Hypophosphatasia in Neonatal Twins With Generalized Skeletal Hypomineralisation

Hypophosphatasia (HPP) is a rare genetic disorder characterised by impaired bone mineralisation and reduced activity of ALP (alkaline phosphatase), resulting from mutations in the ALPL gene [1, 2]. It exhibits a broad spectrum of clinical symptoms, ranging from severe perinatal forms to milder adult-onset types, with features including skeletal abnormalities similar to rickets, premature tooth loss and, in severe cases, respiratory complications due to chest malformations [3]. The incidence of HPP varies globally. In North America and Europe, the prevalence of severe forms of HPP is estimated at about one in 300 000 live births, whereas in China, cases of severe perinatal and infantile HPP are rarely reported [4, 5]. Diagnosing HPP can be complex due to its variable clinical presentation. Severe cases are often identified in early infancy and are associated with a high mortality rate, exceeding 50% within the first year of life [6, 7]. However, milder forms may not become apparent until later, leading to potential delays in treatment [8]. Diagnosis generally involves a mix of clinical assessment, radiographic examination and biochemical testing, focusing on ALP levels. The complexity is compounded by the fact that symptoms may overlap with other metabolic bone diseases, making early and accurate diagnosis challenging. Ultimately, a definitive diagnosis of HPP requires gene sequencing to identify mutations in the ALPL gene [9]. The case of the twin neonates with HPP described in this article demonstrated clinical improvement following enzyme replacement therapy, with no adverse events occurring during the treatment. It underscores the significance of early diagnosis and active management in improving the prognosis of neonatal HPP, particularly in populations where the condition is less frequently reported. The parents of the twin patients are not consanguineous and have no significant familial history of metabolic or skeletal disorders. Prenatal ultrasonography revealed disproportionate biparietal diameters and long bones in the twins, with femoral shortening. At 37 weeks and 3 days of gestation, an emergency caesarean section was performed due to maternal membrane rupture. Five minutes post-birth, the infants exhibited severe respiratory distress, necessitating intubation and ventilation. Umbilical cord blood analysis showed reduced tissue non-specific alkaline phosphatase (TNSALP) activity (sister: < 10 IU/L, younger sister: < 10 IU/L, normal range: 125–250 IU/L) and elevated levels of pyridoxal phosphate (sister: 617.4 nmol/L, younger sister: 558.9 nmol/L, normal range: 30–150 nmol/L), with hypercalcemia at 3.30 and 3.10 mmol/L (normal range: 2.25–2.67 mmol/L). Other parameters including blood phosphate, phosphates and parathyroid hormone were within normal limits (Table 1). Skeletal radiographs obtained shortly after birth demonstrated generalised skeletal hypomineralisation involving the skull, ribs and long bones with a severe rickets-like appearance (Figure 1). Given the radiographic findings and low ALP activity, the twins were clinically diagnosed with HPP and commenced on enzyme replacement therapy with asfotase alfa (STRENSIQ by Alexion Pharmaceuticals Inc., the only approved medication for paediatric HPP patients) at a dose of 2 mg/kg, thrice weekly subcutaneous injections starting on Day 3. Genetic sequencing later identified compound heterozygous mutations in the ALPL in the twins, with each mutation inherited from a different parent. Following the replacement therapy, ALP activity in the twins increased to 15 and 18 IU/L by Day 7. By Day 14, serum calcium levels decreased to 2.90 and 2.70 mmol/L, respectively. At the 2-month mark, ALP activity reached the lower normal limit, recorded at 83 and 91 IU/L. Radiologically, there was a notable improvement in the mineralisation of the trunk and limb bones, increased bone density and amelioration of rib lesions. Despite these clinical improvements, CT imaging in March showed multiple pathological fractures in the ribs of both sisters, indicating a possible healing phase (Figure 2). Currently, the twin sisters continue to be monitored as inpatients. The clinical presentation of the twins, including severe respiratory distress and radiological signs of insufficient bone mineralisation, initially suggested a spectrum of metabolic bone diseases. However, the distinct biochemical profile, marked by low TNSALP activity and elevated pyridoxal