We report a branch site mutation in the gene of the enzyme tyrosine hydroxylase (TH): a −24t
> a substitution two bases upstream of the adenosine in the branchpoint sequence (BPS) of intron
11. As normal lariat formation is therefore prevented, alternative splicing takes place: use of the BPS
of intron 12 results in skipping of exon 12, whereas the use of a cryptic branch site in intron 11 leads
to partial retention of this intron in the mRNA. This leads in both cases to an aberrant protein
product. In the one case, skipping of exon 12 results in the absence of 32 amino acids. In the other,
retention of 36 nucleotides of intron 11 in the mRNA results in the incorporation of twelve additional
amino acids. The functional consequences of this mutation for the patient, who is also heterozygous
for another previously identified mutation, become apparent in a severe clinical phenotype.