Role of omega-3 fatty acids in brain development and function: potential implications for the pathogenesis and prevention of psychopathology

The principle omega-3 fatty acid in brain, docosahexaenoic acid (DHA), accumulates in the brain during perinatal cortical expansion and maturation. Animal studies have demonstrated that reductions in perinatal brain DHA accrual are associated with deficits in neuronal arborization, multiple indices of synaptic pathology including deficits in serotonin and mesocorticolimbic dopamine neurotransmission, neurocognitive deficits, and elevated behavioral indices of anxiety, aggression, and depression. In primates and humans, preterm delivery is associated with deficits in fetal cortical DHA accrual, and children/adolescents born preterm exhibit deficits in cortical gray matter maturation, neurocognitive deficits particularly in the realm of attention, and increased risk for attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. Individuals diagnosed with ADHD or schizophrenia exhibit deficits in cortical gray matter maturation, and medications found to be efficacious in the treatment of these disorders increase cortical and striatal dopamine neurotransmission. These associations in conjunction with intervention trials showing enhanced cortical visual acuity and cognitive outcomes in preterm and term infants fed DHA, suggest that perinatal deficits in brain DHA accrual may represent a preventable neurodevelopmental risk factor for the subsequent emergence of psychopathology.     

Very-long-chain polyunsaturated fatty acids accumulate in phosphatidylcholine of fibroblasts from patients with Zellweger syndrome and acyl-CoA oxidase1 deficiency

Peroxisomes are subcellular organelles that function in multiple anabolic and catabolic processes, including β-oxidation of very-long-chain fatty acids (VLCFA) and biosynthesis of ether phospholipids. Peroxisomal disorders caused by defects in peroxisome biogenesis or peroxisomal β-oxidation manifest as severe neural disorders of the central nervous system. Abnormal peroxisomal metabolism is thought to be responsible for the clinical symptoms of these diseases, but their molecular pathogenesis remains to be elucidated. We performed lipidomic analysis to identify aberrant metabolites in fibroblasts from patients with Zellweger syndrome (ZS), acyl-CoA oxidase1 (AOx) deficiency, D-bifunctional protein (D-BP) and X-linked adrenoleukodystrophy (X-ALD), as well as in peroxisome-deficient Chinese hamster ovary cell mutants. In cells deficient in peroxisomal biogenesis, plasmenylethanolamine was remarkably reduced and phosphatidylethanolamine was increased. Marked accumulation of very-long-chain saturated fatty acid and monounsaturated fatty acids in phosphatidylcholine was observed in all mutant cells. Very-long-chain polyunsaturated fatty acid (VLC-PUFA) levels were significantly elevated, whilst phospholipids containing docosahexaenoic acid (DHA, C22:6n-3) were reduced in fibroblasts from patients with ZS, AOx deficiency, and D-BP deficiency, but not in fibroblasts from an X-ALD patient. Because patients with AOx deficiency suffer from more severe symptoms than those with X-ALD, accumulation of VLC-PUFA and/or reduction of DHA may be associated with the severity of peroxisomal diseases.     

Omega-3 fatty acids and neuropsychiatric disorders

Epidemiological evidence suggests that dietary consumption of the long chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), commonly found in fish or fish oil, may modify the risk for certain neuropsychiatric disorders. As evidence, decreased blood levels of omega-3 fatty acids have been associated with several neuropsychiatric conditions, including Attention Deficit (Hyperactivity) Disorder, Alzheimer's Disease, Schizophrenia and Depression. Supplementation studies, using individual or combination omega-3 fatty acids, suggest the possibility for decreased symptoms associated with some of these conditions. Thus far, however, the benefits of supplementation, in terms of decreasing disease risk and/or aiding in symptom management, are not clear and more research is needed. The reasons for blood fatty acid alterations in these disorders are not known, nor are the potential mechanisms by which omega-3 fatty acids may function in normal neuronal activity and neuropsychiatric disease prevention and/or treatment. It is clear, however, that DHA is the predominant n-3 fatty acid found in the brain and that EPA plays an important role as an anti-inflammatory precursor. Both DHA and EPA can be linked with many aspects of neural function, including neurotransmission, membrane fluidity, ion channel and enzyme regulation and gene expression. This review summarizes the knowledge in terms of dietary omega-3 fatty acid intake and metabolism, as well as evidence pointing to potential mechanisms of omega-3 fatty acids in normal brain functioning, development of neuropsychiatric disorders and efficacy of omega-3 fatty acid supplementation in terms of symptom management.     

