This article includes discussion of single enzyme defects of peroxisomal beta-oxidationACOX1 deficiency, Acyl-CoA oxidase deficiency, D-bifunctional protein deficiency, HSD17B4 deficiency, and thiolase deficiency. The foregoing terms may include synonyms, similar disorders, variations in usage, and abbreviations.
Enzymes in the peroxisomal fatty acid beta-oxidation pathway not only help maintain very long-chain and branched-chain fatty acid homeostasis but also participate in bile acid synthesis. In this article, the author highlights 2 patients with mutations in peroxisomal beta-oxidation pathway proteins that manifest primarily as defects in bile acid synthesis.
• Recognized “single enzyme defects of peroxisomal beta-oxidation” include deficiencies of acyl-CoA oxidase 1 (ACOX1), acyl-CoA oxidase 2 (ACOX2), D-bifunctional protein (HSD17B4), sterol-carrier protein X (SCP2), alpha-methylacyl-CoA racemase (AMACR), and ATP-binding cassette transporter protein family D member 3 (ABCD3).
• Of these rare diseases, ACOX1 and HSD17B4 deficiencies are most common and typically present in infancy, whereas the more rare SCP2 and AMACR deficiencies are primarily found in adults.
• Neonatal hypotonia and neonatal seizures refractory to conventional therapy are the most consistent manifestations of ACOX1 and HSD17B4 deficiencies.
• Biochemical analyses, including plasma very long-chain fatty acids, branched-chain fatty acids, and bile acid intermediates, are essential for establishing a diagnosis.
• No effective treatment is currently available for deficiencies of ACOX1 or HSD17B4.
Historical note and terminology
Peroxisomes are cell organelles present in nearly all eukaryotic cells (Hruban et al 1972; Novikoff et al 1973). They were first described as "microbodies" in 1954 during electron-microscopic examination of mouse kidney cells (Rhodin 1954). Peroxisomes contain numerous enzymes that participate in multiple metabolic pathways (Osmundsen et al 1991; Mannaerts and van Veldhoven 1993). However, it was not until 1973 that an association between the presence of peroxisomes, their function, and human disease was established (Goldfischer et al 1973).
In some peroxisomal disorders, such as Zellweger syndrome, neonatal adrenoleukodystrophy, and infantile Refsum disease, peroxisomes are absent, decreased in number, or severely abnormal, resulting in defects in the multiple metabolic pathways found in this organelle (Gould et al 2001). These diseases are referred to as disorders of peroxisomal biogenesis. In contrast, in a subgroup of peroxisomal disorders the organelles are present but distinct peroxisomal enzymes are absent or defective; these conditions are referred to as single enzyme defects (Wanders et al 2001).
In 1976, Lazarow and de Duve showed that a fatty acid beta-oxidation system different from the mitochondrial system is present in peroxisomes (Lazarow and de Duve 1976). Subsequent research has shown 2 complete sets of beta-oxidation enzymes in this organelle (Wanders et al 2001). Defects in several proteins that participate in peroxisomal beta-oxidation that lead to severe diseases in humans have been described (Moser et al 2001; Wanders et al 2001). One is the adrenoleukodystrophy protein, deficient in the most frequent peroxisomal disorder, X-linked adrenoleukodystrophy (Moser et al 2001). Although peroxisomal beta-oxidation of very long-chain fatty acids is deficient in this disorder, the defective protein is a peroxisomal membrane protein of unknown function that belongs to the superfamily of ATP-binding cassette transmembrane transporters. Defects in 3 enzymes whose roles in peroxisomal fatty acid beta-oxidation are now well established (acyl-CoA oxidase, D-bifunctional protein, and alpha-methylacyl-CoA racemase) have only been described since 1988. Deficiency of a fourth enzyme, 3-ketoacyl-CoA thiolase, was previously reported, but this entity is now thought not to exist. In 2006, a patient with confirmed deficiency of peroxisomal sterol carrier protein X, which also has 3-ketoacyl-CoA thiolase activity, was described (Ferdinandusse et al 2006b).
