Alternative titles; symbols
HGNC Approved Gene Symbol: ALPL
SNOMEDCT: 20756002, 30174008, 55236002, 708672004;
Cytogenetic location: 1p36.12 Genomic coordinates (GRCh38): 1:21,508,984-21,578,410 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 1p36.12 | Hypophosphatasia, adult | 146300 | Autosomal dominant; Autosomal recessive | 3 |
| Hypophosphatasia, childhood | 241510 | Autosomal recessive | 3 | |
| Hypophosphatasia, infantile | 241500 | Autosomal recessive | 3 | |
| Odontohypophosphatasia | 146300 | Autosomal dominant; Autosomal recessive | 3 |
Alkaline phosphatases are membrane-bound glycoproteins that hydrolyze various monophosphate esters at a high pH optimum (Weiss et al., 1986). Liver/bone/kidney alkaline phosphatase, also known as tissue-nonspecific alkaline phosphatase, acts physiologically as a lipid-anchored phosphoethanolamine (PEA) and pyridoxal-5-prime-phosphate (PLP) ectophosphatase (Fedde and Whyte, 1990).
See ALPQTL1 (171720) for information on quantitative trait loci influencing the plasma level of alkaline phosphatase.
Using quantitative inhibition and thermostability studies, Goldstein et al. (1980) presented evidence for the existence of at least 3 distinct forms of human alkaline phosphatase (EC 3.1.3.1): intestinal (ALPI; 171740), placental (ALPP; 171800), and liver/bone/kidney (ALPL). Harris et al. (1974) found no genetic variants by electrophoretic means.
Weiss et al. (1986) cloned a cDNA for liver/bone/kidney alkaline phosphatase from a human osteosarcoma cell line. The deduced 524-amino acid ALPL protein has a presumed signal peptide of 17 amino acids and a predicted molecular mass of 57.2 kD. The ALPL protein shares 52% sequence identity with placental alkaline phosphatase.
Fedde and Whyte (1990) demonstrated that the alkaline phosphatase of skin fibroblasts is ALPL, the tissue-nonspecific type, and that it is active toward millimolar concentrations of the putative natural substrates phosphoethanolamine and pyridoxal-5-prime-phosphate. Both activities were deficient in hypophosphatasia (see 241500) fibroblasts. They presented evidence that normal fibroblast ALP is linked to the outside of the plasma membrane; thus, the enzyme acts physiologically as a lipid-anchored PEA and PLP ectophosphatase.
Weiss et al. (1988) and Matsuura et al. (1990) found that the ALPL gene exists in single copy in the haploid genome and contains 12 exons distributed over more than 50 kb. As compared with the gene isolated using a bone-type ALPL cDNA (Weiss et al., 1988), liver-type ALPL mRNA was found to have another leader exon about 3.4 kb upstream from exon 2 and alternative splicing in the first exon was indicated (Matsuura et al., 1990).
Using human-rodent somatic cell hybrids, Swallow et al. (1985, 1986) mapped the gene for liver/bone/kidney alkaline phosphatase to chromosome 1. Mouse Akp2, which is homologous, is on chromosome 4 between Pgm2 and Pgd. Thus, ALPL might be located on 1p between PGM1 (171900) and PGD (172200).
Using a DNA polymorphism of the ALPL gene, Ardinger et al. (1987) found linkage to Rh (theta = 0.08; lod = 5.50). Multipoint analysis indicated the following order: Rh--3--ALPL--12--GLUT--23--PGM1, with the interlocus intervals as percent recombination in males (the female rate about 2.8 times the male rate).
Weiss et al. (1987) assigned the ALPL locus to 1p36.1-p34 by demonstrating linkage to Rh (maximum lod score = 5.50, theta = 0.10). In the full report, Smith et al. (1988) established the location in 1p36.1-p34 by a combination of Southern blot analysis of hybrid cell DNA, in situ hybridization, and genetic linkage analysis. Linkage to Rh was indicated by a maximum lod score of 15.66 at a recombination fraction of 0.10 and to alpha-L-fucosidase (FUCA1; 612280) by a maximum lod score of 8.24 at a recombination fraction of 0.02.
Hypophosphatasia
In a male infant with hypophosphatasia (241500) who was born of second-cousin parents and died at 3 months of age, Weiss et al. (1988) identified homozygosity for a mutation in the ALPL gene (A162T; 171760.0001). Functional studies demonstrated that the mutation abolished the expression of active enzyme.
