Mechanisms of Disease: genetics of functional ... - Nature

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Mechanisms of Disease: genetics of functional gastrointestinal disorders—searching the genes that matter Birgit Adam, Tobias Liebregts and Gerald Holtmann* S U M M A RY There is evidence to suggest that genetic factors contribute to the manifestation of functional gastrointestinal disorders (FGID). As such, it is important to note that FGID are heterogeneous; they have quite different clinical features and (probably) different underlying pathophysiologic mechanisms. Evidence from family and twin studies indicates that there is clustering of FGID in families and increased concordance in monozygotic compared with dizygotic twins. The clinical features of FGID implicate polymorphisms in the genes that encode adrenergic, opioidergic or serotonergic receptors, as well as in the G-protein β3 subunit (GNB3) gene and serotonin-transporter genes, in their manifestations. As mediators or regulators of mucosal inflammation can trigger events that ultimately result in manifestations of FGID, polymorphisms in genes that encode proteins with immunomodulatory and/or neuromodulatory features (e.g. OPRM1, IL4, IL4R, TNF) might also have a role in the manifestation of FGID. A two-step model for the role of genetic factors in the manifestation of functional gastrointestinal pain can, therefore, be proposed. In the presence of specific hereditary factors, environmental factors that do not usually cause long-term functional alterations are linked to the manifestation of symptoms. KEYWORDS functional gastrointestinal disorders, genetic factors, irritable bowel syndrome, pathophysiology

REVIEW CRITERIA PubMed was searched in February 2006 for full papers published in English-language, peer-reviewed journals, using these keywords (alone or in combination): “irritable bowel syndrome”, “genetic”, and “functional dyspepsia”. We only included full papers published since 1998; however, we have included the most recently published ROME III criteria. The reference list was updated in October 2006.

INTRODUCTION

Functional gastrointestinal disorders (FGID) such as functional dyspepsia and irritable bowel syndrome (IBS) are highly prevalent in Western populations.1,2 On the basis of the Rome III criteria, patients who suffer from functional dyspepsia in the absence of any organic disease are categorized as having postprandial distress syndrome or epigastric pain syndrome.3 IBS is defined as recurrent abdominal pain or discomfort for at least 3 days per month in the past 3 months that is associated with two or more other symptoms such as improvement with defecation; onset of IBS symptoms is associated with a change in frequency of stool and with a change in form of stool.4 There are four subgroups of IBS; this classification is based on the predominant stool pattern according to the Bristol stool-form scale: constipation-predominant IBS, diarrheapredominant IBS, mixed IBS, and unclassified IBS.5,6 There is increasing evidence that inflammation is a risk factor for the development of both functional dyspepsia and IBS.7–9 FGID are also associated with anxiety and depression.10,11 Pathophysiologic abnormalities of FGID include altered visceral sensory function,12,13 altered motility, bacterial overgrowth,14 and altered central nervous system processing. Genetic factors have been postulated to have a major role in the manifestation of FGID. In this article, we review past studies of the genetic factors thought to influence manifestation of FGID. GENERAL PRINCIPLES

B Adam and T Liebregts are Fellows, and G Holtmann is Director of the Department of Gastroenterology, Hepatology and General Medicine, Royal Adelaide Hospital, University of Adelaide, Australia. All also work at the Nerve–Gut Research Laboratory, Hanson Institute, Adelaide, Australia. Correspondence *Royal Adelaide Hospital, Department of Gastroenterology, Hepatology and General Medicine, University of Adelaide, North Terrace, Adelaide, SA 5000, Australia [email protected] Received 13 April 2006 Accepted 24 November 2006 www.nature.com/clinicalpractice doi:10.1038/ncpgasthep0717

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It is unlikely that a single genetic factor causes FGID. It is rather more likely that a genetic factor (or factors) modulates the risk of developing the abnormalities that are characteristic of FGID after exposure to one or more specific environmental factors. This hypothesis is well illustrated by the influence of the GNB3 genotype on the reduction in coronary blood flow after α2-adrenoreceptor stimulation (Figure 1).15 Patients’ responses to this well-defined pharmacologic stimulus are quite variable, but at least part of the variation is due to

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Reduction in coronary blood flow (%)

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Figure 1 The response of coronary blood flow to α2 adrenoceptor stimulation. (A) There is considerable variation in the response to α2 adrenoceptor stimulation, some of which can be attributed to the patient’s GNB3 genotype. (B) Although there is a quite variable response in individuals who carry a CC genotype (left side), those with the TC or TT genotype (right side) have, on average, a significantly greater reduction in blood flow (P = 0.001). Data from Baumgart et al. (1999)15

differences in GNB3 genotype. Indeed, individuals with various GNB3 genotypes (i.e. 825CC, 825TC or 825TT) have significantly different responses to α2-adrenoreceptor stimulation. It is conceivable that, in a similar way, a number of genetic factors determine the usually complex response of the gastrointestinal tract to exogenous stimuli. In patients with functional dyspepsia, a study reported in 2004 that homozygous GNB3 825CC carrier status was significantly associated with unexplained, predominantly upper-abdominal symptoms (Figure 2).16 Genetic factors are thought to have a role in the manifestation of functional dyspepsia and IBS. At present, most studies that investigated the genetic factors associated with FGID have been carried out in patients with IBS. FGID vary in type and frequency between countries, which indicates that in different geographic locations, different environmental and/or genetic factors have roles in the manifestation of FGID. Although geographic variation in the prevalence of IBS has been found, IBS does not predominate in any specific ethnic group, as shown by studies performed in the US in white, African American and Hispanic patients.17–19 It is known that FGID have a twofold to threefold higher prevalence in women than in men; however, it remains unclear whether this finding is related to environmental, lifestyle or physiologic factors, or to the fact that

