Background & aims: Dubin-Johnson syndrome is recessively inherited, conjugated hyperbilirubinemia induced by mutations in the ABCC2/MRP2 gene encoding the canalicular transporter for conjugated bilirubin. Gilbert's syndrome is recessively inherited, unconjugated hyperbilirubinemia caused by decreased conjugation rate of bilirubin associated mostly with homozygous A(TA) 7 TAA variant of the TATAA-box in the UGT1A1 gene promoter. Our aim was to establish the molecular diagnosis in a 3-year-old male with atypical, intermittent, predominantly unconjugated, hyperbilirubinemia.
Methods: 99m Tc-HIDA cholescintigraphy was used for imaging the biliary tree. Expression of ABCC2/MRP2 protein in hepatocytes was investigated immunohistochemically. UGT1A1 and ABCC2/MRP2 genes were sequenced from genomic DNA, and the mutations were verified by fragment analysis, sequencing the cloned exons, and restriction fragment length polymorphism.
Results: Cholescintigraphy revealed delayed visualization of the gallbladder. A brown granular lipopigment differing from melanin-like pigment reported in Dubin-Johnson syndrome was present in hepatocytes, but, otherwise, liver histology was normal. ABCC2/MRP2 protein was not detected on the canalicular membrane of hepatocytes, and 2 novel mutations were found in the ABCC2/MRP2 gene: a heterozygous in-frame insertion-deletion mutation 1256insCT/delAAACAGTGAACCTGATG in exon 10 inherited from the father and a heterozygous deletion 4292delCA in exon 30 inherited from the mother. In addition, the patient was homozygous for -3279T>G and A(TA) 7 TAA mutations in the UGT1A1 gene promoter.
Conclusions: Our patient represents a case of digenic mixed hyperbilirubinemia-a distinct type of constitutive jaundice resulting from coinherited defects in ABCC2/MRP2 and UGT1A1 genes.