Bile acid synthesis disorders
Bile acid synthesis disorders are rare metabolic disorders characterized by defects in the synthesis of bile acids, which are crucial for cholesterol breakdown and the absorption of fats and fat-soluble vitamins. These disorders can lead to the accumulation of abnormal bile acids and intermediary metabolites, causing damage to various organs.
Classification
Bile acid synthesis disorders may be classified as primary or secondary. Major primary BASDs include 3β-hydroxy-Δ5-C27-steroid oxidoreductase deficiency, caused by mutations in the HSD3B7 gene; Δ4-3-oxosteroid 5β-reductase deficiency, caused by mutations in AKR1D1, and cerebrotendinous xanthomatosis, among others. These disorders are typically inherited in an autosomal recessive fashion.Secondary BASDs include disorders of bile acid transport systems, such as progressive familial intrahepatic cholestasis. Peroxisomal diseases such as Zellweger syndrome also affect bile acid production and may be considered secondary BASDs.
Symptoms and signs
Bile Acid Synthesis Disorders present a range of symptoms, often beginning in infancy or early childhood. Jaundice, characterized by yellowing of the skin and eyes, is a common early sign. Growth deficiencies are prevalent, with affected individuals often failing to meet weight and height milestones due to malabsorption of fats and fat-soluble vitamins like A, D, E, and K. This can lead to Vitamin deficiencies, resulting in vision problems, rickets, neurological issues, and blood coagulation problems. Patients may experience cholestasis, a condition where bile flow is interrupted, leading to pale stools, dark urine, and sometimes severe itching. Steatorrhea, or excess fat in stools, is another symptom due to impaired fat digestion. In severe cases, BASDs can progress to liver failure if untreated. The variability in symptom onset and severity depends on the specific genetic defect involved.Diagnosis
Diagnosing Bile Acid Synthesis Disorders requires a comprehensive approach due to their rarity and symptom overlap with other liver diseases. Physicians begin by suspecting BASDs in infants or children presenting with jaundice, unexplained liver disease, or fat-soluble vitamin deficiencies. Initial laboratory tests often include measuring serum bile acids.Advanced diagnostic techniques involve mass spectrometry, Liquid chromatography-tandem mass spectrometry and electrospray ionization-tandem mass spectrometry, to analyze bile acid profiles in urine and blood. These methods identify atypical bile acids and intermediates that accumulate due to enzyme deficiencies. Fast atom bombardment-mass spectrometry is another technique used to detect specific bile acid patterns indicative of BASDs. Genetic testing confirms the diagnosis by identifying mutations in genes responsible for bile acid synthesis enzymes.
Treatment
Treatment for BASDs primarily involves oral bile acid replacement therapy. Cholic acid, approved in 2015, is the standard treatment for patients with single enzyme defects and peroxisomal disorders. This therapy compensates for the lack of primary bile acids, restoring normal liver function and improving symptoms like jaundice and malabsorption.In some cases, ursodeoxycholic acid may be used alongside cholic acid to enhance bile flow, although it is ineffective as a sole treatment. The dosage of cholic acid is carefully monitored to suppress abnormal metabolite production and improve liver biochemistry.
For patients unresponsive to medical therapy or with advanced liver disease, liver transplantation may be necessary. Early intervention with cholic acid has been shown to prevent progression to liver failure and improve long-term health outcomes, allowing many patients to lead normal lives into adulthood.
Epidemiology
BASDs are thought to account for approximately 1-2% of all childhood cholestatic disorders. In Europe, a study found a minimum estimated combined prevalence of 1.13 cases per 10 million people for two common types of BASDs:- 3β-hydroxy-Δ5-C27-steroid dehydrogenase deficiency: 0.99 cases per 10 million
- Δ4-3-oxosteroid 5β-reductase deficiency: 0.14 cases per 10 million