Gene therapy improves dental manifestations in hypophosphatasia model mice

Dental manifestations of hypophosphatasia: translational and clinical advances

Hypophosphatasia (HPP) is an inherited error in metabolism resulting from loss-of-function variants in the ALPL gene, which encodes tissue-nonspecific alkaline phosphatase (TNAP). TNAP plays a crucial role in biomineralization of bones and teeth, in part by reducing levels of inorganic pyrophosphate (PPi), an inhibitor of biomineralization. HPP onset in childhood contributes to rickets, including growth plate defects and impaired growth. In adulthood, osteomalacia from HPP contributes to increased fracture risk. HPP also affects oral health. The dentoalveolar complex, that is, the tooth and supporting connective tissues of the surrounding periodontia, include 4 unique hard tissues: enamel, dentin, cementum, and alveolar bone, and all can be affected by HPP. Premature tooth loss of fully rooted teeth is pathognomonic for HPP. Patients with HPP often have complex oral health issues that require multidisciplinary dental care, potentially involving general or pediatric dentists, periodontists, prosthodontists, and orthodontists. The scientific literature to date has relatively few reports on dental care of individuals with HPP. Animal models to study HPP included global Alpl knockout mice, Alpl mutation knock-in mice, and mice with tissue-specific conditional Alpl ablation, allowing for new studies on pathological mechanisms and treatment effects in dental and skeletal tissues. Enzyme replacement therapy (ERT) in the form of injected, recombinant mineralized tissue-targeted TNAP has been available for nearly a decade and changed the prognosis for those with HPP. However, effects of ERT on dental tissues remain poorly defined and limitations of the current ERT have prompted exploration of gene therapy approaches to treat HPP. Preclinical gene therapy studies are promising and may contribute to improved oral health in HPP.

Different Dental Manifestations in Sisters with the Same ALPL Gene Mutation: A Report of Two Cases

Hypophosphatasia (HPP) is an inherited disease caused by mutation of the alkaline phosphatase (ALPL) gene in an autosomal dominant or an autosomal recessive manner. The main symptoms of HPP are bone hypomineralization and early exfoliation of the primary teeth. Some of the mutations identified in autosomal dominant families are reported to have dominant negative effects. In addition, the penetrance can vary among patients with the same variant even within the same family, resulting in various phenotypes of systemic symptoms. However, differences in dental symptoms between patients with HPP and carriers with the same ALPL variant have not been reported. Herein, we report on two sisters who had the same heterozygous ALPL variant with dominant negative effects. The older sister had bone and dental symptoms and was diagnosed with childhood HPP. In contrast, the younger sister was a carrier with no bone and dental symptoms. It can be inferred that this phenomenon was caused by the difference in penetrance. This case revealed that carriers with the ALPL mutation may have no dental symptoms characteristic of HPP. Because HPP is sometimes progressive, it is very important to carefully monitor carriers to detect the possible onset of dental and systemic symptoms.

Dental manifestation and management of hypophosphatasia

Hypophosphatasia is an inherited metabolic disorder characterized by defective mineralization of bones and teeth with a wide variety of manifestations, ranging from stillbirth to dental symptoms alone. Recently, the prognosis of severe hypophosphatasia patients has been greatly improved by the introduction of enzyme replacement therapy. The typical dental manifestation is early exfoliation of primary teeth due to disturbed cementum formation, so dentures are recommended to ensure that important oral functions are acquired. Some studies have shown that enzyme replacement therapy improves dental mineralization, resulting in the stabilization of periodontal tissues and better growth of tooth roots. A nationwide Japanese survey revealed the common genetic and dental manifestations of patients with mild hypophosphatasia, which markedly differ from those of the severe forms. There may be many undiagnosed mild patients, so dentists should contribute to the early diagnosis by screening possible cases based on the typical finding of early exfoliation of primary teeth. Early diagnosis is important for patients to receive early intervention in both medical and dental fields. The establishment of fundamental dental therapy to solve the dental problems is still underway and is eagerly anticipated.

