Oral Symptoms and Bone Observations in Odonto-Hypophosphatasia

Hypophosphatasia (HPP) is an inherited skeletal disease caused by mutation of the gene encoding tissue non-specific alkaline phosphatase (TNSALP). Odonto-HPP is well known as the mildest of HPP. The manifestations involve only the teeth, such as premature primary teeth exfoliation caused by reduction of alveolar bone, enlarged dental pulp chamber, and dental defects. We report a case of a 9-years-old boy who developed HPP. He was observed from the primary dentition to the mixed dentition period. At initial presentation at our hospital, he had multiple premature exfoliation of primary teeth and reduction of the alveolar bone. HPP was suspected due to the low level of ALP activity in serum, his oral manifestation, and dental history. He was referred to a physician for the final diagnosis. Therefore his compound heterozygote mutations, c.1559 delT (T/delT) and c.407G > A (G/A), were found in TNSALP and he diagnosed with odonto-HPP. Even though these mutations were reported as being involved in odonto-HPP, his mineral densities tended to be lower than that of his age. It is therefore necessary to investigate the bone mineralization level in odonto-HPP without other bone symptoms. Moreover, ongoing enzyme-replacement therapy in odonto-HPP might improve dental abnormality and bone disorders.

Improvement of Bone and Dental Phenotype of Murine Hypophosphatasia Mediated by a Single Injection of Lentiviral Gene Therapy

Background: Alkaline phosphatase has 4 isozymes, tissue non-specific alkaline phosphatase (TNAP), placental alkaline phosphatase (PLAP), intestinal alkaline phosphatase and germ-cell alkaline phosphatase. Hypophosphatasia (HPP) is an inherited skeletal disease caused by mutations of the gene encoding TNAP. Although TNAP is expressed in various tissues, the primary HPP symptoms appear in bones and teeth. The clinical severity of HPP varies widely from the most severe (perinatal, infantile and childhood) to the mildest forms (adult, and odonto-hypophosphatasia). We reported that gene therapy using a single injection of lentiviral vector expressing bone-targeted TNAP (TNAP-D10) is effective in preventing all the skeletal of HPP in TNAP knockout (Alpl–/–) mice as the model of infantile HPP. Objective: In this study we focus on evaluating the efficacy of treatment with gene therapy on the bone and teeth using TNAP-D10 and also we investigate the feasibility of gene therapy using bone-targeted PLAP (PLAP-D10). Methods and Findings: We used Alpl–/–mice that develop skeletal disease at postnatal days 6-8 mimicking the infantile form of human HPP. We injected 100 μl of lentiviral vectors harboring TNALP-D10 (5.0 × 107 TU) or PLAP-D10 (5.0 × 107 TU) to 1-day-old Alpl–/–?mice via the jugular vein. We performed histological analysis and micro-CT evaluation on bone and mandible of Alpl–/–?mice. The alveolar bone, enamel and dentin defects were corrected on treated Alpl–/–?mice by this treatment. Additionally the long bone growth rates (LGR) of long bones were encouraged on treated Alpl–/–?mice compared with untreated mice. Conclusions: These results indicate that the bone-targeted TNAP treatment mediated by lentivirus can correct not only the bone disorder but also the dental symptoms in Alpl–/–. This study also shows that PLAP-D10 can potentially be used to correct HPP disease.