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.
- Cholestasis may be accompanied by jaundice, but pruritus is typically absent . During the clinical examination, an enlarged liver may be palpated. The lack of primary bile acids interferes with the digestion and subsequent absorption of lipophilic food components. Thus, CBAS4 patients may produce pale or clay-colored, acholic stools, and they may also present with symptoms of fat-soluble vitamin deficiency  . The latter comprise nyctalopia and xerophthalmia due to vitamin A deficiency, bone pain and and skeletal deformities because of vitamin D deficiency, and a propensity to bleed due to vitamin K deficiency. Vitamin E deficiency may cause neurological symptoms, thereby adding to the neuropathic phenotype of CBAS4.
- The disease follows a progressive course and affected individuals typically develop a neuropathy in childhood or later in life. Relapsing sensorimotor neuropathy, cerebellar signs like ataxia and dysarthria, pyramidal tract dysfunction, seizures and cognitive decline have all been observed in this regard  . An ophthalmological examination may reveal cataracts, retinitis pigmentosa and pigmentary retinopathy .
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:
- On the one hand, affected individuals benefit from dietary restrictions. A lesser intake of phytanic and pristanic acid reduces the necessity to metabolize these compounds and prevents their accumulation . Phytanic and pristanic acid are mainly obtained from bovine fat, so foods rich in bovine fat should be avoided.
- On the other hand, primary bile acids should be replaced to support the digestion and subsequent absorption of lipids . Replacement of deficient primary bile acids also produces beneficial feedback inhibition of abnormal bile acid production and thus prevents the accumulation of R-stereoisomers of 3α,7α-dihydroxy-5β-cholestanoyl-CoA and 3α,7α,12α-trihydroxy-5β-cholestanoyl-CoA . Permanent replacement of cholic acid has proven effective in normalizing liver function tests and urine composition, and preventing the onset of neurological symptoms . Few data exist in the literature regarding treatment regimens, but Setchell et al. have successfully treated a child suffering from CBAS4 applying 15 mg of cholic acid per kg body weight and day .
In case of persisting fat-soluble vitamin deficiency, the respective vitamins should be supplemented.
AMACR deficiency is a rare disorder that has been documented as few as 7 times up to this day. Due to the lack of sufficient statistic data, a unified prognosis cannot be determined. The overall course of some of the documented patients, however, is described below:
- A study by Ferdinandusse documented 3 patients with AMACR deficiency: two of them were affected by sensory motor neuropathy arising in adulthood, with one of them also being affected by pigmentary retinopathy, epileptic seizures, migraines and depression . The third patient, still in childhood, did not display any symptoms related to an AMACR deficiency but was affected by the Niemann-Pick disease, type C.
- The second group of patients was later described by Setchell : two siblings, who were both affected by AMACR deficiency, exhibited the same genetic mutation with the first three patients described in the former paragraph. Their clinical picture, however, varied: they presented with coagulopathy, liver disease, cholestatic episodes and vitamin deficiency during the first days of their lives. A previous sibling had died at the age of 5 months old and, following the transplantation of their liver to a 2-year old child, the latter exhibited signs of AMACR deficiency, leading to the conclusion that the deceased sibling has also been affected by the condition. The child who received the liver transplant was treated with ursodeoxycholic acid; during follow-up, they remained in good health for the next 8 years, still under treatment with the medication .
Consequently, based on the data that is available up to this day, it can be said that identical genetic mutations can lead to AMACR with a varying clinical picture, age of onset and general prognosis .
Peroxisomes are small organelles that are found in the cytoplasm and mediate the metabolism of multiple lipids, including branched fatty acids (pristanic and phytanic acid). They also take part in the process of bile acid production.
The completion of specific steps in both procedures require the presence of alpha-methyl-acyl-CoA racemase (AMACR); its deficiency leads to the accumulation of intermediate R-isomers of pristanic acid, as well as di- and tri hydrocholestanoic acids (DHCA and THCA) which are intermediate products of the bile acids biosynthesis pathway . AMACR deficiency is caused by a genetic defect, c.154T>C, which is passed down from parents to offspring in an autosomal recessive pattern.
