Primary Bile Acid Malabsorption Research Articles

Primary bile acid malabsorption. Histologic and immunologic study in three patients.

Three patients are presented with a history of chronic watery diarrhea due to bile acid malabsorption, proved by the tauro-23[75Se]selena-25 homocholic acid test and an unequivocal response to cholestyramine therapy. Fecal fat tests, Schilling tests, and barium studies of the small intestine and colon were all normal. Jejunal biopsies were normal but multiple biopsies of the terminal ileum, performed by retrograde ileoscopy, showed uniform crypt hyperplastic villous atrophy and features of colonic metaplasia with increased mononuclear infiltration of the lamina propria. All 3 patients demonstrated evidence of abnormal immune function, including the presence of serum autoantibodies, circulating immune complexes, and hypocomplementemia. One patient had Sjögren's syndrome. This disorder, which might be immunologically mediated, should be called primary bile acid malabsorption and should be distinguished from other ileal disorders.

PTU-141 Bile acid malabsorption: an evaluation of current practice

IntroductionPrimary or idiopathic bile acid malabsorption (BAM) is increasingly recognised as a cause of chronic diarrhoea. Nevertheless, it remains underestimated by many gastroenterologists partly because of the limited use of...

Mechanisms of Diarrhea

Diarrhea is a symptom common to a wide variety of gastrointestinal illnesses, and is an important public health challenge in underdeveloped regions of the world. Normal intestinal absorption is a complex process. Recent research offers new insights into normal physiology and pathophysiology. The role of the enteric nervous system and neurotransmitters in the pathogenesis of diarrhea in inflammatory bowel disease (IBD) and irritable bowel syndrome (IBS) is being actively investigated. In patients with IBD, ileal and sigmoid biopsies showed altered transepithelial sodium and fluid transport, specifically from decreased expression of the NHE3, NHERF-1, and NHE1 epithelial Na channel. This results in changes in normal intestinal electroneutral NaCl absorption and may be an additional factor contributing to the diarrhea in patients with IBD. Physiologic studies in humans suggest that primary bile acid malabsorption may be caused by an abnormal feedback system resulting in the increased bile salts, which may explain the watery diarrhea. Finally, the role of zinc in treatment of infectious diarrhea led to studies of its effect on intracellular human enterocyte ion secretion. Understanding such basic mechanisms may lead to better and novel therapies for treatment of diarrhea.

A New Mechanism for Bile Acid Diarrhea: Defective Feedback Inhibition of Bile Acid Biosynthesis

Primary (idiopathic) bile acid malabsorption (BAM) is a common, yet underrecognized, chronic diarrheal syndrome. Diagnosis is difficult without selenium homocholic acid taurine (SeHCAT) testing. The diarrhea results from excess colonic bile acids, but the pathogenesis is unclear. Fibroblast growth factor 19 (FGF19), produced in the ileum in response to bile acid absorption, regulates hepatic bile acid synthesis. We proposed that FGF19 is involved in bile acid diarrhea and measured its levels in patients with BAM. Blood was collected from fasting patients with chronic diarrhea; BAM was diagnosed by SeHCAT. Serum FGF19 was measured by enzyme-linked immunosorbent assay. Serum 7alpha-hydroxy-4-cholesten-3-one (C4) was determined using high-performance liquid chromatography, to quantify bile acid synthesis. Data were compared between patients and subjects without diarrhea (controls). Samples were taken repeatedly after meals from several subjects. The median C4 level was significantly higher in patients with primary BAM than in controls (51 vs 18 ng/mL; P < .0001). The median FGF19 level was significantly lower in patients with BAM (120 vs 231 pg/mL; P < .0005). There was a significant inverse relationship between FGF19 and C4 levels (P < .0004). Low levels of FGF19 were also found in patients with postcholecystectomy and secondary bile acid diarrhea. Abnormal patterns of FGF19 levels were observed throughout the day in some patients with primary BAM. Patients with BAM have reduced serum FGF19 which may be useful in diagnosis. We propose a mechanism whereby impaired FGF19 feedback inhibition causes excessive bile acid synthesis that exceeds the normal capacity for ileal reabsorption, producing bile acid diarrhea.

