Effect of tetrahydrobiopterin on Phe/Tyr ratios and variation in Phe levels in tetrahydrobiopterin responsive PKU patients

doi:10.1016/j.ymgme.2011.05.011

Molecular Genetics and Metabolism

Volume 104, Issues 1–2, September–October 2011, Pages 89-92

https://doi.org/10.1016/j.ymgme.2011.05.011 Get rights and content

Abstract

Background

Whilst a reduction in blood phenylalanine (Phe) levels is essential in patients with PKU, a decrease in Phe/Tyrosine (Tyr) ratio and fluctuations in blood Phe levels over time have been recently associated with improved neuropsychological outcome. The aim of this study was to identify if Tetrahydrobiopterin (BH₄) offers additional benefit based on the assumption that these 2 factors are beneficial.

Method

Since 2002, 9 patients identified through NBS as BH₄ responsive (BH₄ group) and 25 non-responsive patients (non-BH₄ group) produced a total of 1384 and 4415 samples, respectively, for analysis. Statistical analysis was performed to compare mean and median Phe levels, Tyr levels and Phe/Tyr ratios in BH₄ and non BH₄ responsive patients.

Results

Variations in blood Phe levels were greater in the non-BH₄ group (BH₄: median 338 μmol/L, 95% Confidence Interval (CI) 329–346, mean: 358 μmol/L, CI 350–366; non-BH₄: median 338 μmol/L CI 332–344, mean: 370 μmol/L CI 364–376). Variations in blood Tyr levels were slightly greater in the non-BH₄ group: (BH₄: median 59 μmol/L CI 58–61, mean 67 CI 66–69; non-BH₄: median 62 μmol/L CI 61–63, mean 70 CI 69–71).

The variation in Phe/Tyr ratios was greater in the non-BH₄ group (mean 6.12, CI 5.9–6.3) than in the BH₄ group (Mean 5.44, CI 5.3–5.6), particularly at blood Phe levels > 600 μmol/L.

Conclusion

BH₄ responsive patients have smaller variations in blood Phe levels and tighter Phe/Tyr ratios than non-BH₄ responsive patients, particularly at high blood Phe levels. If decreased fluctuations in Phe levels and a decreased Phe/Tyr ratio are indeed neuro-protective, then BH₄ responsiveness is advantageous over diet alone in PKU. Neuropsychological testing in patients who have been treated with BH₄ long term may be able to ascertain the clinical benefit of these biochemical findings.

Introduction

Dietary restriction of phenylalanine (Phe) has been the basis of treatment of patients with classical phenylketonuria (PKU) for over 40 years, with recommended optimal blood Phe levels slightly differing between centres [1], [2], [3], [4]. Several recent reports indicate that some patients with abnormally low phenylalanine hydroxylase enzyme activity due to a mutation in its encoding gene and a normal BH₄ endogenous production respond to pharmacological doses of BH₄ by lowering or even normalising blood Phe levels [5], [6], [7], [8], [9], [10]. This observation is important that it offers a much needed alternate mode of therapy for some PKU patients [9]. In practise, there is a variability in the extent of BH₄ responsiveness and high doses of BH₄, up to 20 mg/kg/day, have been prescribed in treating some patients [11]. Alternatively, a combination of BH₄ and modified diet has been prescribed to some patients [10], [12].

In addition to ‘optimal’ blood Phe levels, two parameters that have been associated with better clinical outcome have been proposed in recent years as indicators of good metabolic control. The first relates to Phe to Tyrosine (Tyr) (Phe/Tyr) ratios. Luciana et. al. have shown that high Phe/Tyr ratios are associated with poor performance on tests of executive function and suggested that Phe/Tyr ratios could be more important than blood Phe levels only as determinants of long term cognitive performance. They suggested that the Phe/Tyr ratio be incorporated in the evaluation of dietary control of PKU, in addition to the usual monitoring of blood Phe [13]. The second parameter relates to the variability of blood Phe levels over time. Anastasoaie et al. have reported that a decreased variation in Phe levels over time was associated with better neuropsychological outcome and suggested that variability in blood Phe levels should be monitored in addition to the actual blood Phe levels over time [14].

