Review article

Mechanisms of Disease Peripheral Mechanisms in Michael Camilleri, M.D.

From Clinical Enteric Neuroscience Trans- he irritable bowel syndrome (IBS) affects 10 to 20% of the popu­ lational and Epidemiological Research lation in developed countries. Specific peripheral mechanisms result in (C.E.N.T.E.R.), College of Medicine, Mayo Clinic, Rochester, MN. Address reprint re- symptoms of IBS, including abnormal colonic transit and rectal evacuation; quests to Dr. Camilleri at the Mayo Clinic, T intraluminal intestinal irritants, such as maldigested carbohydrates (producing short- Charlton 8-110, 200 First St. SW, Roches- chain fatty acids) or fats, an excess of bile acids, and gluten intolerance; alterations ter, MN 55905, or at camilleri.michael@ mayo.edu. in the microbiome; entero-endocrine-cell products; and genetic susceptibility to inflammation or altered bile acid synthesis. These luminal and mucosal irritants N Engl J Med 2012;367:1626-35. DOI: 10.1056/NEJMra1207068 alter mucosal permeability and cause immune activation or inflammation, which in turn activates local reflexes that alter intestinal motility or secretion. The irritants Copyright © 2012 Massachusetts Medical Society. also stimulate sensory mechanisms, leading to visceral hypersensitivity and pain. A greater understanding of these mechanisms may foster individualized, specific treatments for patients with IBS.

Diagnosis and conventional mechanisms

The diagnosis of IBS is traditionally based on symptoms of recurrent abdominal pain or discomfort at least 3 days per month in the previous 3 months, in associa- tion with two or more of the following: improvement with , an onset associated with a change in the frequency of bowel movements, or an onset associ- ated with a change in the form (appearance) of stool. In the past, it was generally considered that the predominant pathophysiological mechanisms in IBS were ab- normalities intrinsic to the smooth muscle of the gut, visceral hypersensitivity, and central nervous system hypervigilance.1-3 Brain dysfunction and abnormal interac- tion of the peripheral and central nervous system are potential mechanisms causing hypersensitivity in IBS3; however, hypersensitivity 4,5 and central nervous system dysfunction6,7 are not ubiquitous in patients with IBS. Thus, although persistent nociceptive mechanisms are activated in some patients,7 IBS is not “all in the head” in all patients. There is a renaissance in the understanding of IBS: symptoms are not specific to a single etiologic mechanism but are manifestations of several pe- ripheral mechanisms that perturb motor and sensory functions.

Abnormal Colonic Transit and Disorders of Evacuation

Symptoms of -predominant IBS reflect those of chronic constipation,8 and many patients with constipation-predominant IBS can be treated first for con- stipation. About 25% of patients with constipation-predominant IBS (range among studies, 5 to 46) have slow colonic transit.5,9,10 Treatment with intestinal secretagogues (i.e., lubiprostone and linaclotide) or prokinetic agents (e.g., tegaserod) is effective in

