rs3923809 — BTBD9

Second independent intronic BTBD9 variant associated with restless legs syndrome and periodic limb movements during sleep via iron homeostasis and dopaminergic dysregulation

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BTBD9 rs3923809 — A Second RLS Signal in the Iron–Dopamine Gate

BTBD9 (BTB Domain Containing 9) encodes a substrate adaptor for the CUL3-RBX1 E3 ubiquitin ligase complex11 CUL3-RBX1 E3 ubiquitin ligase complex
A molecular machine that tags specific proteins for proteasomal degradation, controlling their cellular abundance. In the nervous system, this protein acts as a gatekeeper of iron homeostasis in dopaminergic circuits — the pathways that coordinate movement, motivation, and the rest–activity cycle. Dysfunction at BTBD9 disrupts iron storage, starves tyrosine hydroxylase of its required cofactor, and impairs dopamine synthesis — the sequence of events that produces the characteristic evening restlessness and periodic limb jerking of restless legs syndrome.

rs3923809 is an intronic variant within BTBD9 on chromosome 6p21.2. It is the most-cited index SNP for the BTBD9 RLS locus in the primary literature, and it sits in partial linkage disequilibrium with the related intronic variant rs9394502. Together they tag overlapping — but not identical — portions of the BTBD9 risk haplotype, meaning both variants contribute independent information about RLS susceptibility. The A allele at rs3923809 is the risk allele; despite being the GRCh38 reference sequence and the global majority allele (~68% frequency), carriers show consistently elevated rates of restless legs syndrome and periodic limb movements of sleep (PLMS) across multiple populations.

The Mechanism

BTBD9 functions as a Cullin-3 adaptor that ubiquitinates iron regulatory protein 2 (IRP2), targeting it for degradation. When BTBD9 is perturbed, IRP2 accumulates and suppresses ferritin expression, reducing cellular iron storage capacity. Because iron is a required cofactor for tyrosine hydroxylase22 tyrosine hydroxylase
The rate-limiting enzyme that converts L-tyrosine to L-DOPA, the immediate precursor of dopamine. Iron binds the enzyme's active site; low iron directly slows dopamine biosynthesis, reduced iron availability translates directly into reduced dopamine synthesis capacity in motor circuits.

In Drosophila, loss of the BTBD9 homologue reproduces all key RLS features: fragmented sleep, increased waking, and heightened locomotor restlessness. Restoring dopamine signalling in these flies rescues the motor phenotype, establishing dopamine deficiency — downstream of iron dysregulation — as the proximal cause of the RLS-like symptoms. Human carriers of BTBD9 risk alleles have measurably lower serum ferritin per risk allele, consistent with the IRP2/ferritin mechanism playing out systemically.

The Evidence

Moore et al. 201433 Moore et al. 2014
Moore H et al. Periodic leg movements during sleep are associated with polymorphisms in BTBD9, TOX3/BC034767, MEIS1, MAP2K5/SKOR1, and PTPRD. Sleep, 2014;37(9):1535-42 examined 1,090 participants from the Wisconsin Sleep Cohort with objective polysomnography, finding rs3923809 A allele the strongest genetic predictor of elevated PLMS (OR=1.65, P=1.5×10⁻⁸) — a genome-wide significant result from a community-based sample, not a clinical RLS cohort.

Winkelman et al. 201544 Winkelman et al. 2015
Winkelman JW et al. Genetic associations of periodic limb movements of sleep in the elderly for the MrOS sleep study. Sleep Med, 2015;16(11):1360-5 replicated this in 2,356 elderly men (OR=1.43, 95% CI 1.26–1.63 per A allele), demonstrating that the association persists in community populations not selected for RLS diagnosis, and that PLMS — often clinically silent — is independently heritable.

The RLS-specific signal was confirmed in multi-population replication. Kemlink et al. 200955 Kemlink et al. 2009
Kemlink D et al. Replication of restless legs syndrome loci in three European populations. J Med Genet, 2009;46(5):315-8 replicated the rs3923809 association across Czech, Austrian, and Finnish cohorts (649 cases, 1,230 controls; OR=1.58, P=4.11×10⁻⁵), concluding that "BTBD9 seems to be the most consistent in its effect on RLS across populations." Kim et al. 201366 Kim et al. 2013
Kim MK et al. Association of restless legs syndrome variants in Korean patients. Sleep, 2013;36(12):1787-91 extended this to an East Asian cohort (320 cases, 320 controls; OR 1.61–1.88, P<0.0001), confirming ethnic generalizability.

