rs115532916 — ACAD9 ACAD9 Ala326Pro

ACAD9 Ala326Pro — A Rare Pathogenic Variant at the Heart of Mitochondrial Energy Production

Most people have never heard of ACAD9, yet it plays two essential roles inside every energy-demanding cell in the body. The protein encoded by ACAD9 — acyl-CoA dehydrogenase family member 9 — acts simultaneously as an enzyme that oxidizes long-chain fatty acids¹¹ enzyme that oxidizes long-chain fatty acids
Long-chain fatty acid oxidation (FAO) converts fat into acetyl-CoA for ATP production; ACAD9 specifically processes very-long-chain acyl-CoA substrates and as an indispensable assembly factor for mitochondrial complex I²² assembly factor for mitochondrial complex I
Complex I (NADH:ubiquinone oxidoreductase) is the first and largest enzyme of the mitochondrial electron transport chain; it transfers electrons from NADH to ubiquinone, pumping protons to drive ATP synthesis. Complex I deficiency is the most common cause of respiratory chain disease in humans. When the Ala326Pro variant disables ACAD9, both functions collapse, and the mitochondrial power supply to the heart, brain, and muscles is severely compromised.

The Mechanism

The c.976G>C substitution (rs115532916) replaces the flexible amino acid alanine at position 326 with the rigid cyclic amino acid proline. Proline is famously disruptive to alpha-helices and beta-sheets — it introduces a kink that few protein structures can accommodate. In ACAD9, this structural disruption abolishes ACAD enzyme activity³³ ACAD enzyme activity
Acyl-CoA dehydrogenase (ACAD) enzymes catalyze the alpha,beta-dehydrogenation of acyl-CoA esters; ACAD9 specifically targets long-chain (C12–C18) substrates, confirmed by functional studies cited in ClinVar and the Schiff et al. analysis of 16 ACAD9 variants.

The loss of ACAD9 protein function also depletes the complex I assembly scaffold. ACAD9 interacts with ECSIT and NDUFAF1⁴⁴ ECSIT and NDUFAF1
ECSIT (evolutionarily conserved signaling intermediate in Toll pathway) and NDUFAF1 (NADH:ubiquinone oxidoreductase complex assembly factor 1) form a trimeric complex with ACAD9 that nucleates the assembly of complex I's membrane arm, and in the absence of functional ACAD9, complex I cannot be fully assembled. The result is a profound biochemical energy deficit: cells cannot efficiently oxidize fats and cannot run the electron transport chain at capacity.

The Evidence

Haack et al. (2010, Nature Genetics)⁵⁵ Haack et al. (2010, Nature Genetics)
Haack TB et al. Exome sequencing identifies ACAD9 mutations as a cause of complex I deficiency. Nat Genet, 2010 established ACAD9 as a complex I deficiency gene using whole-exome sequencing. The original patient carrying Ala326Pro (in compound heterozygosity with a second ACAD9 variant, R532W) developed hypertrophic cardiomyopathy, encephalomyopathy, and lactic acidosis, with muscle complex I activity reduced to 26% of normal. That child died at age two. Restoring wild-type ACAD9 in patient-derived fibroblasts rescued complex I activity, confirming the causal role.

Schiff et al. (2015, Human Molecular Genetics)⁶⁶ Schiff et al. (2015, Human Molecular Genetics)
Schiff M et al. Complex I assembly function and fatty acid oxidation enzyme activity of ACAD9 both contribute to disease severity in ACAD9 deficiency. Hum Mol Genet, 2015 analyzed 16 ACAD9 mutations across 24 patients and found a significant inverse correlation between residual ACAD enzyme activity and phenotypic severity. Both the enzymatic and scaffolding functions are clinically relevant — variants that eliminate only one function produce milder disease than those that eliminate both.

ClinVar lists this variant (VCV000030883) as Pathogenic/Likely pathogenic, with 8 of 9 submissions in agreement across nine independent diagnostic centers including GeneDx, Invitae, Great Ormond Street Hospital, and Baylor Genetics.

A therapeutically important subset of ACAD9 patients responds to high-dose riboflavin⁷⁷ riboflavin
Riboflavin (vitamin B2) is the precursor of FAD (flavin adenine dinucleotide), the essential cofactor bound by ACAD enzyme active sites. In riboflavin-responsive patients, supplemental B2 appears to stabilize residual ACAD9 protein and partially restore complex I assembly (vitamin B2). Gerards et al. (2011, Brain)⁸⁸ Gerards et al. (2011, Brain)
Gerards M et al. Riboflavin-responsive oxidative phosphorylation complex I deficiency caused by defective ACAD9. Brain, 2011 reported improved exercise tolerance and complex I activity in riboflavin-treated ACAD9 patients. However, Nouws et al. (2014)⁹⁹ Nouws et al. (2014)
Nouws J et al. A Patient with Complex I Deficiency Caused by a Novel ACAD9 Mutation Not Responding to Riboflavin Treatment. JIMD Rep, 2014 documented a fatal case where riboflavin supplementation had no effect, indicating that response is mutation-dependent. Variants that cause protein instability (rather than cofactor insufficiency) do not respond. The Ala326Pro variant causes loss of enzyme activity, so responsiveness must be assessed biochemically in each affected individual.

Practical Actions

Because ACAD9 deficiency is autosomal recessive, heterozygous carriers — one Ala326Pro allele plus one normal allele — have sufficient ACAD9 function and do not develop ACAD9 deficiency. The primary clinical significance of heterozygous carrier status is reproductive. If both parents carry a pathogenic ACAD9 variant, each pregnancy has a 25% chance of being affected.

Homozygous or compound heterozygous patients require urgent specialist management. A trial of high-dose riboflavin under metabolic specialist supervision is standard practice, with biochemical response assessed before and after supplementation. Additional interventions studied in riboflavin-non-responsive cases include bezafibrate and nicotinamide riboside, with limited but preliminary evidence of transient benefit.

Interactions

ACAD9 deficiency is caused by biallelic pathogenic variants — two mutations must be present to cause disease. The Ala326Pro variant is documented in compound heterozygosity with R532W in the original Haack et al. case. Any second pathogenic ACAD9 allele combined with Ala326Pro would be expected to produce ACAD9 deficiency. Genetic counseling should characterize both ACAD9 alleles in any suspected case.