Insight in Protein S Deficiency From Mouse Models

Abstract 529

Protein S (ProS) is an important negative regulator of blood coagulation. Its physiological importance is evident in purpura fulminans and other life-threatening thrombotic disorders typical of ProS deficient patients. Our previous characterization of ProS deficiency in mouse models has shown similarities with the human phenotypes: heterozygous ProS-deficient mice (Pros^+/−) had increased thrombotic risk whereas homozygous deficiency in ProS (Pros^−/−) was incompatible with life (Blood 2009; 114:2307-2314). In tissues, ProS exerts cellular functions by binding to and activating tyrosine kinase receptors of the Tyro3 family (TAM) on the cell surface.

To extend the analysis of coagulation defects beyond the Pros^−/− phenotype and add new insights into the sites of synthesis ProS and its action, we generated mice with inactivated ProS in hepatocytes (Pros^lox/loxAlbCre⁺) as well as in endothelial and hematopoietic cells (Pros^lox/loxTie2Cre⁺). Both models resulted in significant reduction of circulating ProS levels and in a remarkable increased thrombotic risk in vivo. In a model of tissue factor (TF)-induced venous thromboembolism (VTE), only 17% of Pros^lox/loxAlbCre⁺ mice (n=12) and only 13% of Pros^lox/loxTie2Cre⁺ mice (n=14) survived, compared with 86% of Pros^lox/lox mice (n=14; P<0.001).

To mimic a severe acquired ProS deficiency, ProS gene was inactivated at the adult stage using the polyI:C-inducible Mx1-Cre system (Pros^lox/loxMx1Cre⁺). Ten days after polyI:C treatment, Pros^lox/loxMx1Cre⁺ mice developed disseminated intravascular coagulation with extensive lung and liver thrombosis.

It is worth noting that no skin lesions compatible with purpura fulminans were observed in any of the above-described models of partial ProS deficiency. In order to shed light on the pathogenesis of purpura fulminans, we exposed the different ProS-deficient mice to warfarin (0.2 mg/day). We observed that Pros^+/−, Pros^lox/loxAlbCre⁺ and Pros^lox/loxTie2Cre⁺ mice developed retiform purpura (characterized by erythematous and necrotic lesions of the genital region and extremities) and died after 3 to 5 days after the first warfarin administration.

In human, ProS is also synthesized by megakaryocytes and hence stored at high concentrations in circulating platelets (pProS). The role of pProS has been investigated by generating megakaryocyte ProS-deficient model using the PF4 promoter as Cre driver (Pros^lox/loxPf4Cre⁺). In the TF-induced VTE model, Pros^lox/loxPf4Cre⁺ (n=15) mice showed a significant increased risk of thrombosis compared to Pros^lox/lox controls (n=14; survival rate 47% and 86%, respectively; P<0.05). Furthermore, preliminary results suggest survival to be associated with higher circulating ProS levels. In order to evaluate the potential role of pProS in thrombus formation, we investigated the thrombotic response to intravenous injection of collagen-epinephrine in vivo and platelet function in vitro. Both in vivo and in vitro experiments showed similar results between Pros^lox/loxPf4Cre⁺ and Pros^lox/lox, indicating that platelet reactivity was not influenced by the absence of pProS. These data suggest that pProS is delivered at the site of thrombosis to inhibit thrombin generation.

We further investigated the ability of ProS to function as a ligand of TAM receptors, by using homozygous and heterozygous deficient mice for both the TAM ligands ProS and Gas6. Gas6^−/−Pros^−/− mice died in utero and showed comparable dramatic bleeding and thrombotic phenotype as described for Pros^−/− embryos.

In conclusion, like complete ProS deficiency, double deficiency in ProS and Gas6 was lethal, whereas partial ProS deficiency was not. Mice partially deficient in ProS displayed a prothrombotic phenotype, including those with only deficiency in pProS. Purpura fulminans did not occur spontaneously in mice with partial Pros deficiency but developed upon warfarin administration.

Thus, the use of different mice models of ProS deficiency can be instrumental in the study of its highly variable thrombotic phenotype and in the investigation of additional roles of ProS in inflammation and autoimmunity through TAM signaling.