Biopterin defect in cofactor regeneration is an inherited (genetic) condition that increases levels of a substance called phenylalanine in the blood. Phenylalanine is one kind of amino acid, which are the building blocks of protein. Your body uses amino acids to make other proteins that it needs to function.

Cofactors are substances that help enzymes work. Tetrahydrobiopterin (BH4), which is a type of biopterin, is a cofactor that helps certain enzymes process phenylalanine and important brain chemicals. Your body needs to build and recycle BH4 correctly for these important enzymes to work. Biopterin defect in cofactor regeneration occurs when the body cannot recycle (regenerate) BH4.

Biopterin defect in cofactor regeneration is one of a group of conditions called tetrahydrobiopterin deficiencies. Another condition in this group is called Biopterin Defect in Cofactor Biosynthesis, which occurs when BH4 is not built correctly.

Two enzymes in your body help recycle BH4. In biopterin defect in cofactor regeneration, one of these two enzymes does not work well. This condition will be more or less severe depending on which enzyme is not working well. Without working BH4, the processes that need this cofactor cannot do important jobs in the body.

Babies with biopterin defect in cofactor regeneration cannot break down all the phenylalanine from the proteins they eat. As a result, phenylalanine builds up to harmful levels in their body. Their body also makes fewer important chemicals for their brain called neurotransmitters. This can cause the signs and symptoms of the condition.

This condition is one type of a group of rare conditions known as tetrahydrobiopterin deficiencies. Fewer than 10 babies are born with tetrahydrobiopterin deficiencies each year in the United States.

Biopterin defect in cofactor regeneration is caused by changes in one of two genes, called PCBD1 or QDPR. These genes give the body instructions to make two enzymes that help recycle BH4, called pterin-4-alpha-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR).

The body cannot recycle enough normal BH4 without working PCBD1 or QDPR genes.Without enough working BH4, the body cannot break down phenylalanine and the brain cannot function as it should.

If DHPR is the enzyme that does not work well, the condition is more severe. If PCD is the enzyme that does not work well, the condition is less severe.

Normal Function

The PCBD1 gene provides instructions for making an enzyme called pterin-4 alpha-carbinolamine dehydratase. This enzyme helps carry out one step in the chemical pathway that recycles a molecule called tetrahydrobiopterin (BH4).

Tetrahydrobiopterin plays a critical role in processing several protein building blocks (amino acids) in the body. For example, it works with the enzyme phenylalanine hydroxylase to convert an amino acid called phenylalanine into another amino acid, tyrosine. Tetrahydrobiopterin is also involved in reactions that produce chemicals called neurotransmitters, which transmit signals between nerve cells in the brain. Because it helps enzymes carry out chemical reactions, tetrahydrobiopterin is known as a cofactor.

When tetrahydrobiopterin interacts with enzymes during chemical reactions, the cofactor is altered and must be recycled to a usable form. Pterin-4 alpha-carbinolamine dehydratase is one of two enzymes that help recycle tetrahydrobiopterin in the body.

Health Conditions Related to Genetic Changes

Tetrahydrobiopterin deficiency

At least nine mutations in the PCBD1 gene have been found to cause tetrahydrobiopterin deficiency. When this condition results from PCBD1 gene mutations, it is known as pterin-4 alpha-carbinolamine dehydratase (PCD) deficiency. PCD deficiency accounts for about 5 percent of all cases of tetrahydrobiopterin deficiency.

Some mutations in the PCBD1 gene change single amino acids in pterin-4 alpha-carbinolamine dehydratase, while other mutations introduce a premature stop signal in the instructions for making this enzyme. Changes in pterin-4 alpha-carbinolamine dehydratase reduce the enzyme's activity, which affects the body's ability to recycle tetrahydrobiopterin. As a result, less of this cofactor is available to participate in chemical reactions such as the conversion of phenylalanine to tyrosine. If phenylalanine is not converted to tyrosine, the excess can build up in the bloodstream and other tissues.

