[Fetal therapy of skeletal dysplasias].

Three types of fetal therapy of skeletal dysplasias, as enzyme replacement, in utero stem cell transplantation, and gene therapy, are reviewed. Clinical trial of recombinant ALP for infantile hypophosphatasia has already started in Japan. In future, such enzyme replacement therapy is expected to be adapted to fetus. There are several reports of mesenchymal stem cell transplantation for osteogenesis imperfecta fetus. These case reports have showed that stem cell transplantation is safe and to some extent works in patients. No clinical trial for gene therapy has been reported. Recently, the study of gene therapy of using HPP fetal mouse showed an excellent therapeutic effect. Fetal therapy of skeletal dysplasias is still the stage of research because of the safety and the ethical issues. However, in order to treat severe cases of skeletal dysplasias which abnormal development has been already completed at birth, fetal therapy at an early stage would be demanded.

The role of phosphatases in the initiation of skeletal mineralization.

Endochondral ossification is a carefully orchestrated process mediated by promoters and inhibitors of mineralization. Phosphatases are implicated, but their identities and functions remain unclear. Mutations in the tissue-nonspecific alkaline phosphatase (TNAP) gene cause hypophosphatasia, a heritable form of rickets and osteomalacia, caused by an arrest in the propagation of hydroxyapatite (HA) crystals onto the collagenous extracellular matrix due to accumulation of extracellular inorganic pyrophosphate (PPi), a physiological TNAP substrate and a potent calcification inhibitor. However, TNAP knockout (Alpl(-/-)) mice are born with a mineralized skeleton and have HA crystals in their chondrocyte- and osteoblast-derived matrix vesicles (MVs). We have shown that PHOSPHO1, a soluble phosphatase with specificity for two molecules present in MVs, phosphoethanolamine and phosphocholine, is responsible for initiating HA crystal formation inside MVs and that PHOSPHO1 and TNAP have nonredundant functional roles during endochondral ossification. Double ablation of PHOSPHO1 and TNAP function leads to the complete absence of skeletal mineralization and perinatal lethality, despite normal systemic phosphate and calcium levels. This strongly suggests that the Pi needed for initiation of MV-mediated mineralization is produced locally in the perivesicular space. As both TNAP and nucleoside pyrophosphohydrolase-1 (NPP1) behave as potent ATPases and pyrophosphatases in the MV compartment, our current model of the mechanisms of skeletal mineralization implicate intravesicular PHOSPHO1 function and Pi influx into MVs in the initiation of mineralization and the functions of TNAP and NPP1 in the extravesicular progression of mineralization.

Hypophosphatasia: molecular testing of 19 prenatal cases and discussion about genetic counseling.

OBJECTIVE: We studied hypophosphatasia (HP) mutations in 19 cases prenatally detected by ultrasonography without familial history of HP. We correlated the mutations with the reported ultrasound signs, and discussed genetic counseling with regard to the particular dominantly inherited prenatal benign form of HP. METHOD: The coding sequence of the tissue nonspecific alkaline phosphatase (TNSALP) gene was analyzed by DNA sequencing, and 3D modeling was used to locate the mutated amino acids with regard to the functional domains of TNSALP. RESULTS: Although reported ultrasound signs were heterogeneous, two mutated alleles were found in 18 of the 19 cases studied, indicating recessive transmission of the disease. Functional domains of TNSALP were affected by 74% of missense mutations. In all the cases, including one with only a heterozygous mutation, molecular, biological, and familial data do not corroborate the hypothesis of prenatal benign HP. The mutation c.1133A>T observed in the prenatal benign form of HP and common in USA was not found in this series. CONCLUSION: The results point out the prenatally detectable allelic heterogeneity of HP. The nature of the detected mutations and the evidence of recessive inheritance do not support these cases being affected with prenatal benign HP.

Specific osseous spurs in a lethal form of hypophosphatasia correlated with 3D prenatal ultrasonographic images.

BACKGROUND: Hypophosphatasia is an osseous dysplasia with highly variable clinical expression, ranging from a recessive lethal prenatal type to late onset dominant short stature with premature shedding of teeth. Lethal forms of hypophosphatasia include short limb dwarfism with lack of ossification, especially on the vertebral bodies, very slender ribs and clavicles, and bowed, short lower extremities, with a bifid aspect of the diaphyses. Alkaline phosphatase is abnormally low in liver, bone, kidney and plasma. METHODS: We present here the prenatal images of a lethal form of hypophosphatasia, diagnosed precociously because of specific osseous spurs in a context of recurrent short limb dwarfism. RESULTS: Prenatal 3D ultrasonography has shown these spurs as early as 18 weeks. Molecular biology found compound heterozygous mutations in the gene TNSALP. CONCLUSION: In a context of short limb dwarfism, the search for these specific osseous spurs orient strongly toward the diagnosis of lethal hypophosphatasia.

