphatase and 25-hydroxy vitamin D values, but the serum alkaline phosphatase activity of their father, mother and grandmother, who showed no clinical symptoms of hypophosphatasia, was exhibited significant decreased. In addition to premature loss of primary teeth, the proband and his younger brother showed low bone mineral density, X-rays showed that they had slight metaphyseal osteoporosis changes, but no additional skeletal abnormalities. Deoxyribonucleic acid sequencing and analysis revealed a single nucleotide polymorphism c.787T>C (p.Y263H) in exon 7 and/or a novel mutation c.-92C>T located at 5’UTR were found in the affected individuals.

Materials and Methods

We examined all individuals of an odonto- hypophosphatasia family by clinical and radiographic examinations as well as laboratory assays. Furthermore, all 12 exons and the exon-intron boundaries of the alkaline phosphatase liver type gene were amplified and directly sequenced for further analysis and screened for mutations.

Conclusion

Our present findings suggest the single nucleotide polymorphism c.787T>C and c.-92C>T should be responsible for the odonto- hypophosphatasia disorders in this family.

Efficacy of anti-sclerostin monoclonal antibody BPS804 in adult patients with hypophosphatasia

BACKGROUND

Hypophosphatasia (HPP) is a rare genetic disorder resulting in variable alterations of bone formation and mineralization that are caused by mutations in the ALPL gene, encoding the tissue-nonspecific alkaline phosphatase (ALP) enzyme.

METHODS

In this phase IIA open-label, single-center, intra-patient, dose-escalating study, adult patients with HPP received 3 ascending intravenous doses of 5, 10, and 20 mg/kg BPS804, a fully human anti-sclerostin monoclonal antibody, on days 1, 15, and 29, respectively. Patients were followed for 16 weeks after the last dose. We assessed the pharmacodynamics, pharmacokinetics, preliminary efficacy, and safety of BPS804 administrations at specified intervals during treatment and follow-up.

RESULTS

Eight patients (mean age 47.8 years) were enrolled in the study (6 females, 2 males). BPS804 treatment increased mean ALP and bone-specific ALP enzymatic activity between days 2 and 29. Transient increases in the bone formation markers procollagen type-I N-terminal propeptide (PINP), osteocalcin, and parathyroid hormone as well as a transient decrease in the bone resorption marker C-telopeptide of type I collagen (CTX-1) were observed. Lumbar spine bone mineral density showed a mean increase by day 85 and at end of study. Treatment-associated adverse events were mild and transient.

CONCLUSION

BPS804 treatment was well tolerated and resulted in increases in bone formation biomarkers and bone mineral density, suggesting that sclerostin inhibition could be applied to enhance bone mineral density, stability, and regeneration in non-life-threatening clinical situations in adults with HPP.

TRIAL REGISTRATION

Clinicaltrials.gov NCT01406977.

FUNDING

Novartis Institutes for BioMedical Research, Basel, Switzerland.

Hypophosphatasia

Hypophosphatasia is a rare disorder due to a mutation in the ALPL gene encoding the alkaline phosphatase (ALP) leading to a diminished activity of the enzyme in bone, liver, and kidney. Hypophosphatasia is a heterogeneous disease, ranging from extreme life-threatening forms revealed at birth in young infants presenting with severely impaired bone mineralization, seizures, and hypercalcemia, to young adults with premature exfoliation of their teeth without any other symptom. We will review the challenges of the clinical, biochemical, radiological, and genetic diagnosis. Schematically, the diagnosis relies on low ALP levels and, in most cases, on the genetic defect in the ALPL gene. An enzyme replacement therapy is now developed for hypophosphatasia; early results in the severe form of the disease are extremely encouraging. However, multidisciplinary care remains the core of treatment of hypophosphatasia encompassing nutritional support, adjustment of calcium and phosphate intake, monitoring of vitamin D levels, careful and personalized physical therapy, and regular dental monitoring and care.

Clinical, biochemical and genetic spectrum of low alkaline phosphatase levels in adults

BACKGROUND

Low serum levels of alkaline phosphatase (ALP) are a hallmark of hypophosphatasia. However, the clinical significance and the underlying genetics of low ALP in unselected populations are unclear.

METHODS

In order to clarify this issue, we performed a clinical, biochemical and genetic study of 42 individuals (age range 20-77yr) with unexplained low ALP levels.

