Overview of bile acid synthesis disorders

Bile acid synthesis disorders (BASDs) are rare genetic conditions that can present as cholestasis, neurologic disease or fat-soluble-vitamin deficiencies. BASDs are responsible for 1-2% of cases of neonatal cholestasis.1

There are nine subtypes of BASDs, all of which result in an abnormal bile acid production and accumulation of bile acids and bile acid intermediaries.1

Bile acid synthesis disorders are classified as either primary or secondary:2

Only primary BASDs are covered here.

The two most frequent defects of primary bile acid synthesis disorders are:3

The mechanism of cholestasis and liver injury is thought to result from:2

The pathophysiology is explained in more detail in the section Pathophysiology

BASDs commonly manifest in infants as cholestasis and can mimic other neonatal liver diseases, including biliary atresia.1 They are sometimes also diagnosed in older children and young adults following unexplained cirrhosis.2

Mode of transmission

These rare, inborn BASDs result from autosomal recessive inheritance.3 The autosomal recessive inheritance pattern is illustrated in the following figure.

Autosomal recessive inheritance pattern

adapted from the Genetic Foundation 2019 4

MOM IS A CARRIER

DAD IS A CARRIER

(no condition)

(no condition)

Child has condition

25% HAVE THE CONDITION

75% OF CHILDREN DON'T HAVE THE CONDITION

Child doesn't have
condition & is not a carrier

Children don't have condition
but are carriers

NON-WORKING GENE

WORKING GENE

Epidemiology

The estimated prevalence of 3β-HSD and Δ4–3-oxoR deficiencies in Europe is 1.13 cases per 10 million:3

However, the estimated prevalence in Europe is likely to be an under-representation due to unawareness of the existence of these diseases and the lack of specialised laboratory facilities.3

Diagnosis

Failure to diagnose and without treatment, they can result in progressive chronic liver disease or liver failure.1

Diagnosis can be difficult because many diseases manifest as neonatal cholestasis or chronic liver disease, and there are no specific clinical features or biomarkers allowing the specific identification of BASDs. However, most patients with BASDs present with:1,5

These are distinctive features as patients with cholestatic liver disease usually have elevated total serum bile acids and γ-glutamyl transpeptidase levels and present with severe pruritus.1 Diagnosis is achieved by connecting certain clinical signs and laboratory results with family history and liver histology, and needs to be confirmed by urinary bile acid analysis with mass spectrometry and genetic testing.2 Further details are provided in the section Diagnosis

3β-hydroxy-Δ5-C27-steroid dehydrogenase (3β-HSD) deficiency

3β-HSD deficiency is caused by mutations in the HSD3B7 gene on chromosome 16p.1

Liver injury occurs because of inadequate synthesis of the normal bile acids required to stimulate bile flow and from the accumulation of toxic bile acids.1

Clinical signs and laboratory results

Most patients with 3β-HSD deficiency are neonates, although age at onset can vary, ranging from 8 weeks to 3 years, with in some cases the disorder even manifesting in adolescents and adults.6 The clinical presentation is also variable.1,2 Typical features may include:1,2,6

The variability of the clinical course of early-onset disease can be illustrated by jaundice initially resolving in some patients who then later in life present with persistent small duct injury or progressive liver disease eventuating in cirrhosis, death, or transplantation.2

Histological signs

The histopathology of 3β-HSD deficiency varies with patient age and correlates with the mode of presentation and rate of disease progression. In the infant, liver histology includes giant cell hepatitis, canalicular bile plugs, hepatocyte bile stasis and portal tract inflammation with varying degrees of fibrosis. In older infants and children, liver biopsy samples may show less pronounced features of giant cell transformation and cholestasis; however, fibrosis becomes more prominent in the portal and periportal areas, and cirrhosis may be present.1,2

Δ4-3-oxosteroid-5β-reductase (Δ4-3-oxoR) deficiency

Δ4-3-oxoR (also known as 5β-reductase) deficiency is an autosomal recessive deficiency of the Δ4-3-oxosteroid-5β-reductase enzyme – encoded by the AKR1D1 gene – causing defective synthesis of the bile acid steroid nucleus.1

