PEX6 Gene

PEX6 Gene

Introduction

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">PEX6 Gene</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>PEX6</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>PEX6</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=PEX6" 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 PEX6 gene (Peroxisome Biogenesis Factor 6), also known as PASD1 or Pex6p, encodes a critical peroxin protein that functions as a AAA ATPase (ATPase Associated with diverse cellular Activities) essential for peroxisome biogenesis and peroxisomal matrix protein import. PEX6 plays a unique role in peroxisome assembly by facilitating the recycling of the peroxin receptor PEX5 from the peroxisomal membrane back to the cytosol, a process essential for continued peroxisomal protein import. Biallelic pathogenic variants in PEX6 cause peroxisome biogenesis disorders (PBDs), particularly those classified in complementation group 6 (CG6), which manifest as Zellweger syndrome spectrum disorders or milder phenotypes such as Heim syndrome ([Matsumoto et al., 2003](https://pubmed.ncbi.nlm.nih.gov/12624732/); [Steinberg et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19385156/)). [@faust2012]

PEX6 Gene

Introduction

<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">PEX6 Gene</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>PEX6</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>PEX6</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=PEX6" 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 PEX6 gene (Peroxisome Biogenesis Factor 6), also known as PASD1 or Pex6p, encodes a critical peroxin protein that functions as a AAA ATPase (ATPase Associated with diverse cellular Activities) essential for peroxisome biogenesis and peroxisomal matrix protein import. PEX6 plays a unique role in peroxisome assembly by facilitating the recycling of the peroxin receptor PEX5 from the peroxisomal membrane back to the cytosol, a process essential for continued peroxisomal protein import. Biallelic pathogenic variants in PEX6 cause peroxisome biogenesis disorders (PBDs), particularly those classified in complementation group 6 (CG6), which manifest as Zellweger syndrome spectrum disorders or milder phenotypes such as Heim syndrome ([Matsumoto et al., 2003](https://pubmed.ncbi.nlm.nih.gov/12624732/); [Steinberg et al., 2009](https://pubmed.ncbi.nlm.nih.gov/19385156/)). [@faust2012]

PEX6 represents one of the most critical components of the peroxisomal import machinery. Its ATPase activity provides the mechanical force necessary for receptor recycling, and without functional PEX6, peroxisomes fail to properly import matrix proteins, leading to the severe multisystem manifestations characteristic of peroxisome biogenesis disorders. [@moser2013]

Gene Structure and Protein

Genomic Organization

The PEX6 gene is located on human chromosome 6p21.1 and spans approximately 20.5 kilobases. It consists of 13 exons encoding a protein of 979 amino acids with a molecular weight of approximately 104 kDa. The gene exhibits a typical housekeeping expression pattern with strong expression in tissues with high peroxisomal activity ([Tam et al., 2003](https://pubmed.ncbi.nlm.nih.gov/12810672/)). [@wanders2015]

Protein Structure

The PEX6 protein is a member of the AAA ATPase family and contains several distinctive domains: [@ito2021]

N-terminal Region: The N-terminal region (approximately 300 amino acids) contains the PEX5 interaction domain. This region is responsible for binding to PEX5 after its ubiquitination and extracting it from the peroxisomal membrane. [@kleinert2022]

AAA ATPase Domain: The core of the protein contains two AAA ATPase modules (AAA-1 and AAA-2), each with characteristic Walker A (P-loop) and Walker B motifs. These domains bind and hydrolyze ATP, providing the mechanical energy for PEX5 extraction. [@steinberg2019]

C-terminal Region: The C-terminal region contains sequences necessary for peroxisomal membrane localization and interaction with PEX10. [@ebberink2011]

AAA+ ATPase Mechanism

PEX6 functions as a hexameric ring complex: [@islinger2012]

• ATP Binding: PEX6 binds ATP in its AAA domains
• Conformational Changes: ATP hydrolysis drives conformational changes in the hexamer
• PEX5 Extraction: These conformational changes pull ubiquitinated PEX5 from the membrane
• ATP Hydrolysis: Hydrolysis of ATP releases PEX6 for another cycle

