The role of 5-HTT LPR and GNβ3 825C>T polymorphisms and gene-environment interactions in irritable bowel syndrome (IBS)

Meta-Analysis of the Association Between 5-Hydroxytryptamine Transporter Gene-Linked Polymorphic Region and Functional Dyspepsia and its Subtypes

Background: Association studies of variations in the 5-hydroxytryptamine (5-HT, serotonin) transporter gene-linked polymorphic region (5-HTTLPR) and functional dyspepsia (FD) have yielded contradictory results. Hence, we performed a meta-analysis to clarify inconsistencies between the 5-HTTLPR polymorphism with FD and it subtypes. Methods: We performed a literature search in PubMed, Embase, Web of Science, Cochrane Library, and CNKI, including articles published until March 2022. We calculated and pooled odds ratios (ORs) with their 95% confidence intervals (CIs) in Stata 15.0. Data extraction was performed in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines and Cochrane Handbook for Systematic Reviews of Interventions. Results: The meta-analysis included six studies, comprising 488 cases and 1513 healthy controls. We did not observe a significant association between the 5-HTTLPR polymorphism and FD in the overall population. In subgroup analyses, the 5-HTTLPR polymorphism was significantly associated with FD-subtype epigastric pain syndrome (EPS) (SS vs. LL+LS, OR = 0.620, 95% CI: 0.414-0.930; SS vs. LS, OR = 0.640, 95% CI: 0.417-0.980; S vs. L, OR = 0.655, 95% CI: 0.471-0.911). However, no association was observed with the other subtype, postprandial distress syndrome (PDS). Conclusion: While the 5-HTTLPR polymorphism had no relationship with FD overall, splitting the disease into its subtypes revealed a clear association with EPS.

[Irritable bowel syndrome in the Russian Federation: results of the ROMERUS multicenter observational study]

BACKGROUND

Irritable bowel syndrome (IBS) is one of the most common functional disorders of the gastrointestinal tract. According to Russian guidelines, a standard examination using laboratory and instrumental evaluation methods, including colonoscopy, should be performed to establish the diagnosis of IBS.

AIM

To characterize the Russian population of IBS patients.

MATERIALS AND METHODS

A multicenter observational prospective study ROMERUS was conducted at 35 clinical centers in the Russian Federation. The study included male and female patients aged 18 to 50 with a diagnosis of IBS based on the Rome IV criteria, with no signs of structural gastrointestinal disease. The follow-up duration was 6 months and included three patients' visits to the study site. During the study, data were collected on patients' demographic and clinical characteristics, medical history, and drug therapy. The secondary parameters included the assessment of the proportion of patients with a diagnosis of IBS confirmed by a standard examination among all patients meeting the Rome IV criteria, the evaluation of the change over time of the IBS symptoms, quality of life (QoL), and adherence to therapy. Characterization of the population was performed using descriptive statistics methods. The standard examination results were presented as the percentage of patients with IBS confirmed by the standard examination among all patients meeting the Rome IV criteria, with a two-sided 95% confidence interval.

RESULTS

The study included 1004 patients with a diagnosis of IBS according to the Rome IV criteria, with 790 (78.7%) patients included in the final analysis. The mean age of patients was 34.0±7.5 years; they were predominantly female (70.4%), Caucasian (99.4%), married (55.1%), urban residents (97.5%) with higher education (64.5%) and a permanent position (74.9%). Patients enrolled in the study have low physical activity and lack a healthy diet. The smoking rate was 26.3%. IBS symptoms with predominant constipation (IBS-C) were observed in 28.1% of patients; 28.9% had IBS with predominant diarrhea (IBS-D), 11.9% had mixed-type IBS, and 31.1% had non-classified IBS. The main IBS symptoms were pain (99.7%), abdominal distension (71.1%), and fullness (36.8%). Biliary tract dysfunction (18.9%) and gastritis (17.2%) were the most frequently reported comorbidities. Prior to enrollment, 28% of patients received drug therapy. The most commonly prescribed drug during the study was mebeverine (54.1%). At 6 months of follow-up, there was a significant reduction of abdominal pain, bloating, and distention, and a twofold reduction in the incidence of constipation and diarrhea in the subgroups of patients with IBS-C and IBS-D, respectively. The overall QoL score measured by the IBS-QoL questionnaire increased from 83.0 to 95.2 points (p<0.05) during the study. In the overall assessment of their condition, 69.6% of patients noted no symptoms and 25.3% reported marked improvement, 35% were asymptomatic according to the physician's overall assessment of the patient's condition, and 51.8% showed significant improvement.

