Gilbert Syndrome 

Background

Gilbert syndrome, the most common inherited cause of unconjugated hyperbilirubinemia, was first described by Augustine Gilbert and Pierre Lereboullet in 1901. This autosomal recessive condition is characterized by intermittent jaundice in the absence of hemolysis or underlying liver disease.

The hyperbilirubinemia is mild. By definition, bilirubin levels in Gilbert syndrome are lower than 6 mg/dL, though most patients exhibit levels lower than 3 mg/dL. Considerable daily and seasonal variations are observed, and bilirubin levels occasionally may be normal in as many as one third of patients.

Gilbert syndrome may be precipitated by dehydration, fasting, menstrual periods, or other causes of stress, such as an intercurrent illness or vigorous exercise. Patients may report vague abdominal discomfort and general fatigue for which no cause is found. These episodes typically resolve spontaneously without curative treatment.

As a rule, Gilbert syndrome can be diagnosed by a thorough history and physical examination and confirmed by standard blood tests. Repeated investigations and invasive procedures are not usually justified for establishing a diagnosis.

Once the diagnosis of Gilbert syndrome is established, the most important aspect of treatment is reassurance. In light of the benign and inconsequential nature of the syndrome, the use of medications to treat patients with this condition is unjustified in clinical practice.

Pathophysiology

Unconjugated hyperbilirubinemia in Gilbert syndrome has long been recognized as due to underactivity of the conjugating enzyme system bilirubin–uridine diphosphate glucuronyl transferase (bilirubin-UGT).^[1]

Bilirubin-UGT is responsible for conjugating bilirubin into bilirubin monoglucuronides and diglucuronides and is located primarily in the endoplasmic reticulum of hepatocytes. It is one of several UGT enzyme isoforms responsible for the conjugation of a wide array of substrates, including carcinogens, drugs, hormones, and neurotransmitters.

Knowledge of these enzymes has been enhanced greatly by characterization of the UGT1 gene locus in humans. The gene that expresses bilirubin-UGT has a complex structure and is located on chromosome 2.^{[2, 3, 4, 5, 6, 7, 8]} There are 5 exons, of which exons 2-5 at the 3' end are constant components of all isoforms of UGT, coding for the UDP-glucuronic acid–binding site.

Exon 1 encodes for a unique region within each UGT and confers substrate specificity. However, multiple (at least 13) exon 1s exist, and to complete the gene, 1 of these exons must be recruited. Exons 1a and 1d encode the variable region for bilirubin UGT1*1 (also known as UGT1A1) and UGT1*2, respectively, with UGT1*1 responsible for virtually all bilirubin conjugation and UGT1*2 playing little, if any, role.

Expression of UGT1*1 depends on a promoter region in a 5' position relative to each exon 1 that contains a TATAA box. Impaired bilirubin glucuronidation therefore may result from mutations in exon 1a, its promoter, or the common exons.

A breakthrough in understanding the genetic basis of Gilbert syndrome was achieved in 1995, when abnormalities in the TATAA region of the promoter were identified. The addition of 2 extra bases (TA) to the TATAA region interferes with binding of the transcription factor IID and results in reduced expression of bilirubin-UGT1 (30% of normal). In the homozygous state, diminished bilirubin glucuronidation is observed, with bile containing an excess of bilirubin monoglucuronide over diglucuronide.^{[2, 9, 10, 11, 12]}

Additional mutations have since been identified. For example, some healthy Asian patients with Gilbert syndrome do not have mutations at the promoter level but are heterozygotes for missense mutations (Gly71Arg, Tyr486Asp, Pro364Leu) in the coding region. These individuals also have significantly higher bilirubin levels than those with the wild-type allele.

Whether reduced bilirubin-UGT activity results from a reduced number of enzyme molecules or from a qualitative enzyme defect is unknown. To compound this uncertainty, other factors (eg, occult hemolysis or hepatic transport abnormalities) may be involved in the clinical expression of Gilbert syndrome. For example, many individuals homozygous for the TATAA defect do not demonstrate unconjugated hyperbilirubinemia, and many patients with reduced levels of bilirubin-UGT, as observed in some granulomatous liver diseases, do not develop hyperbilirubinemia.

