Crigler-Najjar Syndrome

Etiology

Crigler-Najjar syndrome is caused by an absence or decrease level of the enzyme UDP-glucuronosyltransferase due to a genetic defect in the bilirubin-uridine diphosphate glucuronosyltransferase (UGT1A1) gene.[1] The common mutations in type I Crigler-Najjar syndrome is a deletion, alterations in intron splice donor and receptor sites, missense mutation, exon skipping, insertion, or the formation of a stop codon within the UGT1A1 gene leading to complete deficiency of enzyme UDP-glucuronosyltransferase.[2] On the other hand, type II Crigler-Najjar syndrome results from a point mutation in the UGT1A1 gene resulting in decreased production of enzyme UDP-glucuronosyltransferase.[2][3]

Epidemiology

Crigler-Najjar syndrome is a rare disease. It is known to affect 0.6 to 1 in 1 million newborns around the world.[4][5]

Pathophysiology

During heme catabolism, one of the by-products produced is biliverdin. The enzyme biliverdin reductase then converts biliverdin into bilirubin. This bilirubin is unconjugated and is fat-soluble, hence requires its conjugation for its excretion. So, unconjugated bilirubin is carried to the liver by albumin. UDP-glucuronosyltransferase (UGT) enzyme converts unconjugated bilirubin into conjugated bilirubin via the process of glucuronidation in the liver. Genetic mutation is the root mechanism of this disease involving the impairment of conjugation. In the absence of this enzyme, as in Crigler-Najjar syndrome type I, the levels of unconjugated bilirubin will rise, leading to jaundice. Unconjugated bilirubin is water-insoluble, and high levels can deposit in the brain leading to a condition called kernicterus. Severe hyperbilirubinemia usually develops in the first few days of life. Type II Crigler-Najjar syndrome is characterized by decreased activity of UGT, making it less severe. Although some affected infants develop severe hyperbilirubinemia, their jaundice responds well to treatment with phenobarbital.

Unconjugated bilirubin gets accumulated in the plasma. It is eliminated very slowly in small amounts by alternative pathways that consist of direct passage into the bile and small intestine. It cannot be excreted through the urine as it is water-insoluble. These factors contribute to the excessive accumulation of indirect bilirubin in the body resulting in deposition in body tissues such as skin, sclera, brain, etc. At normal physiologic body pH, unconjugated bilirubin binds sufficiently to albumin. But in the absence of conjugation and excretion, the amount of unconjugated bilirubin overwhelms the amount of albumin present in the body. Hence the amount of free (unbound) unconjugated bilirubin rises in the blood. Typically, about 99% of bilirubin is bound to albumin, which doesn't interact with cerebral endothelial. In contrast, free (unbound) unconjugated bilirubin easily crosses the blood-brain barrier and interacts with neural tissue.[6] There are also reports of hepatic parenchymal injury in long term cases. The outcome of long-standing disease has led to liver cirrhosis in many cases.

Histopathology

Previously hepatic histology was considered normal in Crigler-Najjar syndrome. However, recent studies show some pathological changes in histology. Microscopically, bile plugs can be seen occasionally within canaliculi as a result of cholestasis. Crigler-Najjar syndrome can have a hepatic parenchymal injury. A small study reported transplant patients with significant fibrosis seen in the explants.[7][8]

History and Physical

In Crigler-Najjar type I, the newborn may present with jaundice, but the absence of it does not rule out the disease as asymptomatic cases are evident. In healthy born children, persistent jaundice due to unconjugated bilirubin develop shortly within a few days and progress rapidly by the second week. Family history of consanguinity, severe jaundice without evidence of hemolysis, relatives, or family members with exchange transfusion therapy, and liver diseases support the diagnosis. Unconjugated hyperbilirubinemia usually ranges from 20 to 25 mg/dL in type I Crigler-Najjar syndrome. However, significantly elevated unconjugated bilirubin levels up to 50 mg/dL have been reported. Severe unconjugated hyperbilirubinemia (TB >30) can lead to bilirubin-induced neurologic dysfunction (BIND).

