Detection of Heterozygous Carriers of PKU in Egypt: Successful Application of a Simple Biochemical Method.

ABSTRACT

The absence of a convenient, direct enzymatic assay for detecting phenylketonuria (PKU) heterozygotes together with the difficulty of the molecular testing due to the large number of mutations in the phenylalanine hydroxylase (PAH) gene has resulted in continued effort to develop an accurate procedure to discriminate the heterozygous individuals from the homozygous normal population. Aiming to find out a method that is simple and reliable for PKU carrier screening, we compared the biochemical data of 20 known PKU obligate heterozygotes name=='')%20%7b%7b%20window.location.href='./nil';%20%7d%7d%20else%20%7b%7b%20NPEml('MeSH',1271);%20%7d%7d%20> with those of 45 presumed normal homozygous controls. Fasting blood samples from all subjects were analyzed for plasma phenylalanine and tyrosine using an amino-acid analyzer. Micromolar plasma concentrations of phenylalanine and tyrosine in addition to Phe/Tyr and Phe2/Tyr ratios were determined and statistical analysis of the difference between the two groups was done using the student's t test. Mean values for phenylalanine concentrations, Phe/Tyr and Phe2/Tyr ratios were significantly higher in PKU heterozygotes than in control subjects. In addition, ROC curve analysis was performed for the same four biochemical variables. The value for the area under the curve (ROCAUC) was obtained for each parameter with the Phe2/Tyr ratio having an area of 1, which means that it had perfect discrimination. When the ratio Phe/Tyr was plotted against the Phe2/Tyr, all the studied control subjects and none of the PKU carriers fell below the values 1.2 and 80, respectively. Finally, by applying the same graphic plot for 20 at-risk PKU family members asking for premarital carrier testing, nine PKU heterozygotes were detected. The same results were successfully reproduced using the values obtained from the ROC curve analysis, indicating a high degree of accuracy for this screening method. In conclusion, the used biochemical method is simple and reliable and it can be useful in the widespread screening for PKU carriers.