PEX5 Research Articles

G.P.125 - Hereditary spastic paraplegia and peroxisome biogenesis disorders: Case report of a patient with mutations in PEX10 gene

G.P.125 - Hereditary spastic paraplegia and peroxisome biogenesis disorders: Case report of a patient with mutations in PEX10 gene

LOXL1 Hypermethylation in Pseudoexfoliation Syndrome in the Uighur Population.

High prevalence of pseudoexfoliation syndrome (PEX) occurs in the Uighur population. This study investigated DNA methylation of the lysyloxidase-like 1 (LOXL1) gene in Uighur PEX patients with cataracts. The research involved 10 lens capsule specimens from Uighur PEX patients with cataracts and 10 lens capsule specimens from Uighur control patients with age-related cataract (ARC) alone. All specimens were freshly collected during cataract surgery. Methylation status of the CpG islands was analyzed using pyrosequencing. The mRNA levels of LOXL1 were evaluated by quantitative real-time PCR, and protein levels were evaluated by Western blot assay. For all the six chosen CpG islands of the LOXL1 gene promoter, hypermethylation was found in the PEX with cataracts compared to the age-matched ARC group. At the same time, the expression level of LOXL1 mRNA was significantly reduced in the PEX with cataracts group than that in the ARC group, and the expression level of the LOXL1 protein product demonstrated a similar tendency. The susceptible PEX gene LOXL1 undergoes DNA hypermethylation in its promoter region in Uighur PEX with cataracts patients. This indicates that epigenetic regulation might play roles in PEX pathogenesis.

499. Retinal Gene Therapy for Peroxisome Biogenesis Disorders

Peroxisome biogenesis disorders (PBDs) are a group of autosomal recessive disorders most frequently caused by inherited defects in the PEX1 gene, which is required for normal peroxisome assembly and biochemical functions. Approximately 80% of patients fall within the category of Zellweger spectrum disorder (PBD-ZSD), which has an overall incidence of 1:50,000 births in the United States. The majority of patients have milder forms of disease compatible with survival through adulthood; however, they typically show mild to moderate developmental delays, and progressive vision and hearing loss. Thus, therapeutic interventions that prevent or slow down visual loss could have a profound impact on the lives of these individualsHere, we present a retinal gene therapy approach that addresses visual deterioration in patients with milder forms of disease. Optical coherence tomography (OCT) has demonstrated that the cone photoreceptor cells are most significantly affected by loss of peroxisome functions in such patients. These visual phenotypes are recapitulated in a new mouse model of disease that expresses the murine equivalent of most common PEX1 mutation found in patients (PEX1-p.G843D). Electroretinogram (ERG) analyses indicated severe impairment of the cone visual pathway in these homozygous Pex1-mutant mice by 4 months of age with the rod visual system being relatively preserved. Staining retinal sections with peanut agglutinin showed that some cone photoreceptors were retained in the homozygote murine Pex1 -mutant retina at 3 weeks, but were completely degenerated in the adult. Finally, genome-wide expression studies showed specific reduction of photoreceptor cone-specific genes with no differential expression of other cell lineage-specific genes or others that indicate cell death or stress responses.We have developed AAV vectors to deliver normal copies of the 3.85-kb human PEX1 gene to the mammalian retina. These vectors use the rhodopsin kinase 1 (RK1) or truncated cytomegalovirus (CMV) promoters, respectively. Subretinal injections of RK1.PEX1. rAAV9 in healthy mice resulted in robust expression of human PEX1 mRNA. Furthermore, transduction of cultured Pex1 -mutant murine skin fibroblasts with CMV.PEX1.rAAV9 results in the rescue of peroxisome assembly defects in these cells, as determine by the intracellular localization of a modified GFP reporter protein with a peroxisome targeting signal. This indicates that the human PEX1 transgene can complement the genetic defect in the murine Pex1 gene. We are currently testing the ability of the AAV9-PEX1 gene delivery system to complement the retinal gene defect in our Pex1 -mutant mouse model. If successful, this could provide the preliminary data required to begin initial planning for eventual clinical trials in patients.