Clinical signs and laboratory results

The disorder typically presents as neonatal cholestasis. Δ4-3-oxoR deficiency is characterised by increased concentrations of aminotransferases, conjugated hyperbilirubinaemia, normal γ-glutamyl transpeptidase activity and coagulopathy that worsens with disease progression.2

The clinical presentation is similar to that of 3β-HSD deficiency; however, the average age at diagnosis of Δ4-3-oxoR deficiency patients is 3 months.2 In contrast to 3β-HSD deficiency, infants with Δ4-3-oxoR deficiency tend to present more severe liver disease with rapid progression to cirrhosis and death without intervention.2 Liver failure occurs rapidly, accounting for a 50% mortality rate in infants in whom diagnosis is delayed.1 Neonatal liver failure resembling neonatal haemochromatosis is an alternative clinical presentation and is thought to be caused by an impairment in iron excretion, which is usually enhanced by bile acids.1

Histological signs

The histopathology of Δ4-3-oxoR deficiency is typical of neonatal hepatitis including giant cell hepatitis, pseudo-acinar transformation, hepatocellular and canalicular cholestasis, and extramedullary haematopoiesis.1

1. Sundaram SS, Bove KE, Lovell MA, Sokol RJ. Mechanisms of disease: Inborn errors of bile acid synthesis. Nat Clin Pract Gastroenterol Hepatol 2008;5:456-68.

2. Heubi JE, Setchell KDR, Bove KE. Inborn errors of bile acid metabolism. Clin Liver Dis 2018;22:671-87.

3. Jahnel J, Zöhrer E, Fischler B, et al. Attempt to determine the prevalence of two inborn errors of primary bile acid synthesis: results of a European survey. J Pediatr Gastroenterol Nutr 2017;64:864-8.

4. Autosomal recessive. Genetic Support Foundation, 2019. (Accessed April, 2020, at https://www.geneticsupport.org/genetics-101/inheritance-patterns/autosomal-recessive/.)

5. Bile acid synthesis disorders. NORD: National Organization for Rare Disorders, 2017. (Accessed April, 2020, at https://rarediseases.org/rare-diseases/bile-acid-synthesis-disorders/.)

6. Protocole national de diagnostic et de soins : Déficits de synthèse des acides biliaires primaires. Centre de Référence Coordonnateur de l’Atrésie des Voies Biliaires et des Cholestases Génétiques; 2019.

 

TH-BAS10EN/01/02/2024

Enzymatic pathways of bile acid synthesis

classic pathway

adapted from Sundaram 20081 and Monte 20092

Cholesterol

Cholesterol 7α-hydroxylase
CYP7A1

3β-hydroxy-Δ5 -C27-steroid dehydrogenase
HSD3B7

Δ4-3-oxosteroid-5β-reductase
AKR1D1

Δ4-3-oxosteroid-5β-reductase
AKR1D1

3α-hydroxysteroid dehydrogenase
AKR1C4

Sterol 27-hydroxylase
CYP27A1

Bile acid CoA synthetase (BACS)
or very long chain acyl CoA
synthetase (VLCS)

Side-chain modification by
4 peroxisomal enzymes
(AMACR, BCOX, BDP, SCPx)

3α-hydroxysteroid dehydrogenase
AKR1C4

Sterol 27-hydroxylase
CYP27A1

Bile acid CoA synthetase (BACS)
or very long chain acyl CoA
synthetase (VLCS)

Side-chain modification by
4 peroxisomal enzymes
(AMACR, BCOX, BDP, SCPx)

Amino acid N-acyltransferase
(BAAT)

Amino acid N-acyltransferase
(BAAT)

7α-hydroxycholesterol

7α-hydroxy
4 cholesten-3-one

7α-12α-dihydroxy
-4 cholesten-3-one

7α-dihydroxy-5β-
cholestan-3-one

5β-cholestan-
3α,7α -diol

3α,7α-dihydroxy-5β-
cholestanoic acid (DHCA)

