A newly identified platelet and megakaryocyte lysyl oxidase adhesion to collagen axis in human primary myelofibrosis

Controlling platelet function is central to management of various pathologies, including Primary Myelofibrosis (PMF), which is associated with increased incidence of thrombosis and cardiovascular disease. In recent studies we showed that the matrix cross-linking enzyme, Lysyl Oxidase (LOX) is elevated in platelets and megakartocytes of myelofibrotic mice, and transgenic upregulation of LOX increases platelet and megakaryocyte adhesion to monomeric type I collagen (preferred by alpha2β1 collagen receptors), and augments propensity for in vivo thrombosis. Here, we examined the relevance of these findings to human disease, by first determining platelet LOX level, as well as platelet and megakaryocyte adhesion to collagen using samples derived from PMF patients and matching controls. In analyzing 10 PMF platelet samples (5 males and 5 females; 6 JAK2V617F; 4 CALR mutations; age range 30-55; PMF grade 1-3), we found a nearly 20 fold upregulation of LOX expression compared to matching healthy controls (p<0.001). Intriguingly, there was a significant increase in adhesion (plt/mm2) and spreading (pixel2) of PMF platelets relative to control on monomeric, pepsinated acid soluble collagen (PSCI) (p<0.05), while no differences were observed between the samples on native triple helical acid soluble collagen type I collagen (ASCI). To examine the role of LOX in this phenotype, we treated control and PMF-derived human megakaryocytes, differentiated from peripheral blood CD34+ cells, grown in presence or not of LOX inhibitor, β-aminopropionitrile (BAPN) from day 2 of culture. Our preliminary data, based on a cohort of 2 controls and 5 PMF samples, demonstrated that although on ASCI megakaryocyte adhesion is not altered by BAPN treatment both in CTRL and PMF derived megakaryocytes, on PSCI the adhesion of PMF derived megakaryocytes was reduced by about a 50% by BAPN treatment, while the adhesion of CTRL derived MKs was not significantly affected. Taken together, we identified LOX level to be upregulated in human PMF platelets and megakaryocytes, and LOX activity to be important for PMF cells adhesion to collagen. These newly identified properties are highly relevant to megakaryocyte adhesion to the niche, and to platelet activation in PMF.