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.

Abnormal expression of PEX10 gene may be related to epilepsy associated with 1p36 copy number variations

To assess the association of PEX10 gene and 1p36 copy number variations in 1p36 region with concurrent epilepsy through analyzing 3 cases. The karyotypes of 3 patients were determined by high resolution chromosome banding, multiplex ligation dependent probe amplification (MLPA), fluorescence in situ hybridization (FISH) combined with single nucleotide polymorphism array (SNP) technology. Real-time PCR was carried out to determine the mRNA levels of PEX10 gene in peripheral blood of the patients. No abnormality was found upon high resolution karyotyping. MLPA analysis showed that all of the 3 patients had a copy number variation of subtelomeric region in the short arm of chromosome 1, which was confirmed by FISH and SNP chip analyses. Case 1 and case 2 both had an epilepsy phenotype, and their copy number variations have encompassed the PEX10 gene. On the other hand, case 3 has absent epilepsy, and its PEX10 gene copy number was normal. Family investigation confirmed that the chromosome abnormalities in all of the 3 cases were of de novo type. Compared with healthy controls, real-time PCR showed that mRNA of the PEX10 gene was increased in case 1 but decreased in case 2. The abnormal expression of PEX10 gene resulting from copy number variations of 1p36 region may be associated with the epilepsy phenotype.

Infantile Refsum Disease: Influence of Dietary Treatment on Plasma Phytanic Acid Levels.

Infantile Refsum disease (IRD) is one of the less severe of Zellweger spectrum disorders (ZSDs), a group of peroxisomal biogenesis disorders resulting from a generalized peroxisomal function impairment. Increased plasma levels of very long chain fatty acids (VLCFA) and phytanic acid are biomarkers used in IRD diagnosis. Furthermore, an increased plasma level of phytanic acid is known to be associated with neurologic damage. Treatment of IRD is symptomatic and multidisciplinary.The authors report a 3-year-old child, born from consanguineous parents, who presented with developmental delay, retinitis pigmentosa, sensorineural deafness and craniofacial dysmorphisms. While the relative level of plasma C26:0 was slightly increased, other VLCFA were normal. Thus, a detailed characterization of the phenotype was essential to point to a ZSD. Repeatedly increased levels of plasma VLCFA, along with phytanic acid and pristanic acid, deficient dihydroxyacetone phosphate acyltransferase activity in fibroblasts and identification of the homozygous pathogenic mutation c.2528G>A (p.Gly843Asp) in the PEX1 gene, confirmed this diagnosis. Nutritional advice and follow-up was proposed aiming phytanic acid dietary intake reduction. During dietary treatment, plasma levels of phytanic acid decreased to normal, and the patient's development evaluation showed slow progressive acquisition of new competences.This case report highlights the relevance of considering a ZSD in any child with developmental delay who manifests hearing and visual impairment and of performing a systematic biochemical investigation, when plasma VLCFA are mildly increased. During dietary intervention, a biochemical improvement was observed, and the long-term clinical effect of this approach needs to be evaluated.

PEX7 Mutations Cause Congenital Cataract Retinopathy and Late-Onset Ataxia and Cognitive Impairment: Report of Two Siblings and Review of the Literature.

