A PEX6-defective peroxisomal biogenesis disorder with severe phenotype in an infant, versus mild phenotype resembling Usher syndrome in the affected parents

Cellular and Molecular Mechanisms of Pathogenesis Underlying Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are congenital retinal degenerative diseases that have various inheritance patterns, including dominant, recessive, X-linked, and mitochondrial. These diseases are most often the result of defects in rod and/or cone photoreceptor and retinal pigment epithelium function, development, or both. The genes associated with these diseases, when mutated, produce altered protein products that have downstream effects in pathways critical to vision, including phototransduction, the visual cycle, photoreceptor development, cellular respiration, and retinal homeostasis. The aim of this manuscript is to provide a comprehensive review of the underlying molecular mechanisms of pathogenesis of IRDs by delving into many of the genes associated with IRD development, their protein products, and the pathways interrupted by genetic mutation.

The physiological functions of human peroxisomes

Peroxisomes are subcellular organelles that play a central role in human physiology by catalyzing a range of unique metabolic functions. The importance of peroxisomes for human health is exemplified by the existence of a group of usually severe diseases caused by an impairment in one or more peroxisomal functions. Among others these include the Zellweger spectrum disorders, X-linked adrenoleukodystrophy, and Refsum disease. To fulfill their role in metabolism, peroxisomes require continued interaction with other subcellular organelles including lipid droplets, lysosomes, the endoplasmic reticulum, and mitochondria. In recent years it has become clear that the metabolic alliance between peroxisomes and other organelles requires the active participation of tethering proteins to bring the organelles physically closer together, thereby achieving efficient transfer of metabolites. This review intends to describe the current state of knowledge about the metabolic role of peroxisomes in humans, with particular emphasis on the metabolic partnership between peroxisomes and other organelles and the consequences of genetic defects in these processes. We also describe the biogenesis of peroxisomes and the consequences of the multiple genetic defects therein. In addition, we discuss the functional role of peroxisomes in different organs and tissues and include relevant information derived from model systems, notably peroxisomal mouse models. Finally, we pay particular attention to a hitherto underrated role of peroxisomes in viral infections.

Loss of Pex1 in Inner Ear Hair Cells Contributes to Cochlear Synaptopathy and Hearing Loss

Peroxisome Biogenesis Disorders (PBD) and Zellweger syndrome spectrum disorders (ZSD) are rare genetic multisystem disorders that include hearing impairment and are associated with defects in peroxisome assembly, function, or both. Mutations in 13 peroxin (PEX) genes have been found to cause PBD-ZSD with ~70% of patients harboring mutations in PEX1. Limited research has focused on the impact of peroxisomal disorders on auditory function. As sensory hair cells are particularly vulnerable to metabolic changes, we hypothesize that mutations in PEX1 lead to oxidative stress affecting hair cells of the inner ear, subsequently resulting in hair cell degeneration and hearing loss. Global deletion of the Pex1 gene is neonatal lethal in mice, impairing any postnatal studies. To overcome this limitation, we created conditional knockout mice (cKO) using Gfi1^Creor VGlut3^Cre expressing mice crossed to floxed Pex1 mice to allow for selective deletion of Pex1 in the hair cells of the inner ear. We find that Pex1 excision in inner hair cells (IHCs) leads to progressive hearing loss associated with significant decrease in auditory brainstem responses (ABR), specifically ABR wave I amplitude, indicative of synaptic defects. Analysis of IHC synapses in cKO mice reveals a decrease in ribbon synapse volume and functional alterations in exocytosis. Concomitantly, we observe a decrease in peroxisomal number, indicative of oxidative stress imbalance. Taken together, these results suggest a critical function of Pex1 in development and maturation of IHC-spiral ganglion synapses and auditory function.

Characterization of Severity in Zellweger Spectrum Disorder by Clinical Findings: A Scoping Review, Meta-Analysis and Medical Chart Review

Zellweger spectrum disorder (ZSD) is a rare, debilitating genetic disorder of peroxisome biogenesis that affects multiple organ systems and presents with broad clinical heterogeneity. Although severe, intermediate, and mild forms of ZSD have been described, these designations are often arbitrary, presenting difficulty in understanding individual prognosis and treatment effectiveness. The purpose of this study is to conduct a scoping review and meta-analysis of existing literature and a medical chart review to determine if characterization of clinical findings can predict severity in ZSD. Our PubMed search for articles describing severity, clinical findings, and survival in ZSD resulted in 107 studies (representing 307 patients) that were included in the review and meta-analysis. We also collected and analyzed these same parameters from medical records of 136 ZSD individuals from our natural history study. Common clinical findings that were significantly different across severity categories included seizures, hypotonia, reduced mobility, feeding difficulties, renal cysts, adrenal insufficiency, hearing and vision loss, and a shortened lifespan. Our primary data analysis also revealed significant differences across severity categories in failure to thrive, gastroesophageal reflux, bone fractures, global developmental delay, verbal communication difficulties, and cardiac abnormalities. Univariable multinomial logistic modeling analysis of clinical findings and very long chain fatty acid (VLCFA) hexacosanoic acid (C26:0) levels showed that the number of clinical findings present among seizures, abnormal EEG, renal cysts, and cardiac abnormalities, as well as plasma C26:0 fatty acid levels could differentiate severity categories. We report the largest characterization of clinical findings in relation to overall disease severity in ZSD. This information will be useful in determining appropriate outcomes for specific subjects in clinical trials for ZSD.

A Retrospective Study of Hearing Loss in Patients Diagnosed with Peroxisome Biogenesis Disorders in the Zellweger Spectrum

OBJECTIVES

Peroxisome Biogenesis Disorders in the Zellweger Spectrum (PBD-ZSD) are autosomal recessive disorders characterized by defects in peroxisome function, biosynthesis, and/or assembly. Despite its frequent documentation, hearing loss associated with PBD-ZSD has not been extensively characterized. The purpose of this retrospective natural history study was to better characterize the hearing loss associated with PBD-ZSD and to provide additional insight into the evaluation and management of PBD-ZSD patients with hearing loss.

DESIGN

Audiological data from medical records of 42 patients with PBD-ZSD or D-bifunctional protein deficiency were collected from an ongoing longitudinal retrospective natural history study. An initial dataset of 300 audiograms and/or audiometric test results from the 42 patients were used to characterize the degree of hearing loss, type of hearing loss, relationships between air and bone conduction thresholds, age-related changes in hearing loss, and benefit with amplification.

RESULTS

The majority of PBD-ZSD patients in this study presented with moderately-severe to severe hearing loss and relatively slow rates of longitudinal changes in hearing sensitivity. Improvements in hearing thresholds were observed with use of hearing aid amplification. Though bone conduction data were limited, air-bone gaps and air conduction threshold fluctuations observed in several patients suggest there may be an increased occurrence of mixed hearing losses in PBD-ZSD populations.

