Gene Therapy Using Recombinant AAV Type 8 Vector Encoding TNAP-D Improves the Skeletal Phenotypes in Murine Models of Osteomalacia.

Hypophosphatasia (HPP), caused by loss-of-function mutations in the 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 (AAV8-TNAP-D) effectively prevented skeletal disease and prolonged life in mice phenocopying infantile HPP. Here, we aimed to determine the efficacy of AAV8-TNAP-D in improving the skeletal and dental phenotype in the and mouse models of late-onset (adult) HPP and pseudo-HPP, respectively. A single dose of 3 × 10 vector genomes per body (vg/b) was injected intramuscularly into 8-week-old and wild-type (WT) littermates, or into 3-day-old and WT mice, and treatment efficacy was evaluated after 60 days for late-onset HPP mice and after 90 days for mice. Biochemical analysis showed sustained serum alkaline phosphatase activity and reduced plasma PP 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 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 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 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. published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.