Dubin-Johnson syndrome
Summary
Dubin-Johnson syndrome is a benign, autosomal recessive disorder causing conjugated (direct) hyperbilirubinemia due to a defect in the hepatocyte canalicular multidrug resistance protein 2 (MRP2/cMOAT), which impairs excretion of conjugated bilirubin into bile. It is classically associated with a grossly black/dark liver on gross pathology due to accumulation of a pigment resembling melanin (lysosomal deposition of epinephrine metabolites). Patients are typically asymptomatic or have mild jaundice, and liver function tests (aside from bilirubin) are normal.
Detail
Dubin-Johnson syndrome results from a mutation in the ABCC2 gene encoding MRP2 (multidrug resistance-associated protein 2), an ATP-dependent canalicular transporter responsible for exporting conjugated bilirubin and other organic anions from hepatocytes into bile. Loss of function leads to impaired biliary excretion of conjugated bilirubin, causing it to reflux back into the systemic circulation, resulting in conjugated (direct) hyperbilirubinemia. Despite the excretory defect, hepatocyte uptake and conjugation of bilirubin remain normal, and liver enzymes (AST, ALT, ALP) are typically normal, distinguishing it from hepatocellular or obstructive liver disease.
A hallmark feature is the grossly black or dark-colored liver, caused by accumulation of a coarse, granular pigment within hepatocyte lysosomes—thought to be derived from epinephrine metabolites (not bilirubin itself)—giving the liver a distinctive appearance on gross pathology, though liver architecture and histology are otherwise normal.
Clinically, patients present with mild, chronic, or intermittent jaundice, often exacerbated by stress, pregnancy, oral contraceptive use, or intercurrent illness. The condition is typically discovered incidentally or during evaluation of asymptomatic jaundice, often in young adults. Urinary coproporphyrin excretion pattern is diagnostic: total urinary coproporphyrin is normal, but the ratio of coproporphyrin I to III is elevated (>80%), which helps differentiate it from Rotor syndrome, a similar but distinct condition with normal liver color and different coproporphyrin excretion pattern (elevated total urinary coproporphyrin, but normal I:III ratio).
Dubin-Johnson syndrome is a benign condition requiring no treatment beyond reassurance, as it does not lead to liver damage, cirrhosis, or decreased life expectancy. It is important primarily as a differential diagnosis for conjugated hyperbilirubinemia and a classic USMLE association between gross pathology (black liver) and molecular genetics (MRP2/ABCC2 defect).
Sources
- First Aid for the USMLE Step 1
- Robbins and Cotran Pathologic Basis of Disease
- UpToDate: Inherited disorders associated with conjugated hyperbilirubinemia
- Harrison's Principles of Internal Medicine
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