Involvement of impaired phosphate production and aberrant extracellular ATP signaling in the pathogenesis of hypophosphatasia: Analysis of ALPL-Knockout human iPS cell models

Hypophosphatasia (HPP) is caused by inactivating variants of ALPL, the gene encoding tissue non-specific alkaline phosphatase (TNSALP). In order to deepen our understanding of the pathogenic mechanisms of HPP, we herein generated ALPL-knockout (KO) human induced pluripotent stem (iPS) cells by applying CRISPR/Cas9-mediated gene deletion to an iPS clone derived from a healthy subject. We analyzed two ALPL-KO clones, one ALPL-hetero KO clone, and a control clone isogenic except for ALPL. In an osteogenic culture using β-glycerophosphate, which generates inorganic phosphate (Pi) by TNSALP-mediated degradation, ALPL-KO clones showed impaired mineralization, elevated levels of extracellular pyrophosphate (PPi), and reduced levels of extracellular Pi. Osteogenic induction using 3 mM Pi instead of β-glycerophosphate rescued the decreased content of hydroxyapatite in ALPL-KO cells despite the still high levels of extracellular PPi; however, abnormal distribution of hydroxyapatite was noted. Osteoblast lineage cells differentiated from ALPL-KO iPS clones showed the up-regulation of SPP1 and the down-regulation of ANKH and the genes for type III sodium/phosphate co-transporters in the culture using β-glycerophosphate, but not when 3 mM Pi was used. Extracellular ATP levels were elevated in osteoblast lineage cells derived from ALPL-KO iPS clones in both culture conditions, which was associated with the down-regulation of P2X7 encoding a purinergic receptor. Moreover, osteoblast lineage cells differentiated from ALPL-KO iPS clones in the culture using β-glycerophosphate showed a change in cellular response to extracellular Pi. These results suggest that the reduced local production of extracellular Pi and aberrant ATP signaling play substantial roles in the pathogenesis of HPP.