Association of Periodontal Diseases with Genetic Polymorphisms

Periodontal diseases are multifactorial in nature. While microbial and other environmental factors are believed to initiate and modulate periodontal disease progression, there now exist strong supporting data that genetic polymorphis ms play a role in the predisposition to and progression of periodontal diseases. Variat ions in any number or combination of genes that control the development of the periodontal tissues or the competency of the cellular and humoral immune systems could affect an individual's risk fo r d isease. A corollary of this realization is that if the genetic basis of periodontal disease susceptibility can be understood, such information may have diagnostic and therapeutic value. This review aims to update the clin ician about various genetic polymorphis ms associated with periodontal diseases to aid in a better approach to the condition in the future.


Introduction
Periodontal disease is an inflammatory illness that represents the main cause of tooth loss in developed countries, with increasing prevalence in the developing world [1]. Periodontitis is defined as "an inflammatory disease of the supporting tissues of the teeth caused by specific microorganisms or groups of specific microorganis ms, resulting in progressive destruction of the periodontal ligament and alveolar bone with pocket formation, recession, or both [2]. Changes that occur in the alveolar bone are crucial because the destruction of bone is responsible for tooth loss. Periodontal diseases are mu ltifactorial in nature. While microbial and other environmental factors are believed to in itiate and modulate periodontal disease progression, there now exists strong supporting data that genetic and environmental risk factors play a ro le in the predisposition to and progression of periodontal diseases (Fig 1) [3,4,5].
The application of genetic information and technology for the p red ict ion , d iagnos is and t reat ment o f p eriodont al conditions is conceptually compelling. Several features like cytokines, cell-surface receptors, chemokines, enzy mes and others that are related to antigen recognition, the immune system, host response, among others, are determin ed by genetic components; polymorphis ms of wh ich may increase the susceptibility of an indiv idual to periodontal diseases. Identifying genes and their poly morphisms can result in novel diagnostics for risk assessment, early detection of disease and individualized treat ment approaches [6]. Thus, genetic epidemio logy, including knowledge of genetic polymorphis ms, holds promise as one of the tools that may contribute to the understanding of periodontal disease ( Fig  2).

Evidence for the Role of Genetics in Periodontal Disease
Periodontal research has greatly expanded to elucidate the role of genetics in periodontal disease states. Consequently, there has been great interest in identify ing allelic variants of genes that can be used to assess disease risk for periodontal diseases. Reports of genetic poly mo rphisms associated with periodontal disease are continuously increasing (Fig 3).
In the past it was thought that periodontitis would eventually develop in subjects with a longstanding history of poor oral hygiene and gingivit is [7]. Despite this belief, it was recognized that only a portion of the variab ility of d isease in the population could be explained by environmental factors alone. This variation must have been attributable to either unrecognized components of the environment or d ifferences among indiv iduals in their susceptibilities to disease. Because host susceptibility may be defined in terms of genetic variation, the focus of determining disease susceptibility shifted to quantify the genetic aspects of disease. Epidemio logic studies as well as longitudinal clin ical studies of the oral microbial flora supported the development of the theory that although microbial and environmental factors are required for periodontitis, the disease may have a genetic background (Fig 4).   Periodontitis susceptibility may be regulated by a gene polymorphism (dotted line) without ever resulting in developing overt disease. Hence, periodontitis symptoms might be unaffected by a gene polymorphism. However, with multiple effects of environmental factor assaults, the disease may become apparent (solid line from factors A-D). b. Increased predisposition towards periodontal disease may also result as a function of the basal predisposition (dotted line). Hence, higher "predisposition" leads to increased disease activity. as a result of these effects, rate of disease progression (elbow-shaped line) rises significantly following each environmental assault. (Modified from Takashiba S, Naruishi K. Gene polymorphisms in periodontal health and disease. Periodontol 2000Periodontol 200640: 94-106) A study was done as early as 1966 in over 1800 subjects which deduced that certain individuals were mo re at risk for developing periodontitis than others. [8] The same phenomenon was found in two longitudinal studies evaluating the effect of periodontal therapy in patients suffering fro m periodontitis for more than 15 years [9,10]. In a classic longitudinal study of the natural history of periodontitis, Loe et al (1986) found that a relatively small proportion of the population is at risk fo r developing severe forms of periodontitis suggested that not everybody is equally susceptible to periodontitis [11].
The microbial causation of inflammatory periodontal diseases was well established [7,11]. However, it was debated that if periodontitis was simp ly and solely caused by one or more specific periodontal pathogens, the disease should have developed in majo rity of subjects infected by these organisms. In contrast, periodontal pathogens showed a relatively high prevalence in subjects with g ingivitis or minor periodontitis as compared to severe forms. Therefore, the existence of high-risk groups could not be explained by the microbiology alone. These high-risk groups represented around 10-15% o f the population, in whom the disease quickly progressed fro m chron ic gingiv itis to destructive periodontitis [12,13].
This differential risk for periodontitis was consistent with heritable elements of susceptibility and it is possible that periodontitis may be explained by several tens of relatively co mmon h igh-risk poly morphis ms with cumulat ive high-susceptibility profiles [6]. Most genetic research in periodontitis has now focused on gene polymorphis ms that play a role in immunoregulation or metabolism, such as cytokines, cell-surface receptors, chemokines, enzy mes and others that are related to antigen recognition (Table 1).

