Inherited disorder of Bilirubin metabolism: Biochemistry and Genetics

Case objective
a) Discuss an overview of bilirubin metabolism
b) Discuss neonatal physiological jaundice
c) Discuss inborn errors of bilirubin metabolism
d) Discuss biochemical/molecular diagnosis

Case Presentation :
A 48-year-old woman had a history of jaundice for decades, with no related symptoms. A laparoscopic evaluation showed a smooth liver with normal consistency and morphologic features but with a grossly black appearance. A biopsy specimen revealed coarse, deep-brown, pigmented granules on periodic acid–Schiff staining primarily at the canalicular pole of the hepatocytes and especially in the pericentral zones. The laboratory investigation results are given below
Serum Total bilirubin - 4.6 mg/dL
Direct-reacting bilirubin -2.4 mg/dL
Aspartate transaminase - 38 U/L
Alanine transaminase - 25 U/L
Alkaline phosphatase - 45 U/L
Serum Albumin- 4.2 gm/dL
Prothrombin time- 12 seconds

Expression of the multidrug resistance associated protein 2 (MRP2) was absent on anti–MRP2 immunohistochemical analysis

Diagnosis- Dubin–Johnson syndrome was confirmed. This syndrome is an autosomal recessive disorder that is caused by a mutation in MRP2 that results in deficient canalicular expression of MRP2 and impaired secretion of conjugated bilirubin into the bile.

Overview of Bilirubin metabolism

The metabolism of bilirubin can be divided into three different steps:

i) Uptake: The bilirubin is formed in reticuloendothelial cells and released into the circulation. In circulation, the bilirubin binds to albumin at high and low-affinity binding sites and transported to the liver cells. In the hepatocytes, the bilirubin is rapidly sequestered by protein Ligandin Y which enables uptake of bilirubin into hepatocytes against the concentration gradient.

ii) Conjugation: In hepatocytes, the bilirubin is conjugated to UDP-glucuronic acid form bilirubin mono-glucuronides and subsequently converted to bilirubin diglucuronides. The reaction is catalyzed by an inducible enzyme UDP-glucuronosyl transferase (UGT).

iii) Secretion: The conjugated bilirubin is secreted into canalicular space through multidrug resistance transporter protein. The secreted conjugated bilirubin enters the gallbladder and secreted via bile into the intestine.

Figure 1: Overview of bilirubin metabolism




Inherited disorders of bilirubin metabolism:

Gilbert Syndrome:

The Gilbert syndrome is characterized by unconjugated hyperbilirubinemia with serum bilirubin less than 3.0 mg/dL. Gilbert syndrome is associated with a defect in bilirubin uptake and the genetic mechanism is not fully understood. Studies have shown that the mutation in the promoter region of UTGA1A and other missense mutation is associated with Gilbert syndrome. This leads to decreased activity of UGT enzymes in the liver resulting in unconjugated hyperbilirubinemia. This genetic disorder remains undiagnosed until adulthood because of milder hyperbilirubinemia and no specific jaundice. In patients with Gilbert syndrome, fasting causes an increase in serum bilirubin, coloration of eyes and sclera. This is mainly due to the competitive displacement of bilirubin bound to albumin by fatty acid. The liver enzymes are within normal reference interval.

Crigler Najjar Syndrome I:

The Crigler Najjar syndrome-I is characterized by a complete deficiency of enzyme UGT leading to unconjugated hyperbilirubinemia with serum bilirubin above 20 mg/dL. The unconjugated bilirubin above 20 mg/dL crosses the blood-brain barrier; and causes kernicterus. The diagnosis is based on bilirubin greater than 20 mg/dL and normal liver enzymes including aspartate transaminase, alanine transaminase, alkaline phosphatase. Phototherapy is used for reducing bilirubin as it makes bilirubin more soluble in water.

Crigler Najjar Syndrome II:

The Crigler Najjar Syndrome II is characterized by a partial deficiency of enzyme UGT resulting in increased unconjugated bilirubin (5-20 mg/dL). In addition, the unconjugated bilirubin is lowered upon administration of phenobarbital by inducing UGT enzyme. The patients with Type I deficiency are non-responsive to phenobarbital. Apart from unconjugated hyperbilirubinemia, all liver enzyme parameters are normal and kernicterus is absent.

Dubin Johnson syndrome:

Dubin Johnson syndrome is characterized by conjugated hyperbilirubinemia due to defective MRP2 protein. MRP2 protein is responsible for secretion of bilirubin to bile canaliculi. The normal liver enzymes including aspartate transaminase, alanine transaminase, alkaline phosphatase suggestive of normal liver function without any cholestasis. The pigmentation of the liver is observed.

Rotor syndrome:

Rotor syndrome is characterized by conjugated hyperbilirubinemia due to defective secretion of bilirubin to bile canaliculi but the etiology is not known. The normal liver enzymes including aspartate transaminase, alanine transaminase, alkaline phosphatase suggestive of normal liver function without any cholestasis. No pigmentation of the liver is observed.

Take Away Points

Bilirubin conjugation occurs in the liver and the reaction is catalyzed by an inducible enzyme UGT.
Conjugation and phototherapy make bilirubin more water soluble.
Measurement of unconjugated and conjugated bilirubin is used for diagnosis of inherited hyperbilirubinemia.

Reference:
Moni et al. 2016 NEMJ

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