Hypercementosis Associated With ENPP1 Mutations and Generalized Arterial Calcification of Infancy

Objectives: Skeletal and dental mineralization are tightly regulated by levels of extracellular inorganic phosphate (P_i) and pyrophosphate (PP_i). Three regulators of P_i and PP_i concentrations include tissue-nonspecific alkaline phosphatase (TNAP), progressive ankylosis protein (ANK), and ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1). Inactivation of these factors results in inherited mineralization disorders. This study for the first time analyzes effects of decreased PP_i on dental development in individuals with generalized arterial calcification of infancy (GACI) due to ENPP1 loss-of-function mutations.
Methods: Five subjects with GACI were included in this study that employed dental examination, clinical radiography, and high resolution micro-computed tomography (micro-CT) and histology of exfoliated/extracted primary teeth. Teeth from Enpp1 mutant (Enpp1^asj/asj) mice were analyzed in parallel.
Results: Four of five GACI subjects reported a history of infraocclusion, over-retained primary teeth, ankylosis, and/or slow orthodontic movement, suggesting disrupted periodontal mineralization and disturbed tooth movement/exfoliation. All subjects had radiographic evidence of unusually protruding cervical root morphology in primary and/or secondary dentitions. Micro-CT analyses of 10 extracted primary teeth from 3 GACI subjects revealed 4-fold increased cervical cementum thickness (p=0.00007) and a 23% mean increase in cementum density (p=0.009) compared to age-matched healthy control teeth. No differences were found in enamel or dentin densities between GACI and control teeth. Histology revealed dramatically expanded cervical cementum in GACI teeth, including cementocyte-like cells and unusual patterns of cementum resorption and repair. Micro-CT analysis of Enpp1^asj/asj mouse molars revealed 4-fold increased acellular cementum thickness (p=0.002) and 5-fold increased cementum volume (p=0.002) compared to controls, with no changes in enamel or dentin.
Conclusions: These findings report a novel dental phenotype in GACI and identify ENPP1 genetic mutations causative for hypercementosis. The sensitivity of acellular cementum to reduced PP_i levels in both human and mouse teeth establishes this as a well-conserved and fundamental biological process directing cementogenesis across species.