Two loci on chromosome 9 control bile acid composition: evidence that a strong candidate gene, Cyp8b1, is not the culprit

Abstract

An intercross between C57BL/6J and CASA/Rk mice was used to study the genetics of biliary bile acid composition. In parental strains, male C57BL/6J mice had significantly higher cholic acid (CA; 14%) and lower b-muricholic acid (bMC; 27%) than CASA/Rk mice, whereas females did not differ. However, quantitative trait locus analysis of F2 mice revealed no significant chromosome 9 loci in males but loci in females on chromosome 9 for percentage CA (%CA) at 72 centimorgan (cM) [logarithm of the odds (LOD) 5.89] and %bMC at 54 cM (LOD 4.09). Chromosome 9 congenic and subcongenic strains representing CASA/Rk intervals 38-73 cM (9KK) and 68-73 cM (9DKK) on the C57BL/6J background were made. In 9KK and 9DKK males, %CA was increased and %bMC was unchanged, whereas in 9KK but not 9DKK females, %CA was increased and %bMC was decreased. Sterol 12a-hydroxylase (Cyp8b1) channels bile acid precursors into CA and maps at chromosome 9 (73 cM). However, there was no significant difference in Cyp8b1 mRNA or enzymatic activity between parental mice, parental-congenic-subcongenic mice, or highlow biliary %CA F2 mice. In summary, two chromosome 9 loci control sexually dimorphic effects on biliary bile acid composition: a distal (68-73 cM) major determinant in males, and a more proximal (38-68 cM) major determinant in females. In this intercross, Cyp8b1, a strong candidate, does not appear to be responsible. In humans and mice, biliary bile acid composition affects the enterohepatic metabolism of lipids, including the synthesis and transport of bile acids across hepatocytes, the excretion of biliary lipids, and the absorption of sterols from the intestines (1-6). In humans, the two major primary bile acids are chenodeoxycholic acid (CDCA), a dihydroxy bile acid, and cholic acid (CA), a trihydroxy bile acid. The difference between them is hydroxylation of the 12 carbon of the sterol ring, which is carried out by the enzyme sterol 12a-hydroxylase (CYP8B1). As a result, CDCA is more hydrophobic and CA is more hydrophilic. In mice, CA is one of the major primary bile acids; it is formed by CYP8B1-mediated 12 hydroxylation. Muricholic acids comprise the other major murine bile acids, and they are formed from CDCA by 6 hydroxylation and 7 epimerization to generate a-and b-muricholic acid (bMC), with bMC predominating. bMC is even more hydrophilic than CA. Although the mouse does not have CDCA as a major species, it is possible to use this model to identify genes that influence the 12 hydroxylation pathway and uptake of bile acids from the intestines by studying the proportions of CA and bMC in bile. Identifying the genes that influence 12 hydroxylation and bile acid uptake from the intestines is particularly important because they determine the proportions of CA and CDCA and hence the hydrophobicity of the bile acid pool in humans.In humans, the proportions of CA and CDCA vary between individuals (7, 8), and there is some evidence for genetic control. For example, studies in monozygotic and dizygotic twins found a significant pair-...

Single nucleotide polymorphism rs3732860 in the 3′‐untranslated region of CYP8B1 gene is associated with gallstone disease in Han Chinese

“…The Cyp8b1 gene maps to chromosome 9 and is a strong candidate for disease susceptibility in this region. 20 In our study, D3S1266 showed the largest LOD score for the late-onset GD families. D3S1266, together with the flanking chromosomal region, approximately represents the chromosomal location 3p22-21.3, and CYP8B1 is located inside this region, making it a promising candidate for GD susceptibility.…”

“…reported that two chromosome 9 loci are responsible for the biliary bile acid composition in inbred mouse strains. The Cyp8b1 gene maps to chromosome 9 and is a strong candidate for disease susceptibility in this region . In our study, D3S1266 showed the largest LOD score for the late‐onset GD families.…”

Genetic determinants of gut microbiota composition and bile acid profiles in mice

“…We investigated the impact of genetic variation on gut microbiota composition and bile acid (BA) profiles using a cohort of ~400 DO mice maintained on a high-fat high-sucrose diet (45% kcal from fat and 34% from sucrose) for ~22 weeks (range 21–25 weeks), starting at weaning. We previously showed that this diet elicits a wide range of metabolic responses in the eight founder strains that are associated with microbiome changes [18,46]. Furthermore, we incorporated in our analyses previously published clinical weight traits collected from the same DO mice [47].…”

Genetic determinants of gut microbiota composition and bile acid profiles in mice

“…These traits were particularly interesting because both have been shown to be influenced by host genetics by previous studies. Turicibacter has been identified as highly heritable in both mouse and human genetic studies (24,27,45,49), whereas multiple reports have found differences in CA levels as a function of host genotype (18,61). Furthermore, CA levels are influenced by both host genetics and microbial metabolism since it is synthesized by host liver enzymes from cholesterol and subsequently modified by gut microbes in the intestine.…”