Abstract
Hyperbilirubinemia (also known as jaundice), caused by the accumulation of unconjugated bilirubin, is one of the most common clinical diagnoses in both premature and term newborns. Owing to the fact that bilirubin is metabolized solely through glucuronidation by UDP-glucuronosyltransferase (UGT) 1A1, it is now known that immaturity of UGT1A1 in combination with overproduction of bilirubin during the developmental stage acts as a bottleneck to bilirubin elimination and predisposes the infant to high TBS levels. While neonatal jaundice is mostly benign, excessively high levels of serum bilirubin in a small percentage of newborns can cause acute or chronic bilirubin-induced neurologic dysfunction (BIND), potentially progressing to acute encephalopathy leading to irreversible brain damage and death. As a series of hereditary UGT1A1 mutations have been identified that are associated with UGT1A1 deficiency, new evidence has verified that developmental expression of UGT1A1 is a tightly controlled event, and both genetic polymorphisms and developmental regulation of UGT1A1 are major contributing factors determining the severity of hyperbilirubinemia and bilirubin-induced neurotoxicity. This review recapitulates the progress that has been made in recent years in understanding etiology and physiopathology of severe hyperbilirubinemia, investigating molecular mechanisms underlying bilirubin-induced encephalopathy, and searching for potential therapies for treating pathologic hyperbilirubinemia. Several animal models have been developed to make it possible to examine bilirubin-induced neurotoxicity from multiple directions. Moreover, environmental factors that may alleviate or worsen the condition of hyperbilirubinemia are discussed.
Highlights
Newborn hyperbilirubinemia or jaundice, characterized by elevated levels of total serum bilirubin (TSB), is one of the most common clinical diagnoses in neonates, especially in preterm babies
By using hUGT1A1*28 mice, studies have shown that reduction of liver UDPglucuronosyltransferase 1A1 (UGT1A1) gene expression in the developmental stage, which corresponds to the onset of hyperbilirubinemia and high levels of TSB, is actively regulated by pregnane X receptor (PXR)
Results of experiments with Ugt1 mutant mice harboring a mutation in the Ugt1a1 gene showed that accumulated bilirubin in the brain leading to neurotoxicity is manifested as cerebellar abnormalities, hypoplasia, and neuronal cell death
Summary
Newborn hyperbilirubinemia or jaundice, characterized by elevated levels of total serum bilirubin (TSB), is one of the most common clinical diagnoses in neonates, especially in preterm babies. By using hUGT1A1*28 mice, studies have shown that reduction of liver UGT1A1 gene expression in the developmental stage, which corresponds to the onset of hyperbilirubinemia and high levels of TSB, is actively regulated by PXR.
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