The genetic susceptibility/resistance to fluorosis among different inbred mouse strains

Abstract

Fluoridation of community water supplies for the purpose of preventing dental caries remains one of the top 10 public health interventions of the last century. However, exposure (ingestion) of greater than optimal amounts of fluoride from a variety of sources has led to an increase in the prevalence of dental fluorosis. We propose that dental fluorosis represents a complex condition caused by environmental and genetic factors. Purpose: To assess the role of genetics in the pathogenesis of dental fluorosis using genetically separate inbred strains of mice. Methods: Twelve genealogically disparate strains of mice were treated with 0 ppm, 25 ppm, and 50 ppm of fluoride in their drinking water. Each mouse was given weekly dental fluorosis evaluations. After 60 days of treatment, femurs were collected for fluoride analysis. Mandibular incisors were isolated for quantitative light induced fluorescence (QLF) studies and fluoride analysis. Digital and 35 mm images were taken of all mouse incisors in order to apply and compare the Dean's Index and the modified Thylstrup and Fejerskov Index (TFI), both indices of dental fluorosis. Skeletal radiographs were taken on the euthanized mice and later examined for extra skeletal calcifications and other gross bony deformities. Results: Differences in dental fluorosis susceptibility/resistance were identified between the strains, ranging from mild, moderate, to severe dental fluorosis. Furthermore, we found clustering of strains into distinct phenotypic groups. The A/J mouse strain was highly susceptible, with a rapid onset and severe development of dental fluorosis compared with the other strains tested. The 129P3/J mouse strain was least affected with negligible dental fluorosis. From the skeletal radiographs, no gross skeletal lesions or evidence of bone dysplasia were noted. Similar body burden of fluoride, as judged from analysis of mineralized tissues, was seen in all strains despite differences in their predispositions to develop dental fluorosis. Both the Dean's and TF indices are useful for classifying the stage or severity of fluorosis in mice, and there are advantages to the use of digital images over conventional 35 mm slide images. Both indices correlate well with the amount of fluoride exposure during amelogenesis; however, these indices are not promising indicators of fluoride burden during amelogenesis. Conclusions: QLF proved to be an innovative and useful tool for the quantification of dental fluorosis. Furthermore, these observations support the role of a genetic component in the pathogenesis of fluorosis.