Bilirubin, Impaired Conjugation 

Bilirubin is the potentially toxic catabolic product of heme metabolism. There are elaborate physiologic mechanisms for its detoxification and disposition. Understanding these mechanisms is necessary for interpretation of the clinical significance of high serum bilirubin concentrations.

In adults, 250-400 mg of bilirubin is produced daily. Approximately 70-80% of daily bilirubin is derived from degradation of the heme moiety of hemoglobin. The remaining 20-25% is derived from the hepatic turnover of heme proteins, such as myoglobin, cytochromes, and catalase. A small portion of daily bilirubin is derived from the destruction of young or developing erythroid cells.

Bilirubin is poorly soluble in water at physiologic pH because of internal hydrogen bonding that engages all polar groups and gives the molecule an involuted structure. The fully hydrogen-bonded structure of bilirubin is designated bilirubin IX-alpha-ZZ. The intramolecular hydrogen bonding shields the hydrophilic sites of the bilirubin molecule, resulting in a hydrophobic structure. Water-insoluble unconjugated bilirubin is associated with all known toxic effects of bilirubin. Thus, the internal hydrogen bonding is critical in producing bilirubin toxicity and also prevents its elimination.

Conversion of bilirubin IX-alpha to a water-soluble form by disruption of the hydrogen bonds is essential for elimination by the liver and kidney. This is achieved by glucuronic acid conjugation of the propionic acid side chains of bilirubin. Bilirubin glucuronides are water-soluble and are readily excreted in bile. Bilirubin is primarily excreted in normal human bile as diglucuronide; unconjugated bilirubin accounts for only 1-4% of pigments in normal bile.

Unconjugated bilirubin is transported in the plasma bound to albumin. At the sinusoidal surface of the liver, unconjugated bilirubin detaches from albumin and is transported through the hepatocyte membrane by facilitated diffusion. Within the hepatocyte, bilirubin is bound to 2 major intracellular proteins, cytosolic Y protein (ie, ligandin or glutathione S-transferase B) and cytosolic Z protein (also known as fatty acid–binding protein [FABP]). The binding of bilirubin to these proteins decreases the efflux of bilirubin back into the plasma and, therefore, increases net bilirubin uptake.

In order for bilirubin to be excreted into bile and, therefore, eliminated from the body, it must be made more soluble. This water-soluble or conjugated form of bilirubin is produced when glucuronic acid enzymatically is attached to one or both of the propionic side chains of bilirubin IX-alpha (ZZ). Enzyme-catalyzed glucuronidation is one of the most important detoxification mechanisms of the body. Of the various isoforms of the UGT family of enzymes, only one isoform, bilirubin-UGT-1 (BUGT1), is physiologically important in bilirubin glucuronidation.

This attachment occurs through an ester linkage and, therefore, is called esterification. This esterification is catalyzed by the microsomal enzyme bilirubin uridine-diphosphate glucuronosyltransferase (ie, bilirubin-UGT), which is located in the endoplasmic reticulum of the hepatocyte. This reaction leads to the production of water-soluble bilirubin monoglucuronide and bilirubin diglucuronide. Other compounds, such as xylose and glucose, also can undergo esterification with bilirubin.

Bilirubin diglucuronide is the predominant pigment in healthy adult human bile, representing over 80% of the pigment. However, in subjects with reduced bilirubin-UGT activity, the proportion of bilirubin diglucuronide decreases, and bilirubin monoglucuronide may constitute more than 30% of the conjugates excreted in bile. Reduction of conjugating enzyme activity to approximately 30% of normal results in a mild but discernible increase in serum bilirubin concentrations. This conjugation reaction is essential for bilirubin excretion into bile.

Deficiency of bilirubin-UGT leads to ineffective esterification of bilirubin, which, in turn, results in an unconjugated hyperbilirubinemia. Reduced bilirubin conjugation as a result of a decreased or absent UDP-glucuronosyltransferase activity is found in several acquired conditions and inherited diseases, such as Crigler-Najjar syndrome (types I and II) and Gilbert syndrome. Bilirubin conjugating activity is also very low in the neonatal liver.

UGT activity toward bilirubin is modulated by various hormones. Excess thyroid hormone and ethinyl estradiol, but not other oral contraceptives, inhibit bilirubin glucuronidation. In comparison, the combination of progestational and estrogenic steroids results in increased enzyme activity. As some variants of UGT are expressed in the uterus and are involved in the conjugation and excretion of estrogens, McGrath et al hypothesized whether these UGT variants may be associated with endometrial cancer.^[1]

In a case control study nested within the Nurses' Health Study and the Women's Health Study, the investigators studied the association between 5 polymorphisms and endometrial cancer risk using 593 invasive endometrial cancer cases and 1545 healthy controls.^[1] They observed an inverse association between UGT1A1*28 and endometrial cancer risk but no significant associations between individual single nucleotide polymorphisms (SNPs) and UGT1A1 haplotypes and risk of endometrial cancer.