phosphate levels, guided the diagnosis towards HPP. The complexity of diagnosing HPP is compounded by its diverse clinical manifestations and the lack of unified clinical diagnostic standards. HPP is classified into several subtypes based on the age of onset and severity of clinical symptoms: perinatal (including lethal and benign forms), infantile, childhood, adult and odontohypophosphatasia [10]. The lethal perinatal form is the most severe, often resulting in in utero death or respiratory failure shortly after birth [7, 11]. The infantile type is characterised by growth retardation and severe limb deformities, resembling features of rickets such as bowed legs, joint enlargement, rib fractures, progressive thoracic deformities and tracheomalacia [12]. Childhood HPP shows variable phenotypes, with premature loss of primary teeth being a characteristic feature, accompanied by delayed bone mineralisation and stunted growth [13]. It can be challenging to differentiate between infantile and childhood forms. The adult form presents with frequent fractures or pseudofractures and difficulty in fracture healing [14]. Odontohypophosphatasia primarily affects dental health, with early loss of primary teeth, enlarged pulp chambers and less involvement of enamel and dentine, usually without growth retardation or limb deformities [15]. The diagnosis of HPP relies heavily on laboratory tests, radiological examinations and genetic testing due to the variability in its clinical presentation. Serum ALP reduction is a diagnostic marker for HPP, but false positives can occur in early pregnancy, hyperthyroidism or anaemia, necessitating careful clinical correlation [16]. ALPL gene testing is considered the most reliable method for diagnosing HPP, with a detection rate exceeding 95% in severe cases [17]. Additionally, HPP shares clinical features with other metabolic bone diseases like hypophosphatemic rickets and vitamin D-dependent rickets, as well as rare disorders such as ectodermal dysplasia and chondrodysplasia [18, 19]. The pathogenesis of HPP is closely linked to mutations in the ALPL gene, which encodes tissue-non-specific alkaline phosphatase (TNAP). TNAP, a group of isoenzymes expressed in bones, teeth, liver and kidneys, plays a vital role in cellular mineralisation. It does so by hydrolyzing inorganic pyrophosphate (PPi), pyridoxal-5-phosphate (PLP) and phosphoethanolamine (PEA), thereby releasing phosphate and promoting mineralisation. Mutations in the ALPL gene lead to reduced TNAP activity, resulting in elevated levels of PPi, PLP and PEA, which in turn cause mineralisation defects [20, 21]. Consequently, in clinical assessments, serum ALP activity reduction is a primary diagnostic marker for HPP. Additionally, increased serum levels of PLP, urinary PEA and PPi serve as sensitive indicators for HPP diagnosis [10]. Although persistently low serum ALP is a hallmark of hypophosphatasia, low ALP can also be observed in several neonatal settings, including nutritional deficiency (particularly zinc deficiency), severe malnutrition, hypothyroidism, transient neonatal low ALP and acute or chronic illness-related suppression. In clinical practice, these entities are differentiated by the overall biochemical profile, nutritional/endocrine evaluation, clinical context and the presence or absence of characteristic skeletal findings. In our twins, the combination of profoundly reduced ALP/TNSALP activity, markedly elevated pyridoxal-5′-phosphate (PLP), diffuse skeletal hypomineralisation with rachitic metaphyseal changes and subsequent biallelic ALPL variants made alternative causes of low ALP unlikely. Asfotase alfa, a recombinant human bone-targeted alkaline phosphatase, plays a crucial role in improving bone mineralisation, which is fundamental in the management of HPP [22, 23]. The therapy's efficacy in enhancing bone density and improving respiratory and motor functions, as observed in severe HPP cases, was reflected in the twins' treatment outcomes. This improvement in clinical symptoms suggests a direct correlation between the enzyme replacement therapy and the mitigation of some of the most debilitating aspects of HPP [24]. The safety profile of asfotase alfa, as indicated by the absence of adverse events in this case, adds to its viability as a treatment option for neonatal HPP. However, the long-term prognosis for HPP varies, depending on the severity of the disease and the age at which treatment is initiated. In cases like the twins, where early and appropriate intervention