1626 n engl j med 367;17 nejm.org october 25, 2012 The New England Journal of Medicine Downloaded from nejm.org at NCSU HUNT LIBRARY on March 14, 2013. For personal use only. No other uses without permission. Copyright © 2012 Massachusetts Medical Society. All rights reserved. Mechanisms of Disease relieving constipation and associated IBS symp- drate content, appears to have a key role in sensa- toms such as pain and bloating.11 tions of discomfort and pain.20 Disorders of rectal evacuation (spasm of the puborectalis muscle, anismus, and the descend- Malabsorbed or Maldigested Nutrients ing perineum syndrome) cause symptoms of of sugars, such as lactose, fruc- constipation-predominant IBS12 — that is, con- tose, and sorbitol, may mimic the features of IBS, stipation, straining, a sense of incomplete rectal but its prevalence across ethnic groups and races evacuation, bloating, and left-sided abdominal is unclear.21,22 A Norwegian case–control study pain, which are relieved with bowel movement. suggested that IBS and lactose malabsorption Treatment of the evacuation disorder relieves the were separate entities.21 Fructose and sorbitol symptoms of constipation-predominant IBS.13 An malabsorption were observed in patients with IBS evacuation disorder should be suspected when pa- from Denmark,22 but not in patients from the tients with constipation do not have a response Netherlands.23 to first-line therapies such as fiber14 and simple Maldigestion of complex carbohydrates may (e.g., osmotic) . be more prevalent than malabsorption in patients -predominant IBS is associated with with IBS. Fecal short-chain fatty acids, which con- acceleration of colonic transit in 15 to 45% of tain fewer than 6 carbon atoms, are increased in patients.5,10 Several disorders mimic diarrhea- stool from patients with diarrhea-predominant predominant IBS or cause accelerated transit and IBS.24 Short-chain fatty acids or medium-chain should be ruled out, such as food allergy or intol- fatty acids (which contain 6 to 12 carbon atoms) erance, disaccharidase deficiencies, celiac disease, reach the right colon in patients with borderline gluten intolerance without celiac disease, micro- absorptive capacity or rapid transit in the small scopic , and idiopathic bile acid malab- bowel. In a study involving healthy volunteers, sorption.15 2 to 20% of dietary starch escaped absorption in the small bowel,25 providing substrate for the gen- Mechanisms Involved in Bowel eration of short-chain fatty acids by colonic bacte- Irritation ria. The short-chain fatty acid receptor 2 (called free fatty acid receptor 2 [FFA2] or G-protein– Luminal and mucosal factors activate immune, coupled receptor 43) is expressed by enteroendo- motor, and sensory mechanisms in the small in- crine cells and mucosal mast cells in rat intes- testine or colon. Such “irritation” leads to the tine.26 Short-chain fatty acids stimulate colonic symptoms and pathophysiological features of transit and motility through intraluminal release IBS (Fig. 1 and Table 1). of 5-hydroxytryptamine (5-HT)27 from enteroen- docrine cells28 in rats. Short-chain fatty acids also Luminal Factors initiate high-amplitude propagated contractions in Responses to Food Ingestion the colon, propelling colonic content rapidly in Pain is temporally related to eating in patients with dogs.29 An in vitro study in guinea-pig colon IBS,16 especially in those with diarrhea-predomi- showed that the short-chain fatty acid propionate nant IBS who have repeated, high-amplitude, prop- induced transepithelial ion and fluid transport in agated colonic contractions17 (i.e., contractions that mucosal preparations from the distal colon and traverse ≥10 cm of colon and propel colonic con- increased the expression of FFA2, which colocal- tent). Patients with diarrhea-predominant IBS, as izes with chromogranin A–enteroendocrine cells.30 compared with healthy controls, also have Fermentable oligosaccharides, disaccharides, increased ileocolonic transit,18 typically induced monosaccharides and polyols (FODMAPs) are by meals containing fat and providing at least poorly absorbed in the and may 500 kcal.18 In addition, one study involving pa- induce symptoms of IBS31 through the production tients with IBS showed that scores for urge, dis- of short-chain fatty acids and their effects on co- comfort, and pain in response to rectal disten- lonic motility and secretion. Conversely, dietary tion were significantly increased after a 368-kcal manipulation of FODMAP content may reduce IBS meal relative to fasting, though the magnitude of symptoms.32 The role of the colonic microbiota in these differences in sensation was small.19 The these nutrient-induced symptoms requires further fat content of the meal, rather than the carbohy- elucidation.

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likely to have a response to gluten withdrawal as were patients without these genotypes.34 A ran- domized, placebo-controlled trial involving pa- tients who previously reported intolerance to glu- ten and a response to its withdrawal confirmed that gluten was associated with symptoms of IBS.35 In mice sensitized to wheat glycoprotein, gliadin treatment increased responses to contractile and secretory stimuli (carbachol and electrical-field stimulation).36 In a recent randomized, controlled trial of a gluten-containing diet versus a gluten- free diet in patients with diarrhea-predominant IBS, those receiving gluten had increased stool fre- quency and bowel permeability and reduced mes- senger RNA expression of tight-junction proteins in bowel mucosa.37