Bilirubin glucuronidation can also be inhibited by certain antibiotics (eg, novobiocin or gentamicin at serum concentrations exceeding therapeutic levels) and by chronic hepatitis, advanced cirrhosis, and Wilson disease. This area was further examined by Saeki et al^[2] and Saito et al.^[3] These investigators reported a strong association between 5 SNPs within UGT1A1 and total bilirubin levels. Furthermore, results of linear multiple regression analysis on total bilirubin levels followed by analysis of variance showed that at least 13% of the variance in bilirubin levels could be explained by 3 haplotype-tagging SNPs in the UGT1A1 gene.

Three primary diseases result from abnormal bilirubin-UGT production. The severity of the resulting illness depends on the degree of bilirubin-UGT deficiency.

Crigler-Najjar syndrome, also referred to as congenital nonhemolytic jaundice with glucuronosyltransferase deficiency, is a rare, autosomal recessive disorder of bilirubin metabolism. It has been divided into 2 distinct forms (types I and II) based upon the severity of the disease. The molecular defect in Crigler-Najjar syndrome can be caused by a variety of alterations in the coding sequences of the bilirubin-uridine diphosphate glucuronosyltransferase (UGT1A1) gene, and these mutations lead to the production of an abnormal protein, resulting in complete loss or very low levels of hepatic bilirubin-UGT (UGT1A1) activity.

In contrast, the defect in Gilbert syndrome is in the promoter region, the TATAA element, rather than in the gene itself; as a result, reduced amounts of the normal protein are produced.

The absence of bilirubin-UGT leads to Crigler-Najjar type I syndrome. Crigler-Najjar type I syndrome is a disease that causes kernicterus in infants, which usually leads to death. Crigler-Najjar type II syndrome results from decreased levels of bilirubin-UGT. Kernicterus usually does not develop in this disease; therefore, the long-term prognosis is better.

Gilbert syndrome results from decreased levels of bilirubin-UGT. Gilbert syndrome results in a mild hyperbilirubinemia without any clinical sequelae. The long-term prognosis is good.

Other disease states that are seen in the neonatal period consist of physiologic jaundice and breast milk jaundice.

Physiologic jaundice is a mild unconjugated hyperbilirubinemia that affects nearly all newborns and resolves within the first several weeks after birth. It is caused by increased bilirubin production, decreased bilirubin clearance, and increased enterohepatic circulation.

It has been shown that bilirubin production in a term newborn is 2-3 times higher than in adults. This increased production is due to the shorter life span and the greater turnover of neonatal red blood cells. Bilirubin clearance is decreased in newborns, mainly due to the deficiency of the enzyme UGT. UGT activity in term neonates is approximately 1% of that in adults. Also, newborns have fewer intestinal bacteria than adults, resulting in a decreased capacity to reduce bilirubin to urobilinogen and subsequent higher intestinal bilirubin concentrations. In addition, the activity of beta-glucuronidase also is increased, which leads to greater hydrolysis of conjugated to unconjugated bilirubin. The unconjugated bilirubin is reabsorbed from the intestine through the process of enterohepatic circulation, further increasing the bilirubin load in the infant.

Bilirubin and drug metabolism in neonates may also be affected by the influences of ethnicity on UGT1A1 haplotype mutations. A cohort study of 241 consecutive term Asian infants reported that not only was there a variance in the prevalence of hypomorphic haplotypes but the frequency also varied between the different races.^[4] For example, Indian neonates were most likely to have at least one hydromorphic haplotype (64%) compared with Chinese (48%) and Malay neonates (31%).There was also a trend between the number of G71R mutations and the need for phototherapy.

The peak total serum bilirubin level in physiologic jaundice typically is 5-6 mg/dL (86-103 µmol/L), occurs at 48-120 hours of age, and does not exceed 17-18 mg/dL (291-308 µmol/L). Higher levels of unconjugated hyperbilirubinemia are pathologic and occur in various conditions, as previously mentioned.

