Investigation of the Mechanism That Underlies MS32 Minisatellite Instability in Cells That Use the Alternative Lengthening of Telomeres Pathway

Cancer cells escape senescence by activating a telomere maintenance mechanism (TMM) to elongate telomeres and continue dividing. The most common TMM is the enzyme telomerase that adds telomeric repeats. However, some cancer cells activate the Alternative Lengthening of Telomeres (ALT), a recombination-based mechanism to extend shortened telomeres. One of the most peculiar features of ALT+ cells is the instability at the MS32 minisatellite (D1S8), especially as six other minisatellites remained stable in these cells. As MS32 instability correlates with activation of the ALT mechanism, it is likely that the underlying process depends, at least in part, on the same proteins. Thus, a better understanding of the molecular mechanism that underlies ALT may be gained through knowing how and why the MS32 minisatellite becomes unstable in ALT+ cells. Several hypotheses that might explain MS32 instability in ALT+ cells were investigated. In this study it was shown that the instability is restricted to the minisatellite itself and no transcriptional or copy-number changes distinguish this region between ALT+ and non-ALT cells. Interestingly, changes in the DNA methylation-status adjacent to one end of the minisatellite were found, which might indicate that ALT+ cells have a different chromatin conformation around the MS32 minisatellite. Additionally, the mutant molecules arising at MS32 in ALT+ cells seem to derive from intra-allelic processes. Also, EXO1 expression was higher in ALT+ compared to ALT- cells. Thus, our current model proposes that a protein (perhaps hEXO1) involved in lagging-strand synthesis and DNA repair is preferentially recruited to the telomeres in ALT+ cells and this may cause the accumulation of unprocessed 5’ DNA flaps at MS32 during replication. Subsequent DNA repair at MS32, by error-prone processes, may underlie the instability seen in ALT+ cells.