Neonatal seizures and severe hypotonia in a male infant suffering from a defect in peroxisomal beta-oxidation

Autonomous Purkinje cell axonal dystrophy causes ataxia in peroxisomal multifunctional protein-2 deficiency

Peroxisomes play a crucial role in normal neurodevelopment and in the maintenance of the adult brain. This depends largely on intact peroxisomal β-oxidation given the similarities in pathologies between peroxisome biogenesis disorders and deficiency of multifunctional protein-2 (MFP2), the central enzyme of this pathway. Recently, adult patients diagnosed with cerebellar ataxia were shown to have mild mutations in the MFP2 gene, hydroxy-steroid dehydrogenase (17 beta) type 4 (HSD17B4). Cerebellar atrophy also develops in MFP2 deficient mice but the cellular origin of the degeneration is unexplored. In order to investigate whether peroxisomal β-oxidation is essential within Purkinje cells, the sole output neurons of the cerebellum, we generated and characterized a mouse model with Purkinje cell selective deletion of the MFP2 gene. We show that selective loss of MFP2 from mature cerebellar Purkinje neurons causes a late-onset motor phenotype and progressive Purkinje cell degeneration, thereby mimicking ataxia and cerebellar deterioration in patients with mild HSD17B4 mutations. We demonstrate that swellings on Purkinje cell axons coincide with ataxic behavior and precede neurodegeneration. Loss of Purkinje cells occurs in a characteristic banded pattern, proceeds in an anterior to posterior fashion and is accompanied by progressive astro- and microgliosis. These data prove that the peroxisomal β-oxidation pathway is required within Purkinje neurons to maintain their axonal integrity, independent of glial dysfunction.

Lipid homeostasis and inflammatory activation are disturbed in classically activated macrophages with peroxisomal β-oxidation deficiency

Macrophage activation is characterized by pronounced metabolic adaptation. Classically activated macrophages show decreased rates of mitochondrial fatty acid oxidation and oxidative phosphorylation and acquire a glycolytic state together with their pro-inflammatory phenotype. In contrast, alternatively activated macrophages require oxidative phosphorylation and mitochondrial fatty acid oxidation for their anti-inflammatory function. Although it is evident that mitochondrial metabolism is regulated during macrophage polarization and essential for macrophage function, little is known on the regulation and role of peroxisomal β-oxidation during macrophage activation. In this study, we show that peroxisomal β-oxidation is strongly decreased in classically activated bone-marrow-derived macrophages (BMDM) and mildly induced in alternatively activated BMDM. To examine the role of peroxisomal β-oxidation in macrophages, we used Mfp2^-/- BMDM lacking the key enzyme of this pathway. Impairment of peroxisomal β-oxidation in Mfp2^-/- BMDM did not cause lipid accumulation but rather an altered distribution of lipid species with very-long-chain fatty acids accumulating in the triglyceride and phospholipid fraction. These lipid alterations in Mfp2^-/- macrophages led to decreased inflammatory activation of Mfp2^-/- BMDM and peritoneal macrophages evidenced by impaired production of several inflammatory cytokines and chemokines, but did not affect anti-inflammatory polarization. The disturbed inflammatory responses of Mfp2^-/- macrophages did not affect immune cell infiltration, as mice with selective elimination of MFP2 from myeloid cells showed normal monocyte and neutrophil influx upon challenge with zymosan. Together, these data demonstrate that peroxisomal β-oxidation is involved in fine-tuning the phenotype of macrophages, probably by influencing the dynamic lipid profile during macrophage polarization.