Mutation screening of apical sodium-dependent bile acid transporter (SLC10A2): novel haplotype block including six newly identified variants linked to reduced expression

The apical sodium-dependent bile acid transporter (SLC10A2) plays a key role in the reabsorption of luminal bile acids into the enterohepatic circulation. Rare variations in SLC10A2 have been reported to be associated with Crohn's disease, primary bile acid malabsorption and familial hypertriglyceridemia; however, variants associated with reduced SLC10A2 expression have not been reported to date. In this study, we have performed a sequence analysis of SLC10A2 using genomic DNA of 93 individuals. A new haplotype structure was identified including ten variants with complete linkage disequilibrium (LD' = 1.0, r (2) = 1.0) of which six polymorphisms were novel. The sequence variants were confirmed in three independent cohorts (n = 1,290) by a recently established MALDI-TOF MS iPLEX assay. Remarkably, haplotype carriers with the minor allele exhibited significant reduced ileal SLC10A2 expression on mRNA levels (2.6-fold, P = 0.0009) and protein levels (2.4-fold, P = 0.0157). In future studies a single tag SNP selected of this haplotype block will provide reliable genetic testing to investigate systemically the influence of the SLC10A2 haplotype for disease susceptibility and/or drug response.

Exploring possible mechanisms for primary bile acid malabsorption: evidence for different regulation of ileal bile acid transporter transcripts in chronic diarrhoea

Chronic diarrhoea resulting from primary idiopathic bile acid malabsorption (IBAM) is common, but its aetiology is largely unknown. We investigated possible mechanisms, first looking for common sequence variations in the cytoplasmic ileal bile acid-binding protein (IBABP, gene symbol FABP6), and secondly, determining the expression of ileal mucosal transcripts for the apical sodium-linked bile acid transporter (ASBT), IBABP, the putative basolateral transporters, OSTalpha and OSTbeta, and regulatory factors. Genomic DNA was prepared from two cohorts of patients and two control groups; the promoter and exonic regions of FABP6 were sequenced. In intestinal biopsies, transcript expression was measured by quantitative real time-PCR, using ileum from 17 patients and 21 controls. Sequence variations were identified in FABP6, but overall frequencies were similar in patients and controls. Transcripts of ASBT and IBABP, but not OSTalpha and OSTbeta, were expressed at higher levels in ileum than duodenum. The transcription factors farnesoid-X-receptor (FXR) and liver-receptor-homologue (LRH-1) were also more abundant in ileum, as was fibroblast growth factor 19 (FGF19), unlike short heterodimer partner (SHP), c-Fos, or CDX2. No significant differences in mean or median values were found between the groups for any of these transcripts. However, findings on regression analysis suggested that these transporters differ in their regulation, particularly in the relationships of CDX2, LRH-1 and FXR with OSTalpha. Most cases of IBAM are unlikely to be caused by genetic variation in FABP6 or by major differences in transporter transcript expression. Our evidence indicates that other factors, such as regulation of expression of the basolateral bile acid transporter, should be considered as possible causes.

Apical sodium bile acid transporter and ileal lipid binding protein in gallstone carriers

Although a cholesterol supersaturation of gallbladder bile has been identified as the underlying pathophysiologic defect, the molecular pathomechanism of gallstone formation in humans remains poorly understood. A deficiency of the apical sodium bile acid transporter (ASBT) and ileal lipid binding protein (ILBP) in the small intestine may result in bile acid loss into the colon and might promote gallstone formation by reducing the bile acid pool and increasing the amount of hydrophobic bile salts. To test this hypothesis, protein levels and mRNA expression of ASBT and ILBP were assessed in ileal mucosa biopsies of female gallstone carriers and controls. Neither ASBT nor ILBP levels differed significantly between gallstone carriers and controls. However, when study participants were subgrouped by body weight, ASBT and ILBP protein were 48% and 67% lower in normal weight gallstone carriers than in controls (P < 0.05); similar differences were found for mRNA expression levels. The loss of bile transporters in female normal weight gallstone carriers was coupled with a reduction of protein levels of hepatic nuclear factor 1alpha and farnesoid X receptor. In conclusion, in normal weight female gallstone carriers, the decreased expression of ileal bile acid transporters may form a molecular basis for gallstone formation.

Targeted Deletion of the Ileal Bile Acid Transporter Eliminates Enterohepatic Cycling of Bile Acids in Mice