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women access health care more often (Table 1). Nevertheless, experimental studies point to the fact that hormonal influences associated with the menstrual cycle are linked to the symptoms of FGID, and are also linked to changes in visceral sensory thresholds.20 Interestingly, there is very little evidence to suggest that the prevalence of FGID decreases in women after the menopause. FAMILIAL AGGREGATION

Although the clustering of FGID in families has been observed in clinical practice for many years, Whorwell and co-workers were the first group that formally studied familial aggregation of IBS. They found that in a group of 100 IBS patients, 33% reported a family history of IBS compared with only 2% of controls21 (matched for age, sex and social class). Even stronger evidence for familial aggregation of IBS was provided by a US study conducted in Olmsted County residents aged between 30 and 64 years. This study revealed that there was a significant association between having a first-degree relative with abdominal pain or bowel problems and reporting IBS (odds ratio 2.3; 95% CI 1.3–3.9)22 or dyspepsia (odds ratio 1.8; 95% CI 1.05–3.0). There is also evidence that the children of parents who have IBS tend to use health care resources significantly more for gastrointestinal symptoms than the children of parents who do not

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Patients with a given GNB3 genotype (%)

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0– No symptom cluster (N = 161)

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Figure 2 The proportion of individuals with the GNB3 CC genotype is significantly higher in patients who have multiple symptoms of functional gastrointestinal disorders (P C) could be functionally important, because the 779C variant is associated with slower gastric emptying. This observation suggests that, compared with the 779T variant, the 779C substitution results in a greater response to endogenous cholecystokinin, which is known to retard gastric emptying. Owing to the small sample size in this study, however, further confirmatory studies are needed. Cytokines

Transient mucosal inflammation is now believed to be an important trigger for the manifestation of IBS, as shown in animal50,51 as well as human studies.9,52 Although the definition of IBS states that no active inflammation is present to cause symptoms such as abdominal pain, change of bowel-motion pattern and bloating, studies have revealed that there are increased numbers of mast cells present in the terminal ileum of patients with IBS. In addition, patients whose ulcerative colitis is in remission report IBS-like symptoms.53 In many IBS patients, gastrointestinal infections precede the onset of symptoms.54 It has also been shown that the

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number of T lymphocytes and enteroendocrine cells is persistently elevated in patients with postinfectious IBS, which suggests that there is a continuing inflammatory stimulus with increased gut permeability.55 Cytokines are involved in the regulation of the immune and inflammatory response. There are proinflammatory cytokines, such as tumor necrosis factor (TNF) and interferon γ, and antiinflammatory cytokines, such as interleukin 10 (IL-10) and transforming growth factor β1. Study of polymorphisms in the genes that encode these cytokines has revealed the presence of alleles that are associated with high or low cytokine production (‘high’ or ‘low’ producer alleles).56 Individuals can, therefore, be classified by their allele status as high, intermediate or low producers of a particular cytokine: those who are homozygous for the high producer allele make the highest amount, those who are homozygous for the low producer allele make the least amount, whereas heterozygotes produce intermediate amounts. A genetic predisposition to produce high or low amounts of a particular cytokine might alter the individual’s susceptibility to a certain disease, or affect its clinical expression. It has been shown that allele and genotype frequencies for the biallelic polymorphisms of transforming growth factor β are not different in IBS patients and healthy individuals.57 By contrast, there is a significant association between IL10 genotypes and IBS; fewer IBS patients than healthy control individuals have the high-producer genotype.57 The low prevalence of the high-producer genotype in patients with IBS suggests that high levels of IL-10 might have some protective role—or, conversely, that individuals who are predisposed to produce low amounts of this cytokine might be more likely to develop IBS. A genetic predisposition to produce low levels of anti-inflammatory cytokines could mean that control of the inflammatory response might be compromised in some individuals, and might help to explain why gastrointestinal infections, for example, can sometimes lead to continuing problems.57 Another study that recruited patients with IBS diagnosed according to ROME II criteria assessed single-nucleotide polymorphisms in the promoter of the TNF gene, which encodes a proinflammatory cytokine, and the IL10 gene, which encodes an anti-inflammatory cytokine.58 In this study, it has been shown that the high-producer TNF genotype is more