Peroxisomal Disorders: A Review on Cerebellar Pathologies

Peroxisomes are organelles with diverse metabolic tasks including essential roles in lipid metabolism. They are of utmost importance for the normal functioning of the nervous system as most peroxisomal disorders are accompanied with neurological symptoms. Remarkably, the cerebellum exquisitely depends on intact peroxisomal function both during development and adulthood. In this review, we cover all aspects of cerebellar pathology that were reported in peroxisome biogenesis disorders and in diseases caused by dysfunction of the peroxisomal α-oxidation, β-oxidation or ether lipid synthesis pathways. We also discuss the phenotypes of mouse models in which cerebellar pathologies were recapitulated and search for connections with the metabolic abnormalities. It becomes increasingly clear that besides the most severe forms of peroxisome dysfunction that are associated with developmental cerebellar defects, milder impairments can give rise to ataxia later in life.

Early onset epilepsy and inherited metabolic disorders: diagnosis and management

Epileptic encephalopathies presenting in early life present a diagnostic and therapeutic challenge. These disorders present with multiple seizure types that are treatment resistant and associated with significant abnormalities on electroencephalographic studies. The underlying etiology in many cases may be related to an inborn error of metabolism. Efforts to establish the specific diagnosis of a genetic defect or an inborn error of metabolism often results in requests for a vast array of biochemical and molecular tests leading to an expensive workup. In this review, we present the clinician with information that provides a rationale for a selective and nuanced approach to biochemical assays, and initial treatment strategies while waiting for a specific diagnosis to be established. A careful consideration of the presentation, identification of potentially treatable conditions, and consultation with the biochemical genetics laboratory can lead to a greater measure of success while limiting cost overruns. Such a targeted approach is hoped will lead to an early diagnosis and appropriate interventions.

Peroxisomal disorders: the single peroxisomal enzyme deficiencies

Peroxisomal disorders are a group of inherited diseases in man in which either peroxisome biogenesis or one or more peroxisomal functions are impaired. The peroxisomal disorders identified to date are usually classified in two groups including: (1) the disorders of peroxisome biogenesis, and (2) the single peroxisomal enzyme deficiencies. This review is focused on the second group of disorders, which currently includes ten different diseases in which the mutant gene affects a protein involved in one of the following peroxisomal functions: (1) ether phospholipid (plasmalogen) biosynthesis; (2) fatty acid beta-oxidation; (3) peroxisomal alpha-oxidation; (4) glyoxylate detoxification, and (5) H2O2 metabolism.

Peroxisomal multifunctional protein-2 deficiency causes motor deficits and glial lesions in the adult central nervous system

In humans, mutations inactivating multifunctional protein-2 (MFP-2), and thus peroxisomal beta-oxidation, cause neuronal heterotopia and demyelination, which is clinically reflected by hypotonia, seizures, and death within the first year of life. In contrast, our recently generated MFP-2-deficient mice did not show neurodevelopmental abnormalities but exhibited aberrations in bile acid metabolism and one of three of them died early postnatally. In the postweaning period, all survivors developed progressive motor deficits, including abnormal cramping reflexes of the limbs and loss of mobility, with death at 6 months. Motor impairment was not accompanied by lesions of peripheral nerves or muscles. However, in the central nervous system MFP-2-deficient mice overexpressed catalase in glial cells, accumulated lipids in ependymal cells and in the molecular layer of the cerebellum, exhibited severe astrogliosis and reactive microglia predominantly within the gray matter of the brain and the spinal cord, whereas synaptic and myelin markers were not affected. This culminated in degenerative changes of astroglia cells but not in overt neuronal lesions. Neither the motor deficits nor the brain lesions were aggravated by increasing the branched-chain fatty acid concentration through dietary supplementation. These data indicate that MFP-2 deficiency in mice causes a neurological phenotype in adulthood that is manifested primarily by astroglial damage.

