Mutations in the sepiapterin reductase gene cause a novel tetrahydrobiopterin-dependent monoamine-neurotransmitter deficiency without hyperphenylalaninemia

Abstract

Classic tetrahydrobiopterin (BH(4)) deficiencies are characterized by hyperphenylalaninemia and deficiency of monoamine neurotransmitters. In this article, we report two patients with progressive psychomotor retardation, dystonia, severe dopamine and serotonin deficiencies (low levels of 5-hydroxyindoleacetic and homovanillic acids), and abnormal pterin pattern (high levels of biopterin and dihydrobiopterin) in cerebrospinal fluid. Furthermore, they presented with normal urinary pterins and without hyperphenylalaninemia. Investigation of skin fibroblasts revealed inactive sepiapterin reductase (SR), the enzyme catalyzing the final two-step reaction in the biosynthesis of BH(4). Mutations in the SPR gene were detected in both patients and their family members. One patient was homozygous for a TC-->CT dinucleotide exchange, predicting a truncated SR (Q119X). The other patient was a compound heterozygote for a genomic 5-bp deletion (1397-1401delAGAAC) resulting in abolished SPR-gene expression and an A-->G transition leading to an R150G amino acid substitution and to inactive SR as confirmed by recombinant expression. The absence of hyperphenylalaninemia and the presence of normal urinary pterin metabolites and of normal SR-like activity in red blood cells may be explained by alternative pathways for the final two-step reaction of BH(4) biosynthesis in peripheral and neuronal tissues. We propose that, for the biosynthesis of BH(4) in peripheral tissues, SR activity may be substituted by aldose reductase (AR), carbonyl reductase (CR), and dihydrofolate reductase, whereas, in the brain, only AR and CR are fully present. Thus, autosomal recessive SR deficiency leads to BH(4) and to neurotransmitter deficiencies without hyperphenylalaninemia and may not be detected by neonatal screening for phenylketonuria.

Figure 1

Biosynthesis of tetrahydrobiopterin and proposed alternative routes in SR deficiency (for details, see text)

Figure 2

A, Mutations in SPR, in patient 360 and in his father. B, Mutations in SPR and SR cDNA, in patient 229 and in his parents.

Figure 3

Recombinant expression of wild-type and mutant R150G SR proteins in bacterial cells. A, SR activity assayed by determination of biopterin production with sepiapterin as substrate and with different amounts of soluble-protein extracts from E. coli expressing either the wild type, R150G, or no SR (vector control) from the expression vector pGEMEX. B, Western blot analysis using the same extracts as in panel A. Each lane contains 0.05 mg of total protein, and cross-reactive material was detected by use of a rabbit anti–mouse SR antibody (arrow). Note that, although the human recombinant SR is a 28-kD monomer, it runs at ∼34 kD in the 12.5% SDS-PAGE with the molecular-weight standard here used.