First-trimester prenatal diagnosis of hypophosphatasia: experience with 16 cases

Differential diagnosis of perinatal hypophosphatasia: radiologic perspectives

Perinatal hypophosphatasia (HPP) is a rare, potentially life-threatening, inherited, systemic metabolic bone disease that can be difficult to recognize in utero and postnatally. Diagnosis is challenging because of the large number of skeletal dysplasias with overlapping clinical features. This review focuses on the role of fetal and neonatal imaging modalities in the differential diagnosis of perinatal HPP from other skeletal dysplasias (e.g., osteogenesis imperfecta, campomelic dysplasia, achondrogenesis subtypes, hypochondrogenesis, cleidocranial dysplasia). Perinatal HPP is associated with a broad spectrum of imaging findings that are characteristic of but do not occur in all cases of HPP and are not unique to HPP, such as shortening, bowing and angulation of the long bones, and slender, poorly ossified ribs and metaphyseal lucencies. Conversely, absent ossification of whole bones is characteristic of severe lethal HPP and is associated with very few other conditions. Certain features may help distinguish HPP from other skeletal dysplasias, such as sites of angulation of long bones, patterns of hypomineralization, and metaphyseal characteristics. In utero recognition of HPP allows for the assembly and preparation of a multidisciplinary care team before delivery and provides additional time to devise treatment strategies.

Hypophosphatasia: An overview For 2017

Hypophosphatasia (HPP) is the inborn-error-of-metabolism that features low serum alkaline phosphatase (ALP) activity (hypophosphatasemia) caused by loss-of-function mutation(s) of the gene that encodes the tissue-nonspecific isoenzyme of ALP (TNSALP). Autosomal recessive or autosomal dominant inheritance from among >300 TNSALP (ALPL) mutations largely explains HPP's remarkably broad-ranging severity. TNSALP is a cell-surface homodimeric phosphohydrolase richly expressed in the skeleton, liver, kidney, and developing teeth. In HPP, TNSALP substrates accumulate extracellularly. Among them is inorganic pyrophosphate (PPi), a potent inhibitor of mineralization. Superabundance of extracellular PPi explains the hard tissue complications of HPP that feature premature loss of deciduous teeth and often rickets or osteomalacia as well as calcific arthropathies in some affected adults. In infants with severe HPP, blocked entry of minerals into the skeleton can cause hypercalcemia, and insufficient hydrolysis of pyridoxal 5'-phosphate (PLP), the major circulating form of vitamin B₆, can cause pyridoxine-dependent seizures. Elevated circulating PLP is a sensitive and specific biochemical marker for HPP. Also, the TNSALP substrate phosphoethanolamine (PEA) is usually elevated in serum and urine in HPP, though less reliably for diagnosis. Pathognomonic radiographic changes occur in pediatric HPP when the skeletal disease is severe. TNSALP mutation analysis is essential for recurrence risk assessment for HPP in future pregnancies and for prenatal diagnosis. HPP was the final rickets/osteomalacia to have a medical treatment. Now, significant successes using asfotase alfa, a mineral-targeted recombinant TNSALP, are published concerning severely affected newborns, infants, and children. Asfotase alfa was approved by regulatory agencies multinationally in 2015 typically for pediatric-onset HPP.

Parental serum alkaline phosphatase activity as an auxiliary tool for prenatal diagnosis of hypophosphatasia

OBJECTIVE

The objective of this study is to clarify the usefulness of parental alkaline phosphatase (ALP) for prenatal diagnosis of hypophosphatasia (HPP).

METHODS

Maternal (m) and paternal (p) ALP values were measured in 77 cases from a multicenter cohort (fetal skeletal dysplasia forum in Japan) of cases with short limbs on ultrasonography during pregnancy. After birth, X-rays, cord blood ALP, and gene analysis were evaluated to achieve an exact diagnosis. The screening usefulness of ALP was examined retrospectively.

RESULTS

Seventeen cases were eventually diagnosed as HPP and 60 as not HPP; the overall mean m-ALP and p-ALP (standard deviation) values were 133.4 (53) versus 197 (69) IU/L and 149.6 (71.8) versus 231 (61.4) IU/L (p < 0.001). Receiver operating characteristic curve analysis showed that the optimal m-ALP and p-ALP cutoff values were 123 and 165 IU/L, respectively. Presence of at least one of the m-ALP or p-ALP values abnormally low had a sensitivity, specificity, and positive predictive values of 82% (14/17), 93%, and 78%, respectively, for the diagnosis of HPP.

