Hypophosphatasia-associated deficiencies in mineralization and gene expression in cultured dental pulp cells obtained from human teeth

INTRODUCTION

Mutations in the gene ALPL in hypophosphatasia (HPP) reduce the function of tissue nonspecific alkaline phosphatase, and the resulting increase in pyrophosphate (PP(i)) contributes to bone and tooth mineralization defects by inhibiting physiologic calcium-phosphate (P(i)) precipitation. Although periodontal phenotypes are well documented, pulp/dentin abnormalities have been suggested in the clinical literature although reports are variable and underlying mechanisms remains unclear. In vitro analyses were used to identify mechanisms involved in HPP-associated pulp/dentin phenotypes.

METHODS

Primary pulp cells cultured from HPP subjects were established to assay alkaline phosphatase (ALP) activity, mineralization, and gene expression compared with cells from healthy controls. Exogenous P(i) was provided to the correct P(i)/PP(i) ratio in cell culture.

RESULTS

HPP cells exhibited significantly reduced ALP activity (by 50%) and mineral nodule formation (by 60%) compared with the controls. The expression of PP(i) regulatory genes was altered in HPP pulp cells, including reduction in the progressive ankylosis gene (ANKH) and increased ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Odontoblast marker gene expression was disrupted in HPP cells, including reduced osteopontin (OPN), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoprotein (MEPE). The addition of P(i) provided a corrective measure for mineralization and partially rescued the expression of some genes although cells retained altered messenger RNA levels for PP(i)-associated genes.

CONCLUSIONS

These studies suggest that under HPP conditions pulp cells have the compromised ability to mineralize and feature a disrupted odontoblast profile, providing a first step toward understanding the molecular mechanisms for dentin phenotypes observed in HPP.

Correction of hypophosphatasia-associated mineralization deficiencies in vitro by phosphate/pyrophosphate modulation in periodontal ligament cells

BACKGROUND

Mutations in the liver/bone/kidney alkaline phosphatase (ALPL) gene in hypophosphatasia (HPP) reduce the function of tissue non-specific alkaline phosphatase (ALP), resulting in increased pyrophosphate (PP(i)) and a severe deficiency in acellular cementum. We hypothesize that exogenous phosphate (P(i)) would rescue the in vitro mineralization capacity of periodontal ligament (PDL) cells harvested from HPP-diagnosed patients, by correcting the P(i)/PP(i) ratio and modulating expression of genes involved with P(i)/PP(i) metabolism.

METHODS

Ex vivo and in vitro analyses were used to identify mechanisms involved in HPP-associated PDL/tooth root deficiencies. Constitutive expression of PP(i)-associated genes was contrasted in PDL versus pulp tissues obtained from healthy individuals. Primary PDL cell cultures from patients with HPP (monozygotic twin males) were established to assay ALP activity, in vitro mineralization, and gene expression. Exogenous P(i) was provided to correct the P(i)/PP(i) ratio.

RESULTS

PDL tissues obtained from healthy individuals featured higher basal expression of key PP(i) regulators, genes ALPL, progressive ankylosis protein (ANKH), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), versus paired pulp tissues. A novel ALPL mutation was identified in the twin patients with HPP enrolled in this study. Compared to controls, HPP-PDL cells exhibited significantly reduced ALP and mineralizing capacity, which were rescued by addition of 1 mM P(i). Dysregulated expression of PP(i) regulatory genes ALPL, ANKH, and ENPP1 was also corrected by adding P(i), although other matrix markers evaluated in our study remained downregulated.

CONCLUSION

These findings underscore the importance of controlling the P(i)/PP(i) ratio toward development of a functional periodontal apparatus and support P(i)/PP(i) imbalance as the etiology of HPP-associated cementum defects.