Severely Impaired Platelet Function Due to a Novel Mutation of P2Y12 from a Human Subject with No History of Bleeding

Platelets mediate hemostasis through amplifying an initial stimulus and aggregating at a site of injury. The identification of bleeding phenotypes in humans has the potential to reveal new insight into the mechanisms behind platelet signaling, as well as pinpoint new potential therapeutic targets. In fact, the study of bleeding disorders in human subjects has led to the identification of a number of disease states and proteins important to platelet functional responsiveness.

We have recently identified a human subject (PDS25) with a platelet functional disorder that has a normal CBC, and no history of bleeding diathesis. However, platelets from PDS25 have virtually no response to even high concentrations of 2-MeSADP, even though P2Y12 protein expression appears unaltered (Figure 1). Further, platelet reactivity to high doses of agonist for other receptors, such as PAR-4 and GPVI, is normal. Low concentrations of these agonists, which typically rely on feedback for platelet reactivity, elicit an inhibited response using platelets from PDS25 compared to platelets from a healthy control. Consistently, Rap1b activity (Figure 2) was reduced in platelets from PDS25, while VASP phosphorylation was enhanced, suggesting that signaling from the P2Y12 receptor was interrupted by the heterozygous mutation. To determine whether it is the receptor or a downstream signaling component that is dysfunctional and because shape change in response to 2-MeSADP is normal in platelets from PDS25, we co-stimulated 2-MeSADP treated platelets with epinephrine to initiate signaling through G_z, which is very similar to G_i. We found that the response of platelets from PDS25 to 2-MeSADP can be rescued with the addition of epinephrine, suggesting that the signaling components downstream of G_i/zare intact. These data suggest that the aberrant reactivity observed in platelets from PDS25 is due to the P2Y12 receptor. Therefore, we performed a genetic analysis of P2Y12 from PDS25, which revealed a heterozygous mutation of D121N within the conserved DRY motif (Figure 3). We are currently evaluating how this heterozygous mutation confers such a strong phenotype.

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