Congenital Bile Acid Synthesis Defect Type 2

Congenital bile acid synthesis defect type 2 (BAS defect type 2) is an anomaly of bile acid synthesis (see this term) characterized by severe and rapidly progressive cholestatic liver disease, and malabsorption of fat and fat-soluble vitamins.

Epidemiology

Prevalence is unknown. This condition is clearly rare but is the second most common anomaly of BAS on screening infants with cholestasis.

Clinical description

Patients present with neonatal cholestasis and rapid progression to cirrhosis and death in infancy without intervention. The clinical presentation resembles that of congenital BAS defect type 1 (see this term) with hepatosplenomegaly, jaundice, fat-soluble vitamin malabsorption, and steatorrhea. However, the average age at diagnosis is lower, in infancy, and disease progression is more rapid and severe. Liver function tests present elevated serum transaminases (AST, ALT) and gamma-GT, markedly elevated conjugated bilirubinemia and coagulopathy.

Etiology

BAS defect type 2 is caused by a mutation in the delta(4)-3-oxosteroid 5-beta-reductase gene (AKR1D1, 7q32-q33). Transmission is autosomal recessive. Liver injury is thought to be caused by diminished primary bile acid synthesis and hepatotoxicity of accumulated atypical bile acids (delta(4)-3-oxo bile acids).

Diagnostic methods

Diagnosis is based on urine analysis using liquid secondary ionization mass spectrometry (LSIMS) and gas chromatography - mass spectrometry (GC-MS). LSIMS urine analysis reveals elevated amounts of delta(4)-3-oxo bile acids including 3-oxo-7alpha-hydroxy-4-cholenoic and 3-oxo-7alpha, 12alpha-dihydroxy-4-cholenoic acids. Increased production of delta4-3-oxo bile acids occurs in infants during the first few weeks of life and in patients with end-stage liver disease of other causes. It is important to perform repeat LSIMS urine analysis as, on rare occasions, a resolution of the liver disease occurs with disappearance of atypical bile acids.

Differential diagnosis

Differential diagnoses include progressive familial intrahepatic cholestasis, diseases that present with neonatal cholestasis, which includes alpha-1-antitrypsin deficiency (ZZ phenotype), tyrosinemia type 1, biliary atresia, choledochal cyst, cystic fibrosis, Alagille syndrome, galactosemia and hereditary fructose intolerance or diseases that present with growth failure (panhypopituitarism) (see these terms).

Antenatal diagnosis

Antenatal diagnosis can be established by analysis of embryonic tissue when there has been a previously identified sibling. Urine LSIMS on siblings of affected patients may be performed in the first neonatal days and therapy begun before serious morbidity develops.

Management and treatment

Treatment is based on oral bile acid therapy, which leads to gradual resolution of biochemical and histologic abnormalities and prevents progression of the disease. Cholic acid therapy creates a pool of bile acids which stimulates bile flow and facilitates fat soluble vitamin absorption and suppresses atypical bile acid synthesis thereby reducing the production of toxic bile acid metabolic intermediates. Ursodeoxycholic acid (UDCA) may be used but is not the therapy of choice because UDCA does not suppress atypical bile acid synthesis and the toxic metabolites that may injure the liver continue to be produced.

Prognosis

With early treatment, the long-term prognosis is excellent. If a patient is identified with advanced liver disease, cholic acid therapy may not be effective and liver transplantation may be required. Without treatment, the condition is generally fatal.