The mechanistic underpinning comes from Freeman et al. 201277 Freeman et al. 2012
Freeman A et al. Sleep fragmentation and motor restlessness in a Drosophila model of Restless Legs Syndrome. Curr Biol, 2012;22(12):1142-8, which demonstrated that BTBD9 controls brain dopamine levels and iron homeostasis via IRP2-mediated ferritin regulation — directly linking the genetic signal to the dopaminergic pathophysiology of RLS.

Practical Actions

The actionable intervention follows directly from the mechanism. Clinical guidelines for RLS recommend checking serum ferritin and targeting levels above 75 ng/mL — substantially higher than the 12 ng/mL general deficiency cutoff — because brain iron depletion drives symptoms even when hemoglobin is normal. For A allele carriers, this monitoring threshold is especially relevant: the BTBD9 iron-regulatory pathway is already compromised at baseline, making any additional iron deficit disproportionately impactful on dopamine synthesis.

AA homozygotes represent the most common risk group at this locus and should screen actively for RLS and PLMS, particularly as both are significantly underdiagnosed. PLMS in particular disrupts sleep architecture without triggering full awakening — many sufferers report non-restorative sleep without knowing why.

Interactions

rs3923809 and rs9394502 (also BTBD9, chr6:38,484,727) are in partial linkage disequilibrium. Carriers of both risk haplotypes show stronger periodic limb movement severity than either alone — a likely compound effect within the same gene. MEIS1 rs2300478 on chromosome 2p encodes a developmental transcription factor that is an independent RLS risk locus (OR ~1.7–1.9) and acts additively with BTBD9 variants; the combined MEIS1 + BTBD9 risk genotype is present in a significant fraction of clinically diagnosed RLS patients. MAP2K5 rs6494696 represents a third independent RLS locus identified in the Winkelmann 2007 GWAS that compounds further.

Nutrient Interactions

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Genotype Interpretations

What each possible genotype means for this variant.

GG Protective Genotype Beneficial

Two copies of the protective G allele — lowest RLS and PLMS risk at this locus

The GG genotype at rs3923809 confers the lowest RLS and PLMS risk among the three possible genotypes at this locus. The G allele is the ancestral sequence, making GG the evolutionary baseline from which the A-carrying risk haplotype diverged. Although this genotype is uncommon in European populations (~10%), it is more frequent in East Asian populations (~62% carry the GG or AG genotype).

RLS and PLMS are polygenic traits, meaning your overall susceptibility depends on variants across many loci — including MEIS1, MAP2K5, and rs9394502 in the same BTBD9 gene. A protective result here does not exclude RLS risk from other loci.

AG One Risk Allele Intermediate

One copy of the BTBD9 risk allele — moderately elevated RLS and PLMS susceptibility

Under the additive model, each A allele contributes approximately 1.43-fold odds of elevated PLMS (OR=1.43 per allele, Winkelman et al. 2015, PMID 26498236). The BTBD9 iron-dopamine mechanism is dosage-sensitive: partial disruption of IRP2 targeting by one risk allele results in a partial reduction in ferritin expression capacity, translating into mildly reduced iron availability for dopamine synthesis. The practical implication is that iron status amplifies genetic risk — AG carriers with ferritin below 50 ng/mL will likely experience more symptom burden than those with ferritin above 100 ng/mL.

PLMS (periodic limb movements of sleep) is a closely related phenotype that disrupts sleep architecture without causing full awakening. Many AG carriers with unexplained non-restorative sleep may have undetected PLMS.

AA Two Risk Alleles High Risk

Two copies of the BTBD9 risk allele — highest RLS and PLMS susceptibility at this locus

The Moore et al. 2014 Wisconsin Sleep Cohort study (PMID 25142570) established rs3923809 A allele as the strongest genetic predictor of elevated PLMS among five major RLS loci (OR=1.65 per A allele, P=1.5×10⁻⁸). Under the additive model, AA homozygotes carry approximately double the per-allele effect of heterozygotes. Winkelman et al. 2015 in 2,356 elderly men confirmed OR=1.43 per A allele (95% CI 1.26–1.63) for polysomnographically confirmed PLMS, demonstrating clinical-grade replication.

PLMS is particularly important: it disrupts sleep architecture through micro-arousals without causing full awakening. Many AA carriers report non-restorative sleep, daytime fatigue, or an unexplained urge to move the legs at night without meeting the full clinical criteria for RLS. The mechanistic driver — reduced iron availability for dopamine synthesis via the BTBD9/IRP2/ferritin axis — is directly modifiable through iron status optimization.

Serum ferritin is the key modifiable modifier: brain iron deficiency drives dopaminergic dysregulation even when hemoglobin is normal. Clinical RLS guidelines target ferritin above 75 ng/mL in susceptible individuals; some specialists recommend above 100 ng/mL.