Gilbert Syndrome

Definition and Epidemiology

• Goldman-Cecil Medicine, p. 1555
• Tietz Textbook of Laboratory Medicine, 7th Ed.

Pathophysiology and Genetics

Molecular Mechanism

• The most common mutation (in Western populations) is a promoter polymorphism: an extra TA dinucleotide repeat in the TATA box of the UGT1A1 gene, producing 7 TA repeats (UGT1A1*28 allele, written as (TA)₇TAA) instead of the normal 6 repeats ((TA)₆TAA).
• Transcription of the gene is inversely proportional to the number of TA repeats - more repeats = lower transcription = less enzyme activity.
• Subjects with 8 repeats have even higher bilirubin levels than those with 7.
• The result is approximately 30% of normal hepatic glucuronosyltransferase activity remaining - enough to prevent serious hyperbilirubinemia but insufficient to maintain normal bilirubin levels under physiologic stress.
• Asian populations more commonly harbour missense mutations in exon 1 of UGT1A1 rather than the promoter *28 allele.

Allelic Frequencies (UGT1A1*28)

• Goldman-Cecil Medicine, p. 1555
• Tietz Textbook of Laboratory Medicine, 7th Ed.

Inheritance

Biochemical Consequence

• Total bilirubin in serum fluctuates between 1.5 and 4 mg/dL (26-70 µmol/L), almost always unconjugated.
• Rises of 2- to 3-fold occur with fasting, intercurrent illness, stress, or menstruation.
• Despite total biliary bilirubin being reduced, the ratio of bilirubin monoglucuronide to diglucuronide is increased, suggesting an additional defect in the conversion step.
• Urine bilirubin is absent (conjugated bilirubin is not filtered by the kidney in this condition).

Clinical Features

• Typically presents in adolescence, when sex steroids begin altering bilirubin metabolism.
• The classic picture is intermittent mild jaundice (scleral icterus) with no other signs of liver disease.
• Symptoms (when present): mild abdominal discomfort or nausea, but these are non-specific.
• Common precipitants of jaundice episodes:
  • Fasting or caloric restriction
  • Physical or emotional stress
  • Intercurrent infection or illness
  • Menstruation
  • Surgery / anaesthesia
• No hepatosplenomegaly, no stigmata of chronic liver disease.
• Yamada's Textbook of Gastroenterology, 7th Ed.
• Tietz Textbook of Laboratory Medicine, 7th Ed.

Diagnosis

1. Isolated unconjugated hyperbilirubinemia (bilirubin usually <3-4 mg/dL)
2. Normal liver function tests (ALT, AST, ALP, albumin, PT - all normal)
3. No anaemia, no haemolysis
4. No bilirubin in urine (dipstick negative)
5. No other explanation

• Fasting test: Serum bilirubin rises after 48 hours of caloric restriction (400 kcal/day). A rise confirms the diagnosis.
• Phenobarbital test: Phenobarbital induces UGT1A1 activity, normalising bilirubin - a response supports the diagnosis.
• Genetic testing: Identification of UGT1A1*28 homozygosity can confirm.
• The condition is distinguished from chronic hepatitis by the absence of anemia, absence of bilirubinuria, and normal LFTs.

Comparison with Related Disorders

• Quick Compendium of Clinical Pathology, 5th Ed.
• Harper's Illustrated Biochemistry, 32nd Ed.

Drug Interactions and Clinical Significance

Irinotecan (CPT-11) - CRITICAL

• Irinotecan's active metabolite SN-38 is glucuronidated specifically by UGT1A1.
• In Gilbert syndrome, impaired SN-38 glucuronidation leads to accumulation of the toxic metabolite, causing:
  • Severe/intractable diarrhoea
  • Myelosuppression
  • Other serious toxicities
• UGT1A1 genotyping before irinotecan is clinically indicated (FDA label recommends reducing the starting dose for UGT1A1*28 homozygotes).

Raloxifene

• A UGT1A1 substrate; homozygous *28 carriers have 2-fold higher drug exposure and greater pharmacodynamic effects.

Acetaminophen (paracetamol)

• Acetaminophen is primarily glucuronidated via UGT1A1 pathways; Gilbert syndrome patients may have increased susceptibility to acetaminophen hepatotoxicity, as more drug is shunted to the toxic CYP2E1 oxidative pathway.

HIV Protease Inhibitors (e.g., atazanavir, indinavir)

• These inhibit UGT1A1 and can unmask/worsen hyperbilirubinemia in Gilbert patients.
• Goldman-Cecil Medicine, p. 1555
• Tietz Textbook of Laboratory Medicine, 7th Ed.

Potential Protective Effects

• Lower cardiovascular disease risk - attributed to bilirubin's antioxidant properties preventing LDL oxidation.
• Possible protective effects against type 2 diabetes and metabolic syndrome have also been suggested.

Management

• Reassure the patient that they do not have liver disease.
• Educate about precipitants (fasting, illness) that cause jaundice episodes.
• Alert to drug interactions, particularly irinotecan.
• Avoid unnecessary investigations or labelling as liver disease (a common misdiagnosis is chronic hepatitis).
• Phenobarbital can normalise bilirubin but is not used clinically - its use is diagnostic only.

Summary