CONCLUSION

IBS patients in the Russian Federation were characterized. The diagnosis of IBS, established following the Rome IV criteria, is confirmed by the results of a standard examination in 96.3% of patients. The Rome IV criteria for the IBS diagnosis make it possible to establish a diagnosis with a probability of 94.7%. For 6 months of follow-up, there was a clinical improvement with a decrease in the severity of symptoms and a QoL improvement.

Identification of putative transcriptomic biomarkers in irritable bowel syndrome (IBS): Differential gene expression and regulation of TPH1 and SERT by vitamin D

Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders and affects approximately 4% of the global population. The diagnosis of IBS can be made based on symptoms using the validated Rome criteria and ruling out commonly occurring organic diseases. Although biomarkers exist for "IBS mimickers" such as celiac disease and inflammatory bowel disease (IBD), no such test exists for IBS. DNA microarrays of colonic tissue have been used to identify disease-associated variants in other gastrointestinal (GI) disorders. In this study, our objective was to identify biomarkers and unique gene expression patterns that may define the pathological state of IBS. Mucosal tissue samples were collected from the sigmoid colon of 29 participants (11 IBS and 18 healthy controls). DNA microarray analysis was used to assess gene expression profiling. Extraction and purification of RNA were then performed and used to synthesize cDNA. Reverse transcriptase quantitative polymerase chain reaction (RT-qPCR) was employed to identify differentially expressed genes in patients diagnosed with IBS compared to healthy, non-IBS patient-derived cDNA. Additional testing probed vitamin D-mediated regulation of select genes associated with serotonergic metabolism. DNA microarray analyses led to the identification of 858 differentially expressed genes that may characterize the IBS pathological state. After screening a series of genes using a combination of gene ontological analysis and RT-qPCR, this spectrum of potential IBS biomarkers was narrowed to 23 genes, some of which are regulated by vitamin D. Seven putative IBS biomarkers, including genes involved in serotonin metabolism, were identified. This work further supports the hypothesis that IBS pathophysiology is evident within the human transcriptome and that vitamin D modulates differential expression of genes in IBS patients. This suggests that IBS pathophysiology may also involve vitamin D deficiency and/or an irregularity in serotonin metabolism.

Latest Insights on the Pathogenesis of Irritable Bowel Syndrome

The pathogenesis of irritable bowel syndrome is multifactorial and complex. Our understanding of its pathophysiology has evolved, but remains incompletely understood. Symptoms result from a dysregulation of brain-gut interactions. Evidence has identified alterations in central and peripheral (gut) mechanisms in irritable bowel syndrome and the bidirectional communication between the brain and the gut. Pertinent mechanisms include disturbed gut motility, visceral hypersensitivity, altered mucosal and immune function, altered gut microbiota, and altered central nervous system processing. This review addresses factors that increase the risk of irritable bowel syndrome and the central and peripheral mechanisms thought to underlie its symptoms.

Ethnic differences in genetic polymorphism associated with irritable bowel syndrome

Genetic polymorphism is associated with irritable bowel syndrome (IBS) in terms of susceptibility and clinical manifestations. Previous studies have shown that genetic polymorphism might play a key role in the onset and progression of IBS by modulating components of its pathogenesis such as the gut-brain axis, gastrointestinal motility, inflammatory activity, and immune status. Although underlying pathophysiological mechanisms have not been fully clarified, the potential ethnic differences that are present in worldwide genetic studies of IBS deserve attention. This review surveyed numerous studies focusing on IBS-associated single nucleotide polymorphisms, and investigated the ethnic disparities revealed by them. The results demonstrate the need for more attention on ethnic factors in IBS-related genetic studies. Taking ethnic backgrounds into accounts and placing emphasis on disparities potentially ascribed to ethnicity could help lay a solid and generalized foundation for transcultural, multi-ethnic, or secondary analyses in IBS, for example, a meta-analysis. Broader genetic studies considering ethnic factors are greatly needed to obtain a better understanding of the pathophysiological mechanisms of IBS and to improve the prevention, intervention, and treatment of this disease.

