Hypophosphatasia: Validation and expansion of the clinical nosology for children from 25years experience with 173 pediatric patients

Hypophosphatasia (HPP) is caused by loss-of-function mutation(s) within the gene TNSALP that encodes the “tissue-nonspecific” isoenzyme of alkaline phosphatase (TNSALP). In HPP, inorganic pyrophosphate, an inhibitor of mineralization and substrate for TNSALP, accumulates extracellularly often leading to rickets or osteomalacia and tooth loss, and sometimes to craniosynostosis and calcium crystal arthropathies. HPP's remarkably broad-ranging expressivity spans stillbirth from profound skeletal hypomineralization to adult-onset dental problems or arthropathies without bone disease, which is largely explained by autosomal recessive versus autosomal dominant transmission from among several hundred, usually missense, TNSALP mutations. For clinical purposes, this expressivity has been codified according to absence or presence of skeletal disease and then patient age at presentation and diagnosis. Pediatric patients are reported principally with “odonto”, “childhood”, “infantile”, or “perinatal” HPP. However, this nosology has not been tested using a cohort of patients, and the ranges of the clinical and laboratory findings have not been defined and contrasted among these patient groups. To evaluate the extant nosology for HPP in children, we assessed our 25 years experience with 173 pediatric HPP patients. Data were exclusively from inpatient studies. The childhood form of HPP was further designated “mild” or “severe”. Here, we focused on demographic, clinical, and dual-energy X-ray absorptiometry parameters compared to data from healthy American children. The 173-patient cohort comprised 64 individuals with odonto HPP, 38 with mild childhood HPP, 58 with severe childhood HPP, and 13 with infantile HPP. None was a survivor of perinatal HPP. TNSALP analysis revealed a mutation(s) in all 105 probands tested. Thirteen mutations were unique. Most patients represented autosomal dominant inheritance of HPP. Mutant allele dosage generally indicated the disorder's severity. Gender discordance was found for severe childhood HPP; 42 boys versus 16 girls (p = 0.006), perhaps reflecting parental concern about stature and strength. Key disease parameters (e.g., height, weight, numbers of teeth lost prematurely, grip strength, spine and hip bone mineral density) were increasingly compromised as HPP was designated more severe. Although data overlapped successively between the four patient groups, body size (height and weight) differed significantly. Thus, our expanded nosology for HPP in children organizes the disorder's broad-ranging expressivity and should improve understanding of HPP presentation, natural history, complications, and prognosis.