rs121918475 — PROS1 Q279X

PROS1 Q279X — A Stop Codon That Disables the Clotting Brake

Blood clots when you need them to; the rest of the time, a network of natural anticoagulants keeps coagulation in check. Protein S — encoded by the PROS1 gene — is one of those brakes. It acts as a cofactor for activated protein C (APC)¹¹ activated protein C (APC)
APC cleaves and inactivates coagulation Factors Va and VIIIa, shutting down the amplification loop that generates thrombin, the enzyme that forms blood clots. Without sufficient protein S, APC cannot do its job efficiently, and coagulation goes unrestrained. rs121918475 — known as the Q279X or p.Gln279Ter variant — introduces a premature stop codon²² premature stop codon
CAG (glutamine) is changed to TAG (stop) at amino acid position 279 of the canonical PROS1 isoform; position 311 in the longer isoform at a site well before the protein's functional C-terminus, producing a severely truncated, non-functional protein. This is a pathogenic, well-characterized cause of hereditary protein S deficiency.

The Mechanism

PROS1 encodes a 676-amino-acid vitamin K–dependent plasma glycoprotein. The Q279X variant truncates the protein at position 279, eliminating the two Laminin G-like domains³³ Laminin G-like domains
Laminin G domains mediate the interaction between protein S and APC, as well as protein S's direct role in inhibiting Factor Xa and Factor Va independently of APC in the C-terminus that are essential for APC cofactor activity. The truncated product — if it is even stably expressed — retains no meaningful anticoagulant function.

Because Q279X is a stop-gain variant, the truncated transcript also undergoes nonsense-mediated mRNA decay (NMD)⁴⁴ nonsense-mediated mRNA decay (NMD)
A cellular surveillance mechanism that degrades mRNA molecules with premature stop codons to prevent synthesis of aberrant proteins in most mammalian cells, meaning protein S output from the affected allele may be near zero. The result in heterozygotes is typically a Type I protein S deficiency: plasma levels of total protein S antigen, free protein S antigen, and protein S activity are all reduced to roughly 40–60% of normal — consistent with the population-level measurement of 48% of normal in high-risk PROS1 variant carriers from the 2025 UK Biobank study⁵⁵ 2025 UK Biobank study
Chaudhry et al. measured total protein S levels in carriers of high-risk PROS1 variants (functional impact score = 1.0) across 426,436 UK Biobank participants.

The Evidence

This variant has the highest tier of clinical evidence: it is classified Pathogenic in ClinVar (OMIM 176880.0007), it has been confirmed through direct PROS1 mutational screening in affected families, and it is curated by the OMIM-Curated Records review panel. The Q279X change has been documented in multiple independent families and confirmed by haplotype analysis⁶⁶ haplotype analysis
Hurtado et al. showed that Q279X arises on more than one haplotype background, indicating it has occurred as a recurrent de novo mutation rather than descending from a single founder, establishing it as a recurrent pathogenic variant rather than a population-specific founder mutation.

At the population scale, a 2025 JAMA study of 630,000 biobank participants⁷⁷ 2025 JAMA study of 630,000 biobank participants
Chaudhry, Haj, Ryu et al., published April 2025 in JAMA, combining UK Biobank (426,436) and All of Us (204,006) cohorts found that carriers of high-risk PROS1 variants — loss-of-function alleles with a functional impact score of 1.0, in the same tier as Q279X — face an adjusted odds ratio of 14.01 (95% CI 6.98–27.14, P = 9.09 × 10⁻¹¹) for venous thromboembolism, making it one of the strongest single-gene thrombophilia risk factors quantified in unselected population cohorts.

For pregnancy specifically⁸⁸ pregnancy specifically
Croles et al. 2017 meta-analysis of 36 observational studies covering all major thrombophilias during pregnancy, protein S deficiency raises the absolute postpartum VTE risk to 4.2% (95% CrI 0.7–9.4%). In women using combined oral contraceptives⁹⁹ women using combined oral contraceptives
Van Vlijmen et al. 2016 systematic review and meta-analysis; "severe" thrombophilias defined as antithrombin, protein C, or protein S deficiency, VTE risk rises to an absolute rate of 4.3–4.6 per 100 pill-years — a level at which guidelines uniformly recommend against combined hormonal contraceptive use.

The 2023 American Society of Hematology guidelines on thrombophilia testing¹⁰¹⁰ thrombophilia testing
Middeldorp et al., Blood Advances 2023 conditionally recommend testing first-degree relatives of known protein S deficiency carriers before starting combined hormonal contraceptives or other thrombosis-promoting therapies, and before pregnancy.

Practical Implications

A Q279X carrier faces three categories of actionable risk. First, protein S levels should be confirmed by laboratory testing — plasma total and free protein S antigen plus protein S activity — both to quantify the deficiency and to document it for medical providers. Second, situations that further activate the coagulation system (combined hormonal contraceptives, surgery, immobility, pregnancy, postpartum period) carry acutely elevated thrombosis risk that is quantifiable and in most cases preventable with appropriate prophylaxis. Third, thrombosis events themselves may require longer anticoagulation than in non-carriers, since the underlying coagulation imbalance persists after an acute event.

For anticoagulation after a VTE event, full-dose DOACs (rivaroxaban, apixaban)¹¹¹¹ full-dose DOACs (rivaroxaban, apixaban)
Kovac et al. 2024 ISTH SSC communication; hazard ratios for recurrent VTE on full-dose DOACs ranged from 0.3 to 0.75 in cohort studies; low-dose DOACs were not assessed in severe thrombophilia and should be avoided appear comparable to warfarin in severe thrombophilia, with caution against reduced-dose ("half-tablet") regimens due to insufficient evidence.

Interactions

The most clinically important interaction is with Factor V Leiden (rs6025, F5 R506Q)¹²¹² Factor V Leiden (rs6025, F5 R506Q)
Factor V Leiden is the most common inherited thrombophilia (~5% of Europeans); it creates a coagulation Factor Va molecule resistant to cleavage by APC, meaning protein S deficiency and Factor V Leiden attack the same APC-dependent pathway from different sides. A carrier of both Q279X and Factor V Leiden would face a compounded impairment of the APC anticoagulant pathway, with predicted risk exceeding either variant alone. Similarly, the prothrombin G20210A variant (rs1799963)¹³¹³ prothrombin G20210A variant (rs1799963)
Raises plasma prothrombin 30%, pushing the coagulation system toward clotting while protein S deficiency simultaneously weakens the brake would be additive with protein S deficiency on the thrombosis side of the ledger.

Acquired thrombophilic states — antiphospholipid syndrome, active malignancy, nephrotic syndrome (which causes urinary loss of protein S), pregnancy, and immobilization — all compound with Q279X in an additive or synergistic manner. Vitamin K antagonists (warfarin) paradoxically reduce protein S levels as part of their mechanism of action; this is relevant when initiating anticoagulation or managing a carrier who is perioperatively bridged.