Irritable Bowel Syndrome between Molecular Approach and Clinical Expertise-Searching for Gap Fillers in the Oxidative Stress Way of Thinking

Irritable bowel syndrome (IBS) remains to date an intriguing functional gastrointestinal disorder. Recent studies described a multitude of exogenous factors that work together in IBS, gradually impairing intestinal lining cellular metabolism, including oxidative status balance, with or without a genetic background. Although the current biomarkers support the differentiation between IBS subtypes and other functional gastrointestinal disorder, they are mostly non-specific, referring to clinical, biochemical, and inflammatory imbalances. Since IBS could be also the result of deficient signaling pathways involving both gastrointestinal secretion and neuro-vegetative stimulation, IBS makes no exception from the oxidative hypothesis in the pathological mechanisms. Regarding the oxidative stress implication in IBS, the previous research efforts showed controversial results, with some animal models and patient studies reporting clear oxidative imbalance both on systemic and local levels, but still with no concrete evidence to point to a direct correlation between oxidative stress and IBS. Additionally, it seems that a major role could be also attributed to gut microbiota and their ability to shape our bodies and behaviors. Moreover, the genetic features study in IBS patients showed that several genetic similarities point to a possible correlation of IBS with affective spectrum disorders. Thus, we focus here the discussion on the assumption that IBS could in fact be more likely a stress-related disorder rather than a gastrointestinal one.

Celiac disease is uncommon in irritable bowel syndrome in the USA

BACKGROUND AND AIMS

Routine serologic testing for celiac disease (CD) may be useful in irritable bowel syndrome (IBS) patients, but this is controversial. We aimed to compare the prevalence of unrecognized CD in a large cohort of patients with and without IBS.

PARTICIPANTS AND METHODS

This is a family case-control IBS study conducted at a single US academic medical center. Stored serum and DNA were available. Tissue transglutaminase (TTg) immunoglobulin A was performed, followed by indirect immunofluorescence testing for endomysial antibodies with positive or weakly positive TTg results. Individuals were considered to have CD if both results were positive. χ and Fisher's exact tests were used to compare prevalence between the two groups.

RESULTS

Serum samples were studied from 533 cases and 531 controls. In all, 80% of participants were female, with a median age of 50 years; 65% of cases and 0% controls met the Rome criteria for IBS. Previous serological testing for CD had occurred in 142 (27%) cases and 13 (2%) controls, but none had CD on subsequent testing. Six (1.1%) cases versus five (0.9%) controls had positive or weakly positive TTg test. Six cases (1.1%) versus three (0.6%) controls were confirmed to have CD by endomysial antibody (P=0.51).

CONCLUSION

No difference in the prevalence of CD between patients with IBS and patients without IBS at a tertiary medical center was observed. Our findings do not support routine celiac serologic or genetic testing in patients with IBS in all US populations.

G protein beta 3(GNβ3) C825T polymorphism and irritable bowel syndrome susceptibility: an updated meta-analysis based on eleven case-control studies

