Somatic ASXL1 p.R693X mutation identified by next generation sequencing in isolated myeloid sarcoma involving the mediastinum

Isolated myeloid sarcoma (MS) is characterized by extramedullary immature myeloid cell infiltration without bone marrow involvement. The diagnosis of isolated MS is sometimes difficult in cases without expression of typical immunohistochemical markers, such as CD64, MPO or lysozyme. We report a case of isolated MS involving the mediastinum, with negative staining of MPO and lysozyme, which was misdiagnosed for 20 months. A comprehensive analysis in our institution showed MS with a characteristic staining pattern positive for CD34, CD117 and CD33, but negative for MPO, lysozyme, CD3 and CD79a. Next-generation sequencing (NGS) targeting 112 acute myeloid leukemia (AML)- and myelodysplastic syndromes (MDS)-associated genes confirmed the existence of an ASXL1 p.R693X mutation with a frequency of 13.17% of total cells. The patient acquired sustainable remission under the alternative treatment of intermediate-dose cytarabine and decitabine. The ASXL1 p.R693X mutation, a truncated mutation, has been widely reported to be associated with poor prognosis in myeloid malignance. We report the role of this mutation and recommend the utilization of NGS to discover more profound pathobiological information with limited samples, facilitate the diagnosis, and further clarify the uncertainties of prognosis and treatment in more isolated MS patients.