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We report the clinical presentation and risk factors for survival in 175 patients with myeloproliferative neoplasms (MPN) and COVID-19, diagnosed between February and June 2020. After a median follow-up of 50 days, mortality was higher than in the general population and reached 48% in myelofibrosis (MF). Univariate analysis, showed a significant relationship between death and age, male gender, decreased lymphocyte counts, need for respiratory support, comorbidities and diagnosis of MF, while no association with essential thrombocythemia (ET), polycythemia vera (PV), and prefibrotic-PMF (pre-PMF) was found. Regarding MPN-directed therapy ongoing at the time of COVID-19 diagnosis, Ruxolitinib (Ruxo) was significantly more frequent in patients who died in comparison with survivors (p = 0.006). Conversely, multivariable analysis found no effect of Ruxo alone on mortality, but highlighted an increased risk of death in the 11 out of 45 patients who discontinued treatment. These findings were also confirmed in a propensity score matching analysis. In conclusion, we found a high risk of mortality during COVID-19 infection among MPN patients, especially in MF patients and/or discontinuing Ruxo at COVID-19 diagnosis. These findings call for deeper investigation on the role of Ruxo treatment and its interruption, in affecting mortality in MPN patients with COVID-19.

EZH2, a component of the polycomb repressive complex 2, catalyses the trimethylation of histone H3 lysine 27, a chromatin mark associated with transcriptional repression. EZH2 loss-of-function mutations are seen in myeloid neoplasms and are associated with an adverse prognosis. Missense mutations in the SET/CXC domain abrogate catalytic activity as assessed by in vitro histone methylation assays, but missense mutations clustering in the conserved DI and DII regions retain activity. To understand the role of DI and DII mutations, we initially developed a cell-based histone methylation assay to test activity in a cellular context. Murine induced pluripotent stem cells lacking EZH2 were transiently transfected with wild type or mutant EZH2 (n = 15) and any resulting histone methylation was measured by flow cytometry. All DI mutations (n = 5) resulted in complete or partial loss of methylation activity whilst 5/6 DII mutations retained activity. Next, we assessed the possibility of splicing abnormalities induced by exon 8 mutations (encoding DII) using RT-PCR from primary patient samples and mini-gene assays. Exon 8 mutations resulted in skipping of exon 8 and an out-of-frame transcript. We have therefore shown that mutations within regions encoding EZH2 domains DI and DII are pathogenic by loss of function and exon skipping, respectively.

Determining the underlying cause of persistent eosinophilia is important for effective clinical management but remains a diagnostic challenge in many cases. We identified STAT5B N642H, an established oncogenic mutation, in 27/1715 (1.6%) cases referred for investigation of eosinophilia. Of the 27 mutated cases, a working diagnosis of hypereosinophilic syndrome (HES; n = 7) or a myeloid neoplasm with eosinophilia (n = 20) had been made prior to the detection of STAT5B N642H. Myeloid panel analysis identified a median of 2 additional mutated genes (range 0-4) with 4 cases having STAT5B N642H as a sole abnormality. STAT5B N642H was absent in cultured T cells of 4/4 positive cases. Individuals with SF3B1 mutations (9/27; 33%) or STAT5B N642H as a sole abnormality had a markedly better overall survival compared to cases with other additional mutations (median 65 months vs. 14 months; hazard ratio = 8.1; P < 0.001). The overall survival of STAT5B-mutated HES cases was only 30 months, suggesting that these cases should be reclassified as chronic eosinophilic leukemia, not otherwise specified (CEL-NOS). The finding of STAT5B N642H as a recurrent mutation in myeloid neoplasia with eosinophilia provides a new diagnostic and prognostic marker as well as a potential target for therapy.