Tibetans do not exhibit increased hemoglobin concentration at high altitude. We describe a high-frequency missense mutation in the EGLN1 gene, which encodes prolyl hydroxylase 2 (PHD2), that contributes to this adaptive response. We show that a variant in EGLN1, c.[12C>G; 380G>C], contributes functionally to the Tibetan high-altitude phenotype. PHD2 triggers the degradation of hypoxia-inducible factors (HIFs), which mediate many physiological responses to hypoxia, including erythropoiesis. The PHD2 p.[Asp4Glu; Cys127Ser] variant exhibits a lower Km value for oxygen, suggesting that it promotes increased HIF degradation under hypoxic conditions. Whereas hypoxia stimulates the proliferation of wild-type erythroid progenitors, the proliferation of progenitors with the c.[12C>G; 380G>C] mutation in EGLN1 is significantly impaired under hypoxic culture conditions. We show that the c.[12C>G; 380G>C] mutation originated ~8,000 years ago on the same haplotype previously associated with adaptation to high altitude. The c.[12C>G; 380G>C] mutation abrogates hypoxia-induced and HIF-mediated augmentation of erythropoiesis, which provides a molecular mechanism for the observed protection of Tibetans from polycythemia at high altitude.

Thrombocytopenia is common in patients with dengue virus (DENV) infections. With a focus on understanding the possible mechanism of thrombocytopenia in DENV infections we described a direct correlation between activation and depletion of platelets in patients. Our data showed a sharp decrease in platelet counts at day 4 of fever in patients. The high DENV genome copies in platelets correlated directly with the elevated platelet activation along with increased binding of complement factor C3 and IgG on their surface at day 4. Recovery in platelet count was observed on day 10 through day 6 and 8 with simultaneous decrease in platelet activation markers. Further, our in vitro data supported the above observations describing a concentration-dependent increase in platelet activation by DENV serotype-2. The high copy number of DENV2 genome in the platelet pellet correlated directly with platelet activation, microparticle generation and clot formation. Furthermore the DENV2-activated platelets were phagocytosed in large numbers by the monocytes. The DENV2-mediated lysis and clearance of platelets were abrogated in presence of platelet activation inhibitor, prostacyclin. These observations collectively suggest that platelet activation status is an important determinant of thrombocytopenia in dengue infections. A careful strategy of inactivation of platelets may rescue them from rapid destruction during DENV infections.

SummaryVascular occlusion, thromboembolism and strokes are hallmark events in sickle cell disease (SCD). The von Willebrand factor (VWF), largest adhesive protein in circulation, has been implicated as major component in these processes. In SCD, a high level of extracellular haemoglobin (Hb) in plasma has been shown parallely associated with the disease pathogenesis. Investigating the effect of Hb we observed that purified Hb significantly inhibited the ADAMTS-13 cleavage of VWF under static and flow conditions. Hb bound potently to VWF specifically VWFA2 in a saturation-dependent manner with half-maximal binding 24 nM. Inversely, VWFA2 also bound potently to Hb and binding was inhibited by VP1 antibody, which binds to ADAMTS-13 cleavage site on VWF. Microscopic observation also shows that Hb bound specifically to endothelial VWF under flow. Furthermore, the Hb-bound VWF multimers were isolated from plasma. Though, Hb bound also to ADAMTS-13, it is the Hb binding to VWFA2 that prevented the substrate being cleaved by ADAMTS-13. In an observation in a small pool of patients with SCD, high Hb in plasma was inversely correlated with low proteolytic activity of ADAMTS-13. Thus, the observations suggest that the patients with SCD suffer from an acquired ADAMTS-13 deficiency primarily because Hb competitively bound and blocked the proteolysis of VWF, leading to the accumulation of ultra-large VWF multimers in circulation and on endothelium. Therefore, the Hb-VWF interaction may be considered as a therapeutic target for treating thrombotic and vaso-occlusive complications in patients with severe intravascular haemolysis such as those with SCD.

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