Periodontal Diseases Influenced by Genetic Polymorphisms
Given the complex et iology and pathogenesis of periodontal diseases, variations in any number or combination of genes that control the development of the periodontal tissues or the competency of the cellular and humoral immune systems could affect an indiv idual's risk for disease. Evidence fo r a genetic influence on periodontal diseases comes fro m mu ltiple sources including familial aggregation and formal genetic studies of aggressive periodontitis, the association of periodontitis with certain Mendelian inherited diseases, and twin studies of chronic periodontitis (Fig 5) [14].

Chronic Periodonti tis
Chronic periodontitis has been defined as an infect ious disease resulting in inflammat ion within the supporting tissues of the teeth, progressive attachment loss, and bone loss. It is associated with the accumu lation of p laque and calculus and generally has a slow to moderate rate of disease progression, but periods of more rapid destruction may be observed. The disease also may be described by the severity of disease as slight, moderate or severe based on the amount of clin ical attachment loss [2]. Although no clear genetic determinants have been described for patients with chronic periodontitis, a genetic predisposition to more aggressive periodontal breakdown in response to plaque and calculus accumulat ion may exist [15]. Also, clinicians have long suspected that susceptibility to periodontitis differs among racial and ethnic groups.
Investigations have utilized family studies of probands with chronic (adult) periodontitis or younger subjects with mild/incip ient periodontitis. The results suggested that there may be a genetic basis for the less severe forms of periodontitis [16]. The twin model is probably the most powerful method to study genetic aspects of periodontal diseases. The largest twin study included 4908 t win pairs of which, on the basis of questionnaire data, 349 (116 MZ and 233 DZ) pairs reported a history of periodontal disease in one or both pair members. The concordance rates ranged from 0.23 to 0.38 for MZ twins and 0.08 to 0.16 for DZ twins [17]. Michalowicz and co-workers evaluated the periodontal condition (attachment loss, pocket depth, gingival index and plaque index) of 110 adult twin pairs with a mean age of 40 years. The results indicated that between 38% and 82% of the population variance for these measures may be attributed to genetic factors [18]. In a subsequent study the authors estimated that chronic (adult ) periodontitis had approximate ly 50% heritability, which was unaltered following adjustments for behavioural variab les including smoking and utilizat ion of dental care [19]. In epidemiological studies in the Dutch population, it has been suggested that periodontitis aggregates in families [20,21].
Korn man et al reported that a "co mposite" IL-1 genotype consisting of at least one copy of the more rare allele at both an IL-lα and IL-1β loci was associated with severe periodontitis [22]. Karimbu x et al (2012), in their meta-analysis, reported that IL1A and IL1B genetic variations are significant contributors to chronic periodontitis in Caucasians [23].
Fro m both the twin studies and familial studies it can be concluded that the basis for familial aggregation of periodontitis appears to be genetic rather than bacterial, environmental or behavioral in nature. The heritable component for periodontitis was not associated with behaviors such as smoking, utilization of dental care and oral hygiene habits. This implies that genes controlling bio logic mechanis ms, and not behaviors, med iate the genetic influence on disease.

