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Dubin–Johnson syndrome is an autosomal recessive disorder that causes an increase of conjugated bilirubin in the serum without elevation of liver enzymes (ALT, AST). This condition is associated with a defect in the ability of hepatocytes to secrete conjugated bilirubin into the bile, and is similar to Rotor syndrome. It is usually asymptomatic but may be diagnosed in early infancy based on laboratory tests.
The conjugated hyperbilirubinemia is a result of defective endogenous and exogenous transfer of anionic conjugates from hepatocytes into the bile. Impaired biliary excretion of bilirubin glucuronides is due to a mutation in the canalicular multidrug resistance protein 2 (MRP2). A darkly pigmented liver is due to polymerized epinephrine metabolites, not bilirubin.
A hallmark of DJS is the unusual ratio between the byproducts of heme biosynthesis.
- Unaffected subjects have a coproporphyrin III to coproporphyrin I ratio of approximately 3–4:1.
- In patients with DJS, this ratio is inverted with coproporphyrin I being 3–4x higher than coproporphyrin III. Analysis of urine porphyrins show a normal level of coproporphyrin but the I isomer accounts for 80% of the total (normally 25%).
There is plenty of canalicular multi-drug resistant protein that causes bilirubin transfer to bile canaliculi. An isoform of this protein is localized to the apical hepatocyte membrane, allowing transport of glucuronide and glutathione conjugates back into the blood.
High levels of gamma-glutamyl transferase (GGT) help in diagnosing pathologies involving biliary obstruction.
DJS is due to a defect in the multispecific anion transporter (cMOAT) gene (ABC transporter superfamily). It is an autosomal recessive disease and is likely due to a loss of function mutation, since the mutation affects the cytoplasmic / binding domain.
Prognosis is good, and treatment of this syndrome is usually unnecessary. Most patients are asymptomatic and have normal life spans. Some neonates will present with cholestasis. Hormonal contraceptives and pregnancy may lead to overt jaundice and icterus (yellowing of the eyes and skin).