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PHYH gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

PHYH gene

The PHYH gene provides instructions for making an enzyme essential for breaking down phytanic acid within cell structures called peroxisomes.

Normal Function

The PHYH gene provides instructions for making an enzyme called phytanoyl-CoA hydroxylase. This enzyme is critical for the normal function of cell structures called peroxisomes. These sac-like compartments contain enzymes needed to break down many different substances, including fatty acids and certain toxic compounds.

One substance that is broken down in peroxisomes is phytanic acid, a type of fatty acid obtained from the diet (particularly from beef and dairy products). Phytanoyl-CoA hydroxylase is responsible for one of the first steps in breaking down phytanic acid as part of a process known as alpha-oxidation. In subsequent steps, additional enzymes in peroxisomes and other parts of the cell further process this compound into smaller molecules that the body can use for energy.

Researchers suspect that phytanoyl-CoA hydroxylase may have other functions in addition to its role in breaking down phytanic acid. For example, this enzyme appears to help determine the number of peroxisomes within cells and is involved in regulating their activity.

Health Conditions Related to Genetic Changes

Refsum disease

Mutations in the PHYH gene have been found to cause more than 90 percent of all cases of Refsum disease. About 30 mutations in this gene have been identified. These mutations alter the structure or production of phytanoyl-CoA hydroxylase, which reduces the enzyme's activity. A shortage of this enzyme disrupts the breakdown of phytanic acid in peroxisomes. As a result, phytanic acid and related compounds build up in the body's tissues. The accumulation of phytanic acid is toxic to cells, although it is unclear how an excess of this substance affects vision and smell and causes the other specific features of Refsum disease.

Other Names for This Gene

  • LN1
  • LNAP1
  • PAHX
  • PAHX_HUMAN
  • PHYH1
  • phytanic acid oxidase
  • phytanoil-CoA alpha hydroxylase
  • phytanoyl-CoA 2 oxoglutarate dioxygenase
  • phytanoyl-CoA alpha-hydroxylase
  • phytanoyl-CoA dioxygenase, peroxisomal

Frequently Asked Questions

Diagnosis starts with a clinical evaluation and measurement of plasma phytanic acid levels. If elevated, a genetic test for PHYH mutations (often via next‑generation sequencing panels) is performed. Confirmation may also involve a peroxisomal enzyme assay to assess phytanoyl‑CoA hydroxylase activity.
The most effective way is to follow a low‑phytanic acid diet. Avoid high‑fat dairy products (whole milk, cheese, butter), red meats (beef, lamb), and certain fish (e.g., herring, mackerel). Opt for lean poultry, fish low in phytanic acid (cod, tilapia), and plant‑based proteins. Read food labels for “high‑fat” or “full‑fat” indications and consider working with a dietitian to create a meal plan that meets your nutritional needs.
Typical symptoms include progressive ataxia (difficulty walking), loss of vision due to retinitis pigmentosa, hearing loss, and peripheral neuropathy. Sudden worsening of vision, severe dizziness, or seizures warrant immediate medical attention, as they may indicate acute complications.
Yes. In addition to limiting phytanic acid, patients should avoid foods high in long‑chain fatty acids, such as certain nuts and seeds. A dietitian can help ensure adequate intake of essential fatty acids, vitamins A, D, E, and B12, which may be affected by dietary restrictions. Regular monitoring of vitamin levels is advised.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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