DHCA-CoA

Chenodeoxycholic
acid

Glyco or tauro-
chenodeoxycholic acid

Glyco or tauro-
cholic acid

Cholic acid

3α,7α,12α-trihydroxy-5β-
cholestanoic acid (THCA)

5β-cholestan-
3α,7α,12α-triol

7α-12α-dihydroxy-
5β-cholestan-3-one

THCA-CoA

Microsomes

Cytosol

Mitochondria

peroxisomes

hepatocyte

Endoplasmic
reticulum

Sterol 12α-hydroxylase
CYP8B1

AMARC: alpha methylacyl-CoA racemase

BCOX: Branched-chain acyl CoA oxydase

BDP: D-bifunctional protein hydratase 

SCPx: Sterol carrier protein

Alternative pathway

Cholesterol

Oxysterol 7α-hydroxylase
CYP7B1

3β-hydroxy-Δ5-C27-steroid dehydrogenase
HSD3B7

Side-chain modifications

Sterol 27-hydroxylase
CYP27A1

Amino acid N-acyltransferase
(BAAT)

3β-hydroxy-5-cholestanoic acid

3-oxo-7α-hydroxy-4-cholestanoic acid

3β,7α-dihydroxy-5-cholestanoic acid

Chenodeoxycholic acid

Glyco or tauro-
chenodeoxycholic acid

adapted from Sundaram 20081

1. Sundaram SS, Bove KE, Lovell MA, Sokol RJ. Mechanisms of disease: Inborn errors of bile acid synthesis. Nat Clin Pract Gastroenterol Hepatol 2008;5:456-68.

2. Monte MJ, Marin JJG, Antelo A, Vazquez-Tato J. Bile acids: chemistry, physiology, and pathophysiology. World J Gastroenterol 2009;15:804-16

​TH-BAS10EN/01/02/2024

 

Bilirubin metabolism5-7

Unconjugated
bilirubin

Bilirubin is mainly
produced from the
break down of red
blood cells.

Red cell breakdown
produces unconjugated
(“indirect”) bilirubin,
which circulates mostly
bound to albumin,
although some is
“free” and hence
able to enter the brain.

The terms “direct”
and “indirect” refer
to the way laboratories
measure the different
forms.

Unconjugated bilirubin
is metabolised in the
liver to produce
conjugated (“direct”)
bilirubin by uridine
diphosphate
glucuronosyltransferase.

The activity of this
enzyme only rises after
birth to reach adult like
values around the age
of three months.

Conjugation of bilirubin
increases its solubility
and facilitates its
secretion into bile.

Conjugated (“direct”)
bilirubin then passes
into the gut and is
largely excreted.

In the gastrointestinal
tract, bilirubin is modified
by digestive bacteria and
transformed into
urobilinogens . A large
portion remains in the
intestine and is
converted into stercobilin
(responsible for the
brown colour of faeces).
Some is reabsorbed into
the bloodstream and the
remainder is excreted in
the urine (urobilin is
responsible for the
yellow colour of urine).

Reticuloendothelial
system

Red blood cells

albumin

Haeme

Blood

“Indirect bilirubin”

“Direct bilirubin”

Liver

Intestine

Conjugated
bilirubin

Urobilinogens

Uridine
diphosphate
glucuronosyl-
transferase

Excreted in bile

Gut bacteria

Enterohepatic
circulation

Excreted in urine
(urobilin)

Excreted in faeces
(stercobilin)

Unconjugated
bilirubin-albumin
complex

​TH-BAS07EN/01/02/2024

Normal values:

Total bilirubin: 0-1.1 mg/dL

Vitamin A: 20-43 µg/dL

Vitamin E: 2.9 – 16.6 mg/L

Vitamin K1: 80-160 pg/mL

Normal values:

Serum calcium: 8.7 – 9.8 mg/dL

Serum phosphorous: 3.9 – 6.5 mg/dL

PTH: 15 – 65 pg/mL

Normal values:

Vitamin A: 30–120 μg/dL

Vitamin D: 20–100 ng/mL

Vitamin E: 5–20 μg/mL

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REMINDER ABOUT BILE ACIDS

The bile acid family is a group of acid steroids synthesised from cholesterol in the liver. Although their best-known role is to aid with the emulsion, digestion and absorption of fats and liposoluble vitamins, other important physiological roles have been identified.1

On secretion of bile acids into bile canaliculi, osmotic pressure is created that accounts for the bile-acid-dependent fraction of bile flow. Bile acids stimulate biliary lipid secretion and form mixed micelles with biliary phospholipids, allowing the solubilisation of cholesterol and other lipophilic compounds in the bile. The mixed micelles also emulsify dietary fats in the intestines, facilitating their absorption.1

Bile acid synthesis

The primary bile acids, cholic acid and chenodeoxycholic acid are synthesised from cholesterol by an enzymatic cascade involving nearly 20 enzymes and two complementary chemical pathways, the classic “neutral” pathway and the alternative “acidic” pathway.2,3 Although the neutral pathway is believed to be the major pathway for bile acid synthesis in adults, in the first months of life the acidic pathway is thought to be more important.2,5 In humans and under normal conditions, the acidic pathway contributes little (approximately 10%) to the restitution of daily loss of bile acid.1 It may become the major bile acid biosynthetic pathway in patients with liver diseases.1

The primary bile acids produced include cholic acid (CA), which accounts for approximately 70% of the circulating pool of bile acids, and chenodeoxycholic acid (CDCA), which accounts for approximately 30% of the pool.4

Enzymatic pathways of bile acid synthesis

classic pathway

adapted from Sundaram 20083 and Monte 20091

Cholesterol

Cholesterol 7α-hydroxylase
CYP7A1

3β-hydroxy-Δ5 -C27-steroid dehydrogenase
HSD3B7

Δ4-3-oxosteroid-5β-reductase
AKR1D1

Δ4-3-oxosteroid-5β-reductase
AKR1D1

3α-hydroxysteroid dehydrogenase
AKR1C4

Sterol 27-hydroxylase
CYP27A1

Bile acid CoA synthetase (BACS)
or very long chain acyl CoA
synthetase (VLCS)

Side-chain modification by
4 peroxisomal enzymes
(AMACR, BCOX, BDP, SCPx)

3α-hydroxysteroid dehydrogenase
AKR1C4

Sterol 27-hydroxylase
CYP27A1

Bile acid CoA synthetase (BACS)
or very long chain acyl CoA
synthetase (VLCS)

Side-chain modification by
4 peroxisomal enzymes
(AMACR, BCOX, BDP, SCPx)

Amino acid N-acyltransferase
(BAAT)

Amino acid N-acyltransferase
(BAAT)

7α-hydroxycholesterol

7α-hydroxy
4 cholesten-3-one

7α-12α-dihydroxy
-4 cholesten-3-one

7α-dihydroxy-5β-
cholestan-3-one

5β-cholestan-
3α,7α -diol

3α,7α-dihydroxy-5β-
cholestanoic acid (DHCA)

DHCA-CoA

Chenodeoxycholic
acid

Glyco or tauro-
chenodeoxycholic acid

Glyco or tauro-
cholic acid

Cholic acid

3α,7α,12α-trihydroxy-5β-
cholestanoic acid (THCA)

5β-cholestan-
3α,7α,12α-triol

7α-12α-dihydroxy-
5β-cholestan-3-one

THCA-CoA

Microsomes

Cytosol

Mitochondria

peroxisomes

hepatocyte

Endoplasmic
reticulum

Sterol 12α-hydroxylase
CYP8B1

AMACR: alpha methylacyl-CoA racemase

BCOX: Branched-chain acyl CoA oxydase

BDP: D-bifunctional protein hydratase 

SCPx: Sterol carrier protein

The classic pathway, also known as the “neutral” pathway because its intermediate metabolites are neutral sterols, is the main pathway for bile acid synthesis.1-3 It is present only in the liver and synthesises cholic acid and chenodeoxycholic acid.1 This pathway consists of a cascade of reactions catalysed by enzymes located in microsomes, the cytosol, mitochondria and peroxisomes.1 The final step in the synthesis of bile acid is the conjugation of cholic acid and chenodeoxycholic acid to taurine or glycine.3