Increased Levels of Intracolonic Bile Acids Although the is excessively sensitive to the secretory effects of perfused bile acids in patients with IBS,38 the effects of bile acids are mainly man- ifested as cholerheic with diarrhea. A systematic review of the literature suggested that accounts for approximate- ly 30% of cases of diarrhea-predominant IBS.39 In other studies, about 25% of patients with diarrhea- predominant IBS had an elevated rate of either bile acid synthesis (as measured by the fasting concen- tration of serum 7α-hydroxy-4-cholesten-3-one [C4, a bile acid precursor]) or 48-hour total fecal bile acid excretion.40,41 Excess intracolonic bile acid in diarrhea-pre- dominant IBS results from alterations in the en- terohepatic circulation of bile acids. Synthesis of bile acid is regulated homeostatically by feedback inhibition of hepatocytes provided by fibroblast growth factor 19 (FGF19), which is produced by ileal enterocytes. FGF19 is secreted into the portal circulation and binds to fibroblast growth factor Figure 1. Mechanisms Underlying the Irritable Bowel Syndrome (IBS). receptor 4 (FGFR4) and the klotho-β (KLB) recep- A variety of peripheral mechanisms initiate perturbation of gastrointestinal tor on the hepatocyte cell membrane, triggering motor and sensory functions and lead to IBS symptoms. Identification of the peripheral irritants provides an opportunity to prevent or reverse symptoms. intracellular signals that suppress bile acid syn- CNS denotes central nervous system. thesis. Given that a defect in bile acid absorption has been demonstrated in diarrhea-predominant IBS,39 it appears that excessive synthesis of bile acids overcomes ileal absorption capacity, leading Gluten Intolerance to bile acid diarrhea.42 The prevalence of celiac disease among patients Various mechanisms have been suggested to with IBS33 is similar to that among controls. Ce- cause the elevated levels of intracolonic bile acid liac disease–free patients with IBS who carry HLA- reported in diarrhea-predominant IBS or func- DQ2 or HLA-DQ8 genotypes (which confer a pre- tional diarrhea. One potential mechanism is a disposition to celiac disease) were five times as deficiency in ileal secretion of FGF19. Walters et

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Comments and Clinical Correlates predominant IBS, 25% of patients with patients of 25% IBS, predominant IBS constipation-predominant diarrhea ing, pain ing, inant IBS plus incomplete evacuation incomplete plus IBS inant pelvic-floor with reversed straining and retraining inant IBS; may increase fluid secretion fluid increase may IBS; inant mechanism sensory activate or inant IBS; may increase fluid secretion fluid increase may IBS; inant or activate sensory mechanism sensory activate or inant IBS and abdominal pain abdominal and IBS inant diarrhea Affects up to 45% of patients with diarrhea- with patients of 45% to up Affects Contributes to postprandial pain, urgency, pain, postprandial to Contributes Typically associated with diarrhea, bloat- diarrhea, with associated Typically Typical symptoms of constipation-predom ­ of symptoms Typical Typically associated with diarrhea-predom ­ with associated Typically Typically associated with diarrhea-predom ­ with associated Typically Typically associated with diarrhea-predom ­ with associated Typically Associated with abdominal bloating, pain, bloating, abdominal with Associated -

TLR9,

Examples of Factors Involved GUCY2C the gene encoding the guanylate cyclase C receptor; HAPCs high-amplitude cell products (e.g., 5-HT, granins), organic organic granins), 5-HT, (e.g., products cell predis - genetic SCFAs), acids, (bile acids synthesis acid bile impaired to position secretion enterocyte or SNP) ( KLB mutation) ( GUCY2C products, organic acids (bile acids, SCFAs) acids, (bile acids organic products, perineum syndrome perineum (e.g., gluten, FODMAPs), stress FODMAPs), gluten, (e.g., ) and altered feedback regula - feedback altered and TNFSF15 ) SNP) ( KLB synthesis acid bile of tion which increases levels of SCFAs; bile acid bile SCFAs; of levels increases which with patients of 25% (in malabsorption acti - immune IBS); diarrhea-predominant inflam - to predisposition genetic vation; (e.g., activation immune or mation cytes, circulating cytokines circulating cytes, antibiotics or probiotics; microbial probiotics; or antibiotics acids bile by influenced species micutes to bacteroidetes, modified by modified bacteroidetes, to micutes Neuromuscular dysfunction, enteroendocrine- dysfunction, Neuromuscular Fat content of meal, high caloric content caloric high meal, of content Fat Food stimulation of enteroendocrine-cell of stimulation Food Anismus, pelvic-floor , descending descending dyssynergia, pelvic-floor Anismus, Previous , atopy, food intolerance intolerance food atopy, gastroenteritis, Previous Malabsorption of carbohydrates or fats, or carbohydrates of Malabsorption Previous gastroenteritis, mast cells, T lympho T cells, mast gastroenteritis, Previous Increased level of firmicutes or ratio of fir - of ratio or firmicutes of level Increased