Peroxisomal multifunctional protein-2: the enzyme, the patients and the knockout mouse model

The mammalian multifunctional protein-2 (MFP-2, also called multifunctional enzyme 2, D-bifunctional enzyme or 17-beta-estradiol dehydrogenase type IV) was identified by several groups about a decade ago. It plays a central role in peroxisomal beta-oxidation as it handles most, if not all, peroxisomal beta-oxidation substrates. Deficiency of this enzyme in man causes a severe developmental syndrome with abnormalities in several organs but in particular in the brain, leading to death within the first year of life. Accumulation of branched-long-chain fatty acids and very-long-chain fatty acids and a disturbed synthesis of bile acids were documented in these patients. A mouse model with MFP-2 deficiency only partly phenocopies the human disease. Although the expected metabolic abnormalities are present, no neurodevelopmental aberrations are observed. However, the survival of these mice into adulthood allowed to document the importance of this enzyme for the normal functioning of the brain, eyes and testis. In the present review, the identification and biochemical characteristics of MFP-2, and the consequences of MFP-2 dysfunction in humans and in mice will be discussed.

Clinical and biochemical spectrum of D-bifunctional protein deficiency

OBJECTIVE

D-bifunctional protein deficiency is an autosomal recessive inborn error of peroxisomal fatty acid oxidation. Although case reports and small series of patients have been published, these do not give a complete and balanced picture of the clinical and biochemical spectrum associated with this disorder.

METHODS

To improve early recognition, diagnosis, prognosis, and management of this disorder and to provide markers for life expectancy, we performed extensive biochemical studies in a large cohort of D-bifunctional protein-deficient patients and sent out questionnaires about clinical signs and symptoms to the responsible physicians.

RESULTS

Virtually all children presented with neonatal hypotonia and seizures and died within the first 2 years of life without achieving any developmental milestones. However, within our cohort, 12 patients survived beyond the age of 2 years, and detailed information on 5 patients with prolonged survival (> or =7.5 years) is provided.

INTERPRETATION

Biochemical analyses showed that there is a clear correlation between several biochemical parameters and survival of the patient, with C26:0 beta-oxidation activity in cultured skin fibroblasts being the best predictive marker for life expectancy. Remarkably, three patients were identified without biochemical abnormalities in plasma, stressing that D-bifunctional protein deficiency cannot be excluded when all peroxisomal parameters in plasma are normal.

Phenotypic variability (heterogeneity) of peroxisomal disorders

Peroxisomes perform a multitude of biosynthetic and catabolic functions, many of which are related to lipid metabolism. Peroxisomal disorders result either from deficiency of a single peroxisomal enzyme or protein, or from a defect in the complex mechanism of peroxisomal biogenesis, resulting in deficiency of several or multiple peroxisomal functions. These can be assessed by a battery of biochemical assays, enabling a biochemical phenotype to be defined that is specific and diagnostic for each of the peroxisomal disorders. Some peroxisomal disorders have unique and specific clinical phenotypes, which may be diagnostic. Others share patterns of clinical abnormalities (particularly neurological dysfunction, craniofacial dysmorphism, skeletal defects, sensory deafness, retinopathy) consistent with defined clinical phenotypes, but with considerable overlap and heterogeneity. To a certain extent, the clinical features of a particular disorder reflect the accumulation or deficiency of specific metabolites. Thus, the same clinical phenotypes may be caused by both single enzyme defects and PBDs. Furthermore, the same defect may present with different clinical phenotypes. In general, the severity of the clinical phenotype correlates with the degree of biochemical dysfunction. The clinical heterogeneity of peroxisomal disorders constitutes a diagnostic challenge demanding a high index of suspicion on the clinician's part.

