Retrospective analysis of 19 patients with 6-Pyruvoyl Tetrahydropterin Synthase Deficiency: Prolactin levels inversely correlate with growth

Abstract

Background

Pyruvoyl Tetrahydropterin Synthase (PTPS) Deficiency is the most common form of BH4 deficiency resulting in hyperphenylalaninemia. It can have variable clinical severity and there is limited information on the clinical presentation, natural history and effectiveness of newborn screening for this condition.

Methods

Retrospective data (growth and clinical parameters, biochemical and genetic testing results, treatment) were collected from 19 patients with PTPS deficiency in different centers, to evaluate biochemical and clinical outcomes. Descriptive statistics was used for qualitative variables, while linear regression analysis was used to correlate quantitative variables.

Results

Patients with PTPS deficiency had an increased incidence of prematurity (4/18) with an average gestational age only mildly reduced (37.8 ± 2.4 weeks) and low birth weight (−1.14 ± 0.97 SD below that predicted for gestational age). With time, weight and height approached normal. Values. All patients were identified by newborn screening for an elevated phenylalanine level. However, phenylalanine levels were normal in two whose testing was performed at or before 24 h of age. Sapropterin dihydrochloride treatment normalized phenylalanine levels. Molecular testing identified novel variants in the PTS gene, some of which present in more than one affected family. The neurotransmitter derivatives 5-hydroxyindoleacetic acid (5HIAA) and homovanillic acid (HVA) in the CSF were decreased in most cases except in 2 families with the peripheral form of PTPS deficiency. With time, HVA and 5HIAA became abnormally low in two of these patients requiring therapy. Prolactin (whose secretion is inhibited by dopamine) levels were elevated in several patients with PTPS deficiency and inversely correlated with the z-scores for height (p < 0.01) and weight (p < 0.05). Most patients with PTPS deficiency had delayed development early in life, improving around school age with IQs mostly in the normal range, with a small decline in older individuals. From a neurological standpoint, most patients had normal brain MRI and minor EEG anomalies, although some had persistent neurological symptoms.

Discussion

Patients with PTPS deficiency have not only an increased incidence of prematurity, but also decreased birth weight when corrected for gestational age. Hyperphenylalaninemia can be absent in the first day of life. Therapy with sapropterin dihydrochloride normalizes phenylalanine levels and neurotransmitter precursors can improve CSF neurotransmitter metabolites levels. Insufficient dopaminergic stimulation (as seen from elevated prolactin) might result in decreased height in patients with PTPS deficiency. Despite early delays in development, many patients can achieve independence in adult life, with usually normal neuroimaging and EEG.

Introduction

Tetrahydrobiopterin (BH₄) deficiencies are caused by mutations in genes that encode enzymes required for BH₄ biosynthesis or regeneration [1]. BH4 is an essential cofactor for the aromatic amino acid hydroxylases phenylalanine hydroxylase (PAH), tyrosine-3-hydroxylase (TH) and tryptophan-5-hydroxylases (TPH1 and TPH2), as well as of the nitric oxide synthases (NOS). Deficiency of PAH activity causes hyperphenylalaninemia while TH and TPH deficiencies affect the synthesis of monoamine neurotransmitters in the brain [2].

BH₄ is synthesized by the human body by a series of reactions using the enzymes GTP cyclohydrolase I (GTPCH), 6-pyruvoyl tetrahydropterin synthase (PTPS), and sepiapterin reductase (SR). Tetrahydrobiopterin provides electrons during the reaction required to hydroxylate substrates and, as a result, is oxidized to its hydroxyl compound pterin-4α-carbinolamine. Two enzymes pterin-4α-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR) are required to regenerate it to BH₄. Defects in any of these enzymes can cause BH4 deficiency [2,3].

