This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g. "bilirubin_conjugation_transport#Hyperbilirubinaemia"). The key conformance target is the central_effector node "Hyperbilirubinaemia" - the disorder-agnostic convergence point through which every conformer funnels, whichever of the two lesion arms it enters by.
ARM SELECTION IS THE CURATOR'S FIRST DECISION, and the two arms are not interchangeable. Enter at "UGT1A1-Dependent Bilirubin Glucuronidation Deficiency" only for a disorder with UNCONJUGATED hyperbilirubinaemia (Gilbert syndrome - reduced UGT1A1 promoter activity; Crigler-Najjar syndrome types I and II - coding mutations in UGT1A1; physiological and prolonged neonatal jaundice; haemolysis-driven bilirubin overload, which overwhelms rather than lesions the enzyme). Enter at "Conjugated Bilirubin Transport Failure" only for a disorder with CONJUGATED hyperbilirubinaemia (Dubin-Johnson syndrome - ABCC2/MRP2; Rotor syndrome - simultaneous SLCO1B1 and SLCO1B3 loss).
The downstream consequence node "Bilirubin Neurotoxicity and Kernicterus" is reachable ONLY from the unconjugated arm and only at severe elevations. A conforming entry on the transport arm must NOT declare conformance to it: Dubin-Johnson and Rotor syndromes are explicitly benign and non-progressive, and asserting a kernicterus risk for them would be a substantive error, not a conservative one.
Scope boundary against `cholestatic_liver_injury`. Conjugated hyperbilirubinaemia also arises in true cholestasis, where bile acid secretion fails alongside bilirubin export. That is a different disease process and belongs to the cholestatic module: the isolated transport lesions modelled here leave bile acid secretion intact, produce no pruritus, no bile acid retention, and no biliary fibrosis. Do not conform a cholestatic disorder to this module merely because its patients are jaundiced.
Can hepatocyte-directed gene replacement durably restore UGT1A1 activity and replace phototherapy and liver transplantation in Crigler-Najjar syndrome?
KNOWLEDGE GAP
ugt1a1_gene_replacement_unproven
Attached to:
UGT1A1-Dependent Bilirubin Glucuronidation Deficiency
The only curated treatment in this module acts on the downstream hyperbilirubinaemia node and bypasses rather than repairs the enzymatic lesion, and the review literature explicitly frames both standard treatments as having major disadvantages that motivate new strategies. Gene replacement and hepatocyte transplantation target the lesion node itself, but long-term engraftment of transplanted cells remains elusive and vector safety and immunotolerance to the transgene product are unresolved. Conforming entries should curate these as investigational, not established.
Proposed experiments:
Long-term durability study of UGT1A1 gene transfer
Haem Catabolism and the Unconjugated Bilirubin Load
trigger
Unconjugated bilirubin is the final product of haem catabolism, generated chiefly from senescent erythrocyte haemoglobin. Being poorly water-soluble it circulates bound to albumin and must be taken up across the sinusoidal membrane of the hepatocyte before it can be disposed of. This node sets the substrate load the downstream disposal machinery must clear; it is raised above normal by haemolysis and by the physiological erythrocyte turnover of the newborn, which is why those states unmask a marginal conjugation capacity.
Downstream
-
UGT1A1-Dependent Bilirubin Glucuronidation Deficiency
The unconjugated bilirubin load must be glucuronidated by UGT1A1 before it can be excreted; a deficiency of that enzyme is the first lesion arm.
-
Conjugated Bilirubin Transport Failure
When conjugation is intact, the resulting bilirubin glucuronides must be exported and recaptured by canalicular and sinusoidal transporters; loss of those transporters is the second lesion arm.
Show evidence (2 references)
"Jaundice results from the systemic accumulation of bilirubin, the final product of the catabolism of haem."
Establishes bilirubin as the terminal product of haem catabolism and systemic accumulation as the route to the module's clinical readout. Evidence source is OTHER because this is a review of clinical features, pathophysiology, and genetics.