Adult Refsum disease (ARD) is a progressive multisystem disorder that is characterized by retinitis pigmentosa, hearing and smell loss, skeletal deformities, and elevated levels of phytanic acid (PA) in tissues. Peripheral neuropathy, cardiac abnormalities, and skin ichthyosis are considered subsequent manifestations, and are often described in patients not treated with PA dietary restriction. Mutations of the gene encoding the phytanoyl-coenzyme A (CoA) hydroxylase enzyme (PHYH) are the major genetic cause of ARD,1 and in a small number of cases a second gene, PEX7, which encodes peroxin 7 receptor protein (PEX7), has been associated with the ARD phenotype.2 We observed two Italian siblings who presented with late-onset progressive ataxia and cognitive decline. They comprised a 56-year-old woman and her 63-year-old brother, born from consanguineous parents, and presented with infantile bilateral cataract and retinitis pigmentosa. Except for the visual defect, both siblings exhibited normal psychomotor development. In adulthood (at age 40 years for the brother and 53 years for the sister), both patients manifested progressive behavioral and cognitive abnormalities associated with ataxic gait. At the latest examination, at ages 57 and 63 years, respectively, they presented with dementia (Mini-Mental State Examination scores of 19/30 and 13/30, respectively), motor apraxia, ophthalmoparesis, mild dysarthria, ataxic gait, mild limb dysmetria, Babinski sign, and decreased lower-limb deep-tendon reflexes. Neither of the patients had hearing loss, anosmia, or short metacarpals/metatarsals, and only the brother presented with dilated cardiomyopathy. Brain MRI revealed mild diffuse atrophy, and muscle biopsy sampling revealed normal morphology. Nerve conduction studies demonstrated a mild motor demyelinating neuropathy in the lower limbs, and somatosensory evoked potentials revealed a prolongation of the central conduction time. Both patients also presented with increased plasma PA levels (387.5 and 188.9 µmol/L), suggesting a diagnosis of ARD. Mutational screening of PHYH was negative, but there was a novel homozygous mutation in PEX7, p.Leu12Pro, caused by a T>C transition at nucleotide 35 (c.35T>C). This missense variation involved a well-conserved residue, and has not been reported as a single-nucleotide polymorphism (SNP) in the Human Gene Mutation Database, National Center for Biotechnology Information database for SNPs (dbSNP132ver) or Exome Variant Server. In silico analyses predicted the mutation as probably damaging: the scores were 0.98 (PolyPhen-2-HumDiv), 0.635 (PolyPhen-2-HumVar), and 0 (Sorting Tolerant From Intolerant). The parents were not available for segregation analysis, and an asymptomatic brother was found to be heterozygous for this mutation. The patients refused a skin biopsy procedure, preventing investigations of plasmalogen synthesis.3 PEX7 mutations are most frequently found in infants affected by rhizomelic chondrodysplasia punctata type 1 (RCDP1), a congenital and rapidly worsening syndrome that is characterized by rhizomelia, chondrodysplasia punctata, cataract, and severe growth and mental retardation.3 The protein encoded by PEX7, Pex7, is essential for peroxisomal delivery of matrix enzymes containing the peroxisomal targeting signal type 2 in the amino acid sequence, including the phytanoyl-CoA hydroxylase and the plasmalogen synthesis apparatus.4 In both RCDP1 and ARD, patients present with elevated PA levels due to either a defect in the peroxisomal import of the metabolic enzymes or a primary loss-of-function of the phytanoyl-CoA hydroxylase enzyme.4 Several PEX7 mutations have so far been described in association with the RCDP1 phenotype. Only eight patients with PEX7 mutations and the Refsum phenotype have been reported, and in all of these cases the presentation was consistent with the ARD phenotype with a mild disease course (Table 1).2,3,5 In the previously reported cases, the most frequent disease manifestations were retinopathy, cataract, anosmia, hearing loss, and neuropathy (Table 1). In the cases described herein, the ocular abnormalities were also the earliest signs, but neither anosmia nor deafness was observed, while in the fifth decade both siblings developed progressive signs of dementia and ataxia. It is known that the clinical severity of this disease is associated with the residual activities and reduced amounts of Pex7.3 In particular, functional and modeling analyses have demonstrated that missense mutations associated with a severe phenotype are located in within the β sheets, probably causing a disruption of the protein structure.3 The p.Leu12Pro mutation, unlike previous missense mutations, is not located on the WD-40 repeat motif in PEX7, and it can be hypothesized that a less adverse effect on protein structure-allowing correct folding and greater residual activity-can account for the mild phenotype in these two patients. Table 1 Clinical, biochemical, and genetic characteristics of the patients with PEX7 gene mutations associated with Refsum disease phenotype, including the cases described in this study and review of the literature

An ancestral role in peroxisome assembly is retained by the divisional peroxin Pex11 in the yeast Yarrowia lipolytica.

