Genetics of hypophosphatasia

Hypophosphatasia diagnosis: current state of the art and proposed diagnostic criteria for children and adults

BACKGROUND

This manuscript provides a summary of the current evidence to support the criteria for diagnosing a child or adult with hypophosphatasia (HPP). The diagnosis of HPP is made on the basis of integrating clinical features, laboratory profile, radiographic features of the condition, and DNA analysis identifying the presence of a pathogenic variant of the tissue nonspecific alkaline phosphatase gene (ALPL). Often, the diagnosis of HPP is significantly delayed in both adults and children, and updated diagnostic criteria are required to keep pace with our evolving understanding regarding the relationship between ALPL genotype and associated HPP clinical features.

METHODS

An International Working Group (IWG) on HPP was formed, comprised of a multidisciplinary team of experts from Europe and North America with expertise in the diagnosis and management of patients with HPP. Methodologists (Romina Brignardello-Petersen and Gordon Guyatt) and their team supported the IWG and conducted systematic reviews following the GRADE methodology, and this provided the basis for the recommendations.

RESULTS

The IWG completed systematic reviews of the literature, including case reports and expert opinion papers describing the phenotype of patients with HPP. The published data are largely retrospective and include a relatively small number of patients with this rare condition. It is anticipated that further knowledge will lead to improvement in the quality of genotype-phenotype reporting in this condition.

CONCLUSION

Following consensus meetings, agreement was reached regarding the major and minor criteria that can assist in establishing a clinical diagnosis of HPP in adults and children.

Proposed diagnostic criteria for the diagnosis of hypophosphatasia in children and adolescents: results from the HPP International Working Group

Hypophosphatasia (HPP) is a rare inborn error of metabolism that presents variably in both age of onset and severity. HPP is caused by pathogenic variants in the ALPL gene, resulting in low activity of tissue nonspecific alkaline phosphatase (TNSALP). Patients with HPP tend have a similar pattern of elevation of natural substrates that can be used to aid in diagnosis. No formal diagnostic guidelines currently exist for the diagnosis of this condition in children, adolescents, or adults. The International HPP Working Group is a comprised of a multidisciplinary team of experts from Europe and North America who have expertise in the diagnosis and management of patients with HPP. This group reviewed 93 papers through a Medline, Medline In-Process, and Embase search for the terms "HPP" and "hypophosphatasia" between 2005 and 2020 and that explicitly address either the diagnosis of HPP in children, clinical manifestations of HPP in children, or both. Two reviewers independently evaluated each full-text publication for eligibility and studies were included if they were narrative reviews or case series/reports that concerned diagnosis of pediatric HPP or included clinical aspects of patients diagnosed with HPP. This review focused on 15 initial clinical manifestations that were selected by a group of clinical experts.The highest agreement in included literature was for pathogenic or likely pathogenic ALPL variant, elevation of natural substrates, and early loss of primary teeth. The highest prevalence was similar, including these same three parameters and including decreased bone mineral density. Additional parameters had less agreement and were less prevalent. These were organized into three major and six minor criteria, with diagnosis of HPP being made when two major or one major and two minor criteria are present.

Hypophosphatasia: from birth to adulthood

Hypophosphatasia (HPP) is an inherited disease caused by a low activity of tissue-nonspecific alkaline phosphatase, a hydrolase that removes phosphate groups from many molecules. Decreased alkaline phosphatase activity leads to the accumulation of three main metabolites, i.e., pyridoxal 5´-phosphate (PLP), inorganic pyrophosphate (PPi), and phosphoethanolamine. Impairment in PLP dephosphorylation induces seizures, while PPi accumulation inhibits bone mineralization. Clinically, HPP has a wide spectrum of presentations, ranging from neonatal death to an apparent lack of symptoms. This disease is classified into six subtypes according to the age at onset of first signs or symptoms. The clinical manifestations of the disease include rickets-like bone changes, bone demineralization, fragility fractures, reduced muscular strength, chest deformity, pulmonary hypoplasia, nephrolithiasis, nephrocalcinosis, and chondrocalcinosis. Treatment of HPP consists of enzyme replacement therapy. Before this therapy was approved, treatment was palliative and associated with high morbidity and mortality. Asfotase alfa has changed the prognosis of the disease by reducing bone deformity and improving bone mineralization, lung function, and muscle weakness, among other benefits. In adults, teriparatide and anti-sclerostin antibody have been used off-label in selected cases, demonstrating benefit in accelerating fracture healing and in concomitant treatment of osteoporosis. This review summarizes the main aspects of HPP and identifies the particularities of the disease in adult patients.