CONCLUSION

Parental ALP measurement might be an auxiliary tool to hone in the prenatal diagnosis of fetal HPP. © 2017 John Wiley & Sons, Ltd.

Hypophosphatasia - aetiology, nosology, pathogenesis, diagnosis and treatment

Hypophosphatasia is the inborn error of metabolism characterized by low serum alkaline phosphatase activity (hypophosphatasaemia). This biochemical hallmark reflects loss-of-function mutations within the gene that encodes the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP). TNSALP is a cell-surface homodimeric phosphohydrolase that is richly expressed in the skeleton, liver, kidney and developing teeth. In hypophosphatasia, extracellular accumulation of TNSALP natural substrates includes inorganic pyrophosphate, an inhibitor of mineralization, which explains the dento-osseous and arthritic complications featuring tooth loss, rickets or osteomalacia, and calcific arthopathies. Severely affected infants sometimes also have hypercalcaemia and hyperphosphataemia due to the blocked entry of minerals into the skeleton, and pyridoxine-dependent seizures from insufficient extracellular hydrolysis of pyridoxal 5'-phosphate, the major circulating form of vitamin B6, required for neurotransmitter synthesis. Autosomal recessive or dominant inheritance from ~300 predominantly missense ALPL (also known as TNSALP) mutations largely accounts for the remarkably broad-ranging expressivity of hypophosphatasia. High serum concentrations of pyridoxal 5'-phosphate represent a sensitive and specific biochemical marker for hypophosphatasia. Also, phosphoethanolamine levels are usually elevated in serum and urine, though less reliably for diagnosis. TNSALP mutation detection is important for recurrence risk assessment and prenatal diagnosis. Diagnosing paediatric hypophosphatasia is aided by pathognomic radiographic changes when the skeletal disease is severe. Hypophosphatasia was the last type of rickets or osteomalacia to await a medical treatment. Now, significant successes for severely affected paediatric patients are recognized using asfotase alfa, a bone-targeted recombinant TNSALP.

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 anomalies

It is possible to identify many types of skeletal dysplasias and conditions involving limb deformities prenatally using ultrasound. It is likely that in the future, with the advancing technology and discoveries in molecular genetics, specific mutation analysis will become available for many of these conditions. This will make first trimester diagnosis an option in many cases. Because of the complex nature of many of these cases, it may be helpful to use a multidisciplinary approach involving a radiologist and a geneticist at times. In utero radiographs may help clarify a diagnosis. In lethal cases where a specific diagnosis has not been confirmed, it may be helpful postpartum to obtain an autopsy; photographs; complete body radiographs; karyotypic analysis; and specimens of bone, cartilage, and fetal blood for further analysis.

Comparison of serum catalytic activity and immunoreactivity of bone alkaline phosphatase in hypophosphatasia

Hypophosphatasia is a rare bone disorder characterised by low levels of tissue non-specific alkaline phosphatase (TNSALP). Although TNSALP is widespread in virtually all tissues the clinical effects, when produced, seem only to affect the mineralizing tissue such as teeth and skeleton. The skeleton is severely affected in the perinatal form of the disease, when death may occur in utero, or may not be affected in the adult type variety of the disease. We therefore compared the catalytic (cBALP) and immunoreactivity (iBALP) of bone alkaline phosphatase isoenzyme in six families with hypophosphatasia. iBALP was measured using an IRMA method. cBALP was measured after electrophoretic separation of serum alkaline phosphatase isoenzymes on lectin containing agarose gel. The percentage of different isoenzymes was calculated using densitometric scanning and cBALP calculated from the known total serum alkaline phosphatase activity. Results showed cBALP=0.796+3. 269iBALP, r=0.9 p<0.01, in cases of hypophosphatasia. In general, the lower the iBALP and cBALP the more severe the skeletal disease. The bone isoenzyme level predicts the clinical severity of bone disease.

