rs121917746 — SPR Q119X

SPR Q119X — The Hidden Dystonia Gene

Inside every neuron that makes dopamine or serotonin, a small enzyme called sepiapterin reductase (SPR) performs the final step in synthesising tetrahydrobiopterin¹¹ tetrahydrobiopterin
BH4 — an essential cofactor for three key enzymes in neurotransmitter production. Without sufficient BH4, the enzymes that convert tyrosine into dopamine and tryptophan into serotonin grind to a halt. The Q119X variant in SPR — a single nucleotide change that replaces glutamine at position 119 with a premature stop codon — creates a truncated, non-functional protein and is the cause of SPR deficiency (OMIM #612716), a rare but fully treatable neurological disorder.

The Mechanism

BH4 acts as a cofactor for phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylase (TPH). TH and TPH are the rate-limiting enzymes for dopamine and serotonin synthesis respectively. SPR catalyses the reduction of 6-pyruvoyltetrahydropterin to BH4 in the final step of de novo synthesis²² SPR catalyses the reduction of 6-pyruvoyltetrahydropterin to BH4 in the final step of de novo synthesis
sepiapterin reductase is an aldo-keto reductase encoded on chromosome 2p13.2. The Q119X nonsense mutation (c.355C>T, NM_003124.5) truncates the protein at codon 119, abolishing enzyme activity in skin fibroblasts. Peripheral tissues can partially compensate via alternative reductases (aldose reductase, carbonyl reductase, dihydrofolate reductase), but the brain relies predominantly on SPR — explaining why SPR deficiency is neurologically devastating while plasma phenylalanine remains normal, causing it to be invisible to standard PKU neonatal screening³³ invisible to standard PKU neonatal screening
SPR deficiency does not elevate blood phenylalanine, the marker used in universal newborn screening.

The Evidence

Bonafé et al. (2001)⁴⁴ Bonafé et al. (2001) first characterised Q119X in a consanguineous Turkish family: a 14-year-old male homozygous for the variant had psychomotor retardation, spasticity, dystonia, and severely reduced CSF levels of the dopamine metabolite homovanillic acid (HVA) and the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA), with elevated biopterin. Clinical improvement followed levodopa treatment.

Friedman et al. (2012)⁵⁵ Friedman et al. (2012) described 38 patients with confirmed SPR deficiency across 11 countries. Average symptom onset was 7 months, but average time to diagnosis was 9.1 years — most patients were misdiagnosed as having cerebral palsy. The defining features were axial hypotonia, dystonia, oculogyric crises, and diurnal fluctuation (worse in the evening, better after sleep). Patients improved dramatically on levodopa/carbidopa, with further benefit from adding 5-hydroxytryptophan.

GeneReviews (Friedman & Galosi, updated 2025)⁶⁶ GeneReviews (Friedman & Galosi, updated 2025) confirms autosomal recessive inheritance and notes that early treatment — ideally within the first year of life — can reverse developmental delay and prevent permanent cognitive impairment. Levodopa is dosed at 0.1–16 mg/kg/day combined with carbidopa or benserazide; 5-HTP at 1–6 mg/kg/day.

Heterozygous carriers of Q119X are clinically asymptomatic. A population study in Malta — where the carrier frequency reaches ~4–5% due to a founder effect — found that all carrier parents showed no clinical symptoms despite carrying one defective copy of the gene. Farrugia et al. 2007⁷⁷ Farrugia et al. 2007.

Practical Actions

For homozygous individuals (TT) the situation is a medical emergency requiring specialist care. The standard treatment is levodopa/carbidopa plus 5-HTP under neurological supervision, with remarkable outcomes when started early. For heterozygous carriers (CT), current evidence does not support any supplement protocol — carriers are phenotypically normal. The main practical value of knowing carrier status is for reproductive planning: two CT carriers have a 25% chance of an affected (TT) child per pregnancy.

Interactions

SPR deficiency shares its pathway with GTP cyclohydrolase I deficiency (GCH1 mutations, the dominant form of DOPA-responsive dystonia) and other BH4 biosynthesis disorders. All these conditions converge on the same downstream deficiency of dopamine and serotonin. SPR also interacts with the nitric oxide synthase pathway — BH4 is a cofactor for NOS enzymes, so severe SPR deficiency secondarily impairs nitric oxide production, contributing to autonomic dysfunction seen in roughly half of patients.