tumor necrosis factor (ligand) superfamily, member 15. Pathophysiological Effect KLB the gene encoding klotho-β; mRNA messenger RNA; SCFAs short-chain fatty acids; SNP single-nucleotide polymorphism; TNFSF15 the gene encoding altered motility may be a consequence of consequence a be may motility altered secretion altered of colonic HAPCs, increased ileocolonic increased HAPCs, colonic of sensitivity rectal increased transit, and sensory mechanisms sensory and tion of colonic motor function motor colonic of tion (mRNA, protein) of tight-junction of protein) (mRNA, proteins (mRNA, protein) of tight-junction of protein) (mRNA, proteins cosal secretory reflexes and sensory and reflexes secretory cosal mechanisms secretory, and sensory functions sensory and secretory, Accelerated or delayed colonic transit due to due transit colonic delayed or Accelerated Neurally (e.g., vagally) mediated induction mediated vagally) (e.g., Neurally Activation of local secretory or motor reflexes reflexes motor or secretory local of Activation Failure of rectal emptying with reflex inhibi - reflex with emptying rectal of Failure Increased permeability, altered expression altered permeability, Increased Increased permeability, altered expression altered permeability, Increased Increased permeability, activation of submu - of activation permeability, Increased Production of SCFAs, with effects on motor, on effects with SCFAs, of Production

Peripheral Factors in the Biology of Irritable Bowel Syndrome (IBS).* Syndrome Bowel Irritable of Biology the in Factors Peripheral to food ingestion food to bowel and colon and bowel Table 1. Table Peripheral Mechanism Colonic motility Colonic Colonic motor and sensory response sensory and motor Colonic Sensing and responses in the small the in responses and Sensing Rectal evacuation Rectal Small-bowel mucosal permeability mucosal Small-bowel Colonic mucosal permeability mucosal Colonic Mucosal immune activation immune Mucosal Colonic microbiome Colonic propagated contractions; 5-HT 5-hydroxytryptamine; TLR9 the gene encoding toll-like receptor 9; and * FODMAPs denotes fermentable oligosaccharides, disaccharides, monosaccharides, and polyols;

