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The Genetic Bases For Perodontitis

It is increasingly evident that genetic variance is a major determinant of the differential risk for many human diseases (24, 49, 92). While microbial and other environmental factors initiate and modulate periodontal disease, individuals are known to respond differently to common environmental challenges, and this differential response is inf! luenced by the individual's genetic profile. Genes clearly play a role in the predisposition to and progression of periodontal diseases (62, 63, 73, 78, 122, 156). Support for the idea that genetic factors are important determinants of periodontitis susceptibility and progression comes from studies of humans and animals which indicate that genetic factors which impair inflammatory and immune responses in general, affect periodontitis experience specifically. It is hoped that identification of the specific genes causing periodontal disease susceptibility may have diagnostic and therapeutic value.

To understand the potential clinical relevance of genetic variability on periodontitis it is necessary to understand how different genes can contribute to disease. There are estimated to be 20,000–25,000 different genes in the human genome. Genes can exist in different forms or states. Geneticists refer to ! the different forms of a gene as allelic variants, or alleles. Genetic alterations could also change the transcript level of the protein, that is, the polymorphism may occur not in the protein coding region but in the gene promoter region, and thus influence the amount of protein produced by the gene. Allelic variants of a gene differ in their nucleotide sequence. Different allelic forms of a gene can produce an identical protein or different isoforms of a protein. In the former case, the genetic polymorphism does not change the amino acid and the genetic change is said to be 'silent'. In the latter case, a nucleotide change alters the amino acid composition of a protein. The results can range along a continuum of functional consequences, from no observable change in protein function, to a minor change in function, to a dramatic change or obliteration of function. Individual genetic changes that are causal of disease are typically the result of a genetic alteratio! n that dramatically alters a protein's function. Such genetic changes are typically termed mutations and are rare on a population level, typically being present in less than 0.1% of the population. When a specific allele occurs in at least 1% of the population, it is said to be a genetic polymorphism. In contrast to mutations, these more common genetic polymorphisms are usually considered normal variants in the population. Common genetic polymorphisms may change the function of a protein, but usually the change is relatively minor. Consequently, the specific protein products of different alleles may function differently. These differences in physiological functioning of different proteins can be enhanced by certain environmental exposures, e.g. diet, smoking, or microbial factors. If the affected protein functions in a biological process, e.g. inflammatory response to a specific microbial agent, certain polymorphisms may increase or decrease a person's risk for a disease phenotype! . Whether a particular gene variant contributes to a disease phenotype depends upon the magnitude of the effect in contributing to the development of disease. To appreciate the contribution of a genetic variant to a disease it is critical to understand how genes contribute to genetic diseases (Fig. 1). The contribution of an allelic variant to a disease can vary from being deterministic to having only a minor effect on the etiology. The manner and extent to which genetic factors play a role in disease have important implications for identifying the genetic basis of etiology and for utilizing this information for diagnosis and treatment. Geneticists have traditionally divided genetic diseases into two broad groups: 'simple' Mendelian diseases and 'complex' diseases. The distinction of these broad groups is based on the pattern of transmission of the disease, which reflects the manner in which genes contribute to each disease.

Simple 'Mendelian' diseases

Diseases that follow predictable and generally simple patterns of transmission have been called 'Mendelian' conditions. The name reflects the fact that these diseases occur in simple patterns in families and in most cases genetic alterations at a single gene locus are the major determinant of the clinical disease phenotype. These diseases follow a classic Mendelian mode of inheritance (autosomal dominant, autosomal recessive or X-linked). The disease phenotype usually manifests over a broad range of environments, and although environmental factors and other genes can modify the clinical presentation, in most cases the mutation will manifest in a remarkably similar phenotype. The population prevalence of individual Mendelian diseases is rare (typically much less than 0.1%), with the exception of some unique populations that have been isolated from other human populations. Examples of Mendelian! type diseases include amelogenesis imperfecta, Crouzon syndrome, cleidocranio dysplasia, and Papillon–Lefèvre syndrome (Table 1). When the gene responsible for a Mendelian disease has been identified, it is often possible to develop a diagnostic test to identify individuals who carry a disease-causing mutation in the responsible gene. Depending upon the mode of transmission, it is also possible to make fairly specific determinations of the probability of the mutant gene being passed to a child and often it is possible to predict the course of clinical disease.

Complex genetic diseases

Genetically complex diseases differ from simple Mendelian diseases in several important ways. Genetically complex diseases do not follow a simple pattern of familial distribution or transmission. In contrast to 'simple Mendelian traits', which are caused by a single gene, these 'complex traits' are ! the result of the interaction of alleles at multiple different gene loci. Environmental factors are usually etiologically important, and often necessary, in the development of complex diseases. On a population level, complex genetic diseases are much more common than simple Mendelian diseases, and many occur with a population prevalence of greater than 1%. Unlike rare mutations responsible for 'simple' genetic diseases, the allelic variants important for many complex genetic diseases are common in the population. In contrast to mutations that can eliminate a gene product or change the protein product of a gene so significantly that it acts to disrupt other biological processes, the individual genetic variants that are important in complex diseases are much less disruptive, and usually function within the normal range. Many disease-associated genetic polymorphisms are common in the population and can be present at allele frequencies of > 20%, with some disease-associated alle! les reported in > 50% of populations studied (38). The hypothesis that most of the genetic risk for common, complex diseases is due to common polymorphisms at disease loci is known as the Common Disease/Common Variant hypothesis (142). Recently, the idea concept has emerged that at least a portion of susceptibility variants for common diseases are less common (5–10%) but more penetrative (141). The fact that these genetic alterations are not sufficient alone to cause disease has important implications. Disease-associated alleles may be present in a significant proportion of the unaffected general population. Knowledge of the presence of one disease-associated allele in an individual does not provide enough information to make a clinical diagnosis. In contrast to genetic mutations that are often diagnostic for simple Mendel! ian conditions, the presence of a polymorphism in a complex trait can be difficult to interpret, and must be assessed together with other information. It is important to have information about the allele frequency in the population tested, and also to be able to quantify in a meaningful way the magnitude of effect a disease-associated allele has on the disease processes. For this reason some measure of the specificity and sensitivity of a disease-associated allele to predicting disease is desirable. Currently, considerable attention is being focused on the clinical validity and clinical utility of genetic polymorphisms that have been reported to be associated with a disease.