In studies of fibroblasts from 4 unrelated patients with severe (perinatal or infantile) hypophosphatasia (see 241500), Henthorn et al. (1992) identified compound heterozygosity for 8 different mutations in the ALPL gene (171760.0002-171760.0009).
Henthorn and Whyte (1992) reviewed 9 missense mutations in the ALPL gene found in hypophosphatasia. They showed that the 9 amino acid residues affected in these mutations are identical in all mammals examined, indicating evolutionary conservation. Indeed, 3 of the mutations resulted in amino acid substitutions in regions of the polypeptide that are conserved throughout evolution, being identical in mammals, yeast, and bacteria. Four of the mutations were observed in a single patient and the others in only a few. The asp378-to-val mutation (171760.0009) had the highest frequency, being found in 9 of 42 patients and none of 63 controls. Thus, the experience is similar to that in so many other disorders in which many different allelic mutations can produce the same or a similar phenotype.
Mornet et al. (1998) characterized tissue-nonspecific alkaline phosphatase gene mutations in 13 European families affected by perinatal, infantile, or childhood (241510) hypophosphatasia. Eighteen distinct mutations were found, only 3 of which had previously been reported, in North American and Japanese populations. Most of the 15 new mutations were missense mutations, but they also found 2 mutations affecting donor splice sites and 1 nonsense mutation. A missense mutation in the last codon of the putative signal peptide probably affected the final maturation of the protein. Despite extensive sequencing of the gene and its promoter region, only 1 mutation was identified in 2 cases, 1 of which was compatible with a possible dominant effect of certain mutations and the putative role of polymorphisms of the ALPL gene. In 12 of the 13 tested families, genetic diagnosis was possible by characterization of the mutations or by use of polymorphisms as genetic markers. In 2 families where clinical, laboratory, and radiographic data were unclear, the diagnosis of hypophosphatasia could be made, and prenatal diagnosis was performed in 1 case. Thus, severe hypophosphatasia is due to a very large spectrum of mutations in European populations with no prevalent mutation; genetic diagnosis must be performed by extensive analysis of the gene. Combined with previous reports, a total of 43 ALPL mutations had been identified.
Zurutuza et al. (1999) used clinical data, site-directed mutagenesis, and computer-assisted modeling to propose a classification of 32 ALPL gene mutations found in 23 European patients (17 with lethal hypophosphatasia and 6 with nonlethal hypophosphatasia). Transfection studies of the missense mutations found in nonlethal hypophosphatasia showed that 6 of them allowed significant residual in vitro enzymatic activity, suggesting that these mutations corresponded to moderate alleles. Each of the 6 patients with nonlethal hypophosphatasia carried at least 1 of these alleles. The 3-dimensional model study showed that moderate mutations were not found in the active site, and that most of the severe missense mutations were localized in crucial domains such as the active site, the vicinity of the active site, and homodimer interface. Some mutations appeared to be organized in clusters on the surface of the molecule that may represent candidates for regions interacting with the C-terminal end involved in glycosylphosphatidylinositol (GPI) attachment or with other dimers to form tetramers. The results of these studies showed a good correlation between clinical forms of the disease, mutagenesis experiments, and the 3-dimensional structure study, and allowed the authors to distinguish moderate alleles from severe alleles. They also confirmed that the extremely high phenotypic heterogeneity observed in patients with hypophosphatasia was due mainly to variable residual enzymatic activities allowed by missense mutations in the gene.
Mornet (1999) studied 2 large families with adult hypophosphatasia (146300). In 1 family, hypophosphatasia was dominantly inherited and was due to a missense mutation in the ALPL gene. In the other family, hypophosphatasia was recessively transmitted and was due to compound heterozygosity of a missense mutation and a splicing mutation in the ALPL gene. Thus, adult hypophosphatasia can be transmitted as either a dominant or recessive trait.
Mornet (2000) stated that most of the 65 distinct mutations that had been described to that time were missense mutations. Correlations of genotype and phenotype had been established on the basis of clinical data exhibited by the patients, transfection studies, computer-assisted modeling, and examination of biochemical properties of alkaline phosphatase in cultured fibroblasts of patients. Mornet (2000) also discussed the dominantly inherited form of adult hypophosphatasia. It appeared that dominant and recessive hypophosphatasia could in some instances be due to the same mutations. The findings also suggested that parents of patients affected with severe recessive forms of the disease may show, in addition to the well known hypophosphatasemia, undiagnosed mild symptoms corresponding to adult hypophosphatasia or odontohypophosphatasia.