Structure-function relationships in the peroxisome: implications for human disease.

Progress relevant to human peroxisomal disorders over the past 3 years includes improved biochemical delineation of disease phenotypes and new insights into peroxisomal structure and biogenesis. Immunoblotting studies using antibodies to peroxisomal beta-oxidation enzymes have defined mutations affecting each step of the pathway, some with clinical phenotypes as severe as disorders with global peroxisome deficiency. The latter disorders, typified by Zellweger syndrome, often lack matrix proteins but retain major membrane species of 150, 70, 35, and 22 kDa in empty peroxisomal "ghost" structures. The hypothesis that peroxisomal deficiency disorders result from altered targeting or import of peroxisomal matrix proteins has been strengthened by the demonstration of a carboxy terminal peroxisome-targeting signal which is distinct from amino terminal signals directing proteins to mitochondria. A mutation which mistargets alanine/glyoxylate aminotransferase from peroxisomes to mitochondria in primary hyperoxaluria provides a graphic example of these signals. The structural significance of membrane function is supported by the primacy of membrane assembly in normal ontogeny or regenerating liver. The coordinate control, targeting, and striking inducibility of peroxisomal proteins suggests a potential vehicle for gene and enzyme therapy.     

SAR and biological evaluation of SEN12333/WAY-317538: Novel alpha 7 nicotinic acetylcholine receptor agonist

Alpha 7 nicotinic acetylcholine receptor (α₇ nAChR) agonists are promising therapeutic candidates for the treatment of cognitive impairment associated with a variety of disorders including Alzheimer’s disease and schizophrenia. Alpha 7 nAChRs are expressed in brain regions associated with cognitive function, regulate cholinergic neurotransmission and have been shown to be down regulated in both schizophrenia and Alzheimer’s disease. Herein we report a novel, potent small molecule agonist of the alpha 7 nAChR, SEN12333/WAY-317538. This compound is a selective agonist of the α₇ nAChR with excellent in vitro and in vivo profiles, excellent brain penetration and oral bioavailability, and demonstrates in vivo efficacy in multiple behavioural cognition models. The SAR and biological evaluation of this series of compounds are discussed.     

Essential role of docosahexaenoic acid towards development of a smarter brain

Evolution of the high order brain function in humans can be attributed to intake of poly unsaturated fatty acids (PUFAs) of which the ω-3 fatty acid, docosahexaenoic acid (DHA) has special significance. DHA is abundantly present in the human brain and is an essential requirement in every step of brain development like neural cell proliferation, migration, differentiation, synaptogenesis etc. The multiple double bonds and unique structure allow DHA to impart special membrane characteristics for effective cell signaling. Evidences indicate that DHA accumulate in areas of the brain associated with learning and memory. Many development disorders like dyslexia, autism spectrum disorder, attention deficit hyperactivity disorder, schizophrenia etc. are causally related to decreased level of DHA. The review discusses the various reports of DHA in these areas for a better understanding of the role of DHA in overall brain development. Studies involving laboratory animals and clinical findings in cases as well as during trials have been taken into consideration. Additionally the currently available dietary source of DHA for supplementation as nutraceutics with general caution for overuse has been examined.     

Folate and long-chain polyunsaturated fatty acids in psychiatric disease

Schizophrenia, autism and depression do not inherit by Mendel's law, and the search for a genetic basis seems unsuccessful. Schizophrenia and autism relate to low birth weight and pregnancy complications, which are associated with developmental adaptations by "programming". Epigenetics might constitute the basis of programming and depend on folate status and one-carbon metabolism in general. Early folate status of patients with schizophrenia might be compromised as suggested by (i) coinciding incidences of schizophrenia and neural tube defects (NTDs) in the Dutch hunger winter, (ii) coinciding seasonal fluctuations in birth of patients with schizophrenia and NTDs, (iii) higher schizophrenia incidence in immigrants and (iv) higher incidence in methylene tetrahydrofolate reductase 677C-->T homozygotes. Recent studies in schizophrenia and autism point at epigenetic silencing of critical genes or chromosomal loci. The long-chain polyunsaturated fatty acids (LCPUFA), arachidonic acid (AA, from meat) and docosahexaenoic acid (fish) are components of brain phospholipids and modulators of signal transduction and gene expression. Patients with schizophrenia and, possibly, autism exhibit abnormal phospholipid metabolism that might cause local AA depletion and impaired eicosanoid-mediated signal transduction. National fish intakes relate inversely with major and postpartum depressions. Five out of six randomized controlled trials with eicosapentaenoic acid (fish) have shown positive effects in schizophrenia, and 4 of 6 were favorable in depression and bipolar disorders. We conclude that folate and LCPUFA might be important in both the etiology and severity of at least some psychiatric diseases.     

