PEX7 — peroxisomal biogenesis factor 7

The PEX7 gene provides instructions for making a protein that acts like a delivery truck within your cells. Its job is to pick up specific enzymes and transport them into small compartments called peroxisomes. Peroxisomes are like the cell's recycling centers and manufacturing plants; they break down certain fats from our diet and help build special fats called plasmalogens, which are crucial for the health of our brain, bones, and eyes. When the PEX7 gene is mutated, the delivery system breaks down. The necessary enzymes never make it into the peroxisomes, meaning the cell cannot produce plasmalogens or break down specific fats like phytanic acid. This buildup of unprocessed fats and lack of essential building blocks causes damage throughout the body. Depending on how severely the gene's function is affected, this can lead to a very severe condition called Rhizomelic Chondrodysplasia Punctata (RCDP), which causes severe bone growth issues and developmental delays in infants, or a milder condition called Refsum disease, which primarily causes vision loss (retinitis pigmentosa) and nerve damage later in life. PEX7-related disorders are inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the gene—one from each parent—to develop the disease. Parents who carry only one mutated copy typically do not show any symptoms but have a 25% chance of passing the condition on to their children. While there is currently no cure, understanding the specific genetic mutation can help doctors manage symptoms and provide accurate genetic counseling for families.

Gene description: Encodes a peroxisomal targeting signal 2 (PTS2) receptor, essential for importing specific proteins into peroxisomes.

Patient and family guide: The PEX7 gene provides instructions for making a protein that acts like a delivery truck within your cells. Its job is to pick up specific enzymes and transport them into small compartments called peroxisomes. Peroxisomes are like the cell's recycling centers and manufacturing plants; they break down certain fats from our diet and help build special fats called plasmalogens, which are crucial for the health of our brain, bones, and eyes. When the PEX7 gene is mutated, the delivery system breaks down. The necessary enzymes never make it into the peroxisomes, meaning the cell cannot produce plasmalogens or break down specific fats like phytanic acid. This buildup of unprocessed fats and lack of essential building blocks causes damage throughout the body. Depending on how severely the gene's function is affected, this can lead to a very severe condition called Rhizomelic Chondrodysplasia Punctata (RCDP), which causes severe bone growth issues and developmental delays in infants, or a milder condition called Refsum disease, which primarily causes vision loss (retinitis pigmentosa) and nerve damage later in life. PEX7-related disorders are inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the gene—one from each parent—to develop the disease. Parents who carry only one mutated copy typically do not show any symptoms but have a 25% chance of passing the condition on to their children. While there is currently no cure, understanding the specific genetic mutation can help doctors manage symptoms and provide accurate genetic counseling for families.

Gene function: PEX7 is a cytosolic receptor that recognizes and binds to peroxisomal targeting signal 2 (PTS2)-containing proteins. It then facilitates their import into the peroxisome lumen. Peroxisomes are crucial organelles involved in various metabolic processes, including fatty acid oxidation and plasmalogen synthesis, which are vital for retinal health. Defective PEX7 leads to impaired peroxisomal function, impacting photoreceptor viability.

Protein structure: The PEX7 gene encodes the peroxisomal biogenesis factor 7 protein, which is 323 amino acids in length in humans. Structurally, PEX7 is a member of the WD40 repeat protein family. It is characterized by the presence of multiple WD40 domains, which typically fold into a beta-propeller structure. This structural conformation is highly suited for mediating protein-protein interactions. The beta-propeller structure of PEX7 allows it to act as a hub, binding simultaneously to the Peroxisome Targeting Signal type 2 (PTS2) on cargo proteins and to the peroxisomal membrane docking proteins, such as PEX13 and PEX14. A conserved glutamate residue (E77) and other specific motifs within the protein are critical for these interactions. The protein does not have transmembrane domains, consistent with its role as a soluble cytosolic receptor that cycles between the cytosol and the peroxisome.

Molecular function: The PEX7 gene encodes peroxisomal biogenesis factor 7, which functions as the cytosolic receptor for peroxisomal matrix proteins that contain a Peroxisome Targeting Signal type 2 (PTS2). In mammals, the primary cargo for PEX7 is peroxisomal 3-ketoacyl-CoA thiolase, an enzyme essential for the beta-oxidation of specific fatty acids, and alkylglycerone phosphate synthase (AGPS), which is crucial for the biosynthesis of plasmalogens. PEX7 binds to newly synthesized PTS2-containing proteins in the cytosol and transports them to the peroxisomal membrane. There, it interacts with a docking complex, primarily involving the PEX13 and PEX14 proteins, to facilitate the translocation of the cargo into the peroxisomal matrix. After releasing its cargo, PEX7 is recycled back to the cytosol to initiate another round of import. The failure of this import mechanism due to PEX7 mutations leads to the mislocalization and subsequent degradation of PTS2-targeted enzymes. This results in specific biochemical deficiencies: a profound defect in plasmalogen biosynthesis (due to AGPS deficiency) and an inability to properly metabolize branched-chain fatty acids like phytanic acid (due to phytanoyl-CoA hydroxylase deficiency, which requires PTS2 import in some species or is indirectly affected). These metabolic disruptions cause the severe developmental and degenerative symptoms seen in PEX7-related disorders.

