rs9565309 — FBXL3 FBXL3 circadian variant

FBXL3 — The Ubiquitin Clock Hand That Sets Your Morning Preference

Inside every cell, the circadian clock ticks through a 24-hour negative feedback loop: the CLOCK:BMAL1 protein complex drives expression of PER and CRY genes; PER and CRY proteins then feed back to inhibit CLOCK:BMAL1, completing one cycle. The speed of this loop — and therefore whether your internal day runs fast or slow — depends critically on how quickly CRY proteins are cleared. FBXL3 (F-box and leucine-rich repeat protein 3) is the molecular timer that sets this clearance rate. As the substrate- recognition subunit of the SCF^FBXL3 ubiquitin ligase complex¹¹ ubiquitin ligase complex
E3 ubiquitin ligases tag proteins with ubiquitin chains, marking them for destruction by the proteasome, FBXL3 directly binds CRY1 and CRY2 and marks them for proteasomal degradation. More FBXL3 activity → faster CRY turnover → shorter period; less activity → slower CRY turnover → longer period.

The variant rs9565309 (NC_000013.11:g.77002892T>C) sits in the 3' UTR region of the neighboring CLN5 gene and in the regulatory vicinity of FBXL3 on chromosome 13q22.3. It is a GWAS tag SNP — a marker in linkage disequilibrium with the causal variant(s) near or within FBXL3 — rather than the functional mutation itself. The C allele of this tag SNP is associated with increased morningness (earlier chronotype) preference, consistent with slightly enhanced FBXL3-mediated CRY clearance shortening the circadian period toward an earlier phase.

The Mechanism

Mouse genetics established the causal logic cleanly. Two independent loss-of-function alleles of Fbxl3 produce mice with dramatically lengthened circadian periods: the after-hours (Afh) Cys358Ser allele causes ~27-hour free-running rhythms²² the after-hours (Afh) Cys358Ser allele causes ~27-hour free-running rhythms
Godinho et al. Science 2007, and the Overtime (Ovtm) allele causes ~26-hour periods through CRY protein stabilization and global suppression of Per and Cry gene transcription³³ the Overtime (Ovtm) allele causes ~26-hour periods through CRY protein stabilization and global suppression of Per and Cry gene transcription
Siepka et al. Cell 2007. Silencing Fbxl3 in cell culture abolishes clock oscillation only in CRY-expressing cells⁴⁴ Silencing Fbxl3 in cell culture abolishes clock oscillation only in CRY-expressing cells
Busino et al. Science 2007, confirming CRY proteins are the obligate substrates. Translating this to humans: variants near FBXL3 that alter its expression or activity would be expected to shift chronotype in the direction predicted by the mouse data — higher FBXL3 activity tilting earlier, lower activity tilting later.

The rs9565309 C allele's OR of 1.19 for morningness is a subtle but consistent population-level effect. At the individual level, this variant nudges chronotype in the morning direction; it does not override the combined influence of PER3, CRY1, TIMELESS, and dozens of other clock gene variants. The T allele homozygote — the overwhelmingly common genotype (>92% of people) — carries the reference state: no added morningness push from this locus, and a slight statistical lean toward later sleep timing relative to C carriers.

The Evidence

The primary genetic association comes from Jones et al. (Nature Communications, 2019)⁵⁵ Jones et al. (Nature Communications, 2019)
PMID 30696823, a genome- wide association study of chronotype (self-reported morning vs. evening preference) in 697,828 UK Biobank and 23andMe participants — the largest chronotype GWAS published to date. The study increased the number of genome-wide significant chronotype loci from 24 to 351, with rs9565309 near FBXL3 among the hits at OR=1.19 for morningness. Overall, genes near these loci were enriched for circadian regulation pathways; the 5% of individuals carrying the most morningness alleles slept approximately 25 minutes earlier than the 5% carrying the fewest.

The mechanistic foundation rests on three landmark papers from 2007 that independently discovered FBXL3's role using classical forward genetics in mice: Godinho et al.⁶⁶ Godinho et al., Siepka et al.⁷⁷ Siepka et al., and Busino et al.⁸⁸ Busino et al.. Together they established that SCF^{FBXL3-mediated} CRY ubiquitination is a non- redundant step in mammalian circadian oscillation, and that its loss lengthens the free-running period by 2–3 hours. The human GWAS finding is biologically coherent with this mechanism.

The effect allele frequency varies substantially by ancestry: the C allele reaches ~12.8% in East Asian populations versus ~3.3% in Europeans and ~1.4% in Africans. This distribution makes the morningness association more common among East Asian carriers but does not alter the per-allele effect direction.

Practical Actions

The per-allele effect of rs9565309 is modest (OR=1.19 per C allele). T/T homozygotes have neither C allele and thus receive no morningness push from this locus — their sleep timing is governed by other genetic and behavioral factors. The most evidence-supported way to express whatever chronotype you have is through behavioral anchoring: a fixed wake time signals the suprachiasmatic nucleus to stabilize the free-running period against social and environmental drift. For the T/T majority, this is particularly important because without a morningness-promoting allele at this locus, circadian phase is more vulnerable to evening light exposure and irregular schedules.

Evening light management is the highest-leverage behavioral lever. Blue- wavelength light after sunset suppresses melatonin and phase-delays the circadian clock. Individuals without morningness-promoting variants benefit most from reducing this input. Amber-lens glasses or screen filters from approximately 2 hours before desired sleep time limit the phase-delaying signal without requiring lifestyle disruption.

Interactions

FBXL3 operates on CRY1 and CRY2 proteins. Variants in CRY1 and CRY2 themselves therefore interact with the FBXL3 pathway at the substrate level. The most clinically significant is rs184039278 (CRY1Δ11), a rare splice variant in CRY1 that produces a gain-of-function CRY1 protein resistant to normal turnover; even if FBXL3 activity is normal, this variant lengthens circadian period by ~30 minutes and drives Delayed Sleep Phase Disorder. Carriers of CRY1Δ11 who also carry the rs9565309 TT genotype would have both the CRY1 substrate problem and the absence of FBXL3-locus morningness support — potentially compounding the evening-phase tendency.

The PER3 VNTR polymorphism (rs57875989) and common variants in TIMELESS and CLOCK also contribute to chronotype independently. The rs9565309 locus is additive — its contribution to chronotype phenotype combines linearly with other genetic and behavioral influences rather than overriding them.