occurs, the prognosis can be significantly better, with improved quality of life and functionality. Despite the effectiveness of asfotase alfa, HPP currently has no cure and clinical management primarily focuses on symptomatic treatment. Research on HPP treatment has explored various approaches, though with limited success. Attempts to use zinc and magnesium (which can activate ALP) and Teriparatide (an active fragment of human endogenous parathyroid hormone, directly acting on osteoblasts to stimulate bone formation) have yielded unsatisfactory results [14, 25]. Due to the low incidence of HPP and its varied clinical manifestations, conducting large-scale clinical trials remains challenging. Some studies suggest that restricting phosphate intake may benefit HPP treatment [26]. Additionally, the use of medical cannabinoids has been proposed to significantly improve symptoms in patients with adult HPP [27]. An important observation in our case is the occurrence of multiple pathological rib fractures despite early initiation of asfotase alfa and radiographic improvement in mineralisation. Several mechanisms may contribute. First, in severe neonatal HPP, skeletal fragility may remain pronounced during the early treatment phase because improvements in mineral deposition may precede restoration of normal bone architecture and mechanical strength, and bone remodelling may lag behind radiographic gains in density. Second, some rib fractures detected on follow-up CT could represent pre-existing microfractures incurred before or around the start of therapy that became more apparent during subsequent healing and remodelling. Third, the thorax is particularly vulnerable in critically ill neonates due to respiratory mechanics, ventilator-related chest wall stress and handling/positioning, which may predispose to rib injury in the setting of profoundly undermineralised ribs. Similar fracture-related complications have been noted in clinical experience with severe paediatric-onset HPP, emphasising that fracture risk may persist even as mineralisation improves under ERT. Following initiation of asfotase alfa, both infants demonstrated interval clinical stabilisation accompanied by progressive radiographic improvement in skeletal mineralisation. Respiratory status has been monitored closely, with serial assessment of ventilatory requirements and oxygen need over time. Growth trajectory (weight, length and head circumference) and nutritional status are tracked longitudinally, together with feeding tolerance. Given the ongoing risk of skeletal fragility in severe paediatric-onset hypophosphatasia, a multidisciplinary long-term surveillance plan has been established, including skeletal monitoring for fractures, deformity and pain with imaging tailored to symptoms and clinical course; biochemical monitoring of calcium/phosphate homeostasis and therapy response during ongoing asfotase alfa treatment; growth and nutrition follow-up at each visit; dental surveillance beginning in early childhood to monitor tooth eruption and premature tooth loss; and neurodevelopmental follow-up with standardised developmental screening and referral to rehabilitation services when indicated. In conclusion, this case report on HPP in twin neonates provides valuable insights into the complexities of diagnosing and treating this rare genetic disorder. It underscores the importance of early diagnosis, the effectiveness of enzyme replacement therapy and the need for ongoing research and guideline development for managing HPP, particularly in neonatal cases and in regions with lower reporting frequencies. Rui Wang: investigation, data curation, writing original draft, Jiayi Li : conceptualisation, review and editing of manuscript. All authors have read and approved the final version of the manuscript. The authors have nothing to report. The authors declare that the research presented in this manuscript adheres to the ethical principles outlined by the Ethics Committee of West China Second University Hospital, Sichuan University. All procedures involving human participants were conducted in accordance with the ethical standards of Sichuan University and the Declaration of Helsinki (1964), as revised in 2013. Written informed consent was obtained from the parents of the patients for publication of this case report and any accompanying images. The authors declare no conflicts of interest. The data that support the findings of this study are openly available in SCIENCE DATA BANK at https://www.scidb.cn/s/ZnMZFz.