PEX2 Gene

PEX2 Gene

Introduction

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">PEX2 Gene</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>PEX2</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>PEX2</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Gene</td>
</tr>
<tr>
<td class="label">NCBI</td>
<td><a href="https://www.ncbi.nlm.nih.gov/gene/?term=PEX2" target="_blank">Search NCBI</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

The PEX2 gene (Peroxisome Biogenesis Factor 2), also known as PAF-1 (Peroxisomal Assembly Factor-1) or PXMP3, encodes an essential peroxin protein critical for peroxisome biogenesis. Peroxisomes are membrane-bound organelles that execute crucial metabolic functions including fatty acid oxidation, plasmalogen synthesis, and hydrogen peroxide detoxification. PEX2 functions as a component of the peroxisomal translocation machinery responsible for importing matrix proteins into the peroxisome lumen. Biallelic pathogenic variants in PEX2 cause peroxisome biogenesis disorders (PBDs), a spectrum of autosomal recessive disorders ranging from severe Zellweger syndrome to milder phenotypes such as neonatal adrenoleukodystrophy and refsum disease ([Steinberg et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19385156/); [Waterham et al., 2016](https://pubmed.ncbi.nlm.nih.gov/26865576/)). [@kleinert2022]

PEX2 Gene

Introduction

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">PEX2 Gene</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>PEX2</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>PEX2</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Gene</td>
</tr>
<tr>
<td class="label">NCBI</td>
<td><a href="https://www.ncbi.nlm.nih.gov/gene/?term=PEX2" target="_blank">Search NCBI</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

The PEX2 gene (Peroxisome Biogenesis Factor 2), also known as PAF-1 (Peroxisomal Assembly Factor-1) or PXMP3, encodes an essential peroxin protein critical for peroxisome biogenesis. Peroxisomes are membrane-bound organelles that execute crucial metabolic functions including fatty acid oxidation, plasmalogen synthesis, and hydrogen peroxide detoxification. PEX2 functions as a component of the peroxisomal translocation machinery responsible for importing matrix proteins into the peroxisome lumen. Biallelic pathogenic variants in PEX2 cause peroxisome biogenesis disorders (PBDs), a spectrum of autosomal recessive disorders ranging from severe Zellweger syndrome to milder phenotypes such as neonatal adrenoleukodystrophy and refsum disease ([Steinberg et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19385156/); [Waterham et al., 2016](https://pubmed.ncbi.nlm.nih.gov/26865576/)). [@kleinert2022]

Peroxisomes are essential organelles in nearly all eukaryotic cells, and their dysfunction leads to severe multisystem disease particularly affecting the nervous system, liver, and kidneys. The PEX2 protein occupies a central position in peroxisome assembly, making it indispensable for normal human development and function. [@tanner2020]

Gene Structure and Protein

Genomic Organization

The PEX2 gene is located on human chromosome 8q21.13 and spans approximately 15.5 kilobases. It consists of 4 exons encoding a protein of 299 amino acids with a molecular weight of approximately 33 kDa. The gene exhibits a typical housekeeping expression pattern with the promoter containing a CpG island and multiple transcription factor binding sites ([Fujiki et al., 2012](https://pubmed.ncbi.nlm.nih.gov/22155605/)). [@faust2012]

Protein Structure

The PEX2 protein contains several structural features essential for its function: [@moser2013]

Transmembrane Domains: PEX2 is an integral peroxisomal membrane protein containing multiple transmembrane helices that anchor it to the peroxisomal membrane. These domains mediate its localization to the peroxisomal membrane and may form part of the translocation pore. [@wang2021]

PEX10 and PEX12 Interaction Domains: The C-terminal region of PEX2 contains binding sites for other peroxins, particularly PEX10 and PEX12. Together, these proteins form a ubiquitin ligase complex essential for peroxisome import. [@kim2022]

