The gut-brain axis refers specifically to a complex reflex circuit that integrates the bidirectional communication between the cortex and the digestive system through afferent and efferent pathways. Information from the GI tract is collected from receptors in the periphery and input to cortical areas. Then a response is generated downstream, further neuromodulating the actions of the enteric nervous system. A dysfunctional gut-brain axis reaction has been observed to contribute to abnormal GI motility and sensitivity, which is closely related to the pathogenesis of IBS. The existence of ethnic differences of genetic polymorphisms associated with the gut-brain axis has already been observed (Figure 1A), among which genetic polymorphism of the 5-hydroxytryptamine (5-HT) system and adrenergic system are the most well studied.
Figure 1 Genetic polymorphisms involved in several pathophysiological pathways.
A: The polymorphism of 5-hydroxytryptamine receptors, serotonin reuptake transporter and adrenoceptors influences the bidirectional brain–gut axis, which plays essential roles in altering visceral sensitivity and gastrointestinal mobility; B: Genetic polymorphism in voltage-gated sodium channel NaV1.5, an ionic channel in interstitial cells of Cajal, cholecystokinin receptors, cannabinoid receptor, and fatty acid amide hydrolase markedly regulates the motor function of the gastrointestinal tract via the myenteric plexus and smooth muscle cells; C: Genetic polymorphism modulates the level of cytokines [e.g., interleukin (IL)-6, IL-8, IL-10, and tumor necrosis factor-α] released by immune cells, which is closely related to mucosal immune/inflammatory activation and the intestinal immune and inflammatory status. HPA: hypothalamic–pituitary–adrenal; 5-HT: 5-hydroxytryptamine; SERT: serotonin reuptake transporter; FAAH: fatty acid amide hydrolase; CCK: cholecystokinin; IL: Interleukin.
The 5-HT system
5-HT, also known as serotonin, is a paracrine messenger and neurotransmitter that has been particularly investigated in neuropsychiatric diseases. It is also an important mediator in the gut-brain connection. Notably, it has been estimated that about 95% of serotonin in the body is found in the GI tract instead of the brain and 90% of GI serotonin resides in enterochromaffin cells, while the rest is in enteric neurons. As a crucial signaling molecule within the gut-brain axis, 5-HT stimulates enteric afferent nerve fibers of the vagal nerve and regulates the motility and sensitivity of the GI tract in a 5-HT receptor-dependent manner through submucosal and myenteric neurons that respond to serotonin via various receptors. The genetic polymorphism associated with 5-HT metabolism, such as 5-HT receptors and the 5-HT reuptake transporter, has been studied in detail.
Serotonin receptors: There are at least seven subtypes of receptor (5-HT1-7) within the serotonin receptor superfamily, including 5-HT1–4 and 5-HT7 receptors, which are present in the human GI tract. Results emerging from genetic studies have already shown that genetic polymorphism of 5-HT2, 5-HT3, and 5-HT4 receptors are related to IBS.
5-HT2A receptor, a type of phosphatidylinositol-linked receptor involved in sensitivity to pain, can bind to a G-protein-coupled receptor and directly transduce the signal by coupling to phospholipase C turnover in the cerebral cortex. Rat experiments have confirmed that the 5-HT2A receptor is involved in serotonin-induced hyperalgesia, as intradermal injection of exogenous 5-HT2A receptor agonist (a-methyl 5-HT) into footpad produced a decrease of withdrawal latency to heat stimulation. Significant associations were found between the -1438G/A and 102T/C polymorphisms of the 5-HT2A receptor gene and IBS. -1438G/A polymorphism located in the promoter region regulates the genetic transcription of 5-HT2A receptor. A case-control study among Turkish showed that the A/A genotype of the -1438G/A polymorphism and the C/C genotype of the 102T/C polymorphism conferred a high risk of IBS. Furthermore, patients with T/T genotype of 102T/C polymorphism had much higher scores on visual analog scales reflecting abdominal pain. In Greece, Markoutsaki et al also found that the A allele and the AA genotype of -1438G/A polymorphism were closely related to high risk of IBS. However, the Greek study showed no association between 102C/T polymorphism and IBS, and no polymorphism above was found to significantly correlate with the abdominal pain of IBS patients. Therefore, there might be ethnic disparities in genetic polymorphisms of 5-HT2A receptor.
