The type-2 peroxisomal targeting signal.

The type-2 peroxisomal targeting signal (PTS2) is one of two peptide motifs destining soluble proteins for peroxisomes. This signal acts as amphiphilic α-helix exposing the side chains of all conserved residues to the same side. PTS2 motifs are recognized by a bipartite protein complex consisting of the receptor PEX7 and a co-receptor. Cargo-loaded receptor complexes are translocated across the peroxisomal membrane by a transient pore and inside peroxisomes, cargo proteins are released and processed in many, but not all species. The components of the bipartite receptor are re-exported into the cytosol by a ubiquitin-mediated and ATP-driven export mechanism. Structurally, PTS2 motifs resemble other N-terminal targeting signals, whereas the functional relation to the second peroxisomal targeting signal (PTS1) is unclear. Although only a few PTS2-carrying proteins are known in humans, subjects lacking a functional import mechanism for these proteins suffer from the severe inherited disease rhizomelic chondrodysplasia punctata.

Leukodystrophy caused by plasmalogen deficiency rescued by glyceryl 1-myristyl ether treatment.

Plasmalogens are the most abundant form of ether phospholipids in myelin and their deficiency causes Rhizomelic Chondrodysplasia Punctata (RCDP), a severe developmental disorder. Using the Gnpat-knockout (KO) mouse as a model of RCDP, we determined the consequences of a plasmalogen deficiency during myelination and myelin homeostasis in the central nervous system (CNS). We unraveled that the lack of plasmalogens causes a generalized hypomyelination in several CNS regions including the optic nerve, corpus callosum and spinal cord. The defect in myelin content evolved to a progressive demyelination concomitant with generalized astrocytosis and white matter-selective microgliosis. Oligodendrocyte precursor cells (OPC) and mature oligodendrocytes were abundant in the CNS of Gnpat KO mice during the active period of demyelination. Axonal loss was minimal in plasmalogen-deficient mice, although axonal damage was observed in spinal cords from aged Gnpat KO mice. Characterization of the plasmalogen-deficient myelin identified myelin basic protein and septin 7 as early markers of dysmyelination, whereas myelin-associated glycoprotein was associated with the active demyelination phase. Using in vitro myelination assays, we unraveled that the intrinsic capacity of oligodendrocytes to ensheath and initiate membrane wrapping requires plasmalogens. The defect in plasmalogens was rescued with glyceryl 1-myristyl ether [1-O-tetradecyl glycerol (1-O-TDG)], a novel alternative precursor in the plasmalogen biosynthesis pathway. 1-O-TDG treatment rescued myelination in plasmalogen-deficient oligodendrocytes and in mutant mice. Our results demonstrate the importance of plasmalogens for oligodendrocyte function and myelin assembly, and identified a novel strategy to promote myelination in nervous tissue.

A novel type of rhizomelic chondrodysplasia punctata, RCDP5, is caused by loss of the PEX5 long isoform.

Import of peroxisomal matrix proteins, crucial for peroxisome biogenesis, is mediated by the cytosolic receptors PEX5 and PEX7 that recognize proteins carrying peroxisomal targeting signals 1 or 2 (PTS1 or PTS2), respectively. Mutations in PEX5 or 12 other PEX genes cause peroxisome biogenesis disorders, collectively named the Zellweger spectrum disorders (ZSDs), whereas mutations in PEX7 cause rhizomelic chondrodysplasia punctata type 1 (RCDP1). Three additional RCDP types, RCDP2-3-4, are caused, respectively, by mutations in GNPAT, AGPS and FAR1, encoding enzymes involved in plasmalogen biosynthesis. Here we report a fifth type of RCDP (RCDP5) caused by a novel mutation in PEX5. In four patients with RCDP from two independent families, we identified a homozygous frame shift mutation c.722dupA (p.Val242Glyfs(∗)33) in PEX5 (GenBank: NM_001131023.1). PEX5 encodes two isoforms, PEX5L and PEX5S, and we show that the c.722dupA mutation, located in the PEX5L-specific exon 9, results in loss of PEX5L only. Both PEX5 isoforms recognize PTS1-tagged proteins, but PEX5L is also a co-receptor for PTS2-tagged proteins. Previous patients with PEX5 mutations had ZSD, mainly due to deficient import of PTS1-tagged proteins. Similarly to mutations in PEX7, loss of PEX5L results in deficient import of PTS2-tagged proteins only, thus causing RCDP instead of ZSD. We demonstrate that PEX5L expression restores the import of PTS2-tagged proteins in patient fibroblasts. Due to the biochemical overlap between RCDP1 and RCDP5, sequencing of PEX7 and exon 9 in PEX5 should be performed in patients with a selective defect in the import of PTS2-tagged proteins.