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al.43 found that ileal production of FGF19 (re- sibly reflecting the reduced time for bacterial flected in plasma levels of this hormone) was dehydroxylation in the colon due to rapid tran- reduced in patients with chronic diarrhea associ- sit.49,57 Colonic bacteria deconjugate and dehy- ated with bile acid malabsorption. There was an droxylate bile acids to different extents, yet the inverse relationship between fasting serum FGF19 primary or secondary bile acids with at least two levels and serum C4 levels (a finding that is consis- α-hydroxyl groups (chenodeoxycholic, cholic, and tent with increased bile acid synthesis); this in- deoxycholic acids) all induce secretion in mam- verse correlation was confirmed in patients with malian intestine or colonic epithelial-cell mono- diarrhea-predominant IBS.44,45 Another potential layers.58,59 Moreover, chenodeoxycholic acid in- mechanism is mutation of the ileal bile acid trans- duces high-amplitude propagated contractions in porter (also termed apical sodium-dependent bile healthy persons60 and acceleration of colonic tran- acid transporter or solute carrier family 10, mem- sit in patients with constipation-predominant ber 2), resulting in malabsorption of bile acid. This IBS,45 and deoxycholic acid induces colonic in- condition is extremely rare, even among familial flammation and hypersensitivity in rats.61 The cases of bile acid malabsorption.46,47 A third bacterial dehydroxylation of chenodeoxycholic acid potential mechanism is genetic variation in bile to lithocholic acid would theoretically reduce co- acid synthesis or G-protein–coupled bile acid re- lonic secretion, but lithocholic acid constitutes only ceptor 1 (GPBAR1), also known as TGR5 (dis- about 20% of total fecal bile acid.57 Therefore, cussed below). the total proportion of secretory bile acids is only modestly altered by bacterial hydroxylation Microbiota and Organic Acids in the colon. The precise role of the fecal or mucosal microbi- Overall, microbial differences appear to be un- ome in IBS is unclear. An abundance of fir­mic­u­ likely to alter colonic function through changes tes bacteria in the fecal microbiome has been in bile acid metabolism. Conversely, bile acids may observed in patients with IBS, either as the sole modify the microbial content of the colon. For finding48,49 or in combination with a decrease in example, in rats, administration of cholic acid re- bacteroidetes bacteria.50,51 An increase in the ra- sults in cecal microbiota that reflect the ratio of tio of firmicutes to bacteroidetes has been re- firmicutes to bacteroidetes observed in patients ported to be associated with longer colonic transit with IBS.62 time and increased levels of mental depression in IBS.52 Studies of mucosa-associated microbiota Enteroendocrine Signals Arising in patients with diarrhea-predominant IBS have in the Mucosa shown increases in bacteroides and clostridia The release of several peptides and amines, such and a reduction in bifidobacteria.53 as serotonin, from enteroendocrine cells is trig- A role of the microbiome in the causation of gered by luminal factors, such as exogenous di- IBS symptoms is supported by randomized, con- etary amines or tastants or their metabolites (e.g., trolled trials of the minimally absorbed antibi- short-chain fatty acids), and by endogenous chem- otic rifaximin54 and a meta-analysis showing the icals involved in the digestive process, such as bile efficacy of probiotics,55 particularly for abdomi- acids.63,64 The potential role of serotonin in IBS nal pain and bloating. In addition, a probiotic is supported by the increased circulating levels of mixture has been shown to retard colonic transit 5-HT in patients with diarrhea-predominant IBS in patients with diarrhea-predominant IBS.56 and decreased 5-HT levels in patients with con- The fecal microbiome causes changes in co- stipation-predominant IBS; the generally stimu- lonic functions that induce IBS symptoms through latory effects of 5-HT on motor, secretory, and microbial interactions with intraluminal factors. sensory functions65,66; and the effect of selective IBS is characterized by alterations in the profiles serotonergic agonists and antagonists in the treat- of organic acids (e.g., short-chain fatty acids) ment of different IBS phenotypes.11 and the proportion of secondary bile acids (de- Enteroendocrine cells also release granins. oxycholic and lithocholic acids) as compared with Chromogranins and secretogranins are present primary bile acids (cholic and chenodeoxycholic in secretory vesicles of nervous, endocrine, and acids). For example, the proportion of primary bile immune cells. Activation of nicotinic cholinergic acid in stool is higher in patients with diarrhea- receptors (e.g., by acetylcholine released from sub- predominant IBS than in healthy controls, pos- mucosal nerves) induces granin release from these