The ileal apical sodium bile acid cotransporter participates in the enterohepatic circulation of bile acids. In patients with primary bile acid malabsorption, mutations in the ileal bile acid transporter gene (Slc10a2) lead to congenital diarrhea, steatorrhea, and reduced plasma cholesterol levels. To elucidate the quantitative role of Slc10a2 in intestinal bile acid absorption, the Slc10a2 gene was disrupted by homologous recombination in mice. Animals heterozygous (Slc10a2+/-) and homozygous (Slc10a2-/-) for this mutation were physically indistinguishable from wild type mice. In the Slc10a2-/- mice, fecal bile acid excretion was elevated 10- to 20-fold and was not further increased by feeding a bile acid binding resin. Despite increased bile acid synthesis, the bile acid pool size was decreased by 80% and selectively enriched in cholic acid in the Slc10a2-/- mice. On a low fat diet, the Slc10a2-/- mice did not have steatorrhea. Fecal neutral sterol excretion was increased only 3-fold, and intestinal cholesterol absorption was reduced only 20%, indicating that the smaller cholic acid-enriched bile acid pool was sufficient to facilitate intestinal lipid absorption. Liver cholesteryl ester content was reduced by 50% in Slc10a2-/- mice, and unexpectedly plasma high density lipoprotein cholesterol levels were slightly elevated. These data indicate that Slc10a2 is essential for efficient intestinal absorption of bile acids and that alternative absorptive mechanisms are unable to compensate for loss of Slc10a2 function.

Clinical Quiz

A 5-year-old child had a history of chronic diarrhea starting from the first month of life. His parents were related, and three other siblings were in good health. During the pregnancy, dilated bowel loops without polyhydramnios were demonstrated by ultrasound. After 37 weeks' gestation the baby was delivered by caesarean section with an Apgar score of 8/9 and a birth weight of 3200 g. At two weeks of age, meconium absence led to suspicion of Hirschsprung's disease. Thus, anal manometry and histochemical stains of rectal biopsy for acetylcholinesterase were performed showing normal results. Subsequently, in the following months and years, he developed chronic diarrhea (10–12 evacuations of liquid stool daily), without abdominal pain or rectal bleeding. Most bowel movements were associated with urgency or incontinence, and the child could not attend school because of diarrhea. By the age of 5 years, the family had moved to our city and the patient was re-evaluated. More recently, he had been admitted two times to an emergency department for severe dehydration due to gastroenteritis. Except for moderate abdominal distension and mild dehydration, the physical examination was normal. Laboratory tests showed normal erythrocyte sedimentation rate, and electrolytes, immunoglobulin, urea, creatinine, alkaline phosphatase, and albumin levels. Fecal quantitative fat measurement was not performed because of stool liquid consistency. Reducing substances were absent in feces and lactose hydrogen breath test, fecal alfa-1-antitrypsin, xylose absorption, and stool microbiologic tests were normal. Fecal osmotic gap was 64. Jejunal biopsy by endoscopy was performed. Endoscopic and histologic pictures were normal. X-ray evaluation of the intestine was also normal. Electron microscopy of jejunal biopsy specimen is shown in Figure 1.FIG. 1.: Electron microscopy of jejunal biopsy specimen.what is the etiology of diarrhea? A. Glucose and galactose malabsorption B. Congenital chloride diarrhea D. Primary bile acid malabsorption E. Intestinal epithelial dysplasia F. Small bowel overgrowth The electron microscopy of jejunal biopsy specimen was normal, thus excluding intestinal epithelial dysplasia. The patient was affected by congenital chloride diarrhea (B). In fact, fecal Cl−, Na+, and K+ concentrations were 180, 48, and 60 mmol/l respectively. Comment: The two diagnostic criteria of congenital chloride diarrhea (CCD) were fulfilled by the measurement of fecal electrolytes: Cl− content >90 mmol/L and > Na+ + K+ content (1). CCD is an inherited congenital defect of intestinal ion transport due to a mutation in the solute carrier family 26, member 3 (SLC26A3) gene at chromosomal location 7q31, close to the cystic fibrosis transmembrane conductance regulator (CFTR) gene (1,2). In CCD, the Cl−/HCO3− exchanger is absent or defective, causing severe intestinal Cl− malabsorption. Massive losses of Cl− in stools and the related defect in HCO3− secretion induce an acidification of intestinal lumen, which in turn further inhibits Na+ absorption through Na+/H+ exchanger activity. Thus, in the intestinal lumen high electrolyte content leads to osmotic diarrhea (2). Frequently the affected patients do not pass meconium and Hirschsprung's disease could be suspected. Our case is a sporadic form. Sporadic patients from different populations usually have two previously unrecognized unique mutations in their SLC26A3 alleles with no previous suspicion of CCD in the family, that suggests that mutations in the SLC26A3 gene are relatively frequent, and that the actual number of patients may be underestimated. The clinical picture and outcome of the disease could vary significantly from severe neonatal disease with fulminant hypoelectrolytemia and dehydration, to a relatively mild chronic form which remains undiagnosed for long time. Finally, early diagnosis is essential because hyponatremic episodes in infancy may result in mental and psychomotor impairment, and the chronic contraction of the intravascular space could induce renal impairment and gout. Many therapeutic strategies to reduce diarrhea, including cholestyramine and omeprazole, have proven to be ineffective (1,3). Therapy consists of oral supplements of sodium and potassium chloride. Ideally, the intestinal ion concentrations should be only slightly higher than in plasma, enabling net absorption of both electrolytes and water.

