The blasts in the peripheral blood were large with moderate amounts of pale blue agranular cytoplasm. The nuclear membrane was irregular, and the nuclei showed relatively fine chromatin, and a single prominent nucleolus or rarely two nucleoli (Fig. 1a–c). Bone marrow aspirate identified similar cells, and they accounted for >70% of the marrow cells (Fig. 1d,e). Flow cytometry showed expression of CD34, TdT, HLADR, CD123, CD38, and CD79a (39% cells) in the blasts, but the cells were negative CD10, CD19, CD20, cCD22, cIgM, CD1a, CD2, CD3, CD7, CD4, CD8, CD5, cCD3, CD56, CD13, CD14, myeloperoxidase, CD15, lysozyme, CD11b, CD33, CD117, and CD64.
The bone marrow trephine biopsy (BMTB) was hypercellular with a cellularity of >80%. There were sheets of blastic cells that amounted to >80% of the marrow cells (Fig. 1f). These cells expressed CD34, TdT (Fig. 1g), HLADR, CD99, CD79a (Fig. 1h), and PAX5 (Fig. 1i) and were negative for CD117, myeloperoxidase, CD42b, CD68R, CD20, CD10, and CD3. Expression of CD79a and PAX5 demonstrated commitment to B-cell lineage, while most other lineage-specific markers were negative. Based on CD79a and PAX5 expression (demonstrated in ∼40–50% blasts), a diagnosis of B acute lymphoblastic leukaemia (B-ALL) of early precursor (Pro-B) subtype was made. In the absence of the documentation of expression of PAX5 by BMTB, the case would have been classified as acute undifferentiated leukaemia, which is defined by the lack of expression of markers considered specific for lymphoid or myeloid lineages. In BMTB sections, PAX5 expression is considered to be both sensitive and specific for B-cell lineage differentiation especially in the absence of any evidence of myeloid differentiation . PAX5 expression is known to occur in acute myeloid leukaemias (AML) associated with t(8;21) and rarely in other AML .
G-banded chromosome analysis of cultured peripheral blood revealed an abnormal male karyotype, with an extra chromosome 13 and loss of a chromosome 21, in nine of the ten metaphases analyzed (Fig. 1j). These findings were further confirmed by FISH analysis on metaphases and on interphase nuclei. Trisomy 13 is reported to be strongly associated with RUNX1(AML1) mutations and increased FLT3 expression in AML, especially among AML M0 . Whether PAX5 expression had been evaluated in the cases diagnosed as AML M0 with associated trisomy 13 is not clear, as PAX5 expression is generally not evaluated by flow cytometry. Rare cases of B-ALL with trisomy 13 have been reported .