ZEB1 Promotes Alternate Lengthening of Telomeres at Multiple Levels

Approximately 10-15% of cancers rely on Alternate Lengthening of Telomeres (ALT), which is a telomerase-independent, BIR (Break-Induced Replication)-based pathway for maintaining long-term replicative potential.

As ALT is over-represented in mesenchymal-type tumors, we investigated, via RNA-seq, the extent to which the EMT-promoting factor ZEB1 regulates this pathway.

The ALT-associated genes targeted by ZEB1, including PML, RMI2, POLD4, RPA3 (induced), SLX4, and WRN (repressed), in the aggregate, suggest that it regulates ALT at multiple steps in that pathway. ZEB1-deficient cells showed a significant reduction in telomere length as well as in two hallmarks of ALT, C-circle levels and the size and number of ALT-associated PML Bodies (APBs), which are the telomere-aggregating compartments in which BIR occurs. As one of the most highly regulated genetic targets of ZEB1 was the pro-epithelial alternative splicing factor ESRP1, we investigated whether the repression of this factor was required to generate the PML splice variant isoform IV, which is the major structural component of APBs. We found an inverse relationship between the expression of this protein and levels of PML isoform IV mRNA.

These findings suggest a novel role for ZEB1 in promoting ALT both transcriptionally and post-transcriptionally at multiple levels.