Amelogenesis imperfecta (AI) is a group of diverse inherited enamel disorders most often considered as a single trait and not often associated with syndromes or metabolic disorders. However, there have been documented case reports in which AI has been identified in patients with disease states impacting pH homeostasis;conditions such as inherited proximal renal tubular acidosis (pRTA) and cystic fibrosis (CF). Three proteins involved in acid/base transport are expressed in polarized ameloblasts during enamel formation (amelogenesis). Two proteins are the anion exchanger (AE2) and the electrogenic sodium bicarbonate cotransporter (NBCe1). The third protein is the cystic fibrosis transmembrane conductance regulator (CFTR), which is a chloride channel, is directly associated with AE2 function. Mice null or mutant for NBCe1, AE2 or CFTR have abnormal enamel, and a CFTR mutant porcine model also shows enamel defects. It has been reported that human patients with mutations in NBCe1 and CFTR also have enamel abnormalities. All these observations suggest that AE2, NBCe1 and CFTR play important roles in amelogenesis. This grant proposal is aimed towards a better understanding of the critical physiological roles that AE2, NBC1 and CFTR play in amelogenesis. The hypothesis of this proposal is that "the regulated spatiotemporal expression profiles of AE2, CFTR and NBCe1 in ameloblast cells are responsive to changing extracellular pH conditions, and any failure in proper AE2, CFTR and NBCe1 activity will result in disruptions to the enamel structure". Five distinct, but complementary, specific aims are proposed.
Specific aim (SA) 1 will use in situ hybridization to define expression profiles for AE2, NBCe1 and CFTR throughout amelogenesis. SA 2 will use dual immunogold labeling to establish the spatial location of AE2a and CFTR at the apical face of maturation-stage ameloblasts. SA 3 will characterize the forming enamel from AE2-null and NBCe1-null mice using hemi-mandible explants. SA 4 will identify pH-responsive elements in the promoter regions of human SLC4A2 (AE2), CFTR, SLC4A4 (NBCe1). SA 5 will examine whole transcriptome DNA microarray data comparing in vivo gene expression profiles of secretory-stage (normal pH) vs maturation-stage (acidic pH) enamel organ cells. At the conclusion of this study we believe that the mutations to the gene loci encoding the proteins NBCe1, AE2 and CFTR will be considered as causive of AI when a linkage cannot be related to known enamel proteins. This study will raise awareness of dental disease in a large but select group of seriously ill patients where their disease is closely linked to pH regulation.
The connection between pH regulation and enamel mineralization is poorly understood. Animal models, and patients with disease states caused by mutations to genes involved with acid/base homeostasis frequently have dental defects. Three such proteins are the anion exchanger (AE2), the electrogenic sodium bicarbonate cotransporter (NBCe1) and the cystic fibrosis transmembrane conductance regulator (CFTR), which is a chloride channel, is directly associated with AE2 function. Clinical observations suggest that AE2, NBCe1 and CFTR each play important roles in enamel mineralization (amelogenesis). This grant proposes to better understand the physiological and biochemical functions of AE2, NBCe1 and CFTR in amelogenesis. With a better understanding of pH regulation and dental disease, earlier and more accurate dental diagnoses, and more appropriate dental treatment options will be made available to affected individuals.
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