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.

Romosozumab-aqqg in the Treatment of Osteoporosis in a Patient With Hypophosphatasia

Hypophosphatasia is a rare, inherited condition that causes osteomalacia and recurrent fractures. Therapeutic options for osteoporosis in patients with hypophosphatasia are limited because of concerns for a greater likelihood of atypical femoral fractures with antiresorptive agents. We report here the case of a patient with hypophosphatasia and osteoporosis who was treated with romosozumab-aqqg (Romo). An 81-year-old woman presented for management of osteoporosis with multiple fractures. She experienced a decline in bone mineral density over 20 years despite sequential osteoporosis treatment with oral bisphosphonates, hormone replacement therapy, teriparatide, and denosumab. Hypophosphatasia was suspected because of low serum alkaline phosphatase levels and was confirmed by genetic testing. After diagnosing hypophosphatasia, bone mineral density continued to decline and a trial of Romo was begun. After 1 year of Romo therapy, bone mineral density improved by 21%, and 10% at the lumbar spine and total hip, respectively. These changes were substantially greater than what she had experienced with prior teriparatide therapy. Blood alkaline phosphatase remained low on Romo. To our knowledge, this is the first report of a patient with hypophosphatasia and osteoporosis treated with Romo. In our patient, Romo did not significantly impact serum alkaline phosphatase, but improved bone mineral density significantly. In conclusion, Romo is a potential treatment option for osteoporosis in patients with hypophosphatasia for whom limited alternatives exist.

Clinical and Genetic Characteristics of Pediatric Patients with Hypophosphatasia in the Russian Population

(1) Hypophosphatasia (HPP) is a rare inherited disease caused by mutations (pathogenic variants) in the ALPL gene which encodes tissue-nonspecific alkaline phosphatase (TNSALP). HPP is characterized by impaired bone mineral metabolism due to the low enzymatic activity of TNSALP. Knowledge about the structure of the gene and the features and functions of various ALPL gene variants, taking into account population specificity, gives an understanding of the hereditary nature of the disease, and contributes to the diagnosis, prevention, and treatment of the disease. The purpose of the study was to describe the spectrum and analyze the functional features of the ALPL gene variants, considering various HPP subtypes and clinical symptoms in Russian children. (2) From 2014−2021, the study included the blood samples obtained from 1612 patients with reduced alkaline phosphatase activity. The patients underwent an examination with an assessment of their clinical symptoms and biochemical levels of TNSALP. DNA was isolated from dried blood spots (DBSs) or blood from the patients to search for mutations in the exons of the ALPL gene using Sanger sequencing. The PCR products were sequenced using a reagent BigDye Terminator 3.1 kit (Applied Biosystems). Statistical analysis was performed using the GraphPad Prism 8.01 software. (3) The most common clinical symptoms in Russian patients with HPP and two of its variants (n = 22) were bone disorders (75%), hypomyotonia (50%), and respiratory failure (50%). The heterozygous carriage of the causal variants of the ALPL gene was detected in 225 patients. A total of 2 variants were found in 27 patients. In this group (n = 27), we identified 28 unique variants of the ALPL gene, of which 75.0% were missense, 17.9% were frameshift, 3.6% were splicing variants, and 3.6% were duplications. A total of 39.3% (11/28) of the variants were pathogenic, with two variants being probably pathogenic, and 15 variants had unknown clinical significance (VUS). Among the VUS group, 28.6% of the variants (7/28) were discovered by us for the first time. The most common variants were c.571G > A (p.Glu191Lys) and c.1171del (Arg391Valfs*12), with frequencies of 48.2% (13/28) and 11% (3/28), respectively. It was found that the frequency of nonsense variants of the ALPL gene was higher (p < 0.0001) in patients with the perinatal form compared to the infantile and childhood forms of HPP. Additionally, the number of homozygotes in patients with the perinatal form exceeded (p < 0.01) the frequencies of these genotypes in children with infantile and childhood forms of HPP. On the contrary, the frequencies of the compound-heterozygous and heterozygous genotypes were higher (p < 0.01) in patients with infantile childhood HPP than in perinatal HPP. In the perinatal form, residual TNSALP activity was lower (p < 0.0005) in comparison to the infantile and childhood (p < 0.05) forms of HPP. At the same time, patients with the heterozygous and compound-heterozygous genotypes (mainly missense variants) of the ALPL gene had greater residual activity (of the TNSALP protein) regarding those homozygous patients who were carriers of the nonsense variants (deletions and duplications) of the ALPL gene. Residual TNSALP activity was lower (p < 0.0001) in patients with pathogenic variants encoding the amino acids from the active site and the calcium and crown domains in comparison with the nonspecific region of the protein.