Expression pattern: The PEX7 gene is ubiquitously expressed across human tissues, consistent with its fundamental role in peroxisome biogenesis, an essential cellular process. A 1.7-kb transcript is detected in all tissues, with the highest expression levels observed in the pancreas, skeletal muscle, and heart. In these highly expressing tissues, a second, less abundant transcript of approximately 1.5 kb is also present, though its specific functional significance remains unclear. In the context of the eye and inherited retinal diseases, PEX7 is expressed in the retina and the retinal pigment epithelium (RPE). The expression in these ocular tissues is critical for the maintenance of photoreceptor health and function. The progressive retinal degeneration seen in milder PEX7-related disorders (Refsum disease phenotype) underscores the importance of PEX7 and peroxisomal function in the long-term survival and function of retinal cells, likely due to the role of peroxisomes in lipid metabolism and the management of oxidative stress in the retina.

Mutation spectrum: The mutation spectrum of the PEX7 gene includes nonsense, missense, frameshift, and splice-site variants. The most prevalent mutation is a nonsense variant, c.875T>A (p.Leu292Ter or L292X), which accounts for approximately 50% of all mutant alleles in patients with classic Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP1). This mutation leads to a truncated, non-functional protein. Another relatively common variant is the splice-site mutation c.862+1G>C (IVS9+1G>C), which accounts for about 13% of mutant alleles and appears to be a recurrent mutation occurring on multiple haplotypes. The remainder of the mutation spectrum consists largely of private or rare missense and frameshift mutations scattered throughout the gene. Mutations that completely abolish PEX7 function lead to severe RCDP1, while hypomorphic mutations that retain partial function are responsible for milder phenotypes, including Refsum disease.

Pathogenic variants: 1. p.Leu292Ter (c.875T>A) - The most common pathogenic variant, accounting for ~50% of alleles in classic RCDP1; results in a truncated, non-functional protein and severe disease. 2. c.862+1G>C (IVS9+1G>C) - A common splice-site mutation accounting for ~13% of alleles; leads to aberrant splicing and severe RCDP1. 3. p.Ala218Val (c.653C>T) - A missense mutation associated with milder phenotypes, including Refsum disease, due to retained partial protein function. 4. p.Gly217Arg (c.649G>A) - Another missense variant often found in patients with milder, nonclassic presentations or Refsum disease. 5. c.45_52dup - A duplication predicted to cause a frameshift, but functional studies suggest it can undergo frame restoration, leading to a milder phenotype than expected.

Clinical significance: Mutations in the PEX7 gene manifest clinically as a spectrum of peroxisome biogenesis disorders, primarily Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP1) and Refsum disease. Classic (severe) RCDP1 is characterized by profound skeletal abnormalities, including shortening of the proximal long bones (rhizomelia), joint contractures, and characteristic facial features. Patients typically present with severe growth restriction, profound intellectual disability, and bilateral cataracts that are often present at birth or develop shortly after. The disease is typically lethal in the first decade of life, often due to respiratory complications. At the milder end of the spectrum, PEX7 mutations can cause a phenotype indistinguishable from classic Refsum disease (also known as Peroxisome Biogenesis Disorder 9B). This condition is characterized by adult-onset retinitis pigmentosa, peripheral neuropathy, cerebellar ataxia, and anosmia. Patients with this milder presentation often have normal early development and a normal lifespan, with symptoms gradually appearing in late childhood or early adulthood due to the slow accumulation of phytanic acid. Intermediate phenotypes also exist, where patients may have mild skeletal involvement, cataracts, and developmental delays, but survive into adulthood and later develop retinal degeneration and neuropathy.

Inheritance: Autosomal Recessive

Chromosomal location: 6q21

Genotype-phenotype correlations: There is a strong genotype-phenotype correlation in PEX7-related disorders, primarily driven by the residual activity of the mutant PEX7 protein. Null mutations, such as the common nonsense mutation p.Leu292Ter (L292X), result in a complete loss of PEX7 function and are associated with the severe, classic form of Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP1). Patients homozygous for these severe alleles exhibit profound biochemical defects and the classic lethal phenotype. Conversely, missense mutations or specific splice-site variants that allow for the production of some partially functional PEX7 protein are associated with milder phenotypes, such as nonclassic RCDP or Refsum disease. For example, certain alleles have been shown in functional assays to partially restore peroxisomal targeting. Patients carrying at least one of these "mild" alleles typically have less severe biochemical abnormalities, longer survival, and a clinical presentation dominated by later-onset features like retinitis pigmentosa and peripheral neuropathy, rather than severe skeletal dysplasia and early lethality.