Lia-Baldini et al. (2001) stated that in approximately 14% of patients tested in their laboratories, only 1 ALPL 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 (see 146300). The distinction between these 2 situations is of obvious importance to genetic counseling. Lia-Baldini et al. (2001) studied 8 point mutations found in patients with no other detectable mutation. Three of these mutations, G46V, S164L, and I473F, had not previously been described; the others were A99T (171760.0015), R167W, R206W, G232V (171760.0021), and N461I. By means of site-directed mutagenesis, transfections in COS-1 cells, and 3-dimensional modeling, Lia-Baldini et al. (2001) evaluated the possible dominant effect of these 8 mutations. Four exhibited dominant-negative effect by inhibiting the enzymatic activity of the heterodimer, whereas the other 4 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-negative effect were clustered in 2 regions, namely, the active site and an area probably interacting with a region having a particular biologic function such as dimerization, tetramerization, or membrane anchoring.
In a patient with lethal perinatal hypophosphatasia with a unique clinical presentation of convulsions that responded to pyridoxine, Litmanovitz et al. (2002) identified 2 mutations in the ALPL gene. One mutation was considered to be moderate (Zurutuza et al., 1999); the other was novel.
According to Herasse et al. (2002), the glu174-to-lys (E174K; 171760.0008) mutation in the ALPL gene is common in Caucasians, widely distributed geographically, and present in 31% of patients with mild hypophosphatasia.
Odontohypophosphatasia
Herasse et al. (2003) reported 3 patients with odontohypophosphatasia (see 146300) resulting from heterozygosity for a mutation in the ALPL gene; 2 of the mutations had previously been reported (see A99T; 171760.0015), whereas the third, pro91 to leu, was novel (P91L; 171760.0018). The proband with the P91L mutation was 9 years old and the other 2 were 2 years old. The 39-year-old mother of the proband with the P91L mutation had lost her permanent teeth and had hypophosphatasia. The 38-year-old mother of the proband with the A99T mutation had an unusual number of dental caries, numerous treatments of dental root canals, and low serum ALP.
Vitamin B6 Levels
For discussion of a possible association between variation in the ALPL gene and plasma levels of vitamin B6, see 612957.
Using homologous recombination in embryonic stem cells to generate null mutant mice, Waymire et al. (1995) found that mice lacking ALPL (which they referred to as tissue-nonspecific alkaline phosphatase) have normal skeletal development. However, at approximately 2 weeks after birth, homozygous mutant mice developed seizures that were subsequently fatal. Defective metabolism of PLP, characterized by elevated serum PLP levels, resulted in reduced levels of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in the brain. The mutant seizure phenotype could be rescued by the administration of pyridoxal and a semisolid diet. Waymire et al. (1995) observed at about 3 weeks of age both a color difference and malformation of the incisor teeth in the TNAP mutant mice who were administered pyridoxal. Significantly, TNAP is expressed in the tissues within which incisor development takes place. At the growth level, the teeth of the mutants were unusually white and brittle, suggestive of hypocalcification. Between 25 and 60 days of age, mutant animals that responded to pyridoxal injections and were fed a high-fat solid diet started to display signs of paralysis. This progressed to complete inability of the animals to move and death presumably from respiratory and/or cardiac failure. During the time that pyridoxal was being administered, the incisors became progressively more deformed and often broke. (The feeding of a semisolid diet was intended to maximize their food intake.) None of the mutants fed this diet developed symptoms of paralysis; however, if pyridoxal supplementation was withdrawn, all died from seizures within 72 hours. As in humans, mouse TNAP functions as an ectoenzyme to convert PLP to pyridoxal. Waymire et al. (1995) suggested that the fact that pyridoxal is undetectable in the plasma of only rare cases of the severe perinatal form of human hypophosphatasia is probably a reflection of the type of mutation and the degree of residual alkaline phosphatase activity. Some of the TNAP mutants administered pyridoxine responded to the treatment, but the effect on longevity was minimal and mutants succumbed to seizure-related death within 2 weeks. Whyte et al. (1988) observed essentially the same progression in an infant with severe neonatal hypophosphatemia who presented with seizures, had undetectable levels of plasma pyridoxal, and was administered pyridoxine. In mammals, following uptake, the majority of pyridoxine is phosphorylated to form pyridoxine phosphate before being oxidized to yield pyridoxal 5-prime phosphate. The PLP is subsequently converted to pyridoxal by alkaline phosphatase. Very little, if any, pyridoxine is converted directly to pyridoxal. Individuals with hypophosphatasia would not be expected to interconvert pyridoxine to pyridoxal with any degree of efficiency and, based on the findings in the knockout mice by Waymire et al. (1995), administration of pyridoxal would theoretically be more appropriate. It is noteworthy that Waymire et al. (1995) found that pyridoxal administration was relatively ineffective in mice with certain genetic backgrounds. This suggested the existence of genetic modifiers specific to some backgrounds that lead to variable expressivity within the mutants on that background. Macfarlane et al. (1992) found a similar phenomenon in human sibs with hypoplasia. Humans and rodents fed a diet with a high-fat content have elevated levels of circulating alkaline phosphatase whose source is the intestinal isozyme, IAP (ALPI; 171740). Extrapolation from the mouse model suggested to Waymire et al. (1995) that, in those rare cases of hypophosphatasia associated with seizures, it would be better to substitute pyridoxal for pyridoxine, and to administer a high-fat diet to maximize IAP expression which might compensate at least partially for loss of function of the TNAP isozyme. However, because of the high dose of pyridoxal and the paralysis observed in the 'knockout' mice, such treatment would need to be carefully monitored.
