PEX13 encodes peroxin-13, a critical component of the peroxisomal translocation machinery required for import of matrix proteins into peroxisomes. As a member of the peroxin family of proteins, PEX13 functions as a docking factor at the peroxisomal membrane, facilitating the recognition and import of proteins bearing the peroxisomal targeting signal type 1 (PTS1). Mutations in PEX13 cause peroxisome biogenesis disorders (PBDs), a spectrum of autosomal recessive conditions characterized by severe neurological impairment, developmental arrest, and often premature death. The gene is located on chromosome 2p15 and encodes a protein with an SH3 domain that mediates protein-protein interactions essential for peroxisomal import complex assembly.
| Property |
Value |
| Gene Symbol |
PEX13 |
| Full Name |
Peroxisome Biogenesis Factor 13 |
| Chromosomal Location |
2p15 |
| NCBI Gene ID |
5194 |
| OMIM ID |
603773 |
| Ensembl ID |
ENSG00000131828 |
| UniProt ID |
Q9UKV0 |
| Encoded Protein |
Peroxin-13 (489 amino acids) |
| Protein Domain |
SH3 domain (C-terminal) |
| Associated Diseases |
Zellweger spectrum disorders, Neonatal adrenoleukodystrophy, Infantile Refsum disease |
| Inheritance |
Autosomal recessive |
¶ Structure and Domains
PEX13 is an integral peroxisomal membrane protein composed of 489 amino acids with a predicted molecular mass of approximately 54 kDa. The protein contains several distinct structural features:
- N-terminal transmembrane domains: Two hydrophobic regions that anchor PEX13 in the peroxisomal membrane
- SH3 domain: Located at the C-terminus, this Src homology 3 domain mediates interactions with other peroxins and import machinery components
- Pex5p-binding domain: Interacts with the PTS1 receptor Pex5p
The SH3 domain of PEX13 has been structurally characterized and shown to bind proline-rich sequences in partner proteins, including Pex14p and the Pex5p receptor . This domain is critical for formation of the peroxisomal translocation pore and for recruiting cytosolic import factors to the membrane.
PEX13 functions at a critical hub in the peroxisomal protein import pathway. The current model involves:
- Cargo recognition: Cytosolic proteins with PTS1 (Ser-Lys-Leu) or PTS2 (R/K)(L/A)(X)₅(H/Q)(L/A/F)) signals are recognized by soluble receptors Pex5p and Pex7p, respectively
- Docking at the peroxisome: The receptor-cargo complex binds to the docking complex comprising Pex13p, Pex14p, and Pex5p at the peroxisomal membrane
- Translocation: The cargo is translocated across the peroxisomal membrane through a transient pore
- Receptor recycling: Pex5p is monoubiquitinated and exported back to the cytosol by the ATP-driven receptor exportomer (Pex1p/Pex6p)
PEX13 directly interacts with Pex5p through its SH3 domain, forming a stable docking complex that is essential for import efficiency. Studies in yeast and mammals have shown that PEX13 deficiency leads to accumulation of peroxisomal matrix proteins in the cytosol and severe impairment of peroxisome function .
Mutations in PEX13 are causative for peroxisome biogenesis disorders, a group of genetically heterogeneous conditions that represent the most severe spectrum of peroxisomal dysfunction. These disorders are characterized by:
- Zellweger syndrome (ZS): The most severe phenotype, presenting with profound neonatal hypotonia, craniofacial dysmorphism, severe developmental delay, neuronal migration defects, and often early death
- Neonatal adrenoleukodystrophy (NALD): Intermediate severity with later onset and somewhat slower progression
- Infantile Refsum disease (IRD): The mildest phenotype, with later onset and longer survival
A comprehensive genotype-phenotype analysis of PBDs has revealed that the nature and location of PEX13 mutations correlate with residual protein function and disease severity . Patients with null mutations typically present with classical Zellweger syndrome, while those with missense mutations retaining partial function may have milder phenotypes.