CONCLUSION

The results of this retrospective study provide insight into the hearing loss associated with PBD-ZSD, but also emphasize the need for more complete assessments of hearing loss type and middle ear function in these patients. The addition of more comprehensive datasets to the ongoing natural history study will enhance our understanding of the pathophysiology underlying PBD-ZSD and guide the development of targeted evaluation and management recommendations for patients with PBD-ZSD.

Genetics, pathogenesis and therapeutic developments for Usher syndrome type 2

Usher syndrome (USH) is a rare, autosomal recessively inherited disorder resulting in a combination of sensorineural hearing loss and a progressive loss of vision resulting from retinitis pigmentosa (RP), occasionally accompanied by an altered vestibular function. More and more evidence is building up indicating that also sleep deprivation, olfactory dysfunction, deficits in tactile perception and reduced sperm motility are part of the disease etiology. USH can be clinically classified into three different types, of which Usher syndrome type 2 (USH2) is the most prevalent. In this review, we, therefore, assess the genetic and clinical aspects, available models and therapeutic developments for USH2. Mutations in USH2A, ADGRV1 and WHRN have been described to be responsible for USH2, with USH2A being the most frequently mutated USH-associated gene, explaining 50% of all cases. The proteins encoded by the USH2 genes together function in a dynamic protein complex that, among others, is found at the photoreceptor periciliary membrane and at the base of the hair bundles of inner ear hair cells. To unravel the pathogenic mechanisms underlying USH2, patient-derived cellular models and animal models including mouse, zebrafish and drosophila, have been generated that all in part mimic the USH phenotype. Multiple cellular and genetic therapeutic approaches are currently under development for USH2, mainly focused on preserving or partially restoring the visual function of which one is already in the clinical phase. These developments are opening a new gate towards a possible treatment for USH2 patients.

PEX6 Mutations in Peroxisomal Biogenesis Disorders: An Usher Syndrome Mimic

PURPOSE

Peroxisomal biogenesis disorders (PBDs) represent a spectrum of conditions that result in vision loss, sensorineural hearing loss, neurologic dysfunction, and other abnormalities resulting from aberrant peroxisomal function caused by mutations in PEX genes. With no treatments currently available, we sought to investigate the disease mechanism in a patient with a PBD caused by defects in PEX6 and to probe whether overexpression of PEX6 could restore peroxisome function and potentially offer therapeutic benefit.

DESIGN

Laboratory-based study.

PARTICIPANTS

A 12-year-old boy sought treatment with hearing loss and retinopathy. After negative results in an Usher syndrome panel, targeted genetic testing revealed compound heterozygous mutations in PEX6. These included a 14-nucleotide deletion (c.802_815del: p.(Asp268Cysfs∗8)) and a milder missense variant (c.35T→C:(p.Phe12Ser)).

METHODS

Patient-derived skin fibroblasts were cultured, and a PEX6 knockout cell line was developed using clustered regularly interspaced short palindromic repeats and Cas9 technology in HEK293T cells to emulate a more severe disease phenotype. Immunoblot analysis of whole cell lysates was performed to assess peroxisome number. Immunofluorescence studies used antibodies against components of the peroxisomal protein import pathway to interrogate the effects of mutations in PEX6 on protein trafficking.

MAIN OUTCOME MEASURES

Primary outcome measures were peroxisome abundance and matrix protein import.

RESULTS

Peroxisome number was not significantly different between control fibroblasts and patient fibroblasts; however, fewer peroxisomes were observed in PEX6 knockout cells compared with wild-type cells (P = 0.04). Analysis by immunofluorescent microscopy showed significantly impaired peroxisomal targeting signal 1- and peroxisomal targeting signal 2-mediated matrix protein import in both patient fibroblasts and PEX6 knockout cells. Overexpressing PEX6 resulted in improved matrix protein import in PEX6 knockout cells.

CONCLUSIONS

Mutations in PEX6 were responsible for combined hearing loss and retinopathy in our patient. The primary peroxisomal defect in our patient's skin fibroblasts was impaired peroxisomal protein import as opposed to reduction in the number of peroxisomes. Genetic strategies that introduce wild-type PEX6 into cells deficient in PEX6 protein show promise in restoring peroxisome function. Future studies of patient-specific induced pluripotent stem cell-derived retinal pigment epithelium cells may clarify the role of PEX6 in the retina and the potential for gene therapy in these patients.

Peroxisomal Disorders and Their Mouse Models Point to Essential Roles of Peroxisomes for Retinal Integrity

Peroxisomes are multifunctional organelles, well known for their role in cellular lipid homeostasis. Their importance is highlighted by the life-threatening diseases caused by peroxisomal dysfunction. Importantly, most patients suffering from peroxisomal biogenesis disorders, even those with a milder disease course, present with a number of ocular symptoms, including retinopathy. Patients with a selective defect in either peroxisomal α- or β-oxidation or ether lipid synthesis also suffer from vision problems. In this review, we thoroughly discuss the ophthalmological pathology in peroxisomal disorder patients and, where possible, the corresponding animal models, with a special emphasis on the retina. In addition, we attempt to link the observed retinal phenotype to the underlying biochemical alterations. It appears that the retinal pathology is highly variable and the lack of histopathological descriptions in patients hampers the translation of the findings in the mouse models. Furthermore, it becomes clear that there are still large gaps in the current knowledge on the contribution of the different metabolic disturbances to the retinopathy, but branched chain fatty acid accumulation and impaired retinal PUFA homeostasis are likely important factors.

A founder mutation in PEX12 among Egyptian patients in peroxisomal biogenesis disorder

At least 14 distinctive PEX genes function in the biogenesis of peroxisomes. Biallelic alterations in the peroxisomal biogenesis factor 12 (PEX12) gene lead to Zellweger syndrome spectrum (ZSS) with variable clinical expressivity ranging from early lethality to mildly affected with long-term survival. Herein, we define 20 patients derived from 14 unrelated Egyptian families, 19 of which show a homozygous PEX12 in-frame (c.1047_1049del p.(Gln349del)) deletion. This founder mutation, reported rarely outside of Egypt, was associated with a uniformly severe phenotype. Patients showed developmental delay in early life followed by motor and mental regression, progressive hypotonia, unsteadiness, and lack of speech. Seventeen patients had sparse hair or partial alopecia, a striking feature that was not noted previously in PEX12. Neonatal cholestasis was manifested in 2 siblings. Neurodiagnostics showed consistent cerebellar atrophy and variable white matter demyelination, axonal neuropathy in about half, and cardiomyopathy in 10% of patients. A single patient with a compound heterozygous PEX12 mutation exhibited milder features with late childhood onset with gait disturbance and learning disability. Thus, the PEX12 relatively common founder mutation accounts for the majority of PEX12-related disease in Egypt and delineates a uniform clinical and radiographic phenotype.

