Ectopic transcription: an application to the analysis of splicing errors in phenylalanine hydroxylase mRNA

Previous studies have identified various mutations in the phenylalanine hydroxylase (PAH) gene responsible for classical phenylketonuria. These mutations can explain 99% of mutant alleles in the Danish population (1). In many other populations phenylketonuria mutations have yet to be identified, especially in Orientals. For example, 40% of mutant alleles in Chinese and Japanese (unpublished data) and 60% in Turkish phenylketonuria patients (2) carry unknown mutations. This is probably due to the racial heterogeneity of the mutations, absence of predominant mutations and the laboriousness of sequencing all 13 exons of the PAH gene. There may be mutations buried deep in introns, which cause aberrant splicing. Analysis of mRNA is one method of overcoming these difficulties. Sequencing cDNA is less laborious than sequencing all exons. cDNA analysis can detect aberrant splicing even if the causative base substitutions cannot be identified around the exon‐intron boundaries.