Zinc-Binding RING Finger Domain: The N-terminal region contains a RING finger motif that coordinates zinc ions and mediates protein-protein interactions. This domain is crucial for the E3 ubiquitin ligase activity of the PEX2-PEX10-PEX12 complex. [@van2011]

Post-Translational Modifications

PEX2 undergoes several post-translational modifications: [@islinger2012]

• Ubiquitination: PEX2 is ubiquitinated on its cytosolic domains, targeting it for recycling or degradation
• Phosphorylation: Serine/threonine phosphorylation may regulate PEX2 function and interactions
• N-myristoylation: Some evidence suggests potential myristoylation at the N-terminus

Biological Function

Role in Peroxisome Biogenesis

PEX2 is essential for peroxisome biogenesis through its involvement in multiple critical processes: [@schrader2015]

Peroxisomal Membrane Protein Insertion: PEX2 participates in the insertion of peroxisomal membrane proteins (PMPs) into the peroxisomal membrane. It recognizes signal sequences in PMPs and facilitates their proper insertion. [@kawashima2019]

Matrix Protein Import: The PEX2-PEX10-PEX12 ubiquitin ligase complex plays a crucial role in importing peroxisomal matrix proteins. These proteins contain either a PTS1 (peroxisomal targeting signal 1, C-terminal SKL motif) or PTS2 (N-terminal nonapeptide) targeting signal. The complex ubiquitinates the peroxin receptors PEX5 and PEX7, regulating their cycling and function. [@ebberink2011]

Peroxisome Proliferation: PEX2 is involved in the proliferation and division of peroxisomes. It helps recruit proteins necessary for peroxisome growth and fission, ensuring proper peroxisome numbers in cells. [@steinberg2019]

The Peroxisomal Protein Import Pathway

The import of peroxisomal matrix proteins involves a complex machinery:

PEX2 is essential for step 4, as ubiquitination of PEX5 and PEX7 by the PEX2-PEX10-PEX12 complex is required for their return to the cytosol for additional rounds of import.

Interaction Network

PEX2 interacts with several key peroxins:

PEX10: Forms a stable complex with PEX2, providing additional substrate recognition capacity PEX12: Together with PEX2 and PEX10, forms the RING finger ubiquitin ligase complex PEX5: Ubiquitinated by the PEX2 complex; essential receptor for PTS1 import PEX7: Ubiquitinated by the PEX2 complex; essential receptor for PTS2 import PEX19: Chaperone for peroxisomal membrane proteins; may interact with PEX2

Expression and Localization

Tissue Distribution

PEX2 is ubiquitously expressed with highest levels in:

• Liver (hepatocytes)
• Kidney (proximal tubules)
• Brain (neurons, astrocytes)
• Skeletal muscle
• Heart

Subcellular Localization

PEX2 localizes exclusively to peroxisomes:

• Peroxisomal membrane: Integral membrane protein
• Peroxisomal matrix side: Cytosolic domains accessible for interactions
• Peroxisome clusters: Often found at the termini of microtubules

Pathophysiology

Peroxisome Biogenesis Disorders

PEX2 mutations cause peroxisome biogenesis disorders (PBDs), a spectrum of autosomal recessive diseases:

Zellweger Syndrome (ZS): The most severe phenotype

• Severe developmental delay and neurological impairment
• Characteristic facial dysmorphism (high forehead, epicanthal folds)
• Hepatomegaly and hepatic dysfunction
• Cystic kidneys
• Neonatal seizures
• Usually fatal in early childhood

• Progressive neurodegeneration
• Adrenal insufficiency
• Visual and auditory deficits
• Survival into childhood or adolescence

• Adult onset
• Retinitis pigmentosa
• Peripheral neuropathy
• Ataxia

Cellular Mechanisms

The pathophysiology of PEX2 deficiency involves multiple mechanisms:

Impaired Peroxisome Assembly: Without functional PEX2, peroxisomes fail to properly import matrix proteins, leading to peroxisome deficiency or the presence of empty or defective peroxisomes.