Pristanic acid and phytanic acid in plasma from patients with peroxisomal disorders: stable isotope dilution analysis with electron capture negative ion mass fragmentography.

A sensitive and selective stable isotope dilution method was developed for the accurate quantitation of pristanic acid and phytanic acid using electron capture negative ion mass fragmentography on pentafluorobenzyl derivatives. This technique allows detection of 1 pg of each compound and was applied to plasma from healthy controls and patients suffering from various peroxisomal disorders. The age-dependency of phytanic and pristanic acid levels in plasma from healthy controls was demonstrated. The involvement of peroxisomes in the beta-oxidation of pristanic acid was concluded from its accumulation in plasma from patients with peroxisomal deficiencies. Pristanic acid/phytanic acid ratios were markedly increased in bifunctional protein and/or 3-oxoacyl-CoA thiolase deficiency, indicating their role in the (differential) diagnosis of disorders of peroxisomal beta-oxidation.     

Human peroxisomal targeting signal-1 receptor restores peroxisomal protein import in cells from patients with fatal peroxisomal disorders

Two peroxisomal targeting signals, PTS1 and PTS2, are involved in the import of proteins into the peroxisome matrix. Human patients with fatal generalized peroxisomal deficiency disorders fall into at least nine genetic complementation groups. Cells from many of these patients are deficient in the import of PTS1-containing proteins, but the causes of the protein-import defect in these patients are unknown. We have cloned and sequenced the human cDNA homologue (PTS1R) of the Pichia pastoris PAS8 gene, the PTS1 receptor (McCollum, D., E. Monosov, and S. Subramani. 1993. J. Cell Biol. 121:761-774). The PTS1R mRNA is expressed in all human tissues examined. Antibodies to the human PTS1R recognize this protein in human, monkey, rat, and hamster cells. The protein is localized mainly in the cytosol but is also found to be associated with peroxisomes. Part of the peroxisomal PTS1R protein is tightly bound to the peroxisomal membrane. Antibodies to PTS1R inhibit peroxisomal protein-import of PTS1-containing proteins in a permeabilized CHO cell system. In vitro-translated PTS1R protein specifically binds a serine-lysine-leucine-peptide. A PAS8-PTS1R fusion protein complements the P. pastoris pas8 mutant. The PTS1R cDNA also complements the PTS1 protein-import defect in skin fibroblasts from patients--belonging to complementation group two--diagnosed as having neonatal adrenoleukodystrophy or Zellweger syndrome. The PTS1R gene has been localized to a chromosomal location where no other peroxisomal disorder genes are known to map. Our findings represent the only case in which the molecular basis of the protein-import deficiency in human peroxisomal disorders is understood.     

Phytanic acid alpha-oxidation: accumulation of 2-hydroxyphytanic acid and absence of 2-oxophytanic acid in plasma from patients with peroxisomal disorders.

A stable isotope dilution method was developed for the measurement of 2-hydroxyphytanic acid and 2-oxophytanic acid in plasma. In plasma from healthy individuals and from patients with Refsum's disease, 2-hydroxyphytanic acid was found at levels less than 0.2 mumol/l, whereas the acid accumulated in plasma from patients with rhizomelic chondrodysplasia punctata, generalized peroxisomal dysfunction, and a single peroxisomal beta-oxidation enzyme deficiency. In plasma from both healthy controls and patients with peroxisomal disorders, 2-oxophytanic acid was undetectable. Four different groups of diseases were characterized with a defective phytanic acid alpha-oxidation and/or pristanic acid beta-oxidation: 1) Refsum's disease, with a defect at phytanic acid alpha-hydroxylation; 2) rhizomelic chondrodysplasia punctata, with a defect at 2-hydroxyphytanic acid decarboxylation; 3) generalized peroxisomal disorders, with defects at 2-hydroxyphytanic acid decarboxylation and at pristanic acid beta-oxidation; 4) single peroxisomal beta-oxidation enzyme deficiencies, with a defect at pristanic acid beta-oxidation, resulting in an impaired phytanic acid alpha-oxidation by inhibition. The results indicate that 2-hydroxyphytanic acid decarboxylation and pristanic acid beta-oxidation take place in peroxisomes.     