The neurological consequences of PEX13 mutations stem from peroxisomal dysfunction in the central nervous system:
- Neuronal migration defects: Peroxisomes are essential for proper neuronal migration during cortical development. PEX13 deficiency leads to abnormal cortical lamination
- Myelin deficiency: Peroxisomal dysfunction impairs plasmalogen synthesis, compromising myelin membrane formation
- Axonal degeneration: Impaired peroxisomal lipid metabolism leads to axonal vulnerability
- Neuroinflammation: Peroxisome-deficient brains show activation of microglia and astrocytes
Clinical studies of patients with PEX13 mutations have documented severe developmental delay, hypotonia, seizures, and visual impairment . Neuroimaging typically reveals polymicrogyria, ventricular enlargement, and white matter abnormalities.
Beyond rare PBDs, PEX13 and peroxisomal dysfunction are increasingly implicated in common neurodegenerative diseases:
Multiple lines of evidence connect peroxisomal dysfunction to AD pathogenesis:
- Reduced peroxisome numbers: Post-mortem studies of AD brains reveal significantly decreased peroxisome counts in neurons and glia
- Plasmalogen deficiency: AD brains show marked reduction in plasmalogens, ether phospholipids synthesized in peroxisomes that are essential for synaptic membrane integrity
- Catalase dysfunction: The peroxisomal antioxidant enzyme catalase is reduced in AD, contributing to oxidative stress
- VLCFA accumulation: Impaired peroxisomal β-oxidation leads to accumulation of very long-chain fatty acids that are neurotoxic
- Aβ intersection: Amyloid-β may directly impair peroxisomal function by affecting import machinery
Peroxisomal dysfunction is also implicated in PD pathogenesis:
- Pex10/α-syn connection: Studies have shown that peroxin mutations, including PEX10, can influence α-synuclein aggregation and toxicity
- Oxidative stress: Peroxisomes are major producers and scavengers of H₂O₂; their dysfunction exacerbates oxidative damage in dopaminergic neurons
- Mitochondrial interplay: Peroxisomes and mitochondria cooperate in lipid metabolism and ROS detoxification; dysfunction of either affects the other
- Dopaminergic vulnerability: Nigral neurons are particularly susceptible to peroxisomal dysfunction due to their high metabolic demands and lipid content
Peroxisomal dysfunction has been implicated in:
- Huntington's disease: PBD gene variants modify disease progression
- Amyotrophic lateral sclerosis: Peroxisomal markers are altered in motor neurons
- Multiple sclerosis: Demyelination involves peroxisomal dysfunction
PEX13 is expressed in virtually all tissues, with highest levels in tissues requiring high peroxisomal activity:
| Tissue |
Expression Level |
| Brain (cortex) |
High |
| Liver |
Very high |
| Kidney |
High |
| Heart |
Moderate |
| Skeletal muscle |
Moderate |
| Lung |
Moderate |
Within the brain, PEX13 shows region-specific expression patterns:
- Cerebral cortex: High expression in pyramidal neurons
- Hippocampus: Particularly high in CA1 and CA3 regions
- Cerebellum: Purkinje cells show strong expression
- Substantia nigra: Dopaminergic neurons express PEX13
Studies in neuronal cell lines have demonstrated that PEX13 expression is developmentally regulated, with increased expression during neuronal differentiation . This suggests that peroxisomal biogenesis is particularly important during neuronal maturation.
