Genotype‐phenotype correlations and BH₄ estimated responsiveness in patients with phenylketonuria from Rio de Janeiro, Southeast Brazil

Abstract

Background Genetic heterogeneity and compound heterozygosis give rise to a continuous spectrum of phenylalanine hydroxylase deficiency and metabolic phenotypes in phenylketonuria (PKU). The most used parameters for evaluating phenotype in PKU are pretreatment phenylalanine (Phe) levels, tolerance for dietary Phe, and Phe overloading test. Phenotype can vary from a “classic” (severe) form to mild hyperphenylalaninemia, which does not require dietary treatment. A subset of patients is responsive to treatment by the cofactor tetrahydrobiopterin (BH 4 ). Genotypes of PKU patients from Rio de Janeiro, Brazil, were compared to predicted and observed phenotypes. Genotype‐based estimations of responsiveness to BH 4 were also conducted. Methods Phenotype was defined by pretreatment Phe levels. A standard prediction system based on arbitrary assigned values was employed to measure genotype‐phenotype concordance. Patients were also estimated as BH 4 ‐responders according to the responsiveness previously reported for their mutations and genotypes. Results A 48.3% concordance rate between genotype‐predicted and observed phenotypes was found. When the predicted phenotypes included those reported at the BIOPKU database, the concordance rate reached 77%. A total of 18 genotypes from 30 patients (29.4%) were estimated as of potential or probable BH 4 responsiveness. Inconsistencies were observed in genotypic combinations including the common “moderate” mutations p.R261Q, p.V388M, and p.I65T and the mild mutations p.L48S, p.R68S, and p.L249F. Conclusion The high discordance rate between genotype‐predicted and observed metabolic phenotypes in this study seems to be due partially to the high frequency of the so‐called “moderate” common mutations, p.R261Q, p.V388M, and p.I65T, which are reported to be associated to erratic or more severe than expected metabolic phenotypes. Although our results of BH 4 estimated responsiveness must be regarded as tentative, it should be emphasized that genotyping and genotype‐phenotype association studies are important in selecting patients to be offered a BH 4 overload test, especially in low‐resource settings like Brazil.

Phenylalanine Hydroxylase (PAH) Genotyping in PKU Argentine Patients

“…As observed by others, discordant predictions were found mainly in the moderate group or the border of categories assigned. [21,23] Intrinsic conformational changes in the mutated enzymes driven by interallelic interaction may explain to some degree the inaccurate prediction in this group of patients [28,29,30].…”

Phenylketonuria in Portugal: Genotype–phenotype correlations using molecular, biochemical, and haplotypic analyses

“…Dietary restriction of phenylalanine remains the mainstay of treatment but PKU is an active area of research and new treatment options are emerging that might reduce the burden of the difficult and restrictive diet on patients and their families (Enacán et al, 2019; Giovannini et al, 2012; Harding & Blau, 2010; Sumaily & Mujamammi, 2017; Walter et al, 2002). On the contrary, patients with gene variants that determine a high residual enzyme activity (those with the mildest metabolic phenotypes) have a higher probability of responding to BH4 (Bueno et al, 2013; Michals‐Matalon et al, 2007; Rivera et al, 2011; Staudigl et al, 2011; Vieira Neto et al, 2019). In this regard, patients with a genotype known to be non‐BH4‐responsive should not undergo BH4 testing, while patients with a genotype with BH4‐responsive variations may directly proceed to a treatment trial rather than a BH4 loading test.…”

Phenylketonuria: Genes in phenylketonuria, diagnosis, and treatments

“…The encoded PAH enzyme converts phenylalanine to tyrosine 2 . Neto et al (2019) found that mutations in the PAH gene can include deletion with frame shift, in-frame deletion, large deletion, missesnse, nonsense, and splicing sites 5 (Table 2) (Wang et al, 2017). There have been identified PAH 1101 variants in PAHvdb Database; 6 about 50% are missense mutations.…”

“…Reduction of PAH leads to increase of phenylalanine in blood and brain (Van Wegberg, 2017). Four classification of PKU 4 (Neto et al, 2019) severe (higher than 1,200 µM), moderate (between 900 and 1,200 µM), mild (between 600 and 900 µM), and non-PKU mild (lower than 600 µM) (Neto et al, 2019).…”