Dental outcomes for children receiving asfotase alfa for hypophosphatasia

INTRODUCTION

Hypophosphatasia, a genetic disease impeding development of teeth and bones, is associated with premature exfoliation of primary teeth. Hypophosphatasia is caused by mutations in the ALPL gene, which encodes the tissue non-specific form of alkaline phosphatase. Asfotase alfa (Strensiq®) is a human recombinant bone-targeted alkaline phosphatase.

OBJECTIVES

To review development and exfoliation patterns of primary/permanent teeth in a cohort of patients with hypophosphatasia enrolled in an open-label clinical trial of enzyme replacement therapy (ERT) with asfotase alfa.

METHODS

Data were collected from existing study files of a cohort of patients ≤5 years of age with infantile hypophosphatasia. Children were recruited at the Winnipeg site of a global clinical trial and were treated with ERT. Dental information, including the exfoliation/eruption patterns, were recorded at each visit.

RESULTS

Eleven children (7 females, 4 males) participated. Participants enrolled as infants (5 infants; mean age 3.0 ± 2.3 months) prematurely lost significantly fewer teeth to hypophosphatasia than patients recruited as preschoolers (6 preschoolers; mean age 52.5 ± 11.3 months), who started on asfotase alfa at a later age. Conclusion The oral health of children with early onset infantile hypophosphatasia may be improved with early and continued administration of ERT, compared to institution of therapy later in childhood.

PDGFBB-modified stem cells from apical papilla and thermosensitive hydrogel scaffolds induced bone regeneration

Bone defects caused by cancer surgery or trauma have a strong negative impact on human health. Treatment with cell and material-based complexes provides an alternative strategy for the regeneration of damaged bone tissue. The good physical properties and suitable biodegradability of a thermosensitive hydrogel has been shown to act as a valuable scaffold. Platelet derived growth factor BB (PDGFBB) is mainly secreted by platelets and promotes the migration and angiogenesis of mesenchymal stem cells (MSCs). Although PDGFBB is known to indirectly enhance bone repair in vivo, the effects of PDGFBB on stem cells from apical papilla (SCAPs) require further investigation. In our study, the proliferation of cells was investigated by the cell counting kit-8 and live/dead staining methods. The results indicated that PDGFBB promoted the proliferation of SCAPs. Real-time polymerase chain reaction and Western blot experiments were used to detect osteogenic genes and proteins. Moreover, calvarial defects were created in Sprague-Dawley rats and different complexes implanted. The results shown by micro-CT and hematoxylin and eosin analysis demonstrated that the hydrogel combined with lentiviral supernatant-green fluorescent protein-PDGFBB significantly improved new bone formation and mineralization compared with the other three groups. In summary, our research showed that a thermosensitive hydrogel can be used as a scaffold for 3D cell culture, and PDGFBB gene-modified SCAPs can improve bone formation in calvarial defects.

Suitability and limitations of mesenchymal stem cells to elucidate human bone illness

Functional impairment of mesenchymal stem cells (MSCs), osteoblast progenitor cells, has been proposed to be a pathological mechanism contributing to bone disorders, such as osteoporosis (the most common bone disease) and other rare inherited skeletal dysplasias. Pathological bone loss can be caused not only by an enhanced bone resorption activity but also by hampered osteogenic differentiation of MSCs. The majority of the current treatment options counteract bone loss, and therefore bone fragility by blocking bone resorption. These so-called antiresorptive treatments, in spite of being effective at reducing fracture risk, cannot be administered for extended periods due to security concerns. Therefore, there is a real need to develop osteoanabolic therapies to promote bone formation. Human MSCs emerge as a suitable tool to study the etiology of bone disorders at the cellular level as well as to be used for cell therapy purposes for bone diseases. This review will focus on the most relevant findings using human MSCs as an in vitro cell model to unravel pathological bone mechanisms and the application and outcomes of human MSCs in cell therapy clinical trials for bone disease.

