Gilbert's syndrome

Jaundice

Gilbert's syndrome produces an elevated level of unconjugated bilirubin in the bloodstream, but normally has no serious consequences. Mild jaundice may appear under conditions of exertion, stress, fasting, and infections, but the condition is otherwise usually asymptomatic. Severe cases are seen by yellowing of the skin tone and yellowing of the sclera in the eye.

GS has been reported to possibly contribute to an accelerated onset of neonatal jaundice, especially in the presence of increased red blood cell destruction due to diseases such as G6PD deficiency. This situation can be especially dangerous if not quickly treated, as the high bilirubin causes irreversible neurological disability in the form of kernicterus.

Detoxification of certain drugs

The enzymes that are defective in GS - UDP glucuronosyltransferase 1 family, polypeptide A1 (UGT1A1) - are also responsible for some of the liver's ability to detoxify certain drugs. For example, Gilbert's syndrome is associated with severe diarrhea and neutropenia in patients who are treated with irinotecan, which is metabolized by UGT1A1.

While paracetamol (acetaminophen) is not metabolized by UGT1A1, it is metabolized by one of the other enzymes also deficient in some people with GS. A subset of people with GS may have an increased risk of paracetamol toxicity.

Cardiovascular effects

Several analyses have found a significantly decreased risk of coronary artery disease (CAD) in individuals with GS.

Specifically, people with mildly elevated levels of bilirubin (1.1 mg/dl to 2.7 mg/dl) were at lower risk for CAD and at lower risk for future heart disease. These researchers went on to perform a meta-analysis of data available up to 2002, and confirmed the incidence of atherosclerotic disease (hardening of the arteries) in subjects with GS had a close and inverse relationship to the serum bilirubin. This beneficial effect was attributed to bilirubin IXα which is recognised as a potent antioxidant, rather than confounding factors such as high-density lipoprotein levels.

This association was also seen in long-term data from the Framingham Heart Study. Moderately elevated levels of bilirubin in people with GS and the (TA)₇/(TA)₇ genotype were associated with one-third the risk for both coronary heart disease and cardiovascular disease as compared to those with the (TA)₆/(TA)₆ genotype (i.e. a normal, nonmutated gene locus).

Platelet counts and MPV are decreased in patients with Gilbert's Syndrome. The elevated levels of bilirubin and decreasing levels of MPV and CRP in Gilbert's syndrome patients may have an effect on the slowing down of the atherosclerotic process.

Signs and symptoms

Symptoms, whether connected or not to GS, have been reported in a subset of those affected: feeling tired all the time (fatigue), difficulty maintaining concentration, unusual patterns of anxiety, loss of appetite, nausea, abdominal pain, loss of weight, itching (with no rash), and others, but scientific studies found no clear pattern of adverse symptoms related to the elevated levels of unconjugated bilirubin in adults. However, other substances glucuronidized by the affected enzymes in Gilbert's syndrome sufferers could theoretically, at their toxic levels, cause these symptoms. (see below). Consequently, debate exists about whether GS should be classified as a disease. However, Gilbert's syndrome has been linked to an increased risk of gallstones.

Genetics

Gilbert's syndrome is a phenotypic effect, characterized by mild jaundice due to increased unconjugated bilirubin, that arises from several different genotypic variants of the gene for the enzyme responsible for changing bilirubin to the conjugated form.

Gilbert's syndrome is characterized by a 70–80% reduction in the glucuronidation activity of the enzyme, (UGT1A1). The UGT1A1 gene is located on human chromosome 2.

More than 100 variants of the UGT1A1 gene are known, designated as UGT1A1*n (where n is the general chronological order of discovery), either of the gene itself or of its promoter region. UGT1A1 is associated with a TATA box promoter region; this region most commonly contains the genetic sequence A(TA₆)TAA; this variant accounts for about 50% of alleles in many populations. However, several allelic polymorphic variants of this region occur, the most common of which results from adding another dinucleotide repeat TA to the promoter region, resulting in A(TA₇)TAA, which is called UGT1A1*28; this common variant accounts for about 40% of alleles in some populations, but is seen less often, around 3% of alleles, in Southeast and East Asian people and Pacific Islanders.

In most populations, Gilbert's syndrome is most commonly associated with homozygous A(TA₇)TAA alleles. In 94% of GS cases, two other glucuronosyltransferase enzymes, UGT1A6 (rendered 50% inactive) and UGT1A7 (rendered 83% ineffective), are also affected.

However, Gilbert's syndrome can arise without TATA box promoter polymorphic mutations; in some populations, particularly healthy Southeast and East Asians, Gilbert's syndrome is more often a consequence of heterozygote missense mutations (such as Gly71Arg also known as UGT1A1*6, Tyr486Asp also known as UGT1A1*7, Pro364Leu also known as UGT1A1*73) in the actual gene coding region, which may be associated with significantly higher bilirubin levels.

Because of its effects on drug and bilirubin breakdown and because of its genetic inheritance, Gilbert's syndrome can be classed as a minor inborn error of metabolism.

Diagnosis

People with GS predominantly have elevated unconjugated bilirubin, while conjugated bilirubin is usually within the normal range and is less than 20% of the total. Levels of bilirubin in GS patients are reported to be from 20 μM to 90 μM (1.2 to 5.3 mg/dl) compared to the normal amount of < 20 μM. GS patients have a ratio of unconjugated/conjugated (indirect/direct) bilirubin commensurately higher than those without GS.

The level of total bilirubin is often further increased if the blood sample is taken after fasting for two days, and a fast can, therefore, be useful diagnostically. A further conceptual step that is rarely necessary or appropriate is to give a low dose of phenobarbital: the bilirubin will decrease substantially.

Tests can also detect DNA mutations of UGT1A1 by polymerase chain reaction or DNA fragment sequencing.

Differential diagnosis

While Gilbert's syndrome is considered harmless, it is clinically important because it may give rise to a concern about a blood or liver condition, which could be more dangerous. However, these conditions have additional indicators:

Treatment

Where jaundice is an issue, enzyme inductors such as carbamazepine and phenobarbital can reduce unconjugated bilirubin levels and may relieve other associated symptoms of Gilbert's syndrome. Furthermore, it can be minimized by slightly modifying the person's diet.

History

Gilbert's syndrome was first described by French gastroenterologist Augustin Nicolas Gilbert and co-workers in 1901. In German literature, it is commonly associated with Jens Einar Meulengracht.

Alternative, less common names for this disorder include:

Notable cases