Asfotase alfa treatment in perinatal and infantile hypophosphatasia: safe and sustained efficacy

The impact of enzyme replacement therapy on the oral health manifestations of hypophosphatasia among children: a scoping review

PURPOSE

A scoping review to describe the use of enzyme replacement therapy (ERT) in the form of asfotase alfa to decrease the severity of oral manifestations in children with hypophosphatasia (HPP).

METHODS

Six databases were searched using keywords and index terms related to "hypophosphatasia," "children," and "enzyme replacement therapy." Duplicates were removed and two independent reviewers screened the titles and abstracts to identify articles for full-text review. Extracted data was summarised narratively.

RESULTS

The systematic search identified 3548 articles, with 171 suitable for full-text review and a final 22 that met inclusion criteria. Enzyme replacement therapy generally resulted in a reduction in the presence and severity of oral manifestations of HPP. However, numerous studies failed to report specific details regarding the nature of oral health outcomes and there were reported cases of further loss of primary teeth.

CONCLUSIONS

The available evidence suggests that that ERT in the form of asfotase alfa for HPP in infants and young children leads to improved oral health outcomes. It is recommended that the outcomes are improved with earlier initiation of ERT. Further, well-designed clinical research is required to assess oral health improvements and decreased morbidity associated with the early loss of teeth.

Deletion of the Pyrophosphate Generating Enzyme ENPP1 Rescues Craniofacial Abnormalities in the TNAP−/− Mouse Model of Hypophosphatasia and Reveals FGF23 as a Marker of Phenotype Severity

Hypophosphatasia is a rare heritable metabolic disorder caused by deficient Tissue Non-specific Alkaline Phosphatase (TNAP) enzyme activity. A principal function of TNAP is to hydrolyze the tissue mineralization inhibitor pyrophosphate. ENPP1 (Ectonucleotide Pyrophosphatase/Phosphodiesterase 1) is a primary enzymatic generator of pyrophosphate and prior results showed that elimination of ENPP1 rescued bone hypomineralization of skull, vertebral and long bones to different extents in TNAP null mice. Current TNAP enzyme replacement therapy alleviates skeletal, motor and cognitive defects but does not eliminate craniosynostosis in pediatric hypophosphatasia patients. To further understand mechanisms underlying craniosynostosis development in hypophosphatasia, here we sought to determine if craniofacial abnormalities including craniosynostosis and skull shape defects would be alleviated in TNAP null mice by genetic ablation of ENPP1. Results show that homozygous deletion of ENPP1 significantly diminishes the incidence of craniosynostosis and that skull shape abnormalities are rescued by hemi- or homozygous deletion of ENPP1 in TNAP null mice. Skull and long bone hypomineralization were also alleviated in TNAP^−/−/ENPP1^−/− compared to TNAP^−/−/ENPP1^+/+ mice, though loss of ENPP1 in combination with TNAP had different effects than loss of only TNAP on long bone trabeculae. Investigation of a relatively large cohort of mice revealed that the skeletal phenotypes of TNAP null mice were markedly variable. Because FGF23 circulating levels are known to be increased in ENPP1 null mice and because FGF23 influences bone, we measured serum intact FGF23 levels in the TNAP null mice and found that a subset of TNAP^−/−/ENPP1^+/+ mice exhibited markedly high serum FGF23. Serum FGF23 levels also correlated to mouse body measurements, the incidence of craniosynostosis, skull shape abnormalities and skull bone density and volume fraction. Together, our results demonstrate that balanced expression of TNAP and ENPP1 enzymes are essential for microstructure and mineralization of both skull and long bones, and for preventing craniosynostosis. The results also show that FGF23 rises in the TNAP^−/− model of murine lethal hypophosphatasia. Future studies are required to determine if the rise in FGF23 is a cause, consequence, or marker of disease phenotype severity.

Pulmonary Manifestations of Endocrine and Metabolic Diseases in Children

Advances in technology, methodology, and deep phenotyping are increasingly driving the understanding of the pathologic basis of disease. Improvements in patient identification and treatment are impacting survival. This is true in endocrinology and inborn errors of metabolism, where disease-modifying therapies are developing. Inherent to this evolution is the increasing awareness of the respiratory manifestations of these rare diseases. This review updates clinicians, stratifying diseases spirometerically; pulmonary hypertension and diseases with a predisposition to recurrent pulmonary infection are discussed. This division is artificial; many diseases have multiple pathologic effects on respiration. This review does not cover the impact of obesity.

Hypophosphatasia: Biological and Clinical Aspects, Avenues for Therapy

Hypophosphatasia (HPP) is a rare inherited systemic metabolic disease caused by mutations in the tissue-nonspecific alkaline phosphatase (TNSALP) gene. TNSALP is expressed in the liver, kidney and bone, and its substrates include TNSALP inorganic pyrophosphate, pyridoxal-5'-phosphate (PLP)/vitamin B6 and phosphoethanolamine (PEA). Autosomal recessive and dominant forms of the disease result in a range of clinical entities. Major hallmarks are low alkaline phosphatase (ALP) and elevated PLP and PEA levels. Very severe infantile forms of HPP cause premature death as a result of respiratory insufficiency and also present with hypo-mineralisation leading to deformed limbs with, in some cases, the near-absence of bones and skull altogether. Respiratory failure, rib fractures and seizures due to vitamin B6 deficiency are indicative of a poor prognosis. Craniosynostosis is frequent. HPP leads to an unusual presentation of rickets with high levels of calcium and phosphorus, resulting in hypercalciuria, nephrocalcinosis and low ALP levels. Hypercalcaemic crisis, failure to thrive and growth retardation are concerns in infants. Fractures are common in both infantile and adult forms of the disease, concomitantly occurring with unexplained chronic pain and fatigue. Dental clinical presentations, which include the premature loss of teeth, are also commonly found in HPP and specifically manifest as odontohypophosphatasia. A novel enzyme therapy for human HPP, asfotase alfa, which is specifically targeted to mineralised tissues, has been developed in the past decades. While this treatment seems very promising, especially for infantile HPP, many questions regarding its long-term effects, the management of treatment, and any potential secondary adverse effects remain unresolved.