rs422187 — F9 Intronic F9 variant

F9 Intronic Variant — An Intergenic Tag for the Factor IX DVT Haplotype

Coagulation factor IX is the gatekeeper of the intrinsic clotting pathway. Synthesised in the liver and secreted into the circulation as an inactive zymogen, it is activated by factor XIa to form the tenase complex¹¹ tenase complex
a membrane-assembled enzyme comprising activated factor IX (FIXa), cofactor VIIIa, calcium, and phospholipid that amplifies factor X activation approximately 50,000-fold. Severe loss-of-function mutations in F9 cause haemophilia B. A far more common story is the intronic variant rs422187 — an intron-embedded single nucleotide change sitting 362 bp upstream of exon 5 in the F9 pre-mRNA, with no direct protein-coding consequence. Its relevance comes entirely from what it tracks: the rs6048 missense haplotype (Factor IX Malmö) and its documented modest protection against deep vein thrombosis.

Because F9 sits on the X chromosome (Xq27.1), males are hemizygous — they carry exactly one allele at rs422187. Genotyping chips typically report hemizygous males as homozygous in raw data, so a male reported as AA carries one A allele and a male reported as CC carries one C allele.

The Mechanism

The rs422187 variant sits at chrX:139550700 (GRCh38), 421 bp from the rs6048 coding variant at position 139551121. Its HGVS transcript designation is c.521-362A>C — 362 nucleotides into the intron preceding coding position 521. VEP classifies the consequence as intron_variant with MODIFIER impact, predicting no direct disruption to splicing signals or protein sequence. There is no curated ClinVar entry for this variant, consistent with its designation as a population-level tag SNP rather than a functional variant in its own right.

The striking feature of rs422187 is its near-perfect linkage disequilibrium with rs6048 in European populations: r²=0.94 in the combined LETS and MEGA cohorts²² r²=0.94 in the combined LETS and MEGA cohorts
Bezemer et al. Haematologica 2009;94(5):693–9, and r²=1.0, D'=1.0 in CEU samples from 1000 Genomes Phase 3. The allele frequencies mirror each other precisely — the C allele of rs422187 (≈30% European) co-segregates with the G allele of rs6048 on the same haplotype. This complete co-inheritance means the two variants are functionally indistinguishable at current resolution: any causal effect attributed to rs6048 could in principle be mediated by rs422187 or any other variant in tight LD.

Bezemer and colleagues investigated 28 nearby variants alongside rs6048 in their primary analysis. rs422187 showed "similarly associated" DVT risk to rs6048 itself, but rs6048 emerged as the single most strongly associated variant — suggesting the missense change (Thr194Ala) may be causal, with rs422187 serving as a correlated intron tag. However, since factor IX antigen levels, activation peptide levels, and endogenous thrombin potential did not differ between genotype groups, the mechanism of protective effect — whether attributed to rs6048 or rs422187 — remains genuinely unknown.

The Evidence

The primary association evidence comes from the Bezemer 2009 LETS + MEGA case-control analysis³³ Bezemer 2009 LETS + MEGA case-control analysis
Irene D Bezemer et al. "F9 Malmö, factor IX and deep vein thrombosis." Haematologica 2009;94(5):693–9. Combined n=1,849 men (LETS n=380, MEGA n=1,469). The analysis specifically tested rs422187 among 28 nearby variants and found it in near-perfect LD with rs6048, with comparable DVT association. The lead signal (rs6048 G allele) carried an odds ratio of 0.80 (95% CI 0.69–0.93) for DVT protection. Because of the near-perfect LD, rs422187 C allele carriers show essentially the same risk shift.

This locus was subsequently confirmed in two large GWAS studies. The Klarin 2019 genome-wide study⁴⁴ Klarin 2019 genome-wide study
Derek Klarin et al. "Genome-wide association analysis of venous thromboembolism identifies new risk loci and genetic overlap with arterial vascular disease." Nature Genetics 2019;51:1574–1579. Over 650,000 participants, MVP + UK Biobank identified the F9 locus among 33 genomic loci associated with VTE. The Thibord 2022 cross-ancestry meta-analysis⁵⁵ Thibord 2022 cross-ancestry meta-analysis
Florian Thibord et al. "Cross-Ancestry Investigation of Venous Thromboembolism Genomic Predictors." Circulation 2022;146:1225–1242. 81,669 VTE cases across 30 studies replicated the F9 signal across European, African, and Hispanic populations, confirming the population-generalisable nature of the protective association.

Recurrence risk was addressed by Roach et al. 2015⁶⁶ Roach et al. 2015
REH Roach et al. "The F9 Malmö sequence variant and sex-related differences in recurrence risk in patients with a first VTE." J Thromb Haemost 2015;13(10):1815–22. Four European cohorts: n=2,185, which found that the Factor IX Malmö haplotype did not explain the observed sex difference in VTE recurrence. The modest protective effect appears to apply primarily to first VTE events.

The effect is modest — OR ~0.80, equivalent to roughly a 20% reduction in first DVT odds. It does not approach the magnitude of Factor V Leiden (OR ~5–7 in heterozygotes) or prothrombin G20210A (OR ~3). The C/G allele of this locus is a tilt, not a shield.

Practical Actions

For carriers of one or two C alleles at rs422187, the practical implications parallel those of rs6048. The modest DVT protection shifts baseline probability slightly downward but does not override other thrombophilic risk factors. Standard DVT prevention measures — particularly during surgery, prolonged immobility, or when taking combined hormonal contraceptives — remain important regardless of this variant's protective direction.

For AA homozygotes (the most common genotype in most populations), no specific action is indicated — this genotype represents population-average factor IX pathway function, and the variant itself is non-coding. Other coagulation variants (Factor V Leiden, prothrombin G20210A) carry much larger effect sizes and are the primary determinants of inherited thrombophilia risk.

The most variable population is African (C allele frequency ~57%) versus East Asian (C allele frequency <1%), meaning the protective allele is substantially more common in populations of African ancestry than in European or East Asian populations.

Interactions

rs422187 and rs6048 are in near-perfect LD and should be treated as marking the same haplotype. If both appear in a genetic profile, they are reporting the same underlying signal. The most clinically relevant interactions are with other coagulation variants: Factor V Leiden (rs6025, OR ~5–7 for heterozygotes) and prothrombin G20210A (rs1799963, OR ~3) are the dominant determinants of inherited thrombophilia risk. The modest protective effect of the F9 C allele would partially offset, but not neutralise, the elevated risk conferred by these stronger variants.