AMACR deficiency is a rare inborn enzymic deficiency, that has been described in the literature as few as seven times up to this day. It is usually diagnosed during adulthood, with the patients suffering from additional comorbidities, such as sensorimotor neuropathy   . A single case report has described a case of AMACR deficiency diagnosed in a neonate. The c.154T>C genetic mutation that has been found to underlie the condition has been detected in as many as 6 out of the 7 known cases.
The process of β-oxidation that is carried out in the peroxisomes is an indispensable step in the rather complex procedure of molecular degradation. More specifically, β-oxidation contributes to the catabolism of branched fatty acids, VLCFA, polyunsaturated fatty acids and long-chain dicarboxylic acids; prostaglandins and leukotrienes are also catabolized in the peroxisomes . The organelles contain various enzymes, such as two acyl-CoA oxidases, two thiolase and two bifunctional enzymes that are activated by different substrates in order to mediate β-oxidation. Thus, each type of enzyme deficiency leads to the buildup of distinct substrates.
More specifically, the alpha-methylacyl-CoA racemase (AMACR) enzyme is the one that converts (2R)-methyl branched-chain fatty acids into (2S)-methyl branched-chain fatty acids, this conversion creates substrates that can successfully go through β-oxidation in the peroxisome. These newly formed substrates encompass pristanic acid and bile acid intermediates di- and tri hydrocholestanoic acids (DHCA and THCA) and, in the setting of an AMACR deficiency, they accumulate in excessive quantities .
The presence of intermediate products of β-oxidation that are unable to complete the process cause severe symptomatology, such as late-onset cerebral ataxia, adult-onset neuropathy, white matter abnormalities, recurrent encephalopathy and epilepsy    . Under some circumstances, tremor, cataract, lesions in the thalamus and pigmentary retinopathy can arise, while some individuals exhibit signs of cholestasis as early as the first days of their lives  .
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:
- Congenital bile acid synthesis defect type 1 or 3beta-hydroxy-delta5-C27-steroid oxidoreductase deficiency due to mutations in the HSD3B7 gene
- Congenital bile acid synthesis defect type 2 or cholestasis with delta(4)-3-oxosteroid-5-beta-reductase deficiency, resulting from mutations in the AKR1D1 gene
- Congenital bile acid synthesis defect type 3, which has been related to CYP7B1 mutations
- CBAS4, which will be discussed in this article
- Congenital bile acid synthesis defect type 5, a disease that is caused by mutations in the ABCD3 gene
- Congenital bile acid synthesis defect type 6, which is triggered by ACOX2 mutations
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.
Alpha-methylacyl-CoA racemase (AMACR) deficiency is a genetic disease. Individuals affected by it lack an enzyme which is vital for the metabolism of various substances, such as phytanic acid and pristanic acid. The body's inability to process these acids leads to their abnormally high accumulation in the patient's blood and various neurological complications.
AMACR deficiency is not preventable. It is either inherited from the parents in an autosomal recessive way, or it is a result of a spontaneous mutation. The autosomal recessive pattern implies that the affected individual must have inherited two defective genes, one from each parent, in order for the disease to develop.
The condition induces a variety of symptoms, which are all related to the central or peripheral nervous system and include the following:
- Cognitive impairment: progressive loss of cognitive abilities
- Epileptic phenomena
- Encephalopathy: inflammation of the brain
- Stiff muscles
- Ataxia: loss of coordinated movement
- Damage to the peripheral nerves
- Deteriorating vision, or congenital visual impairment, primarily due to defects of the retina
AMACR deficiency is usually suspected when infants fail to grow, are not as active as they should be, are mentally disabled and exhibit elevated liver enzymes and an equally enlarged liver. Diagnostic tests include the detection of the presence of phytanic acid and various other metabolic products in blood that can indicate AMACR. With regard to the treatment of AMACR, patients follow a diet that has been adapted to include absolutely no phytanic and pristanic acid, even though no significant amelioration has been observed. They also receive cholic acid for long periods of time, which has been shown to stop the condition from progressing to severe stages.
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