Lethal perinatal hypophosphatasia caused by a novel compound heterozygous mutation: a case report

Catalyzing precision: unraveling the diagnostic conundrum of tunisian familial hypophosphatasia case through integrative clinical and molecular approaches

Familial Hypophosphatasia presents a complex diagnostic challenge due to its wide-ranging clinical manifestations and genetic heterogeneity. This study aims to elucidate the molecular underpinnings of familial Hypophosphatasia within a Tunisian family harboring a rare c.896 T > C mutation in the ALPL gene, offering insights into genotype-phenotype correlations and potential therapeutic avenues. The study employs a comprehensive approach, integrating biochemical examination, genetic analysis, structural modeling, and functional insights to unravel the impact of this rare mutation. Genetic investigation revealed the presence of the p.Leu299Pro mutation within the ALPL gene in affected family members. This mutation is strategically positioned in proximity to both the catalytic site and the metal-binding domain, suggesting potential functional consequences. Homology modeling techniques were employed to predict the 3D structure of TNSALP, providing insights into the structural context of the mutation. Our findings suggest that the mutation may induce conformational changes in the vicinity of the catalytic site and metal-binding domain, potentially affecting substrate recognition and catalytic efficiency. Molecular dynamics simulations were instrumental in elucidating the dynamic behavior of the tissue-nonspecific alkaline phosphatase isozyme (TNSALP) in the presence of the p.Leu299Pro mutation. The simulations indicated alterations in structural flexibility near the mutation site, with potential ramifications for the enzyme's overall stability and function. These dynamic changes may influence the catalytic efficiency of TNSALP, shedding light on the molecular underpinnings of the observed clinical manifestations within the Tunisian family. The clinical presentation of affected individuals highlighted significant phenotypic heterogeneity, underscoring the complex genotype-phenotype correlations in familial Hypophosphatasia. Variability in age of onset, severity of symptoms, and radiographic features was observed, emphasizing the need for a nuanced understanding of the clinical spectrum associated with the p.Leu299Pro mutation. This study advances our understanding of familial Hypophosphatasia by delineating the molecular consequences of the p.Leu299Pro mutation in the ALPL gene. By integrating genetic, structural, and clinical analyses, we provide insights into disease pathogenesis and lay the groundwork for personalized therapeutic strategies tailored to specific genetic profiles. Our findings underscore the importance of comprehensive genetic and clinical evaluation in guiding precision medicine approaches for familial Hypophosphatasia.

Hypophosphatasia: A Unique Disorder of Bone Mineralization

Hypophosphatasia (HPP) is a rare genetic disease characterized by a decrease in the activity of tissue non-specific alkaline phosphatase (TNSALP). TNSALP is encoded by the ALPL gene, which is abundantly expressed in the skeleton, liver, kidney, and developing teeth. HPP exhibits high clinical variability largely due to the high allelic heterogeneity of the ALPL gene. HPP is characterized by multisystemic complications, although the most common clinical manifestations are those that occur in the skeleton, muscles, and teeth. These complications are mainly due to the accumulation of inorganic pyrophosphate (PPi) and pyridoxal-5′-phosphate (PLP). It has been observed that the prevalence of mild forms of the disease is more than 40 times the prevalence of severe forms. Patients with HPP present at least one mutation in the ALPL gene. However, it is known that there are other causes that lead to decreased alkaline phosphatase (ALP) levels without mutations in the ALPL gene. Although the phenotype can be correlated with the genotype in HPP, the prediction of the phenotype from the genotype cannot be made with complete certainty. The availability of a specific enzyme replacement therapy for HPP undoubtedly represents an advance in therapeutic strategy, especially in severe forms of the disease in pediatric patients.

Clinical and genetic characteristics of hypophosphatasia in Chinese children

BACKGROUND

Hypophosphatasia (HPP) is a rare inherited disorder, which is caused by loss-of-function mutations in the ALPL gene. HPP is a heterogeneous disease that has a wide spectrum of phenotypes. Few studies were carried out in the Chinese population with HPP, especially in children.

METHODS

The clinical and genetic characteristics of 10 Chinese children with HPP who were referred to the Beijing Children's Hospital were described. Previously reported HPP cases of children in China were also reviewed.

RESULTS

A total of 33 cases were identified, which included 2 perinatal lethal HPP, 10 infantile HPP, 10 childhood HPP, and 11 odonto HPP. The male-to-female ratio was 24:9. The average age at onset was 0.69 years (ranged from 2 h after birth to 14 years), while the average age at clinical diagnosis was 3.87 years (ranged from 2 h after birth to 19 years). Serum alkaline phosphatase (ALP) levels were significantly decreased in patients with perinatal lethal/infantile HPP when compared with those with the mild forms of HPP childhood/odonto HPP (P < 0.01). Although serum phosphate levels were not different (P > 0.05), serum calcium levels were elevated, and serum intact parathyroid hormone levels were decreased in patients with perinatal lethal/infantile HPP in comparison with those with the childhood/odonto HPP (P all < 0.01). Genetic analyses identified 40 mutations in 31 HPP cases, including 28 missense mutations, 9 frameshift mutations, 2 splice junction alterations, and 1 regulatory mutation. Of which, 5 novel mutations were identified in our present study: 2 frameshift mutations (p.Arg138GlyfsTer27, p.Leu511Profs*272); 2 missense mutations (p.Ala176Val, p.Phe268Leu), and 1 splice junction alteration (c.297+5G>A). Compound heterozygous mutations accounted for 80.6% of all variants. No mutational "hot-spot" was found. Most mutations of ALPL were located in exons 5, 7, 10, and 3. Notably, subjects that carrying single heterozygous mutations showed milder phenotypes of HPP, while subjects with nonsense mutations were associated with a severer phenotype.