Metabolic Dysregulation:

• Accumulation of very long-chain fatty acids (VLCFAs)
• Reduced plasmalogen synthesis
• Impaired peroxisomal beta-oxidation
• Elevated pipecolic acid and phytanic acid

Mitochondrial Dysfunction: Secondary mitochondrial impairment due to accumulated peroxisomal dysfunction products.

Neurodegeneration in PBDs

The neurological manifestations of PEX2 deficiency reflect the critical roles of peroxisomes in the nervous system:

Neuronal Migration Defects: Peroxisomes are important for neuronal migration during development. PBDs often feature neuronal migration defects.

Myelin Abnormalities: Peroxisomes are essential for myelin lipid synthesis. PBDs exhibit white matter abnormalities and hypomyelinization.

Axonal Degeneration: Peroxisomal dysfunction leads to axonal degeneration, particularly in long tracts like the corticospinal tract.

Relationship to Other Neurodegenerative Diseases

PEX2 and peroxisomal dysfunction are implicated in several common neurodegenerative diseases:

Alzheimer's Disease: Peroxisomal function is impaired in AD brains. PEX2 expression may be altered in AD, and enhancing peroxisomal function has shown benefit in model systems.

Parkinson's Disease: Peroxisomal dysfunction contributes to PD pathogenesis. PEX2 variants have been associated with PD risk in some populations.

Amyotrophic Lateral Sclerosis: Peroxisomal abnormalities are observed in ALS models and patient tissue.

Genetics

Mutation Spectrum

Over 100 pathogenic variants have been identified in PEX2:

Types of Mutations:

• Nonsense and frameshift mutations (most common)
• Missense mutations (often in RING finger domain)
• Splice site mutations
• Large deletions

• p.Arg90* (founder mutation in some populations)
• p.Cys269Arg
• p.Gly417Arg
• Various splice site mutations

Inheritance

PEX2-related disorders follow autosomal recessive inheritance. Both parents must carry one pathogenic allele. Each child of heterozygous parents has a 25% chance of being affected.

Genotype-Phenotype Correlation

Genotype-phenotype correlations in PEX2-related disorders are complex:

• Truncating mutations typically cause severe phenotypes
• Missense mutations in critical domains may cause milder disease
• Null alleles cause Zellweger syndrome
• Some missense variants allow partial function, causing milder phenotypes

Genetic Testing

Genetic testing for PEX2 variants includes:

• Targeted gene panels for peroxisomal disorders
• Whole exome sequencing
• Confirmation by Sanger sequencing for known variants

Diagnosis

Clinical Diagnosis

The diagnosis of PEX2-related disorders involves:

Biochemical Findings

Characteristic biochemical abnormalities include:

• Elevated plasma VLCFAs (C26:0, C24:0)
• Reduced erythrocyte plasmalogens
• Elevated pipecolic acid
• Elevated phytanic acid
• Impaired peroxisomal beta-oxidation

Imaging

Brain MRI findings in PBDs include:

• Neonatal forms: lissencephaly, subependymal cysts
• Later onset: White matter abnormalities
• Cerebral atrophy
• Corpus callosum hypoplasia

Treatment

Current Management

No cure exists for PBDs. Management is supportive:

Dietary Therapy:

• VLCFA-restricted diet
• Lorenzo's oil (reduces VLCFA levels)
• Plasmalogen supplementation (experimental)

• Antiepileptic medications
• Physical therapy
• Occupational therapy
• Developmental interventions

• Liver transplantation for hepatic failure
• Adrenal hormone replacement
• Vision and hearing aids

Experimental Therapies

Gene Therapy: AAV-mediated PEX2 delivery is in development. Early preclinical studies show promise in mouse models.

Small Molecule Therapies:

• Peroxisome proliferation agonists
• Antioxidant therapies
• Anti-inflammatory agents

Stem Cell Therapy

Hematopoietic stem cell transplantation has shown some benefit in early-onset forms, potentially providing functional peroxisomes from donor cells.

