Contribution of variant alleles of ABCB11 to susceptibility to intrahepatic cholestasis of pregnancy

BACKGROUND

Intrahepatic cholestasis of pregnancy (ICP) has a complex aetiology with a significant genetic component. ABCB11 encodes the bile salt export pump (BSEP); mutations cause a spectrum of cholestatic disease, and are implicated in the aetiology of ICP.

METHODS

ABCB11 variation in ICP was investigated by screening for five mutant alleles (E297G, D482G, N591S, D676Y and G855R) and the V444A polymorphism (c.1331T>C, rs2287622) in two ICP cohorts (n = 333 UK, n = 158 continental Europe), and controls (n = 261) for V444A. PCR primers were used to amplify and sequence patient and control DNA. The molecular basis for the observed phenotypes was investigated in silico by analysing the equivalent residues in the structure of the homologous bacterial transporter Sav1866.

RESULTS

E297G was observed four times and D482G once. N591S was present in two patients; D676Y and G855R were not observed. The V444A polymorphism was associated with ICP (allelic analysis for C vs T: OR 1.7 (95% CI 1.4 to 2.1, p<0.001)). In addition, CC homozygotes were more likely to have ICP than TT homozygotes: OR 2.8 (95% CI 1.7 to 4.4 p<0.0001). Structural analyses suggest that E297G and D482G destabilize the protein fold of BSEP. The molecular basis of V444A and N591S was not apparent from the Sav1866 structure.

CONCLUSIONS

Heterozygosity for the common ABCB11 mutations accounts for 1% of European ICP cases; these two mutants probably reduce the folding efficiency of BSEP. N591S is a recurrent mutation; however, the mechanism may be independent of protein stability or function. The V444A polymorphism is a significant risk factor for ICP in this population.

Genetic study of the CD36 gene in a French diabetic population

OBJECTIVES

CD36 is a multifunctional membrane receptor widely expressed in different tissues which binds and internalizes oxidized low-density lipoprotein. In rodents, CD36 gene variations modulate glucose homeostasis and contribute to metabolic syndrome associated with type 2 diabetes but the effects in human are unknown.

METHODS

We screened the entire coding sequence of the CD36 gene in 272 individuals and we genotyped both rare and frequent variants in 454 T2D subjects and 221 controls.

RESULTS

We detected five mutations, P191P and N247S were only found each in one family and did not segregate with diabetes, the three others (A/C-178 in the promoter, A/G-10 in intron 3 and (GGGTTGAGA) insertion in intron 13) being equally frequent in diabetic subjects and in controls. However, adiponectin levels, a marker for insulin sensitivity, were significantly associated with the -178 A/C promoter variant allele (p=0.003, p corrected for multiple testing=0.036), possibly reflecting association with insulin-resistance in the French population.

CONCLUSION

Thus, the -178 A/C SNP promoter mutation in the CD36 gene represents a putative genetic marker for insulin-resistance in the French population, although it does not appear to contribute to the genetic risk for T2D.

Heterozygous MDR3 missense mutation associated with intrahepatic cholestasis of pregnancy: evidence for a defect in protein trafficking

Intrahepatic cholestasis of pregnancy (ICP) is a liver disease of pregnancy with serious consequences for the mother and fetus. Two pedigrees have been reported with ICP in the mothers of children with a subtype of autosomal recessive progressive familial intrahepatic cholestasis (PFIC) with raised serum gamma-glutamyl transpeptidase (gamma-GT). Affected children have homozygous mutations in the MDR3 gene (also called ABCB4 ), and heterozygous mothers have ICP. More frequently, however, ICP occurs in women with no known family history of PFIC and the genetic basis of this disorder is unknown. We investigated eight women with ICP and raised serum gamma-GT, but with no known family history of PFIC. DNA sequence analysis revealed a C to A transversion in codon 546 in exon 14 of MDR3 in one patient, which results in the missense substitution of the wild-type alanine with an aspartic acid. We performed functional studies of this mutation introduced into MDR1, a closely related homologue of MDR3. Fluorescence activated cell sorting (FACS) and western analysis indicated that this missense mutation causes disruption of protein trafficking with a subsequent lack of functional protein at the cell surface. The demonstration of a heterozygous missense mutation in the MDR3 gene in a patient with ICP with no known family history of PFIC, analysed by functional studies, is a novel finding. This shows that MDR3 mutations are responsible for the additional phenotype of ICP in a subgroup of women with raised gamma-GT.

