Differential Gene Expression in Leukemic Cells from Bone Marrow (BM) and Peripheral Blood (PB) of Pediatric Patients with Acute Lymphoblastic Leukemia (ALL).

Although the genetic mechanisms underlying the induction of ALL are similar, the molecular events leading to diverse clinical presentation and course are not well characterized. High PB white blood cell (WBC) counts reflect tumor burden in PB and have been used in the past as a clinical risk factor in ALL. We hypothesized that, given differential microenvironment signaling provided by BM and PB, ALLs with high or low WBC counts (WBChigh and WBClow) might differ with regard to their dependence on the microenvironment. To approach this issue, we investigated gene expression changes in PB blasts as compared to their BM counterparts in patients with precursor B-cell ALL (n=15). Blasts were isolated from BM and PB samples by flow sorting and investigated for mRNA levels using Affymetrix HG U133A microarrays. Gene expression data, normalized by variance stabilization on probe level, were analysed in matched PB-BM pairs using a SAM-like analysis of log ratios in a balanced permutation test. By these procedures, a limited number of 38 genes consistently changed in PB vs BM blasts was identified (false discovery rate < 0.01). Leukemic PB cells were characterized by upregulation of genes encoding for cell adhesion- and trafficking-related proteins (CD11b, CD73, S100A4, EMP3). Furthermore, downregulation of the cell cycle- and proliferation-associated genes (thymidylate synthetase TYMS, kinesin KIF11, topoisomerase TOP2A, ribonucleotide reductase RRM2, kinetochore protein ZWINT) indicated decreased proliferative activity of leukemic cells in PB microenvironment. Finally, the gene encoding for Bcl-2 protein, known to be a powerful downstream mediator of survival signaling from microenvironment in various experimental settings, was consistently downregulated in PB as compared to BM blasts. Within the group of WBClow patients (n=8, mean WBC=17x103/μl), most of these genes retained their statistical significance. By contrast, no consistent expression changes in PB vs BM blasts were found in WBChigh patients (n=7, mean WBC=201x103/μl). In summary, investigation of differential gene expression in leukemic cells from PB and BM indicated decreased levels of proliferative and survival signaling in the PB microenvironment and a higher dependence on this signaling of leukemic cells from WBClow as compared to WBChigh patients.