Rescue of severe infantile hypophosphatasia mice by AAV-mediated sustained expression of soluble alkaline phosphatase

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 5×10 ¹⁰ 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.