Alternative pathway

Cholesterol

Oxysterol 7α-hydroxylase
CYP7B1

3β-hydroxy-Δ5-C27-steroid dehydrogenase
HSD3B7

Side-chain modifications

Sterol 27-hydroxylase
CYP27A1

Amino acid N-acyltransferase
(BAAT)

3β-hydroxy-5-cholestanoic acid

3-oxo-7α-hydroxy-4-cholestanoic acid

3β,7α-dihydroxy-5-cholestanoic acid

Chenodeoxycholic acid

Glyco or tauro-
chenodeoxycholic acid

adapted from Sundaram 2008 and Monte 20091

The alternative pathway involves C27-hydroxylation of cholesterol by sterol 27-hydroxylase as the initial step: side-chain oxidation of cholesterol precedes steroid ring modification.1,3 Thus, acidic intermediate metabolites are formed: this is why this pathway is also known as the “acidic” pathway.1 It primarily produces chenodeoxycholic acid.3

Regulation of bile acid synthesis

Bile acid synthesis is tightly regulated to ensure homeostatic levels of cholesterol are produced and to provide adequate emulsification in the intestine. An excess of bile acids has a negative feedback effect, repressing further synthesis; conversely, when bile acids levels are low, synthesis is increased.2,6

In the ‘neutral’ pathway, the rate-limiting step is the modification of the steroid nucleus, which takes place in hepatic microsomes and is catalysed by cholesterol 7α-hydroxylase (CYP7A1). The neutral pathway facilitates the transformation of cholesterol to cholic acid and chenodeoxycholic acid, which are further conjugated to glycine or taurine and used as substrates for the bile acid transport pump. The canalicular transport of bile acid is the rate-limiting step of bile secretion. Bile acids are recovered from the intestines by the apical sodium-dependent bile acid transporter and return to the liver via the portal blood. The gene encoding CYP7A1 is highly regulated by negative feedback involving farnesoid X receptor (FXR)-dependent induction of fibroblast growth factor 15/19 (FGF15/19) expression by bile acids in the enterocytes. FGF15/19 binds to the fibroblast growth factor receptor 4-β-klotho complex in hepatocytes, activating signalling pathways that transcriptionally repress CYP7A1 expression.2

Enterohepatic circulation of bile acids

Bile acids are mostly restricted to enterohepatic circulation, circulating between the liver, the biliary tree, the intestine, and the portal blood which returns them to the liver. Almost all (95%) the bile acids are recovered from the intestine, mostly in the ileum.1 The liver converts around 500 mg of cholesterol into bile acids per day. This accounts for 90% of the cholesterol that is actively metabolized by the body. The remaining 10% of cholesterol that is synthesised is biosynthesised from steroid hormones.6

Newly synthesised bile acids are secreted into the bile where they are transported to the lumen of the small intestine, where they act as lipid emulsifiers, solubilising nutrients. These nutrients are incorporated into lipoproteins, and are delivered via the portal vein to the liver and metabolised. About 95% of the bile acids are recycled and secreted back into the bile. The remaining 5% are excreted into the faeces.6-7

Enterohepatic circulation of bile acids

adapted from van Mil 20057

ENTEROHEPATIC CYCLE

1. Monte MJ, Marin JJG, Antelo A, Vazquez-Tato J. Bile acids: chemistry, physiology, and pathophysiology. World J Gastroenterol 2009;15:804-16.

2. Jahnel J, Zöhrer E, Fischler B, et al. Attempt to determine the prevalence of two inborn errors of primary bile acid synthesis: results of a European survey. J Pediatr Gastroenterol Nutr 2017;64:864-8.