Polymorphism vs. mutation

A major difference in the genetic basis for simple Mendelian diseases vs. complex genetic diseases is the number of genes involved and the contribution of each gene to the overall disease phenotype. In Mendelian disease, alteration of a! single gene locus can result in disruption of a protein product that has a major physiological impact and therefore may be considered to be deterministic of the disease. The fact that the genetic alteration is predictably associated with a disease phenotype indicates that there is no redundancy or compensation in the particular biological system that can overcome the effect of the underlying genetic defect. In contrast, the genetic alterations that contribute to complex diseases are individually responsible for much more subtle perturbations of protein function. Consequently, it is difficult to establish causal links with genetic polymorphisms that are associated with complex diseases. There is not a one-to-one correlation of the presence of a specific genetic allele and the occurrence of a complex disease, but rather specific alleles are reported to be found more frequently in diseased individuals than in nonaffected controls. Often initial support for the link is the statistical 'association' of an allele with a disease state. It is also important to understand that disease alleles reported to be associated with a complex disease are also found in unaffected individuals. Because multiple (perhaps 5–8) different gene alleles can contribute to a complex disease state, an individual with a complex disease need not have all of the alleles reported to be associated with that disease. Thus, the presence of a disease-associated allele in an individual is not diagnostic for a disease. Environmental factors are also critical to the etiology of most common diseases. Most chronic diseases are of adult onset, and therefore take many years to develop. Genes involved in the innate, inflammatory, and immune responses are often involved. These physiological and biological pathways have many compensatory and redundant aspects, and therefore it is extremely difficult to quantify the effect of any one genetic variant to the disease state. Consequently, if an individu! al is found to have a disease-associated genetic polymorphism, it is often not clear what the clinical significance of this is.

Single nucleotide polymorphisms

The most common type of genetic polymorphism is a Single Nucleotide Polymorphism (SNP). There are an estimated 10 million SNPs in the human genome. SNPs occur throughout the genome, in regions encoding genes as well as in regions without identified open reading frames. SNPs in genes may occur in protein coding regions (exons) and noncoding regions (introns and regulatory regions). Many SNPs that occur in genes have no effect on the encoded protein, but a large number of SNPs do have an effect on the gene product. Whether this change contributes to a disease phenotype depends on the specific consequence of the genetic variant on the function of the gene and its protein product. An SNP is further defined as occurring in at least 1% of the population, and indeed several disease-re! lated polymorphisms occur at much higher frequencies (20–50%).

Clinical and scientific data from a variety of sources suggest that genetic variants are major determinants of syndromic and nonsyndromic periodontitis. To evaluate the quality of supporting studies requires an understanding of the formal genetic analytical methods that have been used. Geneticists use a variety of techniques to demonstrate the genetic basis of disease. Some methods are general, whereas others permit precise identification of genetic variants that cause or contribute to disease. The methods summarized below have been important in the evaluation of genetic diseases including periodontitis.

Familial aggregation

Familial aggregation of a trait or disease can s! uggest genetic etiology. However, families also share many aspects of a common environment, including diet and nutrition, exposures to pollutants, and behaviors such as smoking (active and passive). Certain infectious agents may cluster in families. Thus, familial aggregation may result from shared genes, shared environmental exposures and similar socioeconomic influences. To determine the evidence for genetic factors in familial aggregation of a trait, more formal genetic studies are required. There have been many clinical reports suggesting a familial aggregation of periodontitis, but until recently the research tools to pursue these reports were lacking (16, 66, 73).

Twin studies

Through the phenomenon of twins, in particular monozygous twins, who arise from one fertilized egg, nature has provided a wonderful tool for the examination of genetic influences in disease and for partitioning the relative contribution of genes and environment to a trait. Monozygous twins are genetically identical. Dizygous twins are only as genetically similar as brothers and sisters would be, on average they share 50% of their genes in common (dizygous twins are from two different eggs and two different sperm). Discordance or differences in the presence of disease between monozygous twins must be due to environmental factors. Disease discordance between dizygous twins could arise from both environmental and genetic differences. The difference in concordance between monozygous and dizygous twins for a particular phenotype can be used to estimate the relative contribution of genes (heredity) and environmental factors to a disease and studying disease presentation in twins is often a valuable f! irst step in this process.

Segregation analysis

Genes are passed from parents to children in a predictable manner, and usually segregate in families as predicted by Mendel's laws (127). Geneticists can study the pattern of disease transmission in families using a method called segregation analysis. Segregation analysis evaluates the relative support for different transmission models to determine which model can account for the observed segregation of a trait through families. By sequentially comparing models to each other, segregation analysis identifies the model that best accounts for the observed transmission of a trait in a given population. Geneticists generally apply segregation analyses to determine whether a trait transmission appears to fit a Mendelian or another mode of genetic transmission. When comparing genetic models of transmission, genetic characteristics including mode of transmission (e.g. autosomal, X-linked, dominant, recessive, complex, multilocus, or random environmental), penetrance, phenocopy rates and frequencies for disease and nondisease alleles are some of the characteristics included in the different models evaluated. But segregation analysis does not necessarily provide the true model. Since it is a comparison of two models, segregation analyses are only as good as the models tested. If important assumptions of the model tested are incorrect, this will limit the results. This limitation of segregation analysis must be realized, as it has resulted in inaccurate conclusions for the transmission of at least one form of early onset periodontitis (110). Segregation analysis tests alternative models to try to develop the best characterization of transmission characteristics within a set of data. As such, it is most appropriately applied to data sets of many families to determine the ! best fitting model. Segregation analysis does not find or aim to find a specific gene responsible for a trait.