"Under normal conditions, unconjugated bilirubin is taken up into hepatocytes by transporters of the organic anion-transporting polypeptide (OATP) family, followed by conjugation with glucuronic acid, and ATP-dependent transport into bile."
States the normal three-step disposal pathway - uptake, conjugation, canalicular export - whose successive failure defines the two lesion arms of this module. Evidence source is OTHER because this is a review of hepatobiliary transport physiology.
UGT1A1-Dependent Bilirubin Glucuronidation Deficiency
amplifier
UGT1A1 is the enzyme responsible for glucuronidating bilirubin and so for converting it into a water-soluble, excretable form. Reduced UGT1A1 activity is the lesion of the unconjugated arm. The severity is graded by how much activity remains: reduced promoter activity, as in the common Gilbert genotype, leaves enough capacity for a mild and benign elevation, whereas coding mutations in UGT1A1 abolish or severely reduce activity and produce the Crigler-Najjar phenotype. Conforming entries substitute their specific UGT1A1 lesion here.
Downstream
-
Hyperbilirubinaemia
Bilirubin that cannot be glucuronidated accumulates in plasma in the unconjugated form.
Show evidence (3 references)
"Uridine diphospho-glucuronosyl transferase 1A1 (UGT1A1) is responsible for the glucuronidation of bilirubin; deficiency of this enzyme results in unconjugated hyperbilirubinaemia."
States the enzyme, its reaction, and the specific biochemical phenotype its deficiency produces - the defining claim of this arm. Evidence source is OTHER because this is a review.
"Gilbert syndrome is the mild and benign form of inherited unconjugated hyperbilirubinaemia and is mostly caused by reduced promoter activity of the UGT1A1 gene."
Documents the mild end of the severity gradient and its distinct molecular basis (promoter rather than coding lesion), the substitution the Gilbert syndrome entry makes at this node. Evidence source is OTHER because this is a review.
"The underlying problem in both conditions is impaired bilirubin conjugation and elimination due to a mutation in uridine 5'-diphosphate glucuronyltransferase."
Confirms that Gilbert and Crigler-Najjar syndromes share this single node as their common lesion, differing in degree - which is why they are modelled as one arm rather than two. Evidence source is OTHER because this is a review.
Conjugated Bilirubin Transport Failure
amplifier
When conjugation is intact but the handling of the conjugates fails, bilirubin glucuronides accumulate in plasma. Two non-overlapping transporter lesions produce this. Loss of the ATP-dependent canalicular pump MRP2 (ABCC2) blocks export of mono- and bisglucuronosyl bilirubin into bile; the basolateral pump MRP3, upregulated in that setting, then diverts the conjugates into sinusoidal blood. Loss of both sinusoidal reuptake transporters OATP1B1 and OATP1B3 instead prevents recapture of the conjugates that normally re-enter the hepatocyte. Both lesions are benign and non-progressive - a fact conforming entries must preserve.
Downstream
-
Hyperbilirubinaemia
Bilirubin conjugates that can be neither exported into bile nor recaptured from sinusoidal blood accumulate in plasma in the conjugated form.
Show evidence (4 references)
"Dubin-Johnson syndrome is caused by a defect in the ATP-dependent canalicular transporter, multidrug resistance-associated protein 2 (MRP2), which mediates the export of conjugated bilirubin into bile."
Names the canalicular export lesion and the transporter involved, the substitution a Dubin-Johnson syndrome entry makes at this node. Evidence source is OTHER because this is a review.
"In patients with Rotor syndrome, bilirubin (re)uptake is impaired due to the deficiency of two basolateral/sinusoidal hepatocellular membrane proteins, organic anion-transporting polypeptide 1B1 (OATP1B1) and OATP1B3."
Names the sinusoidal reuptake lesion, mechanistically distinct from the canalicular one though producing the same biochemical pattern. Evidence source is OTHER because this is a review.
"Under pathophysiological conditions such as cholestatic liver injury and MRP2 inhibition, the basolateral efflux pump multidrug resistance protein 3 (MRP3 or ABCC3) is responsible for the occurrence of conjugated hyperbilirubinemia."