The aim of the current study was to test whether BH₄ offers additional benefit to patients with PKU, based on the assumption that the two monitoring parameters mentioned above are truly beneficial. To this end, we compared the Phe/Tyr ratios and blood Phe variability over time in BH₄ responsive patients with those in BH₄ non-responsive patients.

Section snippets

Patients and methods

Since 2002, all newborn babies in Victoria detected as having hyperphenylalaninaemia above 400 μmol/L on first screening test have had a BH₄ load (20 mg/Kg, 6R-5,6,7,8-tetrahydro-l-biopetrin dehydrochloride; Schricks Laboratories, Switzerland) before initiation of treatment, according to previously published protocols [7], [8]. We stratified the patients according to their response to BH₄: Between October 2002 and December 2010, a total of nine newborns were identified as BH₄ responsive (BH₄) and

Results

There were 1384 and 4415 blood samples from the BH₄ and non-BH₄ groups, respectively. Overall, the medians of blood Phe and Tyr levels in both BH₄ responders and non responders were the same (Median Phe BH₄: 338 μmol/L, non-BH₄: 337 μmol/L) but the 95% CI differed between the two groups (Table 1). The general distribution of blood Phe levels and Phe/Tyr ratios in the two groups did not differ significantly but the percentage of samples with Phe levels above 600 μMol/L was slightly higher in the

Discussion

The observation that pharmacological doses of BH₄ can lead to normalisation of blood Phe levels in patients with PKU due to phenylalanine hydroxylase deficiency constituted a significant milestone in the treatment of these patients [5]. It is estimated that BH₄ can be added and partially or completely replace dietary management in a substantial number of patients with PKU [6], [7], [8], [9], and this has also been our experience over eight years [10]. In this study we wished to elucidate the

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Cited by (18)

