Expression of the Chk2 Gene Is Downregulated in Hodgkin’s Lymphoma Cell Lines Via Epigenetic Mechanisms.

Chk2, the mammalian homologue of the yeast Rad53 and Cds1 genes, encodes a nuclear serine/threonine kinase that plays a crucial role in the DNA damage response and helps guard the integrity of the genome by regulating cell-cycle checkpoints, DNA repair and apoptosis. Furthermore Chk2 is regarded as a tumor suppressor gene. Alterations in tumor suppressors that are involved in the DNA damage response have been reported to be frequently involved in the pathogenesis of lymphoid malignancies. We investigated the expression levels of the genes encoding Chk2 in nine cell lines from lymphoid malignancies, including three Hodgkin’s lymphoma (HL) cell lines. We found that all three HL cell lines exhibited a drastic reduction in Chk2 mRNA and protein expression compared to the other cell lines without any apparent mutation of the Chk2 gene. Accumulating evidence demonstrates the importance of posttranslational modification of histone proteins in addition to DNA methylation, as epigenetic mechanisms involved in the organization of chromosomal domains and gene regulation. It is now generally appreciated that hyperacetylated histones H3 and H4 are associated with activated genomic regions, while hypoacetylation of histones H3, H4 and methylation on H3-lysine 9 results in gene repression and silencing. Therefore we tested the possibility that epigenetic mechanisms are involved in this aberrant expression of the Chk2 gene in these cells using the histone deacetylase (HDAC) inhibitors trichostatin A (TsA) and sodium butyrate (SB). Expression of Chk2 in HL cells was restored following treatment with the TsA and SB, or 5Aza-dC. Chromatin-immunoprecipitation (Chip) assays revealed that treatment of HL cells with TsA, SB or 5Aza-dC resulted in increased levels of acetylated histones H3 and H4, and decreased levels of dimethylated H3 lysine 9 at the Chk2 promoter. These results indicate that expression of the Chk2 gene is down-regulated in HL cells via epigenetic mechanisms. Furthermore we examined whether the down regulation of Chk2 gene in HL cells may be involved in apoptosis resistance to irradiation (IR). We found that upregulation of Chk2 in HL cells following treatment by SB resulted in increased susceptibility of the cells to IR. Therefore, our findings suggested that altered epigenetic regulation of Chk2 gene in HL cells results in the down-regulation of Chk2 kinase and abrogates DNA damage response signaling in the cells