Genotype–phenotype correlations in phenylketonuria: PAH variants and BH4 responsiveness for treatment design

Phenylketonuria (PKU) is an autosomal recessive disorder caused by PAH alterations, leading to elevated phenylalanine (Phe) and neurotoxicity. Newborn screening (NBS) and early therapy improve outcomes. This study aimed to characterize the molecular and phenotypic spectrum of PKU, explore genotype–phenotype correlations, and evaluate the relationship between PAH variants and BH4 responsiveness to guide personalized treatment. Medical and genetic records of 171 patients diagnosed with hyperphenylalaninemia via NBS were reviewed. Demographic data, diagnostic Phe and tyrosine (Tyr) levels, BH4 loading test results, and genetic analyses including Sanger sequencing, MLPA, and array CGH were assessed. Allelic and genotype–phenotype values were used to estimate variant severity. 168 carried ≥ 1 PAH variant; three had BH4 metabolism disorders. A total of 63 distinct PAH variants were identified; MLPA detected large deletions in three patients. Five variants (c.1066-11G > A, c.782G > A, c.143T > C, c.898G > T, c.1208 C > T) accounted for > 50% of the cohort. Among 100 patients tested, 41% responded to BH4, with c.782G > A most common (21%). Overall, 68.5% received dietary and/or BH4 therapy. This study contributes to our understanding of PKU’s molecular and phenotypic landscape, highlighting the importance of utilising both genotype and response to the BH4 loading test in the design of personalised treatments, beyond the initial Phe level.