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

Neurochemistry and defects of biogenic amine neurotransmitter metabolism

The purpose of the current review is to present a brief background examining the mechanisms controlling synthesis, storage, release and action of the biogenic amine neurotransmitters and to provide examples of newly defined conditions that expand our awareness of the diversity and complexity of the inherited diseases that affect these important regulators of central and peripheral homeostasis.

Aberrant phenylalanine metabolism in phenylketonuria heterozygotes

The wide variation in phenylalanine hydroxylating capacity observed among patients with phenylketonuria (PKU) is primarily due to allelic heterogeneity at the phenylalanine hydroxylase (PAH) locus. In this study, we examined phenylalanine metabolism after an oral phenylalanine load in 148 carriers of known PAH gene mutations. As a group, heterozygotes formed less tyrosine than normozygotes (p < 0.001), and there was a tendency that carriers of a severe PAH mutation formed less tyrosine than carriers of a mild mutation. Nevertheless, the interindividual variation was extensive, and we identified a group of individuals who formed no or very little tyrosine after the phenylalanine load. This tyrosine response was accompanied by a decreased ability to eliminate the phenylalanine test dose but did not correlate with the intrinsic severity of the mutant PAH allele. Examination of the entire coding region of the PAH gene revealed no additional sequence alterations in these subjects. Our data suggest that a subset of PKU heterozygotes have reduced phenylalanine hydroxylating capacity approaching or equalling the levels observed in genetic compounds with non-PKU mild hyperphenylalaninaemia (MHP). Awareness of this phenotypic overlap between PKU carriers and genetic compounds with two mutant alleles may be useful for clinicians and paediatricians involved in diagnosis and genetic counselling.

Plasma phenylalanine and tyrosine levels revisited in heterozygotes for hyperphenylalaninaemia

We examined the value of the fasting plasma phenylalanine/tyrosine ratio obtained in an ordinary clinical setting for assessing the probability of being a heterozygote for hyperphenylalaninaemia. This biochemical test was found to be of little value in those with a high (66%) prior risk of heterozygosity, because it could not reduce the risk below 12%. However, in a population with a prior risk of only 2%, it discriminates the 3% with a 19% risk from the 97% with a risk of 1.5% or less. This simple method could usefully be applied to such a population, in order to select those at higher risk for further investigation using molecular genetics.

Heterozygous carriers of classical phenylketonuria in a sample of the Turkish population: detection by a spectrofluorimetric method

Blood obtained by finger prick from 209 presumed normal homozygotes and 42 heterozygotes for classical PKU was analyzed for plasma phenylalanine (Phe) and tyrosine (Tyr) by a fluorimetric method. Subjects were tested near midday and 3 hours after a protein-rich breakfast. The plot of Phe/Tyr (micromolar ratio) against Phe2/Tyr, permitted the detection of 11 heterozygotes among 209 controls. The accuracy of this method was checked by computation of a stepwise multivariate discriminant analysis, using Phe and Tyr (mumol/L), Phe/Tyr micromolar ratio and Phe2/Tyr as variables. Ten of the 11 subjects were recovered with a percentage of correct classification of over 90%, while one case had a percentage of 89.45%. The PKU gene frequency was found to be 1/19. This emphasizes the importance of a screening programme for PKU gene carrier status in Turkey.

Significant phenylalanine hydroxylation in vivo in patients with classical phenylketonuria

Indirect measurements have previously suggested that patients with classical phenylketonuria (PKU) do not convert significant amounts of phenylalanine to tyrosine. Low-dose continuous infusion techniques employing [2H5]phenylalanine and [2H2]tyrosine were used to quantitate in vivo phenylalanine hydroxylation in 10 subjects with classical phenylketonuria, 2 with hyperphenylalaninemia (HPA), and 7 controls. Plasma phenylalanine concentration ranged from 523 to 1,540 mumols/liter in PKU, 402 to 533 in HPA, and 49 to 54 in controls. Subjects with classical PKU hydroxylated mean +/- SD 4.8 +/- 2.2 mumols/kg per h (range 0.9-8.4) of phenylalanine to tyrosine and those with HPA 4.4 and 5.3, respectively. These rates were substantial in comparison with those in controls (6.3 +/- 1.6, 3.2-8.2). The significant hydroxylation in PKU and HPA subjects is likely to result from induction of activity of tyrosine hydroxylase towards phenylalanine by the greatly elevated phenylalanine concentration. The presence of such activity in PKU suggests that therapy aimed at promotion of this usually latent hydroxylating capacity may be a future alternative to dietary treatment of PKU.

A simplified test to detect PKU heterozygotes by discriminant analysis in mentally retarded children and their mothers

We have developed classification coefficients and an equation to detect heterozygotes for phenylketonuria. The combination of several variables (Phe, Phe/Tyr, Phe2/Tyr) gave a safe diagnosis in more than 96% of cases. We then computerized a random selection of our population, which was divided into two groups: the first was "selected" to compute discriminant functions, while the second, excluded from computation, was used to check the fitness of our method. Despite the reduction of sample size, 95.2% of unknown subjects were correctly classified. Finally, we used our equation to detect heterozygotes for phenylketonuria in a population of 26 children, affected by non-specific mental retardation, and their mothers. We found a high proportion of carriers for phenylketonuria, defined as subjects having a percent probability of correct classification higher than 90. By this method, heterozygosity was detected in two child-mother couples, four individual children and five mothers.

