Alternative titles; symbols
HGNC Approved Gene Symbol: ACOX2
Cytogenetic location: 3p14.3 Genomic coordinates (GRCh38): 3:58,505,136-58,537,190 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 3p14.3 | Bile acid synthesis defect, congenital, 6 | 617308 | Autosomal recessive | 3 |
The ACOX2 gene encodes a peroxisomal branched-chain acyl-CoA oxidase involved in bile acid synthesis (summary by Vilarinho et al., 2016).
Baumgart et al. (1996) reported the molecular characterization of branched-chain acyl-CoA oxidase. Its composite cDNA sequence, derived from overlapping clones isolated by immunoscreening and hybridization of human liver cDNA expression libraries, consists of 2,225 bp and contains an open reading frame of 2,046 bp, encoding a protein of 681 amino acids with a calculated molecular mass of 76,739 Da. The C-terminal tripeptide of the protein was found to be SKL (ser-lys-leu), a known peroxisome targeting signal. Sequence comparison with the other acyl-CoA oxidases and evolutionary analysis demonstrated that, despite its broader substrate specificity, this branched-chain acyl-CoA oxidase is the human homolog of rat trihydroxycoprostanoyl-CoA oxidase and that separate gene duplication events led to the occurrence in mammals of acyl-CoA oxidases with different substrate specificities. Northern blot analysis demonstrated that, in contrast to the rat gene, the human gene is transcribed also in extra hepatic tissues, such as heart, kidney, skeletal muscle, and pancreas. The highest levels of the 2.6-kb mRNA were found in heart, followed by liver. The enzyme was absent from livers of Zellweger patients, as shown by immunoblot analysis and immunocytochemistry. Palmitoyl-CoA oxidase was also absent, whereas even in autolytic samples of human control livers both acyl-CoA oxidases were present. Baumgart et al. (1996) noted that the deficiency of these enzymes is part of the generalized deficiency in peroxisomal beta-oxidation enzymes in Zellweger syndrome (see 214100).
Baumgart et al. (1996) isolated the rat trihydroxycoprostanoyl-CoA oxidase cDNA sequenced by screening rat liver cDNA expression libraries. The gene contains a 2,046-bp open reading frame encoding a protein of 681 amino acids with a calculated molecular mass of 76,711 Da. This sequence shares 45% amino acid identity with rat palmitoyl-CoA oxidase and 22% with rat pristanoyl-CoA oxidase. The C terminus (his-lys-met) of trihydroxycoprostanoyl-CoA oxidase does not appear to interact with the C-terminal peroxisomal targeting signal 1 (PTS1) import receptor (PEX5; 600414), although the tripeptide fits the rule of conserved variants for targeting of proteins to glycosomes of Trypanosomatidea. Northern analysis of multiple rat tissues revealed a 2.6-kb transcript only in liver and kidney.
Baumgart et al. (1996) assigned the single-copy gene to 3p14.3 by fluorescence in situ hybridization (FISH). Moghrabi et al. (1997) used PCR and rodent/human hybrids to map the gene encoding peroxisomal branched-chain acyl-CoA oxidase to human chromosome 3p21.1-p14.2.
Baumgart et al. (1996) stated that greater than half of the enzymes present in mammalian peroxisomes are associated intimately with lipid metabolism. Thus, peroxisomes are involved in the beta-oxidation of very long straight-chain fatty acids and branched-chain fatty acids, dicarboxylic fatty acids, and eicosanoids. They are also responsible for the beta-oxidation of the side chain of the bile acid intermediates di- and trihydroxycoprostanic acids, resulting in the formation of the primary bile acids (chenodeoxycholic and cholic acid, respectively). Most likely, the different substrates are degraded by distinct beta-oxidation pathways. Peroxisomes in human liver contain 2 distinct acyl-CoA oxidases with different substrate specificities: palmitoyl-CoA oxidase (609751), oxidizing very long straight-chain fatty acids and eicosanoids, and a branched-chain acyl-CoA oxidase, involved in the degradation of long branched fatty acids and bile acid intermediates. The accumulation of branched fatty acids and bile acid intermediates leads to severe mental retardation and death of affected children. Deficiency of acyl-CoA oxidase (palmitoyl-CoA oxidase; ACOX1, 609751) results in pseudoneonatal adrenoleukodystrophy (264470).
In a boy, born of consanguineous Turkish parents, with congenital bile acid synthesis defect-6 (CBAS6; 617308), Vilarinho et al. (2016) identified a homozygous truncating mutation in the ACOX2 gene (Y69X; 601641.0001). The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Patient liver sample showed absence of the ACOX2 protein, consistent with a complete loss of function. Plasma and urine analysis of the patient showed increased levels of the C27 bile acid intermediates DHCA and THCA. However, branched-chain fatty acids, phytanic and pristanic acids, were both normal.