Human peroxisomal disorders

Peroxisomes are single membrane-bound cell organelles performing numerous metabolic functions. The present article aims to give an overview of our current knowledge about inherited peroxisomal disorders in which these organelles are lacking or one or more of their functions are impaired. They are multiorgan disorders and the nervous system is implicated in most. After a summary of the historical names and categories, each having distinct symptoms and prognosis, microscopic pathology is reviewed in detail. Data from the literature are added to experience in the authors' laboratory with 167 liver biopsy and autopsy samples from peroxisomal patients, and with a smaller number of chorion samples for prenatal diagnosis, adrenal-, kidney-, and brain samples. Various light and electron microscopic methods are used including enzyme- and immunocytochemistry, polarizing microscopy, and morphometry. Together with other laboratory investigations and clinical data, this approach continues to contribute to the diagnosis and further characterization of peroxisomal disorders, and the discovery of novel variants. When liver specimens are examined, three main groups including 9 novel variants (33 patients) are distinguished: (1) absence or (2) presence of peroxisomes, and (3) mosaic distribution of cells with and without peroxisomes (10 patients). Renal microcysts, polarizing trilamellar inclusions, and insoluble lipid in macrophages in liver, adrenal cortex, brain, and in interstitial cells of kidney are also valuable for classification. On a genetic basis, complementation of fibroblasts has classified peroxisome biogenesis disorders into 12 complementation groups. Peroxisome biogenesis genes (PEX), knock-out-mice, and induction of redundant genes are briefly reviewed, including some recent results with 4-phenylbutyrate. Finally, regulation of peroxisome expression during development and in cell cultures, and by physiological factors is discussed.

Distinction between peroxisomal bifunctional enzyme and acyl-CoA oxidase deficiencies

The clinical distinction between patients with a disorder of peroxisome assembly (e.g., Zellweger syndrome) and those with a defect in a peroxisomal fatty acid beta-oxidation enzyme can be difficult. We studied 29 patients suspected of belonging to the latter group. Using complementation analysis, 24 were found to be deficient in enoylcoenzyme A hydratase/3-hydroxyacylcoenzyme A dehydrogenase bifunctional enzyme and 5 were deficient in acyl-CoA oxidase. Elevated plasma very long-chain fatty acids (VLCFA), impaired fibroblast VLCFA beta-oxidation, decreased fibroblast phytanic acid oxidation, normal plasmalogen synthesis, normal plasma L-pipecolic acid level, and normal subcellular catalase distribution were characteristic findings in both disorders. The elevation in plasma VLCFA levels and impairment in fibroblast VLCFA beta-oxidation were more severe in bifunctional-deficient than in oxidase-deficient patients. The clinical course in bifunctional deficiency (profound hypotonia, neonatal seizures, dysmorphic features, age at death approximately 9 months) was more severe than in oxidase deficiency (moderate hypotonia without dysmorphic features, development of a leukodystrophy, age at death approximately 4 yr). Based on these findings, accurate early diagnosis of these deficiencies of peroxisomal beta-oxidation enzymes is possible.

Measurement of peroxisomal fatty acid beta-oxidation in cultured human skin fibroblasts

One of the main functions of mammalian peroxisomes is the beta-oxidation of a variety of fatty acids and fatty acid derivatives, including very long-chain fatty acids. Oxidation of these fatty acids is deficient in a number of different peroxisomal disorders, including the disorders of peroxisome biogenesis (Zellweger syndrome, neonatal adrenoleukodystrophy and infantile Refsum disease), X-linked adrenoleukodystrophy and a number of other disorders of peroxisomal beta-oxidation of known and unknown aetiology. Accurate measurement of peroxisomal fatty acid oxidation is of utmost importance for correct postnatal and prenatal diagnosis of these disorders. In this paper we describe a straightforward and accurate assay method to measure the beta-oxidation of palmitic acid (C16:0), hexacosanoic acid (C26:0) and pristanic acid in intact fibroblasts.

Neuropathology of peroxisomal diseases

This paper gives a description of the essential neuropathological techniques applied to the study of metabolic disorders affecting the nervous system. Subsequently, the neuropathological features of a series of peroxisomal disorders are described with special attention being paid to adrenoleukodystrophy.

Practical guide for morphometry of human peroxisomes on electron micrographs

Morphometry of peroxisomes is performed on electron micrographs of ultrathin sections after staining for catalase activity with diaminobenzidine; specific peroxisomal labelling is preferred to guarantee recognition. Peroxisomal number, size, axial ratio and volume parameters are determined and compared to control values. Results from 19 patients with loss of peroxisomal functions are listed. In many patients alterations in peroxisomal morphometric features are found. A brief guideline for interpreting morphometric data is included. Diagnostically relevant morphometric alterations are summarized.

