rs1808593 — NOS3

NOS3 rs1808593 — The Peripheral Circulation Variant

Your NOS3 gene encodes endothelial nitric oxide synthase (eNOS), the enzyme that produces nitric oxide (NO)¹¹ nitric oxide (NO)
a vasodilatory signaling molecule that relaxes blood vessels, maintains blood flow, and protects arterial walls throughout the vascular system. While the most studied NOS3 variant is the coding Glu298Asp (rs1799983), the gene harbors numerous regulatory and intronic polymorphisms that influence eNOS expression or function in subtler ways. rs1808593 is one such intronic variant — its G allele was independently associated with a lower ankle-brachial index (ABI)²² ankle-brachial index (ABI)
a ratio of blood pressure measured at the ankle compared to the arm; low ABI signals reduced arterial blood flow to the legs, the defining measurement for peripheral arterial disease in two replicated studies of hypertensive adults.

The Mechanism

rs1808593 falls within an intron of NOS3 (chromosome 7q35-36). Intronic variants in this region can act as regulatory splicing elements or transcription factor binding sites³³ regulatory splicing elements or transcription factor binding sites
sequences that influence how much eNOS protein is produced or how efficiently the mRNA is processed. The exact molecular mechanism for rs1808593 has not been experimentally defined, but it lies in significant linkage disequilibrium⁴⁴ linkage disequilibrium
the tendency for alleles at nearby loci to be inherited together more often than expected by chance with rs891512, another intronic NOS3 variant in intron 25 that is predicted to impair SF2/ASF splicing factor binding. Whether rs1808593 acts independently, tags a nearby functional variant, or participates in a compound haplotype effect with rs891512 and rs7830 remains to be determined by functional studies.

The Evidence

The primary evidence comes from two complementary genetic studies of hypertensive adults. Kullo et al. (2008)⁵⁵ Kullo et al. (2008)
Association of Polymorphisms in NOS3 with the Ankle-Brachial Index in Hypertensive Adults; Atherosclerosis 2008 genotyped 14 NOS3 polymorphisms in 659 hypertensive subjects (mean age 61 years, 54% women) divided into two independent replication subsets of approximately 330 each. Carriers of the minor G allele had significantly lower mean ABI values than TT homozygotes in both subsets (p=0.0012 and p=0.0302, respectively), a rigorous replication design specifically chosen to reduce false positives. The rs1808593–rs7830 two-SNP haplotype was also significantly associated with ABI in both subsets, with the GG haplotype producing an effect of approximately −0.023 to −0.024 ABI units.

A larger follow-up study by Kullo et al. (2008)⁶⁶ Kullo et al. (2008)
Investigating the complex genetic architecture of ankle-brachial index in non-Hispanic whites; BMC Medical Genomics 2008;1:16 examined 435 SNPs across 112 candidate genes in 1,046 hypertensive subjects and applied three independent validation criteria (FDR <0.30, internal replication, and cross-validation). Of all 435 SNPs tested, only two — rs891512 and rs1808593, both in NOS3 — survived all three filters. rs1808593 explained 1.8% of ABI variance (R²=0.0178, p=0.0001), a substantial result for a common intronic variant in a complex trait.

These findings are biologically plausible given NOS3's established role in peripheral vascular function. Reduced NO bioavailability in peripheral arteries leads to impaired vasodilation, higher vascular resistance, and reduced arterial blood flow to the limbs⁷⁷ Reduced NO bioavailability in peripheral arteries leads to impaired vasodilation, higher vascular resistance, and reduced arterial blood flow to the limbs
the core pathophysiology of peripheral arterial disease, which causes claudication, non-healing wounds, and increased cardiovascular mortality. The evidence to date is rated moderate — replicated in two independent cohorts with a clear biological mechanism in the gene, but limited to hypertensive adults of non-Hispanic white ancestry and not yet validated in other ethnicities or functional studies.

Practical Implications

An ABI below 0.9 defines peripheral arterial disease and signals roughly two- to four-fold increased risk of cardiovascular events. The G allele's association with lower ABI in hypertensive adults means G carriers may have reduced blood flow to their legs even before clinical PAD is diagnosed. Because this variant acts through the NO pathway, the same dietary strategy that supports eNOS function broadly — emphasizing dietary nitrate from beetroot, spinach, and arugula — provides an alternative NO production pathway that bypasses any genetic compromise at the eNOS enzyme itself. Dietary nitrate is converted by oral bacteria to nitrite and then to NO⁸⁸ Dietary nitrate is converted by oral bacteria to nitrite and then to NO
the entero-salivary nitrate-nitrite-NO pathway, active throughout the vasculature including peripheral arteries. Small studies in PAD patients suggest dietary nitrate improves skeletal muscle microvascular function and may modestly benefit coronary blood flow response during exercise, though walking distance improvements have not yet reached significance in trials.

Ankle-brachial index testing is simple and non-invasive; it is recommended for adults over 50 with hypertension or other cardiovascular risk factors, and the earlier PAD is detected, the more opportunity there is for lifestyle intervention and risk reduction.

Interactions

rs1808593 is in significant linkage disequilibrium with rs891512, another intronic NOS3 variant associated with blood pressure and exercise-induced BP response. Carrying the G allele at rs1808593 alongside the A allele at rs891512 may compound vascular risk through overlapping intronic effects on eNOS. Both variants also sit in the same gene as the well-characterized Glu298Asp variant (rs1799983) — the coding variant that accelerates eNOS protein degradation. A carrier of G at rs1808593 plus T at rs1799983 would have both regulatory and structural impairments of the same enzyme, an additive burden on peripheral NO availability.