Biological Function

Role in Peroxisome Biogenesis

PEX6 is essential for peroxisome biogenesis through its involvement in receptor recycling: [@van2011]

PEX5 Recycling: After PEX5 delivers its cargo to the peroxisome lumen, it must return to the cytosol for additional rounds of import. PEX6, in complex with PEX1 (another AAA ATPase), extracts PEX5 from the peroxisomal membrane. This process requires ATP hydrolysis and is essential for peroxisome import. [@schrader2015]

Peroxisomal Matrix Protein Import: By enabling PEX5 recycling, PEX6 ensures the continuous operation of the peroxisomal import machinery. Without PEX6, PEX5 becomes trapped on the peroxisomal membrane, and peroxisome import halts. [@kawashima2019]

Peroxisome Maintenance: PEX6 also participates in peroxisome proliferation and maintenance, helping to maintain proper peroxisome numbers in cells. [@liu2023]

The Peroxisomal Protein Import Cycle

The complete peroxisomal import cycle involves:

PEX6 is essential for steps 5-6, making it indispensable for peroxisome import.

Interaction Network

PEX6 interacts with several key proteins:

PEX1: Forms a heterodimeric AAA ATPase complex with PEX6. Both proteins are required for function, and mutations in either cause peroxisome biogenesis disorders.

PEX5: The primary substrate for PEX6. PEX6 extracts ubiquitinated PEX5 from the peroxisomal membrane.

PEX10: May regulate PEX6 function and localization.

PEX19: Peroxisomal membrane protein chaperone; may interact with PEX6 during membrane protein insertion.

PEX3: Essential for peroxisomal membrane biogenesis; recruits PEX19 and PEX10 to peroxisomes.

Expression and Localization

Tissue Distribution

PEX6 is ubiquitously expressed with highest levels in:

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

Subcellular Localization

PEX6 localizes to peroxisomes:

• Peroxisomal membrane: Peripheral membrane protein on the cytosolic side
• Cytosolic pool: Significant cytosolic pool when not actively extracting PEX5
• Peroxisome-peroxisome contact sites: Found at sites of peroxisome interaction

Pathophysiology

Peroxisome Biogenesis Disorders

PEX6 mutations cause peroxisome biogenesis disorders in complementation group 6 (CG6):

Zellweger Syndrome Spectrum: The classic phenotype

• Severe developmental delay
• Characteristic facial dysmorphism
• Hepatomegaly and hepatic dysfunction
• Retinal dystrophy
• Sensorineural hearing loss
• Variable severity

• Sensorineural hearing loss
• Retinitis pigmentosa
• Developmental delay (variable)
• Sometimes called "variant" Zellweger

• Some PEX6 variants cause single-organ involvement (e.g., isolated retinal dystrophy)

Cellular Mechanisms

PEX6 deficiency leads to peroxisome dysfunction through:

Impaired PEX5 Recycling: Without functional PEX6, PEX5 accumulates on the peroxisomal membrane and cannot be recycled. This halts peroxisomal matrix protein import.

Peroxisome Deficiency: Cells lacking functional PEX6 show either no peroxisomes or peroxisomes lacking matrix proteins.

Metabolic Dysregulation:

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

Neurodegeneration in PEX6-Related Disorders

The neurological manifestations of PEX6 deficiency include:

Neuronal Dysfunction: Peroxisomes are essential for neuronal lipid metabolism and redox homeostasis. Their dysfunction leads to neuronal stress and death.

Myelin Abnormalities: Peroxisomes produce myelin lipids. PBDs exhibit white matter abnormalities and hypomyelinization.

Axonal Degeneration: Peroxisomal dysfunction leads to axonal degeneration, particularly in long tracts.

Visual and Auditory Deficits: The retina and inner ear are particularly vulnerable to peroxisomal dysfunction.

