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Congenital bile acid synthesis defect type 1
Congenital bile acid synthesis defect type 1 (BAS defect type 1) is the most common anomaly of bile acid synthesis (see this term) characterized by variable manifestations of progressive cholestatic liver disease, and fat malabsorption.
ORPHA:79301Classification level: Disorder
Prevalence is unknown but may be around 1-9/1,000,000 for overall BAS defects, excluding cerebrotendinous xanthomatosis.
The clinical presentation is heterogeneous, however most patients present with features of neonatal cholestasis. Clinical features include hepatomegaly with or without splenomegaly, jaundice, fat and fat-soluble vitamin malabsorption, and mild steatorrhea. In most cases, pruritus is absent. Liver function tests present elevated serum transaminases (AST, ALT), conjugated hyperbilirubinemia, and normal gamma-GT. The liver histology shows inflammation, giant cells, evidence of cholestasis, and variable degrees of liver fibrosis. The clinical course of early-onset disease is heterogeneous with some patients resolving jaundice and being identified later in life, or with more fulminant disease that results in death or requires liver transplantation at an early age. The disorder may also present as late-onset chronic cholestasis. In such patients, liver disease is not always evident and patients may have fat-soluble vitamin malabsorption with rickets, corrected by vitamin supplementation, and/or other complications including bleeding diathesis (hematochezia or intracranial bleeding), neuroaxonal dystrophy and night blindness. Serum liver enzymes are initially often normal but later show increases with progression of liver disease to fibrosis. Children and adolescents may also present with extensive fibrosis and/or cirrhosis.
The disease is caused by a mutation in the gene encoding 3-beta-hydroxy-delta-5-C27 steroid oxidoreductase (HSD3B7, 16p12-p11.2). Transmission is autosomal recessive.
Diagnosis is based on detection of sulfate and glycosulfate conjugates of 3-beta-hydroxy-delta-5 bile acids, which are the signature metabolites of this bile acid defect, on liquid secondary ionization mass spectrometry (LSIMS) analysis of urine. Gas chromatography - mass spectrometry (GC-MS) or electroscopy and tandem mass spectrometry may also be used.
Differential diagnoses include progressive familial intrahepatic cholestasis, diseases that present with neonatal cholestasis, which includes alpha-1-antitrypsin deficiency of ZZ phenotype, Alagille syndrome, biliary atresia, cystic fibrosis, and metabolic diseases (tyrosinemia type I, galactosemia, hereditary fructose intolerance) (see these terms), diseases that present with fat and fat soluble vitamin malabsorption, including other liver diseases, and intestinal disease, or diseases that present with growth failure.
Antenatal diagnosis can be made on embryonic tissue obtained when there has been a previously identified sibling. Urine LSIMS in a suspect infant can confirm the diagnosis in the first neonatal days.
Management and treatment
Treatment is based on oral administration of cholic acid which leads to gradual resolution of biochemical and histologic abnormalities and prevents progression of the disease, even in cases with hepatic fibrosis and cirrhosis. Cholic acid therapy stimulates bile flow and suppresses synthesis of atypical bile acids and production of toxic intermediates via the bile acid pathway linked to the pathogenesis of disease.
With early treatment the long-term prognosis is excellent.