On examination, physical findings of jaundice are evident, i.e., yellowish skin and sclera. The stool color is normal. However, there is decreased excretion of fecal urobilinogen due to significantly reduced bilirubin conjugation. Apart from jaundice, other findings are normal in both types with no signs of liver disease. Older children can have scratch marks in the body due to pruritus. In some cases, progressive liver dysfunction and various superimposed toxicities cause hepatosplenomegaly.

BIND results from the toxic effects of bilirubin on the basal ganglia and the brainstem nuclei for oculomotor and auditory function. BIND can be acute or chronic. Acute bilirubin encephalopathy (ABE) initially presents with sleepiness, mild to moderate hypotonia, and a high pitched cry. Persistence of elevated bilirubin levels leads to the progression of ABE that can manifest as fever, lethargy, poor suck, irritability, hypertonia leading to retrocollis, and opisthotonus. Advanced ABE can present with seizures, apnea, and a semi-comatose or comatose state. Death may result from respiratory failure or seizures. Brainstem auditory-evoked responses (BAER) can also help detect acute neurological dysfunction.[9]

Chronic bilirubin encephalopathy (CBE, previously called kernicterus) manifests during the first year of life. CBE presents with choreoathetoid cerebral palsy, sensorineural hearing loss, gaze abnormalities, and enamel hypoplasia. Cognitive function is usually not affected. Brain MRI in these cases demonstrates changes in the cerebellum, hippocampus, and brainstem.[10][11]

Crigler-Najjar type II is the less severe form with less manifestation of clinical symptoms. However, reports exist of bilirubin induced encephalopathy.

Evaluation

Clinically suspected cases of Crigler-Najjar syndrome should undergo serum unconjugated bilirubin level testing. The level of unconjugated bilirubin is high in type I as compared to type II disease. In Crigler-Najjar syndrome type I, the level of unconjugated bilirubin is between 20 to 25 mg/dL, but severe cases can be around 50 mg/dL. In type II, it is usually less than 20 mg/dL. Bile is collected from the duodenum using upper gastrointestinal endoscopy or an orally placed duodenal catheter and analyzed for bilirubin glucuronides by high-performance liquid chromatography (HPLC).

Conjugated bilirubin is either absent or present in trace amount in Crigler-Najjar syndrome type I. In contrast, a significant amount of conjugated bilirubin is present in Crigler-Najjar syndrome type II disease. HPLC of bile obtained from duodenum is the definitive test. Tissue enzyme assay of a liver biopsy can help measure the UGT enzyme level. Urine analysis of bilirubin is absent in Crigler-Najjar type I. Liver function tests usually reveal a normal range of liver enzymes; however, these enzymes may become elevated due to cholestasis.

Phenobarbital administration for two weeks decreases the serum bilirubin concentration in most patients with type II Criggller-Najjar syndrome.[12] Type I disease does not respond to phenobarbital treatment. 

Genetic analysis can be performed in DNA extracted from peripheral blood leukocytes, and buccal scraping, as well as other tissues, and detects the types of mutation in the gene encoding UGT1A1 enzyme. Prenatal diagnosis is possible with genetic analysis of chorionic villus samples or amniotic cells aspirated in amniotic fluid. Diffusion tensor imaging of the brain may help to detect the microstructural gray and white matter changes in Crigler-Najjar type I syndrome.[13] Liver biopsy and histopathologic analysis can help to evaluate the liver cirrhosis in established cases of hepatosplenomegaly.