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.

Hypophosphatasia

Hypophosphatasia (HPP) is an inherited metabolic disease caused by loss-of-function mutations in the tissue non-specific alkaline phosphatase (TNAP) gene. Reduced activity of TNAP leads to the accumulation of its substrates, mainly inorganic pyrophosphate and pyridoxal-5′-phosphate, metabolic aberrations that largely explain the musculoskeletal and systemic features of the disease. More than 400 ALPL mutations, mostly missense, are reported to date, transmitted by either autosomal dominant or recessive mode. Severe disease is rare, with incidence ranging from 1:100,000 to 1:300,000 live births, while the estimated prevalence of the less severe adult form is estimated to be between 1:3100 to 1:508, in different countries in Europe. Presentation largely varies, ranging from death in utero to asymptomatic adults. In infants and children, clinical features include skeletal, respiratory and neurologic complications, while recurrent, poorly healing fractures, muscle weakness and arthropathy are common in adults. Persistently low serum alkaline phosphatase is the cardinal biochemical feature of the disease. Management requires a dedicated multidisciplinary team. In mild cases, treatment is usually symptomatic. Severe cases, with life-threating or debilitating complications, can be successfully treated with enzyme replacement therapy with asfotase alfa.

A two-year follow-up of asfotase alfa replacement in a patient with hypophosphatasia: clinical, biochemical, and radiological evaluation

Hypophosphatasia (HPP) is a rare disease with a high mortality rate in its severe forms. It is caused by mutations within the gene encoding the tissue-nonspecific alkaline phosphatase (TNSALP), an enzyme responsible for bone mineralization. In 2015, the Food and Drug Administration approved the use of asfotase alfa, the first medication showing benefit in the treatment of HPP. We describe a case with a 2-year follow-up of the first Brazilian child treated with asfotase alfa. A 5-year-old boy, born to consanguineous parents, was diagnosed with HPP at the age of 20 months. During prenatal ultrasonography, polyhydramnios and shortening of long bones were detected. After birth, he presented delayed motor development, repeated respiratory infections, and bone deformities. At the age of 2 years and 8 months, he started walking and had already lost his primary teeth. He had reduced levels of alkaline phosphatase (ALP), elevated levels of pyridoxal 5'-phosphate (PLP), and a p.Ala33Val (c.98C>T) missense mutation in homozygosis in the TNSALP gene. His parents and sister also had reduced ALP levels, high PLP levels, and the same mutation in heterozygosis. His father and sister were healthy, and his mother was diagnosed with rickets in childhood, which resulted in short physical stature and lower limb deformities. The patient was started on asfotase alfa at the age of 2 years and 10 months. After 2 years of treatment, he improved his motor skills, had no further episodes of severe respiratory infection, and showed improved radiological findings of rickets, without any severe side effect.