Alkaline Phosphatase Replacement Therapy for Hypophosphatasia in Development and Practice

Hypophosphatasia (HPP) is an inherited disorder that affects bone and tooth mineralization characterized by low serum alkaline phosphatase. HPP is caused by loss-of-function mutations in the ALPL gene encoding the protein, tissue-nonspecific alkaline phosphatase (TNSALP). TNSALP is expressed by mineralizing cells of the skeleton and dentition and is associated with the mineralization process. Generalized reduction of activity of the TNSALP leads to accumulation of its substrates, including inorganic pyrophosphate (PP_i) that inhibits physiological mineralization. This leads to defective skeletal mineralization, with manifestations including rickets, osteomalacia, fractures, and bone pain, all of which can result in multi-systemic complications with significant morbidity, as well as mortality in severe cases. Dental manifestations are nearly universal among affected individuals and feature most prominently premature loss of deciduous teeth. Management of HPP has been limited to supportive care until the introduction of a TNSALP enzyme replacement therapy (ERT), asfotase alfa (AA). AA ERT has proven to be transformative, improving survival in severely affected infants and increasing overall quality of life in children and adults with HPP. This chapter provides an overview of TNSALP expression and functions, summarizes HPP clinical types and pathologies, discusses early attempts at therapies for HPP, summarizes development of HPP mouse models, reviews design and validation of AA ERT, and provides up-to-date accounts of AA ERT efficacy in clinical trials and case reports, including therapeutic response, adverse effects, limitations, and potential future directions in therapy.

Hypophosphatasia: oral cavity and dental disorders

Dental anomalies exist in every subtype of hypophosphatasia (HPP), from the most severe to the most moderate, called odontohypophosphatasia. The forms are defined by the age at onset of the initial symptoms. These anomalies affect all dental mineralized tissues from enamel, dentin and cementum to alveolar bone in a gradient proportional to the severity of the disease. Early loss of the deciduous teeth, before 3 years of age, and then possibly of the permanent teeth, is due to an abnormality of the cementum, the tissue attaching the teeth to alveolar bone, and is the most frequent abnormality. Tooth loss is a very important diagnostic sign and needs to be recognized. Patients with HPP need specialized oral and dental care in coordination with the reference and expert centers. The oral and dental signs and their treatment remain poorly known. The recording of the abnormalities and their treatment in a registry is indispensable in order to enhance patient management and oral and dental health.

Bone-Targeted Alkaline Phosphatase Treatment of Mandibular Bone and Teeth in Lethal Hypophosphatasia via an scAAV8 Vector

Hypophosphatasia is an inherited disease caused by mutations in the gene encoding tissue-nonspecific alkaline phosphatase (TNALP), the major symptom of which is hypomineralization of the bones and teeth. We had recently demonstrated that TNALP-deficient (Akp2^−/−) mice, which mimic the phenotype of the severe infantile form of hypophosphatasia, can be treated by intramuscular injection of a self-complementary (sc) type 8 recombinant adeno-associated virus (rAAV8) vector expressing bone-targeted TNALP with deca-aspartates at the C terminus (TNALP-D₁₀) via the muscle creatine kinase (MCK) promoter. In this study, we focused on the efficacy of this scAAV8-MCK-TNALP-D₁₀ treatment on the mandibular bone and teeth in neonatal Akp2^−/− mice. Upon scAAV8-MCK-TNALP-D₁₀ injection, an improvement of mandibular growth was observed by X-ray analysis. Micro-computed tomography analysis revealed progressive mineralization of the molar root in the treated Akp2^−/− mice, and morphometric parameters of the alveolar bone were improved. These results suggest that the mandibular bones and teeth of hypophosphatasia were effectively treated by muscle directed rAAV-mediated TNALP-D₁₀ transduction. Our strategy would be promising for future hypophosphatasia gene therapy because it induces dentoalveolar mineralization and reduces the risk of tooth exfoliation.