Because of the high frequency of mutations in the Gilbert promoter, heterozygous carriers of Crigler-Najjar syndromes types 1 and 2 can also carry the elongated Gilbert TATAA sequence on their normal allele. Such combined defects can lead to severe hyperbilirubinemia and also help explain the finding of intermediate levels of hyperbilirubinemia in family members of patients with Crigler-Najjar syndrome.

Gilbert syndrome can also frequently coexist with the conditions associated with unconjugated hyperbilirubinemia, such as thalassemia^[13] and glucose-6-phosphate deficiency (G6PD).^{[14, 15]}

Origa et al, in a study of 858 patients with transfusion-dependent thalassemia, found that 30% of patients with thalassemia major had gallstones; they also found evidence that in patients with a combination of thalassemia and the Gilbert syndrome genotype (TA)7/(TA)7 UGT1*1, the latter effected the prevalence of cholelithiasis and influenced the age at which the condition arose.^[16] The authors suggested that in patients with a combination of thalassemia and Gilbert syndrome, biliary ultrasonography should be performed, starting in childhood.

A Greek study of 198 adult patients with cholelithiasis and 152 controls, also found evidence of an association between Gilbert syndrome and the development of cholelithiasis.^[17]

Etiology

Causes of Gilbert syndrome include the following:

• Dehydration
• Fasting - This produces an increase in the plasma unconjugated bilirubin level
• Intercurrent illness, such as a viral infection
• Menstrual periods
• Stress, such as trauma and overexertion

Epidemiology

In the United States, the prevalence of Gilbert syndrome is 3-7%. Worldwide, the prevalence varies considerably, depending on which diagnostic criteria are used (eg, number of bilirubin determinations, method of analysis, bilirubin levels used for diagnosis, and whether the patient was fasting). Estimates of prevalence may be complicated further by molecular genetic studies of polymorphisms in the TATAA promoter region, which affects as many as 36% of Africans but only 3% of Asians.

The clinical phenotype may not be as apparent as the determined genotype, because of environmental influences (eg, alcohol-induced bilirubin glucuronidation) that can reduce bilirubin levels.

Gilbert syndrome is usually diagnosed around puberty, possibly because of the inhibition of bilirubin glucuronidation by endogenous steroid hormones. In older persons, the diagnosis is usually made when unconjugated hyperbilirubinemia is noted on routine blood test results or unmasked by an intercurrent illness or stress. The syndrome occurs predominantly in men, with a male-to-female ratio ranging from 2:1 to 7:1. Gilbert syndrome is not restricted to any ethnic group and occurs in persons of all races.

Prognosis

Gilbert syndrome is a common and benign condition. The bilirubin disposition may be regarded as falling within the range of normal biologic variation. The syndrome has no deleterious associations and an excellent prognosis, and those who have it can lead a normal lifestyle. As further confirmation of its benign nature, studies have reported excellent results in patients undergoing living donor liver transplantation from donors with Gilbert syndrome.^[18]

Patient history

At least 30% of patients are asymptomatic, although nonspecific symptoms, such as abdominal cramps, fatigue, and malaise, are common. Abdominal symptoms in these patients are a poorly defined entity and may be secondary to underlying anxiety. It is true that not all patients with Gilbert syndrome and abdominal symptoms are anxious; nevertheless, they appear to have organic-type discomfort that is hard to characterize and frequently eludes diagnosis.

No relationship exists between these abdominal symptoms and plasma bilirubin levels. Abdominal symptoms apparently may be multifactorial, with underlying anxiety probably playing an important role.

Physical examination

Mild jaundice is present intermittently in some individuals, but no other abnormal physical examination findings are evident. Infants homozygous for Gilbert syndrome may have a greater increase in neonatal jaundice when breastfed or when other disorders of heme metabolism are coinherited.^{[19, 20]}

Gilbert syndrome has a broad differential diagnosis because numerous causes of unconjugated hyperbilirubinemia must be considered. In particular, the possibility of biliary disease must be taken into account.