Rhizomelic Chondrodysplasia Punctata Type 1 Caused by a Novel Mutation in the PEX7 Gene.

Peroxisomes are involved in various metabolic reactions. Rhizomelic chondrodysplasia punctata (RCDP) type 1 is one of the peroxisomal biogenesis disorders caused by mutations in the PEX7 gene and is inherited in an autosomal recessive manner. We present a nine-year-old boy with skeletal abnormalities and dysmorphic facial appearance. The patient was born to parents who were first cousins. Very-long-chain fatty acids and pristanic acid levels were in the normal range, but an elevated phytanic acid level was detected by gas chromatography/mass spectrometry. The PEX7 gene was sequenced in the patient and his parents. A novel homozygous mutation, c.192delT (p.F64Lfs*10), was identified in the patient and was present in heterozygosity in both parents. In conclusion, the clinical presentation and peroxisome profile of the patient suggest that this novel mutation leads to RCDP type 1.

Plasmalogens and fatty alcohols in rhizomelic chondrodysplasia punctata and Sjögren-Larsson syndrome.

Plasmalogens are a special class of ether-phospholipids, best recognized by their vinyl-ether bond at the sn-1 position of the glycerobackbone and by the observation that their deficiency causes rhizomelic chondrodysplasia punctata (RCDP). The complex plasmalogen biosynthetic pathway involves multiple enzymatic steps carried-out in peroxisomes and in the endoplasmic reticulum. The rate limiting step in the biosynthesis of plasmalogens resides in the formation of the fatty alcohol responsible for the formation of an intermediate with an alkyl-linked moiety. The regulation in the biosynthesis of plasmalogens also takes place at this step using a feedback mechanism to stimulate or inhibit the biosynthesis. As such, fatty alcohols play a relevant role in the formation of ether-phospholipids. These advances in our understanding of complex lipid biosynthesis brought two seemingly distinct disorders into the spotlight. Sjögren-Larsson syndrome (SLS) is caused by defects in the microsomal fatty aldehyde dehydrogenase (FALDH) leading to the accumulation of fatty alcohols and fatty aldehydes. In RCDP cells, the defect in plasmalogens is thought to generate a feedback signal to increase their biosynthesis, through the activity of fatty acid reductases to produce fatty alcohols. However, the enzymatic defects in either glyceronephosphate O-acyltransferase (GNPAT) or alkylglycerone phosphate synthase (AGPS) disrupt the biosynthesis and result in the accumulation of the fatty alcohols. A detailed characterization on the processes and enzymes that govern these intricate biosynthetic pathways, as well as, the metabolic characterization of defects along the pathway should increase our understanding of the causes and mechanisms behind these disorders.

CUL4A-DDB1-Rbx1 E3 ligase controls the quality of the PTS2 receptor Pex7p.

Pex7p is the cytosolic receptor for peroxisomal matrix proteins harbouring PTS2 (peroxisome-targeting signal type-2). Mutations in the PEX7 gene cause RCDP (rhizomelic chondrodysplasia punctata) type 1, a distinct PTS2-import-defective phenotype of peroxisome biogenesis disorders. The mechanisms by which the protein level and quality of Pex7p are controlled remain largely unknown. In the present study we show that dysfunctional Pex7p, including mutants from RCDP patients, is degraded by a ubiquitin-dependent proteasomal pathway involving the CRL4A (Cullin4A-RING ubiquitin ligase) complex. Furthermore, we demonstrate that the degradation of dysfunctional Pex7p is essential for maintaining normal PTS2 import, thereby suggesting that CRL4A functions as an E3 ligase in the quality control of Pex7p. Our results define a mechanism underlying Pex7p homoeostasis and highlight its importance for regulating PTS2 import. These findings may lead to a new approach to Pex7p-based therapies for the treatment of peroxisome biogenesis disorders such as RCDP.

