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  • Updated 03.15.2023
  • Released 02.01.1994
  • Expires For CME 03.15.2026

Single enzyme defects of peroxisomal beta-oxidation



Clinical sequencing studies continue to expand our knowledge of the diverse genetic landscape of single enzyme defects of peroxisomal beta-oxidation. As a result of genetic profiling over the past 3 years, several new clinical subtypes with milder presentations have been discovered. Moreover, genetic analyses have provided novel insights into the molecular basis of observed heterogenous clinical phenotypes.

Key points

• Single enzyme defects of peroxisomal beta-oxidation reported in the clinic are acyl-CoA oxidase 1 (ACOX1), acyl-CoA oxidase 2 (ACOX2), D-bifunctional protein (HSD17B4), sterol-carrier protein X (SCP2), alpha-methylacyl-CoA racemase (AMACR), ATP-binding cassette transporter protein family D member 3 (ABCD3), and acyl-CoA binding domain containing protein 5 (ACBD5) deficiencies.

• Inherited deficiencies in the ABCD1, ABCD3, or ACBD5 peroxisomal membrane proteins can also impair peroxisomal beta-oxidation by compromising the import of relevant substrates into the peroxisome matrix for beta-oxidation.

• Single enzyme defects of peroxisomal beta-oxidation are rare genetic disorders that most typically have autosomal recessive modes of transmission.

• Most have highly variable degrees of severity that are influenced by any residual levels of enzymatic activity.

• Although typically affecting multiple organ systems, the majority of the single enzyme defects of peroxisomal beta-oxidation are characterized by neurodevelopmental and neurodegenerative phenotypes that can vary in severity.

• In addition to genetic profiling, biochemical analyses of peroxisomal metabolites are essential for establishing a diagnosis.

Historical note and terminology

Peroxisomes are membrane-bound metabolic organelles present in nearly all eukaryotic cell types and are involved in cell signaling processes (111; 109; 93). They were first described as "microbodies" in the electron-microscopic examination of mouse kidney cells (80). Later, they were named “peroxisomes” by Christian De Duve and Pierre Baudhuin (20). Goldfischer and colleagues first established an association between the presence and functions of peroxisomes and human disease (39). Ensuing investigations over the next five decades led to the discovery of numerous connections between peroxisome dysfunction and the development and progression of rare and common disorders.

Peroxisomal disorders with Mendelian inheritance patterns can be broadly subdivided into those involving (i) defects in peroxisome assembly or (ii) the functions of single enzymes or membrane transporters. The former are currently referred to as peroxisome biogenesis disorders. Peroxisome biogenesis disorders, such as Zellweger spectrum disorder and rhizomelic chondrodysplasia punctata type 1 (RCDP1), are caused by biallelic loss-of-function variants in PEX genes encoding proteins involved in peroxisome biogenesis called peroxins (58). As a result, peroxisomes are improperly assembled and often diminished in number, which results in impaired metabolic functions that affect multiple organ systems (10).

There are several different Mendelian disorders associated with defects in single peroxisome enzymes or membrane transporters. For example, loss-of-function variants in ABCD1, a peroxisomal transmembrane protein that transports very long-chain fatty acids (VLCFA, containing 22 or more carbons) as CoA-esters into the peroxisome where they are catabolized by peroxisomal beta-oxidation, causes the most frequent peroxisomal disorder, X-linked adrenoleukodystrophy (44). Furthermore, there are Mendelian disorders in which peroxisomes are present, but distinct enzymes normally located in their matrix are absent or defective. They are referred to as peroxisomal single enzyme defects (114). Given the wide variety of peroxisomal metabolic functions, single enzyme defects of peroxisomal beta-oxidation can be further classified according to their impact on distinct metabolic pathways and processes. Single enzyme defects of peroxisomal beta-oxidation are the focus of this article. Although most early studies focused on patients with the most severe disease, more recent clinical sequencing has identified longer-surviving patients with hypomorphic alleles that convey residual enzymatic activity and can lessen disease severity.

Lazarow and de Duve were the first to provide evidence of a peroxisomal fatty acyl-CoA oxidizing system (54). It is now known that biallelic loss-of-function variants in any of a group of proteins required for peroxisomal fatty acid beta-oxidation can lead to disease (111; 114; 110). Mendelian disorders caused by inherited deficiencies in enzymatic activities directly involved in peroxisomal beta-oxidation include acyl-CoA oxidase 1 and 2 (ACOX1/2), D-bifunctional protein (HSD17B4), alpha-methylacyl-CoA racemase (AMACR), and sterol-carrier protein X (SCP2). Single enzyme defects of peroxisomal beta-oxidation are relatively understudied disorders. As such, many aspects, such as variations in the initially described phenotypes, the natural history of disease, the characterization of the underlying genetic defects, and the evolution of the clinical manifestations from these genetic defects, are still under investigation.