Other conditions to be considered include the following:

• Hemolysis
• Hematoma
• Acute and chronic liver disease
• Primary hyperbilirubinemia from ineffective erythropoiesis
• Glucuronyl transferase deficiency
• Infections
• Cardiac disease (eg, congestive heart failure, prosthetic heart valves)
• Rhabdomyolysis
• High-altitude living
• Medications (eg, probenecid, rifamycin, other antibiotics)
• Thyrotoxicosis

As a rule, Gilbert syndrome can be diagnosed by a thorough history and physical examination and confirmed by standard blood tests. Repeated investigations and invasive procedures are not usually justified for establishing a diagnosis. Studies can be conducted within the realms of an institutional review board–approved study.

A complete blood count (CBC), including a reticulocyte count and a blood smear, is a useful screening test for excluding hemolysis. Rarely, red blood cell (RBC) abnormalities resembling variegate porphyria have been described in persons with Gilbert syndrome, possibly as a result of the increased hepatocellular bilirubin concentration.

Serum lactate dehydrogenase (LDH) levels are elevated in persons with hemolysis but are normal in those with Gilbert syndrome.

With the exception of unconjugated hyperbilirubinemia, standard liver function test (LFT) results are normal. However, a familial increase in serum alkaline phosphatase (ALP) levels has been reported in persons with Gilbert syndrome.

Imaging studies are not required to confirm a diagnosis of Gilbert syndrome.

Additional diagnostic tests are rarely required, because a diagnosis of Gilbert syndrome can generally be made on the basis of the following findings:

• Unconjugated hyperbilirubinemia noted on several occasions
• Normal results from the complete blood count (CBC), reticulocyte count, and blood smear
• Normal liver function test (LFT) results
• An absence of other disease processes

Nevertheless, certain specialized tests (including some that are of historical interest, as well as the newer molecular genetic techniques) are occasionally performed to confirm a diagnosis of Gilbert syndrome. These tests are described below to introduce the clinician to the broad diagnostic armamentarium available for diagnosing Gilbert syndrome. Recourse to these specialized tests should be rare and is usually difficult to justify in clinical practice, given that the diagnosis of Gilbert syndrome is usually straightforward.

Fasting test

Within 48 hours of the start of a fast, there is usually a 2- to 3-fold rise in the plasma unconjugated bilirubin level, which returns to normal levels within 24 hours after resumption of a normal diet. Although unconjugated bilirubin levels also rise with fasting in patients with hemolysis or liver disease, the magnitude of the rise is less than that observed with Gilbert syndrome. A similar rise in plasma bilirubin is also observed with normocaloric diets deficient in lipids and reverses promptly with lipid replacement.

The precise mechanism of fasting and dietary-induced hyperbilirubinemia remains unclear. The fasting test is primarily of historical interest and has only limited usefulness in the diagnosis of Gilbert syndrome.

Nicotinic acid test

Intravenous (IV) administration of 50 mg of nicotinic acid results in a 2- to 3-fold rise in plasma unconjugated hyperbilirubinemia within 3 hours. The mechanisms are multifactorial and probably related to the following:

• Elevated osmotic fragility of red blood cells
• Increase in splenic production of bilirubin
• Transient inhibition of hepatic bilirubin–uridine diphosphate glucuronyl transferase (bilirubin-UGT) activity
• Increased splenic heme oxygenase activity

Although a similar but less impressive increase is observed in healthy individuals and those with hemolysis or liver disease, the nicotinic test, like the fasting test, does not clearly distinguish patients with Gilbert syndrome from those who are healthy or who have other disease processes.

Phenobarbital test

Phenobarbital and other enzyme inducers of the bilirubin-UGT system will normalize plasma bilirubin levels in patients with Gilbert syndrome. This effect is predominantly due to accelerated bilirubin clearance from enzyme induction but is also due to reduced bilirubin turnover. Steroids can also reduce plasma bilirubin levels in Gilbert syndrome by increasing hepatic uptake and storage of bilirubin.

