Hypophosphatasia is an inherited disorder characterized by defective bone mineralization and a deficiency of tissue-nonspecific alkaline phosphatase (TNSALP) activity. The disease is highly variable in its clinical expression, because of various mutations in the TNSALP gene. In approximately 14% of the patients tested in our laboratory, only one TNSALP gene mutation was found, despite exhaustive sequencing of the gene, suggesting that missing mutations are harbored in intron or regulatory sequences or that the disease is dominantly transmitted. The distinction between these two situations is of importance, especially in terms of genetic counseling, but dominance is sometimes difficult to conclusively determine by using familial analysis since expression of the disease may be highly variable, with parents of even severely affected children showing no or extremely mild symptoms of the disease. We report here the study of eight point mutations (G46 V, A99T, S164L, R167 W, R206 W, G232 V, N461I, I473F) found in patients with no other detectable mutation. Three of these mutations, G46 V, S164L, and I473F, have not previously been described. Pedigree and/or serum alkaline phosphatase data suggested possible dominant transmission in families with A99T, R167 W, and G232 V. By means of site-directed mutagenesis, transfections in COS-1 cells, and three-dimensional (3D) modeling, we evaluated the possible dominant effect of these eight mutations. The results showed that four of these mutations (G46 V, A99T, R167 W, and N461I) exhibited a negative dominant effect by inhibiting the enzymatic activity of the heterodimer, whereas the four others did not show such inhibition. Strong inhibition resulted in severe hypophosphatasia, whereas partial inhibition resulted in milder forms of the disease. Analysis of the 3D model of the enzyme showed that mutations exhibiting a dominant effect were clustered in two regions, viz., the active site and an area probably interacting with a region having a particular biological function such as dimerization, tetramerization, or membrane anchoring.
"Due to the structural properties of the TNAP, some mutations affecting protein structure may exhibit a dominant negative effect. These dominant negative mutations (also called antimorphic mutations) usually result in an altered molecular function due to inhibition of enzymatic activity of the normal monomer by the mutated partner in heterodimers, thus contributing to highly variable clinical phenotypes of HPP . "
[Show abstract][Hide abstract] ABSTRACT: Hypophosphatasia (HPP) is an inherited disorder of mineral metabolism caused by mutations in ALPL, encoding tissue non-specific alkaline phosphatase (TNAP). Here, we report the molecular findings from monozygotic twins, clinically diagnosed with tooth-specific odontohypophosphatasia (odonto-HPP). Sequencing of ALPL identified two genetic alterations in the probands, including a heterozygous missense mutation c.454C>T, leading to change of arginine 152 to cysteine (p.R152C), and a novel heterozygous gene deletion c.1318_1320delAAC, leading to the loss of an asparagine residue at codon 440 (p.N440del). Clinical identification of low serum TNAP activity, dental abnormalities, and pedigree data strongly suggest a genotype-phenotype correlation between p.N440del and odonto-HPP in this family. Computational analysis of the p.N440del protein structure revealed an alteration in tertiary structure affecting the collagen-binding site (loop 422-452), which could potentially impair the mineralization process. Nevertheless, the Probands (compound heterozygous: p.[N440del];[R152C]) feature early-onset and severe odonto-HPP phenotype, whereas the father (p.[N440del];[=]) has only moderate symptoms, suggesting p.R152C may contribute or predispose to a more severe dental phenotype in combination with the deletion. These results assist in defining the genotype-phenotype associations for odonto-HPP, and further identify the collagen-binding site as a region of potential structural importance for TNAP function in the biomineralization.
Bone 06/2013; 56(2). DOI:10.1016/j.bone.2013.06.010 · 3.97 Impact Factor
"However , the homodimeric structure of the enzyme could make possible a dominant negative effect of the mutations, although these are rare in metabolic diseases. However, some forms of hypophosphatasia (MIM 146300, 241500, 241510) and also of cortisone reductase deficiency (MIM 604931) can be transmitted in a dominant manner and in these cases the mutations can exhibit a dominantnegative effect by inhibiting the enzymatic activity of the heterodimer (Lawson et al. 2011; Lia-Baldini et al. 2001). "
[Show abstract][Hide abstract] ABSTRACT: The inherited 5-oxoprolinuria is primarily suggestive of genetic defects in two enzymes belonging to the gamma-glutamyl cycle in the glutathione (GSH) metabolism: the glutathione synthetase (GSS) and the 5-oxoprolinase (OPLAH). The GSS deficiency is the best characterized of the inborn errors of GSH metabolism, whereas the OPLAH deficiency is questioned whether it is a disorder or just a biochemical condition with no adverse clinical effects. Recently, the first human OPLAH mutation (p.H870Pfs) was reported in homozygosis in two siblings who suffered from 5-oxoprolinuria with a benign clinical course. We report two unrelated patients who manifested massive excretion of 5-oxoproline in urine. In both probands, the blood GSH levels were normal and no mutations were found in the GSS gene. The mutational screening of the OPLAH gene, which included the codified sequences, the intronic flanking sequences, the promoter sequence, and a genetic analysis in order to detect large deletions and/or duplications, showed that each patient only harbors one missense mutation in heterozygosis. The in silico analyses revealed that each one of these OPLAH mutations, p.S323R and p.V1089I, could alter the proper function of this homodimeric enzyme. In addition, clinical symptoms manifest in these two probands were not related to GSH cycle defects and, therefore, this study provides further evidence that oxoprolinuria may present as epiphenomenon in several pathological conditions and confound the final diagnosis.
"In molecular diagnosis, the detection of a single heterozygous mutation in a patient with mild HPP means that a second mutation remains undetected (intronic mutations or mutations in the regulatory sequence) or that the heterozygous mutation has a dominant negative effect. We and others previously showed that the measurement of in vitro AP activity in COS cells transfected by the mutated cDNA of ALPL correlates with the HPP phenotype [15-21], and that the dominant negative effect of mutations may be tested by co-transfecting mutated and WT cDNAs [13,14,21]. Such a tool may therefore help to distinguish true heterozygosity from compound heterozygosity with an undetected mutation. "
[Show abstract][Hide abstract] ABSTRACT: Mild hypophosphatasia (HPP) phenotype may result from ALPL 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.
We tested the mutations by site-directed mutagenesis. We also genotyped 8 exonic and intronic ALPL gene polymorphisms in the patients and in a control group in order to detect the possible existence of a recurrent intronic mild mutation.
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.
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.
BMC Medical Genetics 07/2009; 10(1):51. DOI:10.1186/1471-2350-10-51 · 2.08 Impact Factor
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