Gene Therapy Using Recombinant AAV Type 8 Vector Encoding TNAP‐D(10) Improves the Skeletal Phenotypes in Murine Models of Osteomalacia

Hypophosphatasia (HPP), caused by loss‐of‐function mutations in the ALPL gene encoding tissue‐nonspecific alkaline phosphatase (TNAP), is characterized by skeletal and dental hypomineralization that can vary in severity from life‐threatening to milder manifestations only in adulthood. PHOSPHO1 deficiency leads to early‐onset scoliosis, osteomalacia, and fractures that mimic pseudo‐HPP. Asfotase alfa, a life‐saving enzyme replacement therapy approved for pediatric‐onset HPP, requires subcutaneous injections 3 to 6 times per week. We recently showed that a single injection of an adeno‐associated virus vector serotype 8 harboring TNAP‐D(10) (AAV8‐TNAP‐D(10)) effectively prevented skeletal disease and prolonged life in Alpl ( −/− ) mice phenocopying infantile HPP. Here, we aimed to determine the efficacy of AAV8‐TNAP‐D(10) in improving the skeletal and dental phenotype in the Alpl ( Prx1/Prx1 ) and Phospho1 (−/−) mouse models of late‐onset (adult) HPP and pseudo‐HPP, respectively. A single dose of 3 × 10(11) vector genomes per body (vg/b) was injected intramuscularly into 8‐week‐old Alpl ( Prx1/Prx1 ) and wild‐type (WT) littermates, or into 3‐day‐old Phospho1 (−/−) and WT mice, and treatment efficacy was evaluated after 60 days for late‐onset HPP mice and after 90 days for Phospho1 (−/−) mice. Biochemical analysis showed sustained serum alkaline phosphatase activity and reduced plasma PP(i) levels, and radiographic images, micro‐computed tomography (micro‐CT) analysis, and hematoxylin and eosin (H&E) staining showed improvements in the long bones in the late‐onset HPP mice and corrected scoliosis in the Phospho1 ( −/− ) mice. Micro‐CT analysis of the dentoalveolar complex did not reveal significant changes in the phenotype of late‐onset HPP and pseudo‐HPP models. Moreover, alizarin red staining analysis showed that AAV8‐TNAP‐D(10) treatment did not promote ectopic calcification of soft organs in adult HPP mice after 60 days of treatment, even after inducing chronic kidney disease. Overall, the AAV8‐TNAP‐D(10) treatment improved the skeletal phenotype in both the adult HPP and pseudo‐HPP mouse models. This preclinical study will contribute to the advancement of gene therapy for the improvement of skeletal disease in patients with heritable forms of osteomalacia. © 2022 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.