Aggressive Peri odontitis
Aggressive periodontitis was previously classified as early onset periodontitis (EOP). EOP included prepubertal, juvenile and rapidly progressive forms of periodontitis. Aggressive periodontitis is characterized by rapid rate of disease progression seen in an otherwise healthy individual, an absence of large accu mulat ions of plaque and calculus and a family history of aggressive disease suggestive of a genetic trait. Although diagnoses are based on clinical and radiographic criteria, the immunologic and microbio logic profiles can vary substantially within the subforms [1].
Marked aggregation of aggressive periodontitis within families is consistent with a genetic pred isposition to this disease. In a United States survey of 7447 dentate individuals aged 13 years and older, the prevalence of severe disease was low in younger age groups. The prevalence of loss of attachment of ≥ 5 mm was 0.3% among 13-17 and 18-24 year olds, and 6.4% among [25][26][27][28][29][30][31][32][33][34] year o lds in a racially mixed population. In contrast, 40-50% o f siblings in families with aggressive periodontitis were similarly affected [24]. One of the largest and most comprehensive segregation analysis for aggressive periodontitis has been performed by Marazita et al (1994). It was suggested by the authors that the most likely mode of inheritance was autosomal do minant in both African-A merican and Caucasian kindred, with 70% penetrance in African-A mericans and 73% in Caucasians [25]. However, familial patterns of disease may reflect not only a common genetic background but also exposure to common environ mental factors.
Studies have also been conducted on linkage analysis of candidate regions which lie within or near genes that code for enzy mes or regulatory molecules that are likely to be involved in the pathogenesis of disease. The human leukocyte antigens (HLA) have been considered to be candidate markers for aggressive periodontitis because they are involved in regulating immune responses. Two antigens that appear to be consistently associated are HLA-A9 and -B15 [26]. The risk of disease in subjects with HLA-A9 or -B15 is about 1.5 to 3.5 t imes greater than in those lacking these antigens. In contrast, the HLA-A2 appears to be less prevalent in aggressive periodontitis patients than controls, suggesting that this antigen somehow may be protective [27]. An IL1B allele has been reported to be in linkage disequilibriu m with generalized aggressive periodontitis, suggesting that at least one disease allele is at o r very close to this IL1B poly morphism (2q13) [28].
Another study concluded that in West European Caucasians, the Asp299Gly TLR4 gene polymorphis m was associated with a decreased risk of AgP but not CP. Pro moter polymorphis ms of the CD14 gene, however, did not influence susceptibility to inflammatory periodontitis in the population cohorts studied [29]. Li et al provided evidence stating FokI polymo rphism of vitamin D receptor gene might be associated with generalized aggressive periodontitis in Chinese patients. In addition, the carriage of F allele increases the risk of developing generalized aggressive periodontitis [30]. Most studies of aggressive periodontitis (or EOP) to date have had inadequate statistical power to resolve substantial genetic heterogeneity.