Syndromic Inherited Retinal Diseases: Genetic, Clinical and Diagnostic Aspects

Inherited retinal diseases (IRDs), which are among the most common genetic diseases in humans, define a clinically and genetically heterogeneous group of disorders. Over 80 forms of syndromic IRDs have been described. Approximately 200 genes are associated with these syndromes. The majority of syndromic IRDs are recessively inherited and rare. Many, although not all, syndromic IRDs can be classified into one of two major disease groups: inborn errors of metabolism and ciliopathies. Besides the retina, the systems and organs most commonly involved in syndromic IRDs are the central nervous system, ophthalmic extra-retinal tissues, ear, skeleton, kidney and the cardiovascular system. Due to the high degree of phenotypic variability and phenotypic overlap found in syndromic IRDs, correct diagnosis based on phenotypic features alone may be challenging and sometimes misleading. Therefore, genetic testing has become the benchmark for the diagnosis and management of patients with these conditions, as it complements the clinical findings and facilitates an accurate clinical diagnosis and treatment.

Mild form of Zellweger Spectrum Disorders (ZSD) due to variants in PEX1: Detailed clinical investigation in a 9-years-old female

Peroxisomal biogenesis disorders (PBD) are rare autosomal recessive disorders with various degrees of severity caused by hypomorphic mutations in 13 different peroxin (PEX) genes. In this study, we report the clinical and molecular characterization of a 9-years-old female presenting an apparently isolated pre-lingual sensorineural hearing loss (SNHL) and early onset Retinitis Pigmentosa (RP) that may clinically overlap with Usher syndrome. Genetic testing by clinical exome sequencing identified two variants in PEX1: the missense variant c.274G > C; p.(Val92Leu) that was already reported in a PBD patient, and the variant c.2140_2145dup; p.(Ser714_Gln715dup) which is a novel, non-frameshift variant, absent in control databases. On the basis of the molecular analysis, a thorough clinical examination revealed nail and dental abnormalities, a mild cognitive impairment, learning disabilities and poor feeding, apart from the retinal and audiological features initially identified. The clinical and molecular findings led us to the diagnosis of a mild form of PBD. This study further emphasizes that mild forms of PBD can be a differential diagnosis of Usher syndrome and suggests that patients with mild cognitive impairment associated to visual and hearing loss should perform a comprehensive mutation screening that includes PEX genes.

The peroxisomal disorder spectrum and Heimler syndrome: Deep phenotyping and review of the literature

The spectrum of peroxisomal disorders is wide and comprises individuals that die in the first year of life, as well as people with sensorineural hearing loss, retinal dystrophy and amelogenesis imperfecta. In this article, we describe three patients; two diagnosed with Heimler syndrome and a third one with a mild-intermediate phenotype. We arrived at these diagnoses by conducting complete ophthalmic (National Eye Institute), auditory (National Institute of Deafness and Other Communication Disorders), and dental (National Institute of Dental and Craniofacial Research) evaluations, as well as laboratory and genetic testing. Retinal degeneration with macular cystic changes, amelogenesis imperfecta, and sensorineural hearing loss were features shared by the three patients. Patients A and C had pathogenic variants in PEX1 and Patient B, in PEX6. Besides analyzing these cases, we review the literature regarding mild peroxisomal disorders, their pathophysiology, genetics, differential diagnosis, diagnostic methods, and management. We suggest that peroxisomal disorders are considered in every child with sensorineural hearing loss and retinal degeneration. These patients should have a dental evaluation to rule out amelogenesis imperfecta as well as audiologic examination and laboratory testing including peroxisomal biomarkers and genetic testing. Appropriate diagnosis can lead to better genetic counseling and management of the associated comorbidities.

Expanding the Genetic Landscape of Usher-Like Phenotypes

Purpose

Usher syndrome (USH) is a rare disorder characterized by retinitis pigmentosa (RP) and sensorineural hearing loss. Several genes are responsible for the disease, but not all cases are explained by mutations in any of these, supporting the fact that there remain other unknown genes that have a role in the syndrome. We aimed to find the genetic cause of presumed USH patients lacking pathogenic mutations in the known USH genes.

Methods

Whole exome sequencing was performed on a priori USH-diagnosed subjects from nine unrelated families, which had shown negative results for an USH-targeted panel in a previous study.

Results

We identified possible pathogenic variants in six of the studied families. One patient harbored mutations in REEP6 and TECTA, each gene tentatively causative of one of the two main symptoms of the disease, mimicking the syndrome. In three patients, only the retinal degeneration causative mutations were detected (involving EYS, WDR19, and CNGB1 genes). Another family manifested a dementia-linked retinal dystrophy dependent on an allele dosage in the GRN gene. Last, another case presented a homozygous mutation in ASIC5, a gene not yet associated with USH.

Conclusions

Our findings demonstrate that pending cases should be clinically and genetically carefully assessed, since more patients than expected may be either related phenocopies or affected by a more complex disease encompassing additional symptoms rather than classical USH.

Heimler Syndrome

Heimler syndrome is a rare syndrome associating sensorineural hearing loss with retinal dystrophy and amelogenesis imperfecta due to PEX1 or PEX6 biallelic pathogenic variations. This syndrome is one of the less severe forms of peroxisome biogenesis disorders. In this chapter, we will review clinical, biological, and genetic knowledges about the Heimler syndrome.

Mild Zellweger syndrome due to functionally confirmed novel PEX1 variants

Zellweger spectrum disorders (ZSD) constitute a group of rare autosomal recessive disorders characterized by a defect in peroxisome biogenesis due to mutations in one of 13 PEX genes. The broad clinical heterogeneity especially in late-onset presenting patients and a mild phenotype complicates and delays the diagnostic process. Here, we report a case of mild ZSD, due to novel PEX1 variants. The patient presented with an early hearing loss, bilateral cataracts, and leukodystrophy on magnetic resonance (MR) images. Normal results of serum very-long-chain fatty acids (VLCFA) and phytanic acid were found. Molecular diagnostics were performed to uncover the etiology of the clinical phenotype. Using whole exome sequencing, there have been found two variants in the PEX1 gene-c.3450T>A (p.Cys1150*) and c.1769T>C (p.Leu590Pro). VLCFA measurement in skin fibroblasts and C26:0-lysoPC in dried blood spot therefore was performed. Both results were in line with the diagnosis of ZSD. To conclude, normal results of routine serum VLCFA and branched-chain fatty acid measurement do not exclude mild forms of ZSD. The investigation of C26:0-lysoPC should be included in the diagnostic work-up in patients with cataract, hearing loss, and leukodystrophy on MR images suspected to suffer from ZSD.

