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

UROS gene

The UROS gene provides instructions for making an enzyme essential for heme production, and mutations in this gene are linked to congenital erythropoietic porphyria.

Normal Function

The UROS gene provides instructions for making an enzyme known as uroporphyrinogen III synthase. This enzyme is involved in the production of a molecule called heme. Heme is vital for all of the body's organs, although it is most abundant in the blood, bone marrow, and liver. Heme is an essential component of iron-containing proteins called hemoproteins, including hemoglobin (the protein that carries oxygen in the blood).

The production of heme is a multi-step process that requires eight different enzymes. Uroporphyrinogen III synthase is responsible for the fourth step in this process, in which hydroxymethylbilane (the product of the third step) is rearranged to form uroporphyrinogen III. In subsequent steps, four other enzymes produce and modify compounds that ultimately lead to heme.

Health Conditions Related to Genetic Changes

Porphyria

More than 35 mutations in the UROS gene have been found to cause a form of porphyria known as congenital erythropoietic porphyria. Most of these mutations change single protein building blocks (amino acids) in uroporphyrinogen III synthase. The most common UROS gene mutation, which is found in about one-third of all cases of congenital erythropoietic porphyria, replaces the amino acid cysteine with the amino acid arginine at position 73 (written as Cys73Arg or C73R). Several other mutations occur in a nearby region of DNA that regulates the activity of the UROS gene.

Mutations in or near the UROS gene alter the structure and function of uroporphyrinogen III synthase, which reduces the enzyme's activity. A shortage of functional uroporphyrinogen III synthase allows compounds called porphyrins to build up in developing red blood cells. These compounds are formed during the normal process of heme production, but reduced activity of uroporphyrinogen III synthase allows them to accumulate to toxic levels. The excess porphyrins can leak out of developing red blood cells and be transported through the bloodstream to the skin and other tissues. An accumulation of these substances in the skin causes oversensitivity to sunlight and the other characteristic features of congenital erythropoietic porphyria.

Other Names for This Gene

  • Cosynthase
  • HEM4_HUMAN
  • Hydroxymethylbilane hydro-lyase (cyclizing)
  • UROIIIS
  • Uroporphyrinogen co-synthetase
  • Uroporphyrinogen III Cosynthetase
  • uroporphyrinogen III synthase (congenital erythropoietic porphyria)
  • Uroporphyrinogen III Synthetase
  • Uroporphyrinogen Isomerase
  • Uroporphyrinogen-III cosynthase
  • Uroporphyrinogen-III Synthase

Frequently Asked Questions

Congenital erythropoietic porphyria (CEP) is inherited in an autosomal recessive pattern. This means a person must inherit two mutated copies of the UROS gene, one from each parent, to develop the condition. Parents who carry only one mutated copy are typically asymptomatic carriers. If both parents are carriers, there is a 25 percent chance with each pregnancy that the child will inherit two mutated copies and have CEP, a 50 percent chance the child will be a carrier, and a 25 percent chance the child will inherit two normal copies. What are the primary symptoms that distinguish congenital erythropoietic porphyria from other types of porphyria?::The hallmark symptom of CEP is severe photosensitivity, which typically begins in infancy or early childhood. Unlike acute porphyrias that cause abdominal pain or neurological issues, CEP primarily affects the skin. Patients often develop blistering, scarring, and skin fragility on sun-exposed areas such as the hands and face. A distinctive feature is the presence of reddish-brown urine, which turns dark or black upon exposure to light or air due to the accumulation of porphyrins. Severe cases can also lead to anemia and bone abnormalities. Is there a specific treatment to cure the UROS gene mutation, or is management focused on symptom control?::Currently, there is no cure for the underlying genetic mutation in the UROS gene. Management is focused on preventing complications and controlling symptoms. The most critical lifestyle intervention is strict avoidance of sunlight and ultraviolet (UV) exposure, which includes using high-protection sunscreens, wearing protective clothing, and staying indoors during peak sun hours. For patients with severe anemia or bone marrow failure, a bone marrow transplant (hematopoietic stem cell transplant) may be considered as a curative option, though it carries significant risks. How does a deficiency in uroporphyrinogen III synthase affect the production of hemoglobin and overall blood health?::The UROS enzyme is essential for the fourth step in heme biosynthesis. When this enzyme is deficient, the pathway is blocked, causing a buildup of porphyrin precursors that are toxic to tissues, particularly when exposed to light. Simultaneously, the production of heme, which is required to make hemoglobin, is reduced. This leads to ineffective erythropoiesis, where the bone marrow produces red blood cells that are defective and destroyed prematurely. This process results in chronic hemolytic anemia, which can be severe enough to require regular blood transfusions in some patients.
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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