The peroxin Pex11 has a recognized role in peroxisome division. Pex11p remodels and elongates peroxisomal membranes prior to the recruitment of dynamin-related GTPases that act in membrane scission to divide peroxisomes. We performed a comprehensive comparative genomics survey to understand the significance of the evolution of the Pex11 protein family in yeast and other eukaryotes. Pex11p is highly conserved and ancestral, and has undergone numerous lineage-specific duplications, whereas other Pex11 protein family members are fungal-specific innovations. Functional characterization of the in-silico-predicted Pex11 protein family members of the yeast Yarrowia lipolytica, i.e. Pex11p, Pex11Cp and Pex11/25p, demonstrated that Pex11Cp and Pex11/25p have a role in the regulation of peroxisome size and number characteristic of Pex11 protein family members. Unexpectedly, deletion of PEX11 in Y. lipolytica produces cells that lack morphologically identifiable peroxisomes, mislocalize peroxisomal matrix proteins and preferentially degrade peroxisomal membrane proteins, i.e. they exhibit the classical pex mutant phenotype, which has not been observed previously in cells deleted for the PEX11 gene. Our results are consistent with an unprecedented role for Pex11p in de novo peroxisome assembly.

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.

AB186. Association study between polymorphisms of PRMT6, PEX10, SOX5, and non-obstructive azoospermia in the Han Chinese population

A previous genomewide association study of non-obstructive azoospermia (NOA) in the Han Chinese population identified three risk loci (rs12097821, rs2477686, and rs10842262) and provided strong evidence for a genetic influence in male infertility. However, recently, a follow-up study of these single nucleotide polymorphism (SNP) loci in the Japanese population showed that none of them were significantly associated with NOA. Therefore, we conducted an association study, consisting of 550 NOA cases and 555 normal controls, to further validate whether the risk of those three SNPs still existed in an independent Han Chinese male population. The association studies did not support the association of rs12097821 and rs2477686 with NOA for both genotype and allele distributions, but rs10842262 in the SOX5 gene was significantly associated with NOA (chi square =9.31; P=0.0095 and chi square =9.27; P=0.0023, respectively). Our study provides genetic evidence for SOX5 polymorphism in NOA, contributing to predicting males at high risk of NOA in Han Chinese population. Considering genetic differences among populations, future validating studies in independent samples are suggested

Central serotonergic neuron deficiency in a mouse model of Zellweger syndrome

Zellweger syndrome (ZS) is a severe peroxisomal disorder caused by mutations in peroxisome biogenesis, or PEX, genes. A central hallmark of ZS is abnormal neuronal migration and neurodegeneration, which manifests as widespread neurological dysfunction. The molecular basis of ZS neuropathology is not well understood. Here we present findings using a mouse model of ZS neuropathology with conditional brain inactivation of the PEX13 gene. We demonstrate that PEX13 brain mutants display changes that reflect an abnormal serotonergic system – decreased levels of tryptophan hydroxylase-2, the rate-limiting enzyme of serotonin (5-hydroxytryptamine, 5-HT) synthesis, dysmorphic 5-HT-positive neurons, abnormal distribution of 5-HT neurons, and dystrophic serotonergic axons. The raphe nuclei region of PEX13 brain mutants also display increased levels of apoptotic cells and reactive, inflammatory gliosis. Given the role of the serotonergic system in brain development and motor control, dysfunction of this system would account in part for the observed neurological changes of PEX13 brain mutants.