5-HT3 receptor, a member of the superfamily of ligand-gated ion channels, localizes in numerous neurons of the myenteric and submucosal plexus in the GI tract. It mediates the effect of 5-HT on the parasympathetic ganglia, which results in smooth muscle contraction and increased intestinal secretion. Clinical studies have demonstrated that alosetron, a 5-HT3 receptor antagonist, increases the compliance to colorectal distension and effectively alleviates abdominal pain threshold in IBS patients[28,29]. Ondansetron, another 5-HT3 receptor antagonist, markedly increases fasting small bowel water content by either promoting small bowel secretion or reducing small bowel motility. Together, these studies suggest that 5-HT3 receptor might play a role in the development of IBS by altering the visceral sensitivity and GI motility. Through a series of polymerase chain reaction restriction fragment length polymorphism analyses, Gu et al found that SNP of c.−42C>T in the 5-HT3A receptor gene was associated with a higher risk of IBS-D in Asian women; however, no significant difference was shown in the association between T carrier status and IBS among American cohorts in previous studies. The SNP of c.*76G>A in the 5-HT3E receptor gene, another member of the 5-HT3 receptor family, was proven to have a highly significant association with female IBS-D in two independent Caucasian cohorts from United Kingdom and Germany. Moreover, c.*76G>A was shown to modulate the binding of miR-510 to untranslated regions (UTRs) of the 5-HT3E receptor gene in enterocytes and led to elevated expression of the target gene. This study was the first reported example of a cis-regulatory variant affecting microRNA (miRNA)-related expression in the regulation of serotonin receptor gene. The regulation revoked by the SNP may explain the connection between c.*76G>A and higher risk of IBS. Later, Zhang et al found that the variant of c.*76G>A in the 5-HT3E receptor gene was also significantly associated with IBS-D in Chinese women. Immunohistochemistry in their study suggested that the SNP reduced overexpression of the target gene in human colonic mucosal tissues, which underlines the participation of miRNA-related regulation in IBS as well. Thus, unlike the 5-HT3A receptor, there was no direct evidence supporting the ethnic differences in the polymorphism of 5-HT3E receptor. Meta-analysis reconfirmed that the c.*76G>A was significantly associated with the risk of IBS-D in both Asian and non-Asian populations, while the c.-42C>T was only correlated with that in Asians.
5-HT4 receptor, which evokes the 5-HT release of mucosa, degranulation of goblet cell, and Cl− secretion when stimulated, is broadly expressed in the small and large intestines. The role of the 5-HT4 receptor in initiating the peristaltic reflex through myenteric plexus in the human small intestine has already been identified. Wohlfarth et al assessed the genetic heterogeneity of isoform-specific UTRs in British patients (all were of Caucasian origin) and found a relevant SNP of c.*61T>C (rs201253747) residing in the 3′UTR near the stop codon of the gene. This variant was only detected in jejunal biopsy samples from IBS-D patients and proved to increase the risk of IBS-D. To confirm their findings, researchers proceeded to genotype 5 additional cohorts from 4 countries (the United States, Germany, Belgium, and Sweden), and performed a pooled analysis. Strikingly, there was a higher frequency of the variant c.*61T>C in IBS-D patients than in healthy controls and all non-IBS-D patients (P = 0.049, OR = 2.74). It has been further implied that the variant residing in a putative miRNA-binding site in the 3′UTR affects the binding process between the miR-16/miR-103 and 5-HT4 receptor. The disturbance created by the variant putatively impairs the expression of 5-HT4 receptors, which involves in the IBS susceptibility, especially for IBS-D. Caucasian participants from different countries or regions were inclined to present a similar connection between the SNP of c.*61T>C and IBS-D, which might emphasize the importance of ethnic factors in the polymorphism variances rather than geographical discrepancy. Nevertheless, to the best of our knowledge, no related results from other ethnic groups have been reported, and ethnic differences regarding the polymorphisms of 5-HT4 receptor gene warrant further exploration.
Serotonin reuptake transporter: As a transmembrane transport protein, the serotonin reuptake transporter (SERT) modulates the duration and intensity of cumulative impacts from serotonergic neurotransmission by mediating the reuptake of serotonin into the presynaptic neurons. The SERT gene (SLC6A4) locates in chromosome 17q12, and its promoter region contains a genetic polymorphism designated as the “5-HT transporter length polymorphic region” (5HTTLPR), which includes a 44-base-pair deletion or insertion, generating a short (S) or long (L) allele. The short variant of the polymorphism reduces the transcriptional efficiency of the SLC6A4 promoter, further decreases the expression level of the target gene and inhibits serotonin reuptake. Recently, subgroup analyses based on ethnicity in a large-scale meta-analysis proved that the 5HTTLPR insertion/deletion polymorphism was closely related to IBS susceptibility in both Asians and Caucasians. However, a great decrease in the heterogeneity was observed in the Asian subgroup compared to the whole groups, which suggested that there might be an ethnic difference even within the positive correlations between 5HTTLPR polymorphism and IBS risk. Because of limited raw data, the relationship between the 5HTTLPR and the predominant clinical feature of IBS was not statistically analyzed. Previous researches reported that the S/S genotype was predominant in IBS-D patients from Chinese, Indian and Korean[40-42]; furthermore, the serotonin level was higher in rectal biopsy specimens from Indian patients with IBS-D. In contrast, higher frequencies of S/S and L/S genotypes were shared by IBS-C patients from Iran significantly, rather than IBS-D, and even the serotonin levels were proved to be similar in each subtype of IBS, which further proved the involvement of ethnic factors related to 5HTTLPR polymorphism in IBS.
The adrenergic system
The adrenergic receptors, also known as the adrenoceptors, are members of the G-protein-coupled receptor superfamily. The GI tract receives signals from the noradrenaline neuron system via α2-adrenoceptors, which plays an essential role in altering GI mobility and algesthesis. There are three subtypes of α2-adrenoceptors, designated as α2A-, α2B-, and α2C-adrenergic receptors. An earlier study suggested that adrenoceptor polymorphism might be involved in visceral hypersensitivity, as participants with the variant del322–325 in the α2C-adrenergic receptor gene presented a higher pain score in response to the cold presser test than controls. In Caucasians, the polymorphisms of both del322–325 in the α2C-adrenoceptor gene and -1291C>G in α2A-adrenoceptors gene were proven to be associated with IBS-C. Nonetheless, no significant association was identified between the variant -1291C>G and IBS in Turkish cohorts. Moreover, case-control studies conducted in India and South Korea found that the polymorphism of -1291C>G in α2A-adrenergic receptor gene was closely related to IBS-D rather than IBS-C[47,48]. This discrepancy suggested that the development of IBS is possibly directed, at least in part, by unique ethnic variation in the adrenergic system.
Additionally, polymorphisms of the G-protein β3 subunit gene (GNB3), which influences the activity of G-protein-coupled adrenergic receptors, have been extensively investigated concerning its relevance to IBS. Studies performed in South Korea showed that the genotype T/T of 825C/T polymorphism was associated with IBS-C in both children and adults[49,50]. However, other studies in Caucasian populations concluded that there was no significant interaction between the polymorphism of 825C/T and any subtype of IBS[51,52]. Later, in a meta-analysis, Pan et al also failed to show any association between the 825C/T polymorphism and IBS. Nevertheless, after a few years, an updated meta-analysis with a larger sample of researches presents that the SNP of 825C/T was only significantly associated with IBS-C in the Asian population, which further suggested the possibility of ethnic differences in the connection of GNB3 polymorphisms and IBS. In fact, scientists have already obtained evidence that ethnic variation exists in the T-allele frequency of 825C/T polymorphism. The prevalence of the T allele is lowest in Caucasian populations (30.1%-31.9%), while it is higher in Asians (42.3%-47.7%), and the highest in Africans (82.3%-84.1%)[55,56]. The diverse distribution of allele frequency may partly contribute to the inconsistency in polymorphism studies on different ethnic groups. Interestingly, statistics have shown a fairly high frequency of the T allele in African populations. Because no relevant studies have been published connecting the 825C/T polymorphism with IBS in African populations, further investigations are warranted.