Janus kinase 2 (V617F) mutation status, signal transducer and activator of transcription-3 phosphorylation and impaired neutrophil apoptosis in myelofibrosis with myeloid metaplasia

An activating point mutation in Janus kinase 2 ( JAK2 V617F) was recently identified in myelofibrosis with myeloid metaplasia ( MMM). To further elucidate the pathogenic significance, we examined the JAK2 mutation burden, phosphorylation of JAK2 substrates and neutrophil apoptotic resistance. Immunoblotting revealed phosphorylation of signal transducer and activator of transcription-3 ( STAT3) in all four JAK2 with high V617F mutant allele burden and seven of eight with intermediate mutant allele burden, but only one of eight with wild-type JAK2 ( P < 0.001). In contrast, STAT5 phosphorylation was undetectable in patient MMM neutrophils; and phosphorylation of Akt and extracellular signal-regulated kinases ( ERKs) failed to correlate with JAK2 mutation status. Apoptosis was lower in MMM neutrophils ( median 41% apoptotic cells, n = 50) compared to controls ( median 66%, n = 9) or other myeloproliferative disorder patients ( median 53%, n = 11; P = 0.002). Apoptotic resistance in MMM correlated with anemia ( P = 0.01) and the JAK2-V617F ( P = 0.01). Indeed, apoptotic resistance was greatest in MMM neutrophils with high mutant allele burden ( median 22% apoptosis, n 5) than with intermediate burden ( median 39%, n 23) or wild-type JAK2 ( median 47%, n = 22; P = 0.008). These results suggest that mutant JAK2 contributes to MMM pathogenesis by constitutively phosphorylating STAT3 and diminishing myeloid cell apoptosis.