Structural Mapping of Missense Mutations in the Pex1/Pex6 Complex

Peroxisome biogenesis disorders (PBDs) are nontreatable hereditary diseases with a broad range of severity. Approximately 65% of patients are affected by mutations in the peroxins Pex1 and Pex6. The proteins form the heteromeric Pex1/Pex6 complex, which is important for protein import into peroxisomes. To date, no structural data are available for this AAA+ ATPase complex. However, a wealth of information can be transferred from low-resolution structures of the yeast scPex1/scPex6 complex and homologous, well-characterized AAA+ ATPases. We review the abundant records of missense mutations described in PBD patients with the aim to classify and rationalize them by mapping them onto a homology model of the human Pex1/Pex6 complex. Several mutations concern functionally conserved residues that are implied in ATP hydrolysis and substrate processing. Contrary to fold destabilizing mutations, patients suffering from function-impairing mutations may not benefit from stabilizing agents, which have been reported as potential therapeutics for PBD patients.

Two novel mutations of PEX6 in one Chinese Zellweger spectrum disorder and their clinical characteristics

Background

Zellweger spectrum disorder (ZSD) is an autosomal recessive peroxisome biogenesis disorder (PBD) caused by bi-allelic mutations in any of the 13 PEX family genes.

Methods

We reported a Chinese PBD-ZSD patient with compound heterozygous mutations of PEX6 detected by target sequencing and Sanger sequencing. The clinical materials were collected. In silico analysis were used to evaluate the pathogenicity of the two mutations. An updated review summarized the genotype-phenotype correlation of PBD patients with PEX6 mutations.

Results

The patient was diagnosed as PBD-ZSD and displayed retinitis pigmentosa, bilateral sensorineural hearing loss, hypotonia, developmental delay, ovarian and enamel dysplasia. Elevated very long chain fatty acids were shown and a pattern of leukodystrophy was displayed through MRI. The two mutations were novel with p.Cys358* and p.Leu83Pro, both classified as pathogenic according to American College of Medical Genetics and Genomics guideline. Phenotype-genotype correlations were shown in the reported patients with PBD-ZSD continuum.

Conclusions

we reported the first Chinese PBD-ZSD patient with 2 novel mutations in PEX6. Target sequencing and VLFAC were helpful in diagnosis.

Liver disease predominates in a mouse model for mild human Zellweger spectrum disorder

Zellweger spectrum disorders (ZSDs) are autosomal recessive diseases caused by defective peroxisome assembly. They constitute a clinical continuum from severe early lethal to relatively milder presentations in adulthood. Liver disease is a prevalent symptom in ZSD patients. The underlying pathogenesis for the liver disease, however, is not fully understood. We report a hypomorphic ZSD mouse model, which is homozygous for Pex1-c.2531G>A (p.G844D), the equivalent of the most common pathogenic variant found in ZSD, and which predominantly presents with liver disease. After introducing the Pex1-G844D allele by knock-in, we characterized homozygous Pex1-G844D mice for survival, biochemical parameters, including peroxisomal and mitochondrial functions, organ histology, and developmental parameters. The first 20 post-natal days (P20) were critical for survival of homozygous Pex1-G844D mice (~20% survival rate). Lethality was likely due to a combination of cholestatic liver problems, liver dysfunction and caloric deficit, probably as a consequence of defective bile acid biosynthesis. Survival beyond P20 was nearly 100%, but surviving mice showed a marked delay in growth. Surviving mice showed similar hepatic problems as described for mild ZSD patients, including hepatomegaly, bile duct proliferation, liver fibrosis and mitochondrial alterations. Biochemical analyses of various tissues showed the absence of functional peroxisomes accompanied with aberrant levels of peroxisomal metabolites predominantly in the liver, while other tissues were relatively spared. ur findings show that homozygous Pex1-G844D mice have a predominant liver disease phenotype, mimicking the hepatic pathology of ZSD patients, and thus constitute a good model to study pathogenesis and treatment of liver disease in ZSD patients.

Ophthalmic manifestations of Heimler syndrome due to PEX6 mutations

BACKGROUND/AIMS

Pigmentary retinal dystrophy and macular dystrophy have been previously reported in Heimler syndrome due to mutations in PEX1. Here we reported the ocular manifestations in Heimler syndrome due to mutations in PEX6.

MATERIALS AND METHODS

Medical records were reviewed to identify patient demographics, ophthalmic and systemic findings, and results of diagnostic testing including whole genome sequencing.

RESULTS

Patient 1 is 12-year-old boy with a novel mutation c.275T>G (p.Val92Gly) and known mutation c.1802G>A (p.Arg601Gln) in PEX6. Patient 2 is a 7-year-old girl with the same known c.1802G>A (p.Arg601Gln) mutation and another novel missense mutation c.296G>T (p.Arg99Leu). Both patients exhibited a pigmentary retinopathy. Visual acuity in patient 1 was 20/80 and 20/25 following treatment of intraretinal cystoid spaces with carbonic anhydrase inhibitors, while patient 2 had visual acuity of 20/20 in both eyes without intraretinal cysts. Fundus autofluorescence showed a multitude of hyperfluorescent deposits in the paramacular area of both eyes. OCTs revealed significant depletion of photoreceptors in both patients and macular intraretinal cystoid spaces in one patient. Full field electroretinograms showed normal or abnormal photopic but normal scotopic responses. Multifocal electroretinograms were abnormal.

CONCLUSIONS

Heimler syndrome due to biallelic PEX6 mutations demonstrates a macular dystrophy with characteristic fundus autofluorescence and may be complicated by intraretinal cystoid spaces.

Peroxisomal Disorders and Retinal Degeneration

Peroxisomal disorders are a group of inherited metabolic diseases, which can be incompatible with life in the postnatal period or allow survival into adulthood. Retinopathy is a recurrent feature in both the severely and mildly affected patients, which can be accompanied with other ophthalmological pathologies. Thanks to next-generation sequencing, patients originally identified with other inherited blinding diseases were reclassified as suffering from peroxisomal disorders. In addition, new peroxisomal gene defects or disease presentations exhibiting retinal degeneration were recently identified. The pathogenic mechanisms underlying retinopathy in peroxisomal disorders remain unresolved.

