Periodontal Ligament Cells from Patients with Hypophosphatasia Exhibit Limited Mineralization

Objectives: Periodontium has been shown to be sensitive to local levels of inorganic phosphate and pyrophosphate, however, the reason for this unique sensitivity remains unclear. The aim of this study was to comparatively evaluate periodontal ligament (PDL) cells harvested from control patients versus patients with hypophosphatasia (HPP). Methods: PDL tissues were obtained from control (n=3) and HPP (n=2) patients, and cell cultures established by enzymatic digestion. The following in vitro assays were performed: 1) Cell number and viability; 2) Von Kossa and alizarin red staining, and 3) Gene expression by quantitative PCR, at days 1 and 15, for the following genes: Type I collagen (Col I), osteocalcin (OCN), osteopontin (OPN) and bone sialoprotein (BSP). Results: Proliferation and cell viability were comparable in HPP and control cells, except at day 2, where a decrease in cell viability was associated with HPP cells. Both control and HPP cells promoted mineral nodule formation when cultured for 28 days in the presence of ascorbic acid (50µg/ml) and ß-glycerophosphate (5mM); however, alizarin red staining revealed 3-fold greater mineral formation in control versus HPP cells (p<0.05). Expression levels for all genes evaluated increased as a function of time in control cells (p<0.05), whereas only mRNA levels for Col I and OCN increased over time in HPP cells (p<0.05). Intergroup analysis demonstrated that OPN mRNA levels at day 1 and Col-I, BSP, and OPN mRNA levels at day 15 were higher for the control cells compared to HPP cells (p<0.05). Conclusion: Within the limits of this study, the data suggest that there are important differences in the in vitro behavior of control versus HPP cells, including basal expression of cementum-related genes as well as their capacity to promote mineral formation.These findings may bring new insights to treatment of patients presenting deficiencies in phosphate metabolism.