Linkage analysis

Linkage analysis is a technique used to localize the gene for a trait to a specific chromosomal location. Genetic linkage studies are based on the fact that alleles at syntenic gene loci in close proximity on the same chromosome tend to be passed together from generation to generation (i.e. segregate), as a unit. Such genes are said to be 'linked', and violate Mendel's law of independent assortment. Geneticists can apply quantitative analyses to detect this lack of independent assortment of genetic loci, and can use it to map (localize) genes to specific chromosome locations. Over the past 15 years, genetic maps have been developed that show the position of millions of polymorphic genetic loci spanning the human genome (58) (http://www.ncbi.nlm.nih.gov/genome/guide/human/). Scientists can follow a specific trait as it segregates through families of interest and determine whether the trait appears to segregate with a known genetic polymorphism that has been localized to a specific chromosomal location. In this manner, scientists can test whether a trait appears to segregate in a manner consistent with 'linkage' to a known genetic marker. Because the precise chromosomal location of the genetic marker is known, when linkage is detected, the gene responsible for the trait can be placed in the vicinity of the linked genetic polymorphism. Linkage can therefore prove the genetic basis of disease. Linkage is often used as a first step to determine the approximate location of a gene of interest, permitting subsequent studies to identify the mu! tation responsible for a disease trait. Linkage studies have been particularly effective in identifying the genetic basis of simple Mendelian traits (OMIM 2004), where mutation of a single gene can cause a disease. Linkage studies of complex genetic traits have not been as successful for a variety of reasons (52, 171). A limiting factor in the traditional application of linkage to complex diseases is that complex diseases are due the combined effect of 'multiple genes of minor effect'. When multiple genes each contribute a small amount to the disease phenotype, traditional parametric linkage studies are much less powerful. Fortunately, newer adaptations of the linkage approach and the availability of Association testing approaches offer a practical alternative (106, 113, 186).

Association studies

Genes contributing to c! ommon, complex diseases such as periodontitis have proven more difficult to isolate. When multiple, perhaps many, genes act with environmental factors to contribute to disease liability, it is difficult to formulate disease models. In the absence of specific genetic models, the etiology of complex diseases is often conceptualized as due to multiple factors, i.e. several genetic loci interacting with each other to produce an underlying susceptibility, which in turn interacts with additional environmental factors to produce an actual disease state. For complex traits, such as bipolar disorder (11), diabetes (61), obesity (21), and oral–facial clefting (18, 128), traditional parametric linkage analysis has produced either negative results or a plethora of weak, positive results ! not easily replicated. Theoretical research suggests several reasons for the ambiguity of the linkage results in these cases. First, if a disease gene is neither necessary nor sufficient to cause a disease, but rather is a 'modifier gene' that elevates a nonzero baseline risk, conventional parametric linkage analysis may not detect the gene (57). Second, if the relative contribution of a gene to a disease phenotype is small, i.e. the disease susceptibility allele raises the risk by a factor of < 2, linkage analysis using affected sibling pairs will not be powerful enough to detect the gene, given realistic sample sizes (143). Thus, linkage analyses may not be a useful strategy to detect modifier genes or genes that exert small effects – precisely those genes which might be operating in chronic periodontitis and many other complex disorders. Consequently, attention has shift! ed away from model-dependent parametric linkage analysis to model-free, nonparametric 'association' analysis as an alternative means of locating disease susceptibility genes, especially since association studies can sometimes detect weaker effects than can linkage analysis (77).

Two types of association analysis are commonly employed in genetic studies: population-based and family-based (76). The population-based approach utilizes a standard case-control design, in which marker allele frequencies are compared between cases (affected individuals) and controls (either unaffected individuals or individuals randomly chosen from the population). When a positive association is found, several interpretations are possible:

• the associated allele itself is the disease-predisposing allele;
• the associated allele is in linkage disequilibrium with the actual disease-predisposing locus;
• the association is due to population stratification;
• the association is a sampling, or statistical, artifact.

The first two interpretations represent the alternative hypotheses of interest in a gene mapping context. In the first case, the marker itself is the disease-susceptibility locus. This outcome is the rationale behind candidate gene studies, in which alleles of the genes being tested have some a priori expectation of being directly involved in the disease process. Evidence of a positive association c! an be followed up by investigations to establish a functional role. In the second case, the associated allelic polymorphism itself does not play a functional role in causing disease, but rather the polymorphism is in close physical proximity to the gene that does contribute to susceptibility. A classic example is the human leukocyte antigen (HLA) system, in which various HLA haplotypes are associated with a number of diseases, including insulin dependent diabetes mellitus, rheumatoid arthritis, and ankylosing spondylitis (169). There is currently considerable attention being directed towards the clinical use of disease-associated genetic polymorphisms for genetic testing. However, most initial reports of these polymorphisms have not been replicated (75), reinforcing the need to develop acceptable criteria to determine the clinical validity of such reports (52). Fortunately, new approaches hold promise to identify significant disease associations that may be important for understanding susceptibility for complex diseases. There is currently great interest in comprehensively characterizing human SNPs to facilitate evaluation of their role in common diseases.

International HapMap project

The international HapMap project is being conducted to identify and catalog the common genetic variants that occur in human beings (http://www.hapmap.org). The project will describe each of the common SNPs in the human genome. The goal is to determine the genetic location of each SNP and characterize how these genetic variants are distributed in several different population groups. The project is at present studying DNA samples representative of African, Asian, and European ancestry! . By identifying most of the 10 million SNPs that occur in humans, the HapMap project will identify the DNA variants responsible for most of the genetic diversity in humans. The project is not intended to identify specific disease-associated polymorphisms. Instead, it is hoped that by providing such a catalog of common genetic variants, clinicians and scientists can work together to identify SNPs with important disease associations to understand disease etiologies and develop new diagnostic and treatment strategies.

The risk for many diseases, including periodontal diseases, is not borne equally by all individuals in a population (88, 89). A variety of microbial, environmental, behavioral, and systemic disease factors are reported to influence the risk for periodontitis (132). An individual's genetic makeup is a crucial factor influencing their systemic or host response-related risk.

There are reports in the literature on familial aggregation of periodontal diseases, but it is difficult to compare them. Although periodontal disease terminology has changed many times over the years, most familial reports of early onset forms of periodontitis are now referred to as aggressive periodontitis (8, 9, 14, 15, 17, 23, 46, 47, 90, 99, 110, 112, 121, 129, 147, 158, 161, 173). Reports of the familial nature of chronic forms of periodontitis are less frequent but the aggregation within families is consistent with a genetic predisposition. We should remember, however, that familial aggregation of periodontal disease may also reflect exposure to common environmental factors. Shared environmental factors include education, socioeconomic grouping, oral hygiene, shared transmission of bacteria, diseases such as diabetes and environmental features such as passive smoking, sanitation, etc. Some aspects of behavior are determined in part by genetics, which may influence potential modifying factors such as education, lifestyle and, of dental importance, oral hygiene. Complex interactions between genes and environment are difficult! to quantify, but are likely to be important when considering the familial risk for the periodontal diseases.