Treatment / Management

The main target of treatment is a reduction in the level of unconjugated bilirubin using phototherapy and plasmapheresis. Most patients have survival rates beyond puberty without significant brain damage but, eventually, develop kernicterus later in life. The only curative option available presently for Crigler-Najjar type I syndrome is liver transplantation.[14] 

Phototherapy: The mainstay of treatment for Crigler-Najjar type I syndrome is intensive phototherapy. Phototherapy is often part of the treatment of neonatal hyperbilirubinemia.[15] Intensive phototherapy is more effective than conventional because it provides a more effective and quicker response. Intensive phototherapy also shortens the duration of treatment and reduces the late complications.[16] Phototherapy in older children and adults is less effective due to thicker skin, increased skin pigmentation, and less body surface area to body mass.[14]

Plasmapheresis: Plasmapheresis is the most effective process to remove the excess unconjugated bilirubin from the blood during severe hyperbilirubinemia crisis. Plasmapheresis is a process for removing any unwanted substance from the blood. During plasmapheresis, blood is removed from the affected patient, and blood cells are separated from plasma. The plasma is then replaced with the donor plasma, and the blood transfused back into the affected patient. As bilirubin is tightly bound to albumin, removal of albumin during this process leads to a reduction of bilirubin in the blood. 

Orlistat: It is a lipase inhibitor that works more efficiently in combination with calcium phosphate. It is hypothesized to capture the unconjugated intestinal bilirubin and helps in its excretion in proportion to the amount of fat excreted in stools.[17](B3)

Calcium phosphate supplementation: Patients who have undergone phototherapy in Crigler-Najjar type I and supplemented with calcium phosphate demonstrated a reduction of serum bilirubin level by 18%, suggesting that calcium phosphate traps photoproducts of bilirubin excreted in the bile. This hypothesis also has support from animal studies involving rats, that noted decreased serum bilirubin concentration after oral calcium phosphate supplementation, likely from trapping unconjugated bilirubin in the gut.[18](B3)

Liver transplantation: It is the only therapeutic and definitive treatment method in Crigler-Najjar type I. Transplanted liver has a healthy UGT1A1 enzyme for bilirubin conjugation, which rapidly reduces serum bilirubin level. Prophylactic liver transplantation is advisable to prevent kernicterus, as once it occurs, it may not be fully reversible.[19]

Hepatocyte transplantation: Hepatocyte transplantation is an encouraging alternative to liver transplantation. In this process, normal hepatocytes are infused into the portal vein or peritoneal space. It is the temporary method of serum bilirubin reduction with published reports of a 50% reduction in bilirubin level.[20][21](B3)

Gene therapy: Introduction of a normal UGT1A1 gene can potentially cure the genetic defect. It is achievable through ex vivo gene transduction (genes are transduced in cultured hepatocytes) and vector-mediated gene delivery. Adenovirus is the most effectively used vector for gene transfer to liver cells.[4] Plans for clinical trials in humans are presently underway. 

Inhibition of bilirubin production: A single dose of heme oxygenase inhibitors such as tin-protoporphyrin or tin-mesoporphyrin in neonates demonstrated a 76 percent decrease in the bilirubin levels and removed the need for phototherapy.[22][23] However, in adults, this effect is short-lived and hence reserved for acute emergencies.(A1)

Phenobarbitol: Patients with Crigler-Najjar syndrome usually do not require treatment with phenobarbital. However, the patients with jaundice affecting their quality of life receive therapy with phenobarbital, which works by induction of remaining UGT activity. Hence, phenobarbitol has no role in Crigler-Najjar type I. It reduces the serum bilirubin level by 25% in type II patients.[12] Similar benefits are met with clofibrate, but it is contraindicated in pregnancy.

Differential Diagnosis

Other causes of unconjugated hyperbilirubinemia differ from Crigler-Najjar syndrome based on the bilirubin levels and the duration of hyperbilirubinemia.

Increased Production

1. Hemolysis: Hemolysis can occur in sickle cell disease, hereditary spherocytosis, and Rh isoimmunization. The bilirubin level usually does not rise beyond 6 to 8 mg/dL in cases of exclusive hemolysis in newborns. Rh isoimmunization is the condition where fetal red cells are not compatible with the mother's red blood cells. It can also cause pronounced jaundice but is always associated with signs of active hemolysis.
2. Ineffective erythropoiesis: A marked increase in bilirubin level can occur in diseases associated with ineffective erythropoiesis such as thalassemia, pernicious anemia, iron deficiency anemia, lead poisoning, etc.
3. Gastrointestinal bleeding and hematoma: Gastrointestinal tract (GIT) bleeding is associated with a wide range of GI tract pathologies. One case report with hematochezia and jaundice reported polyp formation was responsible for rectal bleeding related to biliary abnormalities.[24] Hemobilia frequently has a link with GI bleeding, which contributes to the hyperbilirubinemia.[25] Resorption of blood from GI bleeding and hematoma correlates with an increased concentration of unconjugated bilirubin.

