rs700651 — BOLL

Intronic variant near the BOLL gene associated with increased intracranial aneurysm susceptibility, replicated across European, Japanese, and Korean populations.

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The Brain Aneurysm Gene at 2q33: How a Common Variant Near BOLL Shapes Your Vascular Risk

Deep within chromosome 2, a modest change in a single DNA letter can shift your lifetime odds of developing an intracranial aneurysm — a balloon-like bulge in a brain artery that, if it ruptures, causes a subarachnoid hemorrhage11 subarachnoid hemorrhage
bleeding into the space surrounding the brain, a life-threatening emergency affecting roughly 500,000 people annually worldwide. The variant rs700651 sits in an intron of the BOLL gene (boule-like RNA binding protein) at chromosome 2q33.1 and was first pinpointed in a landmark 2008 genome-wide association study. It remains one of only a handful of robustly replicated genetic risk factors for this condition.

The Mechanism

BOLL encodes an RNA-binding protein most highly expressed in germ cells, but the 2q33.1 locus appears to influence vascular biology through regulatory mechanisms that extend beyond the BOLL coding sequence itself. Cis-eQTL analysis22 Cis-eQTL analysis
a method linking a DNA variant to changes in nearby gene expression at this locus reveals that the risk-allele genotype is associated with altered expression of SF3B1 (a splicing factor subunit) in thyroid and tibial nerve tissue, and ANKRD44 in testis — suggesting the variant influences RNA splicing regulation in tissues relevant to vascular development and maintenance. Additional genome-wide interaction studies identify strong epistatic interactions between rs700651 and rs1105980 in PTCH1 (the Hedgehog pathway receptor), pointing toward a role for developmental vascular patterning pathways in aneurysm susceptibility.

The mechanism by which intronic variants at 2q33.1 predispose to aneurysm formation is still being characterized. The prevailing model involves altered arterial wall homeostasis — the balance between smooth muscle cell integrity, extracellular matrix remodeling, and inflammatory signaling that determines whether a vessel wall can withstand hemodynamic stress. Risk variants at this locus may subtly shift this balance toward wall weakening, particularly at high-flow branch points in the circle of Willis.

The Evidence

The original discovery came from a multistage GWAS33 multistage GWAS
genome-wide association study: testing millions of DNA variants simultaneously across thousands of people by Bilguvar et al. (2008, Nature Genetics), conducted in Finnish, Dutch, and Japanese cohorts totalling more than 2,100 intracranial aneurysm cases and 8,000 controls. The 2q33.1 locus was among three genome-wide significant hits, with odds ratios of 1.24–1.36 across the identified loci.

An updated meta-analysis44 updated meta-analysis
pooling results from multiple independent studies for greater statistical power by Hong et al. (2019, J Clin Med) specifically refined the rs700651 association across 18,019 individuals spanning European, Japanese, and Korean populations: OR 1.213 (95% CI 1.135–1.296), achieving genome-wide significance. This means each copy of the G allele increases intracranial aneurysm risk by approximately 21%.

A Korean GWAS55 Korean GWAS by Hong et al. (2019) independently replicated rs700651 alongside rs6841581 (EDNRA), confirming that the 2q33.1 signal is not population-specific. A separate shared-genetics analysis66 shared-genetics analysis examined whether the intracranial aneurysm loci also predispose to abdominal or thoracic aortic aneurysm; limited polygenic overlap was found, suggesting the 2q33.1 variant's primary risk is specific to intracranial vessels rather than systemic arterial fragility.

Practical Actions

Carrying one or two G alleles does not determine fate — most G carriers never develop an aneurysm, and most aneurysms are never detected because they remain small and asymptomatic. However, the elevated risk is real and clinically meaningful, especially in combination with modifiable risk factors: hypertension, smoking, and heavy alcohol use are the strongest environmental amplifiers of aneurysm risk, and all are targetable. For GG homozygotes or AG individuals with additional risk factors (family history of aneurysm, polycystic kidney disease, connective tissue disorders), the evidence supports a conversation with a physician about whether a one-time brain MRA screening scan is appropriate.