Radiolabeled chromium test

The radiolabeled chromium test is used to measure red blood cell (RBC) survival. As many as 60% of patients with Gilbert syndrome have a mild and fully compensated state of hemolysis together with increased hepatic heme production. As a result, hyperbilirubinemia may be due to reduced bilirubin clearance and increased production, the latter from increased erythroid or hepatic heme turnover.

Thin-layer chromatography

Thin-layer chromatography is diagnostic for Gilbert syndrome when it shows a significantly higher proportion of unconjugated bilirubin than is seen in individuals with chronic hemolysis or liver disease or those who are healthy. If confirmation of the diagnosis is truly essential, chromatographic determination is of potential use. This shows an increased ratio of bilirubin monoglucuronide to diglucuronide, reflecting reduced bilirubin-UGT activity.

Drug clearance test

In approximately 30% of patients, there is impaired clearance of bromosulfophthalein, indocyanine green, and free fatty acid, suggesting an abnormality in hepatic uptake, transport, or both. Metabolic clearance of tolbutamide is also reduced in persons with Gilbert syndrome, but because the drug does not undergo glucuronidation, hepatic uptake appears to be defective.

Plasma clearance of most drugs that undergo glucuronidation (eg, benzodiazepines) is unaffected. With regard to acetaminophen, however, patients with Gilbert syndrome are a heterogeneous group, with some demonstrating normal metabolism and others exhibiting marked reduction in glucuronidation and an increase in oxidation.^{[21, 22]} These changes suggest that people in this subgroup may be more susceptible to liver injury after an acetaminophen overdose, though no such adverse events have been reported.

Polymerase chain reaction

Polymerase chain reaction (PCR) is a novel and rapid method of identifying genetic polymorphisms in the TATA box of the UGT1*1 gene by using fluorescence resonance energy transfer.

Liver biopsy

Liver biopsies are not performed routinely and are rarely necessary. Histologically, the liver is normal in persons with Gilbert syndrome, except for occasional accumulation of a lipofuscinlike pigment around the terminal hepatic venules.

Once the diagnosis of Gilbert syndrome is established, the most important aspect of treatment is reassurance. The clinician must make it perfectly clear to the patient that the syndrome is essentially benign, is not associated with increased morbidity, has an excellent prognosis, and is associated with normal life expectancy. In light of the benign and inconsequential nature of Gilbert syndrome, the use of medications to treat patients with this condition is unjustified in clinical practice.

Irinotecan toxicity in Gilbert syndrome is of some concern, however. Affected patients show reduced inactivation of the active topoisomerase inhibitor 7-ethyl-10-hydroxycampothecin (SN-38), caused by a mutation in the UDT-1*1 gene promoter. Glucuronidation rates of the active metabolite SN-38 are significantly lower in people who are homozygous and heterozygous for the TA-TATAA variant allele than in those with the wild-type genotype (TATAA).

In Gunn rats (an animal model of Crigler-Najjar syndrome) and in people with Gilbert or Crigler-Najjar syndrome, reduced glucuronidation of SN-38 leads to SN-38 toxicity and causes symptoms such as diarrhea. Preliminary results from clinical trials suggest that screening cancer patients for the UGT1*1 promoter polymorphism may reduce the prevalence of irinotecan toxicity. Until this evidence is available, caution is warranted before irinotecan is prescribed to this subset of patients.

The clinical relevance of pharmacogenetics in Gilbert syndrome is yet to be determined. Although impaired glucuronidation and excretion of certain drugs have been reported, such impairment has not resulted in any adverse clinical events, and the risk probably remains more theoretical than real.^{[23, 24]}

Patients can consume a normal diet, and no restrictions on activity are necessary. They should make an effort to avoid known risk factors for precipitating hyperbilirubinemia (eg, dehydration, fasting).