Several studies have reported an association between GNβ3 C825T polymorphism and irritable bowel syndrome (IBS). However, the results remain inconclusive and controversial, particularly for the data derived from different ethnicities and IBS subtypes. Therefore, we performed an updated meta-analysis to evaluate this association. All eligible case-control studies that met the search criteria were retrieved from multiple databases, and eleven case-control studies were included for detailed evaluation. The pooled odds ratios (ORs) with 95% confidence intervals (95% CIs) were calculated to assess the strengths of the association between GNβ3 C825T polymorphism and susceptibility to IBS and its subtypes. Our meta-analysis found no significantly associations of GNβ3 C825T polymorphism with IBS risk in all populations. Whereas the C allele was demonstrated to be a decreased risk factor for constipation predominant IBS (IBS-C) in allele model. Additionally, the CC genotype was found to be associated with increased diarrhea predominant IBS (IBS-D) risk in recessive model. Subgroup analysis by ethnicity revealed that these associations held true for the Asian subpopulation. In conclusion, this meta-analysis suggests the C allele of GNβ3 C825T might be associated with a decreased risk of IBS-C, and the CC genotype of GNβ3 might be associated with increased IBS-D risk.

Genetics and pharmacogenetics of aminergic transmitter pathways in functional gastrointestinal disorders

Functional gastrointestinal disorders (FGIDs) are highly prevalent syndromes, without evident underlying organic causes. Their pathogenesis is multifactorial in nature, with a combination of environmental and genetic factors contributing to their clinical manifestations, for which most of current treatments are not satisfactory. It is acknowledged that amine mediators (noradrenaline, dopamine and serotonin) play pivotal regulatory actions on gut functions and visceral sensation. In addition, drugs of therapeutic interest for FGIDs act on these transmitter pathways. The present article reviews current knowledge on the impact of genetics and pharmacogenetics of aminergic pathways on FGID pathophysiology, clinical presentations, symptom severity and medical management, in an attempt of highlighting the most relevant evidence and point out issues that should be addressed in future investigations.

The long-term functional consequences of acute infectious diarrhea

PURPOSE OF REVIEW

The study reviews recent publications that build on previous studies showing that acute enteric infection can produce persistent dysfunction in the lower gut (postinfectious irritable bowel syndrome) and proximal gut (postinfectious functional dyspepsia). The review addresses risk factors, the pathophysiological basis of persistent gut dysfunction, and the factors that initiate and maintain it.

RECENT FINDINGS

Recent work has identified several loci of host genetic predisposition to these syndromes that focus attention on host immune responses that may lead to gut dysfunction, including changes in intestinal barrier function and cytokine responses to the initial infection. Human and animal studies identify changes in the serotonergic and cannabinoid pathways regulating visceral pain responses and gut motility. Recent work has also focused attention on the putative role of the intestinal microbiota or dysbiosis in maintaining gut dysfunction and this is reviewed in depth.

SUMMARY

The development of long-term consequences following an acute episode of gastroenteritis reflects a convergence of host factors that include genetic predisposition and psychological factors, as well as the development of intestinal dysbiosis. It is anticipated that future research will generate biomarkers of susceptibility as well as novel microbiota-directed preventive and therapeutic strategies.

Genetic epidemiology of irritable bowel syndrome

Irritable bowel syndrome (IBS) is the most common functional gastrointestinal disorder characterized by presence of abdominal pain or discomfort associated with altered bowel habits. It has three main subtypes - constipation predominant IBS (C-IBS), diarrhea predominant IBS (D-IBS) and IBS with mixed features of both diarrhea as well as constipation (M-IBS). Its pathophysiology and underlying mechanisms remain elusive. It is traditionally believed that IBS is a result of multiple factors including hypersensitivity of the bowel, altered bowel motility, inflammation and stress. Initial studies have shown familial aggregation of IBS suggesting shared genetic or environmental factors. Twin studies of IBS from different parts of world have shown higher concordance rates among monozygotic twins than dizygotic twins, and thus suggesting a genetic component to this disorder. Multiple studies have tried to link single-nucleotide polymorphisms (SNPs) to IBS but there is little evidence that these SNPs are functional. Various molecules have been studied and investigated by the researchers. Serotonin, a known neurotransmitter and a local hormone in the enteric nervous system, has been most extensively explored. At this time, the underlying gene pathways, genes and functional variants linked with IBS remain unknown and the promise of genetically-determined risk prediction and personalize medicine remain unfulfilled. However, molecular biological technologies continue to evolve rapidly and genetic investigations offer much promise in the intervention, treatment and prevention of IBS.