Preperoxisomal vesicles can form in the absence of Pex3

We demonstrate that the peroxin Pex3 is not required for the formation of peroxisomal membrane structures in yeast pex3 mutant cells. Notably, pex3 mutant cells already contain reticular and vesicular structures that harbor key proteins of the peroxisomal receptor docking complex-Pex13 and Pex14-as well as the matrix proteins Pex8 and alcohol oxidase. Other peroxisomal membrane proteins in these cells are unstable and transiently localized to the cytosol (Pex10, Pmp47) or endoplasmic reticulum (Pex11). These reticular and vesicular structures are more abundant in cells of a pex3 atg1 double deletion strain, as the absence of Pex3 may render them susceptible to autophagic degradation, which is blocked in this double mutant. Contrary to earlier suggestions, peroxisomes are not formed de novo from the endoplasmic reticulum when the PEX3 gene is reintroduced in pex3 cells. Instead, we find that reintroduced Pex3 sorts to the preperoxisomal structures in pex3 cells, after which these structures mature into normal peroxisomes.

The Pex1-G844D mouse: A model for mild human Zellweger spectrum disorder

Zellweger spectrum disorder (ZSD) is a disease continuum that results from inherited defects in PEX genes essential for normal peroxisome assembly. These autosomal recessive disorders impact brain development and also cause postnatal liver, adrenal, and kidney dysfunction, as well as loss of vision and hearing. The hypomorphic PEX1-G843D missense allele, observed in approximately 30% of ZSD patients, is associated with milder clinical and biochemical phenotypes, with some homozygous individuals surviving into early adulthood. Nonetheless, affected children with the PEX1-G843D allele have intellectual disability, failure to thrive, and significant sensory deficits. To enhance our ability to test candidate therapies that improve human PEX1-G843D function, we created the novel Pex1-G844D knock-in mouse model that represents the murine equivalent of the common human mutation. We show that Pex1-G844D homozygous mice recapitulate many classic features of mild ZSD cases, including growth retardation and fatty livers with cholestasis. In addition, electrophysiology, histology, and gene expression studies provide evidence that these animals develop a retinopathy similar to that observed in human patients, with evidence of cone photoreceptor cell death. Similar to skin fibroblasts obtained from ZSD patients with a PEX1-G843D allele, we demonstrate that murine cells homozygous for the Pex1-G844D allele respond to chaperone-like compounds, which normalizes peroxisomal β-oxidation. Thus, the Pex1-G844D mouse provides a powerful model system for testing candidate therapies that address the most common genetic cause of ZSD. In addition, this murine model will enhance studies focused on mechanisms of pathogenesis.

Cholesterol biosynthesis and ER stress in peroxisome deficiency

Cholesterol biosynthesis is a multi-step process involving more than 20 enzymes in several subcellular compartments. The pre-squalene segment of the cholesterol/isoprenoid biosynthetic pathway is localized in peroxisomes. This review intends to highlight recent findings illustrating the important role peroxisomes play in cholesterol biosynthesis and maintenance of cholesterol homeostasis. Disruption of the Pex2 gene leads to peroxisome deficiency and widespread metabolic dysfunction. The Pex2(-/-) mouse model for Zellweger syndrome enabled us to evaluate the role of peroxisomes in cholesterol biosynthesis. These studies have shown that Pex2(-/-) mice exhibit low levels of cholesterol in plasma and liver. Pex2(-/-) mice were unable to maintain normal cholesterol homeostasis despite activation of SREBP-2, the master transcriptional regulator of cholesterol biosynthesis, and increased protein levels and activities of cholesterol biosynthetic enzymes. The SREBP-2 pathway remained activated even after normalization of hepatic cholesterol levels in response to bile acid feeding as well as in extrahepatic tissues and the liver of neonatal and longer surviving Pex2 mutants, where cholesterol levels were normal. Several studies have shown that endoplasmic reticulum (ER) stress can dysregulate lipid metabolism via SREBP activation independently of intracellular cholesterol concentration. We demonstrated that peroxisome deficiency activates endoplasmic reticulum stress pathways in Pex2(-/-) mice, especially the integrated stress response mediated by PERK and ATF4 signaling, and thereby leads to dysregulation of the SREBP-2 pathway. Our findings suggest that functional peroxisomes are necessary to prevent chronic ER stress and dysregulation of the endogenous sterol response pathway. The constitutive activation of ER stress pathways might contribute to organ pathology and metabolic dysfunction in peroxisomal disorder patients.

