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Hematological malignancies, including chronic myeloid leukemia (CML), exhibit  ASXL1  mutations; however, the function and molecular mechanism of these mutations remain unclear.  ASXL1  was originally identified as tumor suppressor gene, in which loss of function causes myelodysplastic syndrome (MDS).  ASXL1  mutations are common and associated with disease progression in myeloid malignancies including MDS, acute myeloid leukemia, and similarly in CML. In MDS,  ASXL1  mutations have been associated with poor prognosis; however, the impact of  ASXL1  mutations in CML has not been well described. A 31-year-old male was diagnosed as CML-chronic phase (CP). Laboratory findings showed a white blood cell count of 187,200/µL, with asymptomatic splenomegaly. Blast count was 5.0% in peripheral blood and 7.3% in bone marrow. There was no additional chromosomal abnormality except for t(9;22)(q34;q11.2) by chromosomal analysis. At onset, the Sokal score was 1.4, indicating high risk. The patient received tyrosine kinase inhibitor (TKI) therapy, comprising nilotinib ∼600 mg/day, bosutinib ∼600 mg/day, ponatinib ∼45 mg/day, and dasatinib ∼100 mg/day. Nevertheless, after 1.5 years of continuous TKI therapy, the best outcome was a hematological response. Although additional chromosomal aberrations and  ABL1  kinase mutations were analyzed repeatedly before and during TKI therapy, known genetic abnormalities were not detected. Thereafter, the patient underwent bone marrow transplantation from an HLA 7/8 matched unrelated donor (HLA-Cw 1 locus mismatch, graft-versus-host direction). The patient achieved neutrophil engraftment, 18 days after transplantation, leading to complete remission with an undetectable level of  BCR-ABL1  mRNA. The patient, however, died from graft-versus-host disease and thrombotic microangiopathy after 121 days. Gene sequence analysis of his CML cell before stem cell transplantation revealed  ASXL1  mutations. Physiologically,  ASXL1  contributes to epigenetic regulation. In the CML-CP patient in this case report,  ASXL1  mutation conferred resistance to TKI through obscure resistance mechanisms. Even though a molecular mechanism for TKI resistance in  ASXL1  mutation in CML has remained obscure, epigenetic modulation is a plausible mode of CML disease progression. The clinical impact including prognosis of  ASXL1  for CML is underscored. And the treatment strategy of CML with  ASXL1  mutation has not been established. A discussion of this case was expected to facilitate treatment options.

A Case of Tyrosine Kinase Inhibitor-Resistant Chronic Myeloid Leukemia, Chronic Phase with ASXL1 Mutation