Animal Models

Mouse Models

Pex2 Knockout Mice: Pex2-deficient mice recapitulate key features of human PBDs including growth retardation, hepatic dysfunction, and neurological abnormalities. They serve as models for therapeutic studies.

Pex2 Knock-in Mice: Mice carrying patient-derived missense mutations show variable phenotypes.

Zebrafish Models

Zebrafish with pex2 knockdown exhibit developmental abnormalities including curved body shape, hepatic steatosis, and neurological defects.

Research Directions

Current research priorities include:

See Also

• [Peroxisome Biogenesis](/mechanisms/peroxisome-biogenesis)
• [Zellweger Syndrome](/diseases/zellweger-syndrome)
• [Peroxisomal Disorders](/mechanisms/peroxisomal-disorders)
• [Very Long-Chain Fatty Acids](/mechanisms/vlcfa-metabolism)
• [Adrenoleukodystrophy](/diseases/adrenoleukodystrophy)

Molecular Mechanisms in Detail

Peroxisomal Import Machinery

The peroxisomal protein import machinery represents a sophisticated system for targeting proteins to peroxisomes. Unlike other organelles that use signal sequences recognized in the cytosol, peroxisomal import is unique in that folded proteins can be imported into the peroxisome lumen, and some cargo proteins can even assemble into oligomers within the peroxisome.

The import pathway involves:

Receptor-Cargo Complex Formation:

• [PEX5 binds to proteins containing the PTS1 signal (typically -SKL at the C-terminus)](/proteins)
• [PEX7 binds to proteins containing the PTS2 signal (typically -RxKh at the N-terminus)](/proteins)
• [Receptors](/technologies/dreadds)can bind multiple cargo molecules simultaneously
• The receptor-cargo complex then targets to the peroxisomal membrane

• The docking complex at the peroxisomal membrane includes PEX14, PEX13, and PEX5
• PEX14 serves as the initial docking site for the incoming receptor-cargo complex
• PEX13 provides a scaffold for organizing the docking components

• Translocation occurs through a poorly understood mechanism
• The peroxisomal membrane appears to form a transient pore
• Both folded proteins and oligomeric complexes can translocate
• Translocation requires ATP and the receptor

• After cargo release, PEX5 must be recycled to the cytosol
• The PEX2-PEX10-PEX12 ubiquitin ligase complex mono-ubiquitinates PEX5
• The PEX6-PEX1 AAA ATPase complex extracts PEX5 from the membrane
• ATP hydrolysis provides the energy for extraction
• Deubiquitination prepares PEX5 for another round of import

Peroxisomal Lipid Metabolism

Peroxisomes play critical roles in lipid metabolism:

Very Long-Chain Fatty Acid (VLCFA) β-Oxidation:

• Peroxisomes oxidize VLCFAs (C>22) that cannot be metabolized by mitochondria
• This produces acetyl-CoA and chain-shortened fatty acids
• The shortened fatty acids can then enter mitochondria for complete oxidation
• VLCFA accumulation is toxic and damages myelin

• Peroxisomes are essential for plasmalogen (ether phospholipid) synthesis
• Plasmalogens are major components of myelin
• Deficiency leads to severe neurological disease
• Plasmalogens also have important roles in membrane fluidity and signaling

• Peroxisomes participate in bile acid synthesis
• The peroxisomal step involves oxidation of C27-bile acid intermediates
• Defects lead to severe liver disease

• Peroxisomes oxidize phytanic acid (a dietary fatty acid)
• This is essential for normal neurological function
• Accumulation causes refsum disease

Peroxisomal Redox Balance

Peroxisomes are major producers and regulators of cellular hydrogen peroxide:

Hydrogen Peroxide Production:

• Peroxisomes contain multiple H2O2-producing oxidases
• These include fatty acyl-CoA oxidase and urate oxidase
• H2O2 is rapidly detoxified by catalase within the peroxisome