Management of phenylketonuria in European PKU centres remains heterogeneous
2024, Molecular Genetics and Metabolism
Phenylketonuria (PKU) is a genetic disorder that follows an autosomal recessive inheritance pattern. Dietary treatment is the cornerstone of therapy and is based on natural protein restriction, Phe-free L-amino acid supplements (protein substitutes) and low protein foods. The aim of this project was to collect information about the clinical management of patients with PKU, focusing on understudied or unresolved issues such as blood phenylalanine (Phe) fluctuations and clinical symptoms, particularly gastro intestinal (GI) discomfort and sleep problems.
The survey consisted of 10 open-ended and 12 multiple-choice questions that collected information about size of the PKU population in each center, the center's clinical practices and the outcomes observed by the center concerning adherence, clinical and biochemical abnormalities and clinical symptoms (GI and sleep). The questionnaire was sent to 72 experts from metabolic centers in 11 European countries. Thirty-three centers answered.
The results of this survey provide information about the clinical practice in different age groups, concentrating on dietary tolerance, treatment adherence, and metabolic control. All the centers prescribed a Phe-restricted diet, with Phe-free/low Phe protein substitutes and low protein foods. Daily doses given of protein substitutes varied from 1 to 5, with adherence to the prescribed amounts decreasing with increasing age. Respondents identified that improvement in the flavor, taste, volume and smell of protein substitutes may improve adherence. Finally, the survey showed that clinical symptoms, such as GI discomfort and sleep problems occur in patients with PKU but are not systematically evaluated. Twenty-four-hour Phe fluctuations were not routinely assessed.
The results highlight a strong heterogeneity of approach to management despite international PKU guidelines. More clinical attention should be given to gastrointestinal and sleep problems in PKU.
Phenylalanine hydroxylase deficiency treatment and management: A systematic evidence review of the American College of Medical Genetics and Genomics (ACMG)
2023, Genetics in Medicine
Elevated serum phenylalanine (Phe) levels due to biallelic pathogenic variants in phenylalanine hydroxylase (PAH) may cause neurodevelopmental disorders or birth defects from maternal phenylketonuria. New Phe reduction treatments have been approved in the last decade, but uncertainty on the optimal lifespan goal Phe levels for patients with PAH deficiency remains.
We searched Medline and Embase for evidence of treatment concerning PAH deficiency up to September 28, 2021. Risk of bias was evaluated based on study design. Random-effects meta-analyses were performed to compare IQ, gestational outcomes, and offspring outcomes based on Phe ≤ 360 μmol/L vs > 360 μmol/L and reported as odds ratio and 95% CI. Remaining results were narratively synthesized.
A total of 350 studies were included. Risk of bias was moderate. Lower Phe was consistently associated with better outcomes. Achieving Phe ≤ 360 μmol/L before conception substantially lowered the risk of negative effect to offspring in pregnant individuals (odds ratio = 0.07, 95% CI = 0.04-0.14; P < .0001). Adverse events due to pharmacologic treatment were common, but medication reduced Phe levels, enabling dietary liberalization.
Reduction of Phe levels to ≤360 μmol/L through diet or medication represents effective interventions to treat PAH deficiency.
Effect of BH4 on blood phenylalanine and tyrosine variations in patients with phenylketonuria
2021, Molecular Genetics and Metabolism
Citation Excerpt :
However, data on items like the effect of BH4 on Phe and Tyr variation is still limited. There are some data indicating that BH4 does not only decrease blood Phe concentrations (in BH4 responsive PKU patients) but also results in less variability in blood Phe concentrations [17–19]. However, these data were retrospectively collected.
In patients with phenylketonuria, stability of blood phenylalanine and tyrosine concentrations might influence brain chemistry and therefore patient outcome. This study prospectively investigated the effects of tetrahydrobiopterin (BH4), as a chaperone of phenylalanine hydroxylase on diurnal and day-to-day variations of blood phenylalanine and tyrosine concentrations.
Blood phenylalanine and tyrosine were measured in dried blood spots (DBS) four times daily for 2 days (fasting, before lunch, before dinner, evening) and once daily (fasting) for 6 days in a randomized cross-over design with a period with BH4 and a period without BH4. The sequence was randomized. Eleven proven BH4 responsive PKU patients participated, 5 of them used protein substitutes during BH4 treatment. Natural protein intake and protein substitute dosing was adjusted during the period without BH4 in order to keep DBS phenylalanine levels within target range. Patients filled out a 3-day food diary during both study periods. Variations of DBS phenylalanine and Tyr were expressed in standard deviations (SD) and coefficient of variation (CV).
BH4 treatment did not significantly influence day-to-day phenylalanine and tyrosine variations nor diurnal phenylalanine variations, but decreased diurnal tyrosine variations (median SD 17.6 μmol/l, median CV 21.3%, p = 0.01) compared to diet only (median SD 34.2 μmol/l, median CV 43.2%). Consequently, during BH4 treatment diurnal phenylalanine/tyrosine ratio variation was smaller, while fasting tyrosine levels tended to be higher.
BH4 did not impact phenylalanine variation but decreased diurnal tyrosine and phenylalanine/tyrosine ratio variations, possibly explained by less use of protein substitute and increased tyrosine synthesis.
Pretreatment cognitive and neural differences between sapropterin dihydrochloride responders and non-responders with phenylketonuria