Monogenic Syndromes
Aggressive periodontitis has often been reported inpatient s with certain monogenic syndromes. These conditions are rare (Tab le 2). In many of these conditions the permanent dentition is also affected to a greater or lesser extent.
The most extensively studied of the monogenic conditions is the Papillon-Lefevre syndrome. It is inherited as an autos omal recessive trait. The classical clinical manifestations of Papillon-Lefevre syndrome are hyperkeratotic lesions of the skin, affecting the palmoplantar surfaces, and aggressive periodontitis, affect ing the deciduous and permanent dentitions [31]. The underlying causation of Papillon-Lefevre syndrome has been the subject of considerable debate in the literature. In addit ion to defects in epitheliu m format ion, reduced lymphocyte response in vitro and decreased chemotaxis and phagocytosis of neutrophils and monocytes have been reported in Papillon-Lefevre syndrome patients. Studies involving genome wide lin kage scans of consanguineous families mapped the Papillon-Lefevre syndrome locus to the long arm of chro mosome 11, with the gene encoding lysosomal protease cathepsin C [32]. This enzy me is expressed at high levels in many immune cells including poly morphonuclear leukocytes and macrophages and their precursors. In addit ion, it has been found that cathepsin C is expressed in areas of ep itheliu m often affected by hyperkeratotic lesions such as palms, soles, knees and oral keratinized gingiva [33]. Individuals displaying the Papillon-Lefevre syndrome phenotype are ho mozygous for the trait. Other relatives who are heterozygotes do not show evidence of either palmop lantar hyperkeratosis or severe periodontitis. The finding that mutations of the cathepsin C gene give rise to Papillon-Lefevre syndrome corroborates earlier suggestions of the cause of this syndrome [31].
Another syndrome described by Haim and Munk (1965) has only been observed in the descendents of a religious sect originally fro m India. Th is condition displays the same features of hyperkeratosis and severe aggressive periodontitis as Papillon-Lefevre syndrome, with the addition of arachnodactyly and a peculiar deformity of the terminal phalanges [34]. The Haim-Munk syndrome was also found to be linked to the cathepsin C gene. In other words Papillon-Lefevre syndrome and Haim-Munk syndrome are allelic variants of mutations of the cathepsin C gene.

Refractory Periodontitis
Patients who respond poorly to periodontal treat ment by continuing to lose clinical attach ment are said to have refractory periodontitis. Both, aggressive and chronic periodontitis can be refractory to treatment. Neutrophils fro m refractory disease patients usually display so me functional defect. Kobayashi et al tested the association between neutrophil immunoglobulin G receptor (FcγR) polymorphis ms and adult (now known as chronic) periodontitis in a Japanese population [35]. These polymorphis ms, wh ich reside in genes that encode receptors for the Ig G isotypes, correlate with how efficiently PMNs are able to phagocytose opsonized antigens. The frequency of FcγR genotypes did not differ between chronic periodontitis patients and healthy controls. Among patients, however, one allele (FcγRIIIB-NA2) was found to be more prevalent in those who experienced recurrent disease. There have been very few studies to confirm the association between FcγR genotypes and refractory disease.

Gingi val Enlargement
Gingival enlargement is the overgrowth of the gingiva characterized by an expansion and accumulat ion of the connective tissue with occasional presence of increased number of cells. It is a form of periodontal disease which can be inherited (hereditary gingival fibro matosis (HGF), associated with other diseases characterizing a syndrome, caused by several factors, such as inflammation, leukemia, etc or induced as an adverse effect of systemic drugs, such as phenytoin, cyclosporine or n ifed ipine [1]. The gingival enlargement results in both esthetic and functional problems for affected individuals.
The inheritance condition in wh ich the gingival tissue spontaneously and progressively enlarges is identified as hereditary gingival fibro matosis (HGF). HGF, prev iously known as elephantiasis gingivae, hereditary gingival hyperplasia and hypertrophic gingiva, is traditionally considered an autosomal do minant disease. HGF is a rare disease of infancy characterized by progressive gingival enlargement of normal color and firm consistency that is non-hemorrhagic and asymptomatic. Males and females are equally affected at a phenotype frequency of 1:175,000. HGF results in diastemas, malpositioning of teeth, pro minent lips and open lip posture. Although the gingival enlargement does not directly affect the alveolar bone, the gingival swelling may increase the bacterial p laque accumulat ion, inducing periodontitis and bone resorption and halitosis [36].
The first case was probably reported by Gross in 1856 and although over the past few years increasing efforts have been made to understand the genetic, mo lecular and cellular basis of the gingival enlargement of HGF patients, it has been impaired by the intense clinical, genetic and biologic heterogeneity of the disease. Three different loci have been associated with the isolated form o f HGF: t wo mapping to chromosome 2 (GINGF on 2p 21-22 and GINGF3 on 2p22.3-p23.3), which do not overlap, and one mappping to chromosome 5 (GINGF2 on 5q13-q22). Of these loci, only the SOS1 (son of sevenless one) gene underlying the GINGF locus has been identified [32]. The SOS1 mutation linked to HGF was observed in a large mult igenerational Brazilian family segregating HGF as a highly penetrant autosomal dominant gene. The single insertion mutation, which causes a frame shift and a stop codon resulting in a truncated protein, was segregated in all affected patients and was not identified in unaffected family members or in control subjects [37].
Drug-induced gingival overgro wth has been seen in patients follo wing intake of phenytoin, cyclosporine or nifedipine. Phenytoin, cyclosporine and nifedipine are all metabolized by the hepatic cytochrome P450 en zy mes. Cytochrome P450 genes exh ibit considerable poly morphism, which results in interindividual variation in drug levels. This inherited variation in metabolis m of either drug may influence patient serum and tissue concentrations and hence their gingival response [38].
Another genetic marker that has been investigated in relation to drug-induced gingival overgrowth is the hu man ly mphocyte antigen expression (HLA). Investigation of this marker has been confined to the organ transplant patients since their HLA phenotype is determined p rior to transplantation. Several studies have reported on the relationship between HLA exp ression and the incidence of drug induced gingival overgrowth [38,39]. A study reported that patients who expressed HLA-DR1 are afforded some degree of protection against gingival overgrowth whilst HLA-DR2 may increase the development of this unwanted effect [39]. A trend towards an increased presence of HLA-A19 antigen has also been reported although the relationship was not significant after correction for mu ltip le significance testing. To date only HLA-B37 has been identified as a significant risk factor after correction for the effect of mult iple significance testing and these patients are protected in some way fro m the effects of gingival overgrowth. The concept of molecu lar mimicry in the wider field of periodontal disease or an effect on lymphocyte function have been postulated as the mechanisms that may tie HLA antigens to gingival overgrowth [38].
HGF is frequently an isolated disorder, but in some cases, it is associated with other alterations characterizing a syndrome (Table 3) [36].