Histologic and ultrastructural features in early and advanced phases of Zellweger spectrum disorder (infantile Refsum disease)

Zellweger spectrum disorders (ZSD) are rare autosomal recessive inherited metabolic disorders and include severe (Zellweger syndrome) and milder phenotypes [neonatal adrenoleukodystrophy and infantile Refsum disease (IRD)]. ZSD are characterized by impaired peroxisomal functions and lack of peroxisomes detected by electron microscopy (EM). ZSD are caused by mutations in any of the 14 PEX genes. Patients with ZSD commonly demonstrate nonspecific hepatic symptoms within the first year, often without clinical suspicion of ZSD. Thus, recognition of pathologic findings in the liver is critical for the early diagnosis. We herein demonstrate the histologic and ultrastructural features in liver biopsies in the early and advanced phases from a 16-year-old male with IRD. The initial biopsy at 5 months of age showed a lack of peroxisomes by EM, and this finding played a critical role in the early diagnosis. In contrast, the second biopsy at 14 years of age, after long-term diet therapy, demonstrated significant disease progression with near-cirrhotic liver. In addition to lack of peroxisomes, EM revealed abundant trilamellar inclusions within large angulated lysosomes in many of the hepatocytes and Kupffer cells. Mitochondrial abnormalities were identified only in the second biopsy and were mainly identified in damaged cells; thus they were likely nonspecific secondary changes. This is the first report demonstrating histological and ultrastructural features of liver biopsies in the early and advanced phases from a child with ZSD. Trilamellar inclusions are considered to be an ultrastructural hallmark of ZSD, but they may not be apparent in the early phases.

Mild Zellweger syndrome due to a novel PEX6 mutation: correlation between clinical phenotype and in silico prediction of variant pathogenicity

Zellweger syndrome (ZS) is a consequence of a peroxisome biogenesis disorder (PBD) caused by the presence of a pathogenic mutation in one of the 13 genes from the PEX family. ZS is a severe multisystem condition characterized by neonatal appearance of symptoms and a shorter life. Here, we report a case of ZS with a mild phenotype, due to a novel PEX6 gene mutation. The patient presented subtle craniofacial dysmorphic features and slightly slower psychomotor development. At the age of 2 years, he was diagnosed with adrenal insufficiency, hypoacusis, and general deterioration. Magnetic resonance imaging showed a symmetrical hyperintense signal in the frontal and parietal white matter. Biochemical tests showed elevated liver transaminases, elevated serum very long chain fatty acids, and phytanic acid. After the death of the child at the age of 6 years, molecular diagnostics were continued in order to provide genetic counseling for his parents. Next generation sequencing (NGS) analysis with the TruSight One™ Sequencing Panel revealed a novel homozygous PEX6 p.Ala94Pro mutation. In silico prediction of variant severity suggested its possible benign effect. To conclude, in the milder phenotypes, adrenal insufficiency, hypoacusis, and leukodystrophy together seem to be pathognomonic for ZS.

Identification of a novel mutation in PEX10 in a patient with attenuated Zellweger spectrum disorder: a case report

Background

The peroxisome biogenesis disorders, which are caused by mutations in any of 13 different PEX genes, include the Zellweger spectrum disorders. Severe defects in one of these PEX genes result in the absence of functional peroxisomes which is seen in classical Zellweger syndrome. These patients present with hypotonia and seizures shortly after birth. Other typical symptoms are dysmorphic features, liver disease, retinal degeneration, sensorineural deafness, polycystic kidneys, and the patient does not reach any developmental milestones.

Case presentation

We report a case of a patient with Zellweger spectrum disorder due to a novel mutation in the PEX10 gene, presenting with a mild late-onset neurological phenotype. The patient, an Assyrian girl originating from Iraq, presented with sensorineural hearing impairment at the age of 5 followed by sensorimotor polyneuropathy, cognitive delay, impaired gross and fine motor skills, and tremor and muscle weakness in her teens. Analyses of biochemical markers for peroxisomal disease suggested a mild peroxisomal defect and functional studies in fibroblasts confirmed the existence of a peroxisome biogenesis disorder. Diagnosis was confirmed by next generation sequencing analysis, which showed a novel homozygous mutation (c.530 T > G (p.Leu177Arg) (NM_153818.1)) in the PEX10 gene predicted to be pathogenic.

Conclusions

This case highlights the importance of performing biochemical, functional, and genetic peroxisomal screening in patients with clinical presentations milder than those usually observed in Zellweger spectrum disorders.

Diagnosis of a mild peroxisomal phenotype with next-generation sequencing

Peroxisomal biogenesis disorders (PBD) are caused by mutations in PEX genes, and are typically diagnosed with biochemical testing in plasma followed by confirmatory testing. Here we report the unusual diagnostic path of a child homozygous for PEX1 p.G843D. The patient presented with sensorineural hearing loss, pigmentary retinopathy, and normal intellect. After testing for Usher syndrome was negative, he was found to have PBD through a research sequencing panel. When evaluating a patient with hearing loss and pigmentary retinopathy, mild PBD should be on the differential regardless of cognitive function.

Spectrum of PEX1 and PEX6 variants in Heimler syndrome

Heimler syndrome (HS) consists of recessively inherited sensorineural hearing loss, amelogenesis imperfecta (AI) and nail abnormalities, with or without visual defects. Recently HS was shown to result from hypomorphic mutations in PEX1 or PEX6, both previously implicated in Zellweger Syndrome Spectrum Disorders (ZSSD). ZSSD are a group of conditions consisting of craniofacial and neurological abnormalities, sensory defects and multi-organ dysfunction. The finding of HS-causing mutations in PEX1 and PEX6 shows that HS represents the mild end of the ZSSD spectrum, though these conditions were previously thought to be distinct nosological entities. Here, we present six further HS families, five with PEX6 variants and one with PEX1 variants, and show the patterns of Pex1, Pex14 and Pex6 immunoreactivity in the mouse retina. While Ratbi et al. found more HS-causing mutations in PEX1 than in PEX6, as is the case for ZSSD, in this cohort PEX6 variants predominate, suggesting both genes play a significant role in HS. The PEX6 variant c.1802G>A, p.(R601Q), reported previously in compound heterozygous state in one HS and three ZSSD cases, was found in compound heterozygous state in three HS families. Haplotype analysis suggests a common founder variant. All families segregated at least one missense variant, consistent with the hypothesis that HS results from genotypes including milder hypomorphic alleles. The clinical overlap of HS with the more common Usher syndrome and lack of peroxisomal abnormalities on plasma screening suggest that HS may be under-diagnosed. Recognition of AI is key to the accurate diagnosis of HS.

Characterization of macular structure and function in two Swedish families with genetically identified autosomal dominant retinitis pigmentosa

PURPOSE

To study the phenotype in two families with genetically identified autosomal dominant retinitis pigmentosa (adRP) focusing on macular structure and function.

METHODS

Clinical data were collected at the Department of Ophthalmology, Lund University, Sweden, for affected and unaffected family members from two pedigrees with adRP. Examinations included optical coherence tomography (OCT), full-field electroretinography (ffERG), and multifocal electroretinography (mfERG). Molecular genetic screening was performed for known mutations associated with adRP.

