Subtelomeric DNA hypomethylation is not required for telomeric sister chromatid exchanges in ALT cells

Abstract

Most human tumor cells acquire immortality by activating the expression of telomerase, a ribonucleoprotein that maintains stable telomere lengths at chromosome ends throughout cell divisions. Other tumors use an alternative mechanism of telomere lengthening (ALT), characterized by high frequencies of telomeric sister chromatid exchanges (T-SCEs). Mechanisms of ALT activation are still poorly understood, but recent studies suggest that DNA hypomethylation of chromosome ends might contribute to the process by facilitating T-SCEs. Here, we show that ALT/T-SCE high tumor cells display low DNAmethylation levels at the D4Z4 and DNF92 subtelomeric sequences. Surprisingly, however, the same sequences retained high methylation levels in ALT/T-SCE high SV40-immortalized fibroblasts. Moreover, T-SCE rates were efficiently reduced by ectopic expression of active telomerase in ALT tumor cells, even though subtelomeric sequences remained hypomethylated. We also show that hypomethylation of subtelomeric sequences in ALT tumor cells is correlated with genome-wide hypomethylation of Alu repeats and pericentromeric Sat2 DNA sequences. Overall, this study suggests that, although subtelomeric DNA hypomethylation is often coincident with the ALT process in human tumor cells, it is not required for T-SCE.

CpG-island promoters drive transcription of human telomeres

“…5A). This is consistent with recent observations showing that telomerase-positive human cancer cells carry hypermethylated subtelomeric DNA compared to telomerase-negative cancer or primary human cells (Vera et al 2008;Ng et al 2009;Tilman et al 2009). We then analyzed the methylation state of 61-29-37 repeats in human HCT116 cells knocked-out for DNMT1 (DNMT1 (Figs.…”

Assembly of telomeric chromatin to create ALTernative endings

“…Comparisons between telomeric chromatin of ALT and telomerase expressing cells have revealed that ALT telomeres exhibit a more relaxed and accessible chromatin structure with reduced nucleosome density ( Figure 1B) [49]. Sub-telomeric patterns of DNA methylation appear heterogeneous and at a reduced level [45,50], as do the levels of H3K9me3 [49]. This correlates with the elevated levels of TERRA that are detected in ALT cells ( Figure 1B) [20].…”

DNA polymorphism and epigenetic marks modulate the affinity of a scaffold/matrix attachment region to the nuclear matrix

“…It is known that the methylation level of the D4Z4 array varies between normal cells, FSHD myoblasts and cervical carcinoma cells. 19,[29][30][31][32] However, the methylation status of the FR-MAR sequence was not studied earlier. The total DNA was isolated from several muscular and cervical cell lines and treated as described in Materials and methods section.…”

“…Other genetic and epigenetic abnormalities affecting the subtelomeric region of chromosome 4q have also been observed in various pathologies including the Rett and ICF syndromes, 25 as well as in several types of cancer. [26][27][28][29][30][31][32] Here, we have tested whether mechanisms controlling the association of the FR-MAR to the NM could be similar in different cell types. We have used primary myoblasts from FSHD patients and cervical carcinoma cell lines where the D4Z4 repeat array can be either strongly methylated or demethylated.…”