Interactions

The most clinically relevant interaction is with rs6841581 in EDNRA (endothelin receptor type A, chromosome 4q31.22), which was identified alongside rs700651 in multiple intracranial aneurysm GWAS studies. EDNRA encodes a receptor for endothelin-1, a potent vasoconstrictor that regulates arterial tone and smooth muscle cell proliferation. The two loci appear to operate through distinct but complementary vascular pathways: the 2q33.1 locus likely influences splicing/regulatory biology, while the EDNRA locus directly affects vasoconstrictor signaling. Neither locus directly interacts with the other at the epistatic level — each contributes independently to aneurysm susceptibility. No compound action is warranted because the individual risk contributions are additive rather than synergistic.

A genome-wide interaction analysis identified a strong epistatic signal between rs700651 and rs1105980 in PTCH1 (the Hedgehog pathway receptor, chromosome 9q22.32): lnOR 1.53, p=6.41×10⁻¹¹. This gene-gene interaction suggests that the BOLL locus risk may be particularly amplified in individuals who also carry certain PTCH1 variants — a finding that merits further investigation.

Genotype Interpretations

What each possible genotype means for this variant.

AA Normal

Common protective genotype — typical intracranial aneurysm risk

You carry two copies of the A allele at rs700651, the most common genotype worldwide (~52% of people globally). This is the protective configuration at this locus: you have not inherited the G risk allele that increases intracranial aneurysm susceptibility. Your genetic risk at this particular variant is baseline. Standard cardiovascular risk management applies, and you should be aware of non-genetic risk factors for brain aneurysm — hypertension in particular — as these apply to everyone regardless of genotype.

AG Intermediate

One copy of the risk allele — moderately elevated intracranial aneurysm risk

The G allele at rs700651 was identified in a 2008 Nature Genetics discovery GWAS spanning Finnish, Dutch, and Japanese cohorts, and confirmed in a 2019 meta-analysis (OR 1.213, 95% CI 1.135–1.296) across 18,019 individuals. As a heterozygote, you carry one copy of this allele, conferring an intermediate level of genetic susceptibility.

The clinical significance depends heavily on context. For someone without other risk factors or family history, this finding is informational but not action-driving beyond standard vascular health practices. For someone with first-degree relatives who have had a brain aneurysm or subarachnoid hemorrhage, this genetic finding adds to the clinical picture and supports earlier discussion with a neurologist or vascular specialist about screening MRA (magnetic resonance angiography).

The variant's mechanism appears to involve altered RNA splicing regulation at the 2q33.1 locus, with cis-eQTL effects on SF3B1, a splicing factor subunit. The downstream effect on vascular wall integrity is not yet fully characterized.

GG High Risk

Two copies of the risk allele — highest genetic susceptibility to intracranial aneurysm at this locus

The additive effect of two G copies at rs700651 compounds the per-allele OR of ~1.21 demonstrated in the 2019 meta-analysis of 18,019 individuals (Hong et al., PMID 31100866). GG homozygotes represent the highest genetic risk tier at this locus.

The absolute population lifetime prevalence of intracranial aneurysm is roughly 2–3%, with roughly half of those ever rupturing. GG homozygotes have a meaningfully higher baseline genetic risk than typical, though the majority will still never develop a clinically significant aneurysm. What changes is the risk-benefit calculation for early screening.

Genome-wide interaction data (PMID 36274247) show that rs700651 interacts epistatically with rs1105980 in PTCH1 (Hedgehog pathway receptor), suggesting that the GG genotype's risk may be further amplified by specific PTCH1 variants — a gene-gene interaction worth noting if you have access to your full genome data.

Blood pressure is the strongest actionable modifier: uncontrolled hypertension increases aneurysm rupture risk dramatically regardless of genetic status, and pharmacological antihypertensive therapy is highly effective. Smoking cessation is the second most impactful lifestyle intervention, reducing aneurysm-related risk substantially.