RESULTS

The mode of inheritance was autosomal dominant in both families. The members of the family with a mutation in the PRPF31 (p.IVS6+1G>T) gene had clinical features characteristic of RP, with severely reduced retinal rod and cone function. The degree of deterioration correlated well with increasing age. The mfERG showed only centrally preserved macular function that correlated well with retinal thinning on OCT. The family with a mutation in the RHO (p.R135W) gene had an extreme intrafamilial variability of the phenotype, with more severe disease in the younger generations. OCT showed pathology, but the degree of morphological changes was not correlated with age or with the mfERG results. The mother, with a de novo mutation in the RHO (p.R135W) gene, had a normal ffERG, and her retinal degeneration was detected merely with the reduced mfERG.

CONCLUSIONS

These two families demonstrate the extreme inter- and intrafamilial variability in the clinical phenotype of adRP. This is the first Swedish report of the clinical phenotype associated with a mutation in the PRPF31 (p.IVS6+1G>T) gene. Our results indicate that methods for assessment of the central retinal structure and function may improve the detection and characterization of the RP phenotype.

Zellweger spectrum disorders: clinical manifestations in patients surviving into adulthood

INTRODUCTION

We describe the natural history of patients with a Zellweger spectrum disorder (ZSD) surviving into adulthood.

METHODS

Retrospective cohort study in patients with a genetically confirmed ZSD.

RESULTS

All patients (n = 19; aged 16-35 years) had a follow-up period of 1-24.4 years (mean 16 years). Seven patients had a progressive disease course, while 12 remained clinically stable during follow-up. Disease progression usually manifests in adolescence as a gait disorder, caused by central and/or peripheral nervous system involvement. Nine were capable of living a partly independent life with supported employment. Systematic MRI review revealed T2 hyperintense white matter abnormalities in the hilus of the dentate nucleus and/or peridentate region in nine out of 16 patients. Biochemical analyses in blood showed abnormal peroxisomal biomarkers in all patients in infancy and childhood, whereas in adolescence/adulthood we observed normalization of some metabolites.

CONCLUSIONS

The patients described here represent a distinct subgroup within the ZSDs who survive into adulthood. Most remain stable over many years. Disease progression may occur and is mainly due to cerebral and cerebellar white matter abnormalities, and peripheral neuropathy.

PEX6 is Expressed in Photoreceptor Cilia and Mutated in Deafblindness with Enamel Dysplasia and Microcephaly

Deafblindness is part of several genetic disorders. We investigated a consanguineous Egyptian family with two siblings affected by congenital hearing loss and retinal degeneration, initially diagnosed as Usher syndrome type 1. At teenage, severe enamel dysplasia, developmental delay, and microcephaly became apparent. Genome-wide homozygosity mapping and whole-exome sequencing detected a homozygous missense mutation, c.1238G>T (p.Gly413Val), affecting a highly conserved residue of peroxisomal biogenesis factor 6, PEX6. Biochemical profiling of the siblings revealed abnormal and borderline plasma phytanic acid concentration, and cerebral imaging revealed white matter disease in both. We show that Pex6 localizes to the apical extensions of secretory ameloblasts and differentiated odontoblasts at early stages of dentin synthesis in mice, and to cilia of retinal photoreceptor cells. We propose PEX6, and possibly other peroxisomal genes, as candidate for the rare cooccurrence of deafblindness and enamel dysplasia. Our study for the first time links peroxisome biogenesis disorders to retinal ciliopathies.

Heimler Syndrome Is Caused by Hypomorphic Mutations in the Peroxisome-Biogenesis Genes PEX1 and PEX6

Heimler syndrome (HS) is a rare recessive disorder characterized by sensorineural hearing loss (SNHL), amelogenesis imperfecta, nail abnormalities, and occasional or late-onset retinal pigmentation. We ascertained eight families affected by HS and, by using a whole-exome sequencing approach, identified biallelic mutations in PEX1 or PEX6 in six of them. Loss-of-function mutations in both genes are known causes of a spectrum of autosomal-recessive peroxisome-biogenesis disorders (PBDs), including Zellweger syndrome. PBDs are characterized by leukodystrophy, hypotonia, SNHL, retinopathy, and skeletal, craniofacial, and liver abnormalities. We demonstrate that each HS-affected family has at least one hypomorphic allele that results in extremely mild peroxisomal dysfunction. Although individuals with HS share some subtle clinical features found in PBDs, the diagnosis was not suggested by routine blood and skin fibroblast analyses used to detect PBDs. In conclusion, our findings define HS as a mild PBD, expanding the pleiotropy of mutations in PEX1 and PEX6.

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.

Late-onset Zellweger spectrum disorder caused by PEX6 mutations mimicking X-linked adrenoleukodystrophy

BACKGROUND

Zellweger spectrum disorder is an autosomal recessively inherited multisystem disorder caused by one of the 13 different PEX gene defects resulting in defective peroxisomal assembly and multiple peroxisomal enzyme deficiencies. We report a new patient with late-onset Zellweger spectrum disorder mimicking X-linked adrenoleukodystrophy.

PATIENT DESCRIPTION

This 8.5-year-old boy with normal development until 6.5 years of age presented with bilateral sensorineural hearing loss during a school hearing test. He then developed acute-onset diplopia, clumsiness, and cognitive dysfunction at age 7 years. Magnetic resonance imaging of the brain revealed symmetric leukodystrophy, although without gadolinium enhancement. Elevated plasma very long chain fatty acid levels were suggestive of X-linked adrenoleukodystrophy, but his ABCD1 gene had normal coding sequence and dosage. Additional studies of cultured skin fibroblasts were consistent with Zellweger spectrum disorder. Molecular testing identified disease-causing compound heterozygous mutations in the PEX6 gene supporting the Zellweger spectrum disorder diagnosis in this patient.

CONCLUSIONS

We describe a new patient with late-onset Zellweger spectrum disorder caused by PEX6 mutations who presented with an acute neurodegenerative disease course mimicking X-linked adrenoleukodystrophy. This finding provides an additional reason that molecular confirmation is important for the genetic counseling and management of patients with a clinical and biochemical diagnosis of X-linked adrenoleukodystrophy.

Newly identified milder phenotype of peroxisome biogenesis disorder caused by mutated PEX3 gene

We identified the first patient with infantile Refsum disease (IRD), a milder phenotype of peroxisome biogenesis disorder (PBD) caused by a mutated PEX3, and investigated the clinical, molecular and cellular characterization in this patient. The patient presented psychomotor regression, late-onset leukodystrophy, peripheral neuropathy, hearing impairment, a renal cyst, and renal hypertension and survived until the age of 36. Furthermore, fibroblasts from the patient indicated a mosaic pattern of catalase-positive particles (peroxisomes) and numerous peroxisomal membrane structures. Molecular analysis was homozygous for the D347Y mutation and reduced gene expression of PEX3 which encodes a peroxisomal membrane protein, pex3p, involved in peroxisome assembly at the early stage of peroxisomal membrane vesicle formation, therefore, patients with a mutated PEX3 gene have been reported to have only a severe phenotype of Zellweger syndrome and no or less peroxisomal remnant membrane structure. This is not only a newly identified milder PBD caused by a mutated PEX3 gene but also the first report of a Japanese patient with IRD who had not been diagnosed until over 30years of age, which suggests there must be more variant PBD in patients with degenerative neurologic disorder, and to bring them to light is necessary.