Defects in the enterohepatic circulation of bile acids

ILE ACIDS serve at least two physiological roles, both of which are dependent on their enterohepatic circulation. Bile acid flux is the major driving force of bile production and they are essential for the normal absorption of fats from the gastrointestinal tract. Bile is a mixture of waste products, notably bilirubin and other conjugates, along with bile acids, lipids and inorganic ions, most of which are reabsorbed. Both functions now seem equally important. Whilst bile acids are an essential component of bile they are themselves toxic. The main function of other constituents, notably phospholipids and cholesterol, is to protect against the harmful effects of bile acids, through the formation of mixed micelles. The complete cycle of enterohepatic circulation of bile acids involves transport across two epithelia, as well as passage through the gut and portal blood. Only about 2% of the bile acids excreted daily into the GI tract are lost in the stool. A single transporter appears be responsible for this conservation through uptake in the distal ileum. The apical sodium-dependent bile salt transporter (ASBT) (1,2) is a transmembrane protein similar to the sodiumdependent bile acid transporter found in hepatocytes (NTCP) (3), which is discussed below. The gene is located on chromosome 13q33 and has the symbol SLClOA2 (4). The protein, when expressed in vitro has Ki values in the range of 3.3 PM (chenodeoxycholate) to 41.5 PM (cholate) for bile acids but very high Ki values for a number of other organic anions (5). This contrasts with NTCP, which appears to have a much broader specificity. Recessive, loss of function, mutations in the ASBT gene have been found in patients with primary bile acid malabsorption (4). Bile acids are removed from the portal circulation by hepatocytes. The major bile acid transporter in the basolateral membrane of hepatocytes is the sodium taurocholate co-transporting polypeptide (NTCP) (3). The organic anion transporting polypeptides are capable of transporting bile acids, along with many amphipathic compounds. Whether OATP plays a physiologically significant role in bile acid transport in not clear. Bile acid export from hepatocyes operates against a steep concentration gradient. The biliary bile acid concentration is at least lOOO-fold higher than that in hepatocytes. It is not surprising therefore that the transport of bile acids across this membrane has been shown to be an energy-dependent process. The major transporter is called the bile salt export pump and is encoded by ABCBll (previously called BSEP or SPGP). When the rat cDNA was expressed in vitro, the protein proved to be an effective ATP-dependent bile acid transporter with a Km for taurocholate of -5 ,uM (6). The protein has therefore been named the bile salt export pump. Mutations in the human gene have been found in patients with progressive familial intrahepatic cholestasis with normal levels of serum yGT (7). In those where it has been measured, they also have low levels of biliary bile acids; this correlates well with the low yGT. A similar phenotype has been found in patients with mutations in a P-type ATPase, also located in the hepatocyte canalicular membrane (8). The protein may well be acting as an aminophospholipid flipase. However, it is not clear how mutations in this gene lead to an interruption in the efflux of bile acids from hepatocytes. Most of the major bile acid transporter genes have now been cloned. The intracellular transport of bile acids is less well understood. Recessive mutations have so far been found in three transporter genes, though

New insights into bile acid transport.

The enterohepatic circulation of bile acids is maintained by a series of membrane transport proteins. Recent studies of the cloned sodium bile acid cotransporters have provided new insights into their tissue expression, regulation, and their relationship to cholesterol homeostasis and human diseases such as primary bile acid malabsorption.

Primary bile acid malabsorption: a pathophysiologic and clinical entity?

Primary bile acid malabsorption is defined as chronic diarrhoea with bile acid malabsorption of unknown cause and a symptomatic response to cholestyramine. Convincing evidence of the proposed pathophysiology--a defect of the active bile acid absorption in the distal ileum--has never been substantiated. We found no evidence of a bile acid transport defect across the ileal brush border membrane in 10 patients with primary bile acid malabsorption; moreover, transport was significantly higher than in a control group. In the patients with primary bile acid malabsorption the estimated bile acid pool was significantly larger than in a control group and in a group of patients with ileal disease. In addition, the oro-anal transit time of radiopaque markers was shorter in the primary bile acid malabsorption group than in both other groups. This suggests that the bile acid pool size as well as intestinal motility could play a role in the pathophysiology of primary bile acid malabsorption.