Mild reduction of plasmalogens causes rhizomelic chondrodysplasia punctata: functional characterization of a novel mutation.

Rhizomelic chondrodysplasia punctata (RCDP) is an autosomal recessive disorder due to the deficiency in ether lipid synthesis. RCDP type 1, the most prominent type, is caused by the dysfunction of the receptor of peroxisome targeting signal type 2, Pex7 (peroxisomal biogenesis factor 7), and the rest of the patients, RCDP types 2 and 3, have defects in peroxisomal enzymes catalyzing the initial two steps of alkyl-phospholipid synthesis, glyceronephosphate O-acyltransferase and alkylglycerone phosphate synthase (Agps). We herein investigated defects of two patients with RCDP type 3. Patient 1 had a novel missense mutation, T1533G, resulting in the I511M substitution in Agps. The plasmalogen level was mildly reduced, whereas the protein level and peroxisomal localization of Agps-I511M in fibroblasts were normal as in the control fibroblasts. Structure prediction analysis suggested that the mutated residue was located in the helix α15 on the surface of V-shaped active site tunnel in Agps, likely accounting for the mild defects of plasmalogen synthesis. These results strongly suggest that an individual with mildly affected level of plasmalogen synthesis develops RCDP. In fibroblasts from patient 2, the expression of AGPS mRNA and Agps protein was severely affected, thereby giving rise to the strong reduction of plasmalogen synthesis.

The importance of ether-phospholipids: a view from the perspective of mouse models.

Ether-phospholipids represent an important group of phospholipids characterized by an alkyl or an alkenyl bond at the sn-1 position of the glycerol backbone. Plasmalogens are the most abundant form of alkenyl-glycerophospholipids, and their synthesis requires functional peroxisomes. Defects in the biosynthesis of plasmalogens are the biochemical hallmark of the human peroxisomal disorder Rhizomelic Chondrodysplasia Punctata (RCDP), which is characterized by defects in eye, bone and nervous tissue. The generation and characterization of mouse models with defects in plasmalogen levels have significantly advanced our understanding of the role and importance of plasmalogens as well as pathogenetic mechanisms underlying RCDP. A review of the current mouse models and the description of the combined knowledge gathered from the histopathological and biochemical studies is presented and discussed. Further characterization of the role and functions of plasmalogens will contribute to the elucidation of disease pathogenesis in peroxisomal and non-peroxisomal disorders. This article is part of a Special Issue entitled: Metabolic Functions and Biogenesis of Peroxisomes in Health and Disease.

Impaired neurotransmission in ether lipid-deficient nerve terminals.

Isolated defects of ether lipid (EL) biosynthesis in humans cause rhizomelic chondrodysplasia punctata type 2 and type 3, serious peroxisomal disorders. Using a previously described mouse model [Rodemer, C., Thai, T.P., Brugger, B., Kaercher, T., Werner, H., Nave, K.A., Wieland, F., Gorgas, K., and Just, W.W. (2003) Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice. Hum. Mol. Genet., 12, 1881-1895], we investigated the effect of EL deficiency in isolated murine nerve terminals (synaptosomes) on the pre-synaptic release of the neurotransmitters (NTs) glutamate and acetylcholine. Both Ca(2+)-dependent exocytosis and Ca(2+)-independent efflux of the transmitters were affected. EL-deficient synaptosomes respire at a reduced rate and exhibit a lowered adenosin-5'-triphosphate/adenosine diphosphate (ATP/ADP) ratio. Consequently, ATP-driven processes, such as synaptic vesicle cycling and maintenance of Na(+), K(+) and Ca(2+) homeostasis, might be disturbed. Analyzing reactive oxygen species in EL-deficient neural and non-neural tissues revealed that plasmalogens (PLs), the most abundant EL species in mammalian central nervous system, considerably contribute to the generation of the lipid peroxidation product malondialdehyde. Although EL-deficient tissue contains less lipid peroxidation products, fibroblasts lacking ELs are more susceptible to induced oxidative stress. In summary, these results suggest that due to the reduced energy state of EL-deficient tissue, the Ca(2+)-independent efflux of NTs increases while the Ca(2+)-dependent release declines. Furthermore, lack of PLs is mainly compensated for by an increase in the concentration of phosphatidylethanolamine and results in a significantly lowered level of lipid peroxidation products in the brain cortex and cerebellum.

