Dysregulation of the intrinsic apoptotic pathway mediates megakaryocytic hyperplasia in myeloproliferative neoplasms

Abstract

AimsMegakaryocyte 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.MethodsThe 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.ResultsThe 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 JAK2V617F mutation. Proapoptotic caspase-9 expression showed a positive correlation with platelet count, which was most marked in myelofibrosis and CALR-mutated cases.ConclusionsDisruptions 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.

Effects of polymorphic DNA genes involved in BER and caspase pathways on the clinical outcome of myeloproliferative neoplasms under treatment with hydroxyurea

“…Several studies described modifications in the expression of molecules that participate in the regulation of intrinsic and extrinsic routes of apoptosis, as well as functional studies that showed resistance to apoptosis, indicating that the deregulation of apoptosis in MPNs is a mechanism involved in the pathophysiology and clinic-hematological outcome of these diseases (19,29,30).…”

“…The haematopoietic system is subject to a high cellular turnover rate, which makes it particularly sensitive to disturbance in the apoptosis process (28). Altered control of pro-and anti-apoptotic genes and in the relation with JAK2 or STAT5 signaling routes, seem to lead to myeloaccumulation and myeloproliferation, participating in the pathogenesis of MPNs (19,(29)(30)(31)(32)(33)(34)(35).…”

Programmed Cell Death Deregulation in BCR-ABL1-Negative Myeloproliferative Neoplasms

“…Inhibition of BCL-xL induces profound thrombocytopenia by triggered BAK/BAX-mediated mitochondrial damage, caspase activation, and premature death of MKC [39,40]. In MPNs, a concerted effect resulted from antiapoptotic BCL-xL over-expression and proapoptotic BNIP-3 downregulation was clearly documented [41].…”

“…A greater proportion of myeloproliferative MKC express survivin compared to its reciprocal inhibitor, DIABLO. Survivin seems to be the key mediator of the MKC survival signature in the MPNs and might be a potential therapeutic target [41]. Recently, new evidence suggested that survivin may be involved in the evasion of cell death by manipulation of autophagy [49].…”

“…Caspase-8, a key factor in the extrinsic pathway, together with caspase-9, a key factor in the intrinsic pathway, is implicated in regulating MKC turnover [41]. CALR mutations particularly affect the MKC lineage as indicated by the higher mean number of endogenous MKC colonies in CALR-mutant MPNs than those found in JAK2 V617F-positive and triple-negative cases [54].…”