Mild hypophosphatasia mimicking severe osteogenesis imperfecta in utero: bent but not broken

Hypophosphatasia: is it an underdiagnosed disease even by expert physicians?

INTRODUCTION

Hypophosphatasia (HPP) is caused by mutations in the ALPL that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (ALP). Clinical manifestations range from extreme life-threatening lethal forms to no signs or symptoms at all.

MATERIALS AND METHODS

Consecutive 30,000 outpatients and inpatients with ALP data were screened retrospectively, out of which 1000 patients were found to have low levels of ALP more than once. Then, patients were evaluated for the symptoms and signs of HPP with further biochemical and genetic analyses.

RESULTS

Thirty-seven patients who had severe musculoskeletal pain, recurrent fractures, and tooth anomalies were then screened with substrate and DNA sequencing analyses for HPP. It was determined that eight patients had variants in the ALPL gene. A total of eight different ALPL variants were identified in eight patients. The variants, namely c.244G > C (p.Gly82Arg), c.1444C > T (p.His482Tyr), c.1487A > G (p.Asn493Ser), and c.675_676insCA (p.Met226GlnfsTer52), had not been previously reported.

DISCUSSION

Considering the wide spectrum of clinical signs and symptoms, HPP should be among the differential lists of bone, muscle, and tooth abnormalities at any age.

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.

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.

Perinatal Hypophosphatasia in a Premature Infant

A premature male infant was delivered at 32 weeks' gestation due to category-2 fetal tracing after preterm labor. The physical exam showed shortened and bowed long bones, with calvarium felt in small area of the head. Serum alkaline phosphatase was very low on admission. Extensive metaphyseal abnormalities, bowing of long bones, and poor ossification of all bones were noted on skeletal survey (radiography). Based on ultrasound evidence of "bowing" and long bone fractures at 26 weeks, amniocentesis was performed that later diagnosed hypophosphatasia by genetic testing while ruling out osteogenesis imperfecta. Although ventilated initially, the infant gradually improved with enzyme replacement therapy (ERT) and was extubated to noninvasive ventilation for 6 weeks. Following clinical deterioration with hypoxic respiratory failure secondary to sepsis at 4 months of age, he succumbed to severe pulmonary hypertension, likely secondary to chronic lung disease and prolonged ventilation. Early diagnosis allowed timely initiation of appropriate therapy. Radiological improvement with the therapy showed promising results in this rare disease. Despite specific novel therapy being available, variability in presentation dictates prognosis in this previously universally fatal condition. The potential unknown effects of ERT on pulmonary vascular remodeling need further investigation.

Differential diagnosis of perinatal hypophosphatasia: radiologic perspectives

Perinatal hypophosphatasia (HPP) is a rare, potentially life-threatening, inherited, systemic metabolic bone disease that can be difficult to recognize in utero and postnatally. Diagnosis is challenging because of the large number of skeletal dysplasias with overlapping clinical features. This review focuses on the role of fetal and neonatal imaging modalities in the differential diagnosis of perinatal HPP from other skeletal dysplasias (e.g., osteogenesis imperfecta, campomelic dysplasia, achondrogenesis subtypes, hypochondrogenesis, cleidocranial dysplasia). Perinatal HPP is associated with a broad spectrum of imaging findings that are characteristic of but do not occur in all cases of HPP and are not unique to HPP, such as shortening, bowing and angulation of the long bones, and slender, poorly ossified ribs and metaphyseal lucencies. Conversely, absent ossification of whole bones is characteristic of severe lethal HPP and is associated with very few other conditions. Certain features may help distinguish HPP from other skeletal dysplasias, such as sites of angulation of long bones, patterns of hypomineralization, and metaphyseal characteristics. In utero recognition of HPP allows for the assembly and preparation of a multidisciplinary care team before delivery and provides additional time to devise treatment strategies.

Genetics of hypophosphatasia

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

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.

