Key Points Hemostatic plugs develop a regional architecture defined by the extent of platelet activation and packing density. The regional composition of a hemostatic plug is due to the interaction of local conditions with the platelet-signaling network.

Disorders resulting from HBB (β-globin) gene mutations, mainly sickle cell disease (SCD) and β-thalassemia, become symptomatic postnatally as fetal γ-globin expression from two parologous genes HBG1 and HBG2 falls and adult β-globin increases, thereby shifting red blood cell (RBC) hemoglobin from the fetal (HbF, α2γ2) to adult form (HbA, α2β2). These disorders are alleviated when postnatal expression of fetal γ-globin is maintained. For example, in hereditary persistence of fetal hemoglobin (HPFH), a benign genetic condition, mutations attenuate γ-to-β switching, causing high-level HbF expression throughout life. Co-inheritance of HPFH with β-thalassemia or SCD mutations alleviates their clinical manifestations. Here we performed CRISPR-Cas9-mediated genome editing of human blood progenitors to mutate a 13-nucleotide HBG1/HBG2 promoter sequence, thereby recapitulating a naturally occurring HPFH-associated mutation. Edited progenitors produced RBCs with increased HbF levels that were sufficient to inhibit pathological hypoxia-induced RBC morphology of SCD. Our findings identify a potential DNA target for genome editing-mediated therapy of β–hemoglobinopathies.

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Activated platelets shed surface proteins, potentially modifying platelet function as well as providing a source of bioactive fragments. Previous studies have identified several constituents of the platelet sheddome, but the full extent of shedding is unknown. Here we have taken a global approach, analyzing protein fragments in the supernate of activated platelets using mass spectroscopy and looking for proteins originating from platelet membranes. After removing plasma proteins and microparticles, 1048 proteins were identified, including 69 membrane proteins. Nearly all of the membrane proteins had been detected previously, but only 10 had been shown to be shed in platelets. The remaining 59 are candidates subject to confirmation. Based on spectral counts, protein representation in the sheddome varies considerably. As proof of principle, we validated one of the less frequently detected proteins, semaphorin 7A, which had not previously been identified in platelets. Surface expression, cleavage, and shedding of semaphorin 7A were demonstrated, as was its association with ␣-granules. Finally, cleavage of semaphorin 7A and 12 other proteins was substantially reduced by an inhibitor of ADAM17, a known sheddase. These results define a subset of membrane proteins as sheddome candidates, forming the basis for further studies examining the impact of ectodomain shedding on platelet function. (Blood. 2011;117(1): e15-e26)