Co-Expression of BAALC with ERG, TCF4, and CD133 Indicates a Shared Pathway in Acute Myeloid and Lymphoblastic Leukemia.

Abstract

The human gene BAALC (Brain And Acute Leukemia, Cytoplasmic) is a molecular marker of hematopoietic progenitor cells and is aberrantly expressed in subsets of acute myeloid (AML) and lymphoblastic (ALL) leukemias. High mRNA expression levels of BAALC have been shown to adversely impact outcome in newly diagnosed AML patients (pts) with normal cytogenetics. To gain insight into the functional role of BAALC and its significance to normal hematopoiesis and leukemogenesis we compared gene expression profiles of normal CD34+ progenitors with those of AML and ALL blasts (using oligonucleotide microarrays; HG-U133 plus 2.0, Affymetrix, Santa Clara, CA). First we explored the regulation of BAALC expression during lineage specific maturation of in vitro differentiated human CD34+ bone marrow cells selected from healthy individuals. Microarray analyses were carried out using CD34+ cells stimulated in vitro with EPO, TPO, or G/GM-CSF to induce lineage-specific differentiation. At day 0 of culture and at three different time points during differentiation (days 4, 7, 11) cells were harvested, and if necessary purified by immunomagnetic beads and used for microarray studies. Experiments of all lineages and time points were done in triplicates. A total of 276 genes were identified showing similar changes in expression (with downregulation during differentiation) as BAALC at the three time points in all lineages with a correlation coefficient of R>0.95. This set of 276 BAALC co-expressed genes was investigated in an AML expression dataset generated from 51 adult pts with newly diagnosed de novo AML and normal cytogenetics (Cancer and Leukemia Group B). After exclusion of probesets expressed in fewer than 20% of pt samples, 21 probesets representing 14 named genes 6 of which are known to be involved in AML (BAALC, CD34, CD133, SOX4, ERG, SEPT6) and 4 implicated in lymphoid development (TCF4, SH2D1A, ITM2A, ITM2C) were found to be overexpressed (a significance level of P=0.01 was used) in pts of the highest third compared to pts of the lowest third of BAALC expression values as measured by real-time RT-PCR. We next applied these same 21 BAALC co-expressed probesets to an ALL expression dataset generated from 66 adult pts with newly diagnosed standard risk B-lineage precursor ALL (from the German ALL GMALL study group). A BAALC specific cluster uncovered 7 probesets representing 4 different co-expressed genes: BAALC, CD133, and the transcription factors ERG and TCF4. Thus, applying a BAALC specific expression signature to AML and ALL gene expression profiles revealed 3 genes (CD133, ERG, TCF4), which are highly associated with BAALC in myeloid and lymphoid blasts. Interestingly in non-malignant lymphoid and myeloid cells the oncogeneic ETS transcription factor ERG has shown specificity to immature cells, while its mechanistical role in leukemogenesis remains unknown. ERG and TCF4 may directly regulate BAALC and indicate a specific pathway implicated in leukemogenesis, while co-expression of CD133 and BAALC suggests shared stem cell characteristics. Functional studies are in progress to further explore these findings.