Unlimited cancer cells proliferation requires mechanisms that counteract telomere attrition. In the majority of cancer cells this is possible through up-regulation of telomerase activity. Alternatively, cancer cells use homologous recombination between telomeres. This second mechanism called ALT (Alternative Lengthening of Telomere) implicates proteins that either elongates telomeres or that either prevents telomere loss (maintenance). Among these proteins a complex composed of the topoisomerase III alpha (hTopoIIIα), BLM, RMI1 and 2 is require to repair stalled replication forks.
Little is known about the involvement of the ALT mechanism in B-cell chronic lymphocytic leukemia (B-CLL), which shows low telomerase activity, shelterin defect and telomeric dysfunctions.
We found that hTopoIIIα gene expression is downregulated in almost all B-CLL patients tested (94%; n=31), with 60% of B-CLL patients presenting a two time decreased compared to control samples. That prompted us to build the first CpG islands map of the hTopoIIIα promoter and to characterize those that are methylated in B-CLL patients. To investigate the methylation impact, we first treated CLL cell lines with 5-Azacytidine, a chemical analogue of cytidine that induces hypomethylation, and demonstrated that this treatment increases hTopoIIIα expression. In line, luciferase experiments revealed that SSSI-induced CpG islands methylation inside the TopoIIIα promoter leads to a transcriptional inhibition. Nevertheless, deletion mutants of the TopoIIIα promoter suggested that CpG islands found to be methylated in B-CLL patients are not implicated in TopoIIIα expression regulation through methylation.
Interestingly, we didn't observe a good correlation between mRNA and protein expressions in all B-CLL patients (n=6). Indeed some patients with a moderate amount of mRNA showed low level of the corresponding protein hTopoIIIα (<50% compared to controls; n=3). Downregulation of hTopoIIIα may increase recombination rate between sister chromatid. Hence, experiments aiming to evaluate sister chromatid exchange and telomeric sister chromatid exchange rate on B-CLL patients samples according to their hTopoIIIα expression level have been performed and the data will be discuss.
Our results suggest that there is no canonical mechanism maintaining telomeres in B-CLL but rather an overall recombination arising during replication, due to hTopoIIIα downregulation, leading to a silenced genetic instability in the early step of the disease.
Citation Format: Sandrine Medves, Morgan Auchter, Laetitia Chambeau, Sophie Gazzo, Aurélie Verney, Etienne Moussay, Wim Ammerlaan, Hamid Morjani, Vincent Géli, Valérie Palissot, Gilles Salles, Guy Berchem, Thomas Wenner. Non-canonical telomere maintenance mechanism in B-cell chronic lymphocytic leukemia. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4055. doi:10.1158/1538-7445.AM2013-4055