Hypophosphatasia (HPP) is an inherited “ultra-rare” disease of the bone and mineral metabolism, due to a low activity of the tissue nonspecific isoenzyme of alkaline phosphatase (TNSALP). This enzyme is encoded by ALPL gene (1p36.12; MIM 171760), more than 315 different mutations were identified worldwide. TNSALP hydrolyzes pyrophosphate phosphodiesterase yielding inorganic phosphate, thereby promoting hydroxyapatite formation. In the brain, TNSALP hydrolyzes the phosphorylated version of pyridoxine, allowing this vitamin to cross the blood-brain barrier.
Neonatal lethal HPP is the most severe form of the disease, with estimated prevalence at 1:300,000 in Europe and 1:182,000 in Japan.[3,4] It is radiographically visible in the fetal period as bowed and shorter long bones, low or no skeletal mineralization, and more specific signs such as osteochondral spurs and pretibial dimpling. Newborns at birth present respiratory distress due to chest deformities and lung hypoplasia; skeletal alterations with generalized lack of ossification, caput membranaceum, short and arched limbs, cup-shaped and “moth-eaten” metaphysis, bone spicules and spontaneous fractures, small rib cage with thin ribs; and vitamin B6-dependent epileptic seizures. A pseudo-metabolic alteration can be present: hypercalcemia and hyperphosphatemia and can be complicated by early nephrocalcinosis. A conjunctival bluish color may be observed, enabling differential diagnosis and ruling out osteogenesis imperfecta.[4–6] The key biochemical marker is markedly decreased serum alkaline phosphatase (ALP).
Until very recently, severe perinatal HPP was lethal at birth or in the 1st months of life. The treatment with a recombinant enzyme replacement therapy (ERT), asfotase alfa, had substantially improved bone mineralization, survival, and ventilation-free survival, although few cases are known.[7,8] According to a PubMed search, just 71 cases of neonatal HPP have been published[7–28] since the 1st case was characterized in 1948 by Campbell. Twelve of them received ERT, out of which 10 survived (Table 1) with an average evolution time of 13 months (range from 1 month to 5 years). All the others, except one that his family wanted to go home after 4 days of life, were mostly exitus during the 1st month of life. The case presented here is a newborn affected by severe HPP with complicated management. For publishing, informed consent was obtained by the parents and no other ethical accuracies were necessary.
2 Case report
A male infant was born by cesarean section due to antenatal history of short long bone at 37 weeks of gestation. This was the 5th pregnancy of the mother, aged 35 years, with a healthy daughter from a previous partner and 3 previous abortions with the current partner. There was no history of consanguinity. The antenatal ultrasound at 29 weeks of gestation had shown a fetus with short long bones and generalized hypomineralization suggestive of skeletal dysplasia disorder, considering initially achondroplasia as a potential diagnosis. The parents chose not to undertake any further genetic or invasive test other than gestational ultrasound follow-up.
The birth weight was 2850 g, head circumference 34 cm (both 50th percentile) whereas his length was 42 cm (10th percentile). The Apgar score was 3 at 1 minute and 5 at 5 minutes after birth. He required intubation and mechanical ventilation immediately upon birth due to severe respiratory insufficiency, and showed generalized hypotonia and hypoactivity; however, he was respondent to stimulus and with spontaneous eye opening. On his physical examination was found to have shortened and bowed arms and legs, caput membranaceum, widely open fontanelles, short neck, poor muscle tone, and loose joints. Heart auscultation showed a multifocal systolic murmur II/VI. The external genitalia showed bilateral hydrocele (Fig. 1).
An initial chest X-ray showed thoracic and pulmonary hypoplasia with thin rib bone cortices. Based upon physical examination, prenatal ultrasound and present X-rays, search for hypomineralization defects was initiated. Biochemical analysis showed hypercalcemia (calcium: 11.2 mg/dL) and undetectable serum ALP levels. Urine analysis showed hypercalciuria with increased calcium creatinine ratio (>0.7). Blood count as well as liver function was normal. Further whole-body X-ray was undertaken, showing generalized severe hypomineralization (Fig. 2).
