Genetic variations in cytokine expression: a risk factor for severity of adult periodontitis

Periodontitis is a collection of chronic inflammatory diseases that are caused by specific bacteria. The bacteria activate inflammatory mechanisms in the periodontal tissues that destroy collagen and bone that support the teeth. Although bacteria are essential for the initiation of periodontitis, the quantity and types of bacteria have not been sufficient to explain the differences in disease severity. In recent years, it has become evident that for many common chronic diseases, there are modifying factors that do not cause the disease but rather amplify some disease mechanisms to make the clinical condition more severe. There are now data to suggest that a few factors which amplify the inflammatory process make people susceptible to an increased severity of periodontitis. Studies of untreated disease in Sri Lanka identified 3 patterns of disease progression. Studies in twins suggested that part of the clinical characteristics of periodontitis may be explained by genetic factors, but previous attempts to identify genetic markers for periodontitis have been unsuccessful Some genetic variations (polymorphisms) are commonly found in our population and represent a mechanism by which individuals may exhibit variations within the range of what is considered biologically normal. Since certain cytokines are key regulators of the inflammatory response and are important in periodontitis, we investigated the relationship between genetic variations associated with cytokine production and periodontitis severity. There are several polymorphisms in the cluster of genes that influence IL-1 biological activity. In recent clinical trials, two of these polymorphisms, when found together, have been associated with a significant increase in the risk for severe generalized periodontitis. Genetic association with periodontitis was evident only when smokers were excluded from the analysis, confirming the importance of smoking, and suggesting that both smoking and the IL- I genotype are independent factors in severe periodontitis. It is notable that 1 polymorphism associated with severe periodontitis in our study is also known to correlate with a 2- to 4-fold increase in IL-1 beta production. These findings are consistent with the current model of how genetic factors influence common chronic diseases. If we apply this model to periodontitis, it would involve the following: 1) a disease-initiating factor that would undoubtedly be specific bacteria such as Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans. and Bacteroides forsythus: and 2) modifiers of disease mechanisms that account for the clinical severity, including smoking, the IL-I genotype, certain systemic diseases, and psychosocial stress. The association of the IL-I genotype with severe periodontitis is consistent with several lines of periodontal research. Several studies have suggested there is a substantial genetic influence in periodontal disease. Although specific genetic markers have been identified in the uncommon juvenile forms of periodontitis, previous studies of specific genetic markers in adults with periodontitis have not been encouraging. Many investigators have, however, demonstrated a role for IL-1 in the initiation and progression of periodontitis. For example, IL-1 activates the degradation of the extracellular matrix and bone of the periodontal tissues, and elevated tissue or gingival fluid levels of IL-1 beta have been repeatedly associated with periodontitis. In addition, IL-1 is a strong enhancer of tissue levels of PGE2 and TNF-alpha. The association of severe periodontitis with smoking and the IL-1 genotype suggest a role for these factors in the pathogenesis of periodontitis. The finding that host modifying factors are associated with severe periodontitis suggest a biological mechanism by which some individuals, if challenged by bacterial accumulations, may have a more vigorous immunoinflammatory response, leading to more severe clinical disease. (ABSTRACT