Congenital bile acid synthesis defect type 4 (CBAS4) is a hereditary disorder associated with intrahepatic cholestasis, usually of infantile onset, and progressive sensorimotor neuropathy. Diagnosis rests on bile acid profiles in plasma and urine, measurements of the activity of α-methylacyl-CoA racemase, and sequencing of the AMACR gene. The AMACR gene encodes for the aforementioned enzyme and mutations in this gene have been identified as the cause of CBAS4. Therapy is based on primary bile acid replacement and dietary adaptions, and aims at preventing disease progression and the onset of neurological symptoms.
The clinical presentation of CBAS4 does not allow for its distinction from other types of congenital bile acid synthesis defects, or even from cholestatic liver disease due to any other pathological condition. In this regard, laboratory results of blood sample analyses often provide valuable first indications as to the underlying disease. In detail, hepatic transaminases and conjugated bilirubin concentrations are usually increased in samples obtained from those suffering from congenital bile acid synthesis defects, while γ-glutamyltransferase levels typically remain within reference ranges . Analytical techniques must then be applied to assess the patient's bile acid profiles in plasma and urine. Fast atom bombardment ionization-mass spectrometry, electrospray ionization-tandem mass spectrometry, gas chromatography-mass spectrometry, and liquid chromatography-tandem mass spectrometry have been used to this end   . CBAS4 is associated with an increase of levels of R-stereoisomers of 3α,7α-dihydroxy-5β-cholestanoyl-CoA and 3α,7α,12α-trihydroxy-5β-cholestanoyl-CoA, while S-stereoisomers of those intermediates cannot be detected and primary bile acid concentrations are pathologically reduced . If such samples are tested for their contents in phytanic and pristanic acid, elevated levels will be found . Finally, the sequencing of the AMACR gene allows for the identification of the causal mutation and the confirmation of the diagnosis. This step should not be skipped since precise knowledge regarding the underlying gene defect greatly simplifies diagnostic procedures in family members and a possible prenatal recognition of the disease.
High-throughput sequencing technologies allow for a different approach to diagnosing hereditary liver disease and neuropathies accompanied by non-specific symptoms: Indeed, CBAS4 has been diagnosed based on the results of whole exome sequencing that, in turn, have been confirmed by means of bile acid profiling . The measurement of the activity of α-methylacyl-CoA racemase in determined cells, e.g., in cultured skin fibroblasts, constitutes yet another possibility to diagnose congenital bile acid synthesis defects but isn't typically carried out in the absence of a strong suspicion of CBAS4  . And while magnetic resonance imaging doesn't yield specific results, stroke-like lesions may be observed during encephalopathic episodes and progressive changes of distinct brain sections may be documented over time .
Upon histopathological examination, intralobular cholestasis with giant cell transformation, hepatocytes containing few peroxisomes and undergoing necrosis may be observed  . Giant cell transformation has been reported to be discernible in all symptomatic infants with congenital bile acid synthesis defects and has been proposed as a marker of such diseases. An extensive list of morphological features in a cholestatic liver that suggest a bile acid synthetic defect has been published elsewhere .
An early diagnosis and timely initiation of treatment is important to prevent disease progression and degradation of the nervous system. Therapy relies on two principles:
In case of persisting fat-soluble vitamin deficiency, the respective vitamins should be supplemented.
Clinical symptoms associated with CBAS4 deficiency may be relatively mild , but there are older case reports about infants who presumably suffered from CBAS4 and died of cirrhosis of the liver within their first two years of life  . Fortunately, progress in the diagnosis of congenital bile acid synthesis defects now allows for an early recognition of the disease and patients usually respond well to primary bile acid therapy. Although long-term results cannot yet be provided, such treatment may eventually normalize the life expectancy of CBAS4 patients .
CBAS4 is related to a reduced activity of the enzyme α-methylacyl-CoA racemase, which catalyzes an essential reaction in the synthesis of bile acids. This enzyme is encoded by the AMACR gene, located on the short arm of chromosome 5. Distinct missense mutations have been described in CBAS4 patients, but data is too scarce to derive genotype-phenotype relations .
CBAS4 is inherited in an autosomal recessive manner, i.e., only those with mutations in both alleles of the AMACR gene will develop the disease.
Bile acid synthesis defects are rare disorders. It has been estimated that they account for about 2% of persistent cholestasis in infants . Epidemiological data as to the percentage of late-onset sensorimotor neuropathies due to congenital bile acid synthesis defects cannot be provided. Presumably, they are largely underdiagnosed, and this particularly applies in case of mild disease, as the case may be with CBAS4.