RESULTS

Nine had mild hyperphosphatemia and three had mild hypercalcemia. ALP levels were inversely correlated with serum calcium (r=-0.38, p=0.012), pyridoxal phosphate (PLP; r=-0.51, p=0.001) and urine phosphoethanolamine (PEA; r=-0.49, p=0.001). Although many subjects experienced minor complaints, such as mild musculoskeletal pain, none had major health problems. Mutations in ALPL were found in 21 subjects (50%), including six novel mutations. All but one, were heterozygous mutations. Missense mutations were the most common (present in 18 subjects; 86%) and the majority were predicted to have a damaging effect on protein activity. The presence of a mutated allele was associated with tooth loss (48% versus 12%; p=0.04), slightly lower levels of serum ALP (p=0.002), higher levels of PLP (p<0.0001) and PEA (p<0.0001), as well as mildly increased serum phosphate (p=0.03). Ten individuals (24%) had PLP levels above the reference range; all carried a mutated allele.

CONCLUSION

One-half of adult individuals with unexplained low serum ALP carried an ALPL mutation. Although the associated clinical manifestations are usually mild, in approximately 50% of the cases, enzyme activity is low enough to cause substrate accumulation and may predispose to defects in calcified tissues.

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.

Periodontal Defects in the A116T Knock-in Murine Model of Odontohypophosphatasia

Mutations in ALPL result in hypophosphatasia (HPP), a disease causing defective skeletal mineralization. ALPL encodes tissue nonspecific alkaline phosphatase (ALP), an enzyme that promotes mineralization by reducing inorganic pyrophosphate, a mineralization inhibitor. In addition to skeletal defects, HPP causes dental defects, and a mild clinical form of HPP, odontohypophosphatasia, features only a dental phenotype. The Alpl knockout (Alpl (-/-)) mouse phenocopies severe infantile HPP, including profound skeletal and dental defects. However, the severity of disease in Alpl (-/-) mice prevents analysis at advanced ages, including studies to target rescue of dental tissues. We aimed to generate a knock-in mouse model of odontohypophosphatasia with a primarily dental phenotype, based on a mutation (c.346G>A) identified in a human kindred with autosomal dominant odontohypophosphatasia. Biochemical, skeletal, and dental analyses were performed on the resulting Alpl(+/A116T) mice to validate this model. Alpl(+/A116T) mice featured 50% reduction in plasma ALP activity compared with wild-type controls. No differences in litter size, survival, or body weight were observed in Alpl(+/A116T) versus wild-type mice. The postcranial skeleton of Alpl(+/A116T) mice was normal by radiography, with no differences in femur length, cortical/trabecular structure or mineral density, or mechanical properties. Parietal bone trabecular compartment was mildly altered. Alpl(+/A116T) mice featured alterations in the alveolar bone, including radiolucencies and resorptive lesions, osteoid accumulation on the alveolar bone crest, and significant differences in several bone properties measured by micro-computed tomography. Nonsignificant changes in acellular cementum did not appear to affect periodontal attachment or function, although circulating ALP activity was correlated significantly with incisor cementum thickness. The Alpl(+/A116T) mouse is the first model of odontohypophosphatasia, providing insights on dentoalveolar development and function under reduced ALP, bringing attention to direct effects of HPP on alveolar bone, and offering a new model for testing potential dental-targeted therapies in future studies.

Infantile hypophosphatasia without bone deformities presenting with severe pyridoxine-resistant seizures

An infant carrying a heterozygous c.43_46delACTA and a heterozygous c.668 G>A mutation in the ALPL gene with hypophosphatasia in the absence of bone deformities presented with therapy-resistant seizures. Pyridoxal phosphate was extremely high in CSF and plasma. Pyridoxine treatment had only a transient effect and the severe encephalopathy was fatal. Repeated brain MRIs showed progressive cerebral damage. The precise metabolic cause of the seizures remains unknown and pyridoxine treatment apparently does not cure the epilepsy.

