PEX6 — Peroxisomal biogenesis factor 6

The PEX6 gene provides instructions for making a protein that is essential for the formation and normal function of peroxisomes. Peroxisomes are small, sac-like structures inside our cells that act as recycling centers. They are responsible for breaking down certain types of fats and toxins, and they also help build important fats needed for the brain and nervous system to work properly. The PEX6 protein works like a motor to help transport necessary enzymes into these peroxisomes so they can do their job. When there is a mutation in the PEX6 gene, the peroxisomes cannot form correctly or function properly. As a result, toxic substances build up in the body's cells, and the cells lack the essential fats they need. This leads to a group of conditions known as Zellweger spectrum disorders. Depending on how severely the PEX6 protein is affected, the symptoms can range from very severe, life-threatening problems in newborns (affecting the brain, liver, and kidneys) to milder forms that primarily cause vision loss (retinal dystrophy) and hearing loss later in childhood or adulthood. Because peroxisomes are vital for the health of the retina (the light-sensitive tissue at the back of the eye) and the inner ear, even mild mutations in PEX6 can lead to progressive vision and hearing problems. Currently, treatments focus on managing symptoms, but researchers are actively exploring new therapies, including gene therapy, to help restore peroxisome function.

Gene description: The PEX6 gene encodes peroxisome biogenesis factor 6, a member of the AAA-ATPase family. It plays a crucial role in peroxisome assembly and the import of peroxisomal matrix proteins, and its mutations are a major cause of Zellweger spectrum disorders.

Patient and family guide: The PEX6 gene provides instructions for making a protein that is essential for the formation and normal function of peroxisomes. Peroxisomes are small, sac-like structures inside our cells that act as recycling centers. They are responsible for breaking down certain types of fats and toxins, and they also help build important fats needed for the brain and nervous system to work properly. The PEX6 protein works like a motor to help transport necessary enzymes into these peroxisomes so they can do their job. When there is a mutation in the PEX6 gene, the peroxisomes cannot form correctly or function properly. As a result, toxic substances build up in the body's cells, and the cells lack the essential fats they need. This leads to a group of conditions known as Zellweger spectrum disorders. Depending on how severely the PEX6 protein is affected, the symptoms can range from very severe, life-threatening problems in newborns (affecting the brain, liver, and kidneys) to milder forms that primarily cause vision loss (retinal dystrophy) and hearing loss later in childhood or adulthood. Because peroxisomes are vital for the health of the retina (the light-sensitive tissue at the back of the eye) and the inner ear, even mild mutations in PEX6 can lead to progressive vision and hearing problems. Currently, treatments focus on managing symptoms, but researchers are actively exploring new therapies, including gene therapy, to help restore peroxisome function.

Gene function: PEX6 is a peroxisomal AAA-ATPase that forms a heterohexameric complex with PEX1, anchored to the peroxisomal membrane by PEX26. This complex functions as a protein dislocase, mediating the ATP-dependent extraction and recycling of the PTS1 receptor (PEX5) from the peroxisomal membrane, which is essential for continuous peroxisomal matrix protein import.

Protein structure: PEX6 is a 980-amino acid protein belonging to the AAA-ATPase family. It contains two N-terminal domains (N1 and N2) and two AAA-ATPase domains (D1 and D2) that assemble into a hexameric double-ring structure with PEX1.

Molecular function: At the molecular level, PEX6 functions as an ATPase associated with diverse cellular activities (AAA-ATPase). It forms a double-ring heterohexameric complex with PEX1 (alternating PEX1 and PEX6 subunits). This complex acts as a mechanochemical motor that uses the energy from ATP hydrolysis to physically extract (dislocate) the ubiquitinated peroxisomal targeting signal 1 (PTS1) receptor, PEX5, from the peroxisomal membrane back into the cytosol. This recycling step is strictly required for subsequent rounds of peroxisomal matrix protein import. PEX6 is anchored to the peroxisome membrane via interaction with the integral membrane protein PEX26.

Mutation spectrum: The mutation spectrum of PEX6 includes missense, nonsense, frameshift, splice-site mutations, and large deletions. Missense mutations often retain some residual protein function and are typically associated with milder phenotypes (e.g., Infantile Refsum disease or Heimler syndrome), whereas null mutations (nonsense, frameshift, large deletions) abolish protein function and lead to the severe Zellweger syndrome phenotype.

Clinical significance: Mutations in the PEX6 gene impair the PEX1-PEX6 AAA-ATPase complex, which is essential for peroxisome assembly and matrix protein import. This dysfunction leads to a spectrum of peroxisome biogenesis disorders (Zellweger spectrum disorders), characterized by the accumulation of very long-chain fatty acids and other toxic metabolites. The severity of the disease correlates with the residual function of the PEX6 protein, ranging from fatal neonatal Zellweger syndrome with severe neurological and hepatic dysfunction to milder phenotypes like Heimler syndrome 2, which presents with sensorineural hearing loss and retinal dystrophy.

Inheritance: Autosomal Recessive

Chromosomal location: 6p21.1

Research and therapeutic approaches: Currently, there is no cure for PEX6-related disorders, and management is primarily supportive, focusing on symptom control (e.g., hearing aids, cataract surgery, nutritional support). Experimental therapies under investigation include pharmacological chaperones like arginine, which may improve peroxisome assembly in patients with mild missense mutations. Gene therapy approaches, such as AAV-mediated gene augmentation to restore PEX6 function, are being researched in animal models and show promise for treating vision loss in milder Zellweger spectrum phenotypes.