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.

Emerging therapies for the treatment of rare pediatric bone disorders

In recent years, new therapies for the treatment of rare pediatric bone disorders have emerged, guided by an increasing understanding of the genetic and molecular etiology of these diseases. Herein, we review three such disorders, impacted by debilitating deficits in bone mineralization or cartilage ossification, as well as the novel disease-modifying drugs that are now available to treat these conditions. Specifically, we discuss asfotase alfa, burosumab-twza, and vosoritide, for the treatment of hypophosphatasia, X-linked hypophosphatemia and achondroplasia, respectively. For each skeletal disorder, an overview of the clinical phenotype and natural history of disease is provided, along with a discussion of the clinical pharmacology, mechanism of action and FDA indication for the relevant medication. In each case, a brief review of clinical trial data supporting drug development for each medication is provided. Additionally, guidance as to drug dosing and long-term monitoring of adverse events and pediatric efficacy is presented, to aid the clinician seeking to utilize these novel therapies in their practice, or to become familiar with the healthcare expectations for children receiving these medications through specialized multidisciplinary clinics. The availability of these targeted therapies now significantly augments treatment options for conditions in which past therapy has relied upon less specific, symptomatic medical and orthopedic care.

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.

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.

Asfotase alfa has a limited effect in improving the bowed limbs in perinatal benign hypophosphatasia: A case report

Hypophosphatasia (HPP) is a rare skeletal dysplasia characterized by impaired bone mineralization, caused by loss-of-function mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. Enzyme replacement therapy (ERT) by administration of asfotase alfa was reported to improve the survival rate, bone mineralization, and short stature in the severe form of HPP. However, the effect of asfotase alfa in improving the skeletal phenotypes for the mild form of HPP has not been elucidated. We report a case with perinatal benign HPP who had compound heterozygous mutations of p.F327L and p.R30X in the TNSALP gene. No hypomineralization was seen in the radiographs from the neonatal period, but bowing of the femurs and ulnares bilaterally was persistent. ERT was administered during the age of 7.8 to 10.8 yr, although there was an interruption in the treatment for one year. The bowed femurs and ulnares were not improved by the treatment with asfotase alfa at the age of 10.8 yr. Bone mineral density of the lumbar spine was between –0.5 and –1.0 of the z-score, and the patient’s height was about –2.0 SD during the treatment. Asfotase alfa might have a limited effect in improving the bowed limbs in perinatal benign hypophosphatasia.

Odontohypophosphatasia treated with asfotase alfa enzyme replacement therapy in a toddler: a case report

Hypophosphatasia (HPP) is a rare skeletal disorder caused by loss-of-function mutations in Alkaline Phosphatase, Biomineralization associated (ALPL) gene that encodes tissue-nonspecific alkaline phosphatase. Odontohypophosphatasia (odonto-HPP), a mild form of HPP, is characterized only by oral manifestations including premature exfoliation of deciduous teeth. Enzyme replacement therapy (ERT) is effective in severe HPP cases; however, information about its efficacy for odonto-HPP is limited. A 2-yr-old girl was referred to our hospital for mobility of her deciduous teeth with low serum alkaline phosphatase (ALP) level of 253 U/L (reference range: 410–1,150 U/L) and high urine phosphoethanolamine level of 1,419.9 µmol/g·Cre (7–70 µmol/g·Cre). She had no history of bone fractures; however, several members of her family had low serum ALP levels with a history of pathological fractures. She had a novel heterozygous missense mutation (c.1183A>T, p.Ile395Phe) in ALPL, and therefore, was diagnosed with odonto-HPP. After she was provided ERT to prevent premature exfoliation, no tooth mobility was observed. However, two deciduous teeth exfoliated two months after starting ERT, which was possibly triggered by a bout of common cold. Starting ERT following tooth mobility might be relatively late. Previous studies on experimental mice showed that starting ERT at birth may be effective in preventing premature exfoliation of deciduous teeth.