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

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

Monitoring guidance for patients with hypophosphatasia treated with asfotase alfa

Hypophosphatasia (HPP) is a rare, inherited, systemic, metabolic disorder caused by autosomal recessive mutations or a single dominant-negative mutation in the gene encoding tissue-nonspecific alkaline phosphatase (TNSALP). The disease is associated with a broad range of signs, symptoms, and complications, including impaired skeletal mineralization, altered calcium and phosphate metabolism, recurrent fractures, pain, respiratory problems, impaired growth and mobility, premature tooth loss, developmental delay, and seizures. Asfotase alfa is a human, recombinant enzyme replacement therapy that is approved in many countries for the treatment of patients with HPP. To address the unmet need for guidance in the monitoring of patients receiving asfotase alfa, an international panel of physicians with experience in diagnosing and managing HPP convened in May 2016 to discuss treatment monitoring parameters. The panel discussions focused on recommendations for assessing and monitoring patients after the decision to treat with asfotase alfa had been made and did not include recommendations for whom to treat. Based on the consensus of panel members, this review provides guidance on the monitoring of patients with HPP during treatment with asfotase alfa, including recommendations for laboratory, efficacy, and safety assessments and the frequency with which these should be performed during the course of treatment. Recommended assessments are based on patient age and include regular monitoring of biochemistry, skeletal radiographs, respiratory function, growth, pain, mobility and motor function, and quality of life. Because of the systemic presentation of HPP, a coordinated, multidisciplinary, team-based, patient-focused approach is recommended in the management of patients receiving asfotase alfa. Monitoring of efficacy and safety outcomes must be tailored to the individual patient, depending on medical history, clinical manifestations, availability of resources in the clinical setting, and the clinician's professional judgment.

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.

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.

An anadysplasia-like, spontaneously remitting spondylometaphyseal dysplasia secondary to lamin B receptor (LBR) gene mutations: further definition of the phenotypic heterogeneity of LBR-bone dysplasias

We describe a boy who has an anadysplasia-like spondylometaphyseal dysplasia. By whole exome sequencing he was shown to have compound heterozygous mutations of LBR that codes for the lamin B receptor. He shares many similarities with a case previously described, but in whom the early natural history could not be established [Borovik et al., 2013]. Thus, in addition to Greenberg dysplasia (a perinatal lethal disorder), homozygosity or compound heterozygosity of mutations in LBR can result in a mild, spontaneously regressing bone dysplasia.

Benign prenatal hypophosphatasia: a treatable disease not to be missed

Prenatal bowing of the long bones is often associated with severe bone dysplasias. We report a child who presented marked bowing of the long bones at birth but showed a relatively benign postnatal course with spontaneous improvement of bowing. The fetal imaging showed normal skeletal mineralization and normal chest and abdominal circumferences despite the limb bowing and shortening. Decreased serum alkaline phosphatase activity and elevated urine phosphoethanolamine was biochemically evident, and compound heterozygous mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene were identified, which confirmed the diagnosis of a benign form of prenatal hypophosphatasia. Benign prenatal hypophosphatasia should be considered in the differential diagnosis of congenital bowing of the long bones.

Current concepts in hypophosphatasia: case report and literature review

BACKGROUND.  Hypophosphatasia (HP) is characterized by defective mineralization of bone and teeth because of deficient alkaline phosphatase activity. There are generally six recognized clinical forms, of which the most severe is often lethal prenatally or early in life. In milder forms, such as odontohypophosphatasia (OHP), premature exfoliation of primary teeth may be the only clinical manifestation. CASE REPORT.  A 20-month-old girl was referred to the Specialist Paediatric Salaried Dental Service within the Harrogate and District NHS Foundation Trust with mobility of tooth numbers 71 and 81. Clinical examination revealed grade III mobile 71 and 81, with minimal gingival inflammation and plaque deposits. There were no other dental findings and no significant medical history. Tooth numbers 71 and 81 exfoliated prematurely with no evidence of root resorption, shortly after presentation. Haematological and urinary investigations showed no abnormalities. Histological examination showed a complete absence of cementum. A diagnosis of OHP was made. After 10 months of dental follow-up, no further teeth have increased mobility. CONCLUSION.  Odontohypophosphatasia should be included as a differential diagnosis in children presenting with early loss of primary teeth. The dentist may be the first health care professional to whom the patient presents.