In chronic periodontitis, clinical disease characteristics do not present until the third decade of life, whereas in the aggressive forms of periodontal disease, the presentation can occur much earlier, in the early teens or younger (38). This variability in presentation makes diagnosis difficult, not only in declaring disease but also in detecting patients who are free of the disease and in differentiating between chronic and aggressive forms of periodontitis. The problems associated with the clinical diagnosis of periodontal disease are not uncommon in medical genetics as similar problems arise when studying other delayed onset genetics (16, 139). The problems of genetic model testing in aggressive periodontitis have been highlighted by Bo! ughman et al. (16) who noted that aggressive periodontitis has a variable age of onset and is often not recognized until after puberty. Diagnosis of aggressive periodontitis in older individuals is problematic due to the difficulties of distinguishing between aggressive periodontitis and chronic adult onset forms of periodontitis on the basis of the clinical signs. Similarly, diagnosis in older edentulous individuals is also problematic. The effects of the environment, for example plaque accumulation and smoking, also have major long-term influences on disease experience (Fig. 1) and these confound the diagnosis of aggressive periodontitis. Diagnostic quandaries create significant problems for genetic studies of periodontal disease, limiting attempts to correlate cellular, functional, and immune response variables with early onset periodontitis phenotypes in f! amilies. While there is evidence for a gene of major effect in aggressive periodontitis, early onset forms of periodontitis appear to be etiologically complex and heterogeneous (6, 15, 139, 140, 158). Although bacterial transmission between subjects has been suggested to explain aggressive periodontitis clustering within families, this observation alone is insufficient to account for familial clustering (15). While the heterogeneity paradigm discussed by Potter (139) is borne out in subsequent familial studies of what is now classified as aggressive periodontitis, the striking familial aggregation of the trait is consistent with a significant genetic etiology. Characterization of the specific genetic components in the etiology of this disease requires more formal genetic analyses.

Twin studies

Twin studies have been invaluable in studying the genetic basis of simple and complex traits. Large, worldwide registers of data on twins and their relatives have been established (13). Such twin studies registers offer unique opportunities for selected sampling of quantitative trait loci linkage and association studies. Twin studies of periodontitis, however, have generally been limited in scope and in subject numbers. However, studies of concordance for periodontitis and for clinical indices related to periodontal health and disease generally support a significant heritable component for periodontitis. Most twin studies have studied the more prevalent form of periodontitis, which is chronic periodontitis, as well as chronic gingivitis. Corey et al. (26) studied self-reported periodontal health in 4908 twin pairs and found that 9% of subjects, consisting of 116 identical and 233 nonidentical twin pairs, reported a history of periodontitis. The concordance rate, or level of similarity in disease experience, ranged from 0.23 to 0.38 for monozygous twins, and was much lower (0.08–0.16) for dizygous twins. These findings suggest that heritable factors are important in the reported periodontitis experience. Unfortunately, environmental factors such as smoking status were not factored into this analysis and could introduce a bias towards finding a correlation between twins.

Michalowicz et al. (124) studied dizygous twins reared apart (dizygous-A) and reared together (dizygous-T) and monozygous twins reared apart (monozygous-A) and reared together (monozygous-T). The mean probing depth and clinical attachment level scores were found to vary less for monozygous-T than for dizygous-T twin pairs, further supporting the role of genetics in this d! isease. Michalowicz et al. (123) investigated alveolar bone height and showed significant variations related to genotype. The twin groups had similar smoking histories and oral hygiene practices. It was concluded that genetics plays a role in susceptibility to periodontal disease. In a subsequent study of 117 adult twin pairs, Michalowicz and coworkers estimated genetic and environmental variances and heritability for gingivitis and chronic periodontitis (125). Genetic and environmental variances and heritability were estimated using models with maximum likelihood estimation techniques. Monozygous twins were found to be more similar than dizygous twins for all clinical measures. Statistically significant genetic variance was found for both the severity and the extent of disease. Chronic periodontitis was estimated to have approximately 50% heritability, and was u! naltered after adjusting for behavioral variables, including smoking. Monozygous twins were also more similar than dizygous twins for gingivitis scores but there was no evidence of heritability for gingivitis after behavioral covariates such as utilization of dental care and smoking were incorporated. These results confirm previous studies and indicate that approximately half of the variance for chronic periodontitis is attributable to genetic variance. The basis for the heritability of periodontitis appears to be biological and not behavioral (125).

Segregation analysis

While familial aggregation suggests a heritable component in the etiology of aggressive periodontitis, and twin studies support a genetic component in chronic periodontitis, neither is appropriate to identify the genetic model or specific gene loci involved in these periodontal diseases. Segregation analyses can evaluat! e the relative support for different models to identify the one that most closely represents the clinical data. Segregation analysis is very dependent upon the assumptions of the analyses and there have been few rigorous segregation analyses of aggressive periodontitis. Many are merely studies of one or more families and are statistically underpowered. Genetic segregation analysis needs accurate clinical identification of affected individuals and familial relationships as well as genetic assumptions of the analysis. If inaccurate assumptions or data are used, the outcomes will reflect this. Early studies of aggressive forms of periodontitis were hampered by diagnostic classification issues and by an overrepresentation of affected females (67, 147), falsely supporting X-linked transmission (68). Linkage reports of aggressive periodontitis in an extended kindred from Maryland ! supported an autosomal dominant transmission (14). A segregation analysis of North American families was performed by Marazita and coworkers, who studied more than 100 families segregating aggressive forms of periodontitis; their results supported an autosomal dominant transmission (112). They concluded that autosomal dominant inheritance with approximately 70% penetrance occurred for both Blacks and non-Blacks. The currently held theory on the genetics of aggressive periodontitis is that prepubertal periodontitis, localized aggressive periodontitis, and generalized aggressive periodontitis are probably due to a major gene locus transmitted in an autosomal manner with reduced penetrance; there is evidence for both autosomal recessive (147) and autosomal dominant forms (14, 112! ). It is likely that these aggressive forms of periodontitis are genetically heterogeneous, meaning that while the mutated gene responsible for the condition is likely to be the same in any given family, there are probable several different genetic loci that, if mutated, can cause aggressive periodontitis. The expression 'reduced penetrance' means that some subjects with the genotype may not actually express the phenotype, i.e. the clinical manifestations of aggressive periodontitis, whereas others may express it fully. Environmental factors (such as smoking and plaque control) as well as epigenetic interactions of multiple susceptibility genes may play a large role in whether the phenotype is expressed clinically.