Supplies the mechanism by which a canalicular export block actually raises plasma conjugated bilirubin - MRP3-mediated basolateral diversion - rather than merely trapping pigment in the hepatocyte. Evidence source is OTHER because this is a review.
+ 1 more reference
Hyperbilirubinaemia
central effector
Systemic accumulation of bilirubin, producing jaundice and scleral icterus. This is the disorder-agnostic convergence node of the module and its key conformance target: both lesion arms terminate here, and the conjugation state of the accumulated pigment is what discriminates between them diagnostically. Conforming entries should record which fraction predominates, because that assignment determines whether the downstream neurotoxicity node is reachable at all.
Used by disorders
Rotor Syndrome
as Failure of Hepatic Bilirubin Conjugate Reuptake and Storage
Downstream
-
Bilirubin Neurotoxicity and Kernicterus
Reachable only from the unconjugated arm, and only at severe elevations: unconjugated bilirubin in excess of albumin binding capacity crosses the blood-brain barrier. Conjugated hyperbilirubinaemia does not take this edge.
Show evidence (3 references)
"Bilirubin, a breakdown product of heme, is normally glucuronidated and excreted by the liver into bile. Failure of this system can lead to a buildup of conjugated bilirubin in the blood, resulting in jaundice."
States the convergence this node represents - failure anywhere in the conjugate-and-excrete system produces the same clinical endpoint. Evidence source is OTHER because this is a narrative review.
"Both disorders are benign and not progressive and are characterised by elevated serum levels of mainly conjugated bilirubin."
Records the conjugated pattern and the benign, non-progressive course of the transport arm - the guardrail that stops a conforming Dubin-Johnson or Rotor entry from inheriting the neurotoxicity consequence. Evidence source is OTHER because this is a review.
"Laboratory tests were notable for hyperbilirubinemia (total 7.97 mg/dL, direct 5.37 mg/dL), bilirubinuria, no signs of hemolysis, normal liver tests and lipids profile."
A worked human example of the conjugated pattern at this node with otherwise normal liver tests, illustrating that the transport arm is an isolated bilirubin-handling defect rather than liver disease. Evidence source is HUMAN_CLINICAL because this is a patient case report.
Bilirubin Neurotoxicity and Kernicterus
consequence
At high concentrations, unconjugated bilirubin that exceeds albumin binding capacity crosses the blood-brain barrier and deposits in the basal ganglia and brainstem nuclei, where it is neurotoxic. The resulting acute bilirubin encephalopathy and its chronic sequela, kernicterus, cause severe permanent disability and can be fatal when untreated. This node is the reason Crigler-Najjar syndrome is a medical emergency of the newborn while the other hereditary hyperbilirubinaemias are not, and it is the endpoint that phototherapy exists to prevent.
Show evidence (2 references)
"Crigler-Najjar syndrome is the severe inherited form of unconjugated hyperbilirubinaemia due to mutations in the UGT1A1 gene, which can cause kernicterus early in life and can be even lethal when left untreated."
Establishes that this consequence node is reached specifically from the severe end of the unconjugated arm, and that it is lethal without treatment. Evidence source is OTHER because this is a review.
"Crigler-Najjar syndrome (CN), a rare inherited disorder characterized by failure of bilirubin glucuronidation, can lead to severe disability and death from kernicterus."
Independently confirms the causal chain from glucuronidation failure to kernicterus and its outcome. Evidence source is OTHER because this is a review.
Phototherapy
Platform:
Device
Phototherapy converts unconjugated bilirubin in the skin into water-soluble photoisomers that can be excreted in bile and urine without glucuronidation, thereby bypassing the missing UGT1A1 step rather than restoring it. It acts on the hyperbilirubinaemia convergence node to keep plasma bilirubin below the threshold for neurotoxicity, and it is the mainstay of Crigler-Najjar management. This is a containment therapy, not a cure: the underlying enzymatic lesion is untouched, its efficacy falls as body surface area to mass ratio declines with age, and definitive treatment is liver transplantation. It applies only to the unconjugated arm and confers no benefit in the transport-lesion disorders, which need no treatment.
Evidence: 1