Genetic polymorphism in pathogenesis of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a complex symptom-based disorder without established biomarkers or putative pathophysiology. IBS is a common functional gastrointestinal disorder which is defined as recurrent abdominal pain or discomfort that has at least two of the following symptoms for 3 d per month in the past 3 mo according to ROME III: relief by defecation, onset associated with a change in stool frequency or onset with change in appearance or form of stool. Recent discoveries revealed genetic polymorphisms in specific cytokines and neuropeptides may possibly influence the frequencies and severity of symptoms, as well as the therapeutic responses in treating IBS patients. This review gives new insights on how genetic determinations influence in clinical manifestations, treatment responses and potential biomarkers of IBS.

Association Between SLC6A4 Serotonin Transporter Gene Lainked Polymorphic Region and ADRA2A -1291C>G and Irritable Bowel Syndrome in Korea

Background/Aims

Despite numerous studies on the relation of genetic polymorphisms with irritable bowel syndrome (IBS), the results still remain inconclusive. The aim of this study was to assess the possible association between SLC6A4 serotonin transporter gene linked polymorphic region (5-HTTLPR), ADRA2A −1291C>G, GNB3 825C>T, CCK1R intron 779T>C and TRPV1 945G>C polymorphisms and IBS based on Rome III criteria in Korea.

Methods

Study subjects were prospectively recruited from visitors to Seoul National University Bundang Hospital between July 2009 and January 2014. Ninety-nine IBS patients and 171 healthy controls were enrolled. Polymorphisms of above-mentioned 5 genes were genotyped. Serum serotonin from 101 participants was measured by ELISA and compared according to SLC6A4 5-HTTLPR polymorphisms and IBS subtypes.

Results

Regarding SLC6A4 5-HTTLPR polymorphism, L/L genotype was significantly associated with the total IBS, constipation predominant IBS (IBS-C) and mixture of diarrhea and constipation IBS (IBS-M) (adjusted OR: 4.35, 95% CI: 1.04–16.67; adjusted OR: 11.11, 95% CI: 1.69–50.00 and adjusted OR: 5.56, 95% CI: 1.05–33.33, respectively). Carrying ADRA2A −1291G allele was significantly associated with total IBS and diarrhea predominant IBS (adjusted OR: 3.37, 95% CI: 1.16–9.77 and adjusted OR: 5.64, 95% CI: 1.18–27.01, respectively). IBS-C patients showed reduced level of serum serotonin compared to controls and patients with diarrhea predominant IBS (50.2 ng/mL vs. 69.0 ng/mL and 92.9 ng/mL, P = 0.017 and P = 0.001, respectively).

Conclusions

Genetic polymorphisms of SLC6A4 5-HTTLPR and ADRA2A −1291C>G could be one of the pathophysiological factors of IBS in Korea. Reduced serum serotonin shown in the IBS-C group suggested a role of serotonin in IBS, but large study is needed for confirming genotypic difference in serum serotonin level.

No association of G-protein beta polypeptide 3 polymorphism with irritable bowel syndrome: Evidence from a meta-analysis

AIM: To clarify the associations between G-protein beta polypeptide 3 (GNB3) C825T polymorphism and risk of the irritable bowel syndrome (IBS) by a meta-analysis.

METHODS: We searched relevant studies in PubMed, EMBASE, CNKI, Google Scholar, Ovid and Cochrane library prior to October 2013. The strengths of the associations between GNB3 C825T polymorphism and IBS risk were estimated by odds ratios (ORs) with 95% confidence interval (CIs).

RESULTS: We identified seven case-control studies with 1085 IBS cases and 1695 controls for the analysis. We found no significantly associations of GNB3 C825T polymorphism with IBS risk in the overall population (CC vs TT, OR = 1.12, 95%CI: 0.86-1.45; CC + CT vs TT, OR = 1.17, 95%CI: 0.92-1.49; TT + CT vs CC, OR = 0.93, 95%CI: 0.80-1.08; C vs T, OR = 1.08, 95%CI: 0.97-1.21). Subgroup analysis did not reveal significant associations either in Asian population or Caucasian population. The pooled results of four studies fail to show associations of GNB3 C825T polymorphism with subtypes of IBS (constipation-dominant type, diarrhea-dominant type and mixed type).

