Response of malignant B lymphocytes to ionizing radiation: gene expression and genotype

The human malignant B-lymphocyte cell lines Reh and U698 show arrest in G2 phase after ionizing radiation (IR), but only Reh cells arrest in G1 phase and die by apoptosis. We have used cDNA microarrays to measure changes in gene expression at 2, 4 and 6 hr after irradiation of Reh and U698 cells with 0.5 and 4 Gy in order to begin exploring the molecular mechanisms underlying the phenotypic changes. We also investigated whether gene expression changes could be caused by possible aberrations of genes, as measured by comparative genomic hybridization. Reh cells showed upregulation of CDKN1A that likely mediated the G1 arrest. In contrast, U698 cells have impaired function of TP53 protein and no activation of CDKN1A, suppressing the arrest in G1. The G2 arrest in both cell lines was likely due to repression of PLK1 and/or CCNF. IR-induced apoptosis in Reh cells was probably mediated by TP53 and CDKN1A, whereas a high expression level of MCL1, caused by gene amplification, and activation of the NFKB pathway may have suppressed the apoptotic response in U698 cells. Genes suggested to be involved in apoptosis were activated long before this phenotype was detectable and showed the same temporal expression profiles as genes involved in cell cycle arrest. Our results suggest that differences in functionality and/or copy number of several genes involved in IR-regulated pathways contributed to the phenotypic differences between Reh and U698 cells after IR, and that multiple molecular factors control the radiation response of malignant B lymphocytes.