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Introduction: Hypophosphatasia (HPP) is caused by mutations in the ALPL gene encoding tissue nonspecific alkaline phosphatase (TNSALP) and inherited in either an autosomal recessive or autosomal dominant manner. It is characterized clinically by defective mineralization of bone, dental problems, and low serum ALP levels. In the current report, we demonstrate a novel mutation in the ALPL gene (c.244G > A p.Gly82Arg) in a Japanese family with low serum ALP levels. Materials and methods: The ALPL gene analysis using hybridization capture-based next-generation sequencing was performed. The expression plasmids of the wild type and mutated TNSALP were introduced into COS-7 cells. The enzymatic activity of ALP in the cell lysates was measured using p-nitrophenylphosphate as a substrate. Results: TNSALP with the novel ALPL mutation (c.244G > A p.Gly82Arg) completely lost its enzymatic activity and suppressed that of wild-type TNSALP, corroborating its dominant negative effect. The diagnosis of autosomal dominant HPP was confirmed in three members of the family. Conclusion: Our approach would help to avoid the inappropriate use of bone resorption inhibitors for currently mis- or under-diagnosed HPP, given that the presence of further, yet undetected mutations of the ALPL gene are plausible. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Fibroblast growth factor 23 (FGF23), a central regulator of phosphate and vitamin D metabolism, is mainly produced by osteocytes in bone and exerts its effects on distant organs. Despite its endocrine function, the mechanism controlling serum FGF23 levels is not fully understood. Here we tested the hypothesis that osteoclastic bone resorption may play a role in regulating circulating levels of FGF23, using a mouse model where injections of interleukin (IL)-1β into the subcutaneous tissue over the calvaria induced rapid bone resorption. A significant amount of FGF23 was detected in the extracts from mouse bones, which supports the idea that FGF23 stays in bone for a while after its production. IL-1β-induced bone resorption was associated with elevated serum FGF23 levels, an effect abolished by pre-treatment with pamidronate. Fgf23 expression was not increased in either the calvariae or tibiae of IL-1β-injected mice, which suggests that IL-1β facilitated the entry of FGF23 protein into circulation by accelerating bone resorption rather than increasing its gene expression. The direct effect of IL-1β on bone was confirmed when it increased FGF23 levels in the conditioned media of mouse calvariae in organ culture. Repeated treatment of the cultured calvariae with IL-1β led to a refractory phase, where FGF23 was not mobilized by IL-1β anymore. Consistent with the in vivo results, treatment with IL-1β failed to increase Fgf23 mRNA in isolated primary osteocytes and osteoblasts. These results suggest that FGF23 produced by osteocytes remains in bone, and that rapid bone resorption facilitates its entry into the bloodstream. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Unlabelled: 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. [ABSTRACT FROM AUTHOR]