Proof-of-principle for enhanced dentoalveolar mineralization using exogenous tissue-nonspecific alkaline phosphatase in the Hyp mouse model of X-linked hypophosphatemia.

X-linked hypophosphatemia (XLH) is caused by mutations in the PHEX gene, which leads to increased levels of fibroblast growth factor 23 and hypophosphatemia, contributing to rickets, osteomalacia, and dentoalveolar defects, including severe dentin hypomineralization, thin cementum, and alveolar bone osteomalacia. Current XLH treatment options appear to have limited efficacy on dentoalveolar tissues, suggesting underlying disease mechanisms that remain unchecked. Increased production of inorganic pyrophosphate (PPi) and osteopontin (OPN), both mineralization inhibitors, has been posited to contribute to mineralization defects in XLH. The enzyme, tissue-nonspecific alkaline phosphatase (TNAP) reduces PPi levels via hydrolysis and inactivates OPN by dephosphorylation. Our previous study showed improved alveolar bone socket healing in Hyp mice administered mineralized tissue-targeted TNAP (TNAP-Fc-D10). We hypothesized that increased TNAP would partially ameliorate developmental mineralization defects in XLH by dually reducing PPi levels and dephosphorylating and inactivating OPN. In a proof-of-principle study to investigate pathological mechanisms, we delivered systemic (subcutaneous injection) and local (submucosal injection to mandibles) TNAP-Fc-D10 injections to the Hyp mutant mouse model of XLH from 7 to 60 d postnatal (dpn). While systemic delivery was ineffective at improving dentin or bone properties, micro-CT and histology analyses demonstrated that local delivery of TNAP-Fc-D10 increased dentin thickness, root length, alveolar bone volume, alveolar bone proper (ABP) volume and density, PDL attachment, and acellular cementum thickness, compared to control Hyp mice receiving a sham injection. Dynamic mechanical testing confirmed partially improved mechanical properties in locally treated vs untreated Hyp mice, suggesting incompletely improved periodontal function. Quantitative PCR revealed increased Dspp expression in molars of treated Hyp mice. In conclusion, we found TNAP administration reduced dentoalveolar defects in Hyp mice when delivered locally into dentoalveolar structures, proof-of-principle pointing to a pathological contribution by PPi and/or OPN and highlighting a promising adjunctive approach considering limitations of current treatment modalities.