Peroxisomal leukoencephalopathy.

Peroxisomal leukoencephalopathies include diseases belonging to the Zellweger spectrum and the rhizomelic chondrodysplasia punctata spectrum, as well as some single enzyme defects of peroxisomal ß-oxidation. The authors present information on the clinical and diagnostic approach, and the characteristics of brain magnetic resonance imaging (MRI) in these diseases. MRIs of patients belonging to the Zellweger spectrum may show developmental anomalies and regressive changes consisting of abnormal cerebral white matter. Involvement of the central white matter of the cerebellar hemispheres is frequently seen. The leukoencephalopathy is progressive, with or without peripheral nerve involvement, in patients with a prolonged course of the disease. MRI characteristics in the severe phenotype of rhizomelic chondrodysplasia punctata include supratentorial white matter abnormalities, with a parietooccipital predominance. Demyelinative lesions are the hallmark of the cerebral form of X-linked adrenoleukodystrophy and may appear in a similar way in patients with adrenomyeloneuropathy progressing to a cerebral form. The diagnosis of a peroxisomal disorder can be determined by a battery of biochemical assays in blood and/or urine, and should be confirmed in cultured fibroblasts and DNA analysis. Treatment of the peroxisomal leukoencephalopathies is largely symptomatic, except for boys affected by the cerebral form of X-linked adrenoleukodystrophy in whom a bone marrow/hematopoietic stem cell transplant can be lifesaving, at least in the early stages of the disease.

Defective lipid remodeling of GPI anchors in peroxisomal disorders, Zellweger syndrome, and rhizomelic chondrodysplasia punctata.

Many cell surface proteins in mammalian cells are anchored to the plasma membrane via glycosylphosphatidylinositol (GPI). The predominant form of mammalian GPI contains 1-alkyl-2-acyl phosphatidylinositol (PI), which is generated by lipid remodeling from diacyl PI. The conversion of diacyl PI to 1-alkyl-2-acyl PI occurs in the ER at the third intermediate in the GPI biosynthetic pathway. This lipid remodeling requires the alkyl-phospholipid biosynthetic pathway in peroxisome. Indeed, cells defective in dihydroxyacetone phosphate acyltransferase (DHAP-AT) or alkyl-DHAP synthase express only the diacyl form of GPI-anchored proteins. A defect in the alkyl-phospholipid biosynthetic pathway causes a peroxisomal disorder, rhizomelic chondrodysplasia punctata (RCDP), and defective biogenesis of peroxisomes causes Zellweger syndrome, both of which are lethal genetic diseases with multiple clinical phenotypes such as psychomotor defects, mental retardation, and skeletal abnormalities. Here, we report that GPI lipid remodeling is defective in cells from patients with Zellweger syndrome having mutations in the peroxisomal biogenesis factors PEX5, PEX16, and PEX19 and in cells from patients with RCDP types 1, 2, and 3 caused by mutations in PEX7, DHAP-AT, and alkyl-DHAP synthase, respectively. Absence of the 1-alkyl-2-acyl form of GPI-anchored proteins might account for some of the complex phenotypes of these two major peroxisomal disorders.