MCD encodes peroxisomal and cytoplasmic forms of malonyl-CoA decarboxylase and is mutated in malonyl-CoA decarboxylase deficiency.

Malonyl-CoA decarboxylase (MCD) catalyzes the proton-consuming conversion of malonyl-CoA to acetyl-CoA and CO(2). Although defects in MCD activity are associated with malonyl-CoA decarboxylase deficiency, a lethal disorder characterized by cardiomyopathy and developmental delay, the metabolic role of this enzyme in mammals is unknown. A computer-based search for novel peroxisomal proteins led to the identification of a candidate gene for human MCD, which encodes a protein with a canonical type-1 peroxisomal targeting signal of serine-lysine-leucine(COOH). We observed that recombinant MCD protein has high intrinsic malonyl-CoA decarboxylase activity and that a malonyl-CoA decarboxylase-deficient patient has a severe mutation in the MCD gene (c.947-948delTT), confirming that this gene encodes human MCD. Subcellular fractionation experiments revealed that MCD resides in both the cytoplasm and peroxisomes. Cytoplasmic MCD is positioned to play a role in the regulation of cytoplasmic malonyl-CoA abundance and, thus, of mitochondrial fatty acid uptake and oxidation. This hypothesis is supported by the fact that malonyl-CoA decarboxylase-deficient patients display a number of phenotypes that are reminiscent of mitochondrial fatty acid oxidation disorders. Additional support for this hypothesis comes from our observation that MCD mRNA is most abundant in cardiac and skeletal muscles, tissues in which cytoplasmic malonyl-CoA is a potent inhibitor of mitochondrial fatty acid oxidation and which derive significant amounts of energy from fatty acid oxidation. As for the role of peroxisomal MCD, we propose that this enzyme may be involved in degrading intraperoxisomal malonyl-CoA, which is generated by the peroxisomal beta-oxidation of odd chain-length dicarboxylic fatty acids.     

Modulation of phosphoinositide-protein kinase C signal transduction by omega-3 fatty acids: implications for the pathophysiology and treatment of recurrent neuropsychiatric illness

The phosphoinositide (PI)-protein kinase C (PKC) signal transduction pathway is initiated by pre- and postsynaptic Galphaq-coupled receptors, and regulates several clinically relevant neurochemical events, including neurotransmitter release efficacy, monoamine receptor function and trafficking, monoamine transporter function and trafficking, axonal myelination, and gene expression. Mounting evidence for PI-PKC signaling hyperactivity in the peripheral (platelets) and central (premortem and postmortem brain) tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, coupled with evidence that PI-PKC signal transduction is down-regulated in rat brain following chronic, but not acute, treatment with antipsychotic, mood-stabilizer, and antidepressant medications, suggest that PI-PKC hyperactivity is central to an underlying pathophysiology. Evidence that membrane omega-3 fatty acids act as endogenous antagonists of the PI-PKC signal transduction pathway, coupled with evidence that omega-3 fatty acid deficiency is observed in peripheral and central tissues of patients with schizophrenia, bipolar disorder, and major depressive disorder, support the hypothesis that omega-3 fatty acid deficiency may contribute to elevated PI-PKC activity in these illnesses. The data reviewed in this paper outline a potential molecular mechanism by which omega-3 fatty acids could contribute to the pathophysiology and treatment of recurrent neuropsychiatric illness.     

Peroxisomal disorders.

The concept that there are human disease states that are associated with abnormal peroxisomal function is of recent origin. This is due in part to the relatively recent discovery of the organelle itself by de Duve in 1983, and to the earlier belief that it was a vestigial structure in mammals. The recognition that the organelle is significant in mammals was ushered in by Paul Lazarow's observation that rat peroxisomes catalyze the beta-oxidation of fatty acids. By 1981, more than 40 enzymes had been localized to the peroxisome, and the number continues to grow. Respect for the physiological role of the peroxisome in man has been heightened by our recent recognition that peroxisome malfunction causes profound disturbances. The Zellweger cerebro-hepato-renal syndrome represents the most serious peroxisomal disease. It is associated with malfunction of virtually every organ, and children with the disease usually do not survive beyond the 4th month. Application of newly developed diagnostic techniques has shown that the clinical spectrum and frequency of peroxisomal disorders are greater than had been realized. Eleven separate peroxisomal disorders have now been identified. Our laboratory alone has identified more than 2000 patients. Disturbances of very long chain fatty acid and ether phospholipid metabolism are present in 9 of the 11 peroxisomal disorders. In this presentation, we will provide an overview of the peroxisomal disorders, with emphasis on disturbances of fatty acid and ether lipid metabolism.     