Genetics and molecular basis of human peroxisome biogenesis disorders

Human peroxisome biogenesis disorders (PBDs) are a heterogeneous group of autosomal recessive disorders comprised of two clinically distinct subtypes: the Zellweger syndrome spectrum (ZSS) disorders and rhizomelic chondrodysplasia punctata (RCDP) type 1. PBDs are caused by defects in any of at least 14 different PEX genes, which encode proteins involved in peroxisome assembly and proliferation. Thirteen of these genes are associated with ZSS disorders. The genetic heterogeneity among PBDs and the inability to predict from the biochemical and clinical phenotype of a patient with ZSS which of the currently known 13 PEX genes is defective, has fostered the development of different strategies to identify the causative gene defects. These include PEX cDNA transfection complementation assays followed by sequencing of the thus identified PEX genes, and a PEX gene screen in which the most frequently mutated exons of the different PEX genes are analyzed. The benefits of DNA testing for PBDs include carrier testing of relatives, early prenatal testing or preimplantation genetic diagnosis in families with a recurrence risk for ZSS disorders, and insight in genotype-phenotype correlations, which may eventually assist to improve patient management. In this review we describe the current status of genetic analysis and the molecular basis of PBDs.

Deletion of a single allele of the Pex11β gene is sufficient to cause oxidative stress, delayed differentiation and neuronal death in mouse brain

Impaired neuronal migration and cell death are commonly observed in patients with peroxisomal biogenesis disorders (PBDs), and in mouse models of this diseases. In Pex11β-deficient mice, we observed that the deletion of a single allele of the Pex11β gene (Pex11β^+/− heterozygous mice) caused cell death in primary neuronal cultures prepared from the neocortex and cerebellum, although to a lesser extent as compared with the homozygous-null animals (Pex11β^−/− mice). In corresponding brain sections, cell death was rare, but differences between the genotypes were similar to those found in vitro. Because PEX11β has been implicated in peroxisomal proliferation, we searched for alterations in peroxisomal abundance in the brain of heterozygous and homozygous Pex11β-null mice compared with wild-type animals. Deletion of one allele of the Pex11β gene slightly increased the abundance of peroxisomes, whereas the deletion of both alleles caused a 30% reduction in peroxisome number. The size of the peroxisomal compartment did not correlate with neuronal death. Similar to cell death, neuronal development was delayed in Pex11β^+/− mice, and to a further extent in Pex11β^−/− mice, as measured by a reduced mRNA and protein level of synaptophysin and a reduced protein level of the mature isoform of MAP2. Moreover, a gradual increase in oxidative stress was found in brain sections and primary neuronal cultures from wild-type to heterozygous to homozygous Pex11β-deficient mice. SOD2 was upregulated in neurons from Pex11β^+/− mice, but not from Pex11β^−/− animals, whereas the level of catalase remained unchanged in neurons from Pex11β^+/− mice and was reduced in those from Pex11β^−/− mice, suggesting a partial compensation of oxidative stress in the heterozygotes, but a failure thereof in the homozygous Pex11β^−/− brain. In conclusion, we report the alterations in the brain caused by the deletion of a single allele of the Pex11β gene. Our data might lead to the reconsideration of the clinical treatment of PBDs and the common way of using knockout mouse models for studying autosomal recessive diseases.

Lymphoblastoid cell lines for diagnosis of peroxisome biogenesis disorders

Peroxisome biogenesis disorders (PBDs) are a group of autosomal-recessive developmental and progressive metabolic diseases leading to the Zellweger spectrum (ZS) phenotype in most instances. Diagnosis of clinically suspected cases can be difficult because of extensive genetic heterogeneity and large spectrum of disease severity. Furthermore, a second group of peroxisomal diseases caused by deficiencies of single peroxisomal enzymes can show an indistinguishable clinical phenotype. The diagnosis of these peroxisomal disorders relies on the clinical presentation, the biochemical parameters in plasma and erythrocyte membranes, and genetic testing as the final step. Analysis of patients' cells is frequently required during the diagnostic process, e.g., for complementation analysis to identify the affected gene before sequencing. In the cases with unclear clinical or biochemical presentation, patients' cells are analyzed to prove PBD or to demonstrate biochemical abnormalities that might be elusive in plasma. Cell lines from skin fibroblast that are usually generated for diagnostic workup are not available in all instances, mainly because the required skin biopsy is invasive and sometimes denied by parents. An alternative cellular system has not been analyzed sufficiently. In this study, we evaluated the alternative use of lymphoblastoid cell lines (LCLs), derived from a peripheral blood sample, in the diagnostic process for PBD. LCLs were suitable for immunofluorescence visualization of peroxisomal enzymes, complementation analysis, and the biochemical analysis to differentiate between control and PBD LCL. LCLs are therefore an easily obtainable alternative cellular system for a detailed PBD diagnostic workup with a reliability of diagnostic results equal to those of skin fibroblasts.

Recovery of PEX1-Gly843Asp peroxisome dysfunction by small-molecule compounds

Zellweger spectrum disorder (ZSD) is a heterogeneous group of diseases with high morbidity and mortality caused by failure to assemble normal peroxisomes. There is no therapy for ZSD, but management is supportive. Nevertheless, one-half of the patients have a phenotype milder than classic Zellweger syndrome and exhibit a progressive disease course. Thus, patients would benefit if therapies became available and were instituted early. Recent reports indicate several interventions that result in partial peroxisome recovery in ZSD fibroblasts. To identify drugs that recover peroxisome functions, we expressed a GFP-peroxisome targeting signal 1 reporter in fibroblasts containing the common disease allele, PEX1-p.Gly843Asp. The GFP reporter remained cytosolic at baseline, and improvement in peroxisome functions was detected by the redistribution of the GFP reporter from the cytosol to the peroxisome. We established a high-content screening assay based on this phenotype assay and evaluated 2,080 small molecules. The cells were cultured in chemical for 2 days and then, were fixed and imaged by epifluorescent microscopy on a high-content imaging platform. We identified four compounds that partially recover matrix protein import, and we confirmed three using independent assays. Our results suggest that PEX1-p.G843D is a misfolded protein amenable to chaperone therapy.