Decreased Clearance

1. Liver diseases: Since the liver is involved in conjugation of bilirubin for efficient excretion, liver dysfunction in any disease process can lead to jaundice.
2. Drug-induced: Various drugs can cause liver injury that impairs the conjugating capacity of the liver. Drugs such as amoxicillin, cefazolin, nitrofurantoin, chloramphenicol, isoniazid, etc.[26][27][26]
3. Gilbert syndrome: It is the most common form of inherited hyperbilirubinemia affecting the UGT1A1 gene. Gilbert syndrome results from the defect in the promoter region, rather than the UGT1A1 gene itself; hence leading to a reduced amount of normal protein production. Therefore, the deficiency of enzyme UDP-glucuronosyltransferase is not severe. It can be differentiated from Crigler-Najjar type I by its benign manifestations. 
4. Lucey-Driscoll syndrome: It is also known as transient familial hyperbilirubinemia. It appears to result from due to compounds in the mother's and baby's blood that block the breakdown of bilirubin.
5. Neonatal jaundice and prematurity: In many cases, it is physiologic, but can also result from liver dysfunction, infection, hypothyroidism, metabolic disorders, etc. Physiologic jaundice resolves within the first ten days of life. Higher bilirubin levels present in premature infants due to liver function immaturity.
6. Breast milk jaundice: In some breastfeeding neonates with elevated bilirubin within two weeks of birth, levels come down to a reasonable level over 3 to 12 weeks. It usually lasts longer than physiologic jaundice.

Rotor syndrome and Dubin-Johnson syndrome are two rare inherited disorders causing jaundice, but both entities have high levels of conjugated bilirubin. Rotor syndrome results from alterations in two genes, SLCO1B1 and SLCO1B3, while Dubin-Johnson syndrome results from mutations of the ABCC2 gene.

Prognosis

Crigler-Najjar type I syndrome has a poor prognosis. It may require emergent treatment during the hyperbilirubinemia crisis. Once a patient develops kernicterus, it may be irreversible. Due to some activity of the UGT enzyme, Crigler-Najjar type II syndrome presents with mild symptoms, and can even be asymptomatic, and has a better prognosis than type I.

Complications

Kernicterus: Unconjugated bilirubin is fat-soluble and can cross the cell membranes. At high serum levels, probably above 25 mg/dL, unconjugated bilirubin can cross the blood-brain barrier and cause neurotoxicity. It is more commonly seen in Crigler-Najjar type I syndrome. Still, patients with Crigler-Najjar type II syndrome are not spared, as in a few cases where serum bilirubin level may rise to 40 mg/dL. The most commonly affected areas of the brain include the hippocampus, geniculate bodies, basal ganglia, and nerve nuclei.[28] Kernicterus is preventable with early diagnosis and treatment.

Chronic jaundice: Persistence of disease and delays in diagnosis leads to chronic jaundice. The serum level of bilirubin increases progressively due to the limited excretion of bilirubin. At the later stages, a wide range of tissues have involvement, with significant deposition of bilirubin all over the body.

Cholelithiasis: Increased concentration of unconjugated bilirubin can lead to gallstone formation. As this process takes time, it usually appears later in life.

Liver cirrhosis: Liver cirrhosis develops over a long time. A child born with Crigler-Najjar syndrome has a healthy liver in most cases. The exact mechanism of liver cirrhosis is still unknown. However, it may result from chronic biliary obstruction secondary to cholelithiasis, toxicity from drugs primarily metabolized in the liver, toxicity from phototherapy, and heme degradation products.[29]