In 1988, Poll-The and associates reported the cases of 2 siblings whose phenotype resembled neonatal adrenoleukodystrophy but for whom biopsy findings revealed the presence of peroxisomes in the liver. Additional immunoblotting studies of these patients with "pseudo-neonatal adrenoleukodystrophy" revealed the absence of the peroxisomal beta-oxidation enzyme acyl-CoA oxidase (Poll-The et al 1988).
A patient with a peroxisomal beta-oxidation defect at the level of the bifunctional enzyme was reported by Watkins and colleagues in 1989 (Watkins et al 1989). Although the initial report claimed that the patient lacked L-bifunctional protein, it has been established that he lacked D-bifunctional protein (van Grunsven 1999). Several other documented cases of D-bifunctional protein deficiency have been reported, and this is the most frequently diagnosed of the single enzyme defects of peroxisomal fatty acid beta-oxidation (Moser 1999). Wanders and coworkers divided D-bifunctional protein-deficient patients into 3 subgroups based on the extent of their enzyme deficiency. As the name implies, “bifunctional” protein encompasses 2 primary enzyme activities, enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase. Type I patients lack both activities, whereas type II lack only hydratase activity and type III lack only the dehydrogenase (Wanders et al 2001). In a large series published in 2006, the percentages of patients with types I, II, or III D-bifunctional protein deficiency were reported to be 27%, 38%, and 45%, respectively (Ferdinandusse et al 2006). Exome sequencing has been instrumental in identifying longer-surviving patients with compound heterozygous mutations. Adolescent brothers with milder symptoms were found to have a missense mutation in the hydratase domain of one allele and a missense mutation in the dehydrogenase domain of the other allele, suggesting they represent a new classification (type IV) (McMillan et al 2012).
In 1986, Goldfischer and associates had reported a patient whose clinical presentation and course resembled that of Zellweger syndrome (Goldfischer et al 1986). Peroxisomes were present in liver tissue of this patient and the term "pseudo-Zellweger syndrome" was used to describe this situation where there was a discrepancy between the clinical phenotype and the results of pathological examination. Additional studies at that time suggested that the peroxisomal enzyme 3-ketoacyl-CoA thiolase (ACAA1) was absent in this patient's liver (Schram et al 1987). However, this case was reinvestigated and it was found that thiolase was present in postmortem brain, whereas D-bifunctional protein was absent (Ferdinandusse et al 2002). Mutational analysis confirmed the defect in this patient's HSD17B4 (hydroxysteroid (17-beta) dehydrogenase 4, formerly DBP or D-bifunctional protein) gene. These authors concluded that there is no longer evidence for the existence of thiolase deficiency as a distinct clinical entity.
Other rare causes of peroxisomal beta-oxidation defects continue to be identified. A child with bile acid abnormalities, ataxia, and cognitive impairment was found to have a mutation in ACOX2 via exome sequencing (Vilarinho et al 2016). Routine workup of another child with bile acid abnormalities for the possibility of peroxisomal involvement revealed a lack of ABCD3, an abundant peroxisome membrane protein that facilitates entry of branched-chain fatty acids and bile acid precursors into the organelle (Ferdinandusse et al 2015).
It was not until 2000 that the first diagnosis of alpha-methylacyl-CoA racemase (AMACR) deficiency was reported (Ferdinandusse et al 2000). Unlike acyl-CoA oxidase (ACOX1) deficiency and HSD17B4/D-bifunctional protein deficiency, in which symptoms are present at birth, racemase deficiency is primarily an adult-onset neuropathy. Similarly, the single patient with sterol carrier protein X (SCPX, now named SCP2) deficiency first experienced neurologic symptoms in the second decade of life (Ferdinandusse et al 2006b).
As peroxisomal disorders in general and especially peroxisomal single enzyme defects are still a relatively "young" group of disorders, 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.
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