In a male infant with hypophosphatasia (241500) who was born of second-cousin parents and died at 3 months of age, Weiss et al. (1988) identified homozygosity for a 711G-A transition in exon 6 of the ALPL gene, resulting in an ala162-to-thr (A162T) substitution. Introduction of this mutation into an otherwise normal cDNA by site-directed mutagenesis abolished the expression of active enzyme. The parents and other clinically unaffected relatives were found to be heterozygous for A162T, and the mutation cosegregated with levels of liver/bone/kidney alkaline phosphatase activity that were below the normal range.
In fibroblasts from a 5-month-old boy with hypophosphatasia (241500) who died at 6 months of age, Henthorn et al. (1992) identified compound heterozygosity for 2 mutations in the ALPL gene: a 387C-T transition in exon 4, resulting in an arg54-to-cys (R54C) substitution, and a 1057A-C transversion in exon 9, resulting in an asp277-to-ala (D277A) substitution (171760.0003).
For discussion of the asp277-to-ala (D277A) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Henthorn et al. (1992), see 171760.0002.
In 2 sibs with the mild childhood form of hypophosphatasia and in a 65-year-old woman with adult hypophosphatasia (146300), Henthorn et al. (1992) identified compound heterozygosity for 2 mutations in the ALPL gene: a D277A mutation, and 747G-A transition, resulting in a glu174-to-lys (E174K) substitution (171760.0008).
In fibroblasts from a girl with hypophosphatasia (241500) who died 3 hours after birth, Henthorn et al. (1992) identified compound heterozygosity for 2 mutations in the ALPL gene: a 388G-C transversion in exon 4, resulting in an arg54-to-pro (R54P) substitution, and a 796A-C transversion in exon 6, resulting in a gln190-to-pro (Q190P) substitution (171760.0005).
For discussion of the gln190-to-pro (Q190P) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Henthorn et al. (1992), see 171760.0004.
In a cell line designated MW10 from a patient with hypophosphatasia (241500), Henthorn et al. (1992) found compound heterozygosity for an ala16-to-val (A16V) mutation and a tyr419-to-his mutation (Y419H; 171760.0007).
For discussion of the tyr419-to-his (Y419H) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Henthorn et al. (1992), see 171760.0006.
In fibroblasts from a girl who presented at 2 months of age with severe hypophosphatasia (241500) and died at age 8 months, Henthorn et al. (1992) identified compound heterozygosity for 2 mutations in the ALPL gene: a 747G-A transition in exon 6, resulting in a glu174-to-lys (E174K) substitution, and a 1309A-T transversion in exon 10, resulting in an asp361-to-val (D361V) substitution (171760.0009).
For discussion of the asp378-to-val (D378V) mutation found in compound heterozygous state in the ALPL gene in patients with childhood (241510) or adult (146300) hypophosphatasia by Henthorn et al. (1992), see 171760.0003.
Herasse et al. (2002) investigated whether the E174K mutation had a unique origin or multiple origins arising from de novo mutations by genotyping 3 intragenic polymorphisms in patients with E174K and unaffected related individuals. Because all of the E174K mutations were found on a common ancestral haplotype, the authors suggested that a founder mutation occurred on a single chromosome in northwestern Europe and spread by human migration.