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prevalent in IBS patients than in healthy controls. Although homozygous high-producer genotypes were rare in both IBS patients and healthy controls, the heterozygous genotype, which is also associated with a phenotype of high TNF production, was present in 41% of IBS patients versus only 26% of healthy controls. No differences in IL10 genotype frequencies were found when IBS patients were compared with healthy controls: a high-producer genotype (–1082 GG) was present in 26% of patients and 28% of controls. These findings contrast with the results of Gonsalkorale et al., who showed that there was a significant reduction in frequency of the high-producer IL10 genotype in IBS patients compared with controls (21% versus 32%).57 This difference might be related to variation in genotype frequencies according to ethnicity.59,60 For example, the frequency of the high-producer IL10 genotype is significantly higher in the Irish population (34%) than in Africans (9.5%) or Singapore Chinese (0%).61 There are, however, some limitations to interpretation of the IL10 genotype data.57 There are no data on the ethnic origin of the enrolled individuals, which could serve as an explanation of the contrasting results of IL10 genotype frequencies in the two studies.57,58 Moreover, only one anti-inflammatory cytokine polymorphism was assessed in Gonsalkorale et al.’s study, although it is likely that multiple anti-inflammatory cytokines are involved in the regulation of immune and inflammatory responses. The suggested conclusion—namely, that high production of IL-10 could be a protective factor that prevents the development of IBS after a gastrointestinal infection, by provision of an adequate antiinflammatory cytokine response—needs careful consideration. Moreover, the characteristics of patients with IBS who were enrolled in van der Veek et al.’s study were much more specific, because their recruitment was based on ROME II criteria. By contrast, Gonsalkorale and coworkers included patients who fulfilled ROME I criteria for IBS. Almost 25% of patients who meet ROME I criteria for IBS do not meet the ROME II criteria, owing to insufficient symptom duration or frequency.62 More than 95% of individuals recruited for the study by van der Veek et al. were white, and the frequency of the IL10 –1082 GG highproducer genotype in controls (28%) was similar to that previously reported in the Dutch population (23%).59

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Environmental factors

Normal gastrointestinal function

Infection

No

Stress

Group 1 risk factors present

Inflammation

Yes

No clinical manifestations of functional gastrointestinal disorders

No

Disturbed gastrointestinal function

Group 2 risk factors present

Yes

Clinical manifestations of functional gastrointestinal disorders

Figure 3 A two-step model for the involvement of genetic factors in the pathophysiology of functional gastrointestinal disorders. Environmental factors alone are usually unable to induce the long-term functional changes that cause the onset of symptoms. In the presence of certain hereditary factors (e.g. specific GNB3 genotypes or polymorphisms in proinflammatory cytokine genes), however, these environmental factors can trigger the manifestation of symptoms. Two groups of genetic factors can be identified: group 1 comprises factors that are directly linked to gastrointestinal functions such as sensory or motor function, while group 2 factors, such as polymorphisms in genes that modulate immune and/or neuroimmune functions, cause initiation of the abnormalities that underlie the manifestation of symptoms only in the presence of certain exogenous stressors.

CONCLUSIONS

Although there is evidence to suggest that genetic factors contribute to the manifestations of FGID, there is no single genetic factor that determines disease phenotypes. The search for candidate genes in FGID will be a comprehensive task. Studies that report positive associations between genetic variants and disease have to be considered with caution, because false-positive results are frequent. Genetic associations can be influenced by several factors, such as bias inherent in the study design, poor statistical analysis, and small sample size or overinterpretation of data that ignores the study limitations.63,64 All these factors can result in a lack of reproducibility of data. Emphasis should, therefore, be placed on identifying those limitations that are likely to influence interpretation of the data, to avoid inappropriate conclusions from being drawn. It is likely that several different genetic factors increase the risk of developing FGID, while others decrease this risk; however, even comprehensive knowledge of all potentially related factors would only partly explain the specific disease manifestations experienced by a patient. The manifestations experienced by a given patient probably result from their profile of genetically determined risk factors along with their exposure to environmental

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factors. For example, patients who are exposed to a severe gastrointestinal infection in the presence of specific (and as yet unknown) genetically determined risk factors might have a greatly increased risk, or even the certainty, of developing postinfectious IBS (Figure 3). In individuals with a different genetic profile, such an infection might not have any long-term consequence or might even have a positive effect (e.g. in a patient who suffers from slow-transit constipation due to a polymorphism in a gene that encodes a serotonergic receptor). This complex interaction of genes, environment and phenotype will continue to present a challenge to researchers. It is unlikely, therefore, that a quick fix for FGID will emerge from research into the genetics of these conditions. It is likely, on the other hand, that this research will provide an explanation and theoretical framework for the heterogeneity of clinical manifestations of FGID. This research approach might be considered a comprehensive exercise; however, the development of effective treatments for FGID is hampered by the heterogeneity that is seen in the response to a given therapy, even in relatively small groups of patients. If molecular markers, or more likely a combination of clinical features (such as symptom-precipitating events and symptom pattern) and specific markers, allow us to predict patients’ responses to therapy much will have been gained.

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KEY POINTS ■

The pathogenesis of functional gastrointestinal disorders is probably multifactorial, and includes genetic and environmental factors

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Polymorphisms of genes that encode cytokines and influence immune function are thought to contribute to the onset of symptoms in at least a subgroup of patients with irritable bowel syndrome

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Competing interests The authors declared they have no competing interests.

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