Outcome of perinatal hypophosphatasia in manitoba mennonites: a retrospective cohort analysis

Hypophosphatasia (HPP) is the metabolic bone disease caused by loss-of-function mutation within the gene that encodes the "tissue nonspecific" isoenzyme of alkaline phosphatase (TNSALP). Perinatal HPP is usually fatal due to respiratory insufficiency, and infantile HPP often has a similar outcome although no formal study into the natural history of these severe forms of HPP has been undertaken. We reviewed our 80-year (1927-2007) cohort of 15 Canadian patients with perinatal HPP. All had Mennonite heritage. Family linkage studies indicated that nine were homozygous for a TNSALP disease allele, likely Gly334Asp. Three patients had parents who were carriers for the Gly334Asp allele by mutation analysis. One patient was confirmed by mutation analysis to be homozygous for the TNSALP Gly334Asp mutation. One patient who had only one Mennonite parent was a genetic compound for the Gly334Asp mutation and the Val382Ile mutation. This patient's sibling was also affected. All 15 patients had profound skeletal hypomineralization, severe rickets, and respiratory insufficiency. All died by 9 months of age, usually soon after birth, from pulmonary failure.

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

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

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

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

Prenatal diagnosis and management of intrauterine fracture

Intrauterine fracture is an extremely rare finding, but can occur as the result of maternal trauma, osteogenesis imperfecta (OI), or theoretically other metabolic/structural abnormalities. Increased clinical awareness of the diagnosis and optimal management of these cases can lead to more positive outcomes for the patient and her child. Blunt abdominal trauma late in gestation increases the risk of fetal skull fracture, while a known diagnosis of OI or other abnormalities leading to decreased fetal bone density creates concern for long bone fracture. Biochemical and genetic tests can aid in the prenatal diagnosis of OI, while ultrasound is the best overall imaging modality for identifying fetal fracture of any etiology. When fetal fracture is diagnosed radiologically, specific management is recommended to promote optimal outcomes for mother and fetus, with special consideration given to the mother with OI. With the exception of fetal fractures due to lethal conditions, cesarean delivery is recommended in most cases, especially when fetal or maternal well-being cannot be assured. When a patient presents with risk factors for intrauterine fracture, careful evaluation via thorough history-taking, ultrasonography of the entire fetal skeleton, and laboratory tests should be performed. Heightened awareness of intrauterine fracture allows better postpartum management, whether for simple fracture care or for long-term care of patients with OI or genetic/metabolic abnormalities.

TARGET AUDIENCE

Obstetricians & Gynecologist, Family Physicians.

LEARNING OBJECTIVES

After completion of this educational activity, the reader will be able to compare x-ray, ultrasound modalities and MRI and their utility in diagnosing fetal fracture. Formulate a differential diagnosis for fetal fracture. Propose a delivery plan for a patient whose fetus has a prenatally diagnosed fetal fracture.

Clinical characteristics of perinatal lethal hypophosphatasia: a report of 6 cases