Research and therapeutic approaches: Currently, there are no approved curative therapies for PEX7-related disorders; management is primarily supportive and symptom-directed. For patients with the severe RCDP1 phenotype, treatment focuses on physical therapy for joint contractures, cataract extraction, seizure management, and nutritional support. For patients with the milder Refsum disease phenotype, the primary therapeutic strategy is dietary restriction of phytanic acid. Since phytanic acid is entirely derived from the diet (primarily from dairy, ruminant meats, and certain fish), strict dietary adherence can halt the progression of neuropathy and retinitis pigmentosa, though it cannot reverse existing damage. Plasmapheresis or lipid apheresis may also be used in acute settings to rapidly lower phytanic acid levels. Research into pipeline therapies is ongoing. Plasmalogen replacement therapy is a significant area of investigation, aiming to bypass the biosynthetic defect by providing synthetic plasmalogen precursors orally. Animal studies have shown promise in restoring tissue plasmalogen levels and improving some phenotypes, and clinical trials are being explored. Gene therapy approaches, such as AAV-mediated gene augmentation, are also under investigation in preclinical models (like the Pex7 mouse) to determine if restoring PEX7 expression in specific tissues, such as the retina, can prevent or rescue the degenerative phenotypes. However, no gene therapies for PEX7 are currently in human clinical trials.

Diagnostic testing: Diagnosis of PEX7-related disorders typically begins with clinical evaluation and biochemical testing. Key biochemical markers include profoundly decreased levels of plasmalogens in red blood cells and elevated levels of phytanic acid in plasma, while very long-chain fatty acids (VLCFAs) typically remain normal. This specific biochemical profile distinguishes PEX7 defects from other peroxisome biogenesis disorders like Zellweger syndrome. Molecular genetic testing confirms the diagnosis. This is usually achieved through targeted gene panels for peroxisomal disorders or inherited retinal diseases, or via comprehensive genomic testing such as whole exome sequencing (WES) or whole genome sequencing (WGS). Genetic counseling is essential for affected families, as PEX7-related disorders are inherited in an autosomal recessive manner. Parents of an affected child are obligate carriers and have a 25% chance of having another affected child in each subsequent pregnancy. Carrier screening and prenatal testing are available for at-risk families once the specific pathogenic variants have been identified.

Animal models: Animal models have been crucial for understanding PEX7 function and disease mechanisms. The most prominent model is the Pex7 knockout mouse, which recapitulates many features of severe human Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP1). These mice exhibit severe growth retardation, hypotonia, impaired ossification, bilateral cataracts, and early postnatal lethality. They demonstrate a complete absence of plasmalogens and accumulation of phytanic acid, mirroring the biochemical defects seen in human patients. Hypomorphic Pex7 mouse models, which retain partial gene function, have also been developed to study milder forms of the disease, such as Refsum disease. These models survive longer and develop progressive phenotypes, including cataracts, peripheral neuropathy, and retinal degeneration, allowing researchers to study the long-term consequences of PEX7 deficiency and test potential therapeutic interventions. Studies in these models have shown that incremental increases in Pex7 levels can result in dramatic improvements in the phenotype, highlighting the potential for therapies that restore even partial gene function.

Population genetics: PEX7-related disorders are rare, with classic Rhizomelic Chondrodysplasia Punctata Type 1 (RCDP1) estimated to affect approximately 1 in 100,000 individuals. Based on this incidence, the overall carrier frequency in the general population is estimated to be around 1 in 158. The most common mutation, p.Leu292Ter, is found globally but is particularly prevalent in populations of European descent. Population genetics models suggest that PEX7 variants contribute to the vast majority of RCDP cases in both the United States and Europe. There are no widely recognized founder populations with a significantly higher prevalence of PEX7 mutations, though the recurrent nature of some splice-site mutations suggests they may have arisen independently multiple times.

Selected references: 1. Braverman N, et al. Human PEX7 encodes the peroxisomal PTS2 receptor and is responsible for rhizomelic chondrodysplasia punctata. Nat Genet. 1997;15(4):369-76. PMID: 9090381 2. Motley AM, et al. Mutational spectrum in the PEX7 gene and functional analysis of mutant alleles in 78 patients with rhizomelic chondrodysplasia punctata type 1. Am J Hum Genet. 2002;70(3):612-24. PMID: 11781871 3. Braverman N, et al. Mutation analysis of PEX7 in 60 probands with rhizomelic chondrodysplasia punctata and functional correlations. Hum Mutat. 2002;20(4):284-97. PMID: 12325024 4. van den Brink DM, et al. Identification of PEX7 as the second gene involved in Refsum disease. Am J Hum Genet. 2003;72(2):471-7. PMID: 12522768 5. Braverman NE, et al. PEX7-Related Rhizomelic Chondrodysplasia Punctata. 2001 [Updated 2025]. In: Adam MP, et al., editors. GeneReviews. Seattle (WA): University of Washington, Seattle. PMID: 20301447 6. Fallatah W, et al. A Pex7 Deficient Mouse Series Correlates Biochemical and Clinical Phenotypes in Rhizomelic Chondrodysplasia Punctata. Front Cell Dev Biol. 2022;10:886316. PMID: 35846660