title: Dysregulation of the intrinsic apoptotic pathway mediates megakaryocytic hyperplasia in myeloproliferative neoplasms.
abstract: OBJECTIVE
  Megakaryocyte expansion in myeloproliferative neoplasms (MPNs) is due to uncontrolled proliferation accompanied by dysregulation of proapoptotic and antiapoptotic mechanisms. Here we have investigated the intrinsic and extrinsic apoptotic pathways of megakaryocytes in human MPNs to further define the mechanisms involved.
  
  
  METHODS
  The megakaryocytic expression of proapoptotic caspase-8, caspase-9, Diablo, p53 and antiapoptotic survivin proteins was investigated in bone marrow specimens of the MPNs (n=145) and controls (n=15) using immunohistochemistry. The megakaryocyte percentage positivity was assessed by light microscopy and correlated with the MPN entity, JAK2V617F/CALR mutation status and platelet count.
  
  
  RESULTS
  The proportion of megakaryocytes in the MPNs expressing caspase-8, caspase-9, Diablo, survivin and p53 was significantly greater than controls. A greater proportion of myeloproliferative megakaryocytes expressed survivin relative to its reciprocal inhibitor, Diablo. Differences were seen between myelofibrosis, polycythaemia vera and essential thrombocythaemia for caspase-9 and p53. CALR-mutated cases had greater megakaryocyte p53 positivity compared to those with the JAK2V617Fmutation. Proapoptotic caspase-9 expression showed a positive correlation with platelet count, which was most marked in myelofibrosis and CALR-mutated cases.
  
  
  CONCLUSIONS
  Disruptions targeting the intrinsic apoptotic cascade promote megakaryocyte hyperplasia and thrombocytosis in the MPNs. There is progressive dysfunction of apoptosis as evidenced by the marked reduction in proapoptotic caspase-9 and accumulation of p53 in myelofibrosis. The dysfunction of caspase-9, which is necessary for proplatelet formation, may be the mechanism for the excess thrombocytosis associated with CALR mutations. Survivin seems to be the key protein mediating the megakaryocyte survival signature in the MPNs and is a potential therapeutic target.
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia.
authors.forename: Jacques A J
authors.initials: JA
authors.lastname: Malherbe
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia PathWest Laboratory Medicine, Nedlands, Western Australia, Australia.
authors.forename: Kathryn A
authors.initials: KA
authors.lastname: Fuller
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia PathWest Laboratory Medicine, Nedlands, Western Australia, Australia.
authors.forename: Bob
authors.initials: B
authors.lastname: Mirzai
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia PathWest Laboratory Medicine, Nedlands, Western Australia, Australia.
authors.forename: Simon
authors.initials: S
authors.lastname: Kavanagh
authors.affiliation: Department of Pathology, Faculty of Medicine, University of Hong Kong, Hong Kong, Hong Kong.
authors.forename: Chi-Chiu
authors.initials: CC
authors.lastname: So
authors.affiliation: Department of Pathology & Clinical Biochemistry, Queen Mary Hospital, Hong Kong, Hong Kong.
authors.forename: Ho-Wan
authors.initials: HW
authors.lastname: Ip
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia.
authors.forename: Belinda B
authors.initials: BB
authors.lastname: Guo
authors.affiliation: Jarrett Street Specialist Centre, North Gosford, New South Wales, Australia.
authors.forename: Cecily
authors.initials: C
authors.lastname: Forsyth
authors.affiliation: PathWest Laboratory Medicine, Nedlands, Western Australia, Australia.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Howman
authors.affiliation: School of Pathology and Laboratory Medicine, University of Western Australia, Crawley, Western Australia, Australia PathWest Laboratory Medicine, Nedlands, Western Australia, Australia.
authors.forename: Wendy N
authors.initials: WN
authors.lastname: Erber
keywords: APOPTOSIS; IMMUNOHISTOCHEMISTRY; MYELOPROLIFERATIVE DISEASE