In a Spanish brother and sister with CBAS6, Monte et al. (2017) identified a homozygous missense mutation in the ACOX2 gene (R225W; 601641.0002). The mutation, which was found by direct sequencing of the gene, was present in heterozygous state in the unaffected parents. In vitro functional expression studies in human hepatoblastoma cells showed that the mutant protein was expressed at levels comparable to wildtype and localized properly to the peroxisome, but resulted in significantly decreased production of cholic acid compared to controls. Incubation of hepatoblastoma cells with THCA caused oxidative stress and cell death in a dose-dependent manner, which could be rescued by wildtype ACOX2, but not mutant ACOX2.
Ferdinandusse et al. (2018) identified homozygosity for a 4-bp deletion (c.461_464delTCTG; 601641.0003) in the ACOX2 gene in a patient, born to consanguineous parents, with CBAS6. Studies in patient fibroblasts demonstrated absence of ACOX2 protein expression and reduced branched chain amino acyl-CoA oxidase activity when pristanoyl-CoA was the substrate. Laboratory studies in the patient showed accumulation of C27-bile acid intermediates. The patient had a severe multisystem disorder including a bile acid biosynthesis defect, congenital arthrogryposis, pulmonary hypertension, and respiratory failure, and she died before 1 year of age.
Alonso-Pena et al. (2022) identified homozygosity for the R225W mutation in the ACOX2 gene in 4 patients from 2 generations of a family (case 1) and an unrelated patient (case 2) with CBAS6. Liver biopsies from 2 of the patients demonstrated absence of ACOX2 expression. Huh7 cells with overexpression of ACOX2 with the R225W mutation displayed THCA-induced toxicity compared to Huh7 cells with overexpression of wildtype ACOX2. Alonso-Pena et al. (2022) also identified compound heterozygous mutations in the ACOX gene (R225W; c.456_459del, 601641.0004) in 2 unrelated patients with CBAS6. Liver biopsies from these patients demonstrated absence of ACOX2 expression.
In a boy, born of consanguineous Turkish parents, with congenital bile acid synthesis defect-6 (CBAS6; 617308), Vilarinho et al. (2016) identified a homozygous mutation in the ACOX2 gene (NM_003500), resulting in a tyr69-to-ter (Y69X) substitution. The mutation, which was found by exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. It was not found in the Exome Variant Server (March 2015), 1000 Genomes Project, or ExAC (January 2015) databases, or in 894 control Turkish exomes. Patient liver sample showed absence of the ACOX2 protein, consistent with a complete loss of function.
In a brother and sister, born of parents from neighboring valleys in northern Spain, with congenital bile acid synthesis defect-6 (CBAS6; 617308), Monte et al. (2017) identified a homozygous c.673C-T transition (c.673C-T, NM_003500) in exon 6 of the ACOX2 gene, resulting in an arg225-to-trp (R225W) substitution. The mutation, which was found by direct sequencing of the gene, was present in heterozygous state in the unaffected parents. It was found at a low frequency (0.04%) in the dbSNP database. In vitro functional expression studies in human hepatoblastoma cells showed that the mutant protein was expressed at levels comparable to wildtype and localized properly to the peroxisome, but resulted in significantly decreased production of cholic acid compared to controls. Incubation of hepatoblastoma cells with THCA caused oxidative stress and cell death in a dose-dependent manner, which could be rescued by wildtype ACOX2, but not R225W ACOX2.
In 4 patients from 2 generations of a family (case 1) and an unrelated patient (case 2) with CBAS6, Alonso-Pena et al. (2022) identified homozygosity for the R225W mutation in the ACOX2 gene. The mutation was identified by sequencing of the ACOX2 gene and segregated with disease in the families. Liver biopsies from 2 of the patients demonstrated absence of ACOX2 expression.
In 2 unrelated patients (cases 3 and 4) with CBAS6, Alonso-Pena et al. (2022) identified compound heterozygous mutations in the ACOX2 gene: R225W and a 4-bp deletion (c.456_459del; 601641.0004) resulting in a frameshift and premature termination (Thr154fs). The mutation was identified by sequencing of the ACOX2 gene and segregated with disease in the families. Liver biopsies from the 2 patients demonstrated absence of ACOX2 expression. Both patients had accumulation of C27 bile acids.
