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Mutations of the calreticulin gene (CALRmut) are driver mutations known in the development of myeloproliferative neoplasms (MPN). CALRmut helps to downstream Janus kinase 2/signal transducer and activator of transcription 5 (JAK2/STAT5) signaling through interaction with, and activation of, the thrombopoietin receptor (MPL). Studies have also suggested that CALR expression in normal CD34+ cells increases as they differentiate to megakaryocytes (Mks). However, CALR has a wide-ranging action in protein and calcium homeostasis, and therefore, additional mechanisms may contribute to CALRmut MPN. The MPN Hub recently published an article exploring the mechanism of CALR 52-bp deletion (CALRdel52), which can be found here.
In a newly published study in Blood Advances, Balliu et al,1 investigated the mechanism contributing to CALR-mutated MPN through abnormal activation of the interleukin 6 (IL-6) pathway. The key findings are summarized below.
In vitro and ex vivo experiments using cell lines included granulocyte/macrophage colony-stimulating factor (GM-CSF)-dependent UT7 and GM-CSF and thrombopoietin (TPO)-dependent UT7/mpl. CD34+ cells from healthy donors were collected. CD34+, UT7, and UT7/mpl cell lines and CALR type 1 (DEL) variants were used to generate CALR knockout (KO) cells. CRISPR/CRISPR associated protein 9 (Cas9) genome editing was used for CD34+ and green fluorescent protein-positive (GFP+) transfected cells. Chromatin immunoprecipitation (ChIP) assay was used to assess IL-6 promoter region chromatin occupancy.
These data demonstrate the acquired cytokine independence, involvement in Mk differentiation, increased IL-6 mRNA levels and extracellular release of IL-6 accompanied with higher membrane-associated gp130 and IL-6R in cells, and increase in p-JAK1 and p-STAT3 levels. The study demonstrated an abnormal interaction between mutated CALR and gp130 and IL-6R. Combination of low nanomolar concentrations of TCZ and ruxolitinib led to lesser levels of CFU-Mks. Overall, there may be a potential for target inhibition of IL-6 signaling to have a therapeutic role in CALR- and possibly JAK2V617F-mutated MPN. However, further evaluation is warranted to investigate the therapeutic efficacy of combined JAK2/IL-6 inhibition and paracrine effects of abnormal IL-6 pathway activation in MPN.
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