Concluding Remarks
With recent evidence imp licating the role of periodontal disease on systemic health, an understanding of the potential role of genetic poly morphis ms on periodontal d isease may help in modifying our treat ment plan to improve periodontal health, as well as, general health. Emerging data strongly suggest that host genetic factors are important determinants for periodontitis susceptibility. Genetic tests may prove useful for identifying patients who are most likely to develop periodontal disease, suffer fro m recurrent disease or suffer tooth loss as a result of periodontal disease. Genetic polymorphis ms also result in the development of monogenic syndromes of which periodontal diseases are an important sign.
The present literature review is the first of its kind to focus on the various diseases affecting the periodontium and how genetic polymorphis ms can affect these diseases, unlike previous literature researches wherein the various polymorphis ms have been enlisted and discussed. The idea is to instigate the researcher to approach the disease and investigate the possible polymo rphisms, when present simu ltaneously, that could be aggravating or providing protection against the disease, rather than to identify polymorphis ms and calcu lating the possibility of disease causation or prevention individually.
At present, the search for risk alleles for periodontitis must focus on candidate gene regions. Genome-wide searches for risk alleles are not feasible due to the limited nu mber of genetic markers currently availab le for typing. Single nucleotide polymorphisms are likely to be valuable tools in the search for disease alleles. Recently, genome -wide association studies using single nucleotide polymorphisms or microsatellite poly morphis ms have become realistic due to the development of pro mising high-throughput and cost-effective single nucleotide po ly morphism typing. As suggested by Tamiya et al, microsatellite-based genome-wide association analysis complemented by end-stage single nucleotide poly morphis m typing will be an interesting strategy for genetic dissection of mu ltifactorial pathologies such as common diseases, including periodontitis [40].
Future strategies for the utilizat ion of genetic polymorphis ms in Periodontics will need to consider at least two factors. The first is to perform large-scale genetic analyses in diverse populations using as many target genes and subjects as reasonably possible. Secondly, there needs to be a development of new statistical analytical methods to combine both genetic and environmental factors.
To achieve these goals, mult icenter research studies will be needed to maximize cost-and time-effectiveness. As periodontitis is a mult ifactorial d isease, studies need to be carried out using nonparametric linkage analysis, case-control association analysis and transmission disequilibriu m analysis to determine susceptible or resistant genes for periodontitis. Before the endorsement of any such model as a clin ical screening tool, the models must be reproducible, bio logically plausible and relevant, and of sufficient sensitivity and specificity to warrant applicat ion to clin ical populations.