Novel ALPL genetic alteration associated with an odontohypophosphatasia phenotype

Hypophosphatasia (HPP) is an inherited disorder of mineral metabolism caused by mutations in ALPL, encoding tissue non-specific alkaline phosphatase (TNAP). Here, we report the molecular findings from monozygotic twins, clinically diagnosed with tooth-specific odontohypophosphatasia (odonto-HPP). Sequencing of ALPL identified two genetic alterations in the probands, including a heterozygous missense mutation c.454C>T, leading to change of arginine 152 to cysteine (p.R152C), and a novel heterozygous gene deletion c.1318_1320delAAC, leading to the loss of an asparagine residue at codon 440 (p.N440del). Clinical identification of low serum TNAP activity, dental abnormalities, and pedigree data strongly suggests a genotype-phenotype correlation between p.N440del and odonto-HPP in this family. Computational analysis of the p.N440del protein structure revealed an alteration in the tertiary structure affecting the collagen-binding site (loop 422-452), which could potentially impair the mineralization process. Nevertheless, the probands (compound heterozygous: p.[N440del];[R152C]) feature early-onset and severe odonto-HPP phenotype, whereas the father (p.[N440del];[=]) has only moderate symptoms, suggesting p.R152C may contribute or predispose to a more severe dental phenotype in combination with the deletion. These results assist in defining the genotype-phenotype associations for odonto-HPP, and further identify the collagen-binding site as a region of potential structural importance for TNAP function in the biomineralization.

Compound heterozygosity of two functional null mutations in the ALPL gene associated with deleterious neurological outcome in an infant with hypophosphatasia

Hypophosphatasia (HPP) is a heterogeneous rare, inherited disorder of bone and mineral metabolism caused by different mutations in the ALPL gene encoding the isoenzyme, tissue-nonspecific alkaline phosphatase (TNAP). Prognosis is very poor in severe perinatal forms with most patients dying from pulmonary complications of their skeletal disease. TNAP deficiency, however, may also result in neurological symptoms such as neonatal seizures. The exact biological role of TNAP in the human brain is still not known and the pathophysiology of neurological symptoms due to TNAP deficiency in HPP is not understood in detail. In this report, we describe the clinical features and functional studies of a patient with severe perinatal HPP which presented with rapidly progressive encephalopathy caused by new compound heterozygous mutations in the ALPL gene which result in a functional ALPL "knock out", demonstrated in vitro. In contrast, an in vitro simulation of the genetic status of his currently asymptomatic parents who are both heterozygous for one mutation, showed a residual in vitro AP activity of above 50%. Interestingly, in our patient, the fatal outcome was due to progressive encephalopathy which was refractory to antiepileptic therapy including pyridoxine, rather than hypomineralization and respiratory insufficiency often seen in HPP patients. The patient's cranial MRI showed progressive cystic degradation of the cortex and peripheral white matter with nearly complete destruction of the cerebrum. To our knowledge, this is the first MRI-based report of a deleterious neurological clinical outcome due to a progressive encephalopathy in an infant harboring a functional human ALPL "knock out". This clinical course of disease suggests that TNAP is involved in development and may be responsible for multiple functions of the human brain. According to our data, a certain amount of residual TNAP activity might be mandatory for normal CNS function in newborns and early childhood.

5-Oxoprolinuria in Heterozygous Patients for 5-Oxoprolinase (OPLAH) Missense Changes

The inherited 5-oxoprolinuria is primarily suggestive of genetic defects in two enzymes belonging to the gamma-glutamyl cycle in the glutathione (GSH) metabolism: the glutathione synthetase (GSS) and the 5-oxoprolinase (OPLAH). The GSS deficiency is the best characterized of the inborn errors of GSH metabolism, whereas the OPLAH deficiency is questioned whether it is a disorder or just a biochemical condition with no adverse clinical effects. Recently, the first human OPLAH mutation (p.H870Pfs) was reported in homozygosis in two siblings who suffered from 5-oxoprolinuria with a benign clinical course. We report two unrelated patients who manifested massive excretion of 5-oxoproline in urine. In both probands, the blood GSH levels were normal and no mutations were found in the GSS gene. The mutational screening of the OPLAH gene, which included the codified sequences, the intronic flanking sequences, the promoter sequence, and a genetic analysis in order to detect large deletions and/or duplications, showed that each patient only harbors one missense mutation in heterozygosis. The in silico analyses revealed that each one of these OPLAH mutations, p.S323R and p.V1089I, could alter the proper function of this homodimeric enzyme. In addition, clinical symptoms manifest in these two probands were not related to GSH cycle defects and, therefore, this study provides further evidence that oxoprolinuria may present as epiphenomenon in several pathological conditions and confound the final diagnosis.

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.