Mineralized Tissue and Salivary Gland Physiology Program
Center for Integrative Craniofacial Research
OBJECTIVE: The goal of this Initiative is to stimulate basic research on the genetic basis underlying individual responses to ingested fluoride in regard to its benefits on tooth mineralized tissues as well as potential effects in other physiological systems. This Initiative will encourage studies on: 1) global strategies such as linkage and mapping studies to identify fluoride-responsive genetic variations; 2) candidate gene strategies based on existing knowledge of the mechanisms of fluoride action to identify and characterize functional polymorphisms; 3) the development of animal models with clearly defined differential physiological responses to ingested fluoride at levels relevant to normal human exposures and the association of these phenotypes with genotypes; 4) the development and validation of relevant fluoride-responsive genetic polymorphisms into predictive genetic standards for an individual’s physiological responses to fluoride; and 5) gene-gene and gene-environment interactions that modify the function of fluoride-responsive genetic variants. The knowledge gained from these studies can serve as the basis for future individualized recommendations for total fluoride intake.
BACKGROUND: Since the introduction of fluoride in community drinking water in 1945, there has been a steady decline in the prevalence and severity of dental caries among Americans living in fluoridated communities. In fact, water fluoridation was touted by the Center for Disease Control and Prevention as one of the top ten public health achievements of the twentieth century. However, with this decline in caries prevalence, there has been an increase in dental fluorosis among children living in fluoridated communities. It has been estimated that between 20-80% of these children have mild fluorosis and 4% have moderate to severe fluorosis. A recent analysis of the NHANES datasets indicated that the prevalence of fluorosis among 6-19 year old children increased by 9% since 1987. This increase is thought to be due, in part, to the increase in fluoride intake from sources other than drinking water including fluoridated beverages, supplements and topical fluoride-containing dental products that may be ingested. Factors such as nutrition have been implicated in an increase in an individual’s susceptibility to fluorosis but probably do not fully account for the prevalence observed in the population. On the other hand, although the proper use of fluoride generally reduces dental caries, the extent of its effectiveness varies among different communities.
Ingested fluoride is rapidly and efficiently absorbed through the gastrointestinal system and approximately 35-70% is eventually cleared by the renal system. Most of the fluoride retained in the body is incorporated into bones and teeth. The majority of fluoride related research efforts have focused on tooth enamel. Enamel development begins with the secretion of a set of enamel matrix proteins by ameloblasts that nucleates minerals. Matrix proteins continue to interact with minerals and guide crystal growth. Crystals are aligned in parallel arrays, which further orient themselves to form an interwoven superstructure of hydroxyapatite. Matrix proteins are degraded by proteolytic enzymes and removed leaving enamel that contains 98% minerals. Fluoride is anticariogenic at low concentration. Studies have shown that, during enamel mineralization, fluoride can be intercalated into the enamel crystal structure as fluoroapatite which is more resistant to dissolution than hydroxyapatite. However, excess fluoride results in dental fluorosis characterized by changes in enamel that vary from barely discernible fine white striations to pitted brown lesions. Some studies suggest that fluoride interferes with the activities of enamel matrix protein processing enzymes resulting in a delay in the clearance of matrix proteins during the maturation phase of amelogenesis and incomplete mineralization. Fluoride may also interfere with the maturation and differentiation of ameloblasts.
In regard to its effects on bone, fluoride is considered anabolic in that it stimulates osteoblast proliferation and mineral deposition. However, excessive fluoride can result in an increase in bone density. It has been found that this compromises bone quality and strength such that fluorotic bones have increased fracture rates and delayed fracture repair.
Fluoride might also affect the physiological processes of non-mineralized tissues. Human and animal studies of acute lethal or toxic levels of fluoride show that it affects the gastrointestinal, cardiovascular, renal, reproductive, and neuroendocrine systems. It has also been proposed that fluoride might increase the incidence of osteosarcomas in young males. Most of these studies have been inconclusive and inconsistent. Animal experiments have used relatively high levels of fluoride, whereas well controlled studies using relevant ranges that reflect more typical levels of human exposure are lacking.
The involvement of genetic determinants in fluoride responsiveness has been implicated in several studies. A number of ecological studies of populations around the world living in areas with naturally high levels of fluoride in the water suggest that there is considerable variation in fluorosis among and within these populations. Responsiveness to fluoride cannot be correlated with the total bioburden of fluoride as assayed in urine samples. Heterogeneous responses to fluoride have also been observed in different mouse strains with certain strains exhibiting fluorosis while other strains are unaffected. Several fluoride-responsive genes have been identified but how these genes and their gene products determine physiological responses to fluoride has not been clarified. Nevertheless, these studies strongly suggest that there is a genetic basis underlying the effects of fluoride.
With the completion of the Human Genome Project and rapid advancement of the International HapMap Project, genomic science can begin to be translated into genomic medicine. On the forefront of this progress is the emerging field of pharmacogenetics; an interdisciplinary study of genetic variations that determine heterogeneous responses to drugs and other chemical compounds. Global and high throughput map-based or sequence-based approaches, gene expression profiling and proteomic studies can be utilized to ascertain functionally significant genetic variations in drug responses and to identify polygenic interactions that determine complex drug responses. Genetically engineered animals such as recombinant congenic strains of mice are available to accelerate genetic dissection of complex traits. It is timely to apply these state-of-the-art approaches to fluoride research and to attract new investigators and investigators with diverse expertise to this area. This Initiative focuses on capturing emerging scientific opportunities and technologies to provide the genetic and molecular basis for the physiological outcome of ingested fluoride. Knowledge gained from these studies will pave the way for the identification of specific populations or individuals that will benefit from fluoride supplement and/or exhibit other physiological consequences due to the action of fluoride. This information will also elucidate fundamental mechanisms by which fluoride influences biomineralization.
CURRENT PORTFOLIO OVERVIEW: The current NIDCR portfolio contains one project, “Genetic Determinants of Dental Fluorosis”, which will develop linkage maps of putative fluorosis susceptible loci. There are ten additional active projects related to fluoride research, none of which is a genetics study.
COLLABORATIVE ACTIVITIES: The objectives and content of this Concept Clearance are consistent with the interests of the NIAMS regarding skeletal tissue research and of the NIEHS regarding genetic polymorphisms and exposure assessment.
FUNDING MECHANISMS: This Initiative will utilize the R01 and R21 mechanisms.