BEX1, a Marker for Acute Myeloid Leukemia Cell Lines with MLL Rearrangements.

Patients with acute leukemias carrying MLL rearrangements have a poor prognosis. The tumor cells show characteristic gene expression profiles with increased levels of selected HOX genes (1). We have shown that acute lymphoblastic leukemia (ALL) cell lines with and without MLL rearrangements could likewise be recognized by analysis of HOX gene expression (2). In contrast, MLL wild-type (MLLwt) and mutant (MLLmu) acute myeloid leukemia (AML) cell lines could not be distinguished by analysis of HOX genes, because even wild-type cell lines had a high expression background (2). It was our aim to find out whether MLLwt and MLLmu AML cell lines could be discriminated on the basis of gene expression - other than HOX genes. We performed gene expression analysis with pooled RNAs of MLLmu (n=8) and MLLwt (n=8) cell lines applying high density oligonucleotide Genechips from Affymetrix (HG-U133A). Defensin alpha4 (83x), defensin beta1 (32x), cathepsinG (9x) and FLT3 (7x) genes were overexpressed in MLLmu cell lines, stabilin1 (82x) and galectin10 (55x) in MLLwt cell lines. PCR analysis with individual (non-pooled) cDNAs of the 16 cell lines showed that none of the above genes was exclusively expressed by MLLmu or MLLwt cells. Thus, pooling RNAs has a major disadvantage: many PCRs have to be performed to establish faithful expression profiles for individual samples. BEX1 finally proved to be a gene that was exclusively overexpressed in one group of cell lines. It had been an interesting candidate already after oligonucleotide chip analysis, being both overexpressed (18x) in MLLmu cell lines, and - in contrast to the genes listed above - not a marker of myeloid differentiation. BEX1 is reportedly expressed in brain, testis and ovary, but not in peripheral blood leukocytes, lymph node and bone marrow (3). By RT-PCR analysis we showed that 7/8 MLLmu and 0/8 MLLwt cell lines expressed BEX1. Screening a panel of 54 hematopoetic cell lines gave the same result: BEX1 expression was restricted to MLLmu AML cell lines: 8/11 (73%) MLLmu AML cell lines expressed BEX1, but 43 other hematopoetic cell lines (including Hodgkin′s disease, anaplastic large cell lymphoma, ALL and MLLwt AML cell lines) tested negative. BEX1 expression may depend on the type of MLL rearrangement or the histological background of the cells, as MLLmu ALL cell lines (0/5) also tested negative. It has been shown in the mouse system, that BEX family members may be involved in cell signalling processes. Thus it will be interesting to elucidate whether BEX1 also participates in proliferative/antiapoptotic signalling processes in MLLmu AML cell lines.