2017, Molecular Genetics and Metabolism Reports
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In addition, biochemical [22,25] and genetic [22,26,34] variables may differ between responders and non-responders. For example, in the biochemical domain, Humphrey et al. [25] found that non-responders typically have greater variability in blood Phe and a higher Phe to tyrosine ratio than responders. In the genetic domain, Karačić et al. [26] linked BH4 responsiveness to mutations on the 12q22-24 chromosome of the gene encoding PAH.
Sapropterin dihydrochloride (BH₄) reduces phenylalanine (Phe) levels and improves white matter integrity in a subset of individuals with phenylketonuria (PKU) known as “responders.” Although prior research has identified biochemical and genotypic differences between BH₄ responders and non-responders, cognitive and neural differences remain largely unexplored. To this end, we compared intelligence and white matter integrity prior to treatment with BH₄ in 13 subsequent BH₄ responders with PKU, 16 subsequent BH₄ non-responders with PKU, and 12 healthy controls. Results indicated poorer intelligence and white matter integrity in non-responders compared to responders prior to treatment. In addition, poorer white matter integrity was associated with greater variability in Phe across the lifetime in non-responders but not in responders. These results underscore the importance of considering PKU as a multi-faceted, multi-dimensional disorder and point to the need for additional research to delineate characteristics that predict response to treatment with BH_4.
The challenge of long-term tetrahydrobiopterin (BH<inf>4</inf>) therapy in phenylketonuria: Effects on metabolic control, nutritional habits and nutrient supply
2015, Molecular Genetics and Metabolism Reports
Citation Excerpt :
As a consequence, quality of life, therapy adherence and thus, long-term metabolic control and outcome of these patients might be improved [1,5]. Moreover, there is some evidence for a stabilization of Phe concentrations under BH4 therapy [6,7]. This is of importance as the stability of Phe concentrations in the organism seems to have a relevant influence on long-term cognitive outcome [6,8,9,10].
BH₄-sensitive phenylketonuria (PKU) patients relax their phenylalanine (Phe) restricted diet due to increased Phe tolerance, while keeping dried blood Phe concentrations with in the therapeutic range. We aimed to investigate metabolic control, eating habits and nutrient supply under long-term BH₄-therapy.
Retrospective analysis of mean dried blood Phe concentrations and their variability, food and nutrient intake in BH₄-sensitive patients (n = 8, 3f, age 6.0–16.6 y) under classical dietary treatment for one year and during the three years after initiation of BH₄.
Phe concentrations of BH₄-sensitve PKU patients remained within therapeutic range throughout the observation period, independent of therapeutic regime. Under BH₄, Phe tolerance increased significantly (493.2 ± 161.8 mg/d under classical diet vs 2021.93 ± 897.4 mg/d two years under BH₄; P = 0.004). Variability of Phe concentrations remained unchanged (mean SD; P = 1.000). Patients adjust their food choice and significantly increased their intake of cereals, potatoes, dairy products and meat (P = 0.019, P = 0.016, P = 0.016 and P = 0.016, respectively). Under diet changes after implementation of BH₄ a drop in micronutrient intake (vitamin D, folic acid, iron, calcium, iodine) could be revealed (P = 0.005, P < 0.001, P = 0.004, P = 0.001, P = 0.003, respectively).
BH₄-sensitive PKU patients can achieve good metabolic control under an adjuvant BH₄- or a BH₄ monotherapy. The liberalized diet under BH₄ seems to jeopardize the quality of patients' nutrition, and these patients require close follow-up and special nutrition education to minimize the risk for imbalanced diet and nutrient deficiencies.
6R-tetrahydrobiopterin treated PKU patients below 4years of age: Physical outcomes, nutrition and genotype
2015, Molecular Genetics and Metabolism
Citation Excerpt :
This shift towards less stringent diets improves the quality of life of patients and their families by lessening social and economic burdens associated with these diets [2]. In addition, 6R-BH4 treatment effectively reduces the fluctuation of blood Phe levels, a phenomenon usually observed in PKU patients exclusively and continuously treated with Phe-restricted diets [3–5]. Avoiding high variability in blood Phe levels seems to be crucial, subtle neurological differences having been noted in PKU patients on protein-deprived diets when compared with PKU patients on 6R-BH4 treatment [6].
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.
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.
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 12 months). Patients on 6R-tetrahydrobiopterin increased their phenylalanine intake (from 49.1 [25.6–60.3] to 56.5 [39.8–68.3] mg kg^− 1 day^− 1) and natural protein intake (from 1.0 [0.8–1.7] to 1.5 [1.0–1.8] g kg^− 1 day^− 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.
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.

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^☆: A preliminary report of this study was presented at the 11th ICIEM, San Diego, 2009 (Molecular Genetics and Metabolism, 98: 25, 2009).

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Effect of tetrahydrobiopterin on Phe/Tyr ratios and variation in Phe levels in tetrahydrobiopterin responsive PKU patients☆

Abstract

Background

Method

Results

Conclusion

Introduction

Section snippets

Patients and methods

Results

Discussion

Lancet

Arch. Pediatr.

J. Pediatr.

Mol. Genet. Metab.

Mol. Genet. Metab.

Mol. Genet. Metab.

Mol. Genet. Metab.

Rationale for the German recommendations for phenylalanine level control in phenylketonuria 1997

Eur. J. Pediatr.