rs121918476 — PROS1 Arg561Trp

Protein S Arg561Trp — An Anticoagulant Protein That Cannot Leave the Cell

Protein S is one of the body's key natural anticoagulants. It works as a [cofactor for activated protein C | Activated protein C (APC) inactivates coagulation factors Va and VIIIa; protein S amplifies this process roughly 10-fold (https://pubmed.ncbi.nlm.nih.gov/19809585/)¹¹ https://pubmed.ncbi.nlm.nih.gov/19809585/)], which inactivates the clotting factors that would otherwise sustain a thrombus. When protein S is reduced or absent, activated protein C loses much of its braking power over the coagulation cascade — the balance shifts toward clot formation and thrombosis. The Arg561Trp variant (p.Arg561Trp; c.1681C>T) sits in the C-terminal sex hormone-binding globulin (SHBG)-like domain of protein S, a region essential for the protein's structural stability and its ability to be secreted from liver cells into the bloodstream.

The Mechanism

The arginine-to-tryptophan substitution at position 561 disrupts the folding of the SHBG-like domain²² SHBG-like domain
The C-terminal region of protein S is structurally homologous to sex hormone-binding globulin; it is composed of two laminin G-like (LG) domains that coordinate calcium binding and mediate key protein-protein interactions. Missense mutations in this domain characteristically cause impaired intracellular secretion³³ impaired intracellular secretion
The misfolded mutant protein is retained in the endoplasmic reticulum and degraded rather than exported; this is the predominant mechanism for quantitative Type I protein S deficiency — the misfolded mutant protein never reaches the circulation. Heterozygous carriers produce approximately 50% of normal protein S levels from their one functional allele, which is sufficient for most daily demands but leaves little reserve when the coagulation system is activated by surgery, immobility, pregnancy, or oral contraceptives.

The Arg561Trp variant was identified by Li et al. (2019)⁴⁴ Li et al. (2019)
Li L et al. Clinical Manifestation and Mutation Spectrum of 53 Unrelated Pedigrees with Protein S Deficiency in China. Thrombosis and Haemostasis, 2019 as one of three recurrent hotspot mutations in the Chinese protein S deficiency population. In that cohort of 53 pedigrees, over half of probands (52.8%) experienced recurrent or multi-site thrombotic events, with deep venous thrombosis and pulmonary embolism comprising 82.7% of cases.

The Evidence

Hereditary protein S deficiency is an established thrombophilia. Heterozygous PROS1 loss-of-function increases venous thrombosis risk approximately 10- to 20-fold⁵⁵ approximately 10- to 20-fold
Heeb MJ. Role of the PROS1 gene in thrombosis: lessons and controversies. Thrombosis Research, 2009, compared to the general population. Among all individuals carrying a protein S deficiency-causing PROS1 variant, approximately 50% develop at least one VTE event in their lifetime, while the other 50% remain asymptomatic — reflecting the incomplete penetrance characteristic of this condition. Annual incidence of first venous thrombosis in affected individuals is approximately 1.9%, rising to 6-10% risk of recurrence after an index event.

A large population-scale study found that likely-deleterious missense variants in PROS1 (including Arg561Trp class changes) were associated with OR 1.98 for VTE⁶⁶ OR 1.98 for VTE
compared to OR 14.01 for complete loss-of-function variants (nonsense, frameshift, essential splice site), while complete loss-of-function variants carried a dramatically higher OR of 14.01.

Across East Asian populations, PROS1 mutations are substantially more prevalent in VTE patients than in Caucasian cohorts. Japanese studies found PROS1 mutation frequencies 5-10 times higher in thrombosis patients⁷⁷ 5-10 times higher in thrombosis patients
Kinoshita et al., Clinical Biochemistry, 2005 versus Caucasian populations, and Chinese cohort data consistently place protein S deficiency among the leading inherited thrombophilias in the region.

Free protein S antigen — not total protein S or the functional assay — is the recommended first-line laboratory test for detecting heterozygous deficiency. The International Society on Thrombosis and Haemostasis recommends confirming two or more abnormal results at least four weeks apart before diagnosing hereditary deficiency.

Practical Actions

Heterozygous carriers should have free protein S quantitation measured under appropriate conditions (not during acute illness, pregnancy, or while taking warfarin, as these artificially suppress levels). A confirmed low result, combined with the genetic finding, supports thrombophilia specialist referral for personalized VTE risk stratification.

For carriers who have had a VTE event, direct oral anticoagulants (DOACs such as rivaroxaban or apixaban) are now preferred over warfarin for long-term anticoagulation. Vitamin K antagonists like warfarin suppress all vitamin K-dependent proteins including protein S itself, complicating monitoring and occasionally triggering warfarin-induced skin necrosis in protein S-deficient individuals. DOACs avoid this interaction.

High-risk situations requiring VTE prophylaxis discussion with a physician include: major surgery, prolonged immobilization, air travel exceeding eight hours, pregnancy and the postpartum period, and initiation of estrogen-containing contraceptives or hormone replacement therapy.

Homozygous carriers (AA) face severely reduced or absent protein S and are at risk for neonatal purpura fulminans, a life-threatening thrombotic emergency requiring immediate specialist management. This genotype is vanishingly rare given the allele frequency of approximately 0.002-0.003% in the global population.

Interactions

Protein S deficiency interacts with Factor V Leiden (rs6025) and the prothrombin G20210A variant (rs1799963) in a compound fashion. Individuals carrying both protein S deficiency and Factor V Leiden have VTE risks that approximate or exceed those of Factor V Leiden homozygotes. Similarly, compound heterozygosity for two different PROS1 mutations — for example Arg561Trp and a second PROS1 null allele — causes severe Type I protein S deficiency with protein S activity below 30% and a markedly elevated risk of early-onset, recurrent, and atypical-site thrombosis.