Primary bile acid malabsorption: Histologic and immunologic study in three patients

Three patients are presented with a history of chronic watery diarrhea due to bile acid malabsorption, proved by the tauro-23[75Se]selena-25 homocholic acid test and an unequivocal response to cholestyramine therapy. Fecal fat tests, Schilling tests, and barium studies of the small intestine and colon were all normal. Jejunal biopsies were normal but multiple biopsies of the terminal ileum, performed by retrograde ileoscopy, showed uniform crypt hyperplastic villous atrophy and features of colonic metaplasia with increased mononuclear infiltration of the lamina propria. All 3 patients demonstrated evidence of abnormal immune function, including the presence of serum autoantibodies, circulating immune complexes, and hypocomplementemia. One patient had Sjögren's syndrome. This disorder, which might be immunologically mediated, should be called primary bile acid malabsorption and should be distinguished from other ileal disorders.

Well‐Compensated Primary Bile Acid Malabsorption Presenting as Chronic Nonspecific Diarrhea

SummaryIncreased fecal bile acid loss and defective in vitro ileal bile acid uptake were demonstrated in an 8‐year‐old boy with diarrhea starting in the neonatal period. His continuously normal physical development and good nutritional status are in keeping with a well‐preserved cholic acid pool and normal duodenal bile acid concentration. Isolated bile acid malabsorption can remain well compensated and present as the chronic nonspecific diarrhea syndrome of childhood.

Primary Bile Acid Malabsorption: Defective In Vitro Ileal Active Bile Acid Transport

Two boys with congenital diarrhea, steatorrhea, and growth failure were studied. Preliminary investigations indicated that the enterohepatic circulation of bile acids was interrupted. Radiographically, ileal structure was normal; ileal function was normal when assessed by vitamin B12 absorption. To confirm our clinical suspicion that the patients had an isolated defect of ileal active bile acid transport, peroral terminal ileal biopsies were performed. Ileal mucosa was incubated in vitro in oxygenated Krebs-Ringer bicarbonate buffer containing 10 mM glucose and 0.1, 1.0, or 10.0 mM taurocholic acid at 37 degrees C. Ileal uptake from the patients was 0.10 and 0.34 mumol/g dry wt . min in 0.1 mM taurocholic acid, 1.20 and 2.39 mumol/g dry wt . min in 1.0 mM taurocholic acid, and 21.19 and 11.14 mumol/g dry wt . min in 10.0 mM taurocholic acid. At every concentration, significant (p less than 0.05) reductions were present compared with ileum from 7 ileostomy controls, 0.5 to 27 yr old whose uptake was 1.40 +/- 0.28 mumol/g dry wt . min (mean +/- SEM) at 0.1 mM; 6.36 +/- 1.33 mumol/g dry wt . min at 1.0 mM, and 76.20 +/- 19.30 mumol/g dry wt . min at 10.0 mM taurocholic acid. Ultrastructural examination of the ileal mucosa failed to demonstrate a significant structural abnormality. Significant reduction in ileal uptake of taurocholic acid accompanying clinical and biochemical findings of interruption of the enterohepatic circulation in the absence of mucosal disease suggests that these children have a previously undescribed, congenital transport defect that includes absence of active ileal bile acid transport presenting as diarrhea in infancy.

Serum bile acid response to a test meal stimulus: A sensitive test of ileal function

We assessed ileal functional integrity, as reflected in the capacity to reabsorb the bile acid, cholylglycine, by sequential measurement of postprandial serum levels of this compound. A consistent increase in cholylglycine concentration (mean +/- SEM, peak above fasting baseline = 1.86 +/- 0.14 microM) occurred in eight normal children. Five patients with ileal resection and two with primary bile acid malabsorption had a minimal postprandial increase (0.27 +/- 0.08 microM; P less than 0.005 vs controls) and excess bile acid loss in feces. Similarly, in seven patients with Crohn disease involving the ileum the postprandial increase (0.38 +/- 0.06) was less than controls (P less than 0.001). In eight patients with cystic fibrosis, the administration of pancreatic enzymes normalized the blunted response demonstrated when enzymatic therapy was interrupted suggesting an intact ileal active transport mechanism. Meal-stimulated response of serum cholylglycine concentration is a sensitive indicator of altered ileal integrity and bile acid malabsorption due to ileal resection, inflammation, or dysfunction.

Refractory infantile diarrhea due to primary bile acid malabsorption

A boy is described who had severe, refractory diarrhea beginning soon after birth. Prolonged parenteral alimentation was required to support life. Investigations of bile acid metabolism showed impaired intestinal absorption of bile acids. Contracted bile acid pool sizes resulted, with low intraluminal bile acid concentrations and severe, malabsorption of water and fat. Bile acid malabsorption is a cause of refractory infantile diarrhea.