Early prenatal sonographic diagnosis of congenital hypophosphatasia

A pregnant woman of 14 weeks' gestation was sonographically examined due to large-for-dates uterine size. The ultrasound examination showed poor ossification of all bony structures. All limbs were shortened with no evidence of fractures. The echodensity approximated that of the surrounding organs. No acoustic shadowing was observed. Based on these sonographic findings, skeletal dysplasia and short-limb dwarfism were diagnosed, the most likely condition being congenital hypophosphatasia. Early cordocentesis was successfully performed at 15 weeks' gestation to determine fetal alkaline phosphatase concentration. This was undetectable. The prenatal diagnosis of congenital hypophosphatasia was made. After counselling, the woman decided to opt for termination of pregnancy which was performed vaginally. Post-abortion findings confirmed the prenatal diagnosis. To our knowledge, this is the earliest sonographic diagnosis of this condition reported.

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.

Correlation of alkaline phosphatase (ALP) determination and analysis of the tissue non-specific ALP gene in prenatal diagnosis of severe hypophosphatasia

Prenatal diagnosis of severe hypophosphatasia by mutation analysis of the tissue non-specific alkaline phosphatase (TNSALP) gene is reliable and mostly informative. However, alkaline phosphatase (ALP) assay of CVS may be a useful complementary and independent method, especially when a mutation is unidentified and DNA from the index case is unavailable, rendering impossible the use of DNA polymorphisms as genetic markers of the disease. We report here mutation analysis of the TNSALP gene and ALP assay in nine cases of prenatal diagnosis of severe hypophosphatasia. The results showed a good correlation between ALP assay and DNA analysis in all but one case, which suggested that in at least some cases low values of ALP may correspond to affected fetuses as well as to heterozygotes.

Genetic analysis of hypophosphatasia

A review is presented of the recent advances in: (i) clinical features, (ii) biochemistry and molecular biology of alkaline phosphatase, (iii) genetic defect in hypophosphatasia, and (iv) prenatal diagnosis. Despite the recent progress, the pathogenesis of hypophosphatasia is far from being elucidated. More clinical cases and further characterization of the alkaline phosphatase gene mutations are needed for better understanding of the clinical spectrum of the entity.

First-trimester prenatal molecular diagnosis of infantile hypophosphatasia in a Japanese family

We obtained a prenatal molecular diagnosis during the first trimester in a Japanese woman whose first child (the proband) had been a compound heterozygote for infantile hypophosphatasia. We examined chorionic villus DNA samples obtained at 10 weeks of gestation for the base substitutions detected in the proband DNA using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) and PCR-allele-specific oligonucleotide (ASO) analysis. The genotype of the fetus was the same as that of the proband. The same mobility shift patterns of single strand conformation polymorphism (SSCP) bands were observed in the fetus and the proband. This molecular approach to prenatal diagnosis appears to be more accurate than the enzymatic method and also more accurate and more rapid than the conventional RFLP method.

Infantile hypophosphatasia: successful prenatal assessment by testing for tissue-non-specific alkaline phosphatase isoenzyme gene mutations

We successfully assessed a fetus at risk for lethal infantile hypophosphatasia using amniocyte DNA and allele-specific oligonucleotide (ASO) probes for two missense mutations in the tissue-non-specific alkaline phosphatase isoenzyme (TNSALP) gene. The nucleotide changes had been discovered in a sister who died at 8 months of age from this inborn error of metabolism. The mother was known to carry the 747 (cDNA) G-->A transition, whereas her husband and 5-year-old daughter, who were also healthy, carried the 1309 A-->T transversion. Amniocytes, obtained at 16 weeks' gestation, provided genomic DNA for polymerase chain reaction (PCR) amplification of the appropriate TNSALP gene exons. ASO hybridization revealed absence of the 747A mutation and presence of the 1309T base changes in the fetus, indicating a carrier for hypophosphatasia. At 8 months of age, the offspring was in excellent health and without any radiological evidence of skeletal disease. His serum ALP activity and plasma pyridoxal 5'-phosphate level were decreased and increased, respectively, at levels consistent with the prenatal assessment. The ASO studies were confirmed postnatally using peripheral blood leukocyte DNA. This is the first application of direct mutational analysis to assess a fetus at risk for hypophosphatasia.