Hypophosphatasia: A Novel Mutation Associated with an Atypical Newborn Presentation

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.

Perinatal Hypophosphatasia in a Premature Infant

A premature male infant was delivered at 32 weeks' gestation due to category-2 fetal tracing after preterm labor. The physical exam showed shortened and bowed long bones, with calvarium felt in small area of the head. Serum alkaline phosphatase was very low on admission. Extensive metaphyseal abnormalities, bowing of long bones, and poor ossification of all bones were noted on skeletal survey (radiography). Based on ultrasound evidence of "bowing" and long bone fractures at 26 weeks, amniocentesis was performed that later diagnosed hypophosphatasia by genetic testing while ruling out osteogenesis imperfecta. Although ventilated initially, the infant gradually improved with enzyme replacement therapy (ERT) and was extubated to noninvasive ventilation for 6 weeks. Following clinical deterioration with hypoxic respiratory failure secondary to sepsis at 4 months of age, he succumbed to severe pulmonary hypertension, likely secondary to chronic lung disease and prolonged ventilation. Early diagnosis allowed timely initiation of appropriate therapy. Radiological improvement with the therapy showed promising results in this rare disease. Despite specific novel therapy being available, variability in presentation dictates prognosis in this previously universally fatal condition. The potential unknown effects of ERT on pulmonary vascular remodeling need further investigation.