Prenatal diagnosis of Zellweger syndrome and related disorders: impaired degradation of phytanic acid

Peroxisomes and PPARs: Emerging role as master regulators of cancer metabolism

Cancer is a disease characterized by the acquisition of a multitude of unique traits. It has long been understood that cancer cells divert significantly from normal cell metabolism. The most obvious of metabolic changes is that cancer cells strongly rely on glucose conversion by aerobic glycolysis. In addition, they also regularly develop mechanisms to use lipids and fatty acids for their energy needs. Peroxisomes lie central to these adaptive changes of lipid metabolism. Peroxisomes are metabolic organelles that take part in over 50 enzymatic reactions crucial for cellular functioning. Thus, they are essential for an effective and comprehensive use of lipids' energy supplied to cells. Cancer cells display a substantial increase in the biogenesis of peroxisomes and an increased expression of proteins necessary for the enzymatic functions provided by peroxisomes. Moreover, the enzymatic conversion of FAs in peroxisomes is a significant source of reactive oxygen and nitrogen species (ROS/RNS) that strongly impact cancer malignancy. Important regulators in peroxisomal FA oxidation and ROS/RNS generation are the transcription factors of the peroxisome proliferator-activated receptor (PPAR) family. This review describes the metabolic changes in tumorigenesis and cancer progression influenced by peroxisomes. We will highlight the ambivalent role that peroxisomes and PPARs play in the different stages of tumor development and summarize our current understanding of how to capitalize on the comprehension of peroxisomal biology for cancer treatment.

Peroxisomes: a nexus for lipid metabolism and cellular signaling

Peroxisomes are often dismissed as the cellular hoi polloi, relegated to cleaning up reactive oxygen chemical debris discarded by other organelles. However, their functions extend far beyond hydrogen peroxide metabolism. Peroxisomes are intimately associated with lipid droplets and mitochondria, and their ability to carry out fatty acid oxidation and lipid synthesis, especially the production of ether lipids, may be critical for generating cellular signals required for normal physiology. Here, we review the biology of peroxisomes and their potential relevance to human disorders including cancer, obesity-related diabetes, and degenerative neurologic disease.

Pitfalls in the prenatal diagnosis of peroxisomal beta-oxidation defects by chorionic villus sampling

Variability in the level of expression of very long chain fatty acids (VLCFAs) is documented in cultured chorionic villus (CV) cells derived from two fetuses, one at risk for an unusual peroxisomal fatty acid beta-oxidation defect, and the other at risk for the X-linked form of adrenoleucodystrophy (ALD). Cells from early subcultures of chorionic cells from both cases gave normal values for VLCFA ratios. The results for the fetus at risk for the beta-oxidation defect were interpreted to indicate that the fetus was not affected; however, at birth, the infant was clinically and biochemically affected. In the case of the fetus at risk for X-linked ALD, although VLCFAs were normal in subculture 1, the levels of these fatty acids increased dramatically in subculture 3, suggesting an abnormal fetus. Termination of the pregnancy and subsequent biochemical and morphological follow-up confirmed that the fetus was indeed affected by ALD.

Formic acid is a product of the alpha-oxidation of fatty acids by human skin fibroblasts: deficiency of formic acid production in peroxisome-deficient fibroblasts

Human skin fibroblasts in culture can oxidize beta-methyl fatty acids, such as phytanic acid and 3-methylhexadecanoic acid, to CO2 and water-soluble products. The latter are released largely into the culture medium. The major water-soluble product formed from [1-14C]phytanic and [1-14C]3-methylhexadecanoic acids is [14C]formic acid. As phytanic acid and 3-methylhexadecanoic acids contain beta-methyl groups and theoretically cannot be degraded by beta-oxidation, we postulate that formic acid is formed from fatty acids by alpha-oxidation. The marked reduction in formic acid production from beta-methyl fatty acids in peroxisome-deficient skin fibroblasts suggests that peroxisomes are involved in the generation of C1 units.