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.

Six ALPL gene variants in five children with hypophosphatasia

Background

Hypophosphatasia (HPP) is a rare hereditary disorder characterized by defective bone and tooth mineralization caused by mutations in the alkaline phosphatase (ALPL) gene encoding tissue-nonspecific alkaline phosphatase (TNSALP). Here we performed clinical and molecular studies on 5 HPP children to investigate the pathogenic mechanisms of the ALPL gene variants.

Methods

Clinical and genetic analyses were performed on 5 HPP children, and the loci where ALPL variants were identified. Plasmids containing the relevant loci were constructed. The molecular and cellular mechanisms of the pathogenic ALPL variants were investigated by cellular immunofluorescence, enzyme activity assay, and protein expression assay.

Results

A total of 6 ALPL variants were identified in 5 HPP children: proband 1: c.346G>A (p.A116T); proband 2: c.346G>A (p.A116T)/deletions from c.1097 to c.1099 CCT (p.T366_S367deli) compound heterozygous variant; proband 3: insertion of G from c.1014 to c.1015 (p.H338fs)/c.1446C>A (p.H482Q) compound heterozygous variant; proband 4: c.920C>T (p.P307L); and proband 5: c.883A>G (p.M295V). Twenty-four hours after the HEK-293T was transfected with different variant plasmids, its alkaline phosphatase activity and enzyme protein content were reduced compared with the wild type, and there were differences among different variants. Except for 1014-G-1015+C1446A, the degree of reduction in enzyme activity was negatively correlated with the severity of clinical manifestations. Immunofluorescence revealed that the variants (especially c.883A>G and c.920C>T) caused a decrease in alkaline phosphatase expression in the cellular membrane.

Conclusions

In total, 3 novel variants were identified in these 5 HPP children, the discovery of which will enrich the human ALPL gene mutation database. Different variants in the ALPL gene can downregulate the activity of TNSALP enzyme (and thus affect its function) by affecting protein expression and translational modifications. The same variant may cause clinical manifestations of different severities in different individuals due to the presence of dominant negative effects, alterations in noncoding sequences, blind area of intron regulatory region sequencing, and variations in environmental and individual factors. The molecular mechanisms via which the ALPL gene exerts its expression effect in vivo are highly variable and warrant further investigation.

Hypophosphatasia

Hypophosphatasia (HPP) is a group of inherited disorders characterised by the impaired mineralisation of bones and/or teeth and low serum alkaline phosphatase (ALP) activity. It is caused by a mutation in the ALPL gene encoding the tissue-non-specific isoenzyme of ALP (TNSALP) resulting in a loss of function. The disease is highly heterogenous in its clinical expression ranging from stillbirth without mineralised bone to the mild form of late adult onset with symptoms and signs such as musculoskeletal pain, arthropathy, lower-extremity fractures, premature loss of teeth or an incidental finding of reduced serum ALP activity. A classification based on the age at diagnosis and the presence or absence of bone symptoms was historically used: perinatal, prenatal benign, infantile, childhood, adult and odontohypophosphatasia. These subtypes are known to have overlapping signs and complications. Three forms of HPP distinguishable by their genetic characteristics have been described: severe, moderate and mild. Severe forms of HPP (perinatal and infantile severe) are recessively inherited, whereas moderate HPP may be dominantly or recessively inherited. The biochemical hallmark of HPP is persistently low serum ALP for age and increase in natural substrates of TNSALP, pyridoxal 5'-phosphate and phosphoethanolamine supported by radiological findings. The diagnosis is confirmed by ALPL sequencing. A multidisciplinary team of experts is essential for the effective management. Calcium restriction is recommended in infants/children to manage hypercalcaemia. A targeted enzyme replacement therapy for HPP has become available and correct diagnosis is crucial to allow early treatment.