Hypophosphatasia is a rare inherited disorder caused by deficient tissue-nonspecific alkaline phosphatase activity. It is classified into 6 subtypes, and the perinatal lethal form of hypophosphatasia is the most severe. Patients with this form suffer from various symptoms, including respiratory failure, premature craniosynostosis, rachitic changes in the metaphyses, convulsions and hypercalcemia. This report presents 6 cases of the perinatal lethal form of hypophosphatasia. All of the patients showed shortening of the long bones in utero in ultrasonographic examinations. Two of the six patients died at birth because they could not establish spontaneous breathing. Three of the remaining four patients also died before 1 yr of age. The major cause of death was respiratory failure due to hypoplastic lung. All of the patients, except for the two who died at birth, experienced convulsions in their clinical courses. Vitamin B6 therapy effectively reduced the frequency and severity of convulsions. However, it could not always make the patients convulsion free. Three patients underwent a genetic analysis. The 1559delT mutation, which abolishes Alkaline Phosphatase (ALP) activity, was a hot spot. A homozygous 1559delT mutation was observed in two patients. However, they differed in severity of symptoms. Although a good genotype-phenotype correlation has been reported in hypophosphatasia, the genotype alone does not always predict the life span of the patients. These cases therefore suggested the importance of genetic counseling.

Hypophosphatasia may lead to bone fragility: don't miss it

Hypophosphatasia is an inheritable disorder characterised by defective bone mineralisation due to the impaired activity of tissue-non-specific alkaline phosphatase (AP). Clinical presentation ranges from stillbirth without mineralised bone to pathological fractures in late adulthood. During childhood, the main manifestations include rickets, growth delay and dental problems. Fractures and bone pain usually characterise the adult form. A 9-year-old girl was referred for repetitive fractures after minimal trauma. She had normal growth, normal sclerae, no rickets and minimal dental abnormalities. Her sister had also presented fractures. The proband, her sister and mother had low total and bone-specific AP levels and E435K mutation in exon 12 of the liver/bone/kidney AP gene. Low AP levels must lead to genetic analysis. Bone fragility and repetitive fractures may be symptoms of hypophosphatasia in childhood, which must not be neglected. Associated factors such as vitamin D or calcium deficiency must be prevented. In conclusion, hypophosphatasia must not be forgotten as an aetiological factor of repetitive fractures or bone pain in children and AP activity should be checked accurately.

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

OBJECTIVE

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

METHOD

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

RESULTS

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

CONCLUSION

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

Prosthetic rehabilitation of hypophosphatasia: a case report

Hypophosphatasia is a congenital disease characterized by deficiency of serum and tissue non-specific alkaline phosphatase activity. The disease occurs due to mutations in the liver/bone/kidney alkaline phosphatase gene. Six clinical forms of hypophosphatasia are recognized. Systemic symptoms of the disease are respiratory complications, premature craniosynostosis, widespread demineralization and rachitic changes in the metaphases, stress fractures, chondrocalcinosis and osteoarthropathy. Characteristic dental symptoms are premature deciduous teeth loss, premature exfoliation of fully rooted primary teeth, severe dental caries and alveolar bone loss. This clinical report describes the prosthetic rehabilitation of a twenty two year-old Turkish female patient with hypophosphatasia.

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

CONTEXT

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

OBJECTIVE

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

DESIGN

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

PATIENTS

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

RESULTS

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

CONCLUSIONS

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

Hypophosphatasia

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

Hyperechoic metaphyses in hypophosphatasia: what does it mean?

We report a case of hypophosphatasia diagnosed using US and CT at 29 weeks' gestation and confirmed by molecular analysis. Prenatal US revealed very short fetal limbs and severe demineralization of the skull. The diaphyses were normal, but the metaphyses of the long bones appeared hyperechoic with no posterior shadowing. No fractures or long-bone deformations were observed. Three-dimensional helical CT performed at 29 weeks' gestation provided additional details of the abnormal bones, i.e. irregular and cupped metaphyses that were very similar to the radiological findings of hypophosphatasia described postnatally. To our knowledge, the description of hyperechoic metaphyses in hypophosphatasia is unique and is a consequence of abnormal mineralization of the metaphyses that is specific to this pathology.

Hypophosphatasia

Hypophosphatasia is a rare inherited disorder characterized by defective bone and teeth mineralization, and deficiency of serum and bone alkaline phosphatase activity. The prevalence of severe forms of the disease has been estimated at 1/100 000.