Closely spaced multiple mutations as potential signatures of transient hypermutability in human genes

Data from diverse organisms suggests that transient hypermutability is a general mutational mechanism with the potential to generate multiple synchronous mutations, a phenomenon probably best exemplified by closely spaced multiple mutations (CSMMs). Here we have attempted to extend the concept of transient hypermutability from somatic cells to the germline, using human inherited disease-causing multiple mutations as a model system. Employing stringent criteria for data inclusion, we have retrospectively identified numerous potential examples of pathogenic CSMMs that exhibit marked similarities to the CSMMs reported in other systems. These examples include (1) eight multiple mutations, each comprising three or more components within a sequence tract of <100 bp; (2) three possible instances of "mutation showers"; and (3) numerous highly informative "homocoordinate" mutations. Using the proportion of CpG substitution as a crude indicator of the relative likelihood of transient hypermutability, we present evidence to suggest that CSMMs comprising at least one pair of mutations separated by < or =100 bp may constitute signatures of transient hypermutability in human genes. Although this analysis extends the generality of the concept of transient hypermutability and provides new insights into what may be considered a novel mechanism of mutagenesis underlying human inherited disease, it has raised serious concerns regarding current practices in mutation screening.

A PEX10 defect in a patient with no detectable defect in peroxisome assembly or metabolism in cultured fibroblasts

Zellweger spectrum disorders (ZSD) are diagnosed by biochemical assay in blood, urine and cultured fibroblasts and PEX gene mutation identification. In most cases studies in fibroblasts corroborate results obtained in body fluids. In 1996 Clayton and colleagues described a 10-year old girl with evidence of a peroxisome disorder, based on elevated bile acid metabolites and phytanate. At the time it was not possible to distinguish whether she had a ZSD or a single peroxisomal protein defect. Studies in our laboratory showed that she also had elevated plasma pipecolate, supporting the former diagnosis. Despite the abnormal metabolites detected in blood (phytanate, bile acid intermediates and pipecolate), analysis of multiple peroxisomal pathways in fibroblasts yielded normal results. In addition, she had a milder clinical phenotype than usually associated with ZSD. Since complementation analysis to determine the gene defect was not possible, we screened this patient following the PEX Gene Screen algorithm (PGS). The PGS provides a template for sequencing PEX gene exons independent of complementation analysis. Two mutations in PEX10 were identified, a frameshift mutation inherited from her father and a de novo missense mutation in a conserved functional domain on the other allele. This case highlights that molecular analysis may be essential to the diagnosis of patients at the milder end of the ZSD spectrum. Furthermore, it supports the concept that some tissues are less affected by certain PEX gene defects than brain and liver.

A novel PEX12 mutation identified as the cause of a peroxisomal biogenesis disorder with mild clinical phenotype, mild biochemical abnormalities in fibroblasts and a mosaic catalase immunofluorescence pattern, even at 40 degrees C

Mutations in 12 different PEX genes can cause a generalized peroxisomal biogenesis disorder with clinical phenotypes ranging from Zellweger syndrome to infantile Refsum disease. To identify the specific PEX gene to be sequenced, complementation analysis is first performed in fibroblasts using catalase immunofluorescence. A patient with a relatively mild phenotype of infantile cholestasis, hypotonia and motor delay had elevated plasma very long-chain fatty acids and bile acid precursors, but fibroblast studies revealed normal or only mildly abnormal peroxisomal parameters and mosaic catalase immunofluorescence. This mosaicism persisted even when the incubation temperature was increased from 37 degrees C to 40 degrees C, a maneuver previously shown to abolish mosaicism by exacerbating peroxisomal dysfunction. As mosaicism precludes complementation analysis, a candidate gene approach was employed. After PEX1 sequencing was unrewarding, PEX12 sequencing revealed homozygosity for a novel c.102A>T (p.R34S) missense mutation affecting a partially conserved residue in the N-terminal region important for localization to peroxisomes. Transfection of patient fibroblasts with wild-type PEX12 cDNA confirmed that a PEX12 defect was the basis for the PBD. Homozygosity for c.102A>T was identified in a second patient of similar ethnic origin also presenting with a mild phenotype. PEX12 is a highly probable candidate gene for direct sequencing in the context of a mild clinical phenotype with mosaicism and minimally abnormal peroxisomal parameters in fibroblasts.

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 PEX Gene Screen: molecular diagnosis of peroxisome biogenesis disorders in the Zellweger syndrome spectrum

Peroxisome biogenesis disorders in the Zellweger syndrome spectrum (PBD-ZSS) are caused by defects in at least 12 PEX genes required for normal organelle assembly. Clinical and biochemical features continue to be used reliably to assign patients to this general disease category. Identification of the precise genetic defect is important, however, to permit carrier testing and early prenatal diagnosis. Molecular analysis is likely to expand the clinical spectrum of PBD and may also provide data relevant to prognosis and future therapeutic intervention. However, the large number of genes involved has thus far impeded rapid mutation identification. In response, we developed the PEX Gene Screen, an algorithm for the systematic screening of exons in the six PEX genes most commonly defective in PBD-ZSS. We used PCR amplification of genomic DNA and sequencing to screen 91 unclassified PBD-ZSS patients for mutations in PEX1, PEX26, PEX6, PEX12, PEX10, and PEX2. A maximum of 14 reactions per patient identified pathological mutations in 79% and both mutant alleles in 54%. Twenty-five novel mutations were identified overall. The proportion of patients with different PEX gene defects correlated with frequencies previously identified by complementation analysis. This systematic, hierarchical approach to mutation identification is therefore a valuable tool to identify rapidly the molecular etiology of suspected PBD-ZSS disorders.

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.

Identification of the molecular defect in patients with peroxisomal mosaicism using a novel method involving culturing of cells at 40°C: Implications for other inborn errors of metabolism

The peroxisome biogenesis disorders (PBDs), which comprise Zellweger syndrome (ZS), neonatal adrenoleukodystrophy, and infantile Refsum disease (IRD), represent a spectrum of disease severity, with ZS being the most severe, and IRD the least severe disorder. The PBDs are caused by mutations in one of the at least 12 different PEX genes encoding proteins involved in the biogenesis of peroxisomes. We report the biochemical characteristics and molecular basis of a subset of atypical PBD patients. These patients were characterized by abnormal peroxisomal plasma metabolites, but otherwise normal to very mildly abnormal peroxisomal parameters in cultured skin fibroblasts, including a mosaic catalase immunofluorescence pattern in fibroblasts. Since this latter feature made standard complementation analysis impossible, we developed a novel complementation technique in which fibroblasts were cultured at 40 degrees C, which exacerbates the defect in peroxisome biogenesis. Using this method, we were able to assign eight patients to complementation group 3 (CG3), followed by the identification of a single homozygous c.959C>T (p.S320F) mutation in their PEX12 gene. We also investigated various peroxisomal biochemical parameters in fibroblasts at 30 degrees C, 37 degrees C, and 40 degrees C, and found that all parameters showed a temperature-dependent behavior. The principle of culturing cells at elevated temperatures to exacerbate the defect in peroxisome biogenesis, and thereby preventing certain mutations from being missed, may well have a much wider applicability for a range of different inborn errors of metabolism.