Clinical evidence as well as laboratory studies of periodontitis patients suggest genetic heterogeneity (6, 69). Consequently, the suggestion that aggressive periodontitis may be transmitted in different wa! ys in different families is not a surprise. Some reviews have reported this as problematic, but this is not necessarily the case. There is a common precedent in genetics for a heritable pathologic condition to show different modes of inheritance in different families. Often, once the genetic basis of the condition is understood, it is found that mutations in different genes can cause a clinically similar group of conditions, as occurs in the Ehlers–Danlos syndromes, a heterogeneous group of heritable connective tissue disorders. These conditions involve aberrations of collagen, and several are known to have periodontal disease manifestation. More than 15 forms of Ehlers–Danlos syndromes are known, and these show different inheritance patterns including autosomal dominant, recessive, and X-linked forms. These findings reflect that different genetic loci are capable of causing the disease in dominant and recessive ways. Some of the genes responsible are autosomal and others are X-l! inked, accounting for the different observed modes of transmission. For aggressive forms of periodontitis, the preponderance of the evidence supports autosomal dominant transmission in North America and autosomal recessive transmission in certain European populations (112, 146, 147). These different modes of transmission may reflect genetic heterogeneity such as that seen with Ehlers–Danlos syndromes.

Linkage studies in aggressive periodontitis

Three linkage studies have been performed to date on families with localized aggressive periodontitis. Boughman et al. (14) identified an autosomal dominant form of localized aggressive periodontitis in an extended family from Southern Maryland. In this family, type III dentinogenesis imperfecta and a localized form of aggressive periodontitis were segregating as dominant traits.! Since the gene for dentinogenesis imperfecta-III had been previously localized to chromosome 4, they performed a linkage analysis on this chromosome and demonstrated a relatively close linkage with the suspected locus for aggressive periodontitis (14). This was an important study because it supported autosomal dominant inheritance of a single major gene locus, clearly indicating a major genetic component to the aggressive periodontitis disease etiology. Hart et al. (69) evaluated support for linkage to this region of chromosome 4 in a different population of families (14 African American and 4 Caucasian). Their findings supported genetic locus heterogeneity of aggressive periodontitis, as they excluded a chromosome 4 major gene locus for aggressive periodontitis in the families they studied. Thus, this Brandywine population appears to have a different form of periodontal dis! ease with a different gene being responsible compared with the African-American and Caucasian families studied by Hart and coworkers. These findings support genetic heterogeneity, with at least one gene locus responsible for aggressive periodontitis located on chromosome 4. Recently, Li and coworkers (107) reported evidence of a gene responsible for localized aggressive periodontitis located on chromosome 1q25. To date, a gene of major effect for aggressive periodontitis has not been identified.

Syndromic forms of periodontitis

Significant, irrefutable clinical and laboratory data indicate that genetic variants do predispose to disease states in humans. Severe periodontitis presents as part of the clinical manifestations of a number of monogenic syndromes and the gene mutation and biochemical defect is known for many of these conditions. A commonality of these conditions is that they are inherited as simple Mendelian traits due to genetic alterations of a single gene locus. Examples of these monogenic conditions are given in Table 1. The significance of these conditions is that they clearly demonstrate that a genetic mutation at a single locus can impart susceptibility to periodontitis. Additionally, these conditions illustrate that this genetic susceptibility may segregate by different transmission patterns. Because altered proteins function in different structural and immune pathways, genetic modulation of a variety of different genes can affect a variety of different physiological and cellular pathways (63). These conditions illustrate that the genetic contribution to periodontitis susceptibility is multifaceted, and may potentially involve many different gene loci. However, in contrast to nonsyndromic forms of periodontitis, these conditions have pe! riodontal disease manifestations as part of a collection of syndromic manifestations. In most cases of aggressive periodontitis, individuals present with clinical manifestations of periodontitis, but do not appear to have any other clinical disease manifestations. This is not inconsistent with a genetic disease etiology. Expression of genes can vary in different tissues, and mutations of a ubiquitously expressed gene can result in a tissue-specific condition. Recently, mutation of the SOS1 gene has been identified in individuals with hereditary gingival fibromatosis (72). SOS1 is important in determining whether cells grow, divide or differentiate and is ubiquitously expressed. However, the only clinical manifestation of this gene defect appears to be enlargement of the periodontium, an instance of the tissue-specific nature of many diseases. A similar tissue-specific manifestation of a gene defect may occur in nonsyndromic aggressive periodontitis.

Neutrophil functional disorders

Molecular biology has highlighted the important role of several receptors on the polymorphonuclear leukocyte surface in adhesion, and emphasized that defects in the number of these receptors may lead to increased susceptibility to infectious disease (2). Adhesion is crucial to the proper function of the polymorphonuclear leukocyte because it affects phagocytosis and chemotaxis which, if deficient, might predispose to severe periodontal destruction. Page et al. (131) proposed that the generalized form of prepubertal periodontitis (this disease has generalized and localized forms) is a localized oral manifestation of the leukocyte adhesion deficiency syndromes. More recent studies have indicated that generalized and localized prepubertal periodontitis occur in otherwise healthy children (17, 151). Leukocyte adhesion deficiency occurs in two forms, leukocyte adhesion deficiency syndrome type 1 and leukocyte adhesion deficiency syndrome type 2, both of which are autosomal recessive traits. Circulating leukocytes have reduced or defective surface receptors and do not adhere to vascular endothelial cells; thus they do not accumulate in sites of inflammation where they are needed. Reports of leukocyte adhesion deficiency syndromes indicate that although the blood vessels are full of neutrophils, the disease sites lack sufficient leukocytes to combat the microbial challenge and thus infections ensue rapidly in these patients. Affected homozygotes suffer from acute recurrent infections that are commonly fatal in infancy. Those surviving will develop severe periodontitis, which will begin as the primary dentition erupts (174).

Other disorders of neutrophil fun! ction are associated with severe forms of periodontal destruction. The Chediak–Higashi syndrome is a rare disease transmitted as an autosomal recessive trait. Those affected are very susceptible to bacterial infections due to alterations in the functional capacity of the polymorphonuclear leukocyte. Humans (59) and other animals (105) with Chediak–Higashi syndrome exhibit generalized, severe gingivitis and extensive loss of alveolar bone and premature loss of teeth (168). The polymorphonuclear leukocyte chemotactic and bactericidal functions are thought to be abnormal in these patients.