The synthesis of bile acids comprises several reactions that take place in distinct cell organelles. An essential step of bile acid synthesis is the isomerization of R-stereoisomers of the C25-methyl group of C27-bile acid intermediates to S-stereoisomers as part of the peroxisomal β-oxidation  . This reaction is catalyzed by the enzyme α-methylacyl-CoA racemase. It is indispensable because the enzyme catalyzing the following reaction, namely branched-chain acyl-CoA oxidase, can only process S-stereoisomers . Thus, in case of α-methylacyl-CoA racemase deficiency, R-stereoisomers of C27-bile acid intermediates accumulate. These are 3α,7α-dihydroxy-5β-cholestanoyl-CoA and 3α,7α,12α-trihydroxy-5β-cholestanoyl-CoA . Under physiological conditions, those intermediates would eventually be converted to chenodeoxycholic acid and cholic acid, but this doesn't apply to patients suffering from CBAS4. These individuals hardly produce any primary bile acids, which entails alterations of the physical properties of their bile: Its modified composition interferes with biliary secretion causing cholestasis and malabsorption of lipophilic food components like fat-soluble vitamins . Of note, it could not yet be clarified which pathways lead to the formation of minor quantities of chenodeoxycholic acid and cholic acid in CBAS4 patients  .
At the same time, α-methylacyl-CoA racemase is required for the isomerization of methyl-branched fatty acids such as pristanic acid  . Only after this isomerization takes place, pristanic acid can be degraded. In humans, pristanic acid is either of dietary origin or derives from the α-oxidation of phytanic acid. Therefore, in patients suffering from CBAS4, an accumulation of phytanic and pristanic acid can be observed.
Affected families may benefit from genetic counseling. The latter does, however, require precise knowledge regarding the underlying mutation. In some cases of CBAS4, familial anamnesis revealed a history of liver disease and/or fat-soluble vitamin deficiency in living or deceased relatives  , but the identification of carriers and the prenatal diagnosis of the disease is not feasible without targeted genetic analyses.
Congenital bile acid synthesis defects are rare genetic disorders resulting from mutations in distinct genes that cause enzyme deficiencies. The respective enzymes are required for bile acid synthesis and a reduction of their activity interferes with the production and release of bile from hepatocytes. Accordingly, intrahepatic cholestasis is a clinical hallmark of bile acid synthesis defects.
In detail, the following congenital bile acid synthesis defects are distinguished:
CBAS4 has also been called "intrahepatic cholestasis with defective conversion of trihydroxycoprostanic acid to cholic acid" and although this term is rather hard to remember, it points out the main pathophysiological event triggering the disease: Due to a deficiency of α-methylacyl-CoA racemase, which is required for the conversion of R-stereoisomers of methylacyl-CoA esters to their respective S-stereoisomers and vice versa, the bile acid synthesis from cholesterol is impaired. Only S-stereoisomers can be catabolized to bile acids by peroxisomal β-oxidation, which results in shortening of C27-bile acid intermediates by three mature carbon atoms to mature C24-bile acids . Consequently, R-stereoisomers accumulate and primary bile acids cannot be produced.
Bile acids are synthesized in the liver and are the secreted into the small intestine. They facilitate the digestion and subsequent absorption of lipids ingested with foods. Bile acid synthesis itself is a complex process involving several reactions, with each of them being catalyzed by a specific enzyme. Due to mutations in the respective genes, some people suffer from congenital enzyme deficiencies and bile acid synthesis defects. In detail, congenital bile acid synthesis defect type 4 (CBAS4) is caused by a mutation in the AMACR gene.
The clinical presentation of CBAS4 is rather heterogeneous. On the one hand, affected infants may develop cholestasis and jaundice, and eventually die of liver failure. On the other hand, CBAS4 patients may not experience any complaints until childhood or even adulthood. Late-onset CBAS4 is characterized by neurological symptoms, e.g., sensorimotor neuropathy, ataxia and dysarthria, seizures and cognitive decline. The disease follows a progressive course if treatment is not initiated in a timely manner.
In order to diagnose the disease, blood and urine samples have to be obtained and analyzed. In CBAS4 patients, plasma and urine contain specific intermediates of bile acid synthesis that cannot be metabolized due to the deficiency of α-methylacyl-CoA racemase, the enzyme encoded by the AMACR gene. The diagnosis may be confirmed by sequencing of the AMACR gene and identifying the causal mutation. Treatment is based on two principles: Those bile acids that cannot be produced have to be replaced by applying primary bile acids throughout life, and CBAS4 patients should follow certain dietary restriction to reduce the required level of enzyme activity. Such treatment has only been applied for a few years, so long-term outcomes cannot yet be evaluated - but interim results are highly promising. From what has been seen so far, this type of therapy prevents disease progression and the onset of neurological symptoms.