3. Sundaram SS, Bove KE, Lovell MA, Sokol RJ. Mechanisms of disease: Inborn errors of bile acid synthesis. Nat Clin Pract Gastroenterol Hepatol 2008;5:456-68.

4. Ashby K, Navarro Almario EE, Tong W, Borlak J, Mehta R, Chen M. Review article: therapeutic bile acids and the risks for hepatotoxicity. Aliment Pharmacol Ther 2018;47:1623-38.

5. Clayton PT. Disorders of bile acid synthesis. J Inherit Metab Dis 2011;34:593-604.

6. Russell DW. The enzymes, regulation, and genetics of bile acid synthesis. Annu Rev Biochem 2003;72:137-74.

7. van Mil SW, Houwen RH, Klomp LW. Genetics of familial intrahepatic cholestasis syndromes. J Med Genet 2005;42:449-63.

​TH-BAS10EN/01/02/2024

Normal values:

Vitamin A: 1,09-3,07 μmol/L

Vitamin E: 25-42 μmol/L

AST: <39 U/L

ALT: <34 U/L

GGT: <38 U/L

Total bilirubin: <17 μmol/L

Conjugated bilirubin level: <5 μmol/L

Normal values:

AST: < 45 mU/mL

International normalized ratio: < 1.2

Prothrombin ratio: 70-120 %

Factor VII: 70-120 %

Factor X: 70-120 %

Vitamin E: 300-1200 μg/dL

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Causes of jaundice in newborn or infant5

Jaundice of the newborns or infants

Unconjugated hyperbilirubinaemia

Mechanical haemolysis

Other causes of haemolysis

Intramedullary haemolysis

Infection-related haemolysis

Immune haemolysis

Constitutional haemolysis

Haemolysis

Default in the conjugation of
indirect bilirubin (UGT1A gene)

Rare genetic deficiency of canalicular
or sinusoidal specific transporter of
conjugated bilirubin

Moderate or severe hereditary
deficiency of enzyme activity

Enzyme immaturity or
enzymatic inhibition

Hepatocellular insufficiency

Cholestasis (decreased bile flow)

Other (hypothyroidism, trisomy 21)

Conjugated hyperbilirubinaemia

​TH-BAS07EN/01/02/2024

Bilirubin metabolism5-7

Important things to check: the degree and duration of stool discolouration. The colour of the stool should ideally be recorded after elimination of any component likely to change the colour and with the help of a colour chart:2, 14

This chart is used with the kind permission of the AMFE, Association Maladies Foie Enfants, a French association dedicated to liver diseases in children.3

You can also refer to the stool chart provided by the Children’s Liver Disease Foundation (CLDF), a UK charity committed to fighting all childhood liver diseases :

CLDF-Yellow-Alert-Stool-Chart

Persistently pale coloured stools may indicated liver disease.14  Complete and prolonged discolouration of stools for 7 days is suggestive of biliary atresia until proven otherwise. However, the sign is not specific to biliary atresia and can also occur, among others, in conditions such as cystic fibrosis, Alagille syndrome, alpha-1 antitrypsin deficiency and neonatal sclerosing cholangitis.2

​TH-BAS08EN/01/02/2024

What causes (rare) cholestasis
in paediatric patients

?

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Created by

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Content reviewed by experts in paediatric gastroenterology and hepatology.

This website was created by Theravia. Theravia is a leading international pharmaceutical laboratory specializing in rare or neglected diseases. Formed through the merger of Addmedica and CTRS, we are dedicated to address the unmet medical needs of patients with these challenging conditions

03/2024

What causes (rare) cholestasis
in paediatric patients ?

Menu

To access the platform and the
diagnostic algorithms,
please confirm:

If you are not a healthcare professional, please do not access the website as the content is not suitable.
We invite you to continue your research through another website

This website was created by Theravia. Theravia is a leading international pharmaceutical laboratory specializing in rare or neglected diseases. Formed through the merger of Addmedica and CTRS, we are dedicated to address the unmet medical needs of patients with these challenging conditions

Created by

Content reviewed by experts in paediatric gastroenterology and hepatology.