Autopsy findings in two siblings with infantile Refsum disease

Recognition of adrenal atrophy during a review of autopsy findings in two sisters who died at 8 months and 3 1/2 years prompted estimation of very long chain fatty acids, phytanic acid and pristanic acid on wet liver fixed in formalin for 12 years. These were shown to be markedly increased and defects in multiple peroxisomal functions and decrease in particulate catalase were shown in cultured fibroblasts, confirming an abnormality of peroxisomal biogenesis. The patients had presented with failure to thrive, recurrent diarrhoea and vomiting, poor mental development, retinal pigmentation, blindness and in the older patient deafness, with only mild dysmorphic features. Autopsy in the older patient showed adrenal atrophy, cirrhosis, and foamy histiocytes in multiple organs. The brain showed no demyelination, little cytoarchitectural abnormality, occasional perivascular histiocytes in the grey matter and meninges and prominent Purkinje cells in the molecular layer of the cerebellum. In the younger patient the changes were very subtle in spite of the marked clinical similarity. Despite the young age at death the clinicopathological features are most suggestive of infantile Refsum disease. In many situations anatomical pathology can be very useful in the recognition and study of peroxisomal disorders.

Peroxisomal disorders

Peroxisomal disorders occur more frequently and have a wider range of clinical manifestations than has been realized in the past. Precise diagnosis can be achieved with non-invasive biochemical assays and all can be diagnosed prenatally, thus providing the option of genetic counseling. Specific therapy is being evaluated for one of these disorders (adrenoleukodystrophy). In addition to these clinical advances, a great deal of information has been gained recently about the biogenesis and normal function of peroxisomes. These latter advances have been stimulated, and in part created, by the study of human disease states. It is for these reasons that a review of these human disease states is relevant for the clinical biochemist.

Molecular species of phosphatidylcholine containing very long chain fatty acids in human brain: enrichment in X-linked adrenoleukodystrophy brain and diseases of peroxisome biogenesis brain

Molecular species of phosphatidylcholine containing unsaturated (i.e., monoenoic and polyenoic) 32- to 40-carbon (very long chain) fatty acids (VLCFA-PC) are present in normal human brain, the fatty acid composition changing significantly with development. There is a marked increase in the concentration and a change in the polyenoic VLCFA composition of these molecular species in brains of patients with inherited defects in peroxisomal biogenesis [Zellweger's syndrome, neonatal adrenoleukodystrophy (ALD), and infantile Refsum's disease]. In contrast, there is a marked increase in monoenoic VLCFA-PC in X-linked ALD whereas molecular species containing polyenoic VLCFA are minor components.

Glyceryl ethers in peroxisomal disease

1-O-Alkyl and 1-O-alk-1-enyl (plasmalogens) glyceryl ether lipid levels were measured in post-mortem brain and/or liver biopsies from 7 patients with ultrastructural and biochemical evidence of a defect in peroxisomal biogenesis and/or enzymological evidence of a disturbance in ether lipid synthesis. Near normal levels of both species of glyceryl ether lipids were found in neonatal adrenoleukodystrophy and infantile Refsum's disease but marked deficiencies were found in Zellweger's syndrome and rhizomelic chondrodysplasia punctata, the latter manifesting the most profound reduction in ether lipid levels. These observations suggest that little ether lipid biosynthesis occurs in vivo in rhizomelic chondrodysplasia punctata or Zellweger's syndrome. However, in some phenotypes with apparently gross reductions in peroxisomal numbers, e.g. neonatal adrenoleukodystrophy and infantile Refsom's disease, there is significant ether lipid synthesis in liver and brain.

Defective oxidation of pristanic acid by fibroblasts from patients with disorders in propionic acid metabolism

The alpha-methyl fatty acid, pristanic acid (2, 6, 10, 14 tetramethylpentadecanoic acid) is oxidised rapidly by normal skin fibroblasts in culture to CO2 and to water-soluble metabolites. The latter are secreted into the culture medium. Fibroblasts from patients with propionyl CoA carboxylase, and to a lesser extent from patients with methylmalonyl CoA mutase defects, show reductions in the amount of CO2 released, although the production of water-soluble metabolites is not affected. Our data indicate that propionic acid is produced from pristanic acid, and ultimately from its immediate precursor phytanic acid. As phytanic and pristanic acids are significant components of diets rich in ruminant fats, it is likely that they may contribute to the accumulation of propionate and its metabolites in disorders of propionate metabolism.