The symptoms are highly variable in their clinical expression, which ranges from stillbirth without mineralized bone to early loss of teeth without bone symptoms. Depending on the age at diagnosis, six clinical forms are currently recognized: perinatal (lethal), perinatal benign, infantile, childhood, adult and odontohypophosphatasia. In the lethal perinatal form, the patients show markedly impaired mineralization in utero. In the prenatal benign form these symptoms are spontaneously improved. Clinical symptoms of the infantile form are respiratory complications, premature craniosynostosis, widespread demineralization and rachitic changes in the metaphyses. The childhood form is characterized by skeletal deformities, short stature, and waddling gait, and the adult form by stress fractures, thigh pain, chondrocalcinosis and marked osteoarthropathy. Odontohypophosphatasia is characterized by premature exfoliation of fully rooted primary teeth and/or severe dental caries, often not associated with abnormalities of the skeletal system.

The disease is due to mutations in the liver/bone/kidney alkaline phosphatase gene (ALPL; OMIM# 171760) encoding the tissue-nonspecific alkaline phosphatase (TNAP). The diagnosis is based on laboratory assays and DNA sequencing of the ALPL gene. Serum alkaline phosphatase (AP) activity is markedly reduced in hypophosphatasia, while urinary phosphoethanolamine (PEA) is increased. By using sequencing, approximately 95% of mutations are detected in severe (perinatal and infantile) hypophosphatasia.

Genetic counseling of the disease is complicated by the variable inheritance pattern (autosomal dominant or autosomal recessive), the existence of the uncommon prenatal benign form, and by incomplete penetrance of the trait. Prenatal assessment of severe hypophosphatasia by mutation analysis of chorionic villus DNA is possible. There is no curative treatment for hypophosphatasia, but symptomatic treatments such as non-steroidal anti-inflammatory drugs or teriparatide have been shown to be of benefit. Enzyme replacement therapy will be certainly the most promising challenge of the next few years.

Metaphyseal chondrodysplasia McKusick type in a Chinese fetus, caused by novel compound heterozygosity 64T> A and 79G >T in RMRPgene

We present the first confirmed case by molecular analysis of a metaphyseal chondrodysplasia, McKusick type, in a 22-week fetus. Two novel compound heterozygous mutations, 64T> A and 79G > T, were found in the highly conserved regions of the RMRP gene. Twenty-two heterozygous g.1018 T> C mutations, two homozygous g.1018 T> C mutations, two heterozygous insertion mutations g.799_g.800insC and one heterozygous insertion mutation g.849_g.850insT were found among 100 normal controls. Careful radiological examination of the fetus for skeletal dysplasia allowed definitive diagnosis, proper genetic counselling and future prenatal diagnosis.

Bilateral transverse (Bowdler) fibular spurs with hypophosphatasia in an adolescent girl

Hypophosphatasia is a clinically heterogeneous inheritable disorder characterized by defective bone mineralization and the deficiency of serum and tissue liver/bone/kidney alkaline phosphatase activities. Due to the mineralization defect of the bones, various skeletal findings can be radiologically observed in hypophosphatasia. Bowing and Bowdler spurs of long bones are the characteristic findings. The Bowdler spurs reported on in the previous pertinent literature were observed in the perinatal aged patients and these lesions have rarely involved adolescents. We herein report on a 14-year-old girl with fibular Bowdler spurs.

Common mutations F310L and T1559del in the tissue-nonspecific alkaline phosphatase gene are related to distinct phenotypes in Japanese patients with hypophosphatasia

A total of 22 Japanese patients with hypophosphatasia were included in a study analysing the relationship between mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene and the severity of the phenotype in Japanese patients with hypophosphatasia. The enzymatic activity of some of the identified mutant TNSALP proteins was also examined using corresponding expression vectors. Eighteen mutations, including 6 novel ones, were identified in the patients. Among them, the common mutations were F310L and T1559del. Of note, five patients with F310L mutation in one of the alleles exhibited a relatively mild phenotype without respiratory complications despite its perinatal onset. In contrast, the T1559del mutation is associated with perinatal lethal and infantile forms when not found in patients with the F310L mutation. The genotype-phenotype relationship was, to some extent, consistent with the enzymatic activity of the mutant ALP proteins; mutations K207E and G409C found in a surviving case of infantile hypophosphatasia, as well as F310L, retained some residual activities, whereas T1559del caused a complete loss of activity.