6-Pyruvoyltetrahydrobiopterin synthase (6-PTPS) deficiency is the most common form of BH₄ deficiency (65% of all cases). 6-PTPS catalyzes a magnesium- and zinc- dependent reaction that converts 7,8-dihydroneotriphosphate to 6-pyruvoyl-tetrahydropterin. This was initially discovered in patients with hyperphenylalaninemia and progressive neurological deterioration despite optimal metabolic control of phenylalanine levels [3].

Most patients with PTPS deficiency present severe manifestations with truncal hypotonia and increased limb tone with pronated hand posture. Loss of head control may precede the onset of a progressive neurological degenerative disease leading to hypertonicity, especially in the lower extremities. Difficulty in swallowing, oculogyric crises, somnolence, irritability, hyperthermia, seizures, and impaired neurophysiological development may occur. There may be bradykinesia and episodic “lead pipe” or “cogwheel” rigidity. Symptoms of generalized dystonia with marked diurnal fluctuations can occur in adult patients. Patients with the mild “peripheral” forms of PTPS deficiency may have normal CSF neurotransmitters in the first months of life, but can still progress to have very low CSF neurotransmitter levels between 1 and 2 years of age [[4], [5], [6], [7]]. While these disorders were identified about 40 years ago [6], there are still gaps in knowledge regarding natural history and optimal treatment.

The study of the first 50 patients indicated that BH4 deficiencies affected about 2% of all hyperphenylalaninemic babies [8]. Usually their tolerance to dietary phenylalanine was high and their clinical symptoms responded to neurotransmitter precursors if started early in life. Diagnosis was accomplished by measurement of urinary pteridines and the activity of dihydropteridine reductase (DHPR) in blood spots [8].

The frequency of BH₄ deficiency varies greatly in different countries, with a high incidence (up to 1:11,000) of PTPS deficiency in China and Japan [9,10]. Implementation of newborn screening decreased the median age of diagnosis [10] and therapy with sapropterin dihydrochloride (average 7.9 mg/kg per day) normalized phenylalanine levels in all patients with PTPS deficiency and reduced them in one patient with DHPR deficiency [5].

An international survey of 626 patients with BH₄ deficiencies indicated that some patients with GTPCH deficiency (mild variants and dominant forms) and all with SR deficiency had normal blood phenylalanine levels [11]. Up to 57% of neonates with BH₄ deficiencies had already symptoms at time of diagnosis, with hypotonia, delays in development, and age-dependent movement disorders. Therapy included a combination of L-dopa/carbidopa and 5-hydroxytryptophan in all conditions in addition to sapropterin dihydrochloride in PTPS and GTPCH deficiencies. Folinic acid and a low phenylalanine diet were utilized in DHPR deficiency [11].

To further complicate the issue, some patients with PTPS deficiency have only a mild defect, with no changes in CNS neurotransmitters. In Italy, a cohort of 19 patients with PTPS deficiency was divided into those with relatively normal cerebrospinal fluid (CSF) neurotransmitter metabolites (5-hydroxyindolacetic acid (5-HIAA) and homovanillic acid (HVA) (mild variant) and those with decreased levels (severe form) [4]. In the mild form, levels of CSF HVA remained in the normal range, but there was an asymptomatic mild decline of CSF 5-HIAA in all subjects after the second year of life. Patients with the severe form had a higher diagnostic level of blood phenylalanine compared to mild forms. Variants in the PTS gene predicted to cause a lesser impairment of PTPS enzyme activity were associated with milder biochemical abnormalities [4]. Therapy failed to fully restore neurotransmitter metabolites in the CSF in the severe form [4]. Patients with milder forms had normal neurological outcome, while most patients with the severe form were already neurologically impaired at time of diagnosis. Early correction (<2 months of life) of monoamine neurotransmitter levels by medical therapy was associated with a better prognosis and a continuous improvement in the neuropsychiatric symptoms [12].

Here we present the natural history of a group of patients with PTPS deficiency collected from three different centers showing how the condition evolves over time and that some symptoms remain difficult to control with treatment. Our cohort includes some patients from a recent published Italian study and extends the analysis of data.