HYPERPHENYLALANINEMIA WITH HIGH-LEVELS OF 7-BIOPTERIN IS ASSOCIATED WITH MUTATIONS IN THE PCBD GENE ENCODING THE BIFUNCTIONAL PROTEIN PTERIN-4A-CARBINOLAMINE DEHYDRATASE AND TRANSCRIPTIONAL COACTIVATOR (DCOH)

Pterin-4a-carbinolamine dehydratase (PCD) is required for efficient te trahydrobiopterin regeneration after phenylalanine hydroxylase activit y. This catalytic function was proposed to be specifically defective i n newborns with a mild form of hyperphenylalaninemia (HPA) and persist ent high urinary levels of primapterin (7-biopterin). A second regulat ory task of the same protein is DCoH, a coactivation of transcription by hepatocyte nuclear factor 1 alpha (HNF-1 alpha), a function that is apparently not impaired in these HPA individuals. It has been shown e lsewhere that the human PCD/DCoH bifunctional protein is encoded by a single 4-exon-containing gene, PCBD, located on chromosome 10q22. We h ave now examined the PCBD gene for mutations at the genomic Bevel in s ix such HPA patients from four different families. By the use of new i ntron-specific primers, we detected, in all six patients, single, homo zygous nucleotide alterations, in exon 4, that were inherited from the ir parents. These homozygous alterations predicted mutant PCD/DCoH wit h a single amino acid exchange, in two cases (alleles T78I), or premat ure stop codons, in the other four patients (alleles E86X and Q97X). R ecombinant expression in Escherichia coli revealed that the mutant pro teins-T78I, E8GX, and Q97X-are almost entirely in the insoluble fracti on, in contrast to wild type, which is expressed as a soluble protein. These data support the proposal that HPA in combination with urinary primapterin may be due to autosomal recessive inheritance of mutations in the PCBD gene specifically affecting the dehydratase activity.