CONCLUSION

In Japanese patients, the common mutations F310L and T1559del are associated with the relatively mild and lethal forms of hypophosphatasia, respectively. Our results may enhance the importance of genotyping patients with hypophosphatasia to predict their prognosis.

Neonatal lethal osteochondrodysplasia with low serum levels of alkaline phosphatase and osteocalcin

Neonatal lethal skeletal dysplasias are rare and typically involve thoracic malformations and severe limb shortening. We report on a newborn boy manifesting an osteochondrodysplasia associated with fatal respiratory insufficiency who had normal lung volumes and extremity lengths. His disorder featured aberrant skeletal patterning and defective ossification including a severely osteopenic skull, apparent absence of clavicles, and clefting of the mandible and vertebrae. Serum alkaline phosphatase and osteocalcin levels were markedly low. Biochemical studies suggested parathyroid insufficiency probably from critical illness. Histopathology at autopsy excluded impaired mineralization of skeletal matrix, but endochondral bone formation appeared disorganized with growth plate clustering of chondrocytes in hypertrophic zones and in zones of provisional calcification. Parathyroid glands were not found. Despite features of two distinctive heritable entities, hypophosphatasia and cleidocranial dysplasia, the cumulative findings did not match either condition, and no mutations were found in either the tissue nonspecific ALP isoenzyme or core-binding factor genes, respectively, or in the genes encoding osteocalcin or the osteoblast transcription factor osterix. This patient could represent the extreme of cleidocranial dysplasia (a disorder not always associated with structural mutation in core-binding factor A1), but more likely he defines a unique osteochondrodysplasia disrupting both intramembranous and endochondral bone formation.

[Genetics of hypophosphatasia]

Hypophosphatasia is a rare inherited disorder characterized by defective bone and teeth mineralization and deficiency of serum and bone alkaline phosphatase activity. The symptoms are highly variable in their clinical expression, which ranges from stillbirth without mineralized bone to early loss of teeth without bone symptoms. Currently, there is no treatment for the disease. Recent developments in molecular biology allow to better understand the genetics of the disease, especially its transmission that may be recessive or dominant, to improve genetic counseling and molecular diagnosis, and offer new perspectives of treatment.

Prenatal diagnosis of isolated femoral bent bone skeletal dysplasia: problems in differential diagnosis and genetic counseling

Severe localized and symmetric bowing of the femora, in the absence of other significant skeletal or nonskeletal abnormalities, is a rare prenatal ultrasound finding. A 38-year-old woman was referred at 19 weeks gestation and ultrasound of the fetus showed severe shortening, and marked symmetric bowing of the femora. A provisional diagnosis of kyphomelic dysplasia (KD) was made. The patient elected termination of pregnancy and post mortem assessments were most consistent with kyphomelic dysplasia. KD is bent-bone skeletal dysplasia that, in contrast to campomelic dysplasia, involves principally the femora with relative sparing of the remainder of the skeleton. KD can be difficult to distinguish, particularly from symmetric cases of femoral hypoplasia unusual facies syndrome (FH-UFS), and few prenatal diagnoses have been reported. Because KD is thought to an be autosomal recessive disorder, the possibility that definitive diagnosis may not be possible prenatally, and even by postmortem assessment in cases choosing to abort, is an important counseling consideration.

