rs121434369 — GCDH R402W (Arg402Trp)

GCDH R402W — Europe's Most Common Glutaric Acidemia Allele

Glutaryl-CoA dehydrogenase (GCDH) is a mitochondrial enzyme that breaks down three amino acids — lysine, hydroxylysine, and tryptophan — in the final steps of their catabolism. When GCDH fails, its substrates (glutaric acid and 3-hydroxyglutaric acid¹¹ 3-hydroxyglutaric acid
3-OHG is a potent endogenous neurotoxin that selectively injures the striatum during metabolic stress) accumulate and trigger striatal necrosis²² striatal necrosis
destruction of the caudate nucleus and putamen, the basal ganglia structures controlling voluntary movement, resulting in a severe dyskinetic movement disorder during encephalopathic crises.

The R402W variant (c.1204C>T, p.Arg402Trp) is the single most common pathogenic GCDH allele in European populations, accounting for approximately 40% of alleles in patients of German origin and 12-16% of Caucasian GA1 alleles across Europe. Haplotype analysis by Busquets et al.³³ Haplotype analysis by Busquets et al. demonstrated that all European R402W alleles trace back to a single ancestral founder mutation, explaining its unusual geographic concentration.

This is an autosomal recessive variant: one copy (CT genotype) produces an unaffected carrier; two copies or compound heterozygosity with another pathogenic GCDH allele causes glutaric acidemia type 1 (GA1).

The Mechanism

The c.1204C>T transition converts a coding-strand C to T, replacing arginine (a basic, positively charged residue) with tryptophan (bulky, aromatic) at position 402 of the mature GCDH protein — within the αDC (C-terminal alpha-helical) domain critical for subunit assembly. Keyser et al. 2008⁴⁴ Keyser et al. 2008 showed that the R402W protein is expressed at only 12% of wild-type signal intensity, undergoes rapid intramitochondrial degradation, and cannot form the functional GCDH homotetramer. The result is effectively complete loss of GCDH enzymatic activity.

Without functional GCDH, lysine catabolism stalls. Glutarylcarnitine accumulates in blood — the biomarker detected on newborn screening — while glutaric acid and 3-hydroxyglutaric acid build up in brain tissue. During any catabolic stress (febrile illness, fasting, surgery), the surge of these organic acids selectively injures the striatum⁵⁵ striatum
the basal ganglia nuclei (caudate, putamen, globus pallidus) that regulate movement, causing acute encephalopathic crises. If crises occur, the resulting dystonia is typically irreversible.

The biochemical phenotype of R402W is "high excretor" — R402W homozygotes produce large amounts of urinary glutaric acid, unlike some other GCDH alleles that cause low-excretor phenotypes where routine urine organic acid analysis may appear normal.

The Evidence

GA1 is an established autosomal recessive condition (OMIM 231670) with an estimated birth incidence of 1 in 30,000–100,000 live births. The R402W variant carries ClinVar classification of Pathogenic (Variation ID 2085, 2-star review status with criteria provided by multiple submitters) and is listed as OMIM allelic variant 608801.0004.

The impact of early detection is dramatic. Strauss et al. 2020⁶⁶ Strauss et al. 2020 followed 168 GA1 patients over 31 years: striatal degeneration occurred in 90% of unscreened patients, 47% of screened patients on protein restriction only, and just 7% of patients managed from birth with lysine-free formula plus emergency IV treatment during febrile episodes. The critical intervention window is the first six years of life — no neurological injuries occurred in the Strauss cohort after 19 months in the best-managed group.

A meta-analysis of 647 GA1 patients⁷⁷ meta-analysis of 647 GA1 patients by Boy et al. 2021 confirmed: 74.7% of newborn-screened patients remained asymptomatic versus only 9.6% of clinically diagnosed patients. Quality of therapy — adherence to the emergency protocol during febrile illness — was the strongest predictor of neurological outcome in the screened population.

Practical Actions

For carriers (CT genotype): no clinical management of GA1 is required. One functional GCDH allele is sufficient for adequate enzyme activity. The practical implication is reproductive: each child of two GCDH carriers has a 25% chance of inheriting two pathogenic alleles and developing GA1. Partner testing and genetic counseling are the key interventions.

For affected individuals (TT genotype, or CT compound heterozygous with another GA1 allele): confirmed GA1 requires immediate referral to a metabolic medicine specialist. The 2023 international guidelines⁸⁸ 2023 international guidelines specify a low-lysine diet (using lysine-free amino acid formula), L-carnitine supplementation (100 mg/kg/day in infants), and an emergency protocol authorizing IV glucose plus IV L-carnitine during any febrile episode. Dietary restriction is maintained strictly through age 6, then gradually relaxed as the striatum exits its vulnerability window.

Interactions

R402W most commonly causes GA1 as a homozygote (TT) or in compound heterozygosity with another pathogenic GCDH allele. The most clinically important compound heterozygous combination in European populations is R402W + R88C (rs142967670) — both alleles are independently classified Pathogenic in ClinVar, and compound heterozygotes have the same clinical syndrome as R402W homozygotes. Christensen et al. 2004⁹⁹ Christensen et al. 2004 confirmed no genotype-phenotype correlation for clinical outcome in GA1: crisis prevention, not which specific mutations are present, determines neurological prognosis.

Secondary carnitine deficiency occurs mechanistically in all untreated GA1: excess glutaric acid is conjugated to carnitine and excreted as glutarylcarnitine, depleting free carnitine needed for fatty acid oxidation. L-carnitine supplementation directly addresses this depletion, which is why it is part of the standard treatment regimen across all GA1 alleles.