Pterin-4 alpha-carbinolamine dehydratase deficiency

Pterin-4 alpha-carbinolamine dehydratase (PCD) deficiency, also known as PCBD deficiency or BH4-deficient hyperphenylalaninemia type D, is a rare autosomal recessive disorder of phenylalanine metabolism caused by mutations in the PCBD1 gene. This enzyme plays a role in the regeneration of tetrahydrobiopterin (BH4), an essential cofactor for phenylalanine hydroxylase, which converts phenylalanine to tyrosine. When PCD is deficient, the recycling of BH4 is impaired, leading to mild to moderate hyperphenylalaninemia (elevated phenylalanine levels in the blood). The condition is typically detected through newborn screening programs that identify elevated phenylalanine levels. PCD deficiency is considered the mildest form of BH4-deficient hyperphenylalaninemia. Unlike other BH4 deficiency disorders, PCD deficiency generally does not cause significant neurological impairment because the block in BH4 regeneration is partial and alternative metabolic pathways can partially compensate. Most affected individuals present with transient or benign hyperphenylalaninemia in the neonatal period. Key biochemical findings include elevated levels of primapterin (7-biopterin) in the urine, which is a hallmark diagnostic marker. Some patients may show mildly elevated phenylalanine on newborn screening that normalizes or remains only mildly elevated over time. The clinical course is generally benign, and many patients do not require treatment. In some cases, a phenylalanine-restricted diet or BH4 supplementation may be considered, particularly if phenylalanine levels remain persistently elevated. Neurotransmitter deficiency is not typically a feature of this condition, distinguishing it from more severe forms of BH4 deficiency. Long-term prognosis is generally favorable, with most individuals having normal cognitive and neurological development.

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