Breast milk jaundice results from increased enterohepatic circulation. It is thought to result from an unidentified component of human milk that enhances intestinal absorption of bilirubin. One possible mechanism for hyperbilirubinemia in breast-fed infants compared to formula-fed infants is the increased concentration of beta-glucuronidase in breast milk. Beta-glucuronidase deconjugates intestinal bilirubin, increasing its ability to be absorbed (ie, increasing enterohepatic circulation). Blocking the deconjugation of bilirubin through beta-glucuronidase inhibition may provide a mechanism to reduce intestinal absorption of bilirubin in breast-fed infants; however, this has yet to be proven.

Frequency

United States

The frequency of the various diseases resulting from impaired bilirubin conjugation is based on the disease process. Crigler-Najjar type I syndrome is rare in the US population; reported cases number only in the hundreds. Crigler-Najjar type II syndrome also is an uncommon disease.

Gilbert syndrome affects approximately 3-7% of the population.

Breast milk jaundice affects approximately 0.5-2.4% of live births, and there is a familial incidence of 13.9%, indicating that, in some cases, a unique genetic factor may be expressed.

Mortality/Morbidity

The various diseases resulting from impaired bilirubin conjugation have differing prognoses.

• The Crigler-Najjar type I syndrome usually results in death during infancy, with only a few patients surviving to adolescence. The cause of death is encephalopathy from kernicterus.
• Crigler-Najjar type II syndrome results in elevated unconjugated bilirubin levels, but patients with Crigler-Najjar type II syndrome tend to live into old age and are not at risk for kernicterus. The morbidity associated with the disease is low. Many patients with Crigler-Najjar type II syndrome are unaffected by this condition.
• Gilbert syndrome is a benign disorder associated with no increase in mortality or morbidity. Patients who are affected are anicteric, with a normal life expectancy. Patients usually do not experience complications from their hyperbilirubinemia. Fasting, febrile illness, alcohol, or exercise can exacerbate jaundice in patients with Gilbert syndrome. Hemolysis and mild icterus usually occur at times of stress, starvation, and infection.
• However, clinicians need to be aware that patients with Gilbert syndrome may be at higher risk of developing toxicity from certain medications such as irinotecan and protease inhibitors such as azatanavir and indinavir that can inhibit UGY metabolism.^[5] A study by Lankisch et al reported that the risk of severe hyperbilirubinemia with indinavir was associated with genetic variants of UGT1A3 abd UGT1A7 genes in addition to Gilbert syndrome (UGT1A1*28).^[6] Kweekel et al also reported that patients who were more likely to develop side effects of irinotecan toxicity such a life-threatening neutropenia and diarrhea were more likely to have underlying liver disease, hepatic conjugation disorders, or UGT1A1*28 genotype.^[7] However, due to a lack of prospective data, the relationship between the UGT1A1 genotype and irinotecan toxicity remains unclear, although the irinotecan product label recommends reducing irinotecan dose in patients with this genotype.
• Physiologic jaundice is a benign disorder as well.
• Breast milk jaundice is also a benign disorder. The bilirubin levels need to be checked to avoid complications of hyperbilirubinemia. However, late neurodevelopment or hearing defects have not been observed in neonates, thus enabling the pediatrician to encourage continuation of breastfeeding in most cases of healthy infants with breast milk jaundice.

Race

In Gilbert syndrome, differences exist in the mutation of the UGT1A1 gene in certain ethnic groups. As mentioned previously, the TATAA element in the promoter region is the most common site of mutation in the Caucasian population.

No racial predilection exists for Crigler-Najjar syndrome.

A racial variation exists in the development of neonatal jaundice. A common mutation in the UGT gene (Gly71Arg) leads to an increased incidence of severe neonatal hyperbilirubinemia (approximately 20%) in Asians.

Sex

No sexual predilection exists in Crigler-Najjar type I and II syndromes. In Gilbert syndrome, males are affected more frequently than females. Neonatal physiologic jaundice is increased in males. Breast milk jaundice is seen equally in males and females.

Age

All of the diseases are inherited and, therefore, are present from birth. The age at which symptoms appear may vary.

• In Crigler-Najjar type I syndrome, symptoms appear within the first few days of life, and most patients die in infancy. A few patients have survived into adolescence.
• In Crigler-Najjar type II syndrome, patients have jaundice during the first few years of life.
• Patients with Gilbert syndrome usually are diagnosed when aged 10-30 years.
• Physiologic jaundice affects nearly all newborns, occurs in the first 2-5 days, and resolves within the first several weeks after birth.
• Breast milk jaundice typically begins after the first 3-5 days, peaks within 2 weeks after birth, and progressively declines to normal levels over 3-12 weeks.