To date, all reported cases of peroxisomal acyl-CoA oxidase deficiency are due to loss-of-function variants in the ACOX1 or ACOX2 genes (114). ACOX1 is involved in the peroxisomal catabolism of straight-chain substrates, including VLCFAs, dicarboxylic acids, and polyunsaturated fatty acids (30). Poll-The and colleagues were the first to report acyl-CoA deficiency in the case of two siblings who began their neonatal period with severe muscle hypotonia and seizures (77). Their shared features included delayed psychomotor development and eventual progressive neurologic regression, sensorineural hearing deficits, and an abnormal electroretinogram. Although they showed progressive cerebral white-matter demyelination, there were no signs of cortical malformation. By 2021, over 30 patients with ACOX1 deficiency with variable ranges of severity had been reported (65). Since then, an additional three patients with ACOX1 deficiency have been reported (05). Severe early-onset ACOX1 deficiency can involve neonatal hypotonia, seizures, progressive white matter disease, impaired vision and hearing, hepatomegaly, severely delayed psychomotor development, and a shortened lifespan (27; 11; 114). More moderately affected siblings that showed mild global developmental delay from infancy have subsequently been reported (65). They began to regress at 5 years of age and gradually manifested with cerebellar ataxia, dysarthria, pyramidal signs, and dysphasia (65). An even milder form has been described in adult siblings, 52 and 55 years of age, both of whom were nonambulatory, with mild to moderate cognitive impairment and impaired vision and brain MRI showing profound atrophy of the brainstem and cerebellum in both patients (25). Subsequently, three patients with the same de novo dominant gain-of-function ACOX1 p.N237S variant were described (16). All three presented with a progressive myeloneuropathy with sensorineural hearing loss; onset ranged from 3 to 12 years of age (16). The authors proposed naming the disease caused by the de novo gain-of-function ACOX1 pathogenic variant Mitchell disease in honor of the first patient studied. It is also referred to as Mitchell syndrome (

In contrast to ACOX1, ACOX2 is involved in the peroxisomal catabolism of CoA-esters of branched-chain fatty acids (BCFAs) and C27-bile acid intermediates (30). Only four cases of ACOX2 deficiency have been reported to date (101; 64; 30; 124). The most severely affected patient displayed seizures 2 days after birth and died before 1 year of age (30). The authors noted the patient was born from a consanguineous union and that additional genetic causes could not be excluded. A more moderately affected 8-year-old child showed intermittently elevated transaminase levels, liver fibrosis, elevated bile acid intermediates, slurred speech, vertical gaze palsy, slight dysmetria, and mild gait ataxia as well as mild cognitive impairment (101; 124). Two mildly affected siblings (13 and 16 years old) showed hypertransaminasemia that peaked on exposure to drugs, such as nonsteroidal anti-inflammatory drugs (NSAIDs) and D-penicillamine in one and anti-flu medication in the other (64).

D-bifunctional protein deficiency (also called DBP deficiency or HSD17B4 deficiency) is the most frequently diagnosed single enzyme defect of peroxisomal fatty acid beta-oxidation, with over 130 cases reported in the literature to date (13). DBP is caused by biallelic loss-of-function variants in the HSD17B4 gene that encodes 17-beta-estradiol dehydrogenase, often referred to as D-bifunctional protein (106). DBP/HSD17B4 is involved in the catabolism of VLCFAs, BCFAs, and bile acid intermediates (28). Watkins and colleagues were the first to report a patient with a peroxisomal bifunctional enzyme deficiency (117). Although this initial report claimed the patient lacked L-bifunctional protein, it was later established that they lacked D-bifunctional protein (96).

Wanders and colleagues divided patients with DBP deficiency into three subgroups based on the extent of their enzyme deficiency (104). D-bifunctional protein encompasses two primary enzyme activities: enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase (104). Type I patients lack both activities, whereas type II patients lack only hydratase activity, and type III patients lack only dehydrogenase activity. In a series of 110 severely affected patients typically surviving less than 36 months, individuals with types I, II, or III DBP deficiency were reported (28c). The results suggested that the amount of residual DBP activity correlates with the severity of medical phenotypes and that a genotype-phenotype correlation exists. Adolescent brothers with milder symptoms were found to have a missense variant in the hydratase domain of one allele and a missense variant in the dehydrogenase domain of the other allele, suggesting they represent a new classification (type IV) (63). Since 2017, numerous reports have emerged of severe (07; 13; 53; 82; 15; 123; 120), moderate (24; 46; 53; 05; 122), and milder (14; 60; 59) forms of DBP deficiency.

