Bile acids, which are synthesized from cholesterol in the liver and secreted with bile into the small intestine, play an important role in efficient digestion and absorption of dietary fats and fat-soluble vitamins. Most bile acids are reabsorbed in the distal part of the ileum and cycle back to the liver via the portal venous system. Cholestasis, or impairment of bile flow, leads to malabsorption of fats and vitamins and the toxic bile acids cause chronic liver failure. Hereditary cholestasis can be caused by deficiencies in primary bile acid synthesis, or in the secretion and absorption of bile acids in the enterohepatic circulation.
One of the familial cholestasis syndromes is FIC1 disease, which is associated with mutations in ATP8B1. FIC1 disease patients can present with episodic bouts of cholestasis with no permanent liver damage, also called Benign Recurrent Intrahepatic Cholestasis (BRIC) or with non-remitting cholestasis which progresses to severe permanent liver damage (progressive familial Intrahepatic Cholestasis type 1 (PFIC1). The latter group of patients die in the first decade of life unless liver transplantation is performed. BRIC and PFIC1 patients present with jaundice, pruritus, and diarrhea and are both characterized by normal serum g-glutamyltransferase activity and high concentrations of bile salts in serum.
The ATP8B1 gene product encodes FIC1, which is a member of the P4 P-type ATPase subfamily. A schematic representation of the protein positioned in the plasma membrane is shown in figure 1. This subfamily is well conserved during evolution and is present in all eukaryotes. It consists of at least 14 transporters that are thought to flip aminophospholipids from the outer to the inner plasma membrane leaflet, but this function is questioned. How this molecular function relates to the phenotypes of the FIC1 disease patients remains elusive.
Further studies to elucidate FIC1's function will be essential to unravel the pathogenesis of FIC1 disease. Such studies will also have a general impact on our understanding of the molecular mechanisms of bile formation and may therefore improve clinical management of both hereditary and acquired forms of cholestasis.
We have investigated the localization of FIC1 in the liver, because in FIC1 disease patients the cholestasis is the most prominent symptom.
We performed double labeling experiments on murine liver sections using antibodies to cytokeratin 7 as a marker for cholangiocytes and C219 as a marker for the canalicular membrane of hepatocytes. A colocalization experiment of FIC1 (a, green fluorescence) and cytokeratin 7 (b, red fluorescence) in murine liver is shown. A computer-based superimposition by image processing yielded an overlay (Fig. 2c, yellow) of the cytokeratin 7 and FIC1-specific signals at the murine cholangiocytes of a bile ductule, bd, and a common bile duct membrane, cbd (Fig. 2c, insert). FIC1 localized predominantly to the apical membrane of the cholangiocytes. A separate colocalization experiment showed that FIC1 staining is perfectly overlapping with C219-specific immunoreactivity in murine hepatocytes (Fig. 2 d-f). Liver sections of a PFIC1 patient were devoid of FIC1 staining.
Its localization in the canalicular membrane and cholangiocytes taken together with the clinical and biochemical characteristics of PFIC1 and BRIC patients suggests that FIC1 may play an essential role in bile formation. However, direct evidence concerning the biochemical function of FIC1 is lacking at this moment.
The project group consists of the following people:
Saskia van Mil – Novel insights in familial intrahepatic cholestasis syndromes. 19 May 2004.
Janneke Stapelbroek - ATP8B1 deficiency. Pathophysiology and treatment of a cholestatic syndrome with extrahepatic features. 13 October 2009.
Lieke van der Velden - Cellular consequences of ATP8B1 deficiency. 28 juni 2010
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here to find additional information on ATP8B1 genomic organization and primers used for screening and sequencing ATP8B1 mutations
Klomp LW, Vargas JC, van Mil SW, Pawlikowska L, Strautnieks SS, van Eijk MJ, Juijn JA, Pabon-Pena C, Smith LB, DeYoung JA, Byrne JA, Gombert J, van der Brugge G, Berger R, Jankowska I, Pawlowska J, Villa E, Knisely AS, Thompson RJ, Freimer NB, Houwen RH, Bull LN. Characterization of mutations in ATP8B1 associated with hereditary cholestasis. Hepatology. 2004 Jul;40(1):27-38.
last update: 9 July 2010