Stable overproducer of hepatitis B surface antigen in the methylotrophic yeast Hansenula polymorpha due to multiple integration of heterologous auxotrophic selective markers and defect in peroxisome biogenesis

Two methods of multicopy integrant selection in the methylotrophic yeast Hansenula polymorpha based on the use of heterologous yeast auxotrophic genes have been used to isolate effective overproducers of hepatitis B surface antigen (HBsAg). One selection marker was described earlier for this yeast, the Saccharomyces cerevisiae URA3 gene, whereas the second selection marker was developed by us, the Pichia pastoris ADE1 gene with shortened native promoter. Sequential use of both selection markers produced stable transformants containing up to 30 integration cassettes with HBsAg gene. Deletion of PEX3 gene coding for peroxine involved in the early step of peroxisome formation substantially increased the production of HBsAg in glucose medium as compared to the parental strain. Maximal production of HBsAg in Δpex3 strain was nearly 8-9% of the total cell protein.

Arginine improves peroxisome functioning in cells from patients with a mild peroxisome biogenesis disorder

BackgroundZellweger spectrum disorders (ZSDs) are multisystem genetic disorders caused by a lack of functional peroxisomes, due to mutations in one of the PEX genes, encoding proteins involved in peroxisome biogenesis. The phenotypic spectrum of ZSDs ranges from an early lethal form to much milder presentations. In cultured skin fibroblasts from mildly affected patients, peroxisome biogenesis can be partially impaired which results in a mosaic catalase immunofluorescence pattern. This peroxisomal mosaicism has been described for specific missense mutations in various PEX genes. In cell lines displaying peroxisomal mosaicism, peroxisome biogenesis can be improved when these are cultured at 30°C. This suggests that these missense mutations affect the folding and/or stability of the encoded protein. We have studied if the function of mutant PEX1, PEX6 and PEX12 can be improved by promoting protein folding using the chemical chaperone arginine.MethodsFibroblasts from three PEX1 patients, one PEX6 and one PEX12 patient were cultured in the presence of different concentrations of arginine. To determine the effect on peroxisome biogenesis we studied the following parameters: number of peroxisome-positive cells, levels of PEX1 protein and processed thiolase, and the capacity to β-oxidize very long chain fatty acids and pristanic acid.ResultsPeroxisome biogenesis and function in fibroblasts with mild missense mutations in PEX1, 6 and 12 can be improved by arginine.ConclusionArginine may be an interesting compound to promote peroxisome function in patients with a mild peroxisome biogenesis disorder.

Peroxisome biogenesis disorders: Biological, clinical and pathophysiological perspectives

The peroxisome biogenesis disorders (PBD) are a heterogeneous group of autosomal recessive disorders in which peroxisome assembly is impaired, leading to multiple peroxisome enzyme deficiencies, complex developmental sequelae and progressive disabilities. Mammalian peroxisome assembly involves the protein products of 16 PEX genes; defects in 14 of these have been shown to cause PBD. Three broad phenotypic groups are described on a spectrum of severity: Zellweger syndrome is the most severe, neonatal adrenoleukodystrophy is intermediate and infantile Refsum disease is less severe. Another group is Rhizomelic chondrodysplasia punctata spectrum. Recently, atypical phenotypes have been described, indicating that the full spectrum of these disorders remains to be identified. For most patients, there is a correlation between clinical severity and effect of the mutation on PEX protein function. Diagnosis relies on biochemical measurements of peroxisome functions and PEX gene sequencing. There are no targeted therapies, although management protocols have been suggested and research endeavors continue. In this review we will discuss peroxisome biology and PBD, and research contributions to pathophysiology and treatment.