These diseases related to anomalies of leukocyte and polymorphonuclear leukocyte function are excellent examples of how monogenic defects can cause periodontitis through a clearly attributable mechanism. The host response is made up of a! vast number of processes, all of which are under genetic control and all of which are feasible candidates for irregularities that may result in variability in the clinical presentation of periodontal disease. Deficiency in neutrophil numbers (neutropenias)

A further neutrophil deficiency is found in infantile genetic agranulocytosis, a rare autosomal recessive disease where polymorphonuclear leukocyte numbers are very low and which has been associated with aggressive periodontitis (144).

Cohen's syndrome is another autosomal recessive syndrome and is characterized by mental retardation, obesity, dysmorphia, and neutropenia. Individuals with Cohen's syndrome show more frequent and extensive alveolar bone loss than do age-, sex-, and mental ability-matched controls (1).

Not all neutropenias result in periodontal disease. Familial benign c! hronic neutropenia has variable expressivity and although several individuals within a family may be neutropenic, not all are affected by recurrent infections or periodontal disease (34). These findings might be explained by the variable genetic expression of the disorder or by the variable effects of the environment (such as plaque or smoking) on these patients. Genetic defects of structural components

Papillon–Lefèvre Syndrome is a condition in which the cardinal clinical features are severe periodontitis and great variation in the severity and extent of palmar plantar hyperkeratosis (56, 60, 70). Genetic linkage studies narrowed the Papillon–Lefèvre gene locus to chromosome 11 and subsequent mutational analyses permitted identification of mutations in the cathepsin C gene in patients with Papillon–Lefèvre syndrome (64, 170). Subsequent studies have identified more than 40 different cathepsin C mutations in individuals from many different ethnic groups (65). This is an excellent example of the success of genetic studies in contributing to the identification of a gene defect of periodontal importance. Genetic linkage studies permitted localization of the gene defect to a specific chromosome, permitting focused mutational analyses on genes within an area of the chromosome. These focused analyses uncovered gene mutations in the cathepsin C gene. Mutations of this gene are associated with the loss of protease activity of the cathepsin C protein. Additional work has demonstrated that Papillon–Lefèvre syndrome and Haim–Munk syndrome (a slightly different clinical variant within the Papillon–Lefèvre syndrome group of disorders) are allelic variants of cathepsin C gene mutations, as predicted by Gorlin et al. (56, 65, 182, 183).

Ehlers–Danlos syndrome refers to a collection of connective tissue disorders characterized by defective collagen synthesis. Ehlers–Danlos types IV and VIII are related to an increased susceptibility to periodontitis (71, 108) and are inherited in an autosomal dominant manner. Clinical characteristics of type VIII Ehlers–Danlos syndrome include fragility of the oral mucosa and blood vessels, and a severe form of aggressive periodontitis (3).

Other genetic conditions related to defects in structural components that maintain a healthy periodontium include the very rare Weary–Kindler syndrome and hypophosphatasia. Aggressive periodontitis has been reported in Weary–Kindler syndrome where abnormalities of the epidermal keratinocytes occur (176). Patients with hypophosphatasia have a decreased serum alkaline phosphatase and the presence of phosphoethanolamine in the urine (48). In these patients, there is severe loss of alveolar bone and premature loss of the primary teeth (12, 19), particularly anteriorly (7). There is histologic evidence of enlarged pulp chambers and a disturbance in cementogenesis, the cementum being either absent or hypoplastic. Gingival fibroblasts are also deficient in alkaline phosphatase. The lack of connective tissue attachment between the tooth and bone accounts for the early spontaneous exfoliation of the primary teeth. Baab et al. (7) described a family where all three children manifested premature exfoliation of the primary teeth similar to that seen in prepu! bertal periodontitis (133). These children were assigned a diagnosis of hypophosphatasia on the basis of alkaline phosphatase and phosphoethanolamine levels. Baab et al. (7) noted that their data suggest an autosomal dominant mode of transmission and suggested that hypophosphatasia might be considered in the etiology of some forms of aggressive periodontitis.

In complex diseases, genetic variants at multiple gene loci contribute to overall disease susceptibility. As such, a simple cause and effect relationship between a particular genetic allele and a disease is not possible. Support for an etiologic role for genes (Table 1), complex genetic diseases which are not diachronic periodontitis, is based on the statistically significant association of specific genetic alleles in individuals with disease (cases) compared to individuals without disease (controls). This mathematical association is not necessarily biological or physiological. Studies reporting such associations vary in design and rigor. Association studies ideally should evaluate large numbers in population-based studies and have the power to detect a significant association. The issues of allele frequency in the population studied, case control design, and popu! lation stratification are very important but are unfortunately often omitted from dental studies. This is not acceptable, and studies performed without sufficient rigor need to be interpreted in this light (52, 75, 109). The overstating of results has become commonplace such that rigorous, scientifically principled approaches are needed to guard against unfounded and erroneous conclusions. Tables 2–5 report a multitude of studies evaluating associations of genetic polymorphisms with periodontitis. The variation in design and power of these studies raises questions regarding the appropriateness of incorporating these studies into rigorous analyses such as the meta-analysis techniques used in Cochrane-style

systematic reviews.

Gene polymorphisms of host response elements and periodontitis The genetic basis of many a! spects of the periodontal host response has been discussed in reference to genetic disorders predisposing to periodontal disease. The aim of this section is to generally summarize the potential influence of innate, inflammatory, and immunological genetic variations and to consider where the most promising candidates lie from the viewpoint of a genetic diagnostic approach to periodontitis (Tables 2–5).

There is a genetic basis to many aspects of the periodontal host response. Mutations of genes have been identified in Mendelian inherited syndromes that have significant periodontitis findings (Table 1), clearly indicating that single gene defects can predispose to periodontitis. Data from human and animal studies indicate that genetic variance can influence the innate, inflammatory, and immunological response to microbial infection. The availability of the ! annotated human genome and technological advances in genotyping have made it possible to genotype allelic variants and to test for association in case–control studies. This has facilitated studies to evaluate the support for / against the association of an array of genetic polymorphisms with periodontitis periodontitis (Tables 2–5). It is important to realize that most of these studies are underpowered to draw definitive conclusions.

