Developmental onset of bilirubin-induced neurotoxicity involves Toll-like receptor 2-dependent signaling in humanized UDP-glucuronosyltransferase1 mice.

Biological and signaling events that connect developmentally induced hyperbilirubinemia to bilirubin-induced neurological dysfunction (BIND) and CNS toxicity in humans are poorly understood. In mammals, UDP-glucuronosyltransferase 1A1 (UGT1A1) is the sole enzyme responsible for bilirubin glucuronidation, a rate-limiting step necessary for bilirubin metabolism and clearance. Humanized mice that express the entire UGT1 locus (hUGT1) and the UGT1A1 gene, develop neonatal hyperbilirubinemia, with 8-10% of hUGT1 mice succumbing to CNS damage, a phenotype that is presented by uncontrollable seizures. We demonstrate that neuroinflammation and reactive gliosis are prominent features of bilirubin brain toxicity, and a disturbed redox status resulting from activation of NADPH oxidase is an important contributing mechanism found in BIND. Using knock-out mice and primary brain cells, we connect a key pattern recognition receptor, Toll-like receptor 2 (TLR2), to hyperbilirubinemia-induced signaling. We illustrate a requirement for TLR2 signaling in regulating gliosis, proinflammatory mediators, and oxidative stress when neonatal mice encounter severe hyperbilirubinemia. TLR2-mediated gliosis strongly correlates with pronounced neuroinflammation in the CNS with up-regulation of TNFα, IL-1ß, and IL-6, creating a pro-inflammatory CNS environment. Gene expression and immunohistochemistry staining show that hUGT1/Tlr2(-/-) mice fail to activate glial cells, proinflammatory cytokines, and stress response genes. In addition, bilirubin-induced apoptosis was significantly enhanced by blocking TLR2 signaling indicating its anti-apoptotic property. Consequently, a higher neonatal death rate (57.1%) in hUGT1/Tlr2(-/-) mice was observed when compared with hUGT1 mice (8.7%). These results suggest that TLR2 signaling and microglia neuroinflammation are linked to a repair and/or protection mode against BIND.

Vigintiphobia revisited.

In this review the historical tenets and evidence-based clinical research in support of a bilirubin exchange threshold of >20 mg/dL for the healthy term neonate are revisited. In addition, a hypothesis is ventured that recent cases of kernicterus are related in part to changes in population factors coupled with genetic predispositions that have unmasked an unappreciated potential for marked neonatal hyperbilirubinemia.

Lack of predictive indices in kernicterus: a comparison of clinical and pathologic factors in infants with or without kernicterus.

A review of 398 neonatal autopsies at Downstate Medical Center revealed 27 cases of kernicterus during the seven-year period from 1971 through 1977. With the current intensive care of the sick newborn, kernicterus continues to occur, mainly in premature infants with relatively low levels of serum bilirubin (mean of 11.5 mg/100 ml). To understand the factors contributing to the development of kernicterus, clinical and pathologic findings in 27 infants with kernicterus were compared to 103 "control" infants with retrospectively. Birth weight, gestational age, sex, and Apgar scores were comparable in both groups. The duration of survival was significantly shorter in infants with kernicterus than in the control infants. The clinical signs and symptoms of kernicterus were nonspecific and the premortem diagnosis of kernicterus was not suspected in most of the cases. There were no significant differences in the peak serum bilirubin values, incidence of hypothermia, hypoglycemia, convulsions, anemia, infection, use of phototherapy, transfusion and exchange transfusion in the two groups. Serum albumin values and bilirubin binding capacity measured by 2-(4-hydroxybenzeneazo)benzoic acid were significantly lower in the kernicteric group although the bilirubin-albumin molar ratio was equal in both groups. The incidences of severe acidosis and hypoxic encephalopathy were significantly higher in the kernicteric infants. In this study, acidosis, hypoxia, hypoalbuminemia, and low bilirubin binding capacity were seen more often in kernicteric infants than in control infants. However, analysis of previously suggested risk factors failed to identify any single factor or combination of factors which could be predictive to the development of kernicterus.