CONCLUSION: The present study suggests no associations of GNB3 C825T polymorphism with IBS risk.

Irritable bowel syndrome: Emerging paradigm in pathophysiology

Irritable bowel syndrome (IBS) is one of the most common gastrointestinal disorders, characterized by abdominal pain, bloating, and changes in bowel habits. These symptoms cannot be explained by structural abnormalities and there is no specific laboratory test or biomarker for IBS. Therefore, IBS is classified as a functional disorder with diagnosis dependent on the history taking about manifested symptoms and careful physical examination. Although a great deal of research has been carried out in this area, the pathophysiology of IBS is complex and not completely understood. Multiple factors are thought to contribute to the symptoms in IBS patients; altered gastrointestinal motility, visceral hypersensitivity, and the brain-gut interaction are important classical concepts in IBS pathophysiology. New areas of research in this arena include inflammation, postinfectious low-grade inflammation, genetic and immunologic factors, an altered microbiota, dietary factors, and enteroendocrine cells. These emerging studies have not shown consistent results, provoking controversy in the IBS field. However, certain lines of evidence suggest that these mechanisms are important at least a subset of IBS patients, confirming that IBS symptoms cannot be explained by a single etiological mechanism. Therefore, it is important to keep in mind that IBS requires a more holistic approach to determining effective treatment and understanding the underlying mechanisms.

The serotonin transporter gene polymorphism (5-HTTLPR) and irritable bowel syndrome: a meta-analysis of 25 studies

Background

The results of previous studies assessing the association between the 5-HTTLPR polymorphism of serotonin transporter gene and irritable bowel syndrome (IBS) are inconsistent. The aim of this study was to clarify the association between the 5-HTTLPR mutation and the presence of IBS and its subtypes with a meta-analysis of 25 studies.

Methods

A thorough search for case–control studies evaluating the association between the 5-HTTLPR polymorphism of serotonin transporter gene and the presence of IBS was carried out in four electronic databases. A meta-analysis was performed in accordance with the Cochrane Handbook for systemic reviews.

Results

A total of 25 articles with 3443 IBS cases and 3359 controls were included into our meta-analysis. No significant association was found between this polymorphism and IBS in all populations. Whereas the LL genotype was demonstrated to be a risk factor for constipation predominant IBS (IBS-C) development in the overall population (LL vs SS: OR = 1.570, 95% CI = 1.147-2.148, P = 0.005, Bon = 0.030; LL vs LS: OR = 1.658, 95% CI = 1.180-2.331, P = 0.004, Bon = 0.024; LL vs LS/SS: OR = 1.545, 95% CI = 1.187-2.012, P = 0.001, Bon = 0.006). In the analysis of different ethnicities, L allele and LL genotype were significantly associated with increased IBS-C risk in the East Asian population (L vs S: OR = 1.487, 95% CI = 1.139-1.941, P = 0.003, Bon = 0.018; LL vs SS: OR = 2.575, 95% CI = 1.741-3.808, P = 0.000, Bon = 0.000; LL vs LS: OR = 3.084, 95% CI = 2.017-4.715, P = 0.000, Bon = 0.000; LL vs LS/SS: OR = 2.759, 95% CI = 1.933-3.938, P = 0.000, Bon = 0.000), but not in the Caucasian population.

Conclusions

Different from the conclusions of the earlier meta-analyses, the 5-HTTLPR mutation affects IBS-C but not IBS-D and IBS-M development and this effect only exists in the East Asian population but not other populations.