Hypophosphatasia: Current Literature for Pathophysiology, Clinical Manifestations, Diagnosis, and Treatment

Hypophosphatasia (HPP) is a rare inherited bone disorder identified by impaired bone mineralization. There are seven subtypes of HPP mainly characterized by their age of onset. These subtypes consist of perinatal (prenatal) benign, perinatal lethal, infantile, childhood, adult, odontohypophosphatasia, and pseudohypophosphatasia. Due to limited awareness of the condition, either misdiagnosis or delayed diagnosis is common. Furthermore, the condition is frequently treated with contraindicated drugs. This literature illustrates the most recent findings on the etiology, pathophysiology, clinical manifestations, diagnosing, and treatment for HPP and its subtypes. The etiology of the disease consists of loss-of-function mutations of the ALPL gene on chromosome one, which encodes for tissue nonspecific isoenzyme of alkaline phosphatase (TNAP). A decrease of TNAP reduces inorganic phosphate (Pi) for bone mineralization and allows for an increase in inorganic pyrophosphate (PPi) and phosphorylated osteopontin (p-OPN), which further reduces bone mineralization. The combination of these processes softens bone and mediates a clinical presentation similar to rickets/osteomalacia. HPP has an additional wide range of clinical features depending on its subtype. Although a concrete diagnostic guideline has not yet been established, many studies have supported a similar method of identifying HPP. Clinical features, radiological findings, and/or biomarker levels of the disorder should raise suspicion and encourage the inclusion of HPP as a differential diagnosis. Biomarkers, especially alkaline phosphatase (ALP), are major contributors to diagnosis. However, genetic testing is done for definitive diagnosis. The primary treatment for HPP is the reintroduction of TNAP as a recombinant enzyme called asfotase alfa. There are additional pharmaceutical treatments and in some cases, surgical intervention may be indicated. Pharmaceutical therapies such as bisphosphonates, denosumab, potent antiresorptive agents, and vitamin D are contraindicated in adults with HPP. We hope to raise awareness for HPP in order to prevent delayed diagnosis or misdiagnosis. We plan to encourage appropriate care and avoid treatments that may be contraindicating. We also encourage the development of a diagnostic guideline that will promote a consistently favorable patient prognosis.

Dental manifestations of hypophosphatasia in children and the effects of enzyme replacement therapy on dental status: A series of clinical cases

The most frequent dental signs of hypophosphatasia in children are premature loss of primary teeth, decrease in height of alveolar bone, and malocclusions. Enzyme replacement therapy with Asfotase alfa might be associated with stabilization of dental status.

Effect of Asfotase Alfa on Muscle Weakness in a Japanese Adult Patient of Hypophosphatasia with Low ALP Levels

A 40-year-old Japanese woman presented to our hospital with general fatigue and muscle weakness. She had a history of premature loss of deciduous teeth at 4 years old, her serum alkaline phosphatase (ALP) activity was as low as 91 U/L, and radiologic studies revealed thoracic deformity and sacroiliac calcification. Genetic sequencing revealed a heterozygous c.1559delT mutation in the tissue non-specific alkaline phosphatase gene (ALPL). Based on these findings, she was diagnosed with hypophosphatasia (HPP), and treatment with asfotase alfa, a recombinant human tissue-nonspecific alkaline phosphatase (TNSALP), was initiated. After six months of treatment with asfotase alfa, improvements were observed in the SF-36 score, six-minute walk distance, and grasping power. Although the overdiagnosis needs to be avoided, HPP should be considered in patients with undiagnosed musculoskeletal symptoms and a low serum ALP activity.

Interference of Asfotase Alfa in Immunoassays Employing Alkaline Phosphatase Technology

BACKGROUND

Asfotase alfa (STRENSIQ®, Alexion Pharmaceuticals, Inc.) is the only approved treatment for patients with pediatric-onset hypophosphatasia, a disease caused by a mutation in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. ALP is often used as signaling system in routine immunoassays. Because asfotase alfa contains the active site of the full ALP enzyme, it can catalyze the substrate as the antibody-conjugated ALP would within an assay. Therefore, its presence in a treated patient's sample may generate false positive or false negative results. We investigated whether the presence of asfotase alfa within a sample induced interference in immunoassays that utilize ALP or alternative detection systems.

METHODS

Asfotase alfa was added to samples at concentrations from 0.08-5 µg/mL and analysed on various immunoassays following manufacturer's instructions.

RESULTS

Asfotase alfa was detected in all ALP assays but ALKP1 (RayBiotech). We observed no changes in normetanephrine and noradrenaline (IBL) at any asfotase alfa concentration. However, asfotase alfa notably interfered in an oxytocin (ENZO) assay in nonextracted samples. Extraction using a C18 column eliminated the interference. No interference was observed on automated analyzers using alternative detection system (COBAS fT4 and TSH; Advia Centaur FSH, fT4; Architect LH; FSH). Immulite 2000 fT4, TSH, testosterone and hCG (ALP-based) showed no interference. However, the presence of asfotase alfa resulted in a dose-dependent increase of Troponin I signal.

