MEF2D-BCL9 B-Lymphoblastic Leukemia Blast Morphology Does Not Always Mimic Mature B-Cell Leukemia.

MEF2D (myocyte enhancer factor 2D)-rearranged acute lymphoblastic leukemia (ALL) has recently been documented by transcriptome sequencing in B-cell precursor ALL. It is associated with older age of onset (median: 14 y), and characterized by very early relapse and poorer outcomes than other B-cell precursor ALL groups. According to report by Suzuki and colleagues, all 4 cases of MEF2D-BCL9-fusion ALL among 59 children with relapsed or primary refractory ALL had leukemic blasts morphologically mimicking mature B-cell leukemia cells. However, we display morphologically different blast populations in 2 patients with MEF2D-BCL9-rearranged ALL. Mature B-cell leukemia-like morphology would aid the detection of MEF2D-BCL9 fusion, but not all cases might have typical morphology.