In vitro and in vivo plasmalogen replacement evaluations in rhizomelic chrondrodysplasia punctata and Pelizaeus-Merzbacher disease using PPI-1011, an ether lipid plasmalogen precursor.

BACKGROUND: Childhood peroxisomal disorders and leukodystrophies are devastating diseases characterized by dysfunctional lipid metabolism. Plasmalogens (ether glycerophosphoethanolamine lipids) are decreased in these genetic disorders. The biosynthesis of plasmalogens is initiated in peroxisomes but completed in the endoplasmic reticulum. We therefore undertook a study to evaluate the ability of a 3-substituted, 1-alkyl, 2-acyl glyceryl ether lipid (PPI-1011) to replace plasmalogens in rhizomelic chrondrodysplasia punctata type 1 (RCDP1) and rhizomelic chrondrodysplasia punctata type 2 (RCDP2) lymphocytes which possess peroxisomal mutations culminating in deficient plasmalogen synthesis. We also examined plasmalogen synthesis in Pelizaeus-Merzbacher disease (PMD) lymphocytes which possess a proteolipid protein-1 (PLP1) missense mutation that results in abnormal PLP1 folding and it's accumulation in the endoplasmic reticulum (ER), the cellular site of the last steps in plasmalogen synthesis. In vivo incorporation of plasmalogen precursor into tissue plasmalogens was also evaluated in the Pex7 mouse model of plasmalogen deficiency. RESULTS: In both RCDP1 and RCDP2 lymphocytes, PPI-1011 repleted the target ethanolamine plasmalogen (PlsEtn16:0/22:6) in a concentration dependent manner. In addition, deacylation/reacylation reactions resulted in repletion of PlsEtn 16:0/20:4 in both RCDP1 and RCDP2 lymphocytes, repletion of PlsEtn 16:0/18:1 and PlsEtn 16:0/18:2 in RCDP2 lymphocytes, and partial repletion of PlsEtn 16:0/18:1 and PlsEtn 16:0/18:2 in RCDP1 lymphocytes. In the Pex7 mouse, oral dosing of labeled PPI-1011 demonstrated repletion of tissue levels of the target plasmalogen PlsEtn 16:0/22:6 with phospholipid remodeling also resulting in significant repletion of PlsEtn 16:0/20:4 and PlsEtn 16:0/18:1. Metabolic conversion of PPI-1011 to the target plasmalogen was most active in the liver. CONCLUSIONS: Our data demonstrate that PPI-1011 is activated (removal of 3-substitution) and converted to PlsEtn in vitro in both RCDP1 and RCDP2 lymphocytes and in vivo in the Pex7 mouse model of RCPD1 effectively bypassing the peroxisomal dysfunction present in these disorders. While PPI-1011 was shown to replete PlsEtns 16:0/x, ether lipid precursors of PlsEtn 18:0/x and PlsEtn 18:1/x may also be needed to achieve optimal clinical benefits of plasmalogen replacement in these complex patient populations. In contrast, only limited plasmalogen replacement was observed in PMD lymphocytes suggesting that the effects of protein misfolding and accumulation in the ER negatively affect processing of plasmalogen precursors in this cellular compartment.

Nonsense suppressor therapies rescue peroxisome lipid metabolism and assembly in cells from patients with specific PEX gene mutations.

Peroxisome biogenesis disorders (PBDs) are multisystemic autosomal recessive disorders resulting from mutations in PEX genes required for normal peroxisome assembly and metabolic activities. Here, we evaluated the potential effectiveness of aminoglycoside G418 (geneticin) and PTC124 (ataluren) nonsense suppression therapies for the treatment of PBD patients with disease-causing nonsense mutations. PBD patient skin fibroblasts producing stable PEX2 or PEX12 nonsense transcripts and Chinese hamster ovary (CHO) cells with a Pex2 nonsense allele all showed dramatic improvements in peroxisomal very long chain fatty acid catabolism and plasmalogen biosynthesis in response to G418 treatments. Cell imaging assays provided complementary confirmatory evidence of improved peroxisome assembly in G418-treated patient fibroblasts. In contrast, we observed no appreciable rescue of peroxisome lipid metabolism or assembly for any patient fibroblast or CHO cell culture treated with various doses of PTC124. Additionally, PTC124 did not show measurable nonsense suppression in immunoblot assays that directly evaluated the read-through of PEX7 nonsense alleles found in PBD patients with rhizomelic chondrodysplasia punctata type 1 (RCDP1). Overall, our results support the continued development of safe and effective nonsense suppressor therapies that could benefit a significant subset of individuals with PBDs. Furthermore, we suggest that the described cell culture assay systems could be useful for evaluating and screening for novel nonsense suppressor therapies.