CONCLUSION

The presence of asfotase alfa must be taken into consideration when analyzing blood samples in treated patients to avoid any risk of misinterpretation of false positive/negative results. It is essential that assays be tested for this possible interference.

Case Report: Efficacy of Reduced Doses of Asfotase Alfa Replacement Therapy in an Infant With Hypophosphatasia Who Lacked Severe Clinical Symptoms

BACKGROUND

Hypophosphatasia is a rare bone disease characterized by impaired bone mineralization and low alkaline phosphatase activity. Here, we describe the course of bone-targeted enzyme replacement therapy with asfotase alpha for a female infant patient with hypophosphatasia who lacked apparent severe clinical symptoms.

CASE PRESENTATION

The patient exhibited low serum alkaline phosphatase (60 U/L; age-matched reference range, 520-1,580) in a routine laboratory test at birth. Further examinations revealed skeletal demineralization and rachitic changes, as well as elevated levels of serum calcium (2.80 mmol/L; reference range, 2.25-2.75 mmol/L) and ionic phosphate (3.17 mmol/L; reference range, 1.62-2.48 mmol/L), which are typical features in patients with hypophosphatasia. Sequencing analysis of the tissue-nonspecific alkaline phosphatase (TNSALP) gene identified two pathogenic mutations: c.406C>T, p.Arg136Cys and c.979T>C, p.Phe327Leu. Thus, the patient was diagnosed with hypophosphatasia. At the age of 37 days, she began enzyme replacement therapy using asfotase alpha at the standard dose of 6 mg/kg/week. Initial therapy from the age of 37 days to the age of 58 days substantially improved rickets signs in the patient; it also provided immediate normalization of serum calcium and ionic phosphate levels. However, serum ionic phosphate returned to a high level (2.72 mmol/L), which was presumed to be a side effect of asfotase alpha. Thus, the patient's asfotase alfa treatment was reduced to 2 mg/kg/week, which allowed her to maintain normal or near normal skeletal features thereafter, along with lowered serum ionic phosphate levels. Because the patient exhibited slight distal metaphyseal demineralization in the knee at the age of 2 years and 6 months, her asfotase alfa treatment was increased to 2.4 mg/kg/week. No signs of deterioration in bone mineralization were observed thereafter. At the age of 3 years, the patient's motor and psychological development both appeared normal, compared with children of similar age.

CONCLUSION

This is the first report in which reduced doses of asfotase alfa were administered to an infant patient with hypophosphatasia who lacked apparent severe clinical symptoms. The results demonstrate the potential feasibility of a tailored therapeutic option based on clinical severity in patients with hypophosphatasia.

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.

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.

A case of perinatal hypophosphatasia with a novel mutation in the ALPL gene: clinical course and review of the literature

Hypophosphatasia (HPP) is a metabolic bone disease characterized by failure of bone calcification and vitamin B6 dependent seizures. It is caused by loss-of-function mutations in the ALPL gene. A newborn girl required respiratory support by nasal-directional positive airway pressure at birth, and pyridoxine hydrochloride administration for vitamin B6-dependent seizures observed from day two. Umbilical cord blood showed low alkaline phosphatase (ALP) activity and high pyridoxal phosphate levels. Radiographs showed severe rickets-like appearance of the bones. Genetic analysis of the ALPL gene revealed compound heterozygous mutations, c.1559delT/p.Ser188Pro. We diagnosed her with perinatal severe HPP, and started the patient on asfotase alfa from day six. Following enzyme replacement therapy (ERT), skeletal mineralization and respiratory insufficiency improved with no remarkable side-effects. Crying vital capacity (CVC) was used to evaluate respiratory status, which continuously improved from 13.3 mL/kg (day 22) to 20.6 mL/kg (day 113). Since no seizures occurred, pyridoxine hydrochloride was tapered off at one year of age. Strategies to manage perinatal severe HPP cases following ERT have not been established till date. A review of the literature shows that CVC may be a good indicator for weaning from ventilatory support. In addition, ERT will most likely enable withdrawal of pyridoxine treatment.

