6R-tetrahydrobiopterin treated PKU patients below 4years of age: Physical outcomes, nutrition and genotype

Abstract

Background and Aims: Phenylalanine-restricted diets have proven effective in treating phenylketonuria. However, such diets have occasionally been reported to hinder normal development. Our study aimed to assess whether treating 0-4-year-old phenylketonuric patients with 6R-tetrahydrobiopterin might prevent growth retardation later in life. Methods: We conducted a longitudinal retrospective study which examined anthropometric characteristics of phenylketonuric patients on 6R-tetrahydrobiopterin therapy (22 subjects), and compared them with a group of phenylketonuric patients on protein-restricted diets (44 subjects). Nutritional issues were also considered. We further explored possible relationships between mutations in the PAH gene, BH4 responsiveness and growth outcome. Results: No significant growth improvements were observed in either the group on 6R-tetrahydrobiopterin treatment (height Z-score: initial= -0.57±1.54; final=-0.52±1.29; BMI Z-score: initial=0.17±1.05; final=0.18±1.00) or the diet-only group (height Z-score: initial=-0.92±0.96; final= -0.78±1.08; BMI Z-score: initial=0.17±0.97; final=-0.07±1.03) over the 1-year observation period. Furthermore, we found no significant differences (p>0.05) between the two groups at any of the time points considered (0, 6 and 12months). Patients on 6R-tetrahydrobiopterin increased their phenylalanine intake (from 49.1 [25.6-60.3] to 56.5 [39.8-68.3] mgkg-1day-1) and natural protein intake (from 1.0 [0.8-1.7] to 1.5 [1.0-1.8] gkg-1day-1), and some patients managed to adopt normal diets. Higher phenylalanine and natural protein intakes were positively correlated with better physical outcomes in the diet-only group (p<0.05). No correlation was found between patient genotype and physical outcomes, results being similar regardless of the nutritional approach used. We did not detect any side effects due to 6R-tetrahydrobiopterin administration. Conclusions: Our study indicates that treating 0-4-year-old phenylketonuric patients with 6R-tetrahydrobiopterin is safe. However, poor developmental outcomes were observed, despite increasing the intake of natural proteins. Genotype could be a valid predictor of tetrahydrobiopterin-responsiveness, since patients who carried the same genotype responded similarly to the 6R-tetrahydrobiopterin loading test. On the other hand, harbouring 6R-tetrahydrobiopterin responsive genotypes did not predispose patients to better physical outcomes.