Ferdinandusse and colleagues reported the first cases of AMACR deficiency (26; 35; 62). Two patients presented with adult-onset sensory motor neuropathy. One also had epileptic seizures and pigmentary retinopathy, whereas the other patient had upper motor neuron signs in the legs. A third patient was a child who was diagnosed with Niemann-Pick disease type C (NPC) complementation group 1 and showed progressive neurologic signs at 18 months of age. Since then, additional adult patients (17; 49; 87; 22; 89; 42) and juvenile-onset cases (98; 84; 92; 03; 51) have been described in the literature.

To date, there has been only one report of a patient with a sterol carrier protein X (SCPx, now named SCP2) deficiency (28b). In this case, the patient was homozygous for a frameshift variant in the SCP2 gene. SCPx has an amino-terminal thiolase domain and a carboxy-terminal sterol carrier protein 2 (SCP2) domain, and after SCPx protein import into the peroxisomal matrix, a proteolytic processing event gives rise to two separate proteins with thiolase (important for peroxisomal beta-oxidation) and SCP2 activities, respectively. The resulting thiolase can catabolize straight chain VLCFAs, BCFAs, and bile acid intermediates (114). The patient first experienced neurologic symptoms, including encephalopathy with dystonia as well as motor and peripheral neuropathies, in the second decade of life.

In 1986, Goldfischer and colleagues reported a patient whose clinical presentation and course resembled that of severe Zellweger spectrum disorder (38). Peroxisomes were present in their liver tissue, which ruled out a severe Zellweger spectrum disorder. Additional studies at that time suggested that the peroxisomal beta-oxidation enzyme 3-ketoacyl-CoA thiolase (ACAA1) was absent in this patient's liver (83). Later, it was found that peroxisomal 3-ketoacyl-CoA thiolase was present in postmortem brain, whereas D-bifunctional protein was absent (36). Pathogenic variants in the HSD17B4 gene were found and a diagnosis of DBP deficiency was made. The authors concluded that there is no longer evidence for the existence of ACAA1 peroxisomal thiolase deficiency as a distinct clinical entity (36).

Lastly, genetic deficiencies have been noted in three peroxisome membrane proteins that are transporters critical for normal peroxisomal beta-oxidation (ie, ABCD3, ACBD5, and ABCD1 deficiencies). The latter is the cause of the most common peroxisomal disorder, X-linked adrenoleukodystrophy, which is the subject of a separate review. Although there are a limited number of patients reported with ABCD3 and ACBD5 deficiencies, they are discussed in this review.

To date, there has been one report of a patient with an ABCD3 deficiency (33). ABCD3 is an abundant peroxisome membrane protein that facilitates entry of dicarboxylic acids, BCFAs, and bile acid intermediates into the peroxisome matrix for metabolism (33; 78). ABC (ATP-binding cassette) protein family members are involved in active transport and can have ATPase activity; thus, ABCD3 is included as a single enzyme defect of peroxisomal beta-oxidation. A patient presented with jaundice at 6 months and was hospitalized at 1.5 years of age with fever, gastroenteritis, hepatosplenomegaly, and severe anemia (33). By 4 years of age, the patient deteriorated rapidly with liver cirrhosis and hepatopulmonary syndrome and died 5 days after a liver transplantation.

To date, seven patients with a deficiency in ACBD5, a protein required for normal peroxisomal beta-oxidation, have been reported (31; 121; 09; 40). ACBD5 is a peroxisomal membrane protein with a membrane-spanning region and a cytosolic acyl-CoA binding domain proposed to sequester very long-chain fatty acyl-CoAs (ie, activated VLCFAs) in the cytosol and facilitate their transport into the peroxisomal matrix for catabolism (31; 121; 47). Although it has never been reported to have intrinsic enzymatic activity, it is mentioned in this review due to its role in peroxisomal beta-oxidation. ACBD5 also plays an important role in inducing contact site formation between peroxisomes and the endoplasmic reticulum (19). The first report of a putative ACBD5 deficiency involved three affected siblings with cone-rod dystrophy and psychomotor delay associated with significant white matter involvement (01). Later, a patient with ACBD5 deficiency who was born with a cleft palate and, at 7 months of age, showed a retinal dystrophy was reported (31). By 9 years of age, she showed a progressive leukodystrophy and ataxia. She was confirmed to be homozygous for a ACBD5 frameshift variant and showed elevated VLCFAs in blood and cultured skin fibroblasts. Since then, two siblings (40) and one adult with ACBD5 deficiency have been reported (09).

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