Active search of adult patients with persistently low serum alkaline phosphatase levels for the diagnosis of hypophosphatasia

Objective

Alkaline phosphatase (ALP) is the main laboratory marker of hypophosphatasia (HPP), a rare disease unknown to most physicians. The prevalence of HPP has been widely discussed in the literature due to the diverse phenotypes of HPP. The purpose of this study was to search for patients with hypophosphatasemia based on previous biochemistry tests and reevaluate them to confirm the diagnosis of HPP.

Methods

A total of 289,247 biochemical tests for ALP in adults were performed from 2015 to 2019 in two tertiary hospitals in Rio de Janeiro were reviewed (Clementino Fraga Filho University Hospital - HUCFF - and Bonsucesso Federal Hospital - BFH).

Results

A total of 1,049 patients were identified with ALP levels below 40 U/L, and 410 patients had hypophosphatasemia confirmed by at least two exams. After the active search of medical reports and/or interviews based on structured questionnaires, 398 subjects were excluded due to secondary causes of reduced ALP. The remaining 12 patients were invited to attend the medical consultation at HUCFF, accompanied by at least one first-degree relative. None of the patients or their relatives had a history or clinical manifestations consistent with HPP. Serum ALP was within reference values in all relatives, but persistently low in further laboratory evaluation in all the 12 patients, in whom secondary causes were ruled out. Thus, we cannot exclude the possibility that they might carry the mutations associated with HPP.

Conclusion

Further image evaluations and genetic testing would be appropriate to confirm this asymptomatic adult form of HPP.

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.

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.

Insights into dental mineralization from three heritable mineralization disorders

Teeth are comprised of three unique mineralized tissues, enamel, dentin, and cementum, that are susceptible to developmental defects similar to those affecting bone. X-linked hypophosphatemia (XLH), caused by PHEX mutations, leads to increased fibroblast growth factor 23 (FGF23)-driven hypophosphatemia and local extracellular matrix disturbances. Hypophosphatasia (HPP), caused by ALPL mutations, results in increased levels of inorganic pyrophosphate (PP_i), a mineralization inhibitor. Generalized arterial calcification in infancy (GACI), caused by ENPP1 mutations, results in vascular calcification due to decreased PP_i, later compounded by FGF23-driven hypophosphatemia. In this perspective, we compare and contrast dental defects in primary teeth associated with XLH, HPP, and GACI, briefly reviewing genetic and biochemical features of these disorders and findings of clinical and preclinical studies to date, including some of our own recent observations. The distinct dental defects associated with the three heritable mineralization disorders reflect unique processes of the respective dental hard tissues, revealing insights into their development and clues about pathological mechanisms underlying such disorders.

Perinatal severe hypophosphatasia: a case report

Objectives

Hypophosphatasia is a rare, inherited metabolic disorder characterized by low serum alkaline phosphatase activity. It is caused by mutations in the ALPL gene, which encodes tissue-nonspecific alkaline phosphatase (TNSALP) [1], [2], [3]. The degree of skeletal malformation varies, but the severe form carries a very poor prognosis.

Case presentation

This study reports a male neonate diagnosed with infantile hypophosphatasia (HPP). Genetic analysis showed two heterozygous missense variants at nucleotides c.977G>T (protein Gly326Val) and c.862+4A>G (IVS8+4A>G) (protein NA). The two mutations originated separately from the parents, consistent with autosomal recessive infantile HPP. The pathogenic variant was ALPL exon-9-heterozygous, and the other allele was ALPL IVS8-heterozygous, a variant of uncertain significance.

Conclusions

This case of infantile HPP was caused by two heterozygous mutations. One of those is a novel genetic mutation needed for further study. Genetic consultation is recommended for future offspring of affected parents.