Clinical Outcome Assessments: Use of Normative Data in a Pediatric Rare Disease

Pediatric rare diseases present unique challenges in clinical trial design and in selection of clinical outcome assessments (COAs) used to support claims in medical product labeling. COAs that discriminate level of function relative to a normative sample are particularly important in the pediatric rare disease setting because the literature is often void of natural history data. Pediatric rare disease clinical trials will often include a wide age distribution. Gross and fine motor skills, communication, cognition, and independence in activities of daily living vary by age, and it may be difficult to distinguish between treatment effect and change due to developmental maturation. Asfotase alfa was granted breakthrough therapy designation and subsequently approved for the treatment of hypophosphatasia (HPP; a genetic metabolic musculoskeletal disorder) and is used in this discussion to illustrate COA selection in a pediatric rare disease. Multiple COAs with normative data in HPP clinical trials for asfotase alfa are presented. The assessment instruments included the Bayley Scales of Infant and Toddler Development-Third Edition, the Bruininks-Oseretsky Test of Motor Proficiency, Second Edition, the Childhood Health Assessment Questionnaire, the Pediatric Outcomes Data Collection Instrument, handheld dynamometry, the 6-minute walk test, and the Modified Performance-Oriented Mobility Assessment-Gait scale. Multiple end points were required to adequately capture the impact of asfotase alfa treatment on the multiple systems affected in HPP. These data illustrate the importance of using multiple COAs that provide normative data and to use COAs early in the drug development process for rare pediatric disease.

Successful Asfotase Alfa Treatment in an Adult Dialysis Patient With Childhood-Onset Hypophosphatasia

Hypophosphatasia is an inherited disease characterized by reduced alkaline phosphatase activity, extracellular accumulation of inorganic pyrophosphate, and impaired bone mineralization. Asfotase alfa (AA) is a recombinant human alkaline phosphatase therapy approved for treatment of pediatric-onset hypophosphatasia. Studies show promising outcome in AA-treated children with hypophosphatasia; however, data on adults with pediatric-onset hypophosphatasia are scarce. We report on a 59-year-old woman with childhood-onset hypophosphatasia and a history of multiple fractures and orthopedic procedures. Owing to renal failure (histological diagnosis: focal segmental glomerulosclerosis), hemodialysis was started in 2013. By the end of 2015, the patient was unable to walk, could only stand for 30 seconds, and was completely dependent on help for activities of daily living. After 13 months of AA therapy, the patient showed a dramatic increase in quality of life (increased mobility), reduction in pain medication, and a significant improvement in bone mineralization throughout the skeleton, including consolidation of existing fractures and no occurrence of new fractures. This case report demonstrates a relevant therapeutic success of AA treatment in an adult hemodialysis patient with childhood onset of hypophosphatasia.

Recurrent Metatarsal Fractures in Postmenopausal Woman With Low Serum Alkaline Phosphatase: A Rare Diagnosis Not to Miss

Hypophosphatasia (HPP) is a rare inborn error of metabolism due to a loss-of-function mutation in the gene for tissue nonspecific isoenzyme of alkaline phosphatase (ALP) that results in low levels of ALP. The clinical presentation of HPP is variable and in adults can easily be misdiagnosed as other forms of osteomalacia. We present a case of a 53-year-old Caucasian female who was evaluated for recurrent metatarsal fractures. She reported her first metatarsal fracture at age 21, and since then had at least 8 more metatarsal fractures over her lifetime, most without injury other than weight bearing. She reported history of gait disturbance as a child and dental issues (spacing and loosening). Laboratory tests showed normal serum calcium, phosphorus, and parathyroid hormone, but low serum ALP <20 IU/L and elevated N-telopeptide. Foot X-ray showed several healed and nonhealed metatarsal fractures, and bone densitometry revealed osteopenia. She was treated with calcium and vitamin D. A year later she had a new metatarsal fracture and a nontraumatic pelvic fracture. Teriparatide therapy was attempted but not tolerated. Due to suspicion of HPP vitamin B₆ levels were checked and found to be elevated at 263 µg/L. Given her clinical presentation and low ALP levels with elevated vitamin B₆, the diagnosis of HPP was made. Clinicians should be attentive to a history of recurrent low trauma fractures, premature loss of deciduous teeth, and persistently low serum ALP to suspect this diagnosis. Early case detection with the availability of recent Food and Drug Administration-approved asfotase alfa may avoid years of undiagnosed morbidity.