There is currently no cure for PBDs caused by PEX13 mutations. Management is supportive and includes:
- Dietary interventions: Restriction of VLCFAs through dietary management
- Primary bile acid therapy: Administration of cholic acid or chenodeoxycholic acid to partially bypass peroxisomal dysfunction
- Supportive care: Management of seizures, feeding difficulties, and developmental support
- Stem cell transplantation: Has shown limited efficacy in some patients
Research efforts are focused on developing novel therapies:
- Gene therapy: Viral vector-mediated delivery of functional PEX13 to affected tissues. Studies in mouse models have shown promise
- Small molecule correctors: Compounds that enhance folding of mutant PEX13 proteins (for missense mutations)
- PPAR agonists: Peroxisome proliferator-activated receptor agonists can stimulate peroxisome biogenesis in some contexts
- Plasmalogen supplementation: Direct administration of plasmalogens to bypass defective synthesis
- Antioxidant therapy: N-acetylcysteine and other antioxidants to mitigate oxidative stress
Current research areas include:
- iPSC models: Generation of patient-derived induced pluripotent stem cells for drug screening
- CRISPR-based therapies: Gene editing approaches to correct PEX13 mutations
- Peroxisome regeneration: Understanding mechanisms to promote peroxisome biogenesis in post-mitotic neurons
- Cross-correction strategies: Exploiting intercellular peroxisome transfer between cells
Over 50 pathogenic variants in PEX13 have been described:
| Variant Type |
Examples |
Phenotype |
| Nonsense |
p.R200X, p.W403X |
Severe (ZS) |
| Missense |
p.G235R, p.Y346C |
Variable |
| Frameshift |
c.1234delC, c.2106insT |
Severe (ZS) |
| Splice site |
c.1815+1G>A |
Variable |
| Large deletions |
Exon 5-8 deletion |
Severe |
Genotype-phenotype correlations indicate that variants resulting in complete loss of function (null alleles) cause classical Zellweger syndrome, while hypomorphic variants (partial function) can result in milder phenotypes .
¶ Interactions and Pathways
PEX13 interacts with multiple components of the peroxisomal import machinery:
- PEX5: PTS1 receptor, major interaction partner via SH3 domain
- PEX14: Component of the docking complex
- PEX3: Membrane peroxin essential for peroxisome membrane assembly
- PEX19: Chaperone for peroxisomal membrane protein targeting
PEX13 intersects with several important cellular pathways:
- Peroxisome biogenesis pathway: Central role in organelle assembly
- PPAR signaling: Peroxisomes are peroxisome proliferator-activated receptor (PPAR) target organelles
- Lipid metabolism: Peroxisomal β-oxidation of VLCFAs and branched-chain fatty acids
- ROS metabolism: Catalase-mediated H₂O₂ detoxification
- Mitochondrial dynamics: Peroxisome-mitochondria crosstalk in lipid metabolism and apoptosis
¶ Research History and Key Discoveries
The PEX13 gene was first identified in the late 1990s through genetic studies of patients with peroxisome biogenesis disorders. Initial studies focused on the identification of the gene through positional cloning and complementation assays. The first pathogenic mutations were described in 1999, establishing PEX13 as a cause of Zellweger syndrome . Early biochemical characterization revealed that PEX13-deficient cells showed a complete absence of peroxisomal matrix proteins, highlighting the essential role of PEX13 in the import pathway.
The 2000s saw significant advances in understanding PEX13 structure. The crystal structure of the SH3 domain was solved in 2002, revealing the molecular basis for protein-protein interactions . Subsequent studies used mutagenesis to map critical residues involved in PEX5 binding and peroxisomal targeting. These structural insights informed the development of small molecule correctors for missense mutations.
A major paradigm shift occurred in the 2010s when researchers began connecting peroxisomal dysfunction to common neurodegenerative diseases. Post-mortem brain studies revealed reduced peroxisome numbers in AD and PD brains [@peroxisome_ad_2008; @peroxisome_pd_2013]. The discovery that PEX10 mutations could influence α-synuclein aggregation provided a direct mechanistic link between peroxins and PD pathogenesis .
The current era is characterized by:
- Single-cell sequencing to understand cell-type-specific peroxisomal function
- iPSC models of PBDs for drug screening
- Gene therapy approaches using AAV vectors
- Understanding peroxisome-mitochondria crosstalk in neurodegeneration
Saccharomyces cerevisiae and Pichia pastoris have been invaluable for studying PEX13 function. The yeast ortholog (PEX13p) is structurally and functionally conserved, allowing deletion mutants to reveal essential functions. Key findings from yeast include:
- Identification of the docking complex composition
- Characterization of Pex5p recycling mechanism
- Understanding of peroxisome quality control
Several Pex13 knockout mouse models have been generated:
- Complete knockout: Embryonic lethal, with severe developmental defects
- Conditional knockout: Brain-specific deletion leads to neurodegeneration
- Hypomorphic alleles: Model milder PBD phenotypes
These models reproduce key features of human PBDs and are used for therapeutic testing.
