Fine mapping of dental fluorosis quantitative trait loci in mice

Fluoride (F) in various chemical forms and exposures has been repeatedly shown to have actions on the development and homeostasis of mineralized tissues (1, 2). Excessive systemic fluoride during critical periods of tooth enamel development (amelogenesis) can result in an undesirable developmental defect of tooth enamel, dental fluorosis (enamel fluorosis). Dental fluorosis (DF) is characterized by increased porosity (subsurface hypomineralization) with a loss of enamel translucency and increased opacity (3). It is generally accepted that increasing DF severity correlates with increasing F exposure. DF remains highly prevalent world-wide. As recent as 2005 23% of persons in the United States aged 6–39 years had very mild or greater enamel fluorosis (4). We have demonstrated that genetic background plays an integral role in responses of mineralized tissues to systemic fluoride (5–10). Genetically diverse inbred strains of mice when provided equivalent exposures to F in their environment show strain specific DF severities (5, 9). DF can be considered a complex condition influenced by environmental (F exposure) and host factors (genetics). The later is determined by multiple genes with additive effects (11). Quantitative trait locus (QTL) detection mapping has been extensively used in mapping loci associated with complex traits. Generally large panels of animals and large QTL effects improve the power to detect and map QTLs. Two traditional QTL mapping approaches that can yield large panels are F2 and backcross strategies. In mice F2 panels are created from a two generation cross between two parental inbred strains which have measurable differences for a particular trait. In the case of DF we used the 129P3/J (DF resistant strain) and the A/J (DF susceptible strain)(5, 11). Recently we identified QTLs on chromosomes (Chr) 2 and 11 in mice that are strongly associated with DF susceptibility (11). On Chr 2 a QTL designated as Dfs1 (MGI Accession ID: MGI: 4418088) was detected with an interval of approximately 77.62 Mb and flanked by markers rs13476589 and UT_2_156.443943. A second QTL on Chr 11 designated Dfs2 (MGI Accession ID: MGI: 4418089) was detected with an interval of approximately 74.29 Mb and flanked by markers rs3708339 and rs6161623. The QTLs Dfs1 and Dfs2 combined contain over 2600 genes. The present study had two goals. First, to refine the two DF QTL intervals through the use of increased marker densities on Chr 2 and Chr 11 and by two strain haplotype analysis between the parental A/J and 129P3/J strains. Second, to test a candidate gene Accn1 which encodes an amiloride-sensitive cation channel 1, neuronal (degenerin) and resides on Chr 11 near the QTL peak detected.