Proceedings of the 2019 Santa Fe Bone Symposium: New Concepts in the Care of Osteoporosis and Rare Bone Diseases

The 20th annual Santa Fe Bone Symposium was held August 9-10, 2019, in Santa Fe, New Mexico, USA. This is an annual meeting devoted to clinical applications of recent advances in skeletal research that impact the care of patients with osteoporosis, metabolic bone diseases, and inherited bone diseases. Participants included practicing and academic physicians, fellows, advanced practice providers, fracture liaison service (FLS) coordinators, clinical researchers, and bone density technologists. The symposium consisted of lectures, case presentations, and panel discussions, with an emphasis on learning through interaction of all attendees. Topics included new approaches in the use of anabolic agents for the treatment osteoporosis, a review of important events in skeletal health over the past year, new and emerging treatments for rare bone diseases, the use of genetic testing for bone diseases in clinical practice, medication-associated causes of osteoporosis, new concepts in the use of estrogen therapy for osteoporosis, new Official Positions of the International Society for Clinical Densitometry, skeletal consequences of bariatric surgery, and update on the progress and potential of Bone Health TeleECHO, a virtual community of practice using videoconferencing technology to link healthcare professionals for advancing the care of osteoporosis worldwide. Sessions on rare bone diseases were developed in collaboration with the Rare Bone Disease Alliance. Symposium premeetings included an FLS workshop by the National Osteoporosis Foundation and others devoted to the use of new therapeutic agents for the care of osteoporosis and related disorders.

Two novel mutations in the ALPL gene of unrelated Chinese children with Hypophosphatasia: case reports and literature review

OBJECTIVE

Hypophosphatasia (HPP) is an inherited disorder of defective skeletal mineralization caused by mutations in the ALPL gene that encodes the Tissue Non-specific Alkaline Phosphatase (TNSALP). It is subdivided into six forms depending on the age of onset: perinatal lethal, prenatal benign, infantile, childhood, adult, and odonto HPP. Among these, infantile HPP is characterized by early onset and high frequency of lethal outcome. Few studies have reported the phenotype and genetic characteristics of HPP in Chinese children.

CASE PRESENTATION

Three forms of HPP were identified in four unrelated patients from four different Chinese families, including one lethal infantile (patient 1), two childhood (patient 2 and 3) and one odonto HPP (patient 4). Six variants in the ALPL gene were identified, including five missense mutations and one frameshift mutation. Of which, none were reported previously in the Chinese population, and two were novel (c.359G > C: p.G120A and c.1017dupG: p.H340AfsX3). Patient 1 carrying a novel homozygous (c.359G > C) mutation showed respiratory distress and pneumonia at first day of his life. He presented nearly negligible level of serum ALP activity, overall skeletal hypominaralization and died at 3 months old. Patient 2, 3 and 4 were compound heterozygotes with decreased serum ALP activity. Patient 2 and 3 presented premature loss of deciduous teeth, muscle weakness and bone pain, whereas patient 4 had early loss of deciduous teeth only. All four pedigrees exhibited autosomal recessive pattern of inheritance.

CONCLUSIONS

In this study, six mutations in the ALPL gene were found in four Chinese HPP patients, two of which were novel: c.359G > C in exon 5 and c.1017dupG in exon 10. Our results strongly indicated that the novel mutation c.359G > C might be disease-causing and associated with severe infantile form of HPP.

Hypophosphatasia: Canadian update on diagnosis and management

Hypophosphatasia (HPP) is a rare inherited disorder of bone and mineral metabolism caused by loss of function mutations in the ALPL gene. The presentation in children and adults can be extremely variable and natural history is poorly understood particularly in adults. Careful patient evaluation is required with consideration of pharmacologic intervention in individuals meeting criteria for therapy.

INTRODUCTION

The purposes of this review are to present current evidence regarding the diagnosis and management of hypophosphatasia in children and adults and provide evidence-based recommendations for management.

METHOD

A MEDLINE, EMBASE, and Cochrane database search and literature review was completed. The following consensus recommendations were developed based on the highest level of evidence as well as expert opinion.