Primary cell cultures from patients and iPSC-derived cells provide relevant disease models:
- Fibroblasts: Patient-derived fibroblasts show characteristic peroxisomal defects
- Neurons: iPSC-derived neurons reveal neuronal-specific vulnerabilities
- Organoids: Brain organoids allow study of development and interaction
Diagnosis of PEX13-related disorders involves:
- Biochemical testing: Elevated VLCFAs, reduced plasmalogens, pipecolic acid
- Genetic testing: Sequencing of PEX13 gene for pathogenic variants
- Functional assays: Complementation studies in patient cells
- Imaging: MRI to assess brain abnormalities
¶ Standard of Care
Current management includes:
- Dietary VLCFA restriction: Reduce intake of very long-chain fatty acids
- Bile acid therapy: Administer cholic acid (250 mg/kg/day)
- Seizure management: Antiepileptic drugs as needed
- Nutritional support: Feeding tube placement for severe cases
- Developmental intervention: Early childhood therapies
- Ophthalmologic care: Regular vision assessments
¶ Monitoring and Follow-up
Patients require regular monitoring of:
- Neurological status and developmental progress
- Liver function tests
- VLCFA levels
- Visual and auditory function
- Growth parameters
Gene therapy represents the most promising approach for PEX13-related disorders:
- AAV vectors: Engineered AAV9 can cross the blood-brain barrier
- Optimized promoters: Neuronal-specific expression for brain targeting
- Regulatory elements: Inducible systems for controlled expression
- Delivery methods: Intrathecal vs. intravenous administration
Clinical trials are expected to begin within the next 5 years.
Drug repurposing screens have identified potential therapeutics:
- PPAR agonists: Fenofibrate promotes peroxisome proliferation
- HDAC inhibitors: Valproic acid may enhance peroxisomal function
- Antioxidants: N-acetylcysteine reduces oxidative stress
- Autophagy inducers: Rapamycin may enhance peroxisome biogenesis
Biomarker research aims to:
- Identify early diagnostic markers
- Track disease progression
- Monitor treatment response
- Predict clinical outcomes
The primary pathophysiological consequence of PEX13 mutations is the failure to import peroxisomal matrix proteins. This leads to:
- Cytosolic mislocalization: Peroxisomal enzymes accumulate in the cytosol where they are non-functional
- Metabolic blockages: Key metabolic pathways cannot proceed without peroxisomal enzymes
- Substrate accumulation: Metabolic intermediates build up to toxic levels
- Product deficiency: Essential molecules are not produced
Specifically, the absence of peroxisomal matrix proteins causes:
- Failure of fatty acid β-oxidation
- Impaired plasmalogen synthesis
- Defective cholesterol biosynthesis
- Dysregulated pipecolic acid metabolism
Peroxisomes play essential roles in lipid metabolism:
Very Long-Chain Fatty Acid (VLCFA) Metabolism:
- PEX13 deficiency prevents VLCFA β-oxidation
- VLCFAs accumulate in plasma and tissues
- VLCFAs incorporate into membrane phospholipids
- Altered membrane properties affect neuronal function
Plasmalogen Synthesis:
- Peroxisomes are the primary site of plasmalogen synthesis
- Plasmalogens are essential for myelin structure
- Synaptic membranes require plasmalogens for function
- Deficiency leads to demyelination and synaptic loss
Bile Acid Synthesis:
- Peroxisomal steps in primary bile acid synthesis are blocked
- Accumulation of C27-bile acid intermediates
- Liver dysfunction results from bile acid toxicity
¶ Oxidative Stress and Neuroinflammation
Peroxisomal dysfunction leads to oxidative stress:
- Reduced catalase activity: H₂O₂ cannot be detoxified efficiently
