Gene Therapy Improves Dental Manifestations in Akp2^-/- Hypophosphatasia Mice

Objectives: Hypophosphatasia is characterized by defective bone mineralization and caused by mutations in the gene encoding tissue non-specific alkaline phosphatase (TNALP). As for dental manifestations, premature loss of primary teeth due to disturbed cementum formation is well known. Recently, gene therapy for TNALP knockout mice (Akp2^-/-), which display a phenocopy of hypophosphatasia, was shown to successfully improve general conditions. The purpose of the present study was to analyze the effects of gene therapy on dental tissues in those mice.
Methods: Following sub-lethal irradiation (4 Gy) at the age of 2 days, Akp2^-/- mice underwent gene therapy using bone marrow cells transduced with a lentiviral vector expressing a bone-targeted form of TNALP injected into the jugular vein. One month later, the mice were euthanized, then the mandibles were extirpated and performed micro-CT analysis to assess alveolar bone formation. They were conventionally processed for paraffin embedding, and immunohistochemical analysis of osteopontin, a marker of acellular cementum in incisor sections, was performed. Wild type and Akp2^-/- mice without treatment at the age of 1 month were also analyzed.
Results: Micro-CT analysis showed increased alveolar bone mineral density in Akp2^-/- mice that underwent gene therapy nearly similar level in wild type. Improvement in cementum formation was also identified in treated Akp2^-/- mice and osteopontin was detected immunohistochemically on the incisor root surfaces. However, that formation and expression did not recover to an adequate level in wild type mice. In addition, abnormal enamel and dentin formation was found in the treated Akp2^-/- mice.
Conclusions: Our findings suggest that the gene therapy could improve not only general but also dental conditions, especially alveolar bone mineral density and cementum formation, in hypophosphatasia model mice, whereas abnormal formation of enamel and dentin could be caused by sub-lethal irradiation.