Effect of dietary polyunsaturated fatty acids on the expression of peroxisomal ABC transporters

Peroxisomal ABC transporters encoded by the ABCD genes are thought to participate in the import of specific fatty acids in the peroxisomal matrix. ABCD1 deficiency is associated with X-linked adrenoleukodystrophy (X-ALD), the most frequent peroxisomal disorder which is characterized by the accumulation of saturated very-long-chain fatty acids (VLCFA). ABCD2 (the closest homolog of ABCD1) and ABCD3 have been shown to have partial functional redundancy with ABCD1; only when overexpressed, they can compensate for VLCFA accumulation. Other lipids, for instance polyunsaturated fatty acids (PUFA), should be possible candidate substrates for the ABCD2 and ABCD3 gene products, ALDRP and PMP70 respectively. Moreover, PUFA, which are known regulators of gene expression, could therefore represent potent inducers of the ABCD genes. To test this hypothesis, littermates of n-3-deficient rats were subjected to an n-3-deficient diet or equilibrated diets containing ALA (alpha-linolenic acid, 18:3n-3) as unique source of n-3 fatty acids or ALA plus DHA (docosahexaenoic acid, 22:6n-3) at two different doses. We analyzed the expression of peroxisomal ABC transporters and of the peroxisomal acyl-CoA oxidase gene 1 (Acox1) in adrenals, brain and liver. Whatever the diet, we did not observe any difference in gene expression in adrenals and brain. However, the hepatic expression level of Abcd2 and Abcd3 genes was found to be significantly higher in the n-3-deficient rats than in the rats fed the ALA diet or the DHA supplemented diets. This was accompanied by important changes in hepatic fatty acid composition. In summary, the hepatic expression of Abcd2 and Abcd3 but not of Abcd1 and Abcd4 appears to be highly sensitive towards dietary PUFA. This difference could be linked to the substrate specificity of the peroxisomal ABC transporters and a specific involvement of Abcd2 and Abcd3 in PUFA metabolism.     

Biochemical Aspects of Neurodegeneration in Human Brain: Involvement of Neural Membrane Phospholipids and Phospholipases A<Subscript>2</Subscript>

Neural membrane phospholipids are hydrolyzed by a group of enzymes known as phospholipases. This process results in the generation of second messengers such as arachidonic acid, eicosanoids, platelet activating factor, and diacylglycerols. High levels of these metabolites are neurotoxic and are associated with neurodegeneration. The collective evidence from many studies suggests that neural membrane phospholipid metabolism is disturbed in neural trauma and neurodegenerative diseases. This disturbance is caused by the stimulation of phospholipases A₂. Stimulation of these enzymes produces changes in membrane permeability, fluidity, and alteration in ion homeostasis. Low calcium influx produces mild oxidative stress and results in neurodegeneration promoted by apoptosis, whereas a calcium overload generates high oxidative stress and causes neurodegeneration associated with necrosis. Alterations in phospholipid metabolism along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for neurodegeneration in ischemia, spinal cord trauma, head injury, and Alzheimer disease. The synthesis of phospholipases A₂ inhibitors that cross the blood-brain barrier without harm may be useful for the treatment of acute neural trauma and neurodegenerative diseases.Special issue dedicated to Dr. Lawrence F. Eng.     

Developmental vitamin D deficiency alters the expression of genes encoding mitochondrial, cytoskeletal and synaptic proteins in the adult rat brain

Epidemiology has highlighted the links between season of birth, latitude and the prevalence of brain disorders such as multiple sclerosis and schizophrenia. In line with these data, we have hypothesized that ‘imprinting’ with low prenatal vitamin D could contribute to the risk of these two brain disorders. Previously, we have shown that transient developmental hypovitaminosis D induces permanent changes in adult nervous system. The aim of this study was to examine the impact of prenatal hypovitaminosis D on gene expression in the adult rat brain. Vitamin D deficient female rats were mated with undeprived males and the offspring were fed with a control diet after birth. At Week 10, gene expression in the progeny's brain was compared with control animals using Affymetrix gene microarrays. Prenatal hypovitaminosis D causes a dramatic dysregulation of several biological pathways including oxidative phosphorylation, redox balance, cytoskeleton maintenance, calcium homeostasis, chaperoning, post-translational modifications, synaptic plasticity and neurotransmission. A computational analysis of these data suggests that impaired synaptic network may be a consequence of mitochondrial dysfunction. Since disruptions of mitochondrial metabolism have been associated with both multiple sclerosis and schizophrenia, developmental vitamin D deficiency may be a heuristic animal model for the study of these two brain diseases.     

