A high rate of telomeric sister chromatid exchange occurs in chronic lymphocytic leukaemia B-cells

Cancer cells protect their telomere ends from erosion through reactivation of telomerase or by using the Alternative Lengthening of Telomere (ALT) mechanism that depends on homologous recombination. Chronic lymphocytic leukaemia (CLL) B cells are characterized by almost no telomerase activity, shelterin deregulation and telomere fusions. To characterize telomeric maintenance mechanisms in B-CLL patients, we measured their telomere length, telomerase expression and the main hallmarks of the ALT activity i.e. C-circle concentration, an extra-chromosomal telomere repeat (ECTR), and the level of telomeric sister chromatid exchange (T-SCE) rate. Patients showed relative homogenous telomere length although almost no TERT transcript and nearly no C-circle were evidenced. Nevertheless, compared with normal B cells, B-CLL cells showed an increase in T-SCE rate that was correlated with a strong down-regulation of the topoisomerase III alpha (TOP3A) expression, involved in the dissolution of Holliday Junctions (HJ), together with an increased expression of SLX1A, SLX4, MUS81 and GEN1, involved in the resolution of HJ. Altogether, our results suggest that the telomere maintenance mechanism of B-CLL cells do not preferentially use telomerase or ALT. Rather, the rupture of the dissolvasome/resolvasome balance may increase telomere shuffling that could homogenize telomere length, slowing telomere erosion in this disease.

Abstract 4055: Non-canonical telomere maintenance mechanism in B-cell chronic lymphocytic leukemia

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

The human epidermal growth factor receptor (EGFR) gene in European patients with advanced colorectal cancer harbors infrequent mutations in its tyrosine kinase domain

BACKGROUND

The epidermal growth factor receptor (EGFR), a member of the ErbB family of receptors, is a transmembrane tyrosine kinase (TK) activated by the binding of extracellular ligands of the EGF-family and involved in triggering the MAPK signaling pathway, which leads to cell proliferation. Mutations in the EGFR tyrosine kinase domain are frequent in non-small-cell lung cancer (NSCLC). However, to date, only very few, mainly non-European, studies have reported rare EGFR mutations in colorectal cancer (CRC).

METHODS

We screened 236 clinical tumor samples from European patients with advanced CRC by direct DNA sequencing to detect potential, as yet unknown mutations, in the EGFR gene exons 18 to 21, mainly covering the EGFR TK catalytic domain.

RESULTS

EGFR sequences showed somatic missense mutations in exons 18 and 20 at a frequency of 2.1% and 0.4% respectively. Somatic SNPs were also found in exons 20 and 21 at a frequency of about 3.1% and 0.4% respectively. Of these mutations, four have not yet been described elsewhere.

CONCLUSIONS

These mutation frequencies are higher than in a similarly sized population characterized by Barber and colleagues, but still too low to account for a major role played by the EGFR gene in CRC.