Several features of the host's innate immune system which may contribute to genetic susceptibility to aggressive periodontitis have been outlined already and include epithelial, connective tissue, and fibroblast defects. Functional defects or a deficient number of polymorphonuclear leukocytes, as discussed previously, have profound effects on the host's susceptibility to periodontitis. Another aspect of the host inflammatory response, proinflammatory cytokines, has attracted much attention as potentially crucial variants influencing the host response in periodontitis. This review will not attempt to summarize the plethora of SNP studies in periodontal disease but will summarize tabularly (Tables 2–5) the areas of current research and focus on the interleukin-1 polymorphism by way of example and illustration of the SNP periodontal literature.

Interleukin-1 gene polymorphisms in periodontal disease

The interleukin-1 gene polymorphisms associated with periodontitis are a useful example for considering the strengths and limitations of using gene polymorphisms in disease association studies in the periodontal diseases. In 1997, Kornman et al. (100) found an association between polymorphisms in the genes encoding for interleukin-1a (- 889) and interleukin-1ß (+ 3953! ) (termed the 'composite genotype') and an increased severity of periodontitis. This initial study spawned numerous publications and has been highly influential in creating interest in gene polymorphisms and periodontal disease. The specific genotype of the polymorphic interleukin-1 gene cluster (periodontitis susceptibility trait, PST or 'composite genotype') was only associated with severity of periodontitis in nonsmokers, and distinguished individuals with severe periodontitis from those with mild disease (odds ratio 18.9 for ages 40–60 years, but wide confidence intervals of 1.04–343.05). Functionally, the specific periodontitis-associated interleukin-1 genotype constitutes a variant in the interleukin-1ß gene that is associated with high levels of interleukin-1 production (136, 137). Kornman et al. (100) found that 86.0% of the severe periodontitis in patie! nts was accounted for by either smoking or the interleukin-1 genotype. Similar results were reported by McDevitt et al. (115) and from smaller studies by McGuire et al. (117), Laine et al. (101), and Gore et al. (55) (who suggested that the two polymorphisms within the 'composite genotype' may be in linkage disequilibrium). Other contradictory reports such as Meisel et al. (119) stated that the 'composite genotype' showed a strong interaction with smoking, an established risk factor for periodontitis (93), whereas nonsmokers, even when genotype positive, were not at any increased risk. A similarly contradictory study (134) of 132 periodontitis patients who were age- and sex-matched with controls, did not show any association between the 'composite genotype' and periodontitis. The prognostic utility of the interleukin-1 genotype on chronic periodontitis progression following nonsurgical therapy was performed by Ehmke et al. (37). Of the 33 patients studied, 16 had the susceptible 'composite genotype' reported by Kornman et al. (100). Following 2 years of periodontal maintenance care, no differences in tooth or attachment loss were detected between those with or without the genotype. Equivocal studies such as those of Cullinan et al. (28) demonstrated an interaction of the interleukin-1 positive genotype with age, smoking, and P. gingivalis, which suggests that interleukin-1 genotype is a con! tributory but nonessential risk factor for periodontal disease progression in this population. Cattabriga et al. (20) reported no significant differences in tooth loss in patients with the interleukin-1 genotype after 10 years in a nonsmoking, well-maintained periodontal population. De Sanctis et al. (31) demonstrated that genotype expression did not affect guided tissue regeneration treatment response at 1 year, but had a great impact on long-term stability (year 4). In a 3-year period, patients with a positive interleukin-1 genotype lost about 50% of the clinical attachment level gained in the first year and were about 10 times more likely to experience = 2 mm clinical attachment loss when compared to oral hygiene-matched genotype-negative patients. Indeed, numerous studies, as depicted in Tables 2–5, show associations and lack of associations across different and similar populations for various forms of chronic and aggressive periodontitis.

The polymorphisms in the interleukin-1 gene cluster linked with periodontitis (100) are found in approximately 30% of the European population. However, the prevalences are dramatically lower in Chinese (2.3%) and thus the usefulness of the 'composite genotype' of allele 2 of both interleukin-1a (+ 4845) and interleukin-1ß (+ 3954) for determining susceptibility in Chinese patients is dubious (5).

Interleukin-1 polymorphism in aggressive periodontitis

Hodge et al. (80) examined interleukin-1a and interleukin-1ß genetic polymorphisms in unrelated European white Cauca! sian patients with generalized early onset periodontitis and found no significant differences between patients and controls for any of the 'composite genotypes' described by Kornman et al. (100). No significant differences were found between patients and controls whether smoking was included as a covariate or not. It was concluded that there was a lack of association between the interleukin-1 polymorphisms and aggressive periodontitis, which questions the utility of these candidate genes as markers of susceptibility. This was a relatively homogeneous Scottish population and the results, although negative, merely reflect the lack of utility of this 'composite genotype' test in this population.

Other studies on the 'composite genotype' reported by Kornman et al. (100) and aggressive periodontitis have had similarly mixed results. For example, the stu! dies by Diehl et al. (35, 36) actually found that allele 1 rather than allele 2 of the interleukin-1ß + 3953 exhibited polymorphism. Furthermore, Parkhill et al. (135) investigated the frequency of polymorphisms in the genes encoding interleukin-1ß in Caucasians with aggressive periodontitis compared to controls. The frequency of interleukin-1ß genotypes homozygous for allele 1 of the interleukin-1ß + 3953 SNP was found to be significantly increased in aggressive periodontitis patients (P = 0.025). Upon stratification for smoking status, a significant difference was found in the interleukin-1ß genotype distribution between aggressive periodontitis smokers and control smokers (F-exact test, P = 0.02), but not between aggressive periodontitis nonsmokers and control nonsmokers. The interleuki! n-1ß 1 / 1 genotype occurred at a higher frequency in aggressive periodontitis smokers (odds ratio = 4.9) than in control smokers. These findings of Parkhill et al. (135), in contrast to those of Hodge et al. (81), found that an interleukin-1ß genotype in combination with smoking is associated with aggressive periodontitis.

