TRF2-independent chromosome end protection during pluripotency

Mammalian telomeres protect chromosome ends from aberrant DNA repair(1). TRF2, a component of the telomere-specific shelterin protein complex, facilitates end protection through sequestration of the terminal telomere repeat sequence within a lariat T-loop structure(2,3). Deleting TRF2 (also known as TERF2) in somatic cells abolishes T-loop formation, which coincides with telomere deprotection, chromosome end-to-end fusions and inviability(3-9). Here we establish that, by contrast, TRF2 is largely dispensable for telomere protection in mouse pluripotent embryonic stem (ES) and epiblast stem cells. ES cell telomeres devoid of TRF2 instead activate an attenuated telomeric DNA damage response that lacks accompanying telomere fusions, and propagate for multiple generations. The induction of telomere dysfunction in ES cells, consistent with somatic deletion of Trf2 (also known as Terf2), occurs only following the removal of the entire shelterin complex. Consistent with TRF2 being largely dispensable for telomere protection specifically during early embryonic development, cells exiting pluripotency rapidly switch to TRF2-dependent end protection. In addition, Trf2-null embryos arrest before implantation, with evidence of strong DNA damage response signalling and apoptosis specifically in the non-pluripotent compartment. Finally, we show that ES cells form T-loops independently of TRF2, which reveals why TRF2 is dispensable for end protection during pluripotency. Collectively, these data establish that telomere protection is solved by distinct mechanisms in pluripotent and somatic tissues. Experiments in mouse pluripotent embryonic and epiblast stem cells show that TRF2 is dispensable for telomere protection specifically specifically in the pluripotent cells that form during early embryonic development, when cells form T-loops independently of this protein.

Automated summary

This study demonstrates that in mouse pluripotent embryonic stem cells and epiblast stem cells, TRF2 is largely dispensable for telomere protection, with cells exhibiting an attenuated telomeric DNA damage response and the ability to propagate for multiple generations even in the absence of TRF2. Cells exiting pluripotency rapidly switch to TRF2-dependent end protection, and embryos lacking Trf2 arrest before implantation with evidence of DNA damage response signaling and apoptosis in the non-pluripotent compartment. The formation of T-loops independently of TRF2 in ES cells explains why TRF2 is not essential for end protection during pluripotency.