This information is provided by the National Institutes of Health (NIH) Genetic and Rare Diseases Information Center (GARD).
Tetrahydrobiopterin (BH4) deficiency causes the body to build up an abnormally high level of phenylalanine, one of the building blocks of proteins. In addition, BH4 deficiency leads to low levels of certain neurotransmitters, chemical messengers that control many body functions. Symptoms can range from very mild to severe. Babies with BH4 deficiency appear normal at birth but may develop neurological symptoms such as abnormal muscle tone, poor head control, seizures, and delayed motor development. Without treatment, the condition can cause permanent intellectual disability. BH4 deficiency is caused by pathogenic variants in any one of several genes including the GCH1, PCBD1, PTS, and QDPR genes. It is inherited in an autosomal recessive pattern. Diagnosis is based on the symptoms, clinical exam, and blood and urine tests. BH4 deficiency is sometimes diagnosed based on the results of an abnormal newborn screening test. Treatment depends on the genetic cause and severity, and may include a low phenylalanine diet, oral BH4 supplementation, and neurotransmitter replacement.
For more information, visit GARD.