Chondrodysplasia punctata: a clinical diagnostic and radiological review.

Chondrodysplasia punctata (CDP) is associated with a number of disorders, including inborn errors of metabolism, involving peroxisomal and cholesterol pathways, embryopathy and chromosomal abnormalities. Several classification systems of the different types of CDP have been suggested earlier. More recently, the biochemical and molecular basis of a number of CDP syndromes has recently been elucidated and a new aetiological classification has emerged. Here we provide an updated version with an overview of the different types of CDP, a discussion of the aetiology and a description of the clinical and radiographic findings. An investigative guideline to help determine the exact diagnosis in new cases is also presented.

The crucial step in ether phospholipid biosynthesis: structural basis of a noncanonical reaction associated with a peroxisomal disorder.

Ether phospholipids are essential constituents of eukaryotic cell membranes. Rhizomelic chondrodysplasia punctata type 3 is a severe peroxisomal disorder caused by inborn deficiency of alkyldihydroxyacetonephosphate synthase (ADPS). The enzyme carries out the most characteristic step in ether phospholipid biosynthesis: formation of the ether bond. The crystal structure of ADPS from Dictyostelium discoideum shows a fatty-alcohol molecule bound in a narrow hydrophobic tunnel, specific for aliphatic chains of 16 carbons. Access to the tunnel is controlled by a flexible loop and a gating helix at the protein-membrane interface. Structural and mutagenesis investigations identify a cluster of hydrophilic catalytic residues, including an essential tyrosine, possibly involved in substrate proton abstraction, and the arginine that is mutated in ADPS-deficient patients. We propose that ether bond formation might be orchestrated through a covalent imine intermediate with the flavin, accounting for the noncanonical employment of a flavin cofactor in a nonredox reaction.

Peroxisome biogenesis disorders.

Defects in PEX genes impair peroxisome assembly and multiple metabolic pathways confined to this organelle, thus providing the biochemical and molecular bases of the peroxisome biogenesis disorders (PBD). PBD are divided into two types--Zellweger syndrome spectrum (ZSS) and rhizomelic chondrodysplasia punctata (RCDP). Biochemical studies performed in blood and urine are used to screen for the PBD. DNA testing is possible for all of the disorders, but is more challenging for the ZSS since 12 PEX genes are known to be associated with this spectrum of PBD. In contrast, PBD-RCDP is associated with defects in the PEX7 gene alone. Studies of the cellular and molecular defects in PBD patients have contributed significantly to our understanding of the role of each PEX gene in peroxisome assembly.

The import receptor Pex7p and the PTS2 targeting sequence.

This chapter concerns one branch of the peroxisome import pathway for newly-synthesized peroxisomal proteins, specifically the branch for matrix proteins that contain a peroxisome targeting sequence type 2 (PTS2). The structure and utilization of the PTS2 are discussed, as well as the properties of the receptor, Pex7p, which recognizes the PTS2 sequence and conveys these proteins to the common translocation machinery in the peroxisome membrane. We also describe the recent evidence that this receptor recycles into the peroxisome matrix and back out to the cytosol in the course of its function. Pex7p is assisted in its functioning by several species-specific auxiliary proteins that are described in the following chapter.

Phytanic acid: production from phytol, its breakdown and role in human disease.

