Dr. McCrae's study explores why patients with PNH are at increased risk for the development of thrombosis, or blood clots, that may affect arteries or veins and cause events such as pulmonary emboli or stroke.
One mechanism that may contribute to the development of thrombosis in PNH is the formation of microparticles. Microparticles are small cell-derived membrane fragments that are constitutively released from cells, but released in increased amounts as a consequence of cellular activation or damage. Several studies have reported that elevated levels of microparticles circulate in patients with PNH. Unlike previous studies, we are examining the levels of circulating microparticles in freshly-obtained blood samples (within two hours of sample collection) from patients with PNH. This is important as we have found that freeze/thawing of microparticles can significantly and unpredictably alter microparticle levels compared to fresh samples. To date, we have examined the microparticle levels in eleven PNH patients and 29 normal controls. We have analyzed the levels of circulating endothelial cell, platelet, monocyte, and red blood cell-derived microparticles and also examined whether these microparticles express tissue factor.
Our preliminary studies suggest that red blood cell-derived microparticle and tissue factor levels may be higher in PNH patients compared to normal controls. The endothelial cell, platelet, and monocyte-derived microparticle levels do not appear significantly different from control levels. This is an unexpected finding, but must be considered in light of the fact that all but one of our PNH patients have been treated with eculizumab (Soliris®), which would be expected to prevent complement-mediated cellular damage and minimize microparticle release. It is indeed interesting that in many several PNH patients, microparticle numbers appear lower than the levels detected in normal controls. We hypothesize that this is due to the fact that the alternative pathway (AP) of complement activation is constantly activated even in normal individuals, and inhibition of this pathway by eculizumab actually reduces AP activation to a level below that in control patients. Further analysis and collection of samples, as originally proposed, is needed to refine these conclusions.
We are also currently examining whether circulating microparticles in PNH patients express tissue factor. Tissue factor plays a critical role in the initiation of coagulation and thrombosis. During the past year, we have developed methodology for measuring tissue factor on microparticles. Preliminary data suggests that tissue factor is indeed expressed in the circulating microparticles. We predict that patients with the highest microparticle tissue factor expression will be at greatest risk for the development of thrombosis. It is possible that measurements of this nature may be of use in dictating dosing intervals of eculizumab. Again, further analysis is warranted before conclusions can be drawn.
Originally from Maine, Dr. McCrae earned his undergraduate degree from Dartmouth College and his MD degree from Duke University. After his residency in Internal Medicine at Duke, Dr. McCrae completed his fellowship in Hematology/Oncology at the University of Pennsylvania. During this time, he did a postdoctoral research fellowship that initiated his interest in studying the biology behind antiphospholipid syndrome (APS), a clinical disorder characterized by blood clotting in both arteries and veins and recurrent fetal loss. Today, his laboratory continues research in that area, as well as in the function of a protein called kininogen, which has a known role in the pathway that controls blood clotting. Subsequent studies have shown that kininogen also plays a role in regulating angiogenesis, a process through which new blood vessels are formed. Altogether, Dr. McCrae's research focuses not only on factors involved in blood biology, but also on the cells that line blood vessels. His dual role as physician and as a researcher gives him a unique perspective on how biology of the blood and related systems influence human health and disease.