Heterozygote detection in phenylketonuria

Phenylalanine hydroxylase gene in psychiatric patients: screening and functional assay of mutations

BACKGROUND

Reports relating phenylalanine kinetics and metabolism to psychiatric disorders led us to undertake the comprehensive screening of the phenylalanine hydroxylase (PAH) coding region and functional testing of discovered mutations in a sample of psychiatric patients and healthy control subjects.

METHODS

Genomic DNA from psychiatric patients and control subjects was assayed for sequence variants in all PAH coding regions and splice junctions. In vivo functional analysis of mutations was conducted by assessing the kinetics and conversion to tyrosine of a standardized phenylalanine dose and by measuring fasting pterin levels.

RESULTS

A known missense mutation was observed in a schizoaffective subject, and a novel missense mutation was discovered in four subjects with schizophrenia and one normal subject. The schizoaffective patient heterozygous for the known A403V mutation showed the lowest rate of phenylalanine kinetics and lowest conversion to tyrosine in the patient sample. The four schizophrenic patients heterozygous for the novel K274E mutation showed significantly decreased phenylalanine kinetics, reduced conversion to tyrosine, and increased synthesis of the PAH cofactor tetrahydrobiopterin compared with schizophrenic subjects without the mutation.

CONCLUSIONS

The study findings suggest that larger scale studies are warranted to test the relationship of the PAH genotype with a psychiatric phenotype.

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.

In vivo assessment of mutations in the phenylalanine hydroxylase gene by phenylalanine loading: characterization of seven common mutations

Mutations in the gene encoding phenylalanine hydroxylase (PAH) cause persistent hyperphenylalaninaemia. To date, more than 200 point mutations and microdeletions have been characterized. Each mutation has a particular quantitative effect on enzyme activity and recessive expression of different mutant alleles results in a marked interindividual heterogeneity of metabolic and clinical phenotypes. In this paper we demonstrate how a simple clinical test can be used to evaluate the correlation between mutation genotype and phenylalanine metabolism. In hyperphenylalaninaemic patients with known PAH mutation genotype, we have investigated phenylalanine turnover in vivo by measuring the ability to eliminate a test dose of L-phenylalanine. All patients could be considered functionally hemizygous for one of their mutant alleles by carrying on the other allele a mutation that is known to completely abolish PAH activity and encode a peptide with no immunoreactivity. Seven mutations (R408W, IVS-12nt1, R261Q, G46S, Y414C, A104D, and D415N) were characterized by oral phenylalanine loading, each mutation being represented by at least three patients. The elimination profile determined for a 3-day period provides a measure to compare residual activity of the mutant proteins and to assign each mutation to a particular metabolic phenotype. The established relation between genotype and phenotype may enable prediction of the severity of the disease by genotype determination in the newborn period. This will aid in the management of hyperphenylalaninaemia and may improve prognosis.

CONCLUSION

The possibility of predicting the residual enzyme activity by DNA analysis performed already in the newborn period allows the prompt implementation of a diet that is adjusted to the degree of PAH deficiency. This may improve management and prognosis of hyperphenylalaninaemia.

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.

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.

"Peripheral" tetrahydrobiopterin deficiency with hyperphenylalaninaemia due to incomplete 6-pyruvoyl tetrahydropterin synthase deficiency or heterozygosity

Four patients in three families with "peripheral" tetrahydrobiopterin deficiency were investigated. They were characterized biochemically by a tetrahydrobiopterin-responsive hyperphenylalaninaemia, a high neopterin/biopterin ratio in urine and plasma, and normal or elevated concentrations of biopterin, homovanillic acid, and 5-hydroxyindole acetic acid in cerebrospinal fluid. From measurements of the activity of erythrocyte 6-pyruvoyl tetrahydropterin synthase (PTS, formerly called phosphate-eliminating enzyme) and phenylalanine loading tests in the patients and their parents, one patient was demonstrated to be heterozygous for PTS deficiency. The others were obviously genetic compounds (allelism) with incomplete PTS deficiency. Three of the children developed normally, two of them under treatment with tetrahydrobiopterin. In the latter two patients, significantly lower concentrations of biopterin, homovanillic acid, and 5-hydroxyindole acetic acid in cerebrospinal fluid were noted at age 7 months (when treatment was interrupted) than those observed at 3 and 5 weeks, respectively. The infant who is heterozygous for PTS deficiency was born small for gestational age and showed a moderately delayed psychomotor development. It is concluded that "peripheral" tetrahydrobiopterin deficiency is caused by a partial PTS deficiency with sufficient activity to cover the tetrahydrobiopterin requirement of tyrosine 3-hydroxylase and trytophan 5-hydroxylase in brain but not enough for phenylalanine 4-hydroxylase in liver. For therapy, tetrahydrobiopterin, 2-5 mg/kg in a single oral dose per day, is recommended to keep plasma phenylalanine normal. A careful observation of the mental development is indicated.

A simple method for detection of heterozygous carriers of the gene for classic phenylketonuria

Plasma phenylalanine (Phe) and tyrosine (Tyr) concentrations were measured by high-performance liquid chromatography in arteriolar blood obtained by finger prick from 78 normal volunteers and 36 heterozygote carriers of phenylketonuria (PKU). Subjects were tested at midday, 3 to 4 hours after their breakfast. Values for the micromolar ratio of Phe/Tyr and Phe2/Tyr for control subjects fell below 1.2 and 80, respectively; values for all carriers of PKU were higher for one or both ratios. This method should be applicable to widespread screening of the general population for carriers of the gene for PKU.

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.