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Molecular mechanistic explanation for the spectrum of cholestatic disease caused by the S320F variant of ABCB4
Author(s) -
Andress Edward J.,
Nicolaou Michael,
Romero Marta R.,
Naik Sandhia,
Dixon Peter H.,
Williamson Catherine,
Linton Kenneth J.
Publication year - 2014
Publication title -
hepatology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 5.488
H-Index - 361
eISSN - 1527-3350
pISSN - 0270-9139
DOI - 10.1002/hep.26970
Subject(s) - computational biology , disease , chemistry , medicine , biology , pathology
ABCB4 flops phosphatidylcholine into the bile canaliculus to protect the biliary tree from the detergent activity of bile salts. Homozygous‐null ABCB4 mutations cause the childhood liver disease, progressive familial intrahepatic cholestasis, but cause and effect is less clear, with many missense mutations linked to less severe cholestatic diseases. ABCB4 S320F , in particular, is described in 13 patients, including in heterozygosity with ABCB4 A286V , ABCB4 A953D , and null mutants, whose symptoms cover the spectrum of cholestatic disease. We sought to define the impact of these mutations on the floppase, explain the link with multiple conditions at the molecular level, and investigate the potential for reversal. ABCB4 S320F , ABCB4 A286V , and ABCB4 A953D expression was engineered in naïve cultured cells. Floppase expression, localization, and activity were measured by western blot, confocal microscopy, and lipid transport assays, respectively. ABCB4 S320F was fully active for floppase activity but expression at the plasma membrane was reduced to 50%. ABCB4 A286V expressed and trafficked efficiently but could not flop lipid, and ABCB4 A953D expressed poorly and was impaired in floppase activity. Proteasome inhibition stabilized nascent ABCB4 S320F and ABCB4 A953D but did not improve plasma membrane localization. Cyclosporin‐A improved plasma membrane localization of both ABCB4 S320F and ABCB4 A953D , but inhibited floppase activity. Conclusion : The level of ABCB4 functionality correlates with, and is the primary determinant of, cholestatic disease severity in these patients. ABCB4 S320F homozygosity, with half the normal level of ABCB4, is the tipping point between more benign and potentially fatal cholestasis and makes these patients more acutely sensitive to environmental effects. Cyclosporin‐A increased expression of ABCB4 S320F and ABCB4 A953D , suggesting that chemical chaperones could be exploited for therapeutic benefit to usher in a new era of personalized medicine for patients with ABCB4‐dependent cholestatic disease. (H epatology 2014;59:1921–1931)