Relationship to Other Neurodegenerative Diseases

PEX6 and peroxisomal dysfunction are implicated in common neurodegenerative diseases:

Alzheimer's Disease: Peroxisomal function is impaired in AD. PEX6 expression may be dysregulated, and enhancing peroxisomal function is a therapeutic target.

Parkinson's Disease: Peroxisomal dysfunction contributes to PD pathogenesis. PEX6 variants have been associated with PD risk in some populations.

Retinitis Pigmentosa: PEX6 variants can cause isolated retinal degeneration, highlighting the eye's sensitivity to peroxisomal dysfunction.

Genetics

Mutation Spectrum

Over 80 pathogenic variants have been identified in PEX6:

Types of Mutations:

• Missense mutations (most common; often in AAA domains)
• Nonsense and frameshift mutations
• Splice site mutations
• Large deletions

• p.Tyr629Cys (founder mutation in some populations)
• p.Arg760Trp
• p.Gly843Asp
• Various AAA domain missense mutations

Inheritance

PEX6-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 are complex:

• Missense mutations in AAA domains typically cause milder phenotypes
• Null/truncating mutations cause severe Zellweger syndrome
• Some variants cause tissue-specific disease (e.g., retina-only)

Genetic Testing

Genetic testing for PEX6 variants includes:

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

Diagnosis

Clinical Diagnosis

The diagnosis of PEX6-related disorders involves:

Biochemical Findings

Characteristic biochemical abnormalities include:

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

Imaging

Brain MRI findings include:

• White matter abnormalities
• Cerebral atrophy
• Corpus callosum hypoplasia (variable)
• Cerebellar atrophy (in some cases)

Treatment

Current Management

No cure exists for PBDs. Management is supportive:

Dietary Therapy:

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

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

• Liver support (including transplantation in severe cases)
• Vision and hearing aids
• Hormone replacement as needed

Experimental Therapies

Gene Therapy: AAV-mediated PEX6 delivery is in development. Preclinical studies in mouse models show promise. Key challenges include achieving sufficient peroxisome numbers, tissue-specific targeting, and avoiding immune responses.

Small Molecule Therapies:

• Peroxisome proliferation agonists (fibrates, statins)
• Antioxidant therapies (CoQ10, vitamin E)
• Pharmacological chaperones for missense variants
• Anti-inflammatory agents
• Combination approaches

• Recombinant peroxisomal enzymes
• Targeting to relevant tissues
• Limited by peroxisome membrane permeability

• Hematopoietic stem cell transplantation
• Mesenchymal stem cell approaches
• Liver cell transplantation
• Limitations: donor availability, rejection risk

Stem Cell Therapy

Hematopoietic stem cell transplantation has been explored with some success in early-onset forms.

Animal Models

Mouse Models

Pex6 Knockout Mice: Pex6-deficient mice recapitulate key features of human PBDs. They show peroxisome deficiency, growth retardation, and neurological abnormalities. They serve as models for therapeutic studies.

Pex6 Knock-in Mice: Mice carrying patient-derived mutations show variable phenotypes.

Zebrafish Models

Zebrafish with pex6 knockdown exhibit developmental abnormalities including curved body shape, hepatic steatosis, and neurological defects.

Molecular Mechanisms in Detail

The AAA ATPase Complex

PEX6 functions as part of a heterodimeric AAA ATPase complex with PEX1:

Structural Organization:

• PEX6 and PEX1 form a hexameric ring structure
• Each protein contributes three AAA modules to the ring
• The ring provides the mechanical force for PEX5 extraction
• ATP binding and hydrolysis drive conformational changes

• ATP binding induces conformational changes in the AAA domains
• ATP hydrolysis provides the energy for extraction
• ADP release resets the complex for another cycle
• The cycle is tightly coordinated between PEX6 and PEX1

• Ubiquitinated PEX5 binds to the PEX6-PEX1 complex
• ATP hydrolysis drives conformational changes in the AAA ring
• These changes pull PEX5 from the peroxisomal membrane
• The extraction requires only a single round of ATP hydrolysis

Peroxisomal Membrane Dynamics

PEX6 is involved in peroxisomal membrane dynamics:

Peroxisome Division:

• PEX6 participates in peroxisome fission
• It helps recruit the division machinery
• This ensures proper peroxisome numbers

• PEX6 helps remove damaged peroxisomes
• It participates in pexophagy (peroxisome autophagy)
• This maintains a healthy peroxisome population

Relationship to Peroxisomal Disorders

PEX6 mutations are among the more common causes of PBDs:

Frequency:

• PEX6 mutations account for approximately 10-15% of PBDs
• PEX6-related disorders are second only to PEX10 in frequency
• The p.Tyr629Cys mutation is a founder in some populations

• PEX6 mutations can cause the full spectrum of PBDs
• Genotype-phenotype correlations are complex
• Some variants cause tissue-specific disease

PEX6 in Common Neurodegenerative Diseases

Beyond rare PBDs, PEX6 plays roles in common neurodegenerative diseases:

Alzheimer's Disease:

• Peroxisomal function declines with age in AD brains
• PEX6 expression is reduced in AD neurons
• Enhancing peroxisomes reduces amyloid pathology in models
• VLCFA accumulation contributes to oxidative stress

• PEX6 variants increase PD risk in some populations
• Peroxisomal dysfunction contributes to alpha-synuclein toxicity
• Mitophagy and peroxisome quality control intersect
• PEX6 activity affects mitochondrial function

• Peroxisomal abnormalities in ALS motor neurons
• PEX6 may help clear TDP-43 aggregates
• Energy metabolism is impaired in ALS
• Peroxisome enhancement is protective in models

Research Methods

Biochemical Assays

Peroxisomal Function Tests:

• VLCFA levels in plasma and fibroblasts
• Plasmalogen levels in erythrocytes
• Catalase activity
• Peroxisomal beta-oxidation assays

• PEX6 ATPase activity
• PEX1 ATPase activity
• PEX5 ubiquitination status

Cellular Models

Patient Fibroblasts:

• Most accessible patient tissue
• Show peroxisome deficiency
• Useful for therapeutic screening
• Can be transformed for immortalization

• Differentiation to neurons, astrocytes
• Liver cells for hepatic phenotype
• Retinal cells for visual phenotype
• Disease modeling in relevant tissues

Animal Models

Mouse Models:

• Complete knockout is embryonic lethal
• Conditional knockouts available
• Knock-in models for missense mutations
• Phenotypic characterization ongoing

• Morpholino knockdown possible
• CRISPR knockout available
• In vivo imaging of peroxisomes
• Drug screening platforms

Therapeutic Development

Gene Therapy Approaches:

• AAV vectors for PEX6 delivery
• Tissue-specific promoters
• Optimization of expression levels
• Combination with other peroxins

• Peroxisome proliferation compounds
• ATPase modulators
• Antioxidants
• Anti-inflammatory agents

Clinical Management

Comprehensive Care Team

Neurology:

• Seizure control
• Developmental interventions
• Physical therapy
• Occupational therapy

• Regular eye exams
• Visual rehabilitation
• Low vision aids

• Hearing tests
• Hearing aids
• Auditory training

• Liver function monitoring
• Nutritional counseling
• Management of hepatomegaly

Family Support

• Genetic counseling
• Support groups
• Educational resources
• Financial planning assistance

Outcome Measures

Clinical Trials:

• Motor function assessments
• Cognitive testing
• Visual and auditory function
• Liver function tests

• VLCFA levels
• Plasmalogen levels
• Pipecolic acid
• Neurofilament light chain

Emerging Research and Future Directions

Novel Therapeutic Targets

Perfluorocarbon Compounds:

• Experimental oxygen carriers
• May compensate for peroxisomal dysfunction
• Under investigation in model systems

• 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
• Diffusion tensor imaging

• Neurofilament light chain (NfL)
• VLCFA ratios
• Pipecolic acid levels

Patient Registries

Global PBD Registry:

• Natural history data collection
• Clinical trial recruitment
• Standardized care protocols

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)
• [PEX5 Gene](/genes/pex5)

References