rs3923809 — BTBD9

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.