Safety and efficacy of treatment with asfotase alfa in patients with hypophosphatasia: Results from a Japanese clinical trial

OBJECTIVE

Hypophosphatasia (HPP) is a rare skeletal disease characterized by hypomineralization and low alkaline phosphatase activity. Asfotase alfa (AA) has been recently developed to treat HPP complications. This study evaluated its safety and efficacy in Japan.

DESIGN

Open-label, multicentre, prospective trial. Patients were enrolled in 11 hospitals from June 2014 to July 2015.

PATIENTS

Thirteen patients (9 females, 4 males) ages 0 days to 34 years at baseline were enrolled and treated with AA (2 mg/kg three times weekly subcutaneously in all but one patient). All had ALPL gene mutations. HPP forms were perinatal (n=6), infantile (n=5), childhood (n=1) and adult (n=1).

MEASUREMENTS

Safety determined from adverse events (AEs) and laboratory data was the primary outcome measure. Efficacy was assessed as a secondary outcome measure from overall survival, respiratory status, rickets severity and gross motor development.

RESULTS

Injection site reactions were the most frequent AEs. Serious AEs possibly related to treatment were convulsion and hypocalcaemia observed in a patient with the perinatal form. In addition, hypercalcaemia and/or hyperphosphatemia was observed in three patients with the infantile form and a low-calcium and/or low-phosphate formula was given to these patients. With respect to efficacy, all patients survived and the radiographic findings, developmental milestones and respiratory function improved.

CONCLUSION

Asfotase alfa therapy improved skeletal, respiratory and physical symptoms with a few serious AEs in patients with HPP. Our results add support to the safety and efficacy of AA therapy for HPP patients.

Pathophysiology of hypophosphatasia and the potential role of asfotase alfa

Hypophosphatasia (HPP) is an inherited systemic bone disease that is characterized by bone hypomineralization. HPP is classified into six forms according to the age of onset and severity as perinatal (lethal), perinatal benign, infantile, childhood, adult, and odontohypophosphatasia. The causative gene of the disease is the ALPL gene that encodes tissue-nonspecific alkaline phosphatase (TNAP). TNAP is expressed ubiquitously, and its physiological role is apparent in bone mineralization. A defect in bone mineralization can manifest in several ways, including rickets or osteomalacia in HPP patients. Patients with severe forms suffer from respiratory failure because of hypoplastic chest, which is the main cause of death. They sometimes present with seizures due to a defect in vitamin B₆ metabolism resulting from the lack of alkaline phosphatase activity in neuronal cells, which is also lethal. Patients with a mild form of the disease exhibit rickets or osteomalacia and a functional defect of exercise. Odontohypophosphatasia shows only dental manifestations. To date, 302 mutations in the ALPL gene have been reported, mainly single-nucleotide substitutions, and the relationships between phenotype and genotype have been partially elucidated. An established treatment for HPP was not available until the recent development of enzyme replacement therapy. The first successful enzyme replacement therapy in model mice using a modified human TNAP protein (asfotase alfa) was reported in 2008, and subsequently success in patients with severe form of the disease was reported in 2012. In 2015, asfotase alfa was approved in Japan in July, followed by in the EU and Canada in August, and then by the US Food and Drug Administration in the USA in October. It is expected that therapy with asfotase alfa will drastically change treatments and prognosis of HPP.

Enzyme replacement therapy for congenital hypophosphatasia allows for surgical treatment of related complex craniosynostosis: a case series

Hypophosphatasia (HPP) is a rare inherited disorder of bone metabolism that results in the loss of function of the gene coding for tissue-nonspecific alkaline phosphatase (TNSALP). Patients with HPP have defective bone mineralization as well as craniosynostosis that can be seen in the infantile and childhood forms of this disease. Traditionally, HPP has had a poor prognosis, with few children surviving to exhibit the phenotype of clinical craniosynostosis that requires surgical intervention. Here, the authors report on new advancements in enzyme replacement therapy (ERT) for children affected by HPP, allowing these patients to survive and undergo surgery to address complex craniosynostosis. The authors discuss their case series of 4 HPP patients treated at their institution with ERT who have undergone successful surgical intervention for craniosynostosis. These children had no complications related to their surgeries and exhibited decreased neurological symptoms following cranial vault remodeling. This study reveals that ERT administered either pre- or post- operatively paired with cranial vault remodeling strategies can yield improved neurological outcomes in children affected by HPP.