CONCLUSIONS

HPP is a rare disease with often delayed diagnosis, and the incidence of HPP in China may be seriously underestimated. The present study expands the phenotypic and genotypic spectrum and the understanding of HPP in Chinese children. These findings will be useful for clinical assessment and shorten the diagnosis time for pediatric HPP in China.

Hypophosphatasia: Current Literature for Pathophysiology, Clinical Manifestations, Diagnosis, and Treatment

Hypophosphatasia (HPP) is a rare inherited bone disorder identified by impaired bone mineralization. There are seven subtypes of HPP mainly characterized by their age of onset. These subtypes consist of perinatal (prenatal) benign, perinatal lethal, infantile, childhood, adult, odontohypophosphatasia, and pseudohypophosphatasia. Due to limited awareness of the condition, either misdiagnosis or delayed diagnosis is common. Furthermore, the condition is frequently treated with contraindicated drugs. This literature illustrates the most recent findings on the etiology, pathophysiology, clinical manifestations, diagnosing, and treatment for HPP and its subtypes. The etiology of the disease consists of loss-of-function mutations of the ALPL gene on chromosome one, which encodes for tissue nonspecific isoenzyme of alkaline phosphatase (TNAP). A decrease of TNAP reduces inorganic phosphate (Pi) for bone mineralization and allows for an increase in inorganic pyrophosphate (PPi) and phosphorylated osteopontin (p-OPN), which further reduces bone mineralization. The combination of these processes softens bone and mediates a clinical presentation similar to rickets/osteomalacia. HPP has an additional wide range of clinical features depending on its subtype. Although a concrete diagnostic guideline has not yet been established, many studies have supported a similar method of identifying HPP. Clinical features, radiological findings, and/or biomarker levels of the disorder should raise suspicion and encourage the inclusion of HPP as a differential diagnosis. Biomarkers, especially alkaline phosphatase (ALP), are major contributors to diagnosis. However, genetic testing is done for definitive diagnosis. The primary treatment for HPP is the reintroduction of TNAP as a recombinant enzyme called asfotase alfa. There are additional pharmaceutical treatments and in some cases, surgical intervention may be indicated. Pharmaceutical therapies such as bisphosphonates, denosumab, potent antiresorptive agents, and vitamin D are contraindicated in adults with HPP. We hope to raise awareness for HPP in order to prevent delayed diagnosis or misdiagnosis. We plan to encourage appropriate care and avoid treatments that may be contraindicating. We also encourage the development of a diagnostic guideline that will promote a consistently favorable patient prognosis.

Case Report: Efficacy of Reduced Doses of Asfotase Alfa Replacement Therapy in an Infant With Hypophosphatasia Who Lacked Severe Clinical Symptoms

BACKGROUND

Hypophosphatasia is a rare bone disease characterized by impaired bone mineralization and low alkaline phosphatase activity. Here, we describe the course of bone-targeted enzyme replacement therapy with asfotase alpha for a female infant patient with hypophosphatasia who lacked apparent severe clinical symptoms.

CASE PRESENTATION

The patient exhibited low serum alkaline phosphatase (60 U/L; age-matched reference range, 520-1,580) in a routine laboratory test at birth. Further examinations revealed skeletal demineralization and rachitic changes, as well as elevated levels of serum calcium (2.80 mmol/L; reference range, 2.25-2.75 mmol/L) and ionic phosphate (3.17 mmol/L; reference range, 1.62-2.48 mmol/L), which are typical features in patients with hypophosphatasia. Sequencing analysis of the tissue-nonspecific alkaline phosphatase (TNSALP) gene identified two pathogenic mutations: c.406C>T, p.Arg136Cys and c.979T>C, p.Phe327Leu. Thus, the patient was diagnosed with hypophosphatasia. At the age of 37 days, she began enzyme replacement therapy using asfotase alpha at the standard dose of 6 mg/kg/week. Initial therapy from the age of 37 days to the age of 58 days substantially improved rickets signs in the patient; it also provided immediate normalization of serum calcium and ionic phosphate levels. However, serum ionic phosphate returned to a high level (2.72 mmol/L), which was presumed to be a side effect of asfotase alpha. Thus, the patient's asfotase alfa treatment was reduced to 2 mg/kg/week, which allowed her to maintain normal or near normal skeletal features thereafter, along with lowered serum ionic phosphate levels. Because the patient exhibited slight distal metaphyseal demineralization in the knee at the age of 2 years and 6 months, her asfotase alfa treatment was increased to 2.4 mg/kg/week. No signs of deterioration in bone mineralization were observed thereafter. At the age of 3 years, the patient's motor and psychological development both appeared normal, compared with children of similar age.

CONCLUSION

This is the first report in which reduced doses of asfotase alfa were administered to an infant patient with hypophosphatasia who lacked apparent severe clinical symptoms. The results demonstrate the potential feasibility of a tailored therapeutic option based on clinical severity in patients with hypophosphatasia.