Prenatal and perinatal diagnosis of peroxisomal disorders

Peroxisomes play an essential role in human cellular metabolism. Peroxisomal disorders, a group of genetic diseases caused by peroxisomal dysfunction, can be classified into three groups: (1) disorders of peroxisome biogenesis with a generalized loss of peroxisomal functions (Zellweger syndrome, neonatal adrenoleukodystrophy, infantile Refsum disease, hyperpipecolic acidaemia); (2) disorders with a loss of multiple peroxisomal functions (rhizomelic chondrodysplasia punctata and Zellweger-like syndrome; (3) disorders with loss of a single peroxisomal function (X-linked adrenoleukodystrophy, peroxisomal thiolase deficiency, bifunctional protein deficiency, acyl-CoA oxidase deficiency, classic Refsum disease, hyperoxaluria type I and acatalasaemia). Prenatal diagnosis is indicated in all these genetic disorders with the exception of classic Refsum disease, most types of hyperoxaluria type I and acatalasaemia. A variety of techniques is available now for the prenatal diagnosis of peroxisomal disorders in the first or second trimester of gestation. Prenatal diagnosis was performed by us in 70 pregnancies at risk for a disorder of peroxisome biogenesis, three for rhizomelic chondrodysplasia punctata, four for X-linked adrenoleukodystrophy and two for a defect in peroxisomal beta-oxidation. Fourteen affected fetuses were identified; no false negative cases were obtained.

Prenatal diagnosis of rhizomelic chondrodysplasia punctata

Plasmalogen biosynthesis and phytanic acid oxidation activity were measured in cultured chorionic villus samples or amniocytes from four pregnancies at risk for the rhizomelic form of chondrodysplasia punctata (RCDP). Normal results were obtained in three of the samples and post-natal examination or fetal ultrasound studies confirmed that the fetuses were unaffected. Chorionic villus culture in one case demonstrated defective plasmalogen biosynthesis and lack of phytanic acid oxidation. Pregnancy was interrupted at 10 weeks. Immunoblot studies of post-mortem fetal tissues showed that peroxisomal 3-oxoacyl-coenzyme A thiolase was present in the unprocessed form, a finding we had previously demonstrated in RCDP. These results establish that RCDP can be identified prenatally.

Prenatal diagnosis of Zellweger syndrome by measurement of very long chain fatty acid (C26:0) beta-oxidation in cultured chorionic villous fibroblasts: implications for early diagnosis of other peroxisomal disorders

In this paper we show that cultured chorionic villous fibroblasts efficiently catalyse the peroxisomal beta-oxidation of hexacosanoic acid (cerotic acid), a saturated very long chain fatty acid containing 26 carbon atoms. Hexacosanoic beta-oxidation was found to be strongly impaired in cultured chorionic villous fibroblasts from a Zellweger foetus. This finding indicates that measurement of peroxisomal beta-oxidation can be used (in addition to measurement of acyl-CoA:dihydroxyacetone phosphate acyltransferase, de novo plasmalogen biosynthesis, the amount of particle-bound catalase and phytanic acid oxidase) for prenatal diagnosis in the first trimester of Zellweger syndrome, infantile Refsum disease and neonatal adrenoleukodystrophy. The method should be equally applicable to the early prenatal diagnosis of disorders in which there is a deficiency of a single peroxisomal beta-oxidation enzyme. Such diseases include X-linked adrenoleukodystrophy (peroxisomal very long chain fatty acyl CoA ligase deficiency), 'pseudo-Zellweger syndrome' (peroxisomal 3-oxoacyl-CoA thiolase deficiency) and 'pseudo-neonatal adrenoleukodystrophy' (acyl-CoA oxidase deficiency).