Heterozygous carriers in the relatives of a case of phenylketonuria

Different phenotypes for phenylalanine hydroxylase deficiency

Abnormalities related to the hydroxylation of phenylalanine to tyrosine are classified into three phenotypes based on the amount of dietary phenylalanine tolerated to keep serum phenylalanine within therapeutic levels, i.e., 180-425 micronmol/l(3-7 mg/100 ml) : (1) Classical phenylketonuria (PKU) tolerating approximately 17% of a normal intake of phenylalanine or approximately 0.12 mmol per kg body weight; (2) mild PKU with a tolerance some 50% higher; and (3) persistent hyperphenylalaninaemia (HPA) with serum phenylalanine values within therapeutic levels on a daily intake of greater than or equal to 0.7 mmol phenylalanine per kg body weight. Oral phenylalanine loading was performed on 100 heterozygotes for these abnormalities and 33 normal homozygotes. The slope of the rise in serum tyrosine multiplied by the maximum serum tyrosine concentration over the maximum phenylalanine concentration was the most powerful discriminant (D/-s, 3.54; overlapping 2.4%). Three heterozygous phenotypes were distinguished by this discriminant, and a significant correlation was observed between the phenotypic combination of the parents and the phenotype of their affected child. In particular, parents of children with classical PKU were clearly distinguished from heterozygotes for the other two abnormalities.

Heterozygote detection in phenylketonuria

Phenylalanine loading was carried out on 105 parents of children with phenylalanine hydroxylase deficiency and 33 apparently normal individuals with no family history of phenylketonuria. The best discriminant was found to be the logarithmic transformation of the slope of the rise in serum tyrosine multiplied by the maximum serum tyrosine concentration over the maximum serum phenylalanine concentration obtained after an oral load with a pure solution of L-phenylalanine. The overlap between heterozygotes for penylketonuria and normal homozygotes was 2.4 percent. The distribution of the discriminant values suggested three heterozygous phenotypes for phenylalanine hydroxylase deficiency, and the phenotypic combination of parents could be correlated to the phenotype of their affected offspring, i.e. classical phenylketonuria, mild phenylketonuria of hyperphenylalaninemia. The probability of heterozygosity for phenylketonuria was determined by means of the distribution of the discriminant values of the heterozygotes and that of normal homozygotes. The likelihood of being a heterozygote was corrected for the genetic background of the person requiring genetic counseling, and was finally expressed as the percentage probability of being a heterozygote for phenylketonuria.

A method for analysing fertility of heterozygotes for autosomal recessive disorders, with special reference to cystic fibrosis, Tay-Sachs disease and phenylketonuria

Increased fertility of heterozygotes with respect to decreased foetal loss among offspring of heterozygotes has been proposed by several authors as a possible explanation for the high gene frequency of CF, TSD and PKU in certain populations. Studies comparing reproductive outcome of heterozygotes with reproductive performance in the general population or in special control groups have been done on several occasions. These studies, however, are known to be heavily biased, on the one side by the fact that ascertainment of heterozygotes through affected offspinrg will tend to underestimate the relative frequency of smaller families, and on the other side because of the inadequacy of census data for comparison and the biases inherent in selection of control families. Careful analysis of the biases involved provides suggestions for proper corrections. From this a method has been developed which offers a better approach to the study of heterozygote fertility in those autosomal recessive conditions which lack a test for direct heterozygote detection.