Detection of heterozygous carriers for phenylketonuria by a L-[2H5]phenylalanine stable isotope loading test

Oral loading with 25 mg/kg of pentadeuterated L-phenylalanine has been used for the discrimination between normozygous subjects and carriers for phenylketonuria. The test provides five types of data derived from plasma Phe and Tyr concentrations on which the discrimination can be based: fasting phenylalanine/tyrosine ratios, total Phe levels, total Phe/total Tyr ratios, absolute L-[2H5]phenylalanine plasma levels, and L-[2H5]Phe/L-[2H4]Tyr ratios. Absolute L-[2H4]Tyr and total L-Tyr concentrations provide the poorest discrimination with statistical classification errors around 30%. The corresponding classification error of fasting Phe/fasting Tyr ratios was circa 13%, and both labelled Phe/labelled Tyr and total Phe/total Tyr concentration ratios gave minimal errors below 2%.

Phenylketonuria with a progressive neurological disorder not responsive to tetrahydrobiopterin

ABSTRACT. Phenylketonuria with progressive neurological deterioration is now well known, and has usually been shown to be the result of a defect in biopterin metabolism. This report describes a baby with a progressive neurological disorder present from birth and marked hyperphenylalaninaemia. This responded biochemically, but not clinically, to restriction of phenylalanine in the diet but not to the administration of tetrahydrobiopterin, and no evidence of a defect in biopterin metabolism was detected.

The genetic linkage between the PKU locus and the loci for amylase 1, amylase 2, Fy, PGM 1, and Rh and the question of assignment of the PKU locus to chromosome no. 1

The alpha-amylase loci Amy 1 and Amy 2 and other loci on chromosome 1 were investigated for their linkage relationship to the PKU locus. Ten families were informative for the study of linkage between PKU/Amy, 20 for PKU-Fy, 11 for PKU/PGM 1, and 10 for PKU/Rh linkage. The probabilities of linkage at different recombinant fractions were calculated according to Bayes' theorem. The results are in striking contrast with those of Kamaryt et al. who found strong evidence for close linkage between the amylase loci and the PKU locus, whereas with our results close linkage can be excluded; loose linkage is possible but unlikely. The results are discussed with regard to the genetic heterogeneity of phenylketonuria.

Absent phenylalanine hydroxylase activity without phenylketonuria

A male infant is described who never manifested phenylketonuria even though phenylalanine hydroxylase activity was undetectable in liver tissue. Plasma phenylalanine were elevated in the range typical of PKU patients when the baby was at breast and declined with institution of a low phenylalanine diet. Physical and psychomotor development were normal with the baby on the latter treatment. The results indicate that the absence of phenylketonuria does not rule out phenylalanine hydroxylase activity.

Phenylketonuria heterozygote detection in families with affected children

Improved approaches to the problem of heterozygote detection for phenylketonuria (PKU) were developed in this study. The discrimination was based on 85 obligate heterozygotes and 45 controls who were neither pregnant nor on birth control medication. The best separation between hetrozygotes and normals was achieved with a linear 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 3.75%. In the 19 obligate hetrozygotes and 13 controls who were either pregnant or on birth control medication, the best separation was achieved with a linear discriminant function involving the logarithms of the serum concentrations of phenylalanine and tyrosine. The theoretical overlap area was 8.23%. The genetic accuracy of the discriminant function was confirmed by testing the results with parental-child exclusions, segregation analysis, and the frequency of heterozygosity in nonrelated collateral spouses. Finally, there was evidence suggesting that the antihypertensive agent, aldomet, alters serum tyrosine and tryptophan levels.

Serum tyrosine within the first hour after an oral load of phenylalanine

Twenty-seven heterozygotes for phenylketonuria (PKU) and sixteen normal homozygotes were loaded with an amount of L-phenylalanine per body mass = 0.6 mmol/kg. Serum tyrosine concentration increased significantly within 15 min after the intake and the increase was rectilinear within the first 30 min. The initial rate of increase in serum tyrosine in heterozygotes was 0.47 mumol/l/min (range 0.20-0.98 mumol/l/min) and in normal homozygotes 1.2 mumol/l/min (range 0.80-1.9 mumol/l/min). The median serum tyrosine concentration increased within the first hour after an oral phenylalanine load (0.6 mmol/kg) of twelve infants with persistent hyperphenylalaninaemia (HPA), whereas serum tyrosine showed a decrease in forty infants with classical PKU. In ninetten infants with mild PKU serum tyrosine remained unchanged within the first hour after the load and then declined.

Phenylalanine metabolism and intellectual functioning among carriers of phenylketonuria and hyperphenylalaninaemia

All members of 63 families who had phenylketonuric or hyperphenylalaninaemic offspring received a phenylalanine tolerance test and an individual intelligence test. Parent carriers (heterozygotes, n=115) displayed a significant relationship (canonical correlation R=0.75, P is less than 0.05) between their ability to metabolise phenylalanine and their intellectual strengths and weaknesses. Mothers of hyperphenylalaniaemic children did not show this relationship. The number of carrier children (n=40) was too small for multivariate or sex analysis, but significant bivariate correlations were found for this group.

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.

Rapid micromethod for the preparation of leucocyte-free haemolysates for the determinaton of pyruvate kinase and other erythrocyte enzymes

In pyruvate kinase deficiency the high activity of the leucocyte isoenzyme may mask the erythrocyte defect. Separation of leucocytes from erythrocytes bythe commonly used sedimentation or filtration procedures requires rather large volumes of blood, is time-consuming, and sometimes leaves an unsatisfactorily large proportion of leucocytes. In the present study at least 95 % of leucocytes, measured with the lysosomal enzyme beta-N-acetylhexosaminidase as a very sensitive marker for leucocyte lysis, were consistently removed by differential lysis of erythrocytes in hypotonic saline. Haemolysates suitable for erythrocyte enzyme assays can be prepared from very small quantities of capillary blood (10-100 micro1) very rapidly, cheaply, and reproducibly by this method.