SUN-LB64 Urine Phosphoethanolamine Is an Underutilized Biomarker for Hypophosphastasia

Abstract

Hypophosphatasia (HPP) is a rare disease caused by a loss-of-function mutation in the ALPL gene leading to a deficiency in the tissue-non-specific isoenzyme alkaline phosphatase (ALP) and excess of extracellular inorganic pyrophosphate (PPi) and pyridoxal 5’phosphate (PLP). Patients with HPP have a widely variable clinical phenotype, from neonatal seizures and hypomineralization to isolated dental or joint disease in adults. Patients present with ALP activity levels below their age-adjusted reference interval and elevations in PLP. This variable clinical phenotype leads to significant diagnostic challenges, particularly in those with minor disease manifestations. Biomarkers for diagnosing HPP and monitoring patients on enzyme replacement therapy (ERT) are limited. Low ALP activity is not specific for HPP; it can be low in other conditions including untreated hypothyroidism, and other skeletal dysplasias. Accurate PLP assessment is limited in patients on ERT due to hydrolysis of the substrate. Phosphoethanolamine (PEA) is a substrate hydrolyzed by TNSALP and elevated levels of PEA may be observed in HPP, supporting the diagnosis of HPP, but have been reported to be non-specific. We hypothesized that urine PEA levels could be used to diagnose HPP and as a surrogate marker for ERT compliance. We performed a retrospective analysis on 83 adult patients (63F: 20M) presenting for investigation or follow-up of HPP from 7/1/2014 to 1/7/2020. Urine was sent to the Children’s Hospital in Colorado for PEA testing. Patients were diagnosed with HPP if they had ALP levels persistently below the age-adjusted reference interval, were positive for an ALPL gene mutation/had a family history of HPP and had musculoskeletal/dental symptoms consistent with the disease. Patients were negative for HPP if ALP was not persistently low, vitamin B6 levels were normal without supplementation, negative ALPL genetic testing results (when available) and no musculoskeletal/dental symptoms. PEA levels were not considered for diagnosis. The following was collected from the EMR: PEA results, diagnosis, and ERT status. PEA levels were higher in patients with HPP not on ERT (median=117.0, N=39) vs. those negative for HPP (median=24.0, N=15, p<0.0001). The receiver operator curve for PEA in diagnosing HPP had an AUC of 0.94 (SE=0.03, p<0.0001). Specificity was 100% at PEA levels >53.5 ng/ml (sensitivity=79.5%) with an NPV of 65.2% and PPV of 100%. Initial PEA values in patients with HPP and not on ERT were higher (median 117.0, N=39) than patients on ERT (median 65.0, N=16, p=0.004). Patients who began ERT had a decline in PEA levels after treatment (mean decrease 68.1%). PEA is a specific diagnostic marker for HPP in patients undergoing investigation for HPP and may be used as a surrogate marker to monitor ERT compliance. Future studies are necessary to evaluate the association between PEA levels and functional performance.