Proteomic Analysis of Gray Platelet Syndrome by iTRAQ Labelling and Mass Spectroscopy: A Potential New Diagnostic Strategy for Platelet Disorders.

The recent introduction of novel proteomic techniques to evaluate normal human platelet proteins and their modifications following activation may also help to elucidate the causes and improve diagnoses of platelet-related bleeding disorders. Previous studies from this and other laboratories have demonstrated the sensitivity and reliability of this new technique to detect minor changes (+/−20%) in the protein content of platelets. More than 600 proteins shared by normal platelets were identified, and >200 in platelet-derived microparticles (MPs) generated by ionophore or Thrombin Receptor Activating Peptide (TRAP) activation. As a prelude to our ultimate goal of establishing this new technology, we applied the iTRAQ method (multiplexed relative protein quantitation by mass spectrometry) in the study of two unrelated patients with the Gray Platelet Syndrome (GPS).

Megakaryocytes of patients with GPS can synthesize alpha-granule proteins and enclose them within membranes. However, the membranes lack structure linked latency, and enclosed proteins leak out of the organelles before platelets are delivered to circulating blood. Thus, GPS platelets contain empty alpha-granule vacuoles instead of alpha-granules. Due to large changes in platelet properties, it was surprising to find that cytoskeletal proteins of GPS were present in similar abundance to normal platelets (i.e. in the range of 0.80–1.20). Receptor proteins showed a similar distribution. As expected, the content of most alpha-granule proteins in the platelets from the 2 GPS patients was markedly reduced, with individual protein abundance ratios of 0.10 to 0.52 compared to normal platelets. However, certain trans-membrane proteins of alpha-granules (such as vesicle-associated proteins and P-selectin) were better preserved in GPS platelets, with ratios in the range of 0.73 to 1.16 compared to normal platelets. These results showed that many structural components (such as cytoskeleton and receptor proteins) are unchanged in GPS platelets. The results support our previous data indicating that detection of a change of as little as 20% may indicate a pathological change in the platelet proteome that may impact normal function. Our results therefore demonstrate that highly sensitive methods, capable of detecting small protein changes, may be needed to fully appreciate pathological disorders. The iTRAQ technique is able to detect low level changes and may be a leading tool that can generate new insights into the molecular pathophysiology and genetic variations of platelet disorders.