1630 n engl j med 367;17 nejm.org october 25, 2012 The New England Journal of Medicine Downloaded from nejm.org at NCSU HUNT LIBRARY on March 14, 2013. For personal use only. No other uses without permission. Copyright © 2012 Massachusetts Medical Society. All rights reserved. Mechanisms of Disease cells. Chromogranin A can induce the formation vitro, or in Caco-2 monolayers in response to fe- of mobile secretory granules and promote the sort- cal supernatants from patients with IBS (sum- ing and release of other peptide hormones from marized by Rao et al.80). Children with IBS also enteroendocrine cells.67 Chromogranin-derived have evidence of increased proximal-gut and co- peptides secreted by such cells have antimicrobial lonic permeability and low-grade inflammation.81 properties against bacteria, fungi, and yeasts.68 Factors associated with increased mucosal perme- As compared with healthy controls, patients ability and IBS include cow’s milk allergy, previous with IBS, particularly those with rapid colonic nonspecific infection, atopic disease (e.g., rhino- transit, have higher levels of fecal chromogranin conjunctivitis, rhinitis, and eczema),82-84 stress, and A, secretogranin II, and secretogranin III but dietary fat. lower levels of chromogranin B.69 These findings Two lines of investigation support the role of are nonspecific, since increased fecal granins or stress in increased bowel permeability in humans. chromogranin cell density in colonic mucosa is First, when the stress hormone corticotropin- observed in other diarrheal diseases, such as releasing hormone was applied to the serosal lymphocytic colitis70 and celiac disease.71,72 side of colonic mucosal biopsy specimens from Chromogranin A cells also express FFA2 recep- healthy persons, there was increased transcellular tors that respond to short-chain fatty acids.40 uptake of horseradish peroxidase, an effect that Overall, the data are mostly consistent with the was mediated by mast cells.85 Second, stress in- hypothesis that granins (through their role in the duced by having study participants place their sorting and packaging of neuropeptides73) are hands in cold water increased jejunal permeabil- indirect biomarkers, rather than direct stimula- ity in healthy women but not in healthy men.86 tors, of colonic secretion or motility.74 A high-fat diet results in gut-derived endotox- emia87 and may contribute to the immune acti- Consequences of Irritation vation observed in some patients with IBS. Studies of the Colon in animals have shown that emulsified fats in- crease intestinal permeability, causing endotox- Immune Activation with Minimal emia and inflammation.88 Inflammation The link between increased mucosal perme- Öhman and Simrén75 have summarized the evi- ability and IBS is supported by the observation dence of inflammation or immune activation in that increased permeability enhances mucosal in- the blood in at least some subsets of patients flammation and activates local reflex mechanisms, with IBS. The evidence of mast-cell and other im- stimulating secretion and sensory pathways that mune (e.g., cytokine) activation in intestinal or lead to increased visceral sensation.4 colonic mucosa is less consistent.75 Inflamma- tion, manifested as increased levels of T lympho- Genetic Factors cytes in the rectal mucosa in patients with IBS, has been associated with increased intestinal The published literature on potential genetic fac- permeability.76 These data, in addition to the epi- tors conferring a predisposition to IBS is sum- demiologic and clinical observations of postinfec- marized elsewhere.89,90 The reported genetic fac- tious IBS77,78 and colonic mucosal gene-expres- tors of greatest interest confer a predisposition to sion profiles showing functional alterations of inflammation, bile acid synthesis, expression of several components of the host mucosal immune bioactive neuropeptides, and intestinal secretion response to microbial pathogens,79 support a role through a mutation in the guanylate cyclase C of immune activation and altered bowel barrier secretory pathway. function in a subgroup of patients with IBS. Ge- netic susceptibility may confer a predisposition Susceptibility to Inflammation and IBS to immune activation in a subset of patients with Symptoms IBS (discussed below). Among 30 susceptibility loci for Crohn’s disease that are associated with epithelial transport, bar- Increased Mucosal Permeability rier function, bacterial recognition, autophagy, Several studies in adults have documented in- prostaglandin production, and differentiation of creased small-bowel or colonic mucosal perme- interleukin-17–producing helper T cells, Zucchelli ability in vivo, in mucosal biopsy specimens in et al. identified a significant association between