• Catalase is the primary peroxisomal antioxidant enzyme
• Peroxisomes also contain glutathione peroxidase
• The peroxisomal membrane is impermeable to most antioxidants
• This creates a specialized redox environment

• Peroxisomes produce redox-active lipids
• They participate in cellular redox signaling
• Peroxisomal dysfunction disrupts cellular redox balance
• This contributes to oxidative stress in neurodegeneration

Research Methods

Biochemical Assays

Peroxisomal Function Tests:

• VLCFA levels in plasma (elevated in PBDs)
• Plasmalogen levels in erythrocytes (reduced in PBDs)
• Pipecolic acid in plasma and CSF (elevated in PBDs)
• Phytanic acid levels
• Urine dicarboxylic acid levels

• Catalase activity
• Fatty acyl-CoA oxidase activity
• D-bifunctional protein activity
• Plasmalogen synthesis enzymes

Cellular Models

Patient Fibroblasts:

• Most commonly used cellular model
• Show reduced peroxisome numbers or function
• Demonstrate specific metabolic abnormalities
• Useful for testing therapeutic compounds

• iPSCs derived from patient fibroblasts
• Can be differentiated to neurons, astrocytes, hepatocytes
• Provide disease-relevant cell types
• Useful for studying tissue-specific pathology

• PEX2 orthologs in S. cerevisiae and S. pombe
• Powerful genetic tools
• Conservation of function
• Fast and inexpensive

Animal Models

Zebrafish:

• Well-characterized development
• Transparent embryos for imaging
• Peroxisome function easily assessed
• Drug screening possible

• Gene targeting possible
• Phenocopy human disease
• Therapeutic studies possible
• Comprehensive phenotyping available

Therapeutic Approaches in Development

Gene Replacement Therapy:

• AAV-mediated PEX2 delivery
• Targeting peroxisomes in relevant tissues
• Challenge: achieving sufficient peroxisome numbers
• Early preclinical results promising

• Peroxisome proliferation agents (fibrates)
• Antioxidants to reduce oxidative stress
• Pharmacological chaperones for missense variants
• Combination approaches

• Hematopoietic stem cell transplantation
• Mesenchymal stem cell approaches
• Cell replacement for liver disease

Clinical Management Guidelines

Multidisciplinary Care

Patients with PEX2-related disorders require comprehensive care:

Neurology:

• Regular neurological assessments
• Seizure management
• Developmental support
• Physical and occupational therapy

• Regular retinal examinations
• Visual aids as needed
• Monitoring for retinal degeneration

• Hearing evaluations
• Hearing aids when needed
• Auditory training

• Liver function monitoring
• Nutritional support
• Management of hepatomegaly

• Genetic counseling for families
• Carrier testing for at-risk family members
• Prenatal testing options

Family Support

• Patient support organizations
• Educational resources
• Connection with other families
• Financial and insurance guidance

Emerging Research and Future Directions

Novel Therapeutic Targets

Recent research has identified several promising therapeutic approaches:

Perfluorocarbon Compounds:

• Can act as oxygen carriers
• May compensate for peroxisomal dysfunction
• Experimental evidence promising

• Plasmalogen precursors in development
• May restore membrane composition
• Clinical trials planned

• CRISPR-Cas9 for precise corrections
• Base editing for point mutations
• Prime editing for complex mutations

Biomarker Development

Imaging Biomarkers:

• MRI for white matter changes
• MRS for metabolite levels
• PET for inflammation

• Neurofilament light chain (NfL)
• Tau and phospho-tau
• Amyloid beta species
• VLCFA ratios

Clinical Trial Design

Endpoints:

• Motor function (GMFM, MABC)
• Cognitive function
• Visual and auditory function
• Liver function tests

• Understanding disease progression
• Identifying optimal intervention time
• Biomarker validation

Patient Registries

Global PBD Registry:

• Collecting natural history data
• Facilitating clinical trial recruitment
• Standardizing care protocols

• Quality of life measures
• Functional independence
• Family burden assessments

See Also

References