rs387907200 — SPR

SPR Gly102Cys — When the Neurotransmitter Cofactor Factory Runs at 15%

Deep in the brainstem and striatum, three enzymes are quietly producing dopamine, serotonin, and norepinephrine — the neurotransmitters that govern movement, mood, and alertness. All three depend on a single small molecule: tetrahydrobiopterin (BH4)¹¹ tetrahydrobiopterin (BH4)
BH4 is an essential cofactor for phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase — the rate-limiting enzymes in catecholamine and indoleamine biosynthesis. The SPR gene encodes sepiapterin reductase, which catalyzes the final step in BH4 biosynthesis. The rs387907200 variant (c.304G>T, p.Gly102Cys) — found on chromosome 2 at position 72,887,736 on the plus strand — replaces glycine with cysteine at position 102 and simultaneously disrupts splicing at the last nucleotide of exon 1, leaving the enzyme running at approximately 15% of normal activity.

The Mechanism

The c.304G>T transversion affects a semiconserved residue not directly involved in substrate binding or catalysis²² semiconserved residue not directly involved in substrate binding or catalysis
The glycine-102 position sits in a structural loop near the active site; its replacement with the bulkier cysteine partially destabilizes the homodimer interface. Because the mutation sits at the very last nucleotide of exon 1, it also disrupts the splice donor signal. Minigene analysis showed that the mutation results in some splicing abnormalities, although some normal transcripts can still be produced³³ Minigene analysis showed that the mutation results in some splicing abnormalities, although some normal transcripts can still be produced
This "leaky" splicing is why biallelic G102C carriers have a substantially milder phenotype than those with null alleles. The enzyme that does get made carries the Gly102Cys amino acid change, retaining ~15% residual SPR activity. That partial activity is enough to prevent the complete BH4 collapse seen in severe SRD, but not enough to sustain normal neurotransmitter production under physiological demand. When BH4 falls, dihydrobiopterin (BH2) accumulates and directly inhibits tyrosine and tryptophan hydroxylases⁴⁴ dihydrobiopterin (BH2) accumulates and directly inhibits tyrosine and tryptophan hydroxylases
BH2 competes with BH4 at the active site, compounding the neurotransmitter deficit beyond what BH4 depletion alone would cause.

The Evidence

Bonafé et al. (2001)⁵⁵ Bonafé et al. (2001)
PMID 11443547, American Journal of Human Genetics established that SPR mutations cause a novel syndrome: severe CSF dopamine and serotonin deficiency with dystonia, but without hyperphenylalaninemia — meaning standard PKU newborn screening misses it entirely.

Arrabal et al. (2011)⁶⁶ Arrabal et al. (2011)
PMID 21431957, Neurogenetics characterised the c.304G>T variant in three Spanish siblings carrying it in compound heterozygosity with R150G. Their phenotype was strikingly mild: no cognitive delay, and the eldest sister was nearly asymptomatic. Enzyme assays confirmed 15% residual SPR activity for the G102C protein. The leaky splicing mechanism — producing a mixture of normal and aberrant transcripts — explained the inter-sibling variability in symptom severity.

Nakagama et al. (2019)⁷⁷ Nakagama et al. (2019)
PMID 31041399, Neurology: Genetics extended this concept, demonstrating that leaky splicing variants in SPR may cause milder SRD phenotypes that escape detection entirely, broadening the clinical spectrum of the condition.

Yang et al. (2015)⁸⁸ Yang et al. (2015)
PMID 25550200, JPET showed that sulfonamide-class drugs are potent noncompetitive inhibitors of sepiapterin reductase (IC50 31–180 nM for sulfasalazine, sulfathiazole, sulfapyridine, sulfamethoxazole, and chlorpropamide). In neuronal PC12 cells, sulfathiazole at 200 µM markedly suppressed BH4 biosynthesis and downstream dopamine and serotonin production — an effect reversed by BH4 co-administration. This makes sulfa drugs a concrete biochemical hazard for anyone with impaired SPR activity.

Practical Actions

For heterozygous carriers (GT genotype) with one functional copy of SPR, BH4 levels are generally sufficient — heterozygous knockout mice show no significant reduction in brain BH4 or dopamine. The relevant risk for carriers is sulfa drug exposure, which pharmacologically inhibits the already-reduced enzyme pool, and awareness to test children if symptoms consistent with dopa-responsive dystonia emerge.

For homozygous or compound heterozygous individuals (TT in this locus, or GT paired with a second SPR pathogenic variant), the clinical picture is dopa-responsive dystonia with neurotransmitter deficiency. Levodopa combined with carbidopa at 0.1–16 mg/kg/day is the primary treatment⁹⁹ Levodopa combined with carbidopa at 0.1–16 mg/kg/day is the primary treatment
The motor symptoms respond consistently; cognitive outcomes depend heavily on early initiation. Addition of 5-hydroxytryptophan (5-HTP) at 1–6 mg/kg/day restores serotonin arm of the deficiency¹⁰¹⁰ 5-hydroxytryptophan (5-HTP) at 1–6 mg/kg/day restores serotonin arm of the deficiency
5-HTP bypasses the blocked SPR step to supply serotonin independently of BH4. Sulfa drugs, methotrexate (which inhibits dihydropteridine reductase, a related BH4-cycling enzyme), and nitrous oxide should be avoided on mechanistic grounds.

Interactions

rs1876487 and rs2421095 are promoter polymorphisms in the SPR gene associated with reduced SPR transcription (1.4–1.6-fold reduction) and with bipolar disorder risk (OR 5.47 for the risk haplotype pair). These are distinct from the coding variant rs387907200 but share the same pathway: reduced BH4 availability and consequent monoamine deficiency. Individuals carrying both the coding variant and a low-expressing promoter haplotype may have a greater cumulative reduction in SPR output than either variant alone — a potential compound interaction worth flagging for clinical evaluation.