Immunocytochemical localization of peroxisomal proteins in human liver and kidney

The sample preparation and immunocytochemical methods for investigating the presence and subcellular localization of peroxisomal proteins (catalase, the three beta-oxidation enzymes, alanine : glyoxylate aminotransferase and a peroxisomal membrane protein) in human liver biopsies are described. We present a protocol for immunolabelling on ultrathin and semithin sections from the same tissue block, with protein A-colloidal gold as a reporter system. For this purpose, the tissue is embedded in Unicryl, a hydrophilic acrylic resin that is cured by ultraviolet illumination at 2 degrees C. The limitations and possibilities of the methods are discussed together with methodological problems. Cryostat sections of prefixed material should be used for the visualization by light microscopy of cytoplasmic catalase. It is emphasized that immunolabelling for catalase in formalin-fixed archival liver samples and in liver autopsy tissue (in the latter also for the peroxisomal beta-oxidation enzymes) permits visualization of peroxisomes; this can be helpful in diagnosing an index case retrospectively.

Liver and chorion cytochemistry

Microscopic visualization of peroxisomes in chorionic villus cytotrophoblast and in biopsy and autopsy samples of liver and kidney, the presence of enlarged liver macrophages containing lipid droplets insoluble in acetone and n-hexane as well as polarizing inclusions formed by stacks of trilamellar sheets are of diagnostic value in peroxisomal disorders. Methods are presented for evaluating these structures by light microscopy; trilamellar inclusions are only detected by electron microscopy. Macrophage features are preserved in archival paraffin blocks. In adrenal cortex, insoluble lipid, polarizing inclusions and trilamellar structures should be looked for. The stains are easily reproducible, and all reagents are commercially available.

Measurement of dihydroxyacetone-phosphate acyltransferase (DHAPAT) in chorionic villous samples, blood cells and cultured cells

Dihydroxyacetone-phosphate acyltransferase (DHAPAT) is a peroxisomal enzyme catalysing the first step in ether-phospholipid biosynthesis. DHAPAT is deficient in cells from patients suffering from a variety of peroxisomal disorders. Accurate measurement of the activity of this enzyme is of great importance, especially since it is a central parameter in the prenatal diagnosis of the disorders of peroxisome biogenesis, rhizomelic chondrodysplasia punctata and DHAPAT-deficiency. We describe a straightforward and accurate assay allowing the activity of DHAPAT to be measured reliably in chorionic villus samples, blood cells, cultured skin fibroblasts, cultured chorionic villus fibroblasts and cultured amniocytes.

Human liver pathology in peroxisomal diseases: a review including novel data

Results from electron microscopic morphometry, enzyme cytochemistry and immunolocalization in liver biopsies are reviewed. Emphasis is put on the following aspects: 1) relationship between peroxisomal size and enzyme concentration; 2) abnormal enlargement of peroxisomes in many congenital disorders with peroxisomal dysfunction; 3) normal localization of matrix enzymes in several patients with peroxisomal dysfunction, with the exception of catalase, which is mainly cytoplasmic; 4) ghost-like peroxisomes in the liver of several syndromes but not in nine cases labelled as Zellweger; 5) discrepancies between liver and cultured fibroblasts; 6) trilamellar, regularly spaced inclusions, large stacks of which are birefringent, indicate a peroxisomal dysfunction; their absence does not exclude it. The same rule holds for lipid in macrophages which is insoluble in acetone and n-hexane (after fixation). The chemical nature of these two storage materials remains unclear; and 7) proliferation of human peroxisomes is frequent in acquired liver diseases and drug toxicity, but is never accompanied by an increase in size, in contrast to the effect of the fibrates and phthalates in rat and mouse. Novel data from seven peroxisomal patients are included.