Phenylketonuria is a paradigm of monogenic metabolic disorder, relatively common in populations of European extraction (1/11,031 live births in Portugal) and the one for which screening and therapeutic public health programs have been implemented first in developed countries.

The mutation spectrum is extremely wide: more than 500 different mutations of the responsible gene (phenylalanine hydroxylase, PAH) gene have been identified.

The pathogenicity of many has been solidly established, but for the rarer ones some doubts remain on their causative importance. Moreover, for economic reasons, the search for the mutational basis by partial sequencing usually stops whenever a previously putatively pathogenic mutation is detected.

The haplotype background of the mutations is still less studied. All these three factors have prevented a solid approach to the establishment of unbiased phenotype/genotype correlation.

We intend to provide a sounder basis to the epidemiological analyses and therapy efficiency evaluation, by

(a) complete exon sequencing, disentangling disease causing mutations from trivial polymorphisms and providing the basis for the study of intragenic analysis of compensatory or aggravating effects

(b) typing of ancillary markers (non-coding SNPs and STRs) allowing the distinction of identity-by-state from identity- by-descent cases and therefore the evaluation of inbreeding and familial effects as well as the historical demography of the mutations and

(c) the evaluation of cis-acting effects and the detection of undisclosed deletion events through the establishment of extended informative haplotypes. The comprehensive characterization of the PKU cases from a genetic standpoint will contribute to the understanding of the relative weight of genetic variation at the coding region and environmental exposures in the observed phenotypic spectrum

(d) identifying clusters in the geographical distribution of PKU cases in the north of Portugal, according to genotypic, metabolic phenotype and clinical characteristics, and

(e) quantifying the association between genotype and biochemical and clinical profile, accounting for sociodemographic factors and adherence to dietary restrictions.

Using the information obtained from the patients previously diagnosed and followed we will derive models that can provide objective prognostic estimates to supplement clinicians' intuition and judgment when counselling patients and their families about the meaning of health problems and utility of treatment accomplishment.

It can also be useful in identifying groups at high risk for poor outcomes in whom targeted treatment interventions may need a special support to achieve the Phe-restricted diet in order to maximize the potential effect of the Portuguese Neonatal Screening Program.