RESULTS

Hypophosphatasia is a rare inherited disorder of bone and mineral metabolism due to loss of function mutations in the tissue non-specific alkaline phosphatase (ALPL) gene causing reductions in the activity of the tissue non-specific isoenzyme of alkaline phosphatase (TNSALP). Deficient levels of alkaline phosphatase result in elevation of inhibitors of mineralization of the skeleton and teeth, principally inorganic pyrophosphate. The impaired skeletal mineralization may result in elevations in serum calcium and phosphate. Clinical features include premature loss of teeth, metatarsal and subtrochanteric fractures as well as fragility fractures. Poor bone healing post fracture has been observed. Myalgias and muscle weakness may also be present. In infancy and childhood, respiratory and neurologic complications can occur.

CONCLUSIONS

HPP is associated with significant morbidity and mortality. Pharmacologic intervention can result in significant clinical improvement. This Canadian position paper provides an overview of the musculoskeletal, renal, dental, respiratory, and neurologic manifestations of hypophosphatasia. The current state of the art in the diagnosis and management of hypophosphatasia is presented.

Update on the management of hypophosphatasia

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

Epidemiological, Clinical and Genetic Study of Hypophosphatasia in A Spanish Population: Identification of Two Novel Mutations in The Alpl Gene

Hypophosphatasia (HPP) is a genetic disease caused by one or several mutations in ALPL gene encoding the tissue-nonspecific alkaline phosphatase affecting the mineralization process. Due to its low prevalence and lack of recognition, this metabolic disorder is generally confused with other more frequent bone disorders. An assessment of serum total alkaline phosphatase (ALP) levels was performed in 78,590 subjects. Pyridoxal-5′-phosphate (PLP) concentrations were determined and ALPL gene was sequenced in patients potentially affected by HPP. Functional validation of the novel mutations found was performed using a cell-based assay. Our results showed persistently low serum ALP levels in 0.12% of subjects. Among the studied subjects, 40% presented with HPP-related symptoms. Nine of them (~28%) had a history of fractures, 5 (~16%) subjects showed chondrocalcinosis and 4 (~13%) subjects presented with dental abnormalities. Eleven subjects showed increased PLP concentrations. Seven of them showed ALPL gene mutations (2 of the mutations corresponded to novel genetic variants). In summary, we identified two novel ALPL gene mutations associated with adult HPP. Using this protocol, almost half of the studied patients were diagnosed with HPP. Based on these results, the estimated prevalence of mild HPP in Spain could be up to double than previously reported.

A homozygous missense variant in the alkaline phosphatase gene ALPL is associated with a severe form of canine hypophosphatasia

Inherited skeletal disorders affect both humans and animals. In the current study, we have performed series of clinical, pathological and genetic examinations to characterize a previously unreported skeletal disease in the Karelian Bear Dog (KBD) breed. The disease was recognized in seven KBD puppies with a variable presentation of skeletal hypomineralization, growth retardation, seizures and movement difficulties. Exome sequencing of one affected dog revealed a homozygous missense variant (c.1301T > G; p.V434G) in the tissue non-specific alkaline phosphatase gene, ALPL. The identified recessive variant showed full segregation with the disease in a cohort of 509 KBDs with a carrier frequency of 0.17 and was absent from 303 dogs from control breeds. In humans, recessive and dominant ALPL mutations cause hypophosphatasia (HPP), a metabolic bone disease with highly heterogeneous clinical manifestations, ranging from lethal perinatal hypomineralization to a relatively mild dental disease. Our study reports the first naturally occurring HPP in animals, resembling the human infantile form. The canine HPP model may serve as a preclinical model while a genetic test will assist in breeding programs.

