Gene polymorphisms in periodontal health and disease

Gene polymorphisms are locations within the genome that vary in sequence between individuals and are very prevalent, affecting at least 1% of the population (104). Polymorphisms of human genes occur at one or more of the following sites (Fig. 1): 1. the promoter or 5¢-flanking region; 2. the exon(s) or the gene coding regions; 3. the intron(s) or the gene intervening regions; 4. the 3¢-untranslated region (69). The most common form of polymorphisms is the single nucleotide polymorphism, which is a change in a single base pair (bp) in the genomic DNA (104). Single nucleotide polymorphisms can affect gene function. For example, a single nucleotide polymorphism located in a promoter region may influence the amount of mRNA produced. Another class of polymorphism is the simple sequence repeats, of which the common forms are the dinucleotide and trinucleotide repeats (70). The variable number of tandem repeats can also influence the function of a gene, but the repeats are more likely to be linked with a functional polymorphism elsewhere in the gene. A third category of gene polymorphism involves insertions or deletions (116). Insertions and deletions can be as small as 1 base, in which case they may also be classified in the category of single nucleotide polymorphisms, but can also consist of a few bases, one or more exons, or even a whole gene.