Molecular defect causing liver disease in Dubin-Johnson syndrome is_____?

Explanation

## Correct Answer: C. ABCC2 Dubin-Johnson syndrome (DJS) is a benign autosomal recessive disorder of hepatic bilirubin excretion caused by mutations in the **ABCC2 gene** (also called MRP2 or multidrug resistance-associated protein 2). ABCC2 encodes a canalicular membrane transporter responsible for active secretion of conjugated bilirubin, organic anions, and other compounds from hepatocytes into bile. Loss of ABCC2 function prevents the transport of conjugated (direct) bilirubin across the canalicular membrane, leading to its reflux into blood and causing **conjugated hyperbilirubinemia**. Clinically, patients present with jaundice, dark urine, and normal or near-normal liver function tests. The hallmark finding is a **black or dark brown discoloration of the liver** (melanin-like pigment deposition) on histology, which is pathognomonic. Unlike Crigler-Najjar syndrome (which involves unconjugated hyperbilirubinemia), DJS has conjugated bilirubin in serum and urine. The prognosis is excellent with no hepatic dysfunction, and treatment is supportive. This gene defect distinguishes DJS from other inherited cholestatic syndromes and is the molecular basis for the transport defect. ## Why the other options are wrong **A. ATP7B** — ATP7B encodes a copper-transporting ATPase and is mutated in **Wilson disease**, not Dubin-Johnson syndrome. Wilson disease presents with hepatic cirrhosis, neuropsychiatric symptoms, and Kayser-Fleischer rings due to copper accumulation. This is a common NBE trap pairing copper metabolism disorders with cholestatic syndromes. **B. SERPINA1** — SERPINA1 mutations cause **alpha-1 antitrypsin (AAT) deficiency**, leading to early-onset emphysema and progressive liver disease (cirrhosis). AAT deficiency presents with PAS-positive, diastase-resistant globules in hepatocytes. It causes unconjugated or mixed hyperbilirubinemia with hepatic synthetic dysfunction, not the isolated conjugated hyperbilirubinemia of DJS. **D. ATP7A** — ATP7A is mutated in **Menkes disease**, an X-linked copper malabsorption disorder presenting with kinky hair, neurodegeneration, and vascular abnormalities in infants. It does not cause cholestasis or hyperbilirubinemia. This option tests whether students confuse copper metabolism genes with bilirubin transport defects. ## High-Yield Facts - **ABCC2 gene mutation** → loss of MRP2 transporter → impaired conjugated bilirubin secretion into bile → conjugated hyperbilirubinemia in Dubin-Johnson syndrome. - **Black/dark liver pigment** on histology is pathognomonic for DJS; caused by melanin-like lipofuscin deposition, not bilirubin. - **Conjugated (direct) bilirubin** in serum and urine distinguishes DJS from Crigler-Najjar (unconjugated) and Gilbert syndrome (mild unconjugated). - **Prognosis is excellent** in DJS with normal liver synthetic function and no progression to cirrhosis, unlike AAT deficiency or Wilson disease. - **Dubin-Johnson is autosomal recessive**; Rotor syndrome (similar presentation) is caused by SLCO1B1/SLCO1B3 mutations affecting organic anion uptake. ## Mnemonics **DJS = Dark liver + Direct (conjugated) bilirubin** D-J-S → Dark pigment in liver + Direct hyperbilirubinemia. Remember: the liver looks dark (black pigment), and the bilirubin is already conjugated (direct), so it backs up into blood. **ABCC2 = ABC transporter for Bilirubin Conjugate** ABCC2 is an ABC (ATP-binding cassette) transporter that pumps conjugated bilirubin OUT of the hepatocyte into the canaliculus. When it's broken, bilirubin can't leave → reflux into blood. ## NBE Trap NBE pairs Dubin-Johnson with other inherited liver diseases (Wilson, AAT deficiency) to test whether students confuse copper metabolism genes (ATP7B, ATP7A) with bilirubin transport genes. The key discriminator is the **conjugated hyperbilirubinemia** and **black liver pigment** unique to ABCC2 mutations. ## Clinical Pearl In Indian pediatric practice, Dubin-Johnson syndrome is often discovered incidentally during workup of neonatal jaundice or when a child presents with mild jaundice and dark urine. Unlike other inherited cholestatic syndromes, it requires no treatment and has no long-term sequelae—reassurance to parents is the main intervention. The black liver on ultrasound or biopsy is the diagnostic clue that points to ABCC2 defect. _Reference: OP Ghai Pediatrics Ch. 10 (Neonatal Jaundice & Cholestasis); Harrison Ch. 297 (Hyperbilirubinemia); Robbins Ch. 18 (Liver Pathology)_