Alkaline Phosphatase Replacement Therapy for Hypophosphatasia in Development and Practice

Hypophosphatasia (HPP) is an inherited disorder that affects bone and tooth mineralization characterized by low serum alkaline phosphatase. HPP is caused by loss-of-function mutations in the ALPL gene encoding the protein, tissue-nonspecific alkaline phosphatase (TNSALP). TNSALP is expressed by mineralizing cells of the skeleton and dentition and is associated with the mineralization process. Generalized reduction of activity of the TNSALP leads to accumulation of its substrates, including inorganic pyrophosphate (PP_i) that inhibits physiological mineralization. This leads to defective skeletal mineralization, with manifestations including rickets, osteomalacia, fractures, and bone pain, all of which can result in multi-systemic complications with significant morbidity, as well as mortality in severe cases. Dental manifestations are nearly universal among affected individuals and feature most prominently premature loss of deciduous teeth. Management of HPP has been limited to supportive care until the introduction of a TNSALP enzyme replacement therapy (ERT), asfotase alfa (AA). AA ERT has proven to be transformative, improving survival in severely affected infants and increasing overall quality of life in children and adults with HPP. This chapter provides an overview of TNSALP expression and functions, summarizes HPP clinical types and pathologies, discusses early attempts at therapies for HPP, summarizes development of HPP mouse models, reviews design and validation of AA ERT, and provides up-to-date accounts of AA ERT efficacy in clinical trials and case reports, including therapeutic response, adverse effects, limitations, and potential future directions in therapy.

Genetic engineering a large animal model of human hypophosphatasia in sheep

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

Neonatal lethal hypophosphatasia: A case report and review of literature

RATIONALE

Hypophosphatasia (HPP) is a very rare metabolic bone disease caused by loss-of-function mutations in the ALPL gene encoding the tissue nonspecific alkaline phosphatase. The severe neonatal form is considered lethal but insights into manifestations of the disease can help to increase our knowledge of the natural history for an early initiation of treatment and improvement of survival.

PATIENT CONCERNS

We report the case of a newborn in which his fetal imaging showed findings of skeletal dysplasia disorder, considering initially achondroplasia as a potential diagnosis.

DIAGNOSIS

A definitive diagnosis compatible with perinatal lethal HPP was established in the 1st days due to the presentation at birth with thoracic and pulmonary hypoplasia, bone hypomineralization, and undetectable alkaline phosphatase. The genetic analysis identified a new heterozygous c.413G>C mutation and another 1 c.473-2G>C previously described in the ALPL gene.

OUTCOMES

The patient died on the 4th day by clinical course complicated without having started enzyme replacement therapy (ERT). Retrospectively, previous analyzes of the parents already showed both a decreased alkaline phosphatase.

LESSONS

This report highlights the importance of prenatal differential diagnosis of bone dysplasia with the key biochemical marker of alkaline phosphatase in the parents. Substitutive ERT administered very soon after birth, seems to change the prognosis in these patients with neonatal HPP.

HYPOPHOSPHATASIA: CLINICAL ASSESSMENT AND MANAGEMENT IN THE ADULT PATIENT-A NARRATIVE REVIEW

OBJECTIVE

To review literature and present a schematic approach to hypophosphatasia (HPP) evaluation and management applicable to practicing physicians to ease its recognition and diagnosis.

METHODS

Studies were obtained from online databases PubMed and MEDLINE using keyword 'hypophosphatasia.'

RESULTS

HPP is a rare disease characterized by low serum alkaline phosphatase along with diverse musculoskeletal symptoms that mimic different disorders. To date, the prevalence of HPP and its impact on adults has been unrecognized. There is lack of evidence from larger and long-term studies examining the adult type of this condition.

CONCLUSION

It is essential to increase awareness on the complexity of the pathophysiology and clinical features of HPP, which causes debilitating physical conditions that severely affects quality of life. A better comprehension of adult forms of HPP is essential to reduce a delay in diagnosis as well as ensure suitable management.

ABBREVIATIONS

ALP = alkaline phosphatase; HPP = hypophosphatasia; PEA = phosphorethanolamine; PLP = pyridoxal-5-phosphate; PPi = inorganic pyrophosphate; TNSALP/TNAP = tissue-nonspecific alkaline phosphatase.