Two missense mutations causing mild hyperphenylalaninemia associated with DNA haplotype 12.

Area of Science:

• Biochemistry
• Genetics
• Molecular Biology

Background:

• Phenylketonuria (PKU) and hyperphenylalaninemia (HPA) are genetic disorders primarily caused by mutations in the phenylalanine hydroxylase (PAH) gene.
• Over 50-60 mutations are documented in Caucasians, leading to a spectrum of clinical severities.
• Mutations are often associated with specific haplotypes, defined by polymorphic restriction sites within the PAH gene.

Purpose of the Study:

• To investigate the genetic basis of mild HPA in a Swedish population.
• To identify specific mutations linked to haplotype 12, hypothesized to harbor mild variants.
• To characterize the functional impact of newly identified PAH mutations.

Main Methods:

• Sequence analysis of the PAH gene in Swedish PKU/HPA patients.
• Haplotype analysis to correlate mutations with specific genetic backgrounds.
• Eukaryotic expression system to assess the in vitro enzyme activity of mutant PAH proteins.

Main Results:

• Two novel mutations in the PAH gene, A322G and R408Q, were identified and linked to haplotype 12.
• A322G and R408Q mutations were found in four and three affected haplotype 12 alleles, respectively.
• In vitro studies showed A322G mutant PAH retained approximately 75% and R408Q approximately 55% of wild-type enzyme activity, representing the highest in vitro activities reported for this system.

Conclusions:

• The identified mutations, A322G and R408Q, are associated with mild hyperphenylalaninemia in the Swedish population.
• These findings contribute to the understanding of genotype-phenotype correlations in PKU/HPA.
• The study highlights the importance of haplotype analysis in identifying disease-causing mutations.