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ABSTRACT
Hypophosphatasia (HPP) often leads to premature loss of deciduous teeth, due to disturbed cementum formation. We addressed the question to what extent cementum and dentin are similarly affected. To this end, we compared teeth from children with HPP with those from matched controls and analyzed them microscopically and chemically. It was observed that both acellular and cellular cementum formation was affected. For dentin, however, no differences in mineral content were recorded. To explain the dissimilar effects on cementum and dentin in HPP, we assessed pyrophosphate (an inhibitor of mineralization) and the expression/activity of enzymes related to pyrophosphate metabolism in both the periodontal ligament and the pulp of normal teeth. Expression of nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) in pulp proved to be significantly lower than in the periodontal ligament. Also, the activity of NPP1 was less in pulp, as was the concentration of pyrophosphate. Our findings suggest that mineralization of dentin is less likely to be under the influence of the inhibitory action of pyrophosphate than mineralization of cementum.
KEY WORDS: alkaline phosphatase, hypophosphatasia, mineralization.
Received December 22, 2004; Last revision July 27, 2005; Accepted July 27, 2005
INTRODUCTION
Hypophosphatasia (HPP) is a rare, inherited bone disease characterized by reduced activity of the tissue non-specific (liver/bone/kidney) isoenzyme of alkaline phosphatase (TNSALP), due to deactivating mutations within the TNSALP gene (Whyte, 2001). A consistent feature of HPP is premature loss of deciduous teeth (Chapple, 1993), which is attributed to disturbed cementum formation (e.g., Beumer et al., 1973; Kjellman et al., 1973; Lundgren et al., 1991). With respect to the dentition, it is still unclear whether HPP becomes manifest specifically in the cemental tissues, or may also affect the dentin. Although several authors (e.g., Beumer et al., 1973; Hu et al., 2000) have reported that dentin has reduced thickness and mineral content, others (e.g., Baab et al., 1985; El-Labban et al., 1991) did not find any defects. It was the aim of the present study to determine whether cementum and dentin are similarly affected in HPP. We addressed this question in a group of well-defined HPP patients vs. matched healthy controls. Next to microscopic and chemical analyses of the dental hard tissues, we studied the expression/activity of enzymes in the periodontal ligament and pulp that...