Functional analysis of PEX13 mutation in a Zellweger syndrome spectrum patient reveals novel homooligomerization of PEX13 and its role in human peroxisome biogenesis

In humans, the concerted action of at least 13 different peroxisomal PEX proteins is needed for proper peroxisome biogenesis. Mutations in any of these PEX genes can lead to lethal neurometabolic disorders of the Zellweger syndrome spectrum (ZSS). Previously, we identified the W313G mutation located within the SH3 domain of the peroxisomal protein, PEX13. As this tryptophan residue is highly conserved in almost all known SH3 proteins, we investigated the pathogenic mechanism of the W313G mutation and its role in PEX13 interactions and functions in peroxisome biogenesis. Here, we report for the first time that human PEX13 interacts with itself in peroxisomes in living cells. We demonstrate that the import of PTS1 (peroxisomal targeting signal 1) proteins is specifically disrupted when homooligomerization of PEX13 is interrupted. Live cell FRET microscopy in living cells as well as co-immunoprecipitation experiments reveal that the highly conserved W313 residue is important for self-association of PEX13 but is not required for interaction with PEX14, a well-established interaction partner at the peroxisomal membrane. Experiments with truncated constructs indicate that although the W313G mutation resides in the C-terminal SH3 domain, the N-terminal half is necessary for peroxisomal localization, which in turn appears to be crucial for homooligomerization. Furthermore, rescue of homooligomerization in the W313G mutant cells through complementation with truncation constructs restores import of peroxisomal matrix proteins. Taken together, the thorough analyses of a ZSS patient mutation unraveled the general cell biological function of PEX13 and its mechanism in the import of peroxisomal matrix PTS1 proteins.

Child Neurology: Zellweger syndrome

Zellweger syndrome (ZS) is a severe manifestation of disease within the spectrum of peroxisome biogenesis disorders that includes neonatal adrenoleukodystrophy, infantile Refsum disease, and rhizomelic chondroplasia punctata. Patients with ZS present in the neonatal period with a characteristic phenotype of distinctive facial stigmata, pronounced hypotonia, poor feeding, hepatic dysfunction, and often seizures and boney abnormalities. In patients with ZS, a mutation in one of the PEX genes coding for a peroxin (a peroxisome assembly protein) creates functionally incompetent organelles causing an accumulation of very long chain fatty acids (VLCFA), among other complications. Despite an absence of treatment options, prompt diagnosis of ZS is important for providing appropriate symptomatic care, definitive genetic testing, and counseling regarding family planning.

A deleterious mutation in thePEX2gene causes Zellweger syndrome in individuals of Ashkenazi Jewish descent

Zellweger syndrome is known to be caused by numerous mutations that occur in at least 12 of the PEX genes. While phenotypes vary, many are severely debilitating, and death can result in affected newborns. Since the disease follows an autosomal recessive pattern of inheritance, carrier screening can be done for at-risk couples, but the number of potential mutations sites to screen can be daunting. Ethnicity-specific studies can help narrow this range by highlighting mutations that are present at higher percentages in certain populations. In this article, the carrier frequencies for two mutations causative of the severe Zellweger syndrome spectrum phenotype that occur in the PEX2 gene, c.355C>T and c.550del, were studied in individuals of Ashkenazi Jewish descent in order to advise on inclusion in existing carrier screening mutation panels for this population. The screening was performed for 2093 individuals through the use of TaqMan genotyping assays, real-time PCR, and allelic discrimination. Results indicated a carrier frequency of 0.813% (±0.385%) for the c.355C>T mutation and a carrier frequency of 0.00% (±0.00%) for the c.550del mutation. On the basis of these frequencies, we believe that the c.355C>T mutation should be considered for inclusion in carrier screening panels for the Ashkenazi population.