Mild forms of hypophosphatasia mostly result from dominant negative effect of severe alleles or from compound heterozygosity for severe and moderate alleles

<p>Abstract</p> <p>Background</p> <p>Mild hypophosphatasia (HPP) phenotype may result from <it>ALPL </it>gene mutations exhibiting residual alkaline phosphatase activity or from severe heterozygous mutations exhibiting a dominant negative effect. In order to determine the cause of our failure to detect a second mutation by sequencing in patients with mild HPP and carrying on a single heterozygous mutation, we tested the possible dominant effect of 35 mutations carried by these patients.</p> <p>Methods</p> <p>We tested the mutations by site-directed mutagenesis. We also genotyped 8 exonic and intronic <it>ALPL </it>gene polymorphisms in the patients and in a control group in order to detect the possible existence of a recurrent intronic mild mutation.</p> <p>Results</p> <p>We found that most of the tested mutations exhibit a dominant negative effect that may account for the mild HPP phenotype, and that for at least some of the patients, a second mutation in linkage disequilibrium with a particular haplotype could not be ruled out.</p> <p>Conclusion</p> <p>Mild HPP results in part from compound heterozygosity for severe and moderate mutations, but also in a large part from heterozygous mutations with a dominant negative effect.</p

Structural evidences for a functional rôle of human tissue non-specific alkaline phosphatase in bone mineralization.

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

International audienceThe 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

Two novel mutations of the calcium-sensing receptor gene affecting the same amino acid position lead to opposite phenotypes and reveal the importance of p.N802 on receptor activity

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