Cysteine-scanning mutagenesis provides no evidence for the extracellular accessibility of the nucleotide-binding domains of the multidrug resistance transporter P-glycoprotein

Multidrug resistance of cancer cells is, at least in part, conferred by overexpression of P-glycoprotein (P-gp), a member of the ATP-binding cassette (ABC) superfamily of active transporters. P-gp actively extrudes chemotherapeutic drugs from cells, thus reducing their efficacy. As a typical ABC transporter, P-gp has four domains: two transmembrane domains, which form a pathway through the membrane through which substrates are transported, and two hydrophilic nucleotide-binding domains (NBDs), located on the cytoplasmic side of the membrane, which couple the energy of ATP hydrolysis to substrate translocation. It has been proposed that the NBDs of ABC transporters, including the histidine permease of Salmonella typhimurium and the cystic fibrosis transmembrane conductance regulator, are accessible from the extracellular surface of the cell, spanning the membrane directly or potentially contributing to the transmembrane pore. Such organization would have significant implications for the transport mechanism. We determined to establish whether the NBDs of P-gp are exposed extracellularly and which amino acids are accessible, using cysteine-scanning mutagenesis and limited proteolysis. In contrast to other transporters, the data provided no evidence that the P-gp NBDs are exposed to the cell surface. The implications for the structure and mechanism of P-gp and other ABC transporters are discussed.

The Escherichia coli ATP-binding cassette (ABC) proteins

The recent completion of the Escherichia coli genome sequence (Blattner et al., 1997) has permitted an analysis of the complement of genomically encoded ATP-binding cassette (ABC) proteins. A total of 79 ABC proteins makes this the largest paralogous family of proteins in E. coli. These 79 proteins include 97 ABC domains (as some proteins include more than one ABC domain) and are components of 69 independent functional systems (as many systems involve more than one ABC domain). The ABC domains are often, but not exclusively, the energy-generating domains of multicomponent membrane-bound transporters. Thus, 57 of the 69 systems are ABC transporters, of which 44 are periplasmic-binding protein-dependent uptake systems and 13 are presumed exporters. The genes encoding these ABC transporters occupy almost 5% of the genome. Of the 12 systems that are not obviously transport related, the function of only one, the excision repair protein UvrA, is known. A phylogenetic analysis suggests that the majority of ABC proteins can be assigned to 10 subfamilies. Together with statistical and, importantly, biological evidence, this analysis provides insight into the evolution and function of the ABC proteins.

Structure of the multidrug resistance P-glycoprotein

In order to elucidate the mechanism by which the multidrug resistance P-glycoprotein extrudes cytotoxic drugs from the cell, and particularly the number and nature of the drug binding site(s), knowledge of the structure of P-gp is essential. A considerable body of genetic and biochemical data has accrued which gives insights into P-gp structure and function. These data are critically reviewed, particularly in relation to the low resolution structure of P-gp which has recently been determined by electron microscopy. P-gp is one of the best characterised of the ABC transporters and these structure-function studies may have more general implications.

Cloning of the genes encoding thymidine diphosphoglucose 4,6-dehydratase and thymidine diphospho-4-keto-6-deoxyglucose 3,5-epimerase from the erythromycin-producing Saccharopolyspora erythraea

Genes involved in deoxysugar metabolism, encoding thymidine diphospho (TDP)-glucose 4,6-dehydratase (gdh) and a putative TDP-4-keto-6-deoxyglucose 3,5-epimerase (kde), were cloned from the erythromycin (Er)-producing Saccharopolyspora erythraea by means of an oligodeoxynucleotide corresponding to a segment of the purified Gdh protein. Determination of the nucleotide sequence established that kde lies 3' to gdh. The function of gdh was confirmed by an enzymatic assay following expression of the gene in Escherichia coli. Southern analysis indicated that Sa. erythraea contains only one copy of gdh and kde. It was not possible to establish whether these genes are required for Er biosynthesis, but they appear to be essential for cellular metabolism, since resolution of a partial diploid containing a wt and a disrupted copy of gdh always maintained the wt gene. These loci do not lie within or near the known boundaries of the cluster of Er-production and -resistance genes, nor do they appear to be flanked by other deoxysugar biosynthesis genes.

An ABC-transporter from Streptomyces longisporoflavus confers resistance to the polyether-ionophore antibiotic tetronasin

Streptomyces longisporoflavus produces the polyketide-polyether antibiotic, tetronasin, which acts as an ionophore and depolarizes the membrane of bacteria sensitive to the drug. A genomic library of S. longisporoflavus DNA was cloned in Streptomyces lividans and screened to identify tetronasin-resistance determinants. The inclusion of 0.2M NaCl in the growth medium with tetronasin markedly improved the sensitivity of the screen. Two different resistance determinants, designated tnrB (ptetR51) and tnrA (ptetR11) respectively, were identified. The determinant tnrB (ptetR51) but not tnrA (ptetR11), also conferred resistance to tetronasin when cloned into Streptomyces albus. The tnrB determinant was further localized, by subcloning, to a 2.8 kb KpnI fragment. DNA sequence analysis of this insert revealed one incomplete and two complete open reading frames (ORFs 1, 2 and 3). The deduced sequence of the gene product of ORF2 (TnrB2) revealed significant similarity to the ATP-binding domains of the ABC (ATP binding cassette) superfamily of transport-related proteins. The adjacent gene, ORF3, is translationally coupled to ORF2 and would encode a hydrophobic protein (TnrB3) with six transmembrane helices which probably constitutes the integral membrane component of the transporter. The mechanism of tetronasin resistance mediated by tnrB is probably an ATP-dependent efflux system.