Based on clinical data, the suspected diagnosis was severe perinatal HPP. The data were confirmed by biochemical determination of plasma pyridoxal 5′-phosphate (PLP) levels (>200 nmol/L; normal reference range 23–172 nmol/L) and phosphoethanolamine levels in urine (7567 μmol/g, normal reference below 150 μmol/g). With these data, on the 3rd day treatment with ERT asfotase alfa (approved by EMA in 2015) was indicated obtaining parental informed consent. However, some hours later, the patient died of severe respiratory insufficiency secondary to thoracic hypoplasia.
Genetic analysis confirmed 2 heterozygous c.473-2G>C/c.413G>C (p.Arg138Pro) mutations in the ALPL gene. The parent DNA identified both of them heterozygous carrier.
We have presented a new case of neonatal lethal HPP with 2 heterozygous c.473-2G>C/c.413G>C mutations in the ALPL gene. His inheritance pattern was autosomal recessive, like as in severe forms of HPP. In the prenatal period, the findings of bone alteration show considerable overlap with other skeletal dysplasia such as achondrogenesis in our case, similar to what is described in the literature, especially with osteogenesis imperfecta types IIA and IIC, and achondrogenesis type IA. If present, “spurs” of the limbs and pretibial dimpling are diagnostic for HPP.[4,29] In addition, it is also interesting to keep in mind that parental low serum ALP levels of the pregestation and gestation also contributes to the diagnosis of suspicion. During the gestation, serum ALP levels increase during the 2nd half of pregnancy at the expense of placental production of ALP, and to a lesser degree, in the production of bone ALP. ALP levels of the mother and father were 26 and 32 UI/L, respectively (normal range 35–110 IU/L), in the present case.
At birth, skeletal abnormalities, respiratory distress with pulmonary hypoplasia, hypercalcemia established initial suspicion of bone dysplasia. Neurologic signs with epileptic encephalopathy or hemorrhage were not present undetectable PLP in 2 determinations strongly suspected the diagnosis of HPP at 48 hours. In the literature, all the cases published with neonatal lethal HPP have low PLP levels and a correlation between the severity of the disease and the serum PLP level has been reported. However, it is important to reflect that in the first 48 hours PLP concentrations may be those of the mother and may not yet be significantly low. Later results confirmed elevated serum PLP concentration, which represents a sensitive and specific biochemical marker for HPP, elevated phosphoethanolamine levels are also elevated in serum and urine, although less specific. Both are substrates of TNSALP that accumulate endogenously in HPP. Genetic study revealed a known pathogenic mutation and a new mutation c.413G>C (p.Arg138Pro) in exon 5 of the gene, not described in the database http://www.sesep.uvsq.fr/03_hypo_mutations.ph. Bioinformatic analyzes with SIFT and polyphen 2 algorithms predict harmful and possibly harmful behavior, respectively, for the structure and/or function of the protein.
Before treatment with ERT, no patient reported survived more than 1 year and most died in the 1st days of life, the main cause was respiratory failure and in some cases epileptic encephalopathy.[13,23] However, 83.3% of ERT reported cases survived and substantially improved its clinical evolution. Our case died on the 4th day of life without having begun receiving ERT treatment. The HPP in the differential diagnosis was not considered in the pregnancy and the determinations of alkaline phosphatases in the parents were not assessed. This would have facilitated early availability of enzymatic treatment.
In conclusion, earlier diagnosis of perinatal HPP with prenatal recognition and starting ERT as soon as possible after birth may improve outcomes and might have a positive impact on to survival. It is therefore necessary to establish a consensus of recommendations.
Conceptualization: Laura Castells.
Formal analysis: Laura Castells, Pía Cassanello.
Methodology: Laura Castells, Pía Cassanello, José Félix Muñiz.
Supervision: María-Luz Couce.
Writing – original draft: Laura Castells, María José De Castro.
Writing – review & editing: María-Luz Couce.
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