Problems in the biochemical detection of heterozygotes for phenylketonuria

Improved identification of heterozygotes for phenylketonuria using blood neopterin and biopterin

A novel approach that combines information provided by the metabolism of pteridines and that of phenylalanine has been applied to the detection of heterozygotes for phenylketonuria. Phenylalanine, tyrosine, biopterin and neopterin have been measured in serum from normal controls and heterozygotes for classical phenylketonuria, before and after a phenylalanine oral load. Significant differences in neopterin and biopterin mean values in fasting serum and in the mean increase of biopterin induced by the phenylalanine load were found between groups. Inclusion of pteridine data in the discriminant analysis significantly improved the resolution of the classical phenylalanine loading test for the detection of heterozygotes for phenylketonuria.

A simplified PKU gene carrier detection test using fasting blood

By fluorometric analysis of fasting phenylalanine and tyrosine plasma levels, we could discriminate classic gene PKU carriers from non-carriers with 99% confidence in 67 of 74 adults. Results on the remaining seven subjects were non-discriminating. However, we could not determine their carrier status by other accepted testing parameters either (such as phenylalanine dosing). In contrast to the latter, our method: (1) allows for 90% of the population a relatively accurate but more benign test for carriers of the classical PKU gene (requiring only fluorometry on a single fasting blood specimen); (2) identifies the remaining 10% of the population who require the more cumbersome-noxious testing by phenylalanine dosing.

Linkage analysis using heterozygote detection in phenylketonuria

This linkage investigation was undertaken utilizing an improved method for phenylketonuria (PKU) heterozygote detection. This method is based on studies of semi-fasting, noon-time, blood specimens obtained from 85 obligate heterozgotes and 45 controls who were neither pregnant nor on birth control medication. The best separation between heterozygotes and normals was achieved with a discriminant function involving the logarithms of the serum concentrations of phenylalanine, tyrosine and tryptophan. The theoretical overlap area between the distributions of heterozygotes and controls, based on the above function, was between the distributions of heterozygotes and controls, based on the above function, was 3.75%. In 19 obligate heterozygotes and 13 controls who were either pregnant or on birth control medication, the best separation was achieved with a discriminant function involving the logarithms of the serum concentrations of phenylalanine and tyrosine. The theoretical overlap area was 8.23%. These equations identified heterozygotes with sufficient accuracy to permit efficient genetic linkage analysis. We were unable to demonstrate genetic linkage between the PKU locus and 15 common blood, serum, and urinary markers. All but loose linkage (theta greater than 0.3) was excluded for Rh, ABO, Gc, Kidd, and AP. Moderate linkage exclusion (theta less than 0.2) was shown for PGM, Duffy, Hp, MNS, HL--A, and Kell. Close linkage (theta less than 0.1) was excluded for Amy2, 6PGD, P, and ADA. We were unable to find linkage heterogeneity between the Amish and non-Amish populations.

Heterozygote detection in phenylketonuria. Measurement of discriminatory ability and interpretation of the phenylalanine loading test by determination of the heterozygote likelihood ratio

Two tests of heterozygosity for phenylketonuria, the phenylalanine/tyrosine ratio in the plasma after a fast and the response to an oral load of phenylalanine, have been compared in obligate heterozygotes for phenylketonuria and apparent normal homozygotes. The discriminatory ability of the tests is measured using both parametric and non-parametric methods, the correlation between them is determined, and the reproducibility of the loading test is examined. Because complete separation between the genotypes cannot be achieved by these tests, a method of interpretation which avoids possibly erroneous categoric statements and which enables the family history of a subject and his test result to be combined in a quantitative and meaningful way is described.

Classic phenylketonuria: diagnosis through heterozygote detection

Fifty-nine male and female (pregnant and nonpregnant) normal control subjects were differentiated from 18 male and female obligate heterozygotes for classic PKU. The method of genotyping utilized midday semifasting blood P and T quantitated by ion-exchange chromatography, an empiric determinant, P-2/T, and a discriminant function (sigma), in which sigma = a(1)(P)+a(2)(T);a(1) and a(2) were computed by multivariate analysis, This method of genotyping was applied in three clinical settings. In one family hyperphenylalaninemia was differentiated from classic PKU in two sisters with elevated blood P. In a second family maternal PKU was related to microcephaly in two retarded siblings, one of whom was also homozygous for classic PKU. In a third family a diagnosis of classic PKU was established in a normal-appearing infant whose blood P concentration was elevated.