Gene Therapy Using Adeno‐Associated Virus Serotype 8 Encoding TNAP‐D10 Improves the Skeletal and Dentoalveolar Phenotypes in Alpl −/− Mice

Hypophosphatasia (HPP) is caused by loss‐of‐function mutations in the ALPL gene that encodes tissue‐nonspecific alkaline phosphatase (TNAP), whose deficiency results in the accumulation of extracellular inorganic pyrophosphate (PPi), a potent mineralization inhibitor. Skeletal and dental hypomineralization characterizes HPP, with disease severity varying from life‐threatening perinatal or infantile forms to milder forms that manifest in adulthood or only affect the dentition. Enzyme replacement therapy (ERT) using mineral‐targeted recombinant TNAP (Strensiq/asfotase alfa) markedly improves the life span, skeletal phenotype, motor function, and quality of life of patients with HPP, though limitations of ERT include frequent injections due to a short elimination half‐life of 2.28 days and injection site reactions. We tested the efficacy of a single intramuscular administration of adeno‐associated virus 8 (AAV8) encoding TNAP‐D10 to increase the life span and improve the skeletal and dentoalveolar phenotypes in TNAP knockout (Alpl−/−) mice, a murine model for severe infantile HPP. Alpl−/− mice received 3 × 1011 vector genomes/body of AAV8‐TNAP‐D10 within 5 days postnatal (dpn). AAV8‐TNAP‐D10 elevated serum ALP activity and suppressed plasma PPi. Treatment extended life span of Alpl−/− mice, and no ectopic calcifications were observed in the kidneys, aorta, coronary arteries, or brain in the 70 dpn observational window. Treated Alpl−/− mice did not show signs of rickets, including bowing of long bones, enlargement of epiphyses, or fractures. Bone microstructure of treated Alpl−/− mice was similar to wild type, with a few persistent small cortical and trabecular defects. Histology showed no measurable osteoid accumulation but reduced bone volume fraction in treated Alpl−/− mice versus controls. Treated Alpl−/− mice featured normal molar and incisor dentoalveolar tissues, with the exceptions of slightly reduced molar enamel and alveolar bone density. Histology showed the presence of cementum and normal periodontal ligament attachment. These results support gene therapy as a promising alternative to ERT for the treatment of HPP. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).