rs1799958 — ACADS G209S (c.625G>A)

ACADS G209S — A Common Low-Activity Variant That Rarely Causes Clinical Disease

ACADS¹¹ ACADS
acyl-CoA dehydrogenase short-chain; the mitochondrial enzyme that oxidizes short-chain fatty acids (C4–C6, primarily butyryl-CoA) in the first step of mitochondrial beta-oxidation. Located at chr12:120738280 (GRCh38) encodes short-chain acyl-CoA dehydrogenase (SCAD), which initiates oxidation of the shortest-chain fatty acids produced during the breakdown of branched-chain amino acids and even-chain dietary fats. The G209S variant (c.625G>A, p.Gly209Ser) changes glycine to serine at position 209 in the SCAD protein, reducing enzyme stability and activity without abolishing function entirely.

G209S is one of the most common functional variants in the ACADS gene. The A allele reaches ~25% frequency in European populations and ~18-26% globally, making homozygous AA genotypes present in approximately 5-7% of people of European ancestry. Despite this prevalence, overt SCAD deficiency is extremely rare, and most homozygous individuals identified through newborn screening are asymptomatic. ClinVar classifies the A allele as Benign to Likely-Benign.

The Mechanism

SCAD is a homotetramer localized in the mitochondrial matrix. Like all acyl-CoA dehydrogenases, it requires FAD²² FAD
flavin adenine dinucleotide, a redox cofactor that accepts electrons from the acyl-CoA substrate during the first step of fatty acid beta-oxidation for catalytic activity. Glycine-209 lies within the FAD-binding domain; its substitution with the polar, larger serine residue impairs protein folding kinetics and reduces thermal stability.

The G209S enzyme retains substantial residual activity — typically 30-60% of wild-type levels in heterologous expression systems. This partial reduction leads to mildly elevated butyrylcarnitine (C4) in blood, particularly detectable on newborn tandem mass spectrometry screening, but the threshold for clinical disease in most individuals is not reached. The contrast with severe SCAD deficiency alleles (which reduce activity below 10%) is clinically meaningful.

The Evidence

Corydon et al. 1996³³ Corydon et al. 1996
PMID 8725270 first characterized the G625A polymorphism (G209S in current numbering, earlier called G185S) as a common ACADS allele associated with ethylmalonic aciduria and reduced SCAD activity. Many carriers were clinically normal.

Corydon et al. 1998⁴⁴ Corydon et al. 1998
PMID 9582344 characterized the molecular basis: G209S reduces SCAD protein stability and causes temperature-sensitive folding defects; the mutant protein is rapidly degraded after import into mitochondria, grading residual activity by genotype (GG > GA > AA).

van Maldegem et al. 2005⁵⁵ van Maldegem et al. 2005
PMID 15902559 screened 1,036 newborn blood spots and found that 625G>A homozygotes — despite being common (5.5% homozygous) — showed no significant increase in C4-carnitine levels compared to non-carriers, leading to reclassification of this variant as a potential nondisease rather than a disease-causing mutation.

Nagan et al. 2003⁶⁶ Nagan et al. 2003
PMID 12706374 confirmed the population frequency data in the US population, establishing that clinical disease prevalence is orders of magnitude lower than the genotype frequency, which is the defining argument for benign classification.

Practical Actions

For GG individuals (two functional copies of ACADS): normal SCAD enzyme function. No clinical significance for this variant.

For GA heterozygotes: a single G209S allele reduces SCAD activity mildly; no clinical disease is associated with this genotype alone.

For AA homozygotes: biochemical SCAD deficiency with persistently elevated butyrylcarnitine (C4) but typically no clinical symptoms. If you or a child have been flagged on newborn screening for elevated C4, this genotype explains the finding. Management beyond confirmation is generally not required in the absence of symptoms.

Interactions

G209S homozygosity may interact with other metabolic stressors — intercurrent illness, fasting, or mitochondrial dysfunction — to produce symptomatic hypoglycemia in a minority of individuals, though this is not well established. No gene-gene interaction compound actions are defined for this variant given its benign classification.