A sequence variation in the promoter of the placental alkaline phosphatase gene (ALPP) is associated with allele-specific expression in human term placenta.

Placental alkaline phosphatase (PLAP), encoded by the ALPP gene, is produced by the fetal side of the placenta. This enzyme displays strong genetic variability. Some of the variants were reported to be associated with pathology of pregnancy. We show here that the two most common ALPP allelic variants, Pl(1) and Pl(2), differ in mRNA expression level. This differential expression was independent of the parental origin and probably results from linkage disequilibrium with the sequence variation rs2014683G>A in the ALPP gene promoter that was shown to have allele-specific binding patterns to placental nuclear proteins. The possible role of this allelic-specific expression in placenta-related pathology is discussed.

Infantile hypophosphatasia due to a new compound heterozygous TNSALP mutation - functional evidence for a hydrophobic side-chain?

BACKGROUND: Infantile hypophosphatasia (IH) is an inherited disorder characterized by defective bone mineralization and a deficiency of alkaline phosphatase activity. OBJECTIVE/DESIGN: The aim of the study was to evaluate a new compound heterozygous TNSALP mutation for its residual enzyme activity and localization of the comprised amino acid residues in a 3D-modeling. PATIENT: We report on a 4-week old girl with craniotabes, severe defects of ossification, and failure to thrive. Typical clinical features as low serum alkaline phosphatase, high serum calcium concentration, increased urinary calcium excretion, and nephrocalcinosis were observed. Vitamin D was withdrawn and the patient was started on calcitonin and hydrochlorothiazide. Nonetheless, the girl died at the age of 5 months from respiratory failure. RESULTS: Sequence analysis of the patient's TNSALP gene revealed two heterozygous mutations [c.653T>C (I201T), c.1171C>T (R374C)]. Transfection studies of the unique I201T variant in COS-7 cells yielded a mutant TNSALP protein with only a residual enzyme activity (3.7%) compared with wild-type, whereas the R374C variant was previously shown to reduce normal activity to 10.3%. 3D-modeling of the mutated enzyme showed that I201T resides in a region that does not belong to any known functional site. CONCLUSION: We note that I201, which has been conserved during evolution, is buried in a hydrophobic pocket and, therefore, the I>T-change should affect its functional properties. Residue R374C is located in the interface between monomers and it has been previously suggested that this mutation affects dimerization. These findings explain the patient's clinical picture and severe course.

A new mechanism of dominance in hypophosphatasia: the mutated protein can disturb the cell localization of the wild-type protein.

The dominant negative effect of mutations is rare in metabolic diseases and its mechanism has not been studied much. Hypophosphatasia, a bone inherited metabolic disorder, is a good model because the disease can be dominantly transmitted. The gene product activity depends on a homodimeric configuration and many mutations have been reported in the ALPL gene responsible for the disease. Using CFP/YFP-tagged-TNSALP plasmids, transfections in COS cells and confocal fluorescence analyses, we studied the point mutation G232V (c.746G>T). We showed that the G232V protein sequestrates some of the wild-type protein into the cells and prevents it from reaching the membrane where it plays its physiological role.

A case of lethal hypophosphatasia providing new insights into the perinatal benign form of hypophosphatasia and expression of the ALPL gene.

Hypophosphatasia is a rare inherited bone disease caused by mutations in the alkaline phosphatase liver-type gene (ALPL) gene, with extensive allelic heterogeneity leading to a range of clinical phenotypes. We report here a patient who died from severe lethal hypophosphatasia, who was compound heterozygous for the mutation c.1133A>T (D361V) and the newly detected missense mutation c791A>G, and whose parents were both healthy. Because the c.1133A>T (D361V) mutation was previously reported to have a dominant-negative effect and to be responsible for the uncommon perinatal benign form of the disease, we studied the expression of the ALPL gene in this family. Analysis at the messenger RNA (mRNA) level, both quantitative and qualitative, showed that the paternal c.1133A>T (D361V) mutation was associated with over-expression of the ALPL gene and that the maternal c.791A>G mutation lead to complete skipping of exon 7. The results provide an explanation of the lethal phenotype in the patient where the two ALPL alleles are non-functional and in the asymptomatic father where over-expression of the normal allele could counteract the effect of the c.1133A>T (D361V) mutation by providing an increased level of normal mRNA. This may also explain the variable expression of hypophosphatasia observed in parents of patients with the perinatal benign form.

Childhood hypophosphatasia due to a de novo missense mutation in the tissue-nonspecific alkaline phosphatase gene.

Hypophosphatasia is an inherited disorder due to mutations in the bone alkaline phosphatase (ALPL) gene. We report here a patient with childhood hypophosphatasia diagnosed at 1.4 yr because of pectus excavatum, large anterior fontanel, rachitic skeletal changes, and low serum alkaline phosphatase. Sequencing of the ALPL gene produced evidence of two distinct missense mutations, E174K (c.571G>A), of maternal origin, and a de novo mutation, M45I (c.186G>C). The study of various microsatellite polymorphisms ruled out false paternity and therefore confirmed that M45I occurred de novo in the paternal germline or in the early development of the patient. Site-directed mutagenesis showed that M45I results in the absence of in vitro alkaline phosphatase activity, suggesting that the mutation is a severe allele. In conclusion, childhood hypophosphatasia in this patient is the result of compound heterozygosity for the moderate mutation E174K and a novel severe de novo mutation M45I.