In a Pakistani patient, born to consanguineous parents, with congenital bile acid synthesis defect-6 (CBAS6; 617308), Ferdinandusse et al. (2018) identified homozygosity for a 4-bp deletion (c.461_464delTCTG) in the ACOX2 gene, resulting in a frameshift and premature termination (Thr154SerfsTer25). The mutation was identified by sequencing of a panel of 26 genes associated with peroxisomal disease and confirmed by Sanger sequencing. The variant was present in the ExAC database at a frequency of 0.21%, in the ESP database at a frequency of 0.37%, and in the 1000 Genomes Project database at a frequency of 0.1%. Immunoblot analysis in patient fibroblasts demonstrated absence of ACOX2 protein expression.
For discussion of the 4-bp deletion (c.456_459del, NM_003500.4) in the ACOX2 gene, resulting in a frameshift and premature termination (Thr154fs), that was identified in 2 unrelated patients with congenital bile acid synthesis defect-6 (CBAS6; 617308) by Alonso-Pena et al. (2022), see 601641.0002.
Alonso-Pena, M., Espinosa-Escudero, R., Herraez, E., Briz, O., Cagigal, M. L., Gonzalez-Santiago, J. M., Ortega-Alonso, A., Fernandez-Rodriguez, C., Bujanda, L., Calvo Sanchez, M., D Avola, D., Londono, M. C., and 11 others. Beneficial effect of ursodeoxycholic acid in patients with acyl-CoA oxidase 2 (ACOX2) deficiency-associated hypertransaminasemia. Hepatology 76: 1259-1274, 2022. [PubMed: 35395098] [Full Text: https://doi.org/10.1002/hep.32517]
Baumgart, E., Vanhooren, J. C. T., Fransen, M., Marynen, P., Puype, M., Vandekerckhove, J., Leunissen, J. A. M., Fahimi, H. D., Mannaerts, G. P., Van Veldhoven, P. P. Molecular characterization of the human peroxisomal branched-chain acyl-CoA oxidase: cDNA cloning, chromosomal assignment, tissue distribution, and evidence for the absence of the protein in Zellweger syndrome. Proc. Nat. Acad. Sci. 93: 13748-13753, 1996. [PubMed: 8943006] [Full Text: https://doi.org/10.1073/pnas.93.24.13748]
Baumgart, E., Vanhooren, J. C. T., Fransen, M., Van Leuven, F., Fahimi, H. D., Van Veldhoven, P. P., Mannaerts, G. P. Molecular cloning and further characterization of rat peroxisomal trihydroxycoprostanoyl-CoA oxidase. Biochem. J. 320: 115-121, 1996. [PubMed: 8947475] [Full Text: https://doi.org/10.1042/bj3200115]
Ferdinandusse, S., Denis, S., van Roermund, C. W. T., Preece, M. A., Koster, J., Ebberink, M. S., Waterham, H. R., Wanders, R. J. A. A novel case of ACOX2 deficiency leads to recognition of a third human peroxisomal acyl-CoA oxidase. Biochim. Biophys. Acta Molec. Basis Dis. 1864: 952-958, 2018. [PubMed: 29287774] [Full Text: https://doi.org/10.1016/j.bbadis.2017.12.032]
Moghrabi, N. N., Naylor, S. L., Van Veldhoven, P. P., Baumgart, E., Dawson, D. B., Bennett, M. J. Assignment of the human peroxisomal branched-chain acyl-CoA oxidase gene to chromosome 3p21.1-p14.2 by rodent/human somatic cell hybridization. Biochem. Biophys. Res. Commun. 231: 767-769, 1997. [PubMed: 9070889] [Full Text: https://doi.org/10.1006/bbrc.1997.6192]
Monte, M. J., Alonso-Pena, M., Briz, O., Herraez, E., Berasain, C., Argemi, J., Prieto, J., Marin, J. J. G. ACOX2 deficiency: an inborn error of bile acid synthesis identified in an adolescent with persistent hypertransaminasemia. J. Hepatol. 66: 581-588, 2017. [PubMed: 27884763] [Full Text: https://doi.org/10.1016/j.jhep.2016.11.005]
Vilarinho, S., Sari, S., Mazzacuva, F., Bilguvar, K., Esendagli-Yilmaz, G., Jain, D., Akyol, G., Dalgic, B., Gunel, M., Clayton, P. T., Lifton, R. P. ACOX2 deficiency: a disorder of bile acid synthesis with transaminase elevation, liver fibrosis, ataxia, and cognitive impairment. Proc. Nat. Acad. Sci. 113: 11289-11293, 2016. [PubMed: 27647924] [Full Text: https://doi.org/10.1073/pnas.1613228113]