Rescue of Severe Infantile Hypophosphatasia Mice by AAV-Mediated Sustained Expression of Soluble Alkaline Phosphatase

Abstract

Hypophosphatasia (HPP) is an inherited disease caused by a deficiency of tissue-nonspecific alkaline phosphatase (TNALP). The major symptom of human HPP is hypomineralization, rickets, or osteomalacia, although the clinical severity is highly variable. The phenotypes of TNALP knockout (Akp2^-/-) mice mimic those of the severe infantile form of HPP. Akp2^-/- mice appear normal at birth, but they develop growth failure, epileptic seizures, and hypomineralization and die by 20 days of age. Previously, we have shown that the phenotype of Akp2^-/- mice can be prevented by enzyme replacement of bone-targeted TNALP in which deca-aspartates are linked to the C-terminus of soluble TNALP (TNALP-D10). In the present study, we evaluated the therapeutic effects of adeno-associated virus serotype 8 (AAV8) vectors that express various forms of TNALP, including TNALP-D10, soluble TNALP tagged with the Flag epitopes (TNALP-F), and native glycosylphosphatidylinositol-anchored TNALP (TNALP-N). A single intravenous injection of 5×10¹⁰ vector genomes of AAV8-TNALP-D10 into Akp2^-/- mice at day 1 resulted in prolonged survival and phenotypic correction. When AAV8-TNALP-F was injected into neonatal Akp2^-/- mice, they also survived without epileptic seizures. Interestingly, survival effects were observed in some animals treated with AAV8-TNALP-N. All surviving Akp2^-/- mice showed a healthy appearance and a normal activity with mature bone mineralization on X-rays. These results suggest that sustained alkaline phosphatase activity in plasma is essential and sufficient for the rescue of Akp2^-/- mice. AAV8-mediated systemic gene therapy appears to be an effective treatment for the infantile form of human HPP. Matsumoto and colleagues evaluate the therapeutic effects of adeno-associated virus serotype 8 (AAV8) vectors that express various forms of tissue-nonspecific alkaline phosphatase (TNALP) in a mouse model of hypophosphatasia (HPP). A single intravenous injection of 5 × 10¹⁰ vector genomes of AAV8-TNALP in neonatal HPP mice results in prolonged survival and phenotypic correction.