Microvesicles were historically regarded as cellular debris with no apparent function. However, a growing body of experimental data has suggested that microvesicles have numerous biological activities. For example, platelet-derived microvesicles were shown to stimulate selected cells via surface proteins on the microvesicles (e.g., Thromb. Haemost. (1999), 82:794, or J. Biol. Chem. (1999), 274:7545). In other examples, specific effects of bioactive lipids in platelet microvesicles on certain target cells were reported (e.g., J. Biol. Chem. (2001), 276: 19672; or Cardiovasc. Res. (2001), 49(5):88). In still further examples, platelet microvesicles increased adhesion of mobilized CD34+ endothelial cells by transfer of certain microvesicle surface components to the mobilized cells (e.g., Blood (2001), 89:3143).
More recently, microvesicles have also been shown to comprise RNA that at least in part appeared to reflect the RNA content of the cell from which they originate. Microvesicles have also been shown to have significant biological effect on other cells, probably due to the RNA present in the microvesicles, and various examples and aspects for such microvesicles are described in our commonly owned International application (WO2005/121369), which expressly forms part of this application.
In further known reports, microvesicles were described as including non-coding miRNA (microRNA) that could potentially interfere or regulate gene expression in cells with which such microvesicles merge (PLoS November 2008, Vol. 3(11), e3694). Other reports discuss in vitro cell-to-cell signaling via exosomal RNA (Cell Adh Migr 1:3, 156-158; 2007; Cancer Immunol Immunother 2006 July; 55(7):808-18; Blood. 2007 Oct. 1; 110(7):2440-8.). It was also shown that while some exosomal RNA was functional and translatable in a recipient cell, many of the RNA molecules present in the exosomes were not present in the cytoplasm of cells from which the exosomes were though to have originated (Nat Cell Biol. 2007 June; 9(6):654-9). U.S. Pat. No. 6,916,634 teaches that while RNA is generally instable in serum and readily hydrolyzed by RNAses, other RNA is resistant to RNAse attack, presumably due to its varying association with circulating particles. Remarkably, the '634 patent elaborates on the chemically and structurally highly diverse nature of the RNA associated particles (presumably due to their diverse origin and manner of generation) and thus concludes that serum RNA is best isolated in an indiscriminate manner. Thus, even though membrane associated or vesiculated RNA has more recently been reported, there is a large body of contradictory data and hypotheses with respect to the nature, quality, availability, and origin/manner of generation of microvesicles.
Consequently, the enormous diagnostic potential of RNA-containing microvesicles has not been fully recognized in the art, and there is still a need for microvesicle-based diagnostic compositions and methods in which RNA from microvesicles is enriched and differentiated to so obtain a result that is indicative of the condition of tissue or organ from which the microvesicle originated.