Although people with PCD deficiency usually have elevated levels of phenylalanine in the blood, this form of tetrahydrobiopterin deficiency rarely causes significant medical problems. Researchers believe that other enzymes may compensate for the reduced activity of pterin-4 alpha-carbinolamine dehydratase in people with PCBD1 gene mutations.

Other Names for This Gene

• 4-alpha-hydroxy-tetrahydropterin dehydratase
• 6-pyruvoyl-tetrahydropterin synthase/dimerization cofactor of hepatocyte nuclear factor 1 alpha (TCF1)
• Carbinolamine-4a-dehydratase
• DCOH
• Dimerization cofactor of hepatocyte nuclear factor 1-alpha
• Dimerization cofactor of HNF1
• PCBD
• PCD
• Phenylalanine hydroxylase-stimulating protein
• PHS_HUMAN
• Pterin carbinolamine dehydratase
• pterin-4 alpha-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor 1 alpha
• pterin-4 alpha-carbinolamine dehydratase/dimerization cofactor of hepatocyte nuclear factor 1 alpha (TCF1)
• Pterin-4-alpha-carbinolamine dehydratase
• Pterin-4a-carbinolamine dehydratase (dimerization cofactor of hepatic nuclear factor 1-alpha)

Genomic Location

Normal Function

The QDPR gene provides instructions for making an enzyme called quinoid dihydropteridine reductase. This enzyme helps carry out one step in the chemical pathway that recycles a molecule called tetrahydrobiopterin (BH4).

Tetrahydrobiopterin plays a critical role in processing several protein building blocks (amino acids) in the body. For example, it works with the enzyme phenylalanine hydroxylase to convert an amino acid called phenylalanine into another amino acid, tyrosine. Tetrahydrobiopterin is also involved in reactions that produce chemicals called neurotransmitters, which transmit signals between nerve cells in the brain. Because it helps enzymes carry out chemical reactions, tetrahydrobiopterin is known as a cofactor.

When tetrahydrobiopterin interacts with enzymes during chemical reactions, the cofactor is altered and must be recycled to a usable form. Quinoid dihydropteridine reductase is one of two enzymes that help recycle tetrahydrobiopterin in the body.

Health Conditions Related to Genetic Changes

Tetrahydrobiopterin deficiency

More than 30 mutations in the QDPR gene have been found to cause tetrahydrobiopterin deficiency. When this condition results from QDPR gene mutations, it is known as dihydropteridine reductase (DHPR) deficiency. DHPR deficiency accounts for about one-third of all cases of tetrahydrobiopterin deficiency.

Most QDPR gene mutations change single amino acids in quinoid dihydropteridine reductase, although some mutations insert small amounts of DNA into the QDPR gene or disrupt the way the gene's instructions are used to make the enzyme. Changes in quinoid dihydropteridine reductase greatly reduce or eliminate the enzyme's activity. Without enough of this enzyme, tetrahydrobiopterin is not recycled properly. As a result, this cofactor is not available to participate in chemical reactions such as the conversion of phenylalanine to tyrosine. If phenylalanine is not converted to tyrosine, it can build up to toxic levels in the blood and other tissues. Nerve cells in the brain are particularly sensitive to phenylalanine levels, which is why excessive amounts of this substance can cause brain damage.

Additionally, a reduction in quinoid dihydropteridine reductase activity disrupts the production of certain neurotransmitters in the brain. Because neurotransmitters are necessary for normal brain function, changes in the levels of these brain chemicals contribute to intellectual disability in people with DHPR deficiency.

Other Names for This Gene

• DHPR
• DHPR_HUMAN
• Dihydropteridine reductase
• PKU2
• SDR33C1

Genomic Location

Biopterin defect in cofactor regeneration is a genetic condition. Babies inherit it from their biological (birth) parents.