Whyte et al. (2007) stated that the ASP361VAL designation has been changed to ASP378VAL.
For discussion of the asp378-to-val (D378V) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Henthorn et al. (1992), see 171760.0008.
Moore et al. (1999) described a family with mild hypophosphatasia, apparently transmitted as an autosomal dominant trait, in which ultrasonography detected an affected fetus with severe long bone bowing. In contrast to the progressive deterioration typical of both the perinatal and infantile forms of hypophosphatasia, these skeletal defects improved spontaneously during infancy and early childhood. Biochemical evidence of hypophosphatasia was present in this family, and Moore et al. (1999) identified the D378V mutation in the ALPL gene of all affected members.
Whyte et al. (2007) identified heterozygosity for the D378V mutation in a middle-aged woman with adult hypophosphatasia (146300) who was successfully treated with teriparatide.
In individuals with the Mennonite perinatal, lethal form of hypophosphatasia (241500), Greenberg et al. (1993) demonstrated a homoallelic G-to-A transition in exon 10 of the ALPL gene at position 1177, changing a polar glycine to an acidic aspartate (gly317-to-asp, or G317D). A patient with a juvenile form of hypophosphatasia, with a Mennonite mother and a non-Mennonite father, was found to be heteroallelic for a maternally inherited G317D allele and a paternally inherited glu174-to-lys (E174K; 171760.0008) allele. The authors reported finding the same 2 mutations present as a genetic compound in other late-onset hypophosphatasia patients.
In fibroblasts of a neonate with hypophosphatasia (241500), Ozono et al. (1996) identified compound heterozygosity for 2 mutations in the ALPL gene: a 1155T-C transition, resulting in a phe3101-to-leu (F3101L) substitution, and a 1542G-A transition, resulting in a gly439-to-arg (G439R) substitution (171760.0019). The F3101L mutant exhibited an ALPL activity level 65% of normal, while the G439R mutant had no activity. Each parent was heterozygous for 1 of the mutations.
In a Japanese patient with infantile hypophosphatasia (241500), Orimo et al. (1994) identified compound heterozygosity for 2 mutations in the ALPL gene. One mutation, inherited from the mother, was a 1-bp deletion in exon 12 (1735delT). The deletion caused a frameshift downstream from codon 503 (leu), and the normal termination codon at 508 was eliminated. Since a new in-frame termination codon appeared at codon 588 in the mutant DNA, the resultant protein was predicted to have 80 additional amino acids. The other mutation, inherited from the father, was a 1068G-A transition in exon 9, resulting in a glu281-to-lys (E281K) substitution.
Goseki-Sone et al. (1998) studied expression of the mutant 1735delT ALPL cDNA in COS-1 cells. They found that the protein translated from the mutant gene had undetectable ALPL activity, and its molecular size was larger than the wildtype protein. Detection of the mutant protein in cells transfected with the 1735delT mutation with an immunofluorescent method exhibited only a faint signal on the cell surface, but an intense intracellular fluorescence after permeabilization.
Zurutuza et al. (1999) divided hypophosphatasia into lethal and nonlethal types. They studied 32 ALPL mutations found in 23 European patients, 17 with lethal hypophosphatasia (see 241500) and 6 with nonlethal hypophosphatasia (241510). They identified 6 mutations which on transfection studies were shown to allow significant residual in vitro enzymatic activity. The oldest patient in their study, aged 11 years, was a compound heterozygote for arg119 to his (R119H) and gly145 to val (G145V; 171760.0014). The former was apparently a 'moderate' allele, whereas the latter was a 'severe' allele.
For discussion of the gly145-to-val (G145V) mutation in the ALPL gene that was found in compound heterozygous state in a patient with childhood-onset hypophosphatasia (241510) by Zurutuza et al. (1999), see 171760.0013.
In 13 affected members of a 4-generation Texas family segregating autosomal dominant hypophosphatasia in both children (241510) and adults (146300), Hu et al. (2000) identified heterozygosity for an ala99-to-thr (A99T) substitution in the ALPL gene. The mutation was also found in 1 clinically unaffected individual who had an elevated urinary phosphoethanolamine (PEA) level. Lia-Baldini et al. (2001) performed functional studies of the A99T mutation and observed a moderate dominant negative effect. Complete sequencing of the gene in the brother and sister twin probands from the Texas family, including the untranslated exon 1 and intron/exon borders, revealed no mutation other than the heterozygous 346G-A transition in exon 5 of the ALPL gene that results in the A99T substitution.
