(from OMIM) Familial hypercholanemia (60748) is characterized by elevated serum bile acid concentrations, itching, and fat malabsorption. Carlton et al. (2003) studied this disorder in 17 individuals in 12 families of Lancaster County Old Order Amish descent. In 6 families, affected individuals were homozygous for a missense mutation in the gene encoding tight junction protein-2 (TJP2)(not an a/b hydrolase). In 2 families, affected individuals were homozygous for an M76V mutation in BAAT. In 2 families, the affected individuals were homozygous for the TJP2 missense mutation and heterozygous for the BAAT mutation, and in 1 family affected individuals were homozygous for the BAAT mutation and heterozygous for the TJP2 mutation. Carlton et al. (2003) postulated oligogenic inheritance of familial hypercholanemia. In a patient with hypercholanemia, Zhu et al. (2003) identified compound heterozygosity for 2 mutations in the EPHX1 gene, which resulted in a significant decrease in EPHX1 promoter activity.
Familial hypercholanemia (FHC) is characterized by elevated serum bile acid concentrations, itching, and fat malabsorption. We show here that FHC in Amish individuals is associated with mutations in tight junction protein 2 (encoded by TJP2, also known as ZO-2) and bile acid Coenzyme A: amino acid N-acyltransferase (encoded by BAAT). The mutation of TJP2, which occurs in the first PDZ domain, reduces domain stability and ligand binding in vitro. We noted a morphological change in hepatic tight junctions. The mutation of BAAT, a bile acid-conjugating enzyme, abrogates enzyme activity; serum of individuals homozygous with respect to this mutation contains only unconjugated bile acids. Mutations in both TJP2 and BAAT may disrupt bile acid transport and circulation. Inheritance seems to be oligogenic, with genotype at BAAT modifying penetrance in individuals homozygous with respect to the mutation in TJP2.
Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in carcinogen metabolism and is also able to mediate the sodium-dependent uptake of bile acids into hepatocytes. Studies have identified a subject (S-1) with extremely elevated serum bile salt levels in the absence of observable hepatocellular injury, suggesting a defect in bile acid uptake. In this individual, mEH protein and mEH mRNA levels were reduced by approximately 95% and 85%, respectively, whereas the expression and amino acid sequence of another bile acid transport protein (NTCP) was unaffected. Sequence analysis of the mEH gene (EPHX1) revealed a point mutation at an upstream HNF-3 site (allele I) and in intron 1 (allele II), which resulted in a significant decrease in EPHX1 promoter activity in transient transfection assays. Gel shift assays using a radiolabeled oligonucleotide from each region resulted in specific transcription factor binding patterns, which were altered in the presence of the mutation. These studies demonstrate that the expression of mEH is greatly reduced in a patient with hypercholanemia, suggesting that mEH participates in sodium-dependent bile acid uptake in human liver where its absence may contribute to the etiology of this disease.
We have isolated 48 yeast artificial chromosome (YAC) clones from a 4 cM/27 cR region of human chromosome 9q22.3 encompassed by the markers cen-D9S196-D9S173-tel. Within this region, we have assembled a 4.3-Mb YAC contig across the interval cen-FACC-D9S173-tel containing 42 clones. As a first step toward completing the detailed transcription map of the region, we have mapped 9 gene sequences and 10 expressed sequence tags. Fifteen polymorphic microsatellite repeat markers and 17 novel sequence-tagged sites from the region are also described. The mapping of polymorphic simple tandem repeat markers has permitted the integration of existing genetic and physical maps of the region. Together these maps provide a valuable resource for fine structure mapping and DNA sequencing across the region as well as for the identification of disease gene loci and the isolation of novel coding sequences.
Title: Glycine and taurine conjugation of bile acids by a single enzyme. Molecular cloning and expression of human liver bile acid CoA:amino acid N-acyltransferase. Falany CN, Johnson MR, Barnes S, Diasio RB Ref: Journal of Biological Chemistry, 269:19375, 1994 : PubMed
Title: Purification and characterization of bile acid-CoA:amino acid N-acyltransferase from human liver Johnson MR, Barnes S, Kwakye JB, Diasio RB Ref: Journal of Biological Chemistry, 266:10227, 1991 : PubMed
The bile acid-conjugating enzyme, bile acid-CoA: amino acid N-acyltransferase, was purified 480-fold from the soluble fraction of homogenized frozen human liver. Purification was accomplished by a combination of anion exchange chromatography, chromatofocusing, glycocholate-AH-Sepharose affinity chromatography, and high performance liquid chromatography (HPLC) gel filtration. Following purification, the reduced, denatured enzyme migrated as a single 50-kDa protein band by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A similar molecular mass was obtained for the native enzyme by HPLC gel filtration. Elution from the chromatofocusing column suggested an apparent isoelectric point of 6.0 (+/- 0.2). Using a rabbit polyclonal antibody raised against the purified enzyme, Western blot analysis using 100,000 x g human liver supernatant confirmed that the affinity-purified polyclonal antibody was specific for human liver bile acid-CoA:amino acid N-acyltransferase. The purified enzyme utilized glycine, taurine, and 2-fluoro-beta-alanine (a 5-fluorouracil catabolite), but not beta-alanine, as substrates. Kinetic studies revealed apparent Km values for taurine, 2-fluoro-beta-alanine, and glycine of 1.1, 2.2, and 5.8 mM, respectively, with corresponding Vmax values of 0.33, 0.19, and 0.77 mumol/min/mg protein. These data demonstrate that a single monomeric enzyme is responsible for the conjugation of bile acids with glycine or taurine in human liver.
