Genotype-phenotype associations in French patients with phenylketonuria and importance of genotype for full assessment of tetrahydrobiopterin responsiveness

Abstract

BackgroundMutations in Phenylalanine Hydroxylase (PAH) gene cause phenylketonuria. Sapropterin (BH4), the enzyme cofactor, is an important therapeutical strategy in phenylketonuria. However, PAH is a highly polymorphic gene and it is difficult to identify BH4-responsive genotypes. We seek here to improve prediction of BH4-responsiveness through comparison of genotypes, BH4-loading test, predictions of responsiveness according to the literature and types and locations of mutations.MethodsA total of 364 French patients among which, 9 % had mild hyperphenylalaninemia, 17.7 % mild phenylketonuria and 73.1 % classical phenylketonuria, benefited from a 24-hour BH4-loading test and had the PAH gene sequenced and analyzed by Multiplex Ligation Probe Amplification.ResultsOverall, 31.6 % of patients were BH4-responsive. The number of different mutations found was 127, including 26 new mutations. The mutations c.434A > T, c.500A > T, c.529G > C, c.1045 T > G and c.1196 T > C were newly classified as being BH4-responsive. We identified 261 genotypes, among which 46 were newly recognized as being BH4-responsive. Even though patients carry 2 responsive alleles, BH4-responsiveness cannot be predicted with certainty unless they present mild hyperphenylalaninemia. BH4-responsiveness cannot be predicted in patients carrying one responsive mutation only. In general, the milder the phenotype is, the stronger the BH4-response is. Almost exclusively missense mutations, particularly in exons 12, 11 and 8, are associated with BH4-responsiveness and any other type of mutation predicts a negative response.ConclusionsThis study is the first of its kind, in a French population, to identify the phenotype associated with several combinations of PAH mutations. As others, it highlights the necessity of performing simultaneously BH4 loading test and molecular analysis in monitoring phenylketonuria patients.Electronic supplementary materialThe online version of this article (doi:10.1186/s13023-015-0375-x) contains supplementary material, which is available to authorized users.

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

“…() and two compound heterozygotes with a null mutation reported by Jeannesson‐Thivisol et al. () were BH 4 nonresponsive. v In the BIOPKU database, one compound heterozygote with a null mutation was BH 4 responsive.…”

“…() as responsive out of a total of nine patients from the same authors, BIOPKU database, and Jeannesson‐Thivisol et al. (). k Two (4,3%) patients reported at the BIOPKU database as responsive out of a total of 46 patients from this same database, Aldamiz‐Echevarria et al.…”

“…Comparison of the inverse of pretreatment phenylalanine (Phe) levels (mg/dl) with the predicted phenylalanine hydroxylase (PAH) enzyme activity for a total of 81 patients for which mutant PAH residual activity (PRA) for both alleles and pretreatment Phe levels were both known. Mean PRA for each genotype was established by data in BIOPKU database and previously published articles (Aldamiz‐Echevarria et al., ; Eisensmith et al., ; Guldberg et al., ; Jeannesson‐Thivisol et al., ; Pey et al., ; Trunzo et al., ). A significant correlation ( t = 4.79, p < 0.0001) was found by linear regression analysis: y = 0.0007 x + 0.0379, R 2 = 0.225…”

“…(), and Jeannesson‐Thivisol et al. (). g Based on data from 26 patients in the BIOPKU database, 18 homozygotes and eight compound heterozygotes with a null mutation.…”

“…All patients were completely genotyped formerly by our group (Vieira Neto et al, 2018). Table 1 presents the mutations found in these patients and their predicted PAH residual activities (null mutations pointed out) as indicated by Himmelreich et al (2018), from previous publications (Aldamiz-Echevarria et al, 2016;Eisensmith et al, 1996;Guldberg et al, 1998;Jeannesson-Thivisol et al, 2015;Pey et al, 2003;Trunzo et al, 2016) or judged as null in the case of nonsense mutations, frameshift and large deletions whose PAH residual activities were not formerly reported.…”