Herasse et al. (2003) identified the A99T mutation in heterozygous state in a 2-year-old male and his mother; the former had only dental manifestations and low serum alkaline phosphatase and the latter did not have odontohypophosphatasia (146300) but had an unusual number of dental caries and had had numerous treatments of dental root canals.
Sergi et al. (2001) made a postmortem diagnosis of perinatal lethal hypophosphatasia (see 241500) on the basis of radiologic and pathologic changes of a generalized bone mineralization defect including asymmetry of the cervical vertebral arches. The parents were nonconsanguineous. The fetus was found to be a compound heterozygote: the first nucleotide of intron 6 of the ALPL gene was changed from G to A; the other chromosome carried an asn400-to-ser missense mutation in exon 11 (N400S; 171760.0017). The former was of maternal origin and the latter of paternal origin. DNA-based prenatal testing in a subsequent pregnancy following a chorionic villus sampling performed at 10 weeks of gestation showed no mutation and a healthy baby was born at term.
For discussion of the asn400-to-ser (N400S) mutation in the ALPL gene that was found in compound heterozygous state in a fetus with peinatal lethal hypophosphatasia (see 241500) by Sergi et al. (2001), see 171760.0016.
In a 9-year-old boy with Down syndrome (190685) and odontohypophosphatasia (146300) and his 39-year-old mother, Herasse et al. (2003) identified heterozygosity for a 323C-T transition in the ALPL gene, resulting in a pro91-to-leu (P91L) change. The boy had lost 7 deciduous teeth, mostly incisors, beginning at the age of 2 years and had low serum alkaline phosphatase, and his mother had lost her permanent teeth and had a low alkaline phosphatase level, indicating that the findings could not be attributed to Down syndrome.
For discussion of the gly439-to-arg (G439R) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Ozono et al. (1996), see 171760.0011.
For discussion of the glu281-to-lys (E281K) mutation in the ALPL gene that was found in compound heterozygous state in a patient with infantile hypophosphatasia (241500) by Orimo et al. (1994), see 171760.0012.
In a 15-month-old girl with a phenotype suggestive of childhood hypophosphatasia (241510), Lia-Baldini et al. (2001) identified heterozygosity for a 746G-T transversion in exon 7 of the ALPL gene, resulting in a gly232-to-val (G232V) substitution. The mutation was inherited from her father, who had recurrent dental caries in his 3rd decade despite being raised with fluoridated water; the authors suggested that this represented odontohypophosphatasia (146300).
Lia-Baldini et al. (2008) performed functional analysis of the G232V mutation in the ALPL gene and demonstrated that the mutant protein sequestrates some of the wildtype protein in the cells and prevents it from reaching the membrane. These results suggested a new mechanism for dominance in hypophosphatasia, involving mutated homodimers and heterodimers that are sequestrated in the cells with only wildtype homodimers (approximately 25% of the total dimers) able to play a physiologic role in the cell.
Stevenson et al. (2008) described a boy with prenatal presentation of hypophosphatasia (241500) manifesting as long bone deformity detected in utero by sonography at 18 weeks' gestation who had spontaneous pre- and postnatal improvement. He was compound heterozygous for missense mutations in the ALPL gene, one in exon 6 (526G-A, ala176 to thr, A176T) and the other in exon 8 (814C-T, arg272 to cys, R272C). His affected older brother, who carried the same mutations, had childhood hypophosphatasia (241510). His sonography at 20 weeks was reported as normal, and he was first evaluated at 19 months of age for premature tooth loss. At 18 weeks' gestation, the proband presented with sharp angulation of the midshaft of the right humerus, radius, and ulna. All major long bones were subjectively of slightly narrow diameter and poorly ossified. At birth there was bowing of the right arm with dimpling of the skin near the apex of the bowing, but the physical examination was otherwise unremarkable. Evidence of healing and healed fractures in the right humerus, radius, and ulna was seen on radiographs at birth and 6 months of age. At age 17 months he was evaluated for premature tooth loss, and at that time bowing of the right upper arm was present. The female twin of the proband carried neither mutation and was clinically unaffected. Each parent was heterozygous for one of the mutations.
For discussion of the arg272-to-cys (R272C) mutation in the ALPL gene that was found in compound heterozygous state in brothers, one with infantile hypophosphatasia (241500) and the other with childhood hypophosphatasia (241510) by Stevenson et al. (2008), see 171760.0022.
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