Increased serotonin transporter immunoreactivity intensity in the ileum of patients with irritable bowel disease

Irritable bowel syndrome (IBS) is a common chronic gastrointestinal disorder, which represents an economic burden to society and considerably reduces the quality of life of patients. In a previous study, the density of serotonin cells in the ileum of IBS patients was lower compared with control subjects. The present study aimed to further investigate the immunoreactivity intensity of serotonin and serotonin‑selective reuptake transporter (SERT) in the ileum of IBS patients. A total of 98 patients (77 females and 21 males; mean age, 35 years; range, 18‑66 years), which fulfilled Rome III Criteria for IBS, were included in the study. This cohort included 35 patients with diarrhoea‑predominant (IBS‑D), 31 patients with mixed diarrhoea and constipation (M‑IBS) and 32 patients with constipation‑predominant (IBS‑C) symptoms. A total of 27 subjects were included as controls (16 females and 11 males; mean age, 52 years; range, 20‑69 years). Ileal biopsy specimens were immunostained using the avidin‑biotin (ABC) complex method for serotonin and SERT. The immunoreactivity intensity was quantified by computerised image analysis using Olympus cellSens imaging software. No statistical difference of serotonin immunoreactivity intensity was identified in multiple comparisons between controls, IBS‑total, IBS‑D, IBS‑M and IBS‑C. The SERT immunoreactivity intensity was significantly increased in IBS patients as compared with controls, regardless of the subtype. It was concluded that the increase in ileal epithelial content of SERT increases the intracellular uptake of serotonin and its degradation in the gut epithelial cells and consequently decreases the availability of serotonin within the gut mucosa. The low availability of serotonin at its receptors occurred in all IBS subtypes. This may indicate that this abnormality is associated with a common symptom in all IBS subtypes, namely abdominal pain/discomfort. Serotonin acts upon sensory neurons in the submucosal and myenteric ganglia, as well as in the spinal cord, which is in agreement with this hypothesis.

Genes and functional GI disorders: from casual to causal relationship

BACKGROUND

The functional gastrointestinal disorders (FGID), and in particular irritable bowel syndrome (IBS), pose a considerable burden on health care and society, and negatively impact quality of life. These are common conditions of unknown etiology, and symptom-based criteria are currently the sole nosological tools for their clinical classification. Major insight into FGID pathophysiology is therefore needed and, in recent years, increasing hope has been put on genetic research for the identification of causative pathways. This is more advanced in IBS compared with other FGID, but it has still provided often indecipherable results and no unequivocal evidence of a pathogenetic role for any particular gene. Although thousands of genetic variants have been undoubtedly linked to human disease in hundreds of genome-wide association studies (GWAS), no similar effort has yet even been attempted in FGID. If meaningful, robust, and reproducible results are to be obtained for IBS and other FGID, we must shift gear and adopt these powerful hypothesis-free approaches through concerted actions and allocation of adequate resources. Provided these are in place, the major challenge will be, inevitably, the choice of the target phenotype(s) beyond a descriptive symptom-based classification.

PURPOSE

In view of these much awaited developments, salient results and difficulties inherent to IBS gene discovery are briefly summarized here.

Genetic approaches to functional gastrointestinal disorders

Functional gastrointestinal disorders are complex symptom-based disorders without agreed upon biomarkers or pathophysiology. A better understanding of the genetic architecture of these disorders would help to better identify their complex biology and explain the common comorbidity with other disorders of persistent pain, mood, and affect, as well as possibly make it possible to identify subgroups of patients who respond to customized therapies. In contrast to monogenic diseases, polygenic diseases and traits are characterized by the contribution of common variants in a large number of genes, as well as environmental factors, to the vulnerability of an individual. Family and twin studies have clearly established a genetic component in irritable bowel syndrome. Although candidate gene studies have identified a few gene polymorphisms that may be correlated with the syndrome, small sample size, lack of reproducibility in large data sets, and the unreliability of the clinical phenotype require caution when extrapolating to a major role of any of the reported polymorphisms in the pathophysiology of irritable bowel syndrome. Future progress in this area will require better characterization of intermediate phenotypes with large effect size for the clinical phenotype, as well as consideration of gene-gene, environment-gene (epigenetics), and sex-gene interactions, genome-wide association, and whole genome sequencing approaches in large data sets.