Deficiency of sterol regulatory element‐binding protein‐1c induces schizophrenia‐like behavior in mice

Schizophrenia is a hereditary disease that approximately 1% of the worldwide population develops. Many studies have investigated possible underlying genes related to schizophrenia. Recently, clinical studies suggested sterol regulatory element‐binding protein (SREBP) as a susceptibility gene in patients with schizophrenia. SREBP controls cellular lipid homeostasis by three isoforms: SREBP‐1a, SREBP‐1c and SREBP‐2. This study used SREBP‐1c knockout (KO) mice to examine whether a deficiency in SREBP‐1c would affect their emotional and psychiatric behaviors. Altered mRNA expression in genes downstream from SREBP‐1c was confirmed in the brains of SREBP‐1c KO mice. Schizophrenia‐like behavior, including hyperactivity during the dark phase, depressive‐like behavior, aggressive behavior and deficits in social interaction and prepulse inhibition, was observed in SREBP‐1c KO mice. Furthermore, increased volume of the lateral ventricle was detected in SREBP‐1c KO mice. The mRNA levels of several γ‐aminobutyric acid (GABA)‐receptor subtypes and/or glutamic acid decarboxylase 65/67 decreased in the hippocampus and medial prefrontal cortex of SREBP‐1c KO mice. Thus, SREBP‐1c deficiency may contribute to enlargement of the lateral ventricle and development of schizophrenia‐like behaviors and be associated with altered GABAergic transmission.     

Neuroprotective action of omega-3 polyunsaturated fatty acids against neurodegenerative diseases: evidence from animal studies

Studies in animals clearly show that oral intake of docosahexaenoic acid (DHA) can alter brain DHA concentrations and thereby modify brain functions. This provides us with an opportunity to use DHA as a nutraceutical or pharmaceutical tool in brain disorders such as Alzheimer disease (AD) and Parkinson disease (PD). Most of the published epidemiological studies are consistent with a positive association between high reported DHA consumption or high DHA blood levels and a lower risk of developing AD later in life. Such observations have prompted the investigation of DHA in three different transgenic models of AD. These analyses show that animal models of AD are more vulnerable to DHA depletion than controls and that DHA exerts a beneficial effect against pathological signs of AD, including A beta accumulation, cognitive impairment, synaptic marker loss, and hyperphosphorylation of tau. Multiple mechanisms of action can be associated with the neuroprotective effects of DHA and include antioxidant properties and activation of distinct cell signaling pathways. Although the first randomized clinical assays have yet failed to demonstrate convincing beneficial effects of DHA for AD patients, the knowledge gathered in recent years holds out a hope for prevention and suggests that the elderly and people bearing a genetic risk for AD should at least avoid DHA deficiency.     

Defective peroxisome membrane synthesis due to mutations in human PEX3 causes Zellweger syndrome, complementation group G

Zellweger cerebro-hepato-renal syndrome is a severe congenital disorder associated with defective peroxisomal biogenesis. At least 23 PEX genes have been reported to be essential for peroxisome biogenesis in various species, indicating the complexity of peroxisomal assembly. Cells from patients with peroxisomal biogenesis disorders have previously been shown to segregate into >/=12 complementation groups. Two patients assigned to complementation group G who had not been linked previously to a specific gene defect were confirmed as displaying a cellular phenotype characterized by a lack of even residual peroxisomal membrane structures. Here we demonstrate that this complementation group is associated with mutations in the PEX3 gene, encoding an integral peroxisomal membrane protein. Homozygous PEX3 mutations, each leading to C-terminal truncation of PEX3, were identified in the two patients, who both suffered from a severe Zellweger syndrome phenotype. One of the mutations involved a single-nucleotide insertion in exon 7, whereas the other was a single-nucleotide substitution eight nucleotides from the normal splice site in the 3' acceptor site of intron 10. Expression of wild-type PEX3 in the mutant cell lines restored peroxisomal biogenesis, whereas transfection of mutated PEX3 cDNA did not. This confirmed that the causative gene had been identified. The observation of peroxisomal formation in the absence of morphologically recognizable peroxisomal membranes challenges the theory that peroxisomes arise exclusively by growth and division from preexisting peroxisomes and establishes PEX3 as a key factor in early human peroxisome synthesis.