Dental Manifestations Associated with ENPP1 and ABCC6 Mutations

Objectives: Regulators of pyrophosphate (PPi) levels include ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) and ATP Binding Cassette C6 (ABCC6). ENPP1 and ABCC6 mutations are associated with generalized arterial calcification of infancy (GACI). Cementum is sensitive to perturbation in PPi levels, and we reported increased cementum in GACI individuals with ENPP1 mutations. To further understand the role of PPi modulation during tooth development, we investigated effects of ABCC6 vs. ENPP1 loss-of-function on cementogenesis in GACI subjects and murine models.
Methods: Eight subjects with GACI (five with ENPP1 mutations, three with ABCC6 mutations) were included in this study that employed clinical examinations, photography, and radiography. MicroCT and histology were conducted on exfoliated/extracted primary teeth from GACI subjects and 9-month Abcc6^-/-, Enpp1^-/- and Abcc6^-/-, Enpp1^-/- (dKO), and control mouse first molars.
Results: All ENPP1-GACI subjects exhibited dental malocclusion, and two subjects reported slow or ineffective orthodontic tooth movement. Radiographs revealed protruding cervical root morphology suggestive of hypercementosis in ENPP1-GACI subjects, which was absent in ABCC6-GACI subjects. MicroCT analysis of ENPP1-GACI primary teeth showed 350% increased cervical cementum thickness (p<0.01) and 25% increased cementum density (p<0.01) compared to healthy controls and ABCC6-GACI teeth. In mice, microCT revealed 15% increased dentin volume (p<0.05) and 25% increase in total cementum volume (p<0.05) in Enpp1^-/- and dKO mice vs. wildtype and Abcc6^-/- mice. Changes in enamel, alveolar bone, pulp, and periodontal ligament volumes and densities were not detected. Histologically, Abcc6^-/- cementum displayed similar width and morphology to controls, whereas Enpp1^-/- and dKO mice exhibited ten-fold increased acellular cementum width. Enpp1^-/- and dKO teeth were histologically indistinguishable.
Conclusions: These findings reveal that while ENPP1 mutations result in hypercementosis, ABCC6 mutations do not. These data suggest: a) ENPP1 exerts a local PPi regulatory role unaffected by ABCC6 b) reduced systemic PPi levels are insufficient to promote cementogenesis; and c) factors decreasing local PPi levels are potential targets for periodontal therapies.