Phytanic acid is a branched-chain fatty acid that accumulates in a variety of metabolic disorders. High levels of phytanic acid found in patients can exceed the millimolar range and lead to severe symptoms. Degradation of phytanic acid takes place by alpha-oxidation inside the peroxisome. A deficiency of its breakdown, leading to elevated levels, can result from either a general peroxisomal dysfunction or from a defect in one of the enzymes involved in alpha-oxidation. Research on Refsum disease, belonging to the latter group of disorders and characterized by a deficiency of the first enzyme of alpha-oxidation, has extended our knowledge of phytanic acid metabolism and pathology of the disease greatly over the past few decades. This review will centre on this research on phytanic acid: its origin, the mechanism by which its alpha-oxidation takes place, its role in human disease and the way it is produced from phytol.

MRI of the brain and cervical spinal cord in rhizomelic chondrodysplasia punctata.

BACKGROUND: The classic rhizomelic chondrodysplasia punctata (RCDP) phenotype involves a typical facial appearance, cataracts, skeletal dysplasia causing disproportionate somatic growth failure, microcephaly, and severe psychomotor defects. Biochemical abnormalities include impaired plasmalogen biosynthesis in all forms of RCDP and accumulation of phytanic acid in RCDP type 1. A subset of patients has a milder clinical and biochemical phenotype, with less severe neurologic impairment and an incomplete deficiency in plasmalogens. The impact of plasmalogen deficiency on neurologic function is severe, causing spasticity and mental defects, but its pathomechanism is still unknown. The authors specifically focused on myelination because myelin is rich in ethanolamine plasmalogens. OBJECTIVE: To define the neuroimaging characteristics of the genetic peroxisomal disorder RCDP. METHODS: Twenty-one MR images of the brain and cervical spine of 11 patients were evaluated and correlated with neurologic and biochemical profiles. RESULTS: No abnormalities on MRI were seen in the patients with a mild phenotype of RCDP, whereas delayed myelination, ventricular enlargement and increased subarachnoidal spaces, supratentorial myelin abnormalities, and cerebellar atrophy were observed in patients with the severe phenotype of both RCDP type 1 and 3. The severity of both the MRI abnormalities and the clinical phenotype is correlated with the plasmalogen level. CONCLUSIONS: The severe phenotype of rhizomelic chondrodysplasia punctata (RCDP) is accompanied by a specific pattern of both developmental and regressive MRI abnormalities. Plasmalogen levels seem to play an important role in the pathophysiology of CNS abnormalities in RCDP. Increased phytanic acid appears not to be the cause of cerebellar atrophy.

Inactivation of ether lipid biosynthesis causes male infertility, defects in eye development and optic nerve hypoplasia in mice.

Although known for almost 80 years, the physiological role of plasmalogens (PLs), the major mammalian ether lipids (ELs), is still enigmatic. Humans that lack ELs suffer from rhizomelic chondrodysplasia punctata (RCDP), a peroxisomal disorder usually resulting in death in early childhood. In order to learn more about the functions of ELs, we generated a mouse model for RCDP by a targeted disruption of the dihydroxyacetonephosphate acyltransferase gene. The mutant mice revealed multiple abnormalities, such as male infertility, defects in eye development, cataract and optic nerve hypoplasia, some of which were also observed in RCDP. Mass spectroscopic analysis demonstrated the presence of highly unsaturated fatty acids including docosahexaenoic acid (DHA) in brain PLs and the occurrence of PLs in lipid raft microdomains (LRMs) isolated from brain myelin. In mutants, PLs were completely absent and the concentration of brain DHA was reduced. The marker proteins flotillin-1 and F3/contactin were found in brain LRMs in reduced concentrations. In addition, the gap junctional protein connexin 43, known to be recruited to LRMs and essential for lens development and spermatogenesis, was down-regulated in embryonic fibroblasts of the EL-deficient mice. Free cholesterol, an important constituent of LRMs, was found in these fibroblasts to be accumulated in a perinuclear compartment. These data suggest that the EL-deficient mice allow the identification of new phenotypes not related so far to EL-deficiency (male sterility, defects in myelination and optic nerve hypoplasia) and indicate that PLs are required for the correct assembly and function of LRMs.