Skeletal dysplasias with osteopenia in the newborn: the value of alkaline phosphatase.

Alkaline phosphatase is a commonly measured enzyme in clinical practice. Normal, excessively elevated and depressed serum or plasma levels have clinical value in the approach to the differential diagnosis of skeletal dysplasias associated with osteopenia in the newborn period. Two cases are described to illustrate this contention. In the first case we describe a neonate with congenital hypophosphatasia and markedly depressed levels of plasma alkaline phosphate, and in the second case we report a patient with a fracture of the femur, congenital rickets and an elevated alkaline phosphate level. In skeletal dysplasias with osteopenia, the nature of the abnormality in alkaline phosphate values, in association with that of calcium and phosphate, is an invaluable diagnostic aid in differential diagnosis.

Positive maternal serum triple test screening in severe early onset hypophosphatasia.

OBJECTIVES: Hypophosphatasia is a rare heritable inborn error of metabolism characterized by a liver/bone/kidney alkaline phosphatase defective bone mineralization due to mutations in the tissue-non-specific alkaline phosphatase (TNS-ALP) gene. To date 128 mutations are described in the TNS-ALP gene located on the short arm of chromosome 1. The clinical presentation of hypophosphatasia is variable ranging from early onset lethal short-limb dwarfism to a late-onset presentation with fractures in childhood or adulthood. METHODS: We report a pregnancy with a positive maternal serum triple test screening and a post-mortem pathological and molecular diagnosis of perinatal lethal hypophosphatasia. RESULTS: Two heterogeneous missense mutations in the TNS-ALP gene were found, of which one was not previously described. CONCLUSION: This case report adds to the list of fetal malformations found after positive maternal serum triple test screening and reports a previously undescribed mutation in the TNS-ALP gene responsible for hypophosphatasia.

Mild autosomal dominant hypophosphatasia: in utero presentation in two families.

We describe four pregnancies in two families in which mild hypophosphatasia, apparently transmitted as an autosomal dominant trait, manifested in utero as severe long bone bowing. Postnatally, there was spontaneous improvement of the skeletal defects. Recognition of this presentation for hypophosphatasia by family investigation and assessment of the fetal skeleton for degree of ossification and chest size using ultrasonography is important. The prognosis for this condition is considerably better than for more severe forms of hypophosphatasia and for many other disorders that cause skeletal defects with long bone bowing in utero.

Mild hypophosphatasia mimicking severe osteogenesis imperfecta in utero: bent but not broken.

We describe a fifth instance of hypophosphatasia presenting with prenatal findings suggestive of a very severe bone dysplasia but with a subsequently benign course. Spontaneous improvement of long-bone angulation began prenatally. The postnatal course has been encouraging. This sixth clinical form of hypophosphatasia, which we suggest should be called the benign prenatal form of hypophosphatasia, should be added to the differential diagnostic possibilities considered when angulation or bowing of long bones is discovered prenatally.

Histologic and ultrastructural studies on the mineralization process in hypophosphatasia.

Chondroosseous tissue from six infants with infantile hypophosphatasia and six control infants were studied by light, transmission, and scanning electron microscopy. Alkaline phosphatase histochemical reaction of the growth plate was studied in two infants and was greatly reduced when compared to two control infants. Hypertrophic chondrocytes were increased in number with persisting cartilage islets in the metaphysis. In five of the six cases studied, chondrocytes and intercartilagenous intercellular chondroid matrix appeared ultrastructurally normal. Matrix vesicle distribution was similar to that of control subjects, but they were associated with few mineral crystals. In two infants, the matrix vesicles were alkaline phosphatase nonreactive. In the calcifying zone of the growth plate and in the newly formed metaphyseal trabecular bone, cartilagenous calcospherites often were small and the orientation of crystals was nonradial when compared to that of control infants. The mineralization of diaphyseal bone appeared normal. It seems that matrix vesicles are present in hypophosphatasia and that the impaired mineralization of cartilage is due primarily to the deficiency of alkaline phosphatase. In spite of the lack of alkaline phosphatase, secondary mineralization of bone which is not mediated by matrix vesicles was normal.