Enkurin: A novel marker for myeloproliferative neoplasms from platelet, megakaryocyte, and whole blood specimens.

Impaired protein homeostasis, though well established in age-related disorders, has been linked in recent research with the pathogenesis of myeloproliferative neoplasms (MPNs). As yet, however, little is known about MPN-specific modulators of proteostasis, thus impeding our ability for increased mechanistic understanding and discovery of additional therapeutic targets. Loss of proteostasis, in itself, is traced to dysregulated mechanisms in protein folding and intracellular calcium signaling at the endoplasmic reticulum (ER). Here, using ex vivo and in vitro systems (including CD34+ cultures from patient bone marrow, and healthy cord/peripheral blood specimens), we extend our prior data from MPN patient platelet RNA sequencing, and discover select proteostasis-associated markers at RNA and/or protein levels in each of platelets, parent megakaryocytes, and whole blood specimens. Importantly, we identify a novel role in MPNs for enkurin (ENKUR), a calcium mediator protein, implicated originally only in spermatogenesis. Our data reveal consistent ENKUR downregulation at both RNA and protein levels across MPN patient specimens and experimental models, with a concomitant upregulation of a cell cycle marker, CDC20. Silencing of ENKUR by shRNA in CD34+ derived megakaryocytes further confirm this association with CDC20 at both RNA and protein levels; and indicate a likely role for the PI3K/Akt pathway. The inverse association of ENKUR and CDC20 expression was further confirmed upon treatment with thapsigargin (an agent that causes protein misfolding in the ER by selective loss of calcium) in both megakaryocyte and platelet fractions at RNA and protein levels. Together, our work sheds light on enkurin as a novel marker of MPN pathogenesis beyond the genetic alterations; and indicates further mechanistic investigation into a role for dysregulated calcium homeostasis, and ER and protein folding stress in MPN transformation.