• Biopterin defect in cofactor regeneration is an autosomal recessive condition. Babies inherit the condition when each parent passes down the same nonworking gene that causes biopterin defect in cofactor regeneration (PCBD1 or QDPR) to their baby. Only babies with two matching nonworking genes—for example, one nonworking PCBD1 from the mom and one nonworking PCBD1 from the dad—have this condition. Babies with two nonworking genes that do not match—for example, one nonworking PCBD1 from the mom and one nonworking QDPR from the dad—will not have this condition.
  • People with one working copy and one nonworking copy of the PCBD1 or QDPR gene are called carriers.
  • Carriers do not have or develop the condition. However, they may pass down a nonworking copy of the gene to their children.
  • If two parents are carriers of a nonworking copy of the PCBD1 or QDPR gene, they have a 1 in 4 chance of having a child with biopterin defect in cofactor regeneration.
  • Carriers for biopterin defect in cofactor regeneration often do not know they are carriers before having a child with the condition. In most cases, families have no history of the condition until the birth of a child with biopterin defect in cofactor regeneration.
  • Parents who already have a child with biopterin defect in cofactor regeneration still have a 1 in 4 chance of having another child with biopterin defect in cofactor regeneration. This 1 in 4 chance stays the same for all future children.
• Genetic counselors and medical geneticists can help families learn about this condition and the chance of having children with it.

Babies with biopterin defect in cofactor regeneration often seem healthy at birth, but they may develop signs of the condition over time. Eating foods with high amounts of phenylalanine (protein-containing foods), illness, infection, or going a long time without eating can trigger signs of the condition.

Signs of the condition may include the following:

• Delays in development
• Weak muscle tone (hypotonia)
• Difficulty swallowing
• Seizures
• Problems with controlling body temperature
• Sleepiness (lethargy)
• Behavioral problems

Newborn screening for biopterin defect in cofactor regeneration is done using a small amount of blood collected from your baby’s heel. During screening, a special machine measures how much of a substance called phenylalanine is in your baby’s blood. In most cases, babies with high levels of phenylalanine have a condition called classic phenylketonuria (PKU). But, in some rare cases, babies with high levels of phenylalanine are found to have biopterin defect in cofactor regeneration. Whether a baby has PKU or a biopterin defect will be determined during follow-up testing.

What Happens After an Out-of-Range Screening Result?

If your baby’s blood spot screening result for biopterin defect in cofactor regeneration is out-of-range, your baby’s health care provider will contact you. Together, you will discuss next steps and follow-up plans.

An out-of-range screening result does not mean that your baby definitely has the condition. It does mean that your baby needs more follow-up testing.

Your baby may need the following tests after an out-of-range screening result:

• Blood and/or urine tests
• Genetic testing using a blood sample

You should complete any recommended follow-up testing as soon as possible. Babies with this condition can have serious health problems soon after birth if they are not diagnosed and treated quickly.

False-positive newborn screening results for this condition are rare. However, babies who receive supplemental nutrition (also called total parenteral nutrition [TPN] or hyperalimentation) may have out-of-range results.

Newborn screening helps babies lead healthier lives. If your baby has an out-of-range result, follow up with your health care provider quickly. It is important to follow their instructions. Your baby may need to get treatment right away, even if they are not showing signs or symptoms. In some cases, your baby’s health care provider may decide it is best to watch (monitor) your baby to decide next steps. Careful monitoring and early treatment will help your baby stay as healthy as possible.

It is important to talk to your health care provider about which treatment(s) are best for your baby. The goal of treatment is to prevent the health problems caused by this condition.

Treatments may include the following:

• Restricted diet that avoids foods with phenylalanine
• Medical formula and foods
• BH4 supplements to reduce phenylalanine levels
• Medicine to help increase neurotransmitter levels

Children who receive early and ongoing treatment for biopterin defect in cofactor regeneration can have healthy growth and development.