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

The diagnosis of hypophosphatasia in children as a multidisciplinary effort: an expert opinion

Hypophosphatasia (HPP) is a rare genetic disorder in which pathogenic variants of the ALPL gene lead to a marked decrease of tissue non-specific alkaline phosphatase (TNSALP) activity. Although HPP is a systemic disorder, its clinical manifestations are more evident on bones, teeth, muscle and central nervous system. The clinical spectrum ranges from severe forms with extreme skeletal deformities, respiratory impairment, seizures, to very mild forms with onset in late adulthood and few clinical signs. The diagnosis can be suspected by measurement of TNSALP activity, but the insufficient awareness among health professionals and the lack of official guidelines are responsible for delayed diagnosis in children with HPP. The purpose of the current document is to provide an expert opinion directed at optimizing the diagnostic pathway of pediatric HPP. From April to December 2022, a multidisciplinary working group of 6 experts including two pediatric endocrinologists, a pediatric neurologist, a pediatric odontologist, a clinical geneticist, and a molecular biologist gathered in a series of periodic meetings to discuss the main issues related to the diagnosis of HPP in children and formalize an Expert Opinion statement. The experts agreed on a diagnostic trail that begins with the recognition of specific clinical signs, leading to biochemical analyses of TNSALP activity and vitamin B6 serum concentration. Very important are the neurological and dental manifestation of the disease that should be thoroughly investigated. The evaluation of TNSALP activity must consider sex and age variability and low activity must be persistent. Repeated blood measurements are thus necessary. The molecular analysis is then mandatory to confirm the diagnosis and for genetic counseling.

Hypophosphatasia in Japan: ALPL Mutation Analysis in 98 Unrelated Patients

Hypophosphatasia (HPP) is highly variable in clinical expression and is generally classified into six subtypes. Although it would be beneficial to be able to predict the clinical course from the ALPL genotype, studies on this issue are limited. Here, we aimed to clarify the features of Japanese HPP and the relationships between genotype and clinical manifestations. We analyzed 98 unrelated Japanese patients to investigate the percentage of each clinical form, frequently detected mutations, and the relationship between the genotype and phenotype. Some of the identified mutants were characterized by transfection experiments. Perinatal severe form was the most frequent (45.9%), followed by perinatal benign form (22.4%). Among the 196 alleles, p.Leu520ArgfsX86 (c.1559delT) was detected in 89 alleles, and p.Phe327Leu (c.979T>C) was identified in 23 alleles. All of the homozygotes for p.Leu520ArgfsX86 were classified into perinatal severe form, and patients carrying p.Phe327Leu in one of the alleles were classified into perinatal benign or odonto HPP. Twenty of the 22 patients with perinatal benign HPP were compound heterozygous for p.Phe327Leu and another mutation. Most patients with odonto HPP were found to be monoallelic heterozygotes for dominant-negative mutations or compound heterozygotes with mutants having residual activity. The high prevalence of p.Leu520ArgfsX86 and p.Phe327Leu mutations might underlie the high rate of perinatal severe and perinatal benign forms, respectively, in Japanese HPP. Although ALPL genotyping would be beneficial for predicting the clinical course to an extent, the observed phenotypical variability among patients sharing the same genotypes suggests the presence of modifiers.

Clinical Practice Guidelines for Hypophosphatasia

Hypophosphatasia (HPP) is a rare bone disease caused by inactivating mutations in the ALPL gene, which encodes tissue-nonspecific alkaline phosphatase (TNSALP). Patients with HPP have varied clinical manifestations and are classified based on the age of onset and severity. Recently, enzyme replacement therapy using bone-targeted recombinant alkaline phosphatase (ALP) has been developed, leading to improvement in the prognosis of patients with life-threatening HPP. Considering these recent advances, clinical practice guidelines have been generated to provide physicians with guides for standard medical care for HPP and to support their clinical decisions. A task force was convened for this purpose, and twenty-one clinical questions (CQs) were formulated, addressing the issues of clinical manifestations and diagnosis (7 CQs) and those of management and treatment (14 CQs). A systematic literature search was conducted using PubMed/MEDLINE, and evidence-based recommendations were developed. The guidelines have been modified according to the evaluations and suggestions from the Clinical Guideline Committee of The Japanese Society for Pediatric Endocrinology (JSPE) and public comments obtained from the members of the JSPE and a Japanese HPP patient group, and then approved by the Board of Councils of the JSPE. We anticipate that the guidelines will be revised regularly and updated.