Denaturing gradient gel electrophoresis analysis of the tissue nonspecific alkaline phosphatase isoenzyme gene in hypophosphatasia

Hypophosphatasia, a heritable form of rickets/osteomalacia, was first described in 1948. The biochemical hallmark, subnormal alkaline phosphatase (ALP) activity in serum, reflects a generalized disturbance involving the tissue-nonspecific isoenzyme of ALP (TNSALP). Deactivating mutations in the gene that encodes TNSALP have been reported in patients worldwide. Nevertheless, hypophosphatasia manifests an extraordinary range of clinical severity spanning death in utero to merely premature loss of adult teeth. There is no known medical treatment. To delineate the molecular pathology which explains the disease variability and to clarify the pattern(s) of inheritance for mild cases of hypophosphatasia, we developed comprehensive mutational analysis of TNSALP. High efficiency of mutation detection was possible by denaturing gradient gel electrophoresis (DGGE). Primers and conditions were established for all TNSALP coding exons (2-12) and adjacent splice sites so that the amplicons incorporated a GC clamp on one end. For each amplicon, the optimum percentage denaturant was determined by perpendicular DGGE. In 19 severely affected pediatric subjects (having perinatal or infantile hypophosphatasia or early presentation during childhood) from among our large patient population, we detected 2 TNSALP mutations each in 16 patients (84%) as expected for autosomal recessive disease. For 2 patients (11%), only 1 TNSALP mutation was detected by DGGE. However, one subject (who died from perinatal hypophosphatasia) had a large deletion as the second mutation. In the other (with infantile hypophosphatasia), no additional mutation was detected by DNA sequencing of all protein-coding exons. Possibly, she too has a deletion. For the final patient, with unclassifiable hypophosphatasia (5%), we detected only a single mutation which has been reported to cause relatively mild autosomal dominant disease; the other allele appeared to be intact. Hence, DGGE analysis was 100% efficient in detecting mutations in the coding exons and adjacent splice sites of TNSALP in this group of severely affected patients but, as expected, failed to detect a large deletion. To date, at least 78 different TNSALP mutations (in about 70 hypophosphatasia patients) have been reported globally. In our large subset of severely affected patients, we identified 8 novel TNSALP mutations (Ala34Ser, Val111Met, Delta G392, Thr117His, Arg206Gln, Gly322Arg, Leu397Met, and Gly409Asp) and 1 new TNSALP polymorphism (Arg135His) furthering the considerable genotypic variability of hypophosphatasia.

Mutational analysis and functional correlation with phenotype in German patients with childhood-type hypophosphatasia

The tissue-nonspecific alkaline phosphatase (TNSALP) gene from five German family members with childhood-type hypophosphatasia (HOPS) was analyzed using the polymerase chain reaction-single strand conformation polymorphism (PCR-SSCP)-direct sequencing method. Four novel missense mutations (T51M, R54S, L258P, and R374H) and two that had been described previously (A160T and R206W) were detected in the respective patients. Mutation A160T was detected in 3 distinct patients, and a polymorphism V505A that had been described previously was detected in the same allele as L258P mutation in 1 patient and in 2 fathers whose V505A alleles were not transmitted to the probands. No other mutations were found in 2 patients. Transient expression of the mutant proteins in COS-1 cells showed that the four novel mutations and R206W were severe alleles, whereas A160T was a moderate allele. Analysis of its enzymatic activity and genetic transmission patterns confirmed that V505A was a polymorphism. Immunoprecipitation of the transiently expressed proteins showed that levels of the 80-kDa mature form of the enzyme were diminished or absent with the severe alleles; instead, levels of high-molecular mass disulfide-linked aggregates were increased. These results suggest that in compound heterozygotes, the combination of severe and moderate alleles may combine to cause the mild phenotype seen in childhood-type HOPS.

Osteogenesis imperfecta: perspectives and opportunities

The last 2 years have seen additions proposed to the very limited armamentarium of treatments for osteogenesis imperfecta. These include the use of bisphosphonates to decrease bone resorption, growth hormone to augment growth and collagen production, and bone marrow transplantation to create chimeras at the level of the collagen production unit in bone. Although there are optimistic proponents for each strategy, the lack of well-controlled studies and the absence of clearly defined objectives for therapy hinder clear assessment.