Similar negative findings for this 'composite genotype' and both chronic and aggressive periodontitis populations from different racial and ethnic backgrounds have been demonstrated and thus the diagnostic utility of the 'composite genotype' may be restricted to specific populations, i.e. the results do not appear to be applicable globally and across ethnic populations, and certainly not for aggressive periodontitis. Biological plausibility for the composite interleukin-1 polymorphisms

Many investigators have suggested a role for interleukin-1 in the initiation and progression of periodontitis and have quoted in vitro and in vivo studies showing that interleukin-1 activates the degradation of the extracellular matrix and bone of the periodontal tissues. Elevated tissue or gingival fluid levels of interleukin-1ß have been associated with periodontitis. Kornman et al. (100) quotes abstracts of in vitro studies from Pociot et al. (137) and others (104) which claim that the interleukin-1 polymorphism associated with severe periodontitis in the Kornman study is also known to correlate with a two- to fourfold increase in interleukin-1ß production. A problem with this line of reasoning is that interleukin-1 is a proinflammatory cytokine intimately involved in all inflammatory reactions as well as immune and reparative or healing responses and any perturbation of its level so as not to be homeostatically controlled could have widespread consequences not limited to periodontal disease. In addition, interleukin-1 is one of many proinflammatory cytokines (interleukin-6, tumor necrosis factor-a, etc.) with overlapping activities and thus some redundancy exists in the cytokine system. Furthermore, interleukin-1 has many controlling mechanisms which include inhibition of transcription, release controls, and receptor antagonists, and thus is highly regulated such that any polymorphism coding for increased production of this molecule could readily be controlled by the elaborate positive and negative feedback loops associated with its regulation.

The claimed association of severe periodontitis with smoking and the interleukin-1 genotype (in that in smokers the composite interleukin-1 genotype did not influence susceptibility)! poses further problems. Do smoking and the overproduction of interleukin-1 work along the same pathogenic pathway and thus is the action of both factors not additive but renders the other redundant, or is the overall effect of smoking is so overriding that the 'composite genotype' has little or no effect? These possibilities, while feasible, require much more mechanistic knowledge for both risk factors, but especially for the genotype, given that the association with periodontitis is not as established as that found in the literature on smoking (93). Socransky et al. (155) investigated the association between the 'composite genotype' and carriage of periodontal species. They found the mean counts of specific species were higher in general in interleukin-1 genotype positive than in negative subjects. The species detected at higher levels were those frequently ass! ociated with measures of periodontal inflammation. A further study aimed at studying the 'composite genotype' and inflammation was performed by Lang et al. (103). Genotype-negative subjects had a significantly lower percentage of bleeding on probing (P = 0.0097) and it was concluded that the increased bleeding on probing prevalence and incidence observed in interleukin-1 genotype-positive subjects indicates that some individuals have a genetically determined hyperinflammatory response that is expressed in the clinical response of the periodontal tissues.

Shirodaria et al. (154) have taken the research further by attempting an assessment of the functional effect of the 'composite genotype' in terms of the quantity of interleukin-1a protein in gingival crevice fluid of severe chronic periodontitis patients. These researchers found that al! lele 2 at position - 889 of the interleukin-1a gene (one of the alleles linked with susceptibility to periodontitis by Kornman et al. (100)) was associated with a fourfold increase in interleukin-1a as determined by enzyme linked immunoassay. This technique does not demonstrate activity but merely protein presence or absence and would not differentiate protein bound to receptors or inhibitors. Furthermore, it is feasible that inhibitors of proinflammatory cytokines may concomitantly be produced to dampen this effect. The authors noted reduced levels of interleukin-1a protein in heavy smokers regardless of genotype but this may be related to the reduced gingival crevice fluid noted in smokers (93). This is a useful study given that it addresses the in vivo effects of the polymorphism on interleukin-1 protein quantities, However, differences in local g! ingival crevice fluid production among patients, sites, smokers, and across gender add considerable variance to such a study, and these factors have to be considered in the interpretation of the data.

Engebretson et al. (40) also found elevated levels of interleukin-1ß in the gingival crevice fluid in shallow sites of patients who were positive for the 'composite genotype' reported by Kornman et al. (100). Smoking was not considered in the study and no statistically significant differences were noted for deeper pockets. Interestingly, of the 22 chronic periodontitis patients examined, only seven were positive for the susceptible genotype. Mark et al. (114) studied peripheral blood monocytes from 'composite genotype' positive and negative patients to examine whether the interleukin-1^! 6; polymorphism was correlated with increased interleukin-1ß expression by monocytes in response to periodontal bacterial stimulus. Contrary to previous reports, these workers found no significant differences in interleukin-1ß production in response to any stimulant tested. They went on to report marked interindividual variation in the production of interleukin-1ß in both the genotype positive and negative patient groups. Clearly, either the genotype is not important in monocyte production of interleukin-1 or other genetic loci may determine the monocyte interleukin-1 responses. Summary of the findings on the interleukin-1 'composite genotype' in periodontitis It appears that the interleukin-1 'composite genotype' has an equivocal ability to detect susceptibility to periodontitis and may at best be limited in its utility to only specific populations. It would appear from the mixed reports on this 'composite genotype' that:

• it appears irrelevant in aggressive periodontitis;
• it may be in linkage disequilibrium with the gene contributing susceptibility to chronic periodontitis;
• the composite polymorphisms may be part of several involved in the genetic risk for chronic periodontitis;
• the polymorphism is only a useful marker in defined populations (5, 175);
• confirmation of the functional significance of this gene polymorphism remains to be confirmed;
• clinical utilization of the composite polymorphisms for risk assessment and prognostic determination is premature.

Larry replies:

That was a very long article. The first paragraph was the only part that actually had clinical releavance as far as I could see. If you read the entire article it would be easy to ignore the part we are really interested in.

Fact is genetics is an important aspect of periodontitis and a knowledge of the patients family history is essential for risk analysis.

I agree there is no genetic testing available which I would consider using. A simple family history however, I consider essential.

Here is the releavant part of the first paragraph.

The Genetic Bases For Perodontitis

It is increasingly evident that genetic variance is a major determinant of the differential risk for many human diseases (24, 49, 92). While microbial and other environmental factors initiate and modulate periodontal disease, individuals are known to respond differently to common environmental challenges, and this differential response is influenced by the individual's genetic profile. Genes clearly play a role in the predisposition to and progression of periodontal diseases (62, 63, 73, 78, 122, 156). Support for the idea that genetic factors are important determinants of periodontitis susceptibility and progression comes from studies of humans and animals which indicate that genetic factors which impair inflammatory and immune responses in general, affect periodontitis experience specifically

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