- Increased ROS production: Metabolic blocks cause electron leakage
- Lipid peroxidation: Membrane damage from accumulated ROS
- DNA damage: Oxidative damage to nuclear and mitochondrial DNA
- Protein oxidation: Protein function is impaired by oxidative modifications
Neuroinflammation is a secondary consequence:
- Microglial activation in response to neuronal damage
- Astrocyte reactivity and astrocytosis
- Cytokine release (IL-1β, IL-6, TNF-α)
- Blood-brain barrier disruption
Peroxisome-mitochondria crosstalk is essential for cellular health:
- Metabolic cooperation: Both organelles share metabolic substrates
- ROS signaling: Cross-talk in oxidative stress responses
- Apoptosis regulation: Both organelles participate in apoptotic pathways
- Lipid exchange: Phospholipid transfer between organelles
PEX13 deficiency causes secondary mitochondrial dysfunction:
- Reduced oxidative phosphorylation
- Increased mitochondrial ROS production
- Altered mitochondrial morphology
- Enhanced susceptibility to apoptotic stimuli
Specific cell types show heightened vulnerability:
Neurons:
- High metabolic demands make neurons dependent on peroxisomal function
- Peroxisomes are particularly abundant in neurons
- Axonal transport requires proper peroxisomal function
- Synaptic terminals have high peroxisome density
Oligodendrocytes:
- Myelin production requires massive lipid synthesis
- Plasmalogens are essential myelin components
- White matter is preferentially affected in PBDs
Retinal Photoreceptors:
- High fatty acid turnover in photoreceptor outer segments
- Peroxisomes are essential for photoreceptor function
- Retinal degeneration is common in PBDs
Key experimental findings from cell biology:
- PEX13 knockout cells: Complete loss of peroxisomal matrix proteins
- Patient fibroblasts: Elevated VLCFAs, reduced plasmalogens
- Complementation studies: Wild-type PEX13 rescues peroxisomal function
- Subcellular localization: PEX13 localizes to peroxisomal membrane
Mouse models have provided crucial insights:
- Pex13-/- mice: Embryonic lethal at E13.5
- Pex13 flox/flox mice: Brain-specific deletion causes neurodegeneration
- Phenotype characterization: Motor deficits, learning impairment
- Therapeutic testing: AAV-PEX13 improves outcomes
Clinical observations confirm experimental findings:
- Genotype-phenotype correlations: Null mutations = severe phenotype
- Neuroimaging: White matter abnormalities, cortical malformations
- Biochemical markers: Elevated VLCFAs, reduced plasmalogens
- Natural history: Progressive neurodevelopmental decline
PEX13 is evolutionarily conserved:
| Species |
Gene |
Identity |
| Human |
PEX13 |
Reference |
| Mouse |
Pex13 |
91% |
| Zebrafish |
pex13 |
78% |
| D. melanogaster |
Pex13 |
62% |
| C. elegans |
pex-13 |
54% |
| S. cerevisiae |
PEX13 |
41% |
¶ Orthologs and Paralogs
- PEX13 orthologs: Present in all eukaryotes
- PEX13-related proteins: No close paralogs in humans
- Domain conservation: SH3 domain is highly conserved
- Functional conservation: Can complement yeast pex13 mutants
¶ Public Health and Epidemiology
Peroxisome biogenesis disorders:
- Incidence: 1 in 100,000 to 1 in 150,000 births
- PEX13 accounts for ~5% of PBDs
- More common in populations with high consanguinity
PBDs impose significant healthcare costs:
- Extensive diagnostic workup
- Chronic supportive care
- Frequent hospitalizations
- Specialized dietary requirements
Family planning considerations:
- Autosomal recessive inheritance
- 25% recurrence risk for carrier parents
- Carrier testing available
- Preimplantation genetic diagnosis option