RUNX1 rearrangement in mature B-cell acute lymphoblastic leukemia with non-L3 morphology

Abstract

Mature B-cell acute lymphoblastic leukemia (ALL) is defined by the expression of light chain-restricted surface immunoglobulin (sIg) and usually has features of the leukemic phase of Burkitt lymphoma including FAB-L3 morphology and MYC rearrangement. Recently, another distinct entity in childhood mature B-cell ALL has been characterized as non-L3 morphology and KMT2A rearrangement. Here we report an unusual case of mature B-cell ALL that presented with RUNX1 rearrangement. A 65-year-old male was admitted to our department for thorough examination of leukocytosis and thrombocytopenia. The patient's bone marrow was hypercellular and infiltrated with 97.8% myeloperoxidase-negative, medium-to-large-sized blasts without cytoplasmic vacuoles. Immunophenotypes were characterized by the presence of light chain-restricted sIg and the lack of immature markers, indicating a diagnosis of mature B-cell ALL with L2 morphology: sIg-κ+, CD19+, CD20+, CD22+, CD79a+, TdT-, and CD34-. G-banding combined with spectral karyotyping showed the following complex karyotype: 45,X,der(Y;10)(p10;q10),del(13)(q?),inv(21)(p13q22.1). Fluorescence in situ hybridization revealed separated signals of RUNX1 at 21q22.1, whereas rearrangements of MYC and KMT2A were not found. To our knowledge, inv(21)(p13q22.1) involving RUNX1 is a novel cytogenetic aberration and this is the first case of mature B-cell ALL that presented with RUNX1 rearrangement. Thus, RUNX1 may be implicated in the pathogenesis of mature B-cell ALL showing non-L3 morphology without MYC rearrangement.

Keywords: RUNX1 rearrangement; fluorescence in situ hybridization; mature B-cell acute lymphoblastic leukemia; non-L3 morphology.

Fig. 1

Morphologic and immunophenotypic findings of leukemia cells. A Bone marrow smears show medium-to-large lymphoblasts having fine nuclear chromatin, basophilic cytoplasm, and a few nucleoli. Cytoplasmic vacuoles are not seen (May–Grünwald–Giemsa stain, ×1000). B Bone marrow lymphoblasts are negative for myeloperoxidase staining, while myeloid cells are positive. C Flow cytometry analysis of bone marrow lymphoblasts. The percentage of cells delimited by CD45/side scatter gating is 95.5%. The results of two-color analyses for the indicated markers are demonstrated. Lymphoblasts are positive (>20%) for CD19, CD22, CD79a, CD10, CD20, CD38, HLA-DR, and sIg κ-chain.

Fig. 2

Cytogenetic findings of leukemia cells. A G-banded karyotype of leukemia cells: 45,X,-Y,add(10)(p11.2),del(13)(q ?),-21,+mar1. Arrows point to rearranged chromosomes. B Spectral karyotyping (SKY) (right, SKY; left, reverse DAPI). The karyotype is corrected to 45,X,der(Y;10)(p10;q10),del(13)(q?),inv(21)(p13q22.1). Arrows point to rearranged chromosomes.

Fig. 3

Fluorescence in situ hybridization (FISH) analyses of leukemia cells. A FISH using CytoCell RUNX1 breakapart probe on metaphase spreads. The arrows indicate 1) a normal 5’ RUNX1/3’ RUNX1 fusion signal (yellow) on a normal chromosome 21, and 2) a faint 5’ RUNX1 signal (green) at 21q22.1, and a 5’ RUNX1 signal (green) and 3’ RUNX1 signal (red) at 21p13 on the inv(21)(p13q22.1). B FISH using a CytoCell RUNX1 breakapart probe on interphase nuclei. Red/green, green, and yellow signals are observed on an interphase cell. C Schematic representation of the RUNX1 gene and assumed breakpoints in the 21q22.1 region. The position of a CytoCell RUNX1 probe (5’ RUNX1 and 3’ RUNX1 probes) covering the RUNX1 gene on a normal chromosome 21 is presented. The vertical arrow indicates the assumed breakpoint; that is, middle of the region covered by the 5’ RUNX1 probe. D Idiograms showing G-banding patterns of normal chromosome 21 (left) and inv(21)(p13q22.1) (right) at 300-band levels. Chromosome breakpoints are shown by horizontal arrows. The locations of 5’ RUNX1 (green) and 3’ RUNX1 (red) signals are indicated on the right sides of the normal and inverted chromosomes.