The roles of MRP2, MRP3, OATP1B1, and OATP1B3 in conjugated hyperbilirubinemia.

Increased concentrations of bilirubin glucuronides in blood plasma indicate hepatocellular dysfunction. Elucidation of the transport processes of bilirubin conjugates across the basolateral (sinusoidal) and the canalicular plasma membrane domains of hepatocytes has decisively contributed to our current understanding of the molecular basis of conjugated hyperbilirubinemia in human liver diseases. Under normal conditions, unconjugated bilirubin is taken up into hepatocytes by transporters of the organic anion-transporting polypeptide (OATP) family, followed by conjugation with glucuronic acid, and ATP-dependent transport into bile. This efflux across the canalicular membrane is mediated by multidrug resistance protein 2 (MRP2 or ABCC2), which is a 190-kDa glycoprotein transporting with high affinity and efficiency monoglucuronosyl bilirubin and bisglucuronosyl bilirubin into bile. MRP2 is hereditarily deficient in human Dubin-Johnson syndrome. Under pathophysiological conditions such as cholestatic liver injury and MRP2 inhibition, the basolateral efflux pump multidrug resistance protein 3 (MRP3 or ABCC3) is responsible for the occurrence of conjugated hyperbilirubinemia. MRP3 is a glycoprotein with a similar molecular mass as MRP2, with 48% amino acid identity, and with overlapping substrate specificity. Human MRP3 is the only basolateral efflux pump shown to transport bilirubin glucuronides. In human and rat hepatocytes, MRP3/Mrp3 is strongly upregulated under conditions of cholestasis and MRP2 deficiency. This is in line with the concept that basolateral efflux pumps of the hepatocyte compensate for impaired canalicular efflux of compounds into bile and contribute to balance the rate of uptake or synthesis of compounds in hepatocytes with the capacity for efflux into bile.

Mutational analysis of the MRP2 gene and long-term follow-up of Dubin-Johnson syndrome in Japan.

BACKGROUND: Recent studies have indicated that dysfunction or loss of the multidrug resistance protein 2 (MRP2) is the molecular basis of Dubin-Johnson syndrome (DJS). To clarify the genetic basis of the disease and the long-term stability of serum bilirubin levels, we conducted a mutational analysis of the MRP2 gene and followed up serum bilirubin levels in Japanese DJS patients 30 years after they were originally diagnosed, based on traditional criteria. METHODS: Patients were interviewed by telephone, and blood tests, including a genetic analysis of MRP2, were performed on the patients and family members who gave informed consent. RESULTS: Over the 30 years, hyperbilirubinemia remained unchanged in four of the five patients studied, while it worsened in 1 patient whose DJS was complicated by chronic hepatitis C. From an MRP2 gene mutational analysis, six mutations, including the novel mutation 1177C>T, were found. Three patients of a consanguineous family were homozygotes for three mutations (298C>T, 1967+2T>C, and 2439+2T>C). Two patients were compound heterozygotes (1177C>T/2302C>T and 1967+2T>C/2026G>C). A familial study showed no difference in serum bilirubin levels between mutant/wild heterozygotes and wild/wild homozygotes. CONCLUSIONS: The hyperbilirubinemia of four Japanese patients with DJS, one of whom had a novel mutation, 1177C>T, of the MRP2 gene, had not worsened with aging.

Improvement of serum bilirubin levels after venesection in a patient with Dubin-Johnson syndrome and HCV-positive chronic liver disease.

Direct-type hyperbilirubinemia in Dubin-Johnson syndrome is due to the genetic dysfunction of multidrug resistance protein 2. However, serum bilirubin levels may fluctuate as a result of acquired conditions. Iron-reduction therapy by venesection, an alternative to interferon, was performed in a 55-year-old male patient with Dubin-Johnson syndrome complicated by hepatitis C virus-positive chronic liver disease and hepatic iron overload. His pretreatment serum total bilirubin was 10.2 mg/dl, with a dominant direct fraction. The treatment induced a significant reduction in serum total bilirubin, although it remained as high as 7.9 mg/dl. A negative correlation between serum total bilirubin and cumulative bled volume suggested that venesection could suppress bilirubin production from aged erythrocytes. The hepatic iron overload was distributed in hepatocyte lysosomes with Dubin-Johnson granules; thus, it seems that iron removal from the lysosomal granules may also help to reduce serum bilirubin. In conclusion, deep jaundice in a patient with Dubin-Johnson syndrome complicated by hepatitis C virus-positive chronic liver disease and iron overload was partially improved by iron-reduction therapy.

Dubin-Johnson-like black liver with normal bilirubin level.

Black liver is a common finding in Dubin-Johnson syndrome (DJS), which is caused by the lack of multidrug resistance-associated protein 2 (MRP2). Impaired excretion of epinephrine metabolites is believed to be a cause of black liver in DJS. Recently, we experienced a patient with black liver whose serum bilirubin level was normal. Coarse brown granules were observed in the hepatocytes, and this finding closely resembled that observed in DJS. However, the granules were negative for Schmorl staining. The MRP2 gene did not show any mutation. Immunostaining study demonstrated MRP2 protein expression in the liver, and it was localized in the canalicular membranes of hepatocytes. This case illustrates for the first time that DJS is not the only cause of black liver.

Unconjugated bilirubin kinetics in Dubin-Johnson syndrome.

Plasma unconjugated bilirubin kinetics were studied by a two-compartment analysis in 10 patients with Dubin-Johnson syndrome (DJS) and 7 normal controls. The clearance of unconjugated bilirubin from plasma was significantly reduced in the patients with DJS as compared to the controls (p less than 0.01). Kinetic analysis of plasma disappearance data showed an increase in reflux and decrease in conjugation. These results indicate that defects in both conjugated and unconjugated bilirubin metabolism exist in DJS.

Abnormality of oral ursodeoxycholic acid tolerance test in the Dubin-Johnson syndrome.

Serial measurement of serum bile acid levels before and after oral administration of ursodeoxycholic acid (UDCA) was performed in 10 patients with Dubin-Johnson acid levels were significantly elevated in the patients compared with the controls (p less than 0.05). Oral UDCA tolerance studies revealed impaired bile acid clearance from serum in the patients and showed markedly increased serum bile acid levels 120 min after administration (p less than 0.01). These results indicate that some DJS patients have abnormal bile clearance which may be attributable to defects in hepatic uptake and biliary excretion of bile acids.

Severe jaundice due to coexistence of Dubin-Johnson syndrome and hereditary spherocytosis: a case report.

Dubin-Johnson syndrome is a chronic, benign, intermittent jaundice, mostly of conjugated hyperbilirubinemia. The level of bilirubin is not expected to be more than 20 mg/dl in this syndrome. In this article, we report a patient who was evaluated for hyperbilirubinemia and liver function test abnormalities and diagnosed with Dubin-Johnson syndrome coexisting with hereditary spherocytosis. We suggest that other diseases should be investigated if patients with Dubin-Johnson syndrome present with severe hyperbilirubinemia. Dubin-Johnson syndrome accompanied by hemolytic diseases might also have high coproporphyrin levels (as in Rotor's syndrome) than expected in pure Dubin-Johnson syndrome.