Skeletal mineralization: mechanisms and diseases

Skeletal mineralization is initiated in matrix vesicles (MVs), the small extracellular vesicles derived from osteoblasts and chondrocytes. Calcium and inorganic phosphate (Pi) taken up by MVs form hydroxyapatite crystals, which propagate on collagen fibrils to mineralize the extracellular matrix. Insufficient calcium or phosphate impairs skeletal mineralization. Because active vitamin D is necessary for intestinal calcium absorption, vitamin D deficiency is a significant cause of rickets/osteomalacia. Chronic hypophosphatemia also results in rickets/osteomalacia. Excessive action of fibroblast growth factor 23 (FGF23), a key regulator of Pi metabolism, leads to renal Pi wasting and impairs vitamin D activation. X-linked hypophosphatemic rickets (XLH) is the most common form of hereditary FGF23-related hypophosphatemia, and enhanced FGF receptor (FGFR) signaling in osteocytes may be involved in the pathogenesis of this disease. Increased extracellular Pi triggers signal transduction via FGFR to regulate gene expression, implying a close relationship between Pi metabolism and FGFR. An anti-FGF23 antibody, burosumab, has recently been developed as a new treatment for XLH. In addition to various forms of rickets/osteomalacia, hypophosphatasia (HPP) is characterized by impaired skeletal mineralization. HPP is caused by inactivating mutations in tissue-nonspecific alkaline phosphatase, an enzyme rich in MVs. The recent development of enzyme replacement therapy using bone-targeting recombinant alkaline phosphatase has improved the prognosis, motor function, and quality of life in patients with HPP. This links impaired skeletal mineralization with various conditions, and unraveling its pathogenesis will lead to more precise diagnoses and effective treatments.

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.

Current concepts in odontohypophosphatasia form of hypophosphatasia and report of two cases

BACKGROUND

Hypophosphatasia is a rare inherited disease derived from mutations in tissue non-specific alkaline phosphatase genes, with typical oral symptoms including short root anomaly and dysplasia of dentin or cementum.

CASE PRESENTATION

Two young female patients presented with short root anomaly with a history of premature loss of deciduous and/or permanent teeth. The laboratory and imaging investigations were performed. One case was diagnosed as odontohypophosphatasia concurrent with hyperthyroidism, the other was odontohypophosphatasia concurrent with multiple radicular cysts.

CONCLUSION

This report presents two cases of odontohypophosphatasia, a rare disease which is difficult to be diagnosed, and highlights that the history of premature loss of deciduous and/or permanent teeth, oral manifestation and laboratory tests are crucial for clinical diagnosis.

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.

New mouse models for metabolic bone diseases generated by genome-wide ENU mutagenesis

Metabolic bone disorders arise as primary diseases or may be secondary due to a multitude of organ malfunctions. Animal models are required to understand the molecular mechanisms responsible for the imbalances of bone metabolism in disturbed bone mineralization diseases. Here we present the isolation of mutant mouse models for metabolic bone diseases by phenotyping blood parameters that target bone turnover within the large-scale genome-wide Munich ENU Mutagenesis Project. A screening panel of three clinical parameters, also commonly used as biochemical markers in patients with metabolic bone diseases, was chosen. Total alkaline phosphatase activity and total calcium and inorganic phosphate levels in plasma samples of F1 offspring produced from ENU-mutagenized C3HeB/FeJ male mice were measured. Screening of 9,540 mice led to the identification of 257 phenodeviants of which 190 were tested by genetic confirmation crosses. Seventy-one new dominant mutant lines showing alterations of at least one of the biochemical parameters of interest were confirmed. Fifteen mutations among three genes (Phex, Casr, and Alpl) have been identified by positional-candidate gene approaches and one mutation of the Asgr1 gene, which was identified by next-generation sequencing. All new mutant mouse lines are offered as a resource for the scientific community.

Prenatal diagnosis of familial lethal hypophosphatasia using imaging, blood enzyme levels, chorionic villus sampling and archived fetal tissue

Hypophosphatasia is an inheritable disorder characterized by defective bone mineralization and a deficiency of tissue-nonspecific alkaline phosphatase (TNSALP) activity. Screening for mutations in the TNSALP gene allows genetic counseling and prenatal diagnosis of the disease in families with severe forms of hypophosphatasia. A 33-year-old, gravida 4, para 3 Japanese woman was referred to Nagoya City University Hospital for prenatal genetic counseling because of two previous occurrences of fetal bone anomalies. The molecular examination showed that the fetus was homozygous for the TNSALP gene mutation c.1559delT, each parent being heterozygous. Genetic counseling was offered and at the next pregnancy, chorionic villus sampling was performed, whereupon genetic analysis confirmed that the fetus did not carry the familial mutation c.1559delT. Postnatal molecular genetic analysis using the cord tissue can provide a diagnosis of lethal hypophosphatasia and prenatal genetic diagnosis of the TNSALP gene allows time for parental counseling and delivery planning.