No abstract

MPLW515K or W515L mutation plays an important role in the pathogenesis of myeloproliferative neoplasms (MPNs) through signaling molecules of the cytokine receptor axis. Besides MPLW515K or W515L , more than 30 atypical MPL mutations have been reported in patients who are negative for JAK2V617F, MPLW515K/L , and CALR mutations. Here, we aimed to identify the disease‐causing mutations in the triple‐negative case of ET. We described two MPL mutations in patients diagnosed with ET by target sequencing the hotspot mutation region of MPL gene. The MPLA497‐L498ins4 is an insertion mutation detected recurrently in ET patients, and the MPLW515RQ516E is a novel double‐point mutation found in an ET patient. Functional studies of MPLA497‐L498ins4 and MPLW515RQ516E revealed that they are gain‐of‐function mutations. Mutants of MPLA497‐L498ins4 and MPLW515RQ516E promoted autonomous proliferation on Ba/F3 cells in the absence of IL‐3. Autonomous activation of TPO‐R without ligand TPO was observed in MPLA497‐L498ins4 and MPLW515RQ516E mutants. Lower percentage of cells in G1 phase and higher percentage of cells in S phase of two atypical MPL mutants were detected after culturing without any cytokines. These two atypical MPL mutations also presented increase in phosphorylation of signaling proteins including JAK2/STAT, PI3K/AKT, and MAPK/RAS. In summary, the MPLA497‐L498ins4 and MPLW515RQ516E are gain‐of‐function mutations which may be novel driving factors participating in the pathogenesis of triple‐negative MPN.

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In chronic myeloid leukemia (CML) patients, the involvement of the BCR/ABL1 isoform in tyrosine kinase inhibitors (TKIs) resistance has attracted lots of attention. In this work, a novel isoform that encoded truncated protein due to the deletion of ABL1 exon7, 8, and 9 was reported and named BCR/ABL1ΔE7‐8‐9 here. This isoform was detected only in 10.2% of CML patients with inadequate responses to TKIs. BCR/ABL1Δexon7‐8‐9 isoform promoted S phase cell proliferation and reduced the expression of fusion gene and ABL1 phosphorylation level more slowly than that of control cells after TKIs treatment. The novel isoform has the qualities of a functional tyrosine kinase, localized in the cytoplasm, and could not be imported into the nucleus by TKIs. These results indicated that BCR/ABL1Δexon7‐8‐9 showed poorer sensitivity to imatinib and nilotinib than wild‐type BCR/ABL1. According to molecular docking studies, nilotinib and imatinib present different binding sites and have a lower binding capacity with BCR/ABL1ΔE7‐8‐9 protein than the wild type. Our findings suggested that the novel isoform BCR/ABL1ΔE7‐8‐9 may contribute to TKIs resistance in CML due to its weakened TKIs binding ability. It enriched the mechanism of spliceosome involved in TKIs resistance. Monitoring the expression of BCR/ABL1ΔE7‐8‐9 helps guide the treatment of CML patients in the clinic.