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Tetrahydrobiopterin Deficiency – Causes, Symptoms & Treatment

Tetrahydrobiopterin deficiency (BH4 deficiency) is a rare inherited metabolic disorder. Insufficient BH4 causes elevated phenylalanine levels and impaired neurotransmitter production, leading to neurological symptoms.

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Things worth knowing about "Tetrahydrobiopterin Deficiency"

Tetrahydrobiopterin deficiency (BH4 deficiency) is a rare inherited metabolic disorder. Insufficient BH4 causes elevated phenylalanine levels and impaired neurotransmitter production, leading to neurological symptoms.

What Is Tetrahydrobiopterin Deficiency?

Tetrahydrobiopterin deficiency (BH4 deficiency) is a rare, inherited metabolic disorder in which the cofactor tetrahydrobiopterin (BH4) is absent or present in insufficient amounts. BH4 is an essential helper molecule required by several enzymes in the human body. When BH4 is deficient, the metabolism of certain amino acids and neurotransmitters is severely disrupted. The condition belongs to the group of hyperphenylalaninaemias, as it results in elevated blood phenylalanine levels, similar to classic phenylketonuria (PKU).

Causes

Tetrahydrobiopterin deficiency is caused by genetic defects in enzymes involved in the synthesis or recycling of BH4. Depending on the affected enzyme, several subtypes are distinguished:

GTP cyclohydrolase I deficiency (GTPCH deficiency): A defect in BH4 biosynthesis; both autosomal recessive and autosomal dominant forms exist.
6-Pyruvoyltetrahydropterin synthase deficiency (PTPS deficiency): The most common form of BH4 deficiency; affects an early step in BH4 synthesis.
Dihydropteridine reductase deficiency (DHPR deficiency): Impairs BH4 regeneration; frequently associated with cerebral folate deficiency.
Pterin-4a-carbinolamine dehydratase deficiency (PCD deficiency): Usually a mild form with transient hyperphenylalaninaemia.
Sepiapterin reductase deficiency (SR deficiency): A special form in which blood phenylalanine levels are often normal, but neurotransmitter synthesis is impaired.

In all subtypes, BH4 as a cofactor is insufficiently available for the enzymes phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase. This impairs both the breakdown of phenylalanine and the synthesis of the neurotransmitters dopamine and serotonin.

Symptoms

The clinical presentation of tetrahydrobiopterin deficiency varies depending on the subtype and the severity of the enzyme defect. Common symptoms include:

Elevated blood phenylalanine levels (hyperphenylalaninaemia)
Developmental delay and intellectual disability (in untreated cases)
Movement disorders such as tremor and muscle rigidity
Epileptic seizures
Swallowing difficulties and reduced muscle tone (hypotonia)
Impaired motor function and coordination
Behavioral problems and psychiatric symptoms
Sleep disturbances

Without early diagnosis and treatment, severe and irreversible neurological damage can occur.

Diagnosis

The initial diagnosis is often made through newborn screening, which detects elevated phenylalanine levels in the blood. However, since not all cases of hyperphenylalaninaemia are caused by BH4 deficiency, further investigations are required:

Urinary pterin analysis: Measurement of biopterin and neopterin levels to differentiate between subtypes.
DHPR activity assay: Detection of dihydropteridine reductase deficiency from a dried blood spot sample.
Cerebrospinal fluid (CSF) neurotransmitter analysis: Measurement of dopamine and serotonin metabolites to assess neurotransmitter metabolism.
Molecular genetic testing: Identification of the causative gene mutation to confirm the diagnosis.
BH4 loading test: Administration of BH4 followed by phenylalanine monitoring to assess BH4 responsiveness.

Treatment

Treatment of tetrahydrobiopterin deficiency depends on the specific subtype and typically involves a combination of approaches:

BH4 Supplementation

In BH4-responsive forms, sapropterin (synthetic BH4, e.g., marketed as Kuvan) can be administered. It lowers blood phenylalanine levels and improves enzyme function.

Neurotransmitter Precursors

Because dopamine and serotonin synthesis is impaired, precursor substances are given:

L-DOPA (combined with carbidopa): a precursor to dopamine
5-Hydroxytryptophan (5-HTP): a precursor to serotonin

Folinic Acid

In DHPR deficiency, cerebral tetrahydrofolate levels are often reduced, requiring additional supplementation with folinic acid.

Dietary Therapy

In cases with persistent hyperphenylalaninaemia, a low-phenylalanine diet may be required as a complementary measure, similar to the dietary management used in classic PKU.

Early diagnosis and prompt initiation of treatment are critical to preventing or minimizing neurological complications. Patients are typically managed in specialized metabolic centers.

References

Blau N, et al. - Disorders of phenylalanine and tetrahydrobiopterin metabolism. In: Blau N, Duran M, Gibson KM (eds.): Laboratory Guide to the Methods in Biochemical Genetics. Springer, Berlin, 2008.
Opladen T, et al. - Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (BH4) deficiencies. Orphanet Journal of Rare Diseases, 2020. https://doi.org/10.1186/s13023-020-01379-8
National Institutes of Health (NIH) - Tetrahydrobiopterin deficiency. MedlinePlus Genetics. https://medlineplus.gov/genetics/condition/tetrahydrobiopterin-deficiency/

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