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the gene encoding tumor necrosis factor (ligand) homozygosity for the minor allele A at this SNP, superfamily, member 15 (TNFSF15) and IBS phe- as compared with the CC genotype (homozygos- notype in separate populations of Swedish and ity for the major allele C), increases the odds of U.S. patients90; the strongest association was with diarrhea-predominant IBS or IBS with alternat- constipation-predominant IBS (odds ratio for de- ing bowel function and is significantly associ- velopment of the disease, 1.79; 95% confidence ated with accelerated colonic transit in diarrhea- interval, 1.41 to 2.26; P<0.001). In a cohort from predominant IBS.97 the United Kingdom, genetic variations in TNFSF15 The gene controlling the serotonin transporter were protective against diarrhea-predominant (SLC6A4) protein, 5-HTTLPR, is associated with IBS IBS.91 Villani et al. reported that four genes were phenotype in some ethnic groups, but the find- associated with postinfectious IBS in patients in ings are inconsistent. The short allele is associated Walkerton, Ontario, Canada, and these suscepti- with reduced SLC6A4 function. As compared with bility loci included toll-like receptor 9 (TLR9).92 In the long allele, the short allele is associated with a univariate analysis, colonic transit in patients higher ratings of rectal pain98 and increased with IBS was found to be associated with four activation of regional cerebral blood flow during inflammation-susceptibility genes that included painful colorectal distentions.99 TLR9 and the genes encoding cadherin 1 (CDH1) and interleukin 6 (IL6).93 Genetic Mutation in the Guanylate Cycle C Secretory Pathway Genetic Variation in Bile Acid Synthesis Fiskerstrand et al.100 described a Norwegian fam- Genetic variation in KLB, to which FGF19 binds,42 ily with a rare form of familial diarrhea, charac- is associated with diarrhea-predominant IBS and terized by the onset of symptoms in infancy and accelerated colonic transit.94 A functional genetic chronic, relatively mild diarrhea diagnosed as variation in KLB (Arg728Gln), resulting in im- diarrhea-predominant IBS. This dominantly in- paired KLB synthesis, prevents FGF19 from bind- herited, fully penetrant disease is due to a muta- ing to the combined KLB–FGFR4 receptor on the tion in the gene encoding the guanylate cyclase C hepatocyte. This reduces the FGF19 feedback inhi- receptor (GUCY2C), which induces enterocyte se- bition of hepatocyte synthesis of bile acids, result- cretion. The mutation was a heterozygous base ing in more bile acid reaching the bowel and, po- substitution, c.2519G→T, in exon 22 of chromo- tentially, causing accelerated transit and diarrhea. some 12. A second potential mechanism is genetic variation in GPBAR1. The receptor is located on Clinical and Therapeutic myenteric, cholinergic, and nitrergic neurons in Implications the colon and proximal small intestine. Genetic variation in GPBAR1 may be associated with small- IBS is no longer regarded as an idiopathic bowel bowel and colonic transit in healthy persons and dysfunction that originates exclusively from psy- in patients with IBS.95 chological stress or brain dysfunction. Fecal tests (e.g., measurements of fecal calprotectin and lacto- Genetic Variation in expression ferrin levels) or colonic imaging (e.g., colonoscopy) of Neurotransmitters and Cytokines may be indicated clinically to rule out inflamma- Neuropeptide S (NPS) receptor 1 (NPSR1) is ex- tory bowel disease and neoplastic diseases. Pa- pressed by gastrointestinal enteroendocrine cells tients who do not have a response to lifestyle and and induces the production of several neuropep- dietary modifications and symptomatic remedies tides. The NPS–NPSR1 ligand–receptor complex should undergo tests to identify causative factors. is involved in inflammation, anxiety, and noci- These include tests of colonic transit and rectal ception. Three single-nucleotide polymorphisms evacuation in patients with constipation-predomi- (SNPs) of NPSR1 are significantly associated with nant IBS and tests for carbohydrate or fat mal- colonic transit in IBS.96 digestion (leading to the formation of short-chain The endocannabinoid anandamide is inacti- fatty acids), increased bile acid synthesis or in- vated by fatty acid amide hydrolase (FAAH). A creased fecal bile acid excretion, and, possibly, SNP in human FAAH (C385A or rs324420) re- dietary intolerance (e.g., gluten) in patients with duces FAAH expression. FAAH heterozygosity or diarrhea-predominant IBS.

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It is anticipated that the relationship of diet, predominant IBS,11 and possibly probiotics, microbiome, endogenous irritants, barrier func- nonabsorbable antibiotics,54 antiinflammatory tion, immune activation, and genetic factors to agents,10 or tight-junction modulators (e.g., lar- the motor, sensory, secretory, and psychological azotide) for patients with evidence of immune dysfunctions in IBS will be further elucidated. activation or increased mucosal permeability. Future research will address the overall hypothe- 101 sis that, like inflammatory bowel disease, IBS Dr. Camilleri reports receiving consulting fees from Takeda results from interactions between the host and Pharmaceuticals USA, Albany Molecular Research, BioKier, genetic factors. These advances will lead to the Theravance, Alkermes, ARYx Therapeutics, AstraZeneca, Domain Therapeutics, Ironwood Pharmaceuticals, Tranzyme, and NPS development of more specific, individualized treat- Pharmaceuticals; receiving grant support from Rose Pharma, ment that is based on the underlying peripheral Albireo, Tsumura, Second Genome, SK Life Science, Salix Phar- pathophysiological mechanisms, including dietary maceuticals, and Rhythm through his institution; being a coin- ventor of a patent related to enteric delivery of chenodeoxycholic recommendations (e.g., gluten or FODMAP exclu- acid for constipation; and receiving royalties from EnteroMedics sion32), -based treatment of defecation for licensed technology related to vagal-stimulation treatment of disorders,13 bile acid sequestrants and 5-HT an- obesity. No other potential conflict of interest relevant